Sagittal MR image of the knee

Para-sagittal MR images of the brain

Magnetic resonance imaging (MRI) is a medical imaging technique primarily used in Radiology to visualize the structure and function of the body. It provides detailed images of the body in any plane. MRI provides much greater contrast between the different soft tissues of the body than does computed tomography (CT), making it especially useful in neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer) imaging. Unlike CT it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body. Radiofrequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body. MRI can mean: Magnetic Resonance Imaging, a medical imaging technique The mri, a fictional alien species Manchester Royal Infirmary, a hospital in Manchester, England A Member of the Royal Institution This page concerning a three-letter acronym or abbreviation is a disambiguation page—a list of articles associated with the... Medical imaging designates the ensemble of techniques and processes used to create images of the human body (or parts thereof) for clinical purposes (medical procedures seeking to reveal, diagnose or examine disease) or medical science (including the study of normal anatomy and function). ... Image A: A normal chest X-ray. ... Left side of the image has low contrast, the right has higher contrast. ... negron305 Cat scan redirects here. ... Neurology is a branch of medicine dealing with disorders of the nervous system. ... The human musculoskeletal system is the musculoskeletal system that gives us the ability to move. ... See cancer for the biology of the disease, as well as a list of malignant diseases. ... Radiation hazard symbol. ... For other senses of this word, see magnetism (disambiguation). ... The nuclear magnetic moment is the magnetic moment of an atomic nucleus and arises from the spin of the protons and neutrons. ... This article is about the chemistry of hydrogen. ... For other uses, see Atom (disambiguation). ... Radio waves are electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum. ...

Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. In its early years the technique was referred to as nuclear magnetic resonance imaging (NMRI). However, as the word nuclear was associated in the public mind with ionizing radiation exposure it is generally now referred to simply as MRI. Scientists still use the term NMRI when discussing non-medical devices operating on the same principles. One of the contributors to modern MRI, Paul Lauterbur, originally named the technique zeugmatography, a Greek term meaning "that which is used for joining".^[1] The term referred to the interaction between the static and the gradient magnetic fields necessary to create an image, but this term was not adopted. NMR redirects here. ... Radiation hazard symbol. ... Paul Christian Lauterbur, (born May 6, 1929) is an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging (MRI) possible. ...

How MRI works

Brief lay explanation of MRI physics

When a person is in the scanner, the hydrogen nuclei (i.e., protons) found in abundance in the human body in water molecules, align with the strong magnetic field. A radio wave at just the right frequency for the protons to absorb energy pushes some of the protons out of alignment. The protons then snap back to alignment, producing a detectable rotating magnetic field as they do so. Since protons in different tissues of the body (e.g., fat v. muscle) realign at different speeds, the different structures of the body can be revealed.

Gradient fields in the three dimensions allow the scanner to work only with protons from a "slice" at a time, allowing the creation of a whole volume that can be looked at in three dimensions.

Contrast agents may be injected intravenously to show enhancement of blood vessels, tumors or inflammation. Unlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants and cardiac pacemakers are prevented from having an MRI due to effects of the powerful magnetic field and powerful radio waves. Radiocontrast agents (also simply contrast agents or contrast materials) are compounds used to improve the visibility of internal bodily structures in an X-ray image. ... Intravenous therapy or IV therapy is the giving of liquid substances directly into a vein. ... f you all The blood vessels are part of the circulatory system and function to transport blood throughout the body. ... Neoplasia (literally: new growth) is sudden and abnormal growth in a tissue or organ. ... An abscess on the skin, showing the redness and swelling characteristic of inflammation. ...

MRI is used to image every part of the body, but is particularly useful in neurological conditions, disorders of the muscles and joints, for evaluating tumors and showing abnormalities in the heart and blood vessels.

Physics Principles

Modern 3 tesla clinical MRI scanner.

Image File history File linksMetadata Download high-resolution version (1600x1200, 191 KB) Modern high field clinical MRI scanner. ... Image File history File linksMetadata Download high-resolution version (1600x1200, 191 KB) Modern high field clinical MRI scanner. ... SI unit. ...

Nuclear Magnetism (for more details on this topic see Nuclear magnetic resonance)

Subatomic particles such as protons have the quantum mechanical property of spin. Certain nuclei such as ¹H (protons), ²H, ³He, ²³Na or ³¹P, have a non–zero spin and therefore a magnetic moment. In the case of the so-called spin-1/2 nuclei, such as ¹H, there are two spin states, sometimes referred to as "up" and "down". Nuclei such as ¹²C have no unpaired neutrons or protons, and no net spin: however the isotope ¹³C (referred to in this context as "carbon 13") does. NMR redirects here. ... For alternative meanings see proton (disambiguation). ... For a generally accessible and less technical introduction to the topic, see Introduction to quantum mechanics. ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... In physics, the magnetic moment of an object is a vector relating the aligning torque in a magnetic field experienced by the object to the field vector itself. ...

When these spins are placed in a strong external magnetic field they precess around an axis along the direction of the field. Protons align in two energy eigenstates (Zeeman effect) one low-energy, and one high-energy, which are separated by a quantum of energy. Precession redirects here. ... The Zeeman effect (IPA ) is the splitting of a spectral line into several components in the presence of a magnetic field. ...

Resonance and Relaxation

In the static magnetic fields commonly used in MRI, the energy difference between the nuclear spin states corresponds to electromagnetic radiation at radio frequency (rf) wavelengths. Resonant absorption of energy by the protons due to an external oscillating magnetic field will occur at the Larmor frequency for the particular nucleus. It has been suggested that this article or section be merged with Radio waves. ... Larmor precession refers to the precession of the magnetic moments of electrons or atomic nucleii in atoms around the direction of an external magnetic field. ...

The net magnetization vector has two components. The longitudinal magnetization is due to an excess of protons in the lower energy state. This gives a net polarization parallel to the external field. The transverse magnetization is due to coherences forming between the two proton energy states. This gives a net polarization perpendicular to the external field in the transverse plane. The recovery of longitudinal magnetization is called T1 relaxation and the loss of phase coherence in the transverse plane is called T2 relaxation. T1 is thus associated with the enthalpy of the spin system while T2 is associated with its entropy. In physics, atomic coherence is the induced coherence between levels of a multi-level atomic system sometimes observed when it interacts with a coherent electromagnetic field. ... This article or section is in need of attention from an expert on the subject. ... t In thermodynamics and molecular chemistry, the enthalpy or heat content (denoted as H or ΔH, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the useful work obtainable from a closed thermodynamic system under constant pressure. ... For other uses, see: information entropy (in information theory) and entropy (disambiguation). ...

When the radio frequency pulse is turned off, the transverse vector component produces an oscillating magnetic field which induces a small current in the receiver coil. This signal is called the free induction decay (FID). In an idealized nuclear magnetic resonance experiment, the FID decays with a time constant T2, but in practical MRI small differences in the static magnetic field at different spatial locations cause the Larmor frequency to vary across the body creating destructive interference which shortens the FID. The time constant for the observed decay of the FID is called the T2* ("T 2 star") relaxation time, and is always shorter than T2. A free induction decay (FID) curve is generated as excited nuclei relax in an NMR machine. ... NMR redirects here. ...

In MRI, the static magnetic field is caused to vary across the body (a field gradient), so that different spatial locations become associated with different precession frequencies. Usually these field gradients are pulsed, and it is the almost infinite variety of rf and gradient pulse sequences that gives MRI its versatility. Application of field gradient destroys the FID signal, but this can be recovered and measured by a refocusing gradient (to create a so-called "gradient echo" which imparts a T2*-weighting to the signal), or by a radio frequency pulse (to create a so-called "spin-echo" which imparts a T2-weighting to the signal). The whole process can be repeated when T1-relaxation is complete and the thermal equilibrium of the spins is restored, although usually MRI pulse sequences are repeated much faster than this, imparting an additional T1-weighting to the signal.

Typically in soft tissues T1 is around 1 second while T2 and T2* are a few tens of milliseconds, but these values vary widely between different tissues (and different external magnetic fields), giving MRI its tremendous soft tissue contrast.

Imaging

A number of schemes have been devised for combining field gradients and radiofrequency excitation to create an image. One involves 2D or 3D reconstruction from projections, much as in Computed Tomography. Others involve building the image point-by-point or line-by-line. One even uses gradients in the rf field rather than the static field. Although each of these schemes is occasionally used in specialist applications, the majority of MR Images today are created either by the Two-Dimensional Fourier Transform (2DFT) technique with slice selection, or by the Three-Dimensional Fourier Transform (3DFT) technique. Another name for 2DFT is spin-warp. What follows here is a description of the 2DFT technique with slice selection. negron305 Cat scan redirects here. ...

Slice selection is achieved by applying a magnetic gradient in addition to the external magnetic field during the radio frequency pulse. Only one plane within the object will have protons that are on–resonance and contribute to the signal.

A real image can be considered as being composed of a number of spatial frequencies at different orientations. A two–dimensional Fourier transformation of a real image will express these waves as a matrix of spatial frequencies known as k–space. Low spatial frequencies are represented at the center of k–space and high spatial frequencies at the periphery. Frequency and phase encoding are used to measure the amplitudes of a range of spatial frequencies within the object being imaged. In mathematics, the Fourier transform is a certain linear operator that maps functions to other functions. ... In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. ... K-space has conjugate symmetry. ...

The frequency encoding gradient is applied during readout of the signal and is orthogonal to the slice selection gradient. During application of the gradient the frequency differences in the readout direction progressively change. At the midpoint of the readout these differences are small and the low spatial frequencies in the image are sampled filling the center of k-space. Higher spatial frequencies will be sampled towards the beginning and end of the readout filling the periphery of k-space.

Phase encoding is applied in the remaining orthogonal plane and uses the same principle of sampling the object for different spatial frequencies. However, it is applied for a brief period before the readout and the strength of the gradient is changed incrementally between each radio frequency pulse. For each phase encoding step a line of k–space is filled.

Either a spin echo or a gradient echo can be used to refocus the magnetisation.

The 3DFT technique is rather similar except that there is no slice selection and phase-encoding is performed two separate directions.

Another scheme which is sometimes used, especially in brain scanning or where images are needed very rapidly, is called echo-planar imaging (EPI): in this case each rf excitation is followed by a whole train of gradient echoes with different spatial encoding.

Example of a Pulse Sequence

Simplified timing diagram for two-dimensional-Fourier-transform (2DFT) pulse sequence

In the timing diagram, the horizontal axis represents time. The vertical axis represents: (top row) amplitude of radiofrequency pulses; (middle rows) amplitudes of the three orthogonal magnetic field gradient pulses; and (bottom row) receiver analog-to-digital converter (ADC). Radiofrequencies are transmitted at the Larmor frequency of the nuclide to be imaged: for example for ¹H in a magnetic field of 1T, a frequency of 42578100 Hz would be employed. The three field gradients are labeled G_X (typically corresponding to a patient's Left-to-Right direction and colored red in diagram), G_Y (typically corresponding to a patient's Front-to-Back direction and colored green in diagram), and G_Z (typically corresponding to a patient's Head-to-Toe direction and colored blue in diagram). Where negative-going gradient pulses are shown, they represent reversal of the gradient direction, i.e. Right-to-Left, Back-to-Front or Toe-to-Head. For human scanning gradient strengths of 0.001-0.01 T.m^-1 are employed: higher gradient strengths permit better resolution and faster imaging. The pulse sequence shown here would produce a transverse (axial) image.

The first part of the pulse sequence, SS, achieves Slice Selection. A shaped pulse (shown here with a sinc modulation) causes a 90° (π/2 radian) nutation of longitudinal nuclear magnetization within a slab, or slice, creating transverse magnetization. The second part of the pulse sequence, PE, imparts a phase shift upon the slice-selected nuclear magnetization, varying with its location in the Y direction. The third part of the pulse sequence, another Slice Selection (of the same slice) uses another shaped pulse to cause a 180° (π radian) rotation of transverse nuclear magnetization within the slice. This transverse magnetisation refocuses to form a spin echo at a time TE. During the spin echo, a frequency-encoding (FE) or readout gradient is applied, making the resonant frequency of the nuclear magnetization vary with its location in the X direction. The signal is sampled n_FE times by the ADC during this period, as represented by the vertical lines. Typically n_FE of between 128 and 512 samples are taken. Wikipedia does not yet have an article with this exact name. ...

The longitudinal relaxation is then allowed to recover somewhat and after a time TR the whole sequence is repeated n_PE times, but with the phase-encoding gradient incremented (indicated by the horizontal hatching in the green gradient block). Typically n_PE of between 128 and 512 repetitions are made.

The negative-going lobes in G_X and G_Z are imposed to ensure that, at time TE (the spin echo maximum), phase only encodes spatial location in the Y direction.

Typically TE is between 5msec and 100msec, while TR is between 100msec and 2000msec.

After the two-dimensional matrix (typical dimension between 128x128 and 512x512) has been acquired, producing the so-called K-space data, a two-dimensional Fourier transform is performed to provide the familiar MR image.

K-space

See main article K-space

In 1983 Ljunggren^[2] and Tweig^[3] independently introduced the k-space formalism, a technique that proved invaluable in unifying different MR imaging techniques. They showed that the demodulated MR signal S(t) generated by freely precessing nuclear spins in the presence of a linear magnetic field gradient G equals the Fourier transform of the effective spin density i.e. K-space has conjugate symmetry. ...

where:

In other words, as time progresses the signal traces out a trajectory in k-space with the velocity vector of the trajectory proportional to the vector of the applied magnetic field gradient. By the term effective spin density we mean the true spin density corrected for the effects of T₁ preparation, T₂ decay, dephasing due to field inhomogeneity, flow, diffusion, etc. and any other phenomena that affect that amount of transverse magnetization available to induce signal in the RF probe.

From the basic k-space formula, it follows immediately that we reconstruct an image simply by taking the inverse Fourier transform of the sampled data viz. In mathematics, Fourier inversion recovers a function from its Fourier transform. ...

Using the k-space formalism, a number of seemingly complex ideas became simple. For example, it becomes very easy to understand the role of phase encoding (the so-called spin-warp method). In a standard spin echo or gradient echo scan, where the readout (or view) gradient is constant (e.g. G_x), a single line of k-space is scanned per RF excitation. When the phase encoding gradient is zero, the line scanned is the k_x axis. When a non-zero phase-encoding pulse is added in between the RF excitation and the commencement of the readout gradient, this line moves up or down in k-space i.e. we scan the line k_y=constant.

The k-space formalism also makes it very easy to compare different scanning techniques. In single-shot EPI, all of k-space is scanned in a single shot, following either a sinusoidal or zig-zag trajectory. Since alternating lines of k-space are scanned in opposite directions, this must be taken into account in the reconstruction. Multi-shot EPI and fast spin echo techniques acquire only part of k-space per excitation. In each shot, a different interleaved segment is acquired, and the shots are repeated until k-space is sufficiently well-covered. Since the data at the center of k-space represent lower spatial frequencies than the data at the edges of k-space, the T_E value for the center of k-space determines the image's T₂ contrast.

The importance of the center of k-space in determining image contrast can be exploited in more advanced imaging techniques. One such technique is spiral acquisition - a rotating magnetic field gradient is applied, causing the trajectory in k-space to trace out spiral out from the center to the edge. Due to T₂ and T₂ * decay the signal is greatest at the start of the acquisition, hence acquiring the center of k-space first improves contrast to noise ratio (CNR) when compared to conventional zig-zag acquisitions, especially in the presence of rapid movement. Contrast to Noise Ratio, written also as CNR, refers to the ability of an imaging modality such as MRI or fluoroscopy to distinguish between various contrasts of an acquired image and the inherent noise in the image. ...

Since and are conjugate variables (with respect to the Fourier transform) we can use the Nyquist theorem to show that the step in k-space determines the field of view of the image (maximum frequency that is correctly sampled) and the maximum value of k sampled determines the resolution i.e.. The Nyquist-Shannon sampling theorem is the fundamental theorem in the field of information theory, in particular telecommunications. ...

(these relationships apply to each axis [X, Y, and Z] independently).

Scanner construction and operation

Schematic of construction of a cylindrical superconducting MR scanner

The three systems described above form the major components of an MRI scanner: a static magnetic field, an RF transmitter and receiver, and three orthogonal, controllable magnetic gradients. Image File history File links Mri_scanner_schematic_labelled. ... Image File history File links Mri_scanner_schematic_labelled. ...

Magnet

The magnet is the largest and most expensive component of the scanner, and the remainder of the scanner is built around it. Just as important as the strength of the main magnet is its precision. The straightness of the magnetic lines within the center (or, as it is technically known, the iso-center) of the magnet needs to be nearly perfect. This is known as homogeneity. Fluctuations (non-homogeneities in the field strength) within the scan region should be less than three parts per million (3 ppm). Three types of magnets have been used:

• Permanent magnet: Conventional magnets made from ferromagnetic materials (e.g., steel) can be used to provide the static magnetic field. A permanent magnet that is powerful enough to be used in an MRI will be extremely large and bulky; they can weigh over 100 tonnes. But permanent magnet MRIs are very inexpensive to maintain; this cannot be said of the other types of MRI magnets. But there are significant drawbacks to using permanent magnets. They are only capable of achieving relatively weak field strengths compared to other MRI magnets (usually less than 0.4 T), and they are of limited precision and stability. Permanent magnets also present special safety issues; since their magnetic fields cannot be "turned off," ferromagnetic objects are virtually impossible to remove from them once they come into direct contact. Permanent magnets also require special care when they are being brought to their site of installation.
• Resistive electromagnet: A solenoid wound from copper wire is an alternative to a permanent magnet. An advantage is low initial cost, but field strength and stability are limited. The electromagnet requires considerable electrical energy during operation which can make it expensive to operate. This design is essentially obsolete.
• Superconducting electromagnet: When a niobium-titanium alloy is cooled by liquid helium to 4K (−269°C, −452°F) it becomes a superconductor, losing resistance to flow of electrical current. An electromagnet constructed with superconductors can have extremely high field strengths, with very high stability. The construction of such magnets is extremely costly, and the cryogenic helium is expensive and difficult to handle. However, despite its cost, helium cooled superconducting magnets are the most common type found in MRI scanners today.

Most superconducting magnets have their coils of superconductive wire immersed in liquid helium, inside a vessel called a cryostat. Despite thermal insulation, ambient heat causes the helium to slowly boil off. Such magnets, therefore, require regular topping-up with liquid helium. Generally a cryocooler, also known as a coldhead, is used to recondense some helium vapor back into the liquid helium bath. Several manufacturers now offer 'cryogenless' scanners, where instead of being immersed in liquid helium the magnet wire is cooled directly by a cryocooler. SI unit. ... For other uses, see Solenoid (disambiguation). ... Superconducting magnets are electromagnets that are built using superconducting coils. ... General Name, Symbol, Number niobium, Nb, 41 Chemical series transition metals Group, Period, Block 5, 5, d Appearance gray metallic Standard atomic weight 92. ... General Name, symbol, number titanium, Ti, 22 Chemical series transition metals Group, period, block 4, 4, d Appearance silvery grey-white metallic Standard atomic weight 47. ... Helium exists in liquid form only at very low temperatures. ... Superconductivity is a phenomenon occurring in certain materials at low temperatures, characterised by the complete absence of electrical resistance and the damping of the interior magnetic field (the Meissner effect. ... Cryocoolers are refrigerators used to reach cryogenic temperatures. ... Cryocoolers are the devices used to reach cryogenic temperatures. ...

Magnets are available in a variety of shapes. However, permanent magnets are most frequently 'C' shaped, and superconducting magnets most frequently cylindrical. However, C-shaped superconducting magnets and box-shaped permanent magnets have also been used.

Magnetic field strength is an important factor in determining image quality. Higher magnetic fields increase signal-to-noise ratio, permitting higher resolution or faster scanning. However, higher field strengths require more costly magnets with higher maintenance costs, and have increased safety concerns. 1.0 - 1.5T field strengths are a good compromise between cost and performance for general medical use. However, for certain specialist uses (e.g., brain imaging), field strengths up to 3.0 T may be desirable. Signal-to-noise ratio (often abbreviated SNR or S/N) is an electrical engineering concept, also used in other fields (such as scientific measurements, biological cell signaling), defined as the ratio of a signal power to the noise power corrupting the signal. ...

Radio frequency system

The radio frequency (RF) transmission system consists of a RF synthesizer, power amplifier and transmitting coil. This is usually built into the body of the scanner. The power of the transmitter is variable, but high-end scanners may have a peak output power of up to 35 kW, and be capable of sustaining average power of 1 kW. The receiver consists of the coil, pre-amplifier and signal processing system. While it is possible to scan using the integrated coil for transmitting and receiving, if a small region is being imaged then better image quality is obtained by using a close-fitting smaller coil. A variety of coils are available which fit around parts of the body, e.g., the head, knee, wrist, or internally, e.g., the rectum.

A recent development in MRI technology has been the development of sophisticated multi-element phased array coils which are capable of acquiring multiple channels of data in parallel. This 'parallel imaging' technique uses unique acquisition schemes that allow for accelerated imaging, by replacing some of the spatial coding originating from the magnetic gradients with the spatial sensitivity of the different coil elements. However the increased acceleration also reduces the signal-to-noise ratio and can create residual artifacts in the image reconstruction. Two frequently used parallel acquisition and reconstruction schemes are SENSE^[4] and GRAPPA^[5]. A detailed review of parallel imaging techniques can be found here: ^[6] For the ultrasonic and medical imaging application, see phased array ultrasonics. ...

Gradients

Gradient coils are used to spatially encode the positions of protons by varying the magnetic field linearly across the imaging volume. The Larmor frequency will then vary as a function of position in the x, y and z-axes.

Gradient coils are usually resistive electromagnets powered by sophisticated amplifiers which permit rapid and precise adjustments to their field strength and direction. Typical gradient systems are capable of producing gradients from 20 mT/m to 100 mT/m (i.e. in a 1.5 T magnet, when a maximal z-axis gradient is applied the field strength may be 1.45 T at one end of a 1 m long bore, and 1.55 T at the other). It is the magnetic gradients that determine the plane of imaging - because the orthogonal gradients can be combined freely, any plane can be selected for imaging.

Scan speed is dependent on performance of the gradient system. Stronger gradients allow for faster imaging, or for higher resolution; similarly, gradients systems capable of faster switching can also permit faster scanning. However, gradient performance is limited by safety concerns over nerve stimulation.

Image Contrast

In order to understand MRI contrast, it is important to have some understanding of the time constants involved in relaxation processes that establish equilibrium following RF excitation. As the high-energy nuclei relax and realign they emit energy at rates which are recorded to provide information about the material they are in. The realignment of nuclear spins with the magnetic field is termed longitudinal relaxation and the time required for a certain percentage of the tissue's nuclei to realign is termed "Time 1" or T1, which is typically about 1 second at 1.5 tesla main field strength. T2-weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse; the transverse relaxation time is termed "Time 2" or T2, typically < 100 ms for tissue at 1.5 tesla main field strength. A subtle but important variant of the T2 technique is called T2* imaging. T2 imaging employs a spin echo technique, in which spins are refocused to compensate for local magnetic field inhomogeneities. T2* imaging is performed without refocusing. This sacrifices some image integrity (resolution) but provides additional sensitivity to relaxation processes that cause incoherence of transverse magnetization. Applications of T2* imaging include functional MRI (fMRI) or evaluation of baseline vascular perfusion (e.g. cerebral blood flow (CBF)) and cerebral blood volume (CBV) using injected agents; in these cases, there is an inherent trade-off between image quality and detection sensitivity. Because T2*-weighted sequences are sensitive to magnetic inhomogeneity (as can be caused by deposition of iron-containing blood-degradation products), such sequences are utilized to detect subtle areas of recent or chronic intra cranial hemorrhage ("Heme sequence"). In physics and engineering, the time constant usually denoted by the Greek letter , (tau), characterizes the frequency response of a first-order, linear time-invariant (LTI) system. ... Spin-lattice relaxation time, known as T1, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. ... Spin-spin relaxation time, known as T2, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. ... A light wave is an example of a transverse wave. ... Spin-spin relaxation time, known as T2, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. ... One millisecond is one-thousandth of a second. ... Spin echo: Pulse sequence (above) and Signal (below) In nuclear magnetic resonance, spin echo refers to the refocusing of precessing nuclear spin magnetisation by a 180° pulse of resonant radiofrequency. ... Image resolution describes the detail an image holds. ... Functional magnetic resonance imaging (fMRI) is the use of MRI to measure the haemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. ... In physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue. ... Cerebral blood flow, or CBF, is the amount of blood that enters the brain. ... Sensitivity is a statistical measure of how well a binary classification test correctly identifies a condition, whether this be medical screening tests picking up on a disease or quality control in factories deciding if a new product is good enough to be sold. ... Fe redirects here. ...

Image contrast is created by using a selection of image acquisition parameters that weights signal by T1, T2 or T2*, or no relaxation time ("proton-density images"). In the brain, T1-weighting causes the nerve connections of white matter to appear white, and the congregations of neurons of gray matter to appear gray, while cerebrospinal fluid appears dark. The contrast of "white matter," "gray matter'" and "cerebrospinal fluid" is reversed using T2 or T2* imaging, whereas proton-weighted imaging provides little contrast in normal subjects. Additionally, functional information (CBF, CBV, blood oxygenation) can be encoded within T1, T2, or T2*. White matter is one of the two main solid components of the central nervous system. ... Grey matter (or gray matter) is a major component of the central nervous system, consisting of nerve cell bodies, glial cells (astroglia and oligodendrocytes), capillaries, and short nerve cell extensions/processes (axons and dendrites). ... Cerebrospinal fluid (CSF), Liquor cerebrospinalis, is a clear bodily fluid that occupies the subarachnoid space in the brain (the space between the skull and the cerebral cortex—more specifically, between the arachnoid and pia layers of the meninges). ... Pulse oximetry is a non-invasive method which allows health care providers to monitor the oxygenation of a patients blood. ...

Contrast enhancement

Both T1-weighted and T2-weighted images are acquired for most medical examinations; However they do not always adequately show the anatomy or pathology. The first option is to use a more sophisticated image acquisition technique such as fat suppression or chemical-shift imaging.^[7] The other is to administer a contrast agent to delineate areas of interest. Human heart and lungs, from an older edition of Grays Anatomy. ... Look up Contrast in Wiktionary, the free dictionary. ...

A contrast agent may be as simple as water, taken orally, for imaging the stomach and small bowel although substances with specific magnetic properties may be used. Most commonly, a paramagnetic contrast agent (usually a gadolinium compound^[8][9]) is given. Gadolinium-enhanced tissues and fluids appear extremely bright on T1-weighted images. This provides high sensitivity for detection of vascular tissues (e.g. tumors) and permits assessment of brain perfusion (e.g. in stroke). There have been concerns raised recently regarding the toxicity of gadolinium-based contrast agents and their impact on persons with impaired kidney function. Special actions may be taken, such as hemodialysis following a contrast MRI scan for renally-impaired patients. Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Paramagnetism is the tendency of the atomic magnetic dipoles, due to quantum-mechanical spin, in a material that is otherwise non-magnetic to align with an external magnetic field. ... General Name, Symbol, Number gadolinium, Gd, 64 Chemical series lanthanides Group, Period, Block n/a, 6, f Appearance silvery white Standard atomic weight 157. ... It has been suggested that Artificial kidney be merged into this article or section. ... Renal failure or kidney failure is a situation in which the kidneys fail to function adequately. ...

More recently, superparamagnetic contrast agents (e.g. iron oxide nanoparticles^[10][11]) have become available. These agents appear very dark on T2*-weighted images and may be used for liver imaging - normal liver tissue retains the agent, but abnormal areas (e.g. scars, tumors) do not. They can also be taken orally, to improve visualization of the gastrointestinal tract, and to prevent water in the gastrointestinal tract from obscuring other organs (e.g. pancreas). Superparamagnetism refers to materials which become magnetic in the presence of an external magnet, but revert to a non magnetic state when the external magnet is removed. ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ... Silicon nanopowder Nanodiamonds, TEM image A nanoparticle (or nanopowder or nanocluster or nanocrystal) is a small particle with at least one dimension less than 100 nm. ... The liver is the largest internal organ in the human body, and is an organ present in vertebrates and some other animals. ... Gut redirects here. ... The pancreas is a gland organ in the digestive and endocrine systems of vertebrates. ...

Diamagnetic agents such as barium sulfate have been studied for potential use in the gastrointestinal tract, but are less frequently used. Levitating pyrolytic carbon Diamagnetism is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. ... Granulated Barium Sulfate Barium sulfate (or barium sulphate) is the white crystalline solid with the formula BaSO4. ... Gut redirects here. ...

Applications

In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue. One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range. Compare this to CT scans and traditional X-rays which involve doses of ionizing radiation and may increase the risk of malignancy, especially in a fetus. A brain tumor is any intracranial tumor created by abnormal and uncontrolled cell division, normally either in the brain itself (neurons, glial cells (astrocytes, oligodendrocytes, ependymal cells), lymphatic tissue, blood vessels), in the cranial nerves (myelin-producing Schwann cells), in the brain envelopes (meninges), skull, pituitary and pineal gland, or... CT apparatus in a hospital Computed axial tomography (CAT), computer-assisted tomography, computed tomography, CT, or body section roentgenography is the process of using digital processing to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around... A radiograph of a right elbow-joint Radiography is the use of certain types of electromagnetic radiation—usually ionizing—to view objects. ... Radiation hazard symbol. ... When normal cells are damaged or old they undergo apoptosis; cancer cells, however, avoid apoptosis. ...

While CT provides good spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI. Resolving power is the ability of a microscope or telescope to measure the angular separation of images that are close together. ... Contrast resolution is referred to as the ability of an imaging modality such as MRI or fluoroscopy to distinguish between various contrasts of an acquired image. ...

For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, a sequence will take on the property of T2-weighting. On a T2-weighted scan, fat-, water- and fluid-containing tissues are bright (most modern T2 sequences are actually fast T2 sequences). Damaged tissue tends to develop edema, which makes a T2-weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T2-weighted sequence can be converted to a FLAIR sequence, in which free water is now dark, but edematous tissues remain bright. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating diseases, such as multiple sclerosis. This page is about the condition called edema. ... Fluid Attenuated Inversion Recovery (FLAIR) is a pulse sequence used in Magnetic Resonance Imaging. ... Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. ...

The typical MRI examination consists of 5-20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician. For other uses, see Doctor. ...

Specialized MRI scans

Diffusion MRI

Main article: Diffusion MRI

Diffusion MRI measures the diffusion of water molecules in biological tissues.^[12] In an isotropic medium (inside a glass of water for example) water molecules naturally move randomly according to Brownian motion. In biological tissues however, the diffusion may be anisotropic. For example a molecule inside the axon of a neuron has a low probability of crossing the myelin membrane. Therefore the molecule will move principally along the axis of the neural fiber. If we know that molecules in a particular voxel diffuse principally in one direction we can make the assumption that the majority of the fibers in this area are going parallel to that direction. Diffusion MRI is a specific Magnetic Resonace Imaging (MRI) modality that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion. ... Image File history File links Illus_dti. ... Diffusion MRI is a specific Magnetic Resonace Imaging (MRI) modality that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion. ... diffusion (disambiguation). ... Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ... Three different views of Brownian motion, with 32 steps, 256 steps, and 2048 steps denoted by progressively lighter colors. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. ... A voxel (a portmanteau of the words volumetric and pixel) is a volume element, representing a value on a regular grid in three dimensional space. ...

The recent development of diffusion tensor imaging (DTI) enables diffusion to be measured in multiple directions and the fractional anisotropy in each direction to be calculated for each voxel. This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain (using tractography) or to examine areas of neural degeneration and demyelination in diseases like Multiple Sclerosis. Diffusion tensor imaging (DTI) is a new magnetic resonance imaging (MRI)-based technique that allows us to visualize the location, the orientation, and the anisotropy of the brains white matter tracts. ... Tractography Tractography is a procedure to demonstrate the neural tracts. ...

Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic stroke, DWI is highly sensitive to the changes occurring in the lesion.^[13] It is speculated that increases in restriction (barriers) to water diffusion, as a result of cytotoxic edema (cellular swelling), is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5-10 minutes of the onset of stroke symptoms (as compared with computed tomography, which often does not detect changes of acute infarct for up to 4-6 hours) and remains for up to two weeks. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy. Diffusion-weighted imaging is a specific MRI modality that produces in vivo magnetic resonances images of biological tissues weighted with the local caracteristics of water diffusion. ... For other uses, see Stroke (disambiguation). ... negron305 Cat scan redirects here. ...

Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.

Magnetic resonance angiography

Magnetic Resonance Angiography

Magnetic resonance angiography (MRA) is used to generate pictures of the arteries in order to evaluate them for stenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium) or using a technique known as "flow-related enhancement" (e.g. 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood which has recently moved into that plane, see also FLASH MRI. Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method the tissue is now excited inferiorly while signal is gathered in the plane immediately superior to the excitation plane, and thus imaging the venous blood which has recently moved from the excited plane. Image File history File links Mra1. ... Image File history File links Mra1. ... Angiography or arteriography is a medical imaging technique in which an X-ray picture is taken to visualize the inner opening of blood filled structures, including arteries, veins and the heart chambers. ... A stenosis is an abnormal narrowing in a blood vessel or other tubular organ or structure. ... Post surgical photo of brain aneurysm survivor. ... General Name, Symbol, Number gadolinium, Gd, 64 Chemical series lanthanides Group, Period, Block n/a, 6, f Appearance silvery white Standard atomic weight 157. ... FLASH MRI (Fast Low Angle Shot Magnetic Resonance Imaging) is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm and Axel Haase at the Max-Planck-Institut für biophysikalische Chemie in Göttingen, Germany. ...

Magnetic resonance spectroscopy

In vivo ('in the living organism') magnetic resonance spectroscopy (MRS), also known as MRSI (MRS imaging) and volume selective NMR spectroscopy, is a technique which combines the spatially-addressable nature of MRI with the spectroscopically-rich information obtainable from NMR. That is to say, MRI allows one to study a particular region within an organism or sample, but gives relatively little information about the chemical or physical nature of that region (its chief value is in being able to distinguish the properties of that region, how much fat or water is present, relative to those of surrounding regions). MR spectroscopy, however, provides a wealth of information about other biological chemicals ('metabolites') within that region, as would an NMR spectrum of that region. In vivo (that is in the living organism) magnetic resonance spectroscopy is a specialised technique associated with magnetic resonance imaging (MRI). ... In vivo (that is in the living organism) magnetic resonance spectroscopy is a specialised technique associated with magnetic resonance imaging (MRI). ...

Functional MRI

Main article: Functional magnetic resonance imaging

A fMRI scan showing regions of activation in orange, including the primary visual cortex (V1, BA17).

Functional MRI (fMRI) measures signal changes in the brain that are due to changing neural activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2-3 seconds). Increases in neural activity cause changes in the MR signal via T2* changes^[14]; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue. Functional magnetic resonance imaging (fMRI) is the use of MRI to measure the haemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. ... Sample fMRI data This example of fMRI data shows regions of activation including primary visual cortex (V1, BA17), extrastriate visual cortex and lateral geniculate body in a comparison between a task involving a complex moving visual stimulus and rest condition (viewing a black screen). ... Sample fMRI data This example of fMRI data shows regions of activation including primary visual cortex (V1, BA17), extrastriate visual cortex and lateral geniculate body in a comparison between a task involving a complex moving visual stimulus and rest condition (viewing a black screen). ... Brodmann area 17 (primary visual cortex) is shown in red in this image which also shows area 18 (orange) and 19 (yellow) The primary visual cortex (usually called V1) is the most well-studied visual area in the brain. ... Functional Magnetic Resonance Imaging (or fMRI) describes the use of MRI to measure hemodynamic signals related to neural activity in the brain or spinal cord of humans or other animals. ... For other uses, see Brain (disambiguation). ... This article is about cells in the nervous system. ... Blood-oxygen-level dependent or BOLD fMRI is a method of observing which areas of the brain are active at any given time. ... f you all The blood vessels are part of the circulatory system and function to transport blood throughout the body. ... Structure of hemoglobin. ...

While BOLD signal is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling (ASL) or weight the MRI signal by cerebral blood flow (CBF) and cerebral blood volume (CBV). The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in preclinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity.

Here is a video compiled of MRI scans showing two arachnoid cysts: http://www.youtube.com/watch?v=PF_mDsdxSsg

Interventional MRI

Main article: Interventional MRI

The lack of harmful effects on the patient and the operator make MRI well-suited for "interventional radiology", where the images produced by a MRI scanner are used to guide minimally-invasive procedures. Of course, such procedures must be done without any ferromagnetic instruments. Interventional magnetic resonance imaging, also Interventional MRI, is the use of magnetic resonance imaging (MRI) to do interventional radiology procedures. ...

A specialized growing subset of interventional MRI is that of intraoperative MRI in which the MRI is used in the surgical process. Some specialized MRI systems have been developed that allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical procedure is temporarily interrupted so that MR images can be acquired to verify the success of the procedure or guide subsequent surgical work.

Radiation therapy simulation

Because of MRI's superior imaging of soft tissues, it is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. For therapy simulation, a patient is placed in specific, reproducible, body position and scanned. The MRI system then computes the precise location, shape and orientation of the tumor mass, correcting for any spatial distortion inherent in the system. The patient is then marked or tattooed with points which, when combined with the specific body position, will permit precise triangulation for radiation therapy.

Current density imaging

Current density imaging (CDI) endeavors to use the phase information from images to reconstruct current densities within a subject. Current density imaging works because electrical currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during an imaging sequence. To date no successful CDI has been performed using biological currents, but several studies have been published which involve applied currents through a pair of electrodes. Current density imaging (CDI) is an extension of magnetic resonance imaging (MRI), developed at the University of Toronto. ...

Magnetic resonance guided focused ultrasound

In MRgFUS therapy, ultrasound beams are focused on a tissue - guided and controlled using MR thermal imaging - and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65°C, completely destroying it. This technology can achieve precise "ablation" of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment. MRgFUS -Magnetic Resonance guided Focused Ultrasound uses a magnetic resonance imaging (MRI) scanner to identify tissues in the body and focused ultrasound to destroy tumors or fibroids. ... The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similar system in 1742. ... Ablation is defined as the removal of material from the surface of an object by vaporization, chipping, or other erosive processes. ...

Multinuclear imaging

Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance. However, any nucleus which has a net nuclear spin could potentially be imaged with MRI. Such nuclei include helium-3, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. ²³Na and ³¹P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as ³He or ¹²⁹Xe must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. ¹⁷O, ¹³C and ¹⁹F can be administered in sufficient quantities in liquid form (e.g. ¹⁷O-water, ¹³C-glucose solutions or perfluorocarbons) that hyperpolarization is not a necessity. General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... For other uses, see Carbon (disambiguation). ... Distinguished from fluorene and fluorone. ... This article is about the chemical element and its most stable form, or dioxygen. ... For sodium in the diet, see Salt. ... General Name, symbol, number phosphorus, P, 15 Chemical series nonmetals Group, period, block 15, 3, p Appearance waxy white/ red/ black/ colorless Standard atomic weight 30. ... General Name, Symbol, Number xenon, Xe, 54 Chemical series noble gases Group, Period, Block 18, 5, p Appearance colorless Standard atomic weight 131. ... Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. ... Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. ...

Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on ¹H MRI (e.g. lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized ³He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing ¹³C or stabilized bubbles of hyperpolarized ¹²⁹Xe have been studied as contrast agents for angiography and perfusion imaging. ³¹P can potentially provide information on bone density and structure, as well as functional imaging of the brain.

Magnetic Resonance Spectroscopy

Magnetic resonance spectroscopy is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain,^[15] as well as to provide information on tumor metabolism. ^[16] In vivo (that is in the living organism) magnetic resonance spectroscopy is a specialised technique associated with magnetic resonance imaging (MRI). ... Structure of the coenzyme adenosine triphosphate, a central intermediate in energy metabolism. ...

Susceptibility Weighted Imaging (SWI)

Susceptibility Weighted Imaging (also known as SWI), is a new type of contrast in MRI different from spin density, T1, or T2 imaging. This method exploits the susceptibility differences between tissues and uses a fully velocity compensated, three dimensional, rf spoiled, high-resolution, 3D gradient echo scan. This special data acquisition and image processing produces an enhanced contrast magnitude image very sensitive to venous blood, hemorrhage and iron storage. It is used to enhance the detection and diagnosis of tumors, vascular and neurovascular diseases (stroke and hemorrhage, multiple sclerosis, Alzheimer's), and also detects traumatic brain injuries that may not be diagnosed using other methods.^[17]

Experimental MRI techniques

Currently there is active research in several new MRI technologies like magnetization transfer MRI (MT-MRI), diffusion tensor MRI (DT-MRI), Susceptibility Weighted Imaging MRI (SWI), and proton MR spectroscopy, plus recent research in to Dendrimer-enhanced MRI as a diagnostic and prognostic biomarker of sepsis-induced acute renal failure and additional developments based on SWI as an imaging biomarker for tumors, neurological and neurovascular diseases.

With between 72% and 90% accuracy where chance would achieve 0.8%,^[18] fMRI techniques can decide which of a set of known images the subject is viewing.^[19]

Portable instruments

Portable magnetic resonance instruments are available for use in education and field research. Using the principles of Earth's field NMR, they have no powerful polarizing magnet, so that such instruments can be small and relatively inexpensive. Some can be used for both EFNMR spectroscopy and MRI imaging^[20]. The low strength of the Earth's field results in poor signal to noise ratios, requiring relatively long scan times to capture spectroscopic data or build up MRI images. The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...

MRI vs CT

A computed tomography (CT) scanner uses X-rays, a type of ionizing radiation, to acquire its images, making it a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for non-calcified tissue, though MR images can also be acquired from bones and teeth^[21] as well as fossils^[22]. negron305 Cat scan redirects here. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... Radiation hazard symbol. ... It has been suggested that this article or section be merged with Radio waves. ...

CT may be enhanced by use of contrast agents containing elements of a higher atomic number than the surrounding flesh such as iodine or barium. Contrast agents for MRI are those which have paramagnetic properties, e.g. gadolinium and manganese. A contrast medium is a radiopaque substance used to facilitate roentgen visualization of internal structures of the body such as the urogenital sinus. ... For other uses, see Iodine (disambiguation). ... For other uses, see Barium (disambiguation). ... Simple Illustration of a paramagnetic probe made up from miniature magnets. ... General Name, Symbol, Number gadolinium, Gd, 64 Chemical series lanthanides Group, Period, Block n/a, 6, f Appearance silvery white Standard atomic weight 157. ... General Name, symbol, number manganese, Mn, 25 Chemical series transition metals Group, period, block 7, 4, d Appearance silvery metallic Standard atomic weight 54. ...

Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features. (See Application below.)

MRI can generate cross-sectional images in any plane (including oblique planes). In the past, CT was limited to acquiring images in the axial (or near axial) plane. The scans used to be called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic resolution, allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality. This article is about the mathematical construct. ... Isotropy (the opposite of anisotropy) is the property of being independent of direction. ...

For purposes of tumor detection and identification in the brain, MRI is generally superior.^[23][24][25] However, in the case of solid tumors of the abdomen and chest, CT is often preferred due to less motion artifact. However, CT usually is more widely available, faster, much less expensive, and may be less likely to require the person to be sedated or anesthetized.

MRI is also best suited for cases when a patient is to undergo the exam several times successively in the short term, because, unlike CT, it does not expose the patient to the hazards of ionizing radiation.

Economics of MRI

MRI equipment is expensive. New 1.5 tesla scanners often cost between $1,000,000 USD and $1,500,000 USD. New 3.0 tesla scanners often cost between $2,000,000 and $2,300,000 USD. Construction of MRI suites can cost $500,000 USD.

For over a dozen years, MRI scanners have been significant sources of revenue for healthcare providers in the US. This is because of favorable reimbursement rates from insurers, both private and federal government programs. Insurance reimbursement has historically been provided in two components, technical for the actual performance of the MRI scan and professional for the radiologist's review of the images and/or data.

In the US, the 2007 Deficit Reduction Act (DRA) significantly reduced reimbursement rates paid by federal insurance programs for the technical component of many scans, shifting the economic landscape. Many private insurers have followed suit.

Currently, in the US, there is increasing interest in reducing the costs associated with MRI services and simultaneously improving the ability to effectively and efficiently provide MRI examination services to larger numbers of patients with the same equipment.

Safety