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| CATEGORIES ABOUT MAGNETIC RESONANCE IMAGING | |
| magnetic resonance imaging | |
| medical imaging | |
| radiologymagnetic resonance imaging | |
| medical imaging | |
| radiology | |
| nuclear magnetic resonance | |
| medical imaging | |
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cross section through a human head.]] Magnetic resonance imaging (MRI), formerly referred to as ''magnetic resonance tomography (MRT)'' and, in scientific circles, '' Nuclear Magnetic Resonance imaging (NMRI)'' or NMR zeugmatography imaging, is a Non-invasive method used to render images of the inside of an object. It is primarily used in Medical Imaging to demonstrate Pathological or other Physiological alterations of Living Tissues . MRI also has uses outside of the medical field, such as detecting rock permeability to Hydrocarbons and as a Non-destructive Testing method to characterize the quality of products such as Produce and Timber .http://www.mri.cl/index.pl/industrial_stud#355 MRI should not be confused with the NMR Spectroscopy technique used in Chemistry , although both are based on the same principles of Nuclear Magnetic Resonance . The scanners used in medicine have a typical magnetic field strength of 0.3 to 3 Teslas . Construction costs approximately US$ 1 million per tesla and maintenance an additional several hundred thousand dollars per year. BACKGROUND Nomenclature Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. In its early years MRI was referred to as nuclear magnetic resonance imaging (NMRI), but the word nuclear has been associated with Ionizing Radiation exposure, which is not used in an MRI, so to prevent patients from making a negative association between MRI and ionizing radiation, the word has been almost universally removed. Scientists still use the term ''NMR'' when discussing non-medical devices operating on the same principles. One of the inventors of MRI, Paul Lauterbur , originally named the technique ''zeugmatography'', a Greek term meaning "that which is used for joining".Lauterbur, P.C., Nature, 1973; 242:190-191. The term referred to the interaction between the static and the gradient magnetic fields necessary to create an image, but the nomenclature never caught on. Principle clinical MRI scanner.]] Medical MRI most frequently relies on the Relaxation properties of excited Hydrogen Nuclei in water and Lipid s. When the object to be imaged is placed in a powerful, uniform Magnetic Field , the Spins of Atomic Nuclei with a resulting non-zero spin have to arrange in a particular manner with the applied magnetic field according to Quantum Mechanics . Nuclei of hydrogen atoms ( Proton s) have a simple spin 1/2 and therefore align either Parallel or Antiparallel to the magnetic field. The Spin Polarization determines the basic MRI signal strength. For protons, it refers to the population difference of the two Energy States that are associated with the parallel and antiparallel alignment of the proton spins in the magnetic field and governed by Boltzmann Statistics . In a 1.5 T magnetic field (at room temperature) this difference refers to only about one in a million nuclei since the Thermal Energy far exceeds the energy difference between the parallel and antiparallel states. Yet the vast quantity of nuclei in a small volume sum to produce a detectable change in field. Most basic explanations of MRI will say that the nuclei align parallel or anti-parallel with the static magnetic field; however, because of Quantum Mechanical reasons, the individual nuclei are actually set off at an angle from the direction of the static magnetic field. The bulk collection of nuclei can be partitioned into a set whose sum spin are aligned parallel and a set whose sum spin are anti-parallel. The Magnetic Dipole Moment of the nuclei then Precesses around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moments precess is called the Larmor Frequency . The tissue is then briefly exposed to pulses of Electromagnetic energy ( RF pulses) in a plane Perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high-energy state. Or in other words, the steady-state equilibrium established in the static magnetic field becomes perturbed and the population difference of the two energy levels is altered. The frequency of the pulses is governed by the Larmor Equation to match the required energy difference between the two spin states. Image formation In order to selectively image different Voxel s (volume picture elements) of the subject, Orthogonal magnetic gradients are applied. Although it is relatively common to apply gradients in the principal axes of a patient (so that the patient is imaged in x, y, and z from head to toe), MRI allows completely flexible orientations for images. All spatial encoding is obtained by applying magnetic field gradients which encode position within the phase of the signal. In one dimension, a linear phase with respect to position can be obtained by collecting data in the presence of a magnetic field gradient. In three dimensions (3D), a plane can be defined by "slice selection", in which an RF pulse of defined bandwidth is applied in the presence of a magnetic field gradient in order to reduce spatial encoding to two dimensions (2D). Spatial encoding can then be applied in 2D after slice selection, or in 3D without slice selection. Spatially-encoded phases are recorded in a 2D or 3D Matrix ; this data represents the spatial frequencies of the image object. Images can be created from the matrix using the Discrete Fourier Transform (DFT). Typical medical resolution is about 1 mm³, while research models can exceed 1 µm³. Scanner construction and operation 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. Magnet The magnet is the largest and most expensive component of the scanner, and the remainder of the scanner is built around the magnet. Just as important as the strength of the main magnet is its precision. The straightness of flux lines within the centre or, as it is known as, the iso-centre of the magnet, need to be almost perfect. This is known as homogeneity. Fluctuations or, non-homogeneities in the field strength, within the scan region, should be less than three parts-per-million (3 PPM). Three types of magnet have been used:
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 helium. Generally a Cryocooler , also known as a Coldhead is used to recondense some helium vapour 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. 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 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 - 1.5 T 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 3.0T may be desirable. RF system The 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 SNR and can create residual artifacts in the image reconstruction. Two frequently used parallel acquisition and reconstruction schemes are SENSEPruessmann KP, Weiger M, Scheidegger MB, Boesiger P. "SENSE: sensititivy encoding for fast MRI." Magn Reson Med. 1999 Nov;42(5):952-62. PMID 10542355 and GRAPPAGriswold MA, Jakob PM, Heidemann RM, Nittka M, Jellus V, Wang J, Kiefer B, Haase A. "Generalized autocalibrating partially parallel acquisitions (GRAPPA)." Magn Reson Med. 2002 Jun;47(6):1202-10. PMID 12111967 . A detailed review of parallel imaging techniques can be found here: http://cfmriweb.ucsd.edu/ttliu/be280a_05/blaimer05.pdf Gradients Magnetic gradients are generated by three orthogonal coils, oriented in the x, y and z directions of the scanner. These 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 1m 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.
Diffusion weighted imaging (DWI) Le Bihan D, Breton E, Lallemand D, Grenier P, Cabanis E, Laval-Jeantet M. MR imaging of intravoxel incoherent motions: application to diffusion and perfusion in neurologic disorders. Radiology. 1986 Nov;161(2):401-7 PMID 3763909 uses very fast scans with an additional series of gradients (diffusion gradients) rapidly turned on and off. Protons from water diffusing randomly within the brain, via . Contrast enhancement Both T1-weighted and T2-weighted images are acquired for most medical examinations; However they do not always adequately show the agent to delineate areas of interest. A contrast agent may be as simple as following a contrast MRI scan for Renally-impaired patients. More recently, tissue retains the agent, but abnormal areas (e.g. scars, tumors) do not. They can also be taken orally, to improve visualisation of the Gastrointestinal Tract , and to prevent water in the gastrointestinal tract from obscuring other organs (e.g. Pancreas ). Diamagnetic agents such as Barium Sulfate have been studied for potential use in the Gastrointestinal Tract , but are less frequently used. MRI vs CT A Computed Tomography (CT) scanner uses X-ray s, a type of Ionizing Radiation , to acquire its images, making it a good tool for examining tissue composed of elements of a relatively 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. CT may be enhanced by use of Contrast Agents containing elements of a higher atomic number than the surrounding flesh (iodine, barium). Contrast agents for MRI are those which have Paramagnetic properties. One example is Gadolinium . 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). CT was limited to acquiring images in the axial (or near axial) plane in the past. 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. For purposes of tumor detection and identification, MRI is generally superiorMagnetic resonance and computerized tomography of posterior cranial fossa tumors in childhood. Differential diagnosis and assessment of lesion extent] in Italian Colosimo C, Celi G, Settecasi C, Tartaglione T, Di Rocco C, Marano P. (1995) Radiol Med (Torino) 90(4):386-395The clinical and radiological evaluation of primary brain neoplasms in children, Part II: Radiological evaluation. Allen ED, Byrd SE, Darling CF, Tomita T, Wilczynski MA. (1993) J Natl Med Assoc. 85(7):546-553Computed tomography versus magnetic resonance imaging of the brain. A collaborative interinstitutional study. Deck MD, Henschke C, Lee BC, Zimmerman RD, Hyman RA, Edwards J, Saint Louis LA, Cahill PT, Stein H, Whalen JP. (1989) Clin Imaging 13(1):2-15. 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. THE K-SPACE FORMALISM See main article K-space In 1983 LjunggrenLjunggren S. J Magn Reson 1983; 54:338. and Tweig1 independently introduced the k-space formalism, a technique that proved invaluable in unifying different MR imaging techniques. They showed that the demodulated MR signal generated by freely precessing nuclear spins in the presence of a linear magnetic field gradient equals the Fourier transform of the effective spin density i.e. 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 preparation, 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. Using the k-space formalism, a number of seemingly complex ideas become 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. ), a single line of k-space is scanned per RF excitation. When the phase encoding gradient is zero, the line scanned is the 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 =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 value for the center of k-space determines the image's contrast.
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. |
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