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4 MAGNETIC RESONANCE IMAGING
Pages 37-88

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From page 37...
... High-speed imaging pulse sequences, with particular focus on functional imaging, are discussed in this chapter, as are algorithms for image reconstruction; both are promising fields of research. Significant attention is given also to two applications for which MR} has unique potential: blood flow imaging and quantification, and functional neuroimaging based on exploiting (lynamic changes in the magnetic susceptibility.
From page 38...
... 4.! Principles of Magnetic Resonance Imaging Unlike its x-ray counterparts, magnetic resonance imaging (also known as nuclear magnetic resonance (NMR)
From page 39...
... Time domain nuclear magnetic resonance signal from volume element dxdydz in an object of magnetization density Merit) in the presence of a spatial encoding gradient G
From page 40...
... is the time-dependent spatial encoding gradient defined above, which imparts differing phase shifts to spins at different spatial locations. Equation 4.4 shows that the time variation of the spatial frequency vector is determined by the integral over the gradients; the time sequence of the RF pulses and magnetic field gradients is known as the "pulse sequence" of the MRT acquisition.
From page 41...
... Unique to the MRT device is the gradient system, which permits generation of the previously defined time-dependent gradient fields needed for spatial encoding. Both the transmit/receive and gradient systems are under the control of a data acquisition processor that ties into the main central processing unit.
From page 42...
... Permanent and resistive magnets, both iron core and air core, are coste~ective for field strengths below approximately 0.5-0.3 T Permanent magnets, in particular, can have very low operating costs.
From page 43...
... The field must now have homogeneities of the order of 1.0 ppm to avoid signal Toss and distortion. The signal Toss stems from variations in frequency within an imaging voxeii where the resultant destructive phase interference causes enhanced signal decay, significantly reducing the sampling time available.
From page 44...
... The readout bias field following the pulse is made very Tow so that its variations will result in negligible T2 or distortion considerations.2 Susceptibility-induced variations (which scale with readout field strength) should be negligible.
From page 45...
... Several research opportunities are presented by the need to improve the quality of the data and the rate of data acquisition. The data quality could be improved with better coil designs and, in some applications, cooled or superconducting coils, which result in Tower noise.
From page 46...
... Cooler} Receiver Coils for MR Imaging Most SNR calculations in MRI rest on the basic assumption that the patient contributes the dominant part of the noise. This assumption is known to be incorrect in two important cases: .
From page 47...
... Alternative possibilities are tape HTSs or the use of conventional copper coils at liquid nitrogen temperature. Use of Multiple Receivers While the use of multiple acquisition coils operating in parallel could improve data quality, the technological requirements for a practical multi-coiT array are formidable: beyond the issues of building a decoupled array of surface coils, there are considerable demands on the system hardware to be able to deal with data rates that are an order of magnitude higher than those from a conventional receiver coil.
From page 48...
... 4.2.4 Magnetic Field Gradients Spatial encoding in MRT typically uses spatial variations in the static or radio-frequency magnetic field. Because such variations are small compared to those in the static magnetic field Be used for MRT, a set of "graclient coils" can be used to produce a very nearly linear gradient along any direction.
From page 49...
... Local gradient coils (LGCs) , which are smaller than the whole-body gradient coils that are built · 1 1 · 1 _ _ _ _ 1 _ _, v ~ into cllnlca1 imaging systems, are designed for specific purposes or areas of anatomy.
From page 50...
... , inductance, desired stew rate, eddy currents, gradient uniformity, forces and torques, heat dissipation, and nerve stimulation. Geometry: The geometry of [GCs, like that of local RF coils, can be divided into three categories: whole-volume coils, which completely surround the anatomic structure of interest; partial volume coils, which partially surround the structure; and surface gradient coils, which are placed against one side of the structure.
From page 51...
... However, RF coils or shields that have low-resistance closed-DC current paths in close proximity to the gradient coil can support eddy currents and (regrade the gradient field waveforms. This problem must be considered carefully in the design of RF coils and shields for use with EGCs.
From page 52...
... CHAPTER 4. MAGNETIC RESONANCE IMAGING interest.
From page 53...
... of the head, for example, a three-axis gradient coil enables the use of EPT in an otherwise conventional commercial scanner. Using standard gradient amplifiers, the 20-T/s limit imposed by the U.S.
From page 54...
... 5. Morris, P.G., Nuclear Magnetic Resonance Imaging in Medicine and Biology, Oxford University Press, New York, 1986.
From page 55...
... 14 11. Kwong, K.K., Belliveau, J.W., Chester, D.A., et al., Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation, Proc.
From page 56...
... , 277-279. 4.3 Dynamic MR Image Reconstruction Dynamic magnetic resonance imaging today can be interpreted to mean either very fast or essentially continuous scanning.
From page 57...
... In practice, the reconstructed image data are complex-valued due to factors such as main field inhomogeneity and flow effects. Discussed below are four specific strategies that attempt to deal with this more general situation.
From page 58...
... The second echo wit have worse phase errors than the first but the phases of the two are simply related. However, one must then deal with another reason why it is so biscuit to carry out partial Fourier reconstructions: the data sake are the Fourier transform of an object that is spatially varying, hence, all points in sake contain information about all points in pox)
From page 59...
... In the simplest one-dimensional example, modeling the data as a box, one would find the location of the center of the box, its amplitude, and its width. Unlike FFT metho(ls or image processing methods, these methods have no inherent pixel size limitations.
From page 60...
... In keyhole imaging, for example, a limited number of data are recollected as, say, a subject performs a functional imaging task where only a small portion of the image changes. Concepts such as use of shared echoes between scans to speed up spin echo or cardiac cine-imaging are similar to those described in section 4.3.1, but different enough to warrant rethinking certain data acquisition schemes that might otherwise not qualify as real-time imaging.
From page 61...
... 4. I,iang, Z.-P., and I,auterbur, P.C., An efficient method for (lynamic magnetic resonance imaging, IEEE Trans.
From page 62...
... MAGNETIC RESONANCE IMAGING 6. Wu, Z., Chung, H., and Wehrli, F.W., A Bayesian approach to subvoxel tissue classification in NMR microscopic images of trabecular bone, Magn.
From page 63...
... Fourier Velocity Encoding Phase mapping methods yield data in which the mean velocity in each image pixel is measured. Magnetization phase information can also be used to determine the velocity distribution within each pixel.
From page 64...
... Measurement of Wave Speed and Distensibility Increases in arterial pressure during the cardiac cycle arise from the contraction of the heart and are propagated to the vascular tree. If blood vessels were perfectly rigid, the pressure wave would be propagated at approximately the speed of sound in water.
From page 65...
... The use of color coding, familiar to physicians from Doppler ultrasound, has been demonstrated and is potentially helpful. Conclusions Related to MR Imaging of Blood Flow Magnetic resonance imaging can be used in a number of ways to obtain non-invasive quantitative measurements of a variety of physiological flow parameters.
From page 66...
... Mapping of Diffusion Tensor Diffusion measurements may provide usefuT information on tissue microstructure and function, at a scale significantly smaller than the voxel size. Recently, it has been shown that the entire diffusion tensor can be determined in viva, including in the human brain.
From page 67...
... Several examples of imaging parameters that provide more than simply anatomical mapping are discussed in this section. Relaxation Times The contrast in conventional MR images depends on the nuclear magnetic resonance relaxation times of tissues, particularly the longitudinal relaxation time To and the transverse relaxation time T2.
From page 68...
... This new technique has been dubbed "functional magnetic resonance imaging," or IMRT, and was almost immecliately reproduced by other workers in the field, using both conventional gradient echo imaging and EPl. Contrast Mechanism The basis of fMRT lies in the fact that deoxyhemogIobin, found in red blood ceils, acts as nature's own intravascular paramagnetic contrast agent.
From page 69...
... In addition, while it is readily implementable on smaD-bore imaging systems, mollifications to the imaging system are required for human studies. These modifications, which remain an active area of investigation, include the use of resonant gradient systems, dedicated head gradient coils, and modified high-power linear systems.
From page 70...
... Problems with the scanning system that may not greatly affect normal clinical images need to be detected and corrected, since they may degrade the fMRI results much more. Field Strength Considerations As discussed above, focal field gradients induced by differences in magnetic susceptibility between the intra- and extravascular spaces induce intravoxe!
From page 71...
... Based on such calculations, it is possible to predict that the field dependence of the alteration in T2 due to susceptibility-incluced gradients around blood vessels will depend on the diffusional averaging; as such, the change in T2 wid be a function of the motion of the protons in the brain tissue as well as the magnitucle of the field gradients encountered by the tissue protons. The latter is, in turn, dependent on the size of the blood vessels and their orientation relative to the main magnetic fielcI direction.
From page 72...
... Clearly, if field strengths in excess of 1.5 T were shown to be of significant benefit to functional imaging, there would be increased interest in the development of high-field imaging instrumentation. Processing of Functional Images In order for form to become a practical neurological research and clinical investigation tool, a large number of technical issues need to be addressed, including: 1.
From page 73...
... Can fldRT be used to determine when certain neural processes are taking place, as well as where? Characterization of Physiological Noise and Its Effects on fMRI: The dominant "noise" source in fMRT is not thermal noise in the MR receiver coils or eddy currents in the sample volume.
From page 74...
... Observed proton relaxation rates can be affected by compartmentalized paramagnetic contrast agents including deoxygenated hemoglobin, as well as by in-flowing blood. While the
From page 75...
... Because these ions are bound to macromolecules, with their long rotational correlation times, the electric quadrupole moments are modulated by the local electric field gradients to result in very short relaxation times and thus broad lines. This presents technical challenges but offers opportunities as well, because difference in relaxation behavior inside and outside the cell permits differentiation between
From page 76...
... Two of the most commonly used techniques for acquiring OH ST data use volume selection in combination with one-, two-, or three-(limensional phase encoding. The relatively high sensitivity of OH ST has ma(le it possible to obtain data from the brain at a spatial resolution of 1-2 cm3 with a volume coil anti 0.15-0.3 cm3 with phased array or surface coils.
From page 77...
... Thus, it is not clear at this stage which approach is best suited for visual interpretation of clinical studies and how the spectral information will ultimately be best integrated with the imaging data. Injected Paramagnetic Contrast Agents and Hyperpolarized Noble Gases Although this report does not give emphasis to the active field of tracer chemistry as it applies to MRT, positron emission tomography (PET)
From page 78...
... to track the spatial distribution over time of the agent, from which kinetic parameters such as flow or tissue perfusion can be derived using mathematical models of various degrees of complexity. These agents effect a change in the local relaxation rate 1/T~ that is proportional to the concentration of the contrast agent.
From page 79...
... Gradients As the SNR is increased, the gradient strengths required to encode image information become a problem. High-amplitude gradient coils to
From page 80...
... Movement of gradient coils can be mistaken for object motion, and gradients that track object motion would eliminate primary motion artifacts, but are not now practical. Future Applications of in viva MRI Microscopy A rich variety of structural and functional features becomes visible below the millimeter level.
From page 81...
... with EPT on conventional systems for clinical use. The implementation of EPI would require improvements in the gradient hardware (eddy current compensation and high-amplitude, high-slew-rate gradients)
From page 82...
... Other Tissue Parameters · Development of accurate measurement techniques for quantitative relaxation times and their interpretation in terms of clinical diagnosis.
From page 83...
... 4. DumouTin, Ah., Souza, S.P., Hardy, Ad., and Ash, S.A., Quantitative measurement of blood flow using cylindrically localized Fourier velocity encoding, Magn.
From page 84...
... 11. Le Bihan, D., Molecular diffusion nuclear magnetic resonance imaging, Magn.
From page 85...
... 27. Kwong, K.K., Behiveau, J.W., Chesler, D.A., Goldberg, I.E., Weisskoff, R.M., Poncelet, B.P., Kennedy, D.N., Hoppel, B.E., Cohen, M.S., Turner, R., Cheng, H.M., Brady, T.~., and Rosen, B.R., Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation, Proc.
From page 86...
... 32. Ogawa, S., Tank, D.W., Menon, R., Ellermann, J.M., Kim, S-G., Merkie, H., and UgurbiT, K., Intrinsic signal changes accompanying sensory stimulation: Functional brain mapping with magnetic resonance imaging, Proc.
From page 87...
... Aguayo, ] ., Blackband, S., Schoeniger, I., Mattingly, M., and Hintermann, M., Nuclear magnetic resonance imaging of a single cell, Nature 322 (1986)
From page 88...
... MAGNETIC RESONANCE IMAGING 50. Early, T., Roemer, P., Mueller, O., Mogro-Campero, A., Turner, L., and Johnson, G., A high-temperature superconducting receiver for nuclear magnetic resonance microscopy, Science 2 5 9 ( 1993)


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