MRI Physics for Physicians

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Chavhan , Paul S. Babyn , Bejoy Thomas , Manohar M. Shroff , E. Mark Haacke. Technical principles of MR angiography methods. Marko K. Ivancevic , Liesbeth Geerts , William J.

Weadock , Thomas L. Clinical safety of cardiovascular magnetic resonance: cardiovascular devices and contrast agents. A square array of spheres precess b in a Gx gradient for various times a during the gradient application. A plot of gradient area for various times c is used to characterize the spatial frequency Kx of the resulting stripe patterns. In a similar manner, it follows that using a gradient in the Y direction Fig. Thus, by application of a gradient in either the X or Y directions, stripes in the vertical and horizontal directions, respectively, can be generated.

A square array of spheres precess b in a Gy gradient for various times a during the gradient application. A plot of gradient area for various times c is used to characterize the spatial frequency Ky of the resulting stripe patterns. Finally, by using the sequential application of Y and X gradients an oblique stripe pattern can be generated Fig. In this case, the Y gradient first generates a horizontal stripe pattern by causing rows of spheres to precess in synchrony. Thereafter, the application of the X gradient causes columns of spheres to undergo rotations, with the final result being an oblique stripe pattern as seen at time t 2.

Referring to the gradient area plot Fig. A square array of spheres precess b in a Gy gradient followed by a Gx gradient a. A plot of gradient area c for various times is used to characterize the spatial frequencies Kx and Ky of the resulting stripe patterns. The application of gradients to create stripe patterns of varying orientation and spatial frequency and their relation to gradient areas have been shown.

The remaining piece of information needed is how the correct amplitude and phase for each spatial frequency to correctly encode the object is determined.

During the application of the gradients Fig. Usually, the signals are obtained from two coils placed in an orthogonal arrangement near the patient, as discussed above Fig. The signals from these two coils are different due to their relative orientation. In some cases, such as with the use of surface coils, the signal from a single coil can be used to measure phase and magnitude. Figure 19 f details the gradients used and shows a specific Gy waveform, which is followed by a fixed Gx waveform.

The evolution of magnetization a—c for various times under the influence of a Gx gradient after application of a Gy gradient f. During the RF pulse, a slice is selected in the presence of a Gz gradient.

Understanding MRI: basic MR physics for physicians.

Then an incremented Gy gradient precedes the Gx gradient. The NMR signal, or echo, is sampled during the application of the Gx gradient. If Nx by Ny pixels in the image domain are needed, then the pulse sequence requires Ny incremented Gy gradient waveforms after which the echo is sampled Nx times. The time interval between the successive Gy gradients is TR repetition time. TR is the parameter frequently used to control the T1 weighting of the image. Similarly, the time between the selective excitation pulse and the peak of the MRI signal is the TE echo time.

The pulse sequence illustrated in Fig. For example, Fig. In this case, there is no distinction between the gradients in terms of the order and as such the notion of frequency and phase encoding gradients cannot be applied.

The net result is that images can be made that require fewer RF repetitions without major effects on image contrast 63 , Variants of this sequence can be used to image tissues with very small T2 values This can result in a substantial reduction in the time needed to acquire data, and as such is one of the methods of choice for MRI of dynamic processes such as in cardiac imaging Given an understanding of how MR pulse sequences are assembled to provide a means of imaging, it is now possible to illustrate how the choice of different timing parameters can be used to generate images with contrast reflecting different relaxation mechanisms.

This sequence is very similar to the one discussed in Fig.

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The sequence is repeated at intervals of TR seconds. The longitudinal solid lines and transverse magnetization dashed lines are shown for short TR1 and long TR2 repetition times. These are combined with short TE short and long TE long echo times b for two tissues of long T1 and long T2 red and short T1 and short T2 black relaxation times. Images are described in terms of the various factors that define their contrast. As the name implies, proton density reflects the variations in the density of protons that are accessible to MRI.

Small variations in proton density occur among different soft tissues as seen by the MR system. This is a somewhat unfortunate term in that it does not explicitly reflect gravimetric variations in proton density but rather variations in the abundance of protons available for detection by a specific MRI pulse sequence.

The mechanism for achieving different weighting is illustrated in Fig. The longitudinal magnetization Mz for CSF and white matter as a function of time is plotted in this figure. If a very long time is allowed between sequential RF selective excitation pulses TR2 , the magnetization Mz of the red and black tissues converge toward their corresponding equilibrium longitudinal magnetization as reflected through their proton density.

Alternatively, the growth of longitudinal magnetization could be interrupted before either tissue reaches its equilibrium value by using a shorter TR interval TR1. In this case, the CSF curve would have regained a smaller fraction of its equilibrium magnetization compared to that of white matter due to the longer T1 of CSF. In this case, white matter is seen to be brighter than the CSF Fig. Finally, if a longer TE period, for this short TR case, was used, the tissue signals would converge until at some point the contrast between the red and black tissues would vanish.

In this case, the black tissue was biased with a short TR interval, only to have this contrast reduced by a long echo time and an eventual inversion of contrast. While different nuclei can exhibit NMR, protons are the primary means for MRI due to their abundance and large gyromagnetic ratio.

Through the use of RF pulses, transverse magnetization can be selectively generated in specific sections of an object in a fashion akin to that of CT scanners. However, MRI offers the option to create sections in orientations that would be impossible for CT systems and offers unique perspectives of anatomy. The magnetic environment at the molecular level mediates different relaxation mechanisms. Transverse relaxation, T2, arises from low temporal frequency changes in the magnetic environment, while T1 relaxation is primarily influenced by temporal magnetic fluctuations at the spin Larmor frequency.

Large variations in relaxation times have been found between various normal and diseased tissue. Through the use of pulse sequences with different timing and excitation characteristics, these relaxation time differences can be exploited to great advantage to generate exquisite soft tissue contrast. This article is an attempt to present a very basic introduction to the physics of MRI, which hopefully should form the foundation for a more detailed study of the deep MRI literature that has evolved over the past three decades.

The interested reader is urged to use these concepts as a transition to the many excellent texts 41 , 72 , 73 available, which delve deeper into the subject. Hopefully this article will consolidate some basic concepts to help in exploring the continually expanding technical MRI literature and growing array of clinical applications. Volume 35 , Issue 5. The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.

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This article, written for the general hospital physician, describes the basic physics of MRI taking into account the machinery, contrast weighting, spin- and. Understanding MRI: basic MR physics for physicians. Currie S(1), Hoggard N, Craven IJ, Hadjivassiliou M, Wilkinson ID. Author information.

If the address matches an existing account you will receive an email with instructions to retrieve your username. Journal of Magnetic Resonance Imaging. Review: MR Physics for Clinicians. Donald B. Tools Request permission Export citation Add to favorites Track citation. Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Figure 1 Open in figure viewer PowerPoint.

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