Clinical MR scanners

Clinical MR scanners used for MRI all rely on the FFT. By inertia, this has continued for MRS, despite the fact that with the FFT the rich information content of a given tissue cannot be unfolded. The more appropriate spectroscopic methods based on quantum physics should have been transferred to MRS. Fitting has been used in attempts to deal with the lack of quantification of the FFT. However, as we have seen, this is an inadequate approach and in fact is just a form of guesswork. Fundamenfally, it is not a question... 

Fig.4.n T, -weighted MR images of angiogenesis in a 3 mm Vx-2 tumor (rabbit model) obtained on a 1.5 Tesla clinical MR scanner. Perfluorocarbon nanoparticles studded with Gd-chelates and an epitope antagonist are used to confer tar-... 

Clinical MR scanners used for fast imaging experiments may have up to 15 kW RF amplifiers. These high powers are necessary to reduce pulse duration in fast spin-echo imaging sequences. However, care must be taken that the amplifiers are linear otherwise the shaped pulses necessary for slice selection will be distorted and the slice profile degraded. Of course, many manufacturers are aware of this problem and compensate their pulse shapes for the known distortions induced by the RF amplifier so that the final pulse shape delivered to the RF coil is optimal. If slice profiles are inadequate it is always worth checking for non-linearity in the RF amplifiers. 

Spectral simulation tools provide a similar opportunity for MR researchers. In its simplest form, spectral simulation allows changes in a data acquisition to be observed without having to program an MR pulse sequence and scan a volunteer on an actual MR scanner. This is particularly convenient in a clinical research setting where scanner time costs are high and access can be scarce. However, simulation methods additionally provide researchers with the ability to run "experiments" on instruments that are not available at their institution. Variations in the "samples" can also be part of a simulation and the effects of their variability on the MR results calculated. Properly characterized, a spectral simulation can incorporate the effects of any physical equipment or environment on a given sample.20 21... 

Fig.4.4 Animal MR image with 100 urn resolutio obtained on a clinical 3 Tesla MR scanner usin dedicated MR receive coil (Philips Research, Ha burg, Germany).

A recently developed pinhole high-resolution SPECT system and a 1.5 T clinical MRI scanner with a specially developed surface coil were used for rats (n = 9) after injection of the dopamine transporter ligand I-FP-CIT. The SPECT images showed clear striatal uptake. On the MR images, cerebral and extra-cerebral structures could be easily delineated. 

R = 0.77, P = 0.0007) and cadaveric discs (R = 0.83, P < 0.0001). Significant comelation between N-acetyl/Cho and GAG content was also found in cadaver discs (R = 0.64, P = 0.0039). This study demonstrates for the first time that short-echo PRESS on a clinical 3T MR scanner can be used to noninvasively and can reproducibly quantify metabolic changes associated with degeneration of intervertebral discs, (c) 2009 Wiley-Liss, Inc. 

NMR spectroscopy, commonly called magnetic resonance spectroscopy (MRS) in its in vivo application, is continuing to expand its reach in both biomedical research and clinical medicine. Currently, most in vivo MRS utilizes as a straightforward addition to MRI on a clinical scanner. The high sensitivity of permifs relatively small tissue volumes, on the order of 1-8 ml, to be probed in vivo in humans. With water suppression,... 

Phosphorus-31 MRS is continuing to advance and see wider application. The capability to perform P MRS on both clinical and high-held animal scanners is steadily becoming more widespread. Advances such as high magnetic fields and improved RF hardware, data-acquisition approaches, spectrum analysis automation, and pulse sequences promise a technique of increased speed and spectral and spatial resolution, with a widening range of potential applicahons. 

In the clinical environment, an exercise device to provide isotonic resistance to plantarflexion or dorsiflexion has been constructed for use in a whole body scanner. The device was tested in five subjects during 3 min of exercise at 30% of maximum voluntary plantarflexion during MRS. 

MR-guided percutaneous interventions have been clinically established, in particular, with open, low-field MR systems (Koenig et al. 2001 Lewin et al. 1998 Silverman et al. 1995). However, the imaging quality of closed-bore scanners is superior to... 

The former technique was that known as filtered back-projection, which, as originally applied, was a direct MR analogue of the original translate-rotate CT-X-ray scanner developed by Hounsfield. The first version of the other strategy, involving Fourier transformation in two or more directions, was proposed by Ernst and his colleagues in a form that ultimately proved much less useful than the spin warp method that has become the basis of the vast majority of clinical MRI. 

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