By L. Crooks, L. Kaufman, N. Hylton (auth.), Dr. D. H. W. Grönemeyer, Professor Robert B. Lufkin M.D. (eds.)
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Additional resources for Open Field Magnetic Resonance Imaging: Equipment, Diagnosis and Interventional Procedures
Example text
The disadvantages are high power consumption, large cooling demand, a large conductor mass and its consequent weight. When an iron core is used, the larger mass of conductor forces an increase in the physical size of the core, which then results in further weight increases. Permanent Drivers In a permanent magnet the field originates in blocks of permanently magnetized material. Currently available are ferrite magnets for low field operation and Neomax (a rare earth alloy) for higher fields. The attractive aspect of permanent magnet drivers is that they are totally passive, requiring no added services such as electricity, cryogens, or cooling.
In practice, because data acquisition is completed in less than 50 ms, the waiting time can be spent acquiring other slices until it is time to return to the first slice at TR = 500 ms. Many special-function pulse sequences have been developed to address specific imaging needs. The spin-echo pulse sequence has been a standard over the years, but it has some drawbacks. The time spent issuing 180 0 pulses to correct for field inhomogeneities also serves to prolong the TE value. If the 180 pulse is removed and a shorter TE is used, the loss of signal due to field inhomogeneity can be offset to some extent by the reduced T2 decay.
The issue of contrast in MR images is broad and could not be fully covered here. Nevertheless, there are frequent and typical cases that can be used to exemplify the subject. We discuss here three examples of the behavior of contrast at low and high fields. The first example involves the detection of lesions where water content is increased. This is a typical effect seen in tumors, edema and infarcts, considering the case where there is no blood. For these lesions N(H), Tl and T2 are all elevated [2].