Abdominal MR imaging at 3T

Body MR imaging at 3T is in its infancy, and should improve substantially over the next several years. Radiologists need to be aware of several limitations that are based on the laws of physics: Overall, the gain in SNR at 3T will be less than twofold (without protocol alteration) compared with a standard 1.5T MR system because of the increase in T1 at ultrahigh field. Typically, the gain in SNR is greater in T2-weighted sequences than in T1-weighted sequences, because longer TRs allow for a more complete recovery of the longitudinal magnetization, and T2 is independent of B₀. Thus, for example, patients who are referred for an MR cholangiography may benefit from an ultrahigh-field MR examination. Chemical shift artifacts of the first kind are twice as large in ultrahigh-field MR imaging compared with standard 1.5T MR imaging. Conversely, chemical shift artifacts of the second kind do not increase in size, although the timing is altered. The increased difference in resonant frequency between water and fat at 3T also is advantageous because it allows for a better separation of the fat and water peak during MR spectroscopy, and allows better or faster fat suppression using chemical shift techniques, such as fat saturation or water excitation. Susceptibility artifacts are approximately twice as large by volume on 3T MR imaging. Although patients who are referred for a "colon" study may be challenging at ultrahigh field, the search for "gas" (eg, free air or pneumobilia) should be easier. Patients with metal implants should undergo an MR examination at 3T only if the metal-containing device specifically has been proved to be MR safe at this field strength. Usually, standing wave and conductivity effects are not seen in body imaging at a field strength of 1.5T. At 3T, these artifacts are most pronounced in pregnant women in the second and third trimester, because of the large amount of conductive amniotic fluid and the increased size of the abdomen. Therefore, fetal MR imaging generally should not be performed at 3T because of these artifacts and the increased safety concerns. The same holds true for patients with a large amount of ascites, who also are not well suited for an ultrahigh-field MR examination. Except as noted above, most patients can undergo an abdominal MR imaging study at 3T with a reasonable outcome in terms of image quality.