As with CT evaluations, the preoperative evaluation for NSS with MRI also includes both pre- and post-contrast examinations (35). As with most body MRI examinations, the precontrast T1- and T2-weighted images are obtained to evaluate anatomy, to identify abnormalities, and to begin to characterize any identified renal or adrenal lesions (Fig. 4). Following intravenous gadolinium administration, postcontrast gradient-echo T1-weighted sequences are performed in multiple phases to define the enhancement characteristics of any detected lesions and to define the adjacent anatomy for surgical planning. Specifically the arterial, venous, and collecting system anatomy is again assessed.
The importance of patient preparation and general technique in body MRI cannot be understated. Anterior and posterior phased array surface coils should be used to increase the signal and must be positioned over the kidneys. Patient motion during image acquisition, including respiratory motion, leads to image artifacts, including blurring and ghosting in the phase-encoding direction. Eliminating this motion is an important factor in improving imaging quality in body MRI. In addition to eliminating image artifacts from respiratory motion, reproducible suspension of respiration is needed to take advantage of a variety of postprocessing techniques.
In general, motion compensation techniques can be used to eliminate these artifacts on precontrast scans. However, these techniques increase the acquisition time and could preclude the possibility of obtaining postcontrast scans confined to one specific phase of contrast enhancement, particularly the arterial phase. The most sensitive technique used to determine the presence of lesion enhancement is image subtraction. It requires reproducible breath-holding, because to perform accurate image subtraction, the datasets to be manipulated must be nearly perfectly registered in 3-D space. This is especially true for evaluating small lesions. Comparing imaging characteristics of lesions on different sequences requires their identification on similar or, even better, identical slices. Mathematically combining datasets requires near perfect anatomic registration.
In order to obtain motion-free imaging with a specific temporal resolution, the MRI sequences are kept as short as possible and patients are asked to breath-hold at end-expiration during image acquisition. Breath-holding is done during end-expiration because it is more reproducible. Patients tend to exhale to their functional residual capacity and stop, whereas there is significantly more variation in the
Figure 1 (Figure on facing page) Three-phase helical CT of the kidneys. (A) Noncontrast, (B) vascular phase, and (C) parenchymal phase axial images show two renal masses (arrows) in a 64-year-old male with a solitary left kidney. Note that the intrarenal mass is barely visible in (A), and best seen in (C). The parenchymal phase (C) is the most sensitive for lesion detection. Abbreviation: CT, computed tomography.
end-inspiratory lung volume when patients are asked to suspend respiration at endinspiration. This can be a challenge for some patients, but in those who have difficulty suspending respiration at end-expiration, hyperventilation and supplemental oxygen can allow longer breath-holds, yielding much higher quality examinations. Using low-dose anxiolytic medication can allow for diagnostic studies to be obtained in anxious, nervous, or claustrophobic patients who would otherwise be unable to follow instructions.
When studies for surgical planning are performed, increasing the standard contrast material dose helps obtain better venous opacification. It may also be desirable to distend the calyces in order to better assess invasion in patients whose lesions approach the renal sinus. This can be accomplished with the administration of a small dose of intravenous furosemide. This is discussed in more detail in the following section.
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