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Invited Commentary |
Department of Radiology, St Luke Roosevelt Hospital, New York, NY
Renal magnetic resonance (MR) imaging has evolved over the years into a comprehensive method of evaluating the kidneys in particular and the urinary tract in general. With the widespread availability of three-dimensional, T1-weighted, gradient-echo sequences, renal vascular MR imaging is now commonplace (1–3).
In the preceding article , Dr Heiss and colleagues present their experience with these dynamic contrast materialenhanced MR sequences in evaluation of the whole spectrum of renal disease. The authors present a total picture of what the contrast-enhanced, three-dimensional, fast spoiled gradient-echo sequence can do. Evaluation of the aorta or renal arteries and veins to demonstrate dissection, stenoses, thromboses, malformations, or occlusions is easily performed. The renal parenchyma and its function can be both qualitatively and quantitatively evaluated as the study progresses through the arterial corticomedullary and tubular nephrographic and excretory phases.
The functional equivalent of renal nuclear medicine scanning is within the capabilities of this MR imaging sequence (4–6). Pyelonephritis and its complications, as well as reflux nephropathy, renal transplant complications, and renal trauma are well illustrated in the preceding article. Tumors, cysts, polycystic disease, multilocular cystic nephroma, and their distinction from one another are all illustrated and briefly discussed. The staging of renal neoplasms is thoroughly presented.
The authors point out that nonenhanced T1-weighted images and T2-weighted images, including fat-saturated images, are necessary to fully characterize renal masses. MR imaging with T2-weighted sequences and other recently developed sequences (eg, half-Fourier rapid acquisition with relaxation enhancement, or RARE [HASTE; Siemens Medical Systems, Iselin, NJ]; single-shot fast spin echo [SSFE; GE Medical Systems, Milwaukee, Wis]) also helps evaluation of the renal collecting system (7), which is part of any comprehensive renal study. Evaluation of the collecting system and ureter was not, however, discussed in the preceding article.
With the ability to readily perform this kind of comprehensive study, are there any limitations to be aware of? The main deficiency of MR imaging in the renal-urinary tract is the inability to consistently recognize calcifications and calculi.
Since these abnormalities on imaging studies are the most frequently encountered by the radiologist, it is no wonder that nonenhanced helical computed tomography (CT) has become the imaging modality of choice in the evaluation of stone disease (8). Medullary sponge kidney and other causes of nephrocalcinosis might also easily go undetected at MR imaging.
It might therefore be argued that CT can accomplish what contrast-enhanced, three-dimensional, fast spoiled gradient-echo MR imaging can accomplish—and also depict calculi and calcifications. Nonenhanced helical CT and contrast-enhanced helical CT angiography are in fact capable of just that (9,10). MR imaging, however, offers multiplanar capability without the need for as much reconstruction, the ability to use an intravenous contrast agent in patients with azotemia, and specialized vascular sequences such as phase-contrast sequences (3), which are valuable in the grading of stenoses. As a result of its increased contrast resolution, MR imaging can also demonstrate certain cysts that may be indeterminate at CT. Of course, a combination of nonenhanced helical CT and contrast-enhanced fast MR imaging would also be totally comprehensive. The choice of study should therefore depend on the equipment, the radiologic expertise available, and the nature of the clinical problem under investigation.
In conclusion, the article by Heiss et al summarizes most of the current state-of-the-art applications for renal MR imaging, with emphasis on contrast-enhanced fast MR imaging. Although for the most part, their results are comparable to those with nonenhanced helical CT with contrast-enhanced helical CT angiography, the future advantages of MR imaging include functional imaging, with pharmacologic agents such as angiotensin-converting enzyme inhibitors (2) or furosemide (11), and diffusion (12) or echo-planar (13) imaging. The potential value of these advances has yet to be determined.
References
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