In the 1980s and early 1990s, time-of-flight and phase contrast magnetic resonance (MR) techniques were considered the state of the art methods of MR angiography until the advent of contrast-enhanced MR angiography (CE-MRA) introduced in the early to mid 1990s . Many consider CE-MRA MRI to be an appropriate “one-stop shop” imaging exam in the evaluation of a renal donor .
There has been a renewed interest in noncontrast-enhanced (NCE) MRA techniques for several reasons. First, if one could provide a comprehensive MR exam without having to administer gadolinium, the study could be completed with less cost . Second, a small but true risk of acute allergic reaction to gadolinium could be avoided by performing a nonenhanced exam . Finally, gadolinium administration has been associated with the development of nephrogenic systemic fibrosis, especially in those patients with reduced renal function .
The current issue of Academic Radiology contains an article by Dr. Goetti and colleagues that describes their experience with a NCE MRI protocol in the evaluation of potential living renal donors . The authors compared a respiratory-gated three-dimensional gated steady-state free precession (SSFP) NCE-MRA sequence to a CE-MRA three-dimensional spoiled gradient echo sequence and showed that the former sequence depicted more accessory renal arteries than the latter.
The CE-MRA technique as performed by Goetti et al had several limitations that did not allow for optimal detection of small accessory renal arteries. First, the larger field of view and smaller matrix of the CE-MRA compared with the NCE-MRA resulted in a lower in plane resolution of the CE-MRA. Second, the gadolinium was hand injected as opposed to administered with a power injector. The latter technique is preferred to ensure an accurate and precise rate of contrast injection. Finally, the mean breath-hold time of 25 seconds could have been shortened in order to minimize potential breathing or other motion artifacts. With improved software modifications, three second temporal resolution dynamic-enhanced abdominal MRI-MRA is now possible and is being performed in some centers .
Independent of the limitations of the comparison CE-MRA sequence, I am not convinced that the proposed NCE-MRA MRI exam can be considered a sufficient imaging exam to screen renal donors. I site three reasons. First, the craniocaudal dimensions of the gated SSFP exam was only 11 cm. Potential inferior accessory renal arteries that originate outside of the field of view of this sequence would not be detected. The SSFP NCE-MRA sequence incorporated by Laurence et al in their evaluation of renal donors has a mean craniocaudal dimension of 35 cm and would avoid this pitfall . Second, if respiratory gating is not optimized (see Fig 3) the resultant image quality of the NCE-MRA sequence may be nondiagnostic. Third, when evaluating a renal donor, our purview is not limited to the renal vasculature. Focal parenchymal lesions as well as pathological processes outside of the major abdominal organs can be more confidently characterized with the use of gadolinium enhancement. I think the increased cost and risk of gadolinium is justified if it can successfully prevent an altruistic donor from undergoing an unnecessary nephrectomy or ensuring that an appropriate renal harvesting procedure gets performed by accurately identifying all variant renal vascular anatomy.
Finally, if you are considering switching to or continuing the practice of screening renal donors with MR imaging, be aware that MR is not sensitive for the detection of renal calculi. Renal stones smaller than 1 cm are rarely depicted with MR imaging . In a cohort of almost 2000 patients being considered for renal donation at the Mayo Clinic, 10% had asymptomatic renal calculi and lacked the typical risk factors that occur in patients with symptomatic renal stones . Ask your renal transplant surgeon what his or her policy is on transplanting patients with renal calculi. Some centers will remove the kidney in a donor with unilateral stones and perform ex vivo ureteroscopy at the time of transplantation so that the kidney is stone free at the time of implantation into the recipient .
References
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