1 Department of Nephrology and 2 Department of Radiology, Cliniques Universitaires St Luc, Université Catholique de Louvain, Brussels, Belgium
Keywords: angiography; colour Doppler ultrasound; gadolinium; magnetic resonance angiography; transplant renal artery stenosis
Cases
Patient 1
This was a 57-year-old woman with end-stage renal disease (ESRD) due to chronic pyelonephritis who had received a cadaver kidney graft in July 1980 after 42 months of haemodialysis. Maintenance immunosuppression included azathioprine and prednisolone. Hypertension appeared a few months after kidney transplantation (KT) and was treated by methyldopa 250 mg three times a day. A systolo-diastolic murmur was noticed 6 months after KT. A first graft arteriography was performed in 1982. It revealed parietal irregularities with a <20% stenosis involving the first 2 cm of the transplant renal artery stenosis (RAS). In 2000, hypertension became resistant to a treatment combining isradipine and atenolol. Cockroft creatinine clearance and 24 h proteinuria were 72 ml/min and 150 mg, respectively.
A colour Doppler ultrasound (CDU) using a HDI 3000 machine with a 3.5 or 5 MHz sectorial transducer was performed. Peak systolic velocity (PSV) was measured at 3.36 m/s in the ostial part of the transplant renal artery, raising the suspicion of the existence of transplant RAS, since truncal angle-corrected PSV above 2 m/s is a generally accepted criterion for this diagnosis [13]. A magnetic resonance angiography (MRA) was also performed, using a breath-hold gadolinium (GD)-enhanced fast 3D gradient echo sequence with multiplanar and maximum intensity projection reconstructions (3D-GD-MRA) (1.5 Tesla Philips Gyroscan magnet). An 80% post-ostial stenosis of the transplant renal artery was demonstrated (Figure 1A).
|
Patient 2
A 49-year-old woman with ESRD due to haemolytic uraemic syndrome had received a cadaver kidney in September 1991 after 14 months of haemodialysis. Immunosuppression included cyclosporin, azathioprine and prednisolone. Hypertension appeared immediately after KT. In 2000, hypertension became resistant to a combination of bisoprolol, amlodipine, moxonidine, frusemide and quinapril. At that time, Cockroft creatinine clearance was 68 ml/min. A transplant RAS was suspected despite the absence of murmur. CDU was normal. MRA then was performed. Two graft arteries were shown on the 3D GD-enhanced MRA: the superior artery, rapidly dividing into segmentary branches appeared normal, whereas in the inferior artery a >50% stenosis was demonstrated at the intersection between the vertical and the horizontal parts (Figure 2A). However, conventional arteriography failed to show RAS. Three metal clips were noted in the soft tissues adjacent to the inferior artery (Figure 2B
).
|
Discussion
In these two patients with a clinical suspicion of transplant RAS, GD-enhanced MRA led to an erroneous diagnosis of transplant RAS. This diagnosis was also strongly suggested by CDU in the first case but not in the second one. These cases illustrate the pitfalls in the interpretation of transplant artery MRA images and provide the opportunity to discuss the accuracy and place of CDU and GD-enhanced MRA in the diagnosis of transplant RAS.
RAS is a potentially curable cause of arterial hypertension or allograft dysfunction after KT. The reported incidence varies from 1 to 12% [4,5]. The diagnosis of transplant RAS should be suspected whenever arterial hypertension develops or worsens rapidly after KT, especially when a bruit is audible over the graft or when administration of an ACE inhibitor induces a decline in graft function. Conventional arteriography remains the gold standard for the diagnosis of transplant RAS. Therefore, there has been a search for a suitable, non-invasive alternative technique.
CDU is largely used as a first-step diagnostic tool in the diagnosis of transplant RAS. The measurement of PSV is regarded as the best discriminating parameter for the detection of transplant RAS [2]. PSV measures the maximum velocity recorded during the systolic phase of arterial flow.
With a transplant renal artery, PSV has to be corrected according to the angle between the probe and graft arterial axis; this is generally made by inbuilt software. There are some controversies on the diagnostic cut-off value of PSV. Using a value of >1 m/s as positive, Erley et al. [1] reported a sensitivity of 100% and specificity of 75%, while Baxter et al. [2] obtained a sensitivity of 100% and a specificity of 95% with a higher cut-off >2.5 m/s. Tortuosity, arteriovenous fistula, and/or angulation between renal artery and recipient iliac artery may account for high PSV in the absence of stenosis, and hence a false-positive test [3]. Thus, MRA has appeared as a complementary test to CDU and an interesting alternative to angiography.
More recently, MRA has emerged as another valuable non-invasive alternative including a three-dimensional representation of the vascular anatomy without the disadvantages of arteriography. The limitations of MRA include the difficulties of exploring accessory arteries, distal artery segments, and stented arteries [6], arteries adjacent to metallic structures and en-block renal grafts [68]. The 3D phase-contrast MRA technique was the first technique used to study renal graft arteries [9]. Its limitations were essentially associated with blood-flow artefacts that mimic stenosis [3]. The use of gadolinium injection, breath-hold sections, and ultra-rapid image acquisition techniques, as well as the development of new image reconstruction processes were all advances allowing improvement of the imaging quality while diminishing artefacts associated with breathing movements or blood flow [8,10]. Very few studies have been conducted on the reliability of 3D-GD-MRA in comparison to IADSA for the diagnosis of transplant RAS [7,8,1114]. By pooling the cases of the four available prospective series comparing 3D-GD-MRA and IADSA for clinically suspected transplant RAS (studies comprising 7, 6, 17 and 41 patients, respectively), we calculated that 3D-GD-MRA has a 96% sensitivity, a 96% specificity and a 98% negative predictive value [8,1214].
How to explain these two false-positive results of MRA?
In the first patient, a stenosis <20% is seen on the upper edge of the transplant renal artery after the ostium. There is, however, no pressure gradient between the iliac artery and the transplant renal artery, which confirms that there is no haemodynamically significant stenosis. Metal clips are too distant to explain an artefactual image of stenosis. The most likely explanation for the false-positive result with both CDU and MRA is an accelerated blood stream in the ostial area, accounting for high PSV (even when corrected for angulation) at CDU, and a loss of the magnetic signal at MRA mimicking high-grade stenosis (Figure 1).
This is not true for the second patient in whom CDU did not detect any flow acceleration whereas MRA was falsely positive. The metal clips in the close vicinity of the renal artery are probably responsible for the pseudostenosis image. As reported by others [15], metal clips can scatter the magnetic signal and thus mimic stenosis (Figure 2).
These two cases illustrate the pitfalls of GD-enhanced MRA in the diagnosis of transplant RAS. Based on the high sensitivity and availability of CDU, this technique remains the best screening tool. If CDU is doubtful or positive, is it useful to perform MRA?
Available studies report that specificity for MRA is only marginally better than that for CDU in the detection of transplant RAS. Furthermore, there is no study directly comparing CDU and GD-enhanced MRA in this setting.
Only a prospective study showing a significant gain in specificity provided by GD-enhanced MRA on CDU for transplant RAS would establish this technique as a valuable second step test in this indication.
Teaching points
Acknowledgments
We thank C. Van Ruyssevelt, MD, for logistic help and for advice.
Notes
Correspondence and offprint requests to: G. Clerbaux, MD, Department of Nephrology, Université Catholique de Louvain, Av. Hippocrate 10, 1200 Brussels, Belgium. Email: gaso.clerbaux{at}belgacom.net
References