Helical CT angiography in evaluation of live kidney donors

Uday D. Patil1, Ashok Ragavan1, Nadaraj1, Keshava Murthy1, Ravi Shankar1, Bahar Bastani2, and Sudarshan H. Ballal1

Divisions of Nephrology and Radiology at 1 Manipal Hospital, Bangalore, India, and 2 Saint Louis University School of Medicine, Missouri, USA



   Abstract
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Background. Live kidney donor evaluation mandates anatomical and functional assessment of the donor kidney. Helical computed tomography (CT) with advanced 3-D techniques provides detailed description of the vascular, parenchymal, and collecting system.

Methods. We compared the accuracy of helical CT angiography with intra-operative findings in the evaluation of 102 live kidney donors.

Results. Identification of vascular anomalies was best on direct viewing of the axial images using interactive scrolling through the images and cine-loop paging. In 204 kidneys evaluated, a single renal artery was present in 74.5% and a single renal vein in 87.5%. Multiple renal arteries were more common on the left side (31%) vs the right side (20%). Early branching of the arteries was seen with equal frequency ({approx}10%) on either side. Multiple renal veins were more often on the right side (20%) vs the left side (5%), and one patient was found to have double inferior vena cava. CT angiographic findings were concordant with the intra-operative findings in 97% of the cases, missing a small renal vein, an accessory artery that was visualized in retrospect, and a very early branch that was read as accessory artery. CT also revealed cortical cysts (four cases), duplex collecting system (two cases), hydronephrosis (one case), renal stone (one case), and liver haemangioma (two cases).

Conclusion. CT angiography is highly accurate for detecting vascular anomalies, and providing anatomical information. It may serve as the primary tool for donor evaluation.

Keywords: donor evaluation; helical CT scan; renal angiography; transplantation



   Introduction
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
The donor work-up consists of functional and anatomical evaluation of the donor kidney [1]. The anatomical evaluation commonly consists of flush aortography to assess the arterial system, which in some institutions is supplemented with bilateral selective renal angiography. Intravenous pyelography (IVP) is performed as a separate study or in combination with angiography. Abdominal ultrasound is also used in some institutions to screen the donor. Helical CT scan with advanced 3-D display technique provides detailed images of the anatomy of the vascular, parenchymal, and collecting systems, and is challenging the traditional battery of tests performed in donor evaluation.

The purpose of this study was to test the accuracy of helical CT angiography in the evaluation of live kidney donors, as compared with intra-operative findings.



   Subjects and methods
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Between April 1999 and January 2000, 102 potential live kidney donors were evaluated with helical CT angiography at Manipal Hospital, Bangalore, India. The subjects, 37 women and 65 men in the age range of 20–60 years old (mean age 33 years), were screened clinically and with laboratory investigations to rule out any medical contraindications for kidney donation. Ultrasonography of the entire abdomen was also performed in all subjects. None of the subjects had a known history of allergy to iodine. Ninety-six subjects underwent donor nephrectomy.

CT protocol
All studies were performed using CT LX/i Highspeed Advantage Scanner (General Electric Medical Systems, Milwaukee, USA). The subjects were called after 4 h of fasting. Venous access was obtained in the preparation room using an 18–20 G Intracath in the antecubital vein or a large vein in the forearm. The subjects were trained to hold their breath with special attention to avoid diaphragmatic motion. On a frontal scout view, unenhanced helical CT scan was performed using 10-mm section thickness, pitch of 2, 120 kV, 100 to 130 mA, 0.8 s scan time, and image reconstruction at 5-mm intervals from diaphragm to mid pelvis during a single breath-hold. Using these images, the scanning range was determined from the celiac axis to aortic bifurcation, including the entire kidneys bilaterally. The delay between the initiation of the injection and the scan was calculated using the ‘Smart Prep’ option (GE Medical Systems) with a test injection of 10 ml of contrast. It ranged between 10 and 28 s. Delay was not timed in the first 15 subjects.

Non-ionic contrast (Iohexol or Iversol, 130–150 ml, 300 mg iodine/ml) was injected at a rate of 4 ml/s with a power injector. Helical CT scans were performed at 2-mm slices, pitch 1.5–2.0, scan time 0.8 s, 120 kV, 150–180 mA, during suspended inspiration. The scan time ranged from 26 to 35 s. The contrast volume was reduced to 80–100 ml, and the injection rate to 3 ml/s in the subjects who weighed less than 50 kg. The images were then reconstructed at 1-mm intervals with segmented interpolation and a small field of view of 18–22 cm, yielding 180–270 images, which were sent to the workstation on the network. After 5–7 min, a frontal scout view was taken to assess the collecting system and the ureters, and was repeated after 5 min delay if the opacification was incomplete.

CT interpretation and 3-D processing
Interpretation was performed on a Sun Ultrasparc II workstation using Advantage Windows 3.1 software (GE Medical Systems). The images were viewed independently by two radiologists (UDP and AR). The unenhanced as well as the contrast-enhanced axial images were first viewed using cine paging to identify the arterial and venous anatomical variants, the parenchymal abnormalities, and calcifications. The interpretation was by consensus. The data was then loaded for 3-D processing. Surface Shaded Display and Maximum Intensity Projection images were then generated. The vascular anatomy was further displayed using Curved Multiplanar reconstruction.

Nephrectomy
Ninety-six of the 102 patients underwent surgical nephrectomy; five subjects were excluded because of unfavourable CT findings, and one subject was excluded because of medical contraindications. The findings on CT and invasive angiography were used to guide the selection of the donor kidney. Left kidney nephrectomy was preferred if the findings for both kidneys were normal. Intra-operatively, the surgeon noted the number and the locations of the renal arteries and veins, as well as the presence of early branches from the renal arteries.



   Results
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
In the 102 subjects who underwent CT angiography, arterial opacification was good in all subjects with good visualization of the vascular anatomy. We did not find any study that was technically poor.

The anatomical findings for renal vasculature are shown in Table 1Go. The other associated findings were: cortical cysts in four, complete duplex collecting system in two, Ask–Upmark kidney (segmental atrophy) in one, mega-ureter/hydronephrosis in one, renal stone in one, and liver haemangioma in two patients. Six patients were excluded from kidney donation, four because of unfavourable vascular anatomy, one for mega-ureter/hydronephrosis, and one because of medical contraindications.


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Table 1. Vascular anatomy on CT angiograms of 102 subjects

 
Identification of vascular anomalies was best on direct viewing of the axial images using interactive scrolling through the images by one author (UDP), and cine-loop paging by another author (AR). The 3-D Surface Shaded Display (SSD) and the Maximum Intensity Projections (MIP) failed to show the small vessels unless they were first identified on the axial source images. Both authors use curved multiplanar reformatting (MPR) extensively to show the vascular anomalies, and to depict the entire course of all the supernumerary vessels from their origin to their entry into the renal parenchyma. The rest of the display formats like SSD and MIP were used to give an easy 3-D understanding of the anomalies. Figures 1a,Go bGo show some representative images of vascular anomalies depicted by CT angiography.



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Fig. 1. (a) CT angiogram; frontal maximum intensity projection (MIP) image showing five renal arteries on the left side and three on the right; (b) same patient, curved multiplanar reformatting (MPR) image showing three veins forming a distal confluence.

 
Comparison of the CT angiogram and the intra-operative findings
Of the 102 prospective donors, 96 underwent donor nephrectomy. Hence, correlation was possible on 96 renal units. The CT angiogram findings were concordant with the intra-operative findings in all but three of the donors (accuracy rate of 97%). In one subject, a small upper pole renal vein was missed on CT angiogram. This was only the second study performed at our institution. At that time we did not measure the delay between the injection and the scanning. In another subject, an accessory artery to the lower pole of the left kidney was missed on the initial reading, but was well visualized in retrospect. The artery arose from the aorta just above its bifurcation. The patient also had late confluence of multiple accessory renal veins from the lower pole, which joined the inferior vena cava just above its bifurcation. The artery ran posterior to these veins. In another donor the CT finding of an accessory artery arising just superior to the main renal artery was found to be a very early branch arising within 3 mm of the origin of the right renal artery.

The findings in 204 renal units (102 subjects) evaluated by CT angiography are as follows: single renal artery in 74.5% of the renal units, multiple renal arteries in 25.5%, single renal vein in 87.5%, and supernumerary veins in 12.5%. Multiple arteries were more common on the left (31%) than on the right side (20%). Early branching of the arteries was seen with equal frequency on the right side (11%) and the left (9%). Multiple renal veins were seen more often on the right side (20%) than the left side (5%). Late confluence of renal veins was seen in 9% of the right and 4% of the left kidneys.



   Discussion
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Although 3-D reconstruction and displays are quite valuable, we found the axial source images most informative in assessing the anatomical variations, as has been reported earlier [2]. In no case did we find any new information on the reconstructed images that was not evident on the source images. Moreover, the minor accessory vessels and the branch vessels, which were not identified on the 3-D images and often had to be shown, using ‘paint brush’ technique, were best visible on direct viewing of the axial images by interactive scrolling through the images and cine-loop paging. In our series, the incidence of accessory arteries and veins is similar to previous reports [25]. While multiple renal arteries are one and a half times more common on the left than the right side (31 vs 20% respectively), the incidence of multiple renal veins on the right side is four times more than the left side (20 vs 5% respectively). Late confluence of renal veins is also twice as common on the right side than the left side (9 vs 4% respectively). This may be due to the short course of the renal vein on the right side.

In comparison with the intra-operative findings, we found an accuracy rate of 97% with helical CT angiography delineating vascular anatomy. Only three cases were discordant. An accessory artery to the lower pole of the left kidney was missed on the initial CT reading, which was well visualized in retrospect. The artery arose from the aorta just above its bifurcation and the patient also had late confluence of accessory renal veins from the lower pole, which joined the IVC just above its bifurcation. The artery ran posterior to these veins. This was one of our earlier cases, and the vessel was overlooked among the renal venous channels due to our lack of experience. In another case the discrepancy was difficult to resolve—whether it was an early branch within 3 mm of the ostium, or an accessory artery arising very close to the main renal artery ostium. And finally, a small upper pole accessory renal vein was missed in our earlier experience with CT angiography.

Our results are better than previous reports of a concordance rate of 87–95% [2,49]. In two of the largest series, Platt et al. [5] reported a concordance rate of 95% among 117 patients, and Del Pizzo et al. [9] found a concordance rate of 93% among 157 cases. The higher degree of concordance in our series may be explained in part by some differences in the technique of CT angiography. We routinely time the delay between the injection and peak aortic opacification, with a test injection. We have found the delay to vary from 10 to 28 s. Thus, using fixed scan delays of 17 or 20 s may not result in uniform vascular opacification. We also routinely use 2-mm thickness sections with a pitch of 1.5–2, rather than the 3-mm sections used in the majority of the previous reports. It has been documented that for the evaluation of renal artery stenosis, 2-mm slice thickness is better than 3-mm thickness [10]. A thinner slice profile with a segmented interpolation reconstruction at 1-mm intervals would thus be expected to improve the sensitivity for detecting small vessels. The increased signal noise can be overcome to some extent by better X-ray tube loading and better detector performance. With the introduction of the new multislice helical CT units, it may be possible to routinely use 1-mm slice images. These units permit a routine pitch of up to 4 without significant deterioration of the slice profile. This in turn would reduce the breath-holding duration by a factor of 2 or 3 and decreases the problem of motion causing misregistration artefacts.

The advantage of helical CT angiography over conventional angiography is that the former is non-invasive, requiring only an antecubital or forearm venous access, it is performed as an outpatient procedure, and the subject is ambulatory immediately after the procedure, with no loss of working time. This is in contrast to invasive angiography where 6–8 h of absolute bed rest is mandatory.

While helical CT angiography can identify renal vascular anomalies more accurately than invasive angiography, the limitation of CT angiography in identifying fibromuscular dysplasia is well known, and has been emphasized in previous reports [210]. We came across an artefact that appeared to be an arterial stenosis. In one patient the anterior division of the right renal artery showed a 50% stenosis on MIP and SSD images, which was due to a mild diaphragmatic motion causing misregistration of the vessel. We have not come across reports of similar artefacts.

The other disadvantage of CT angiography compared with invasive angiography is the larger volume of contrast used with the former, two to three times more, and hence the potential increased risk of nephrotoxicity. However, contrast-induced renal failure is only rarely encountered in individuals with normally functioning kidneys [11]. This potential disadvantage can be avoided if gadolinium-enhanced magnetic resonance angiography (MRA) is used [12], because gadolinium as a contrast agent does not lead to nephrotoxicity. In a study in which MRA was compared with intra-arterial digital subtraction angiography (DSA) in 24 consecutive potential donors [13], three MR angiograms were technically unacceptable because of inadequate breath-holding. In the remaining patients, MRA visualized all 47 renal arteries, including five accessory arteries (100% sensitivity), but failed to detect a subtle distal stenosis due to fibromuscular dysplasia. Two other studies have also reported 100% correlation between the number of renal arteries detected with MRA and the number of arteries found at surgery [14] and/or intra-arterial DSA [15]. Several reports on MRA [14] and helical (spiral) CT angiography [4,5] for the evaluation of potential renal donors have found that a few small accessory arteries detected by these techniques were missed by DSA. The accuracy of gadolinium-enhanced MRA and helical (spiral) CT angiography in detecting accessory arteries and renal artery stenosis are approximately the same. However, both techniques are less accurate in detecting fibromuscular dysplasia and more distal renal arterial and/or branch vessel lesions, as compared to conventional angiography. Anomalies of the renal veins, renal parenchyma, and collecting system are equally well detected by CT and MR imaging, and both modalities are clearly more suitable for this purpose than conventional angiography.

Advantages of CT angiography over MRA are that inadequate breath-holding causes less severe degradation of image quality, and CT angiography is easier to perform by most technicians in the routine clinical services. The disadvantages of helical CT angiography in comparison to MRA are the use of ionizing radiation and large volume of iodinated contrast material with the former. However, the cost of MRA is significantly higher than CT angiography or conventional angiography.

The cost saving of helical CT angiography as compared with conventional angiography is well documented in the western literature, and ranges from 35 to 50% [4,16], although in India the cost difference is much less and in the order of only 5–10%.

In conclusion, we have found helical CT angiography to be safe and highly accurate in the evaluation of vascular anomalies in potential renal donors. We suggest that CT angiography be the primary technique for this purpose, and that invasive angiography be used as a secondary modality only for problem cases.



   Acknowledgments
 
The authors appreciate the secretarial assistance of Ms Tracy Line.



   Notes
 
Correspondence and offprint requests to: Bahar Bastani MD, Professor of Medicine, Division of Nephrology, Saint Louis University School of Medicine, St Louis, Missouri 63110, USA. Back



   References
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 

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Received for publication: 17.10.00
Revision received 28. 3.01.