Abstract
Background
A routine, multiphase, computed tomography (CT) protocol is associated with high radiation exposure to potential kidney donors. To reduce radiation exposure, several authors have suggested a reduction in the number of phases.
Purpose
To evaluate a low-radiation-dose, dual-phase protocol (i.e. a protocol with an unenhanced phase and combined vascular and excretory phase) for the preoperative evaluation of potential renal donors.
Material and Methods
Sixty-five potential renal donors were divided into two groups. The first group was scanned with a routine quadric-phase protocol (non-contrast, arterial, venous, and delayed), and the second group was scanned with a triple-phase protocol (dual phase protocol + venous phase). In the second group, we replaced CT angiography with a routine abdominal CT technique. In addition to the evaluation of renal arteries, veins, and excretory systems, the radiation dose of the suggested protocol was compared to that of the routine quadric-phase protocol.
Results
The suggested protocol was efficient in the evaluation of renal arteries, veins, and excretory systems in all studied potential renal donors. Renal arteries were well visualized in the combined vascular excretory phase using the routine abdominal CT technique; no significant difference was noted when these results were compared to those obtained from the CT angiography used in the quadric-phase protocol. The mean effective radiation dose of our suggested dual-phase protocol was only 34% of the dose resulting from the routine quadric-phase protocol.
Conclusion
Use of a low-radiation, dual-phase, CT protocol, which relied on both an unenhanced phase and a combined vascular and excretory phase, significantly reduced radiation dose. Furthermore, the proposed protocol provides adequate visualization of renal arteries and veins, and affords sufficient opacification of the urinary tract using improved acquisition triggering.
Living kidney donor transplantation is frequently performed because of the insufficient supply of organs from cadaveric donors and the increased viability of living allografts (1, 2). Preoperative detailed information about the renal donor's vascular anatomy helps in minimizing the risks of bleeding and vascular injury (3).
Traditionally, potential renal donors have been evaluated by conventional renal angiography and excretory radiography. Recently, however, this practice was replaced by computed tomographic angiography (CTA) and computed tomographic urography (CTU) (4, 5).
The introduction of 64-slice, multirow-detector computed tomography (MDCT) has further improved the performance of helical CT. These improvements have not only resulted in increased scanning speed, but they have also led to higher spatial resolution, thinner acquisition, and superior image quality (6).
Preoperative evaluation includes the assessment of arteries and veins, confirmation of renal parenchymal integrity, and assessment of the excretory system. To obtain all of these data, the recommended scanning protocol includes unenhanced, arterial, venous, and excretory phases (7), the combination of which has been associated with high radiation doses (8).
The aim of this work was to evaluate a low-radiation-dose, dual-phase protocol for preoperative evaluation of potential renal donors. Our suggested protocol included a low-radiation, unenhanced phase followed by a well-timed, combined vascular and excretory phase (one phase during which we can evaluate the arteries, veins and the excretory systems). The latter was achieved by splitting the contrast medium dose, while imaging acquisition started after well opacification of the distal ureter.
Material and Methods
Study population
Our study was approved by the hospital ethics committee. Written informed consent was obtained from all potential kidney donors.
During the period between April 2008 and October 2010, a total of 65 potential renal donors underwent a preoperative MDCT examination before transplantation. Sixty-two potential renal donors were operated upon; resulting operative notes were reviewed and were compared to CT findings.
CT imaging technique
Images of all the 65 potential renal donors were obtained using a 64-slice MDCT scanner (Lightspeed VCT; GE Healthcare, Milwaukee, WI, USA).
The potential renal donors were divided into two groups. The first group included 27 renal donors scanned by a routine, quadric-phase, CT protocol, whereas the second group, which included 38 potential donors, was scanned by a triple-phase protocol that was composed of a dual-phase protocol plus a venous phase.
Potential renal donors fasted for at least 6 h; each patient ingested 750 mL of water 15–20 min before scanning to increase distension of the collecting system. The CT procedure was explained to potential donors. All phases of both groups were obtained from the 11th thoracic vertebral body to the pubic symphysis.
In the first group after acquisition of initial scout anteroposterior (AP) and lateral views, a low-radiation-dose, unenhanced phase was performed. This was followed by an arterial phase, using the CTA protocol, and venous and delayed phases, which used the routine abdominal CT protocol (Table 1).
Technical parameters of the different phases
In the second group, after initial scout AP and lateral views were acquired, a low-radiation-dose, unenhanced phase was performed; this was then followed by time-optimized, combined vascular excretory and venous phases, which used the routine abdominal CT protocol (Table 1).
The contrast medium dose was determined based on the patient weight and the following criteria: 100 mL for weights up to 90 kg and 120 mL for weights greater than 90 kg. Using a power injector, a low-osmolarity contrast material, Iopromide (Ultravist 300 mg I/mL; Bayer Schering Pharma, Berlin, Germany), was intravenously administrated at a rate of 3.5 mL/s via an 18-gauge peripheral line in an antecubital vein. In the first group, the start time of arterial-phase scanning was determined with low-radiation-dose, automatic bolus tracking using the following parameters: tube current = 40 mAs; thickness = 5 mm; and rotation time = 0.7 s. Scanning was initiated 5 s after a threshold of 200 Hounsfield units (HU) was reached in the region of interest (i.e. within the abdominal aorta, just above the kidneys). Venous-phase scanning was started 10 s after the end of the arterial phase, whereas the excretory phase was started 5 min after the arterial phase.
In the second group, the calculated contrast medium dose was divided into two doses. First, a dose of 30 mL of contrast was given. Five minutes later, a low-radiation-dose, single cut was acquired with the following parameters: tube current = 40 mAs; thickness = 5 mm; and rotation time = 0.7 s. This image slice was acquired at the distal end of the ureter to achieve contrast medium detection in the lower ureter and urinary bladder. Acquisition was repeated every minute until the contrast medium was visualized; the second contrast medium dose was then administrated at this point. The combined vascular excretory phase was initiated five seconds after a threshold of 200 HU was reached in a region of interest within the abdominal aorta, just above the kidneys. The venous phase was started 10 s after the end of the combined vascular and excretory phase.
CT interpretation
Volumetric imaging data were reconstructed using a thin, 0.625-mm slice thickness and a standard body filter. The reconstructed slices were sent to an external workstation (Advantage Workstation 4.3.; GE Healthcare, Milwaukee, WI, USA). For each CT examination, the source axial images were reviewed, and the CT volume data-set was edited to create an optimal, multiplanar reformation (MPR) in the axial, sagittal, and coronal planes. Three-dimensional CTA included volume rendering (VR), maximum intensity projection (MIP), and CTU.
The arterial and venous systems were graded according to the degree of visualization using the following grading system: 3 = excellent; 2 = good; 1 = adequate; and 0 = poor. To evaluate the possibility of using only the combined vascular and excretory phase for the evaluation of renal arteries and veins, the visualization grades were compared in the arterial phases (the arterial phase of the first group and the combined vascular and excretory phase of the second group) and venous phases. A subjective evaluation of whether the arterial phase or venous phase could be used alone for the evaluation of both renal arteries and veins was performed.
We compared the visualization grade of the renal arteries in the arterial phase of the first group (using the CTA protocol) with the visualization grade of renal arteries in the combined vascular excretory phase of the second group (using the routine abdominal CT protocol).
Excretory systems were evaluated in the excretory phase of the first group and in the combined vascular and excretory phase of the second group based on opacification of the calyces, renal pelvis, upper ureter (lumbar part), and lower ureter (pelvic part).
In general, the less normal kidney was selected for donation. If both kidneys were free from abnormalities, the kidney with less complex renal vascular anatomy was chosen. If both were equal, the left kidney was typically selected because of its longer vein.
Radiation dose evaluation
Effective doses in milliSieverts (mSv) were calculated from the saved dose–length product (DLP) of the quadric-phase protocol (first group), the triple-phase protocol (second group, including the venous phase) and the suggested low-radiation-dose, dual-phase protocol (second group without the venous phase) using the following equation: effective dose (mSv) = 0.015 X DLP (9). The effective dose of the suggested protocol was compared to the routine quadric-phase protocol and triple-phase protocol.
Statistical analysis
Descriptive statistics were calculated for patient age, patient sex, and visualization grade. Statistical analysis was performed using the Minitab software package (Version 12.2 for Windows). The results were expressed as a mean ± standard deviation. A t-test, with a confidence interval of 95%, was used; P values less than 0.05 were accepted as statistically significant.
Results
The study included 65 potential renal donors. This patient group had a mean age of 32.7 years (range 18–45 years) and included 49 men and 16 women. All CT studies were technically satisfactory. Renal donors were randomly divided into two groups. The first group included 27 renal donors, while the second group included 38 renal donors.
Sixty-two potential renal donors underwent surgery. Of these patients, 56 donated their left kidney, while six donated their right kidney because of the complex vascular anatomy of the left kidney. Two potential renal donors were excluded from donation due to the presence of bilateral renal cysts, while an additional potential donor was excluded due to the presence of a splenic infarction and liver cyst (proved to be a sickle-cell trait).
According to the operative notes, all anatomical details of the renal arteries and veins in both groups were well-depicted by the CT studies. Additionally, all accessory arteries were accurately depicted by the CT studies in both groups.
Multiple renal arteries (Fig. 1) were found bilaterally. Although all of them originated from the abdominal aorta, there was an increased incidence on the right side (Table 2). The incidence of early branching was slightly higher on the right side (Fig. 1). Multiple renal veins (Fig. 2) were found in only two potential renal donors (Table 2). Late venous confluence occurred bilaterally, with an increased incidence on the left side (14%).
Multiple, bilateral renal arteries are shown. (a) A coronal, 3D, volume rendering from the combined vascular and excretory phase showing bilateral, double renal arteries, and an opacified renal pelvis and bilateral lumbar ureters is presented. (b) An axial, MIP image of the arterial phase of another renal donor showing a right-lower, polar, precaval, supernumerary artery is presented
(a) An oblique, coronal, MIP image in the arterial phase showing double-right renal arteries and veins; (b) Axial, MIP images in the arterial phase of another renal donor showing a retroaortic, left, renal vein
Renal arteries and veins findings by MDCT
MDCT = multirow-detector computed tomography
Arterial phases were started at 24–33 s (mean = 27.5 ± 2.5) after contrast medium administration, while venous phases (both groups) were started at 45–55 s (mean = 50.1 ± 3.5) following contrast injection. Arterial phases were satisfactory in the evaluation of renal arteries and veins for all (100%) renal donors, while the venous phases were satisfactory for close to half (45%) of renal donors. Renal arteries were better visualized, with a significant difference (P value <0.001), in arterial phases than in venous phases (Table 3), while renal veins were well visualized in both arterial and venous phases (Table 3), with no significant difference between the two phases (P value = 0.159).
Mean grade of visualization of the arteries and veins in the different phases
Significant P value < 0.05
Arterial phases = arterial phase of the first group + combined vascular excretory phase of the second group
The excretory systems (Fig. 3) were better opacified in the time-optimized, combined vascular and excretory phase of the second group than in the excretory phase of the first group (Table 4). No gross anatomical abnormalities were noted in excretory systems.
A coronal, 3D, volume rendering from the combined vascular and excretory phase showing a single renal artery, vein and ureter bilaterally: (a) An anteroposterior view; (b) A posteroanterior view; and (c) A bilateral, single ureter after removal of arteries and veins
Excretory systems opacification by MDCT
MDCT = multirow-detector computed tomography
A routine CT abdominal protocol, which reduced the radiation dose, was used to replace the CTA protocol in the combined vascular and excretory phase. No significant statistical difference was noted (P value = 0.453) between the mean degree of visualization of renal arteries in the arterial phase of the first group using the CTA technique (2.96 ± 0.19) and the combined vascular and excretory phase of the second group using the routine abdominal CT technique (2.91 ± 0.29).
The mean effective radiation dose of the suggested low-radiation, dual-phase protocol was 11.9 ± 3 mSv, which was only 34% of the mean effective dose of the quadric-phase protocol (34.7 ± 3.8 mSv) and 58% of the mean effective dose of the triple-phase protocol (20.4 ± 3.3 mSv). A significant difference was noted between the effective radiation dose of the suggested low-radiation-dose, dual-phase protocol and both quadric-phase and triple-phase protocols (P value <0.001).
Discussion
In this work, we have evaluated a low-radiation-dose, dual-phase protocol that consisted of unenhanced and well-timed, combined vascular and excretory phases. The inclusion of a well-timed, combined vascular and excretory phase allowed for the visualization of the renal arteries and veins and the excretory system in a single phase with a significant reduction in radiation dose.
Adequate vascular enhancement is crucial in obtaining a high-quality angiogram, and it is greatly affected by scanning delay. In the presented work, the start time of the late arterial phase (i.e. the arterial phase of the first group and the well-timed, combined vascular and excretory phase of the second group) was determined using a bolus tracking technique.
The reported accuracy of MDCT in the evaluation of renal arteries and early branching ranges from 93–100% (1, 3, 4, 10); in the evaluation of the venous system, it ranges from 98–100% (1, 8). In the presented work, MDCT was in total (100%) agreement with surgical notes.
Renal arteries were better visualized in the late-arterial phases than in the venous phases, while renal veins were well-visualized in both the late-arterial and venous phases, which is in agreement with Zamboni et al. (8) and Namasivayam et al. (11).
The well-timed, combined vascular and excretory phase of the second group was satisfactory as a single phase for the evaluation of 100% of renal donors. This result stands in contrast to the venous phase, which was satisfactory as a single phase for evaluation of only 45% of the renal donors. The former result is in agreement with Zamboni et al. (8), who reported satisfactory evaluations in 100% of renal donors when the combined vascular and excretory phase was employed.
Previous reports using CT scanograms (with limited resolution) and screen-film radiography have complicated examinations because the patient had to be transferred to an X-ray machine (1) for the evaluation of the excretory system. In the presented work, we used CTU images that were obtained from the well-timed, combined vascular and excretory phase in the second group and from the excretory phase in the first group.
The evaluation of the renal collecting system and ureters requires optimal opacification and distension. The following techniques striving to achieve this have been reported in the literature: abdominal compression, saline infusion, multiple acquisitions, prone position, and furosemide administration (12).
The collecting system and ureter opacification were optimized in the first group by ingestion of 750 mL of water 15–20 min prior to the study. In the second group, after water ingestion and following acquisition of the low-radiation-dose, unenhanced-phase, the first dose of contrast was administrated. This was followed by repeated, very low-radiation-dose, single cuts at the level of the distal ureter, started with a 6-min delay and was repeated every minute until opacification was noted. Using this technique, we achieved 97% opacification of the renal collecting system, 93% opacification of the lumbar ureters and 96% opacification of the pelvic ureters. This technique lead to improved optimization over the results of Kekelidze et al. (13), who reported 91% opacification of the collecting system and 82% opacification of the upper ureters using oral hydration (using 30 mL of contrast material and a 510-s delay). In addition, our results offered improved optimization over the results of Kawamoto et al. (12), who achieved 94.5% opacification of the renal pelvis, 78% opacification of the upper ureters and 58.5% opacification of the lower ureters using oral hydration (6 min post-injection and using 120 mL of contrast and a 240-s delay).
There are a few risks associated with MDCT, the most important of which is its associated high radiation dose, which is especially critical in the case of healthy, young adults. The estimated risk of cancer development as a consequence of diagnostic X-ray has resulted in an increased awareness of the importance of radiation dose. The general rule is to plan a CT scan according to ALARA rules (i.e. as low as reasonably achievable) (8).
Sahani et al. (6) reported their experience using different KV values with 16-slice MDCT. They found that in renal donors, the image quality at 120 KV was similar to that at 140 KV, but with the former KV value resulting in a radiation dose reduction. In the presented work, all potential renal donors were examined using 120 KV, and the image quality was satisfactory in all studies.
In the arterial phase of the first group, we used the CTA technique, while in the well-timed, combined vascular excretory phase of the second group, we used the routine CT abdomen technique. No significant difference was noted between the visualization degree of the renal arteries in the first group and the visualization degree of the renal arteries in the combined vascular excretory phase of the second group. The routine abdominal CT technique was associated with a reduced radiation dose.
In the well-timed, combined vascular excretory phase in the second group, we omitted the venous phase by starting acquisition with late-arterial timing. Additionally, we omitted the excretory phase by splitting the contrast medium into two doses and by starting after adequate opacification of the distal ureter and urinary bladder.
By omitting the venous and excretory phases and using the routine abdominal CT technique, the radiation dose of the suggested protocol was significantly lower than that experienced with routine quadric-phase and triple-phase protocols. Similar findings were suggested by Zamboni et al. (8), but the following differences are noted: (a) they included only the abdomen, while we covered the whole abdomen and pelvis; (b) they did not report on excretory system opacification; (c) they did not include radiation exposure data in their work (they instead used the estimated radiation dose from patients who underwent MDCT examinations for other indications with the same parameters and scanner); and (d) they did not indicate the technical parameters or information regarding whether it was a CTA or routine abdominal CT technique.
The presented work has reduced radiation dose through the use of only 120 KV, low-radiation-dose unenhanced phase and the use of a well-timed, combined vascular and excretory phase (using the routine abdominal CT technique), which replaced the higher-dose arterial venous and excretory phases.
A limitation of the presented work is the limited number of potential renal donors presented in the study. In addition, we used water ingestion to cause diuresis and enhance distension of the excretory system, which is now, a contradictory issue.
In conclusion, the proposed low-radiation-dose, dual-phase, MDCT protocol (composed of a low-radiation-dose, unenhanced phase, and a combined vascular and excretory phase, which was obtained by splitting the contrast medium dose and using a routine abdominal CT technique) significantly reduced the radiation dose. At the same time, our proposed protocol provided adequate visualization of renal arteries and veins and sufficient opacification of the urinary tract by triggering the best timing for scanning.