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Original Research |
1 All authors: Department of Radiology, Massachusetts General Hospital, 55 Fruit St., Boston, MA 02114-2696.
Received April 3, 2005;
accepted after revision December 19, 2005.
Address correspondence to D. V. Sahani
(dsahani{at}partners.org).
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MATERIALS AND METHODS. Sixty-two renal donors (32 men, 30 women) who underwent 16-MDCT were divided into three groups: 18 subjects were studied at 140 kVp (group A); 20, at 120 kVp (group B); and 24, at 100 kVp (group C). Other constant scanning parameters were as follows: detector collimation, 0.625 mm; table feed, 9.375 mm/rotation; gantry rotation time, 500 milliseconds; and automatic current tube modulation (ATCM) using a noise index of 15. A total of 135-140 mL of iodinated contrast material (300 mg I/mL) was administered at 5 mL/s via an 18-gauge cannula, and arterial phase scanning was initiated using a bolus-tracking technique. Two observers evaluated image quality of the axial and 3D images and the visibility of branch order in the superior mesenteric artery (SMA) and renal arteries. Attenuation (in Hounsfield units [H]) in the aorta, SMA, and main renal artery was also measured by placing a region of interest. Radiation dose measurements were based on the scanner-generated CT dose index volume (CTDIvol). Each parameter tested was compared among the three groups using a nonparametric analysis of variance test, and a p value of 0.05 was considered significant.
RESULTS. Differences in the quality of the axial images existed between groups A and C (p < 0.001) and between groups B and C (p < 0.01); the image quality of the 3D images and the visibility of branch order in the SMA and renal arteries were comparable for all groups. The difference in mean attenuation of the aorta, SMA, and renal arteries was significant between groups A and C (p < 0.001) and between groups B and C (p < 0.01). All groups had 100% diagnostic accuracy in identifying the number of renal arteries on the side of nephrectomy. The mean radiation dose in CTDIvol was 25 ± 3 mGy at 140 kVp, 17 ± 4 mGy at 120 kVp, and 12 ± 3 mGy at 100 kVp (p < 0.001).
CONCLUSION. Our initial observations suggest that the image quality of 16-MDCT angiography performed at 120 kVp is similar to that of CT angiography (CTA) performed at 140 kVp in adult kidney donors but with a significant radiation dose reduction. CTA at 100 kVp results in higher image noise but provides diagnostically acceptable images with significant radiation dose reduction compared with CTA at 120 or 140 kVp.
Keywords: kidney transplantation • MDCT angiography • oncologic imaging • radiation dose • renal artery
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Recent reports have suggested that low-peak kilovoltage (kVp, or tube potential) techniques also can be exploited for increased iodine attenuation in vessels and can consequently improve vessel conspicuity [5]. At a low kVp setting, the mean photon energy of the polychromatic X-ray beam is closer to the K-edge of the iodine, resulting in an increased photoelectric effect and decreased Compton scattering that, in effect, translates into a higher mean attenuation value (in Hounsfield units [H]) of iodine [6]. Lowering the kVp can also substantially reduce patient radiation dose as long as the product of tube current and time is maintained at an acceptable level. In addition, the benefit of achieving higher attenuation with a lower kVp than routinely needed for CTA can also permit reducing the volume of iodinated contrast medium that is injected.
The purpose of our study was to compare the performance of 16-MDCT angiography at various peak kilovoltage settings and the impact of those settings on image quality and radiation dose to the patient during CTA evaluation of adult kidney donors.
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The variations in kVp were the result of an adjustment in CT protocols by our department for the purposes of decreasing radiation dose and optimizing image quality. As a part of our department policy for CT examinations, the technologist recorded each patient's body weight, radiation dose delivered to the patient using the CT dose index volume (CTDIvol), the volume of IV contrast medium, and the rate of contrast injection. This study also complied with the Health Insurance Portability and Accountability Act (HIPAA), and approval was obtained from the institution's review board for human studies. The requirement that informed consent be obtained from each patient was waived because the study was retrospective.
CTA
Peak kilovoltage—Based on the kVp selected— 140, 120,
or 100—for CTA, the study population was divided into three
groups—that is, a 140-kVp group, 120-kVp group, and 100-kVp group. The
140-kVp group included 18 donors (nine men, nine women; age range, 28-64
years; mean age, 46 years; median age, 45 years; weight range, 130-215 lb
[59-97 kg]; mean weight, 167 lb [75 kg]; median weight, 160 lb [72 kg]) who
underwent CTA performed at 140 kVp. The 120-kVp group included 20 donors (11
men, nine women; age range, 29-66 years; mean age, 45 years; median age, 48
years; weight range, 130-200 lb [59-90 kg]; mean weight, 170 lb [77 kg];
median weight, 160 lb [72 kg]) who underwent CTA performed at 120 kVp. The
100-kVp group included 24 donors (12 men, 12 women; age range, 28-75 years;
mean age, 46 years; median age, 48 years; weight range, 120-190 lb [54-86 kg];
mean weight, 165 lb [74 kg]; median weight, 156 lb [70 kg]) who underwent CTA
performed at 100 kVp. There was no statistically significant difference in the
age, sex, or weight among the three groups based on Kruskal-Wallis test for
age and weight and Fisher's exact test for sex.
Other CT parameters—Except kVp, the scanning parameters were kept constant in all the patients and were as follows: The milliamperage (mA) selection was based on automatic current tube modulation (ACTM) at a noise index of 15; the gantry rotation time was 500 milliseconds and the detector collimation and table feed were 0.625 mm and 9.375 mm/rotation, respectively. In all of the patients, the maximum tube current-time product the system could deliver was set to 210 mAs.
Contrast administration—Unenhanced CT of the abdomen was first performed using a 10-mm slice thickness. A bolus-tracking technique with automated scan-triggering software (SmartPrep, GE Healthcare) was used for initiating arterial phase scanning in all of the patients. A region of interest (ROI) was placed in the abdominal aorta at the level of the celiac artery, and a threshold of 125 H for peak enhancement in the abdominal aorta was arbitrarily selected for triggering arterial phase imaging. The technical parameters of SmartPrep were identical in all groups. A kVp of 140 was used in all patients to obtain the initial monitoring scans of SmartPrep before triggering arterial phase imaging. A total of 135-140 mL of iodinated contrast (iopamidol [Isovue 300 mg I/mL, Bristol-Myers Squibb]) was then administered at 5 mL/s via an 18-gauge cannula placed in an antecubital vein of the arm. During the arterial phase, the abdomen was scanned from the level of diaphragm to the iliac crest while the patient held his or her breath in end expiration.
Image reconstruction—Images were reconstructed at a 1.25-mm thickness with no overlap for viewing and at a 0.625-mm thickness with 50% overlap for image postprocessing. A commercially available workstation (Advantage Windows, version 4.0, GE Healthcare) was used for image postprocessing. Both the 2D and 3D reconstructions were created by a specially trained image-processing technologist using a standard protocol for all of the examinations. The reformations included maximum-intensity-projection (MIP), subvolume MIP, and volume-rendered images for depicting the renal arteries.
Image Analysis
Two fellowship-trained abdominal and vascular radiologists, each having
more than 8 years of experience in radiology, evaluated the CT data in
consensus. The observers were blinded to patient demographics and the
technical parameters of the examination. The axial images, 2D data set, and 3D
data set were evaluated independently using qualitative and quantitative
methods.
Qualitative evaluation—For the qualitative evaluation, the following image parameters were assessed: The quality of the axial images was recorded using a 5-point scale (1 = unacceptable, 2 = suboptimal, 3 = diagnostically acceptable, 4 = good, and 5 = excellent quality), and the visibility of the branch order (first order, second order, or third order) of the superior mesenteric artery (SMA) and renal arteries was recorded. The quality of the 3D reformations was also assessed on a 5-point scale (1 = poor quality, 3 = diagnostically sufficient quality, 5 = excellent quality). In addition, the axial images were evaluated for the number of renal arteries supplying each kidney and the branching pattern of the renal arteries. If an accessory renal artery was detected, its diameter and location (superior pole, hilar, or inferior pole) were recorded.
Quantitative evaluation—For the quantitative evaluation, attenuation (in Hounsfield units) was measured in the aorta, SMA, and main renal artery. An ROI was placed to match the size of the arterial lumen on the aorta at the level of SMA; on the SMA beyond its origin; and on the right and left main renal arteries just beyond their origin. The SD of the ROI placed on the gallbladder was used to calculate image noise. The size of the ROI ranged from 13 to 25 mm2. The contrast-to-noise ratio (CNR) was calculated using the following formula: (attenuation of main renal artery) - (attenuation of gallbladder) - (image noise in gallbladder).
Radiation Dose
Radiation dose measurements were based on the scanner-generated
CTDIvol values during arterial phase scanning. In addition, the
delivered tube current-time product (mAs) at the level of the SMA was recorded
for each patient. The actual dose (dose-length product in mGy · cm) was
not calculated because this value is affected by the subject's anatomy and it
is not useful to compare radiation doses among different protocols.
Statistical Analysis
Each parameter tested was compared among the three groups using a standard
parametric one-way analysis of variance F test, and a p value of 
0.05 was considered significant. The Tukey-Kramer multiple comparisons test
was then performed to compare each group with another group, and a p
value of 0.05 was considered significant. The entire statistical analysis
was performed using Graph-Pad InStat (version 3.00, GraphPad Software) for
Windows 95 (Microsoft).
In the patients who later underwent nephrectomy, the accuracy of CTA was calculated by comparing the findings on CTA with the observations made at surgery. The number of renal arteries in each donor was recorded on CTA, and this number was compared with the surgical findings. A simple 2 x 2 table was used to assess the accuracy of CTA in detecting the number of renal arteries.
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Visibility of Branch Order of SMA and Main Renal Artery
The mean scores for the visibility of branch order in the SMA and main
renal artery were 5.2 and 3.4 in the 140-kVp group, 4.9 and 3.2 in the 120-kVp
group, and 4.9 and 3.3 in the 100-kVp group. No significant difference among
the three groups for the visibility of branch order of the SMA (p =
0.6) or the main renal artery (p = 0.9) was observed.
Quality of the 3D Reformations
The mean scores for the quality of the 3D reformations among the 140-,
120-, and 100-kVp groups were 4.80, 4.75, and 4.66, respectively
(Table 1, Figs.
1A,
1B,
2A,
2B,
3A, and
3B). There was no significant
difference among the three groups for the quality of the 3D reformations
(p > 0.5).
Noise of the Axial Images
Objective noise measured as the SD of gallbladder density in Hounsfield
units was 21.8 ± 9.7 H in the 140-kVp group, 21.9 ± 5.0 H in the
120-kVp group, and 47.9 ± 20.4 H in the 100-kVp group. There was a
significant difference in objective noise among the three groups (p
< 0.05). This was obvious between the 140- and 100-kVp groups and also
between the 120- and 100-kVp groups. However, there was no significant
difference in objective noise between the 140- and 120-kVp groups.
CNR
The mean CNR was 15.1 in the 140-kVp group, 17.1 in the 120-kVp group, and
9.7 in the 100-kVp group. There was a significant difference in CNR among the
three groups (p < 0.05). The observed difference was most obvious
between the 140- and 100-kVp groups and between the 120- and 100-kVp groups.
However, there was no significant difference in CNR between the 140- and
120-kVp groups.
Attenuation of Aorta, SMA, and Main Renal Artery
The mean attenuation values of the aorta were 283 ± 45 H, 340
± 38 H, and 399 ± 72 H among the 140-, 120-, and 100-kVp groups,
respectively (Table 1). The
mean attenuation values of the SMA were 287 ± 47 H, 340 ± 45 H,
and 402 ± 69 H among the 140-, 120-, and 100-kVp groups, respectively.
The mean attenuation values of the main renal artery were 292 ± 62 H,
338 ± 26 H, and 384 ± 67 H among the 140-, 120-, and 100-kVp
groups, respectively. There was a significant difference in the mean
attenuation values of the aorta, SMA, and main renal artery among the three
groups (p < 0.01). There was a significant difference in the mean
attenuation of the aorta, SMA, and main renal artery between the 140- and
100-kVp groups and between the 120- and 100-kVp groups. However, there was no
significant difference in the mean attenuation of the aorta, SMA, and main
renal artery between the 140- and 120-kVp groups.
Diagnostic Accuracy
In the 140-kVp group, 10 accessory renal arteries were identified in seven
of the 18 patients. Three patients had one accessory renal artery on the
right, two patients had one accessory renal artery on the left, and one
patient had one accessory renal artery on each side. The remaining patient had
two accessory renal arteries on the right and one on the left. All 18 patients
had undergone nephrectomy. CTA had an accuracy of 100% in diagnosing the
number of renal arteries on the side of the nephrectomy.
In the 120-kVp group, six accessory renal arteries were identified in five of the 20 patients. Two patients had accessory renal arteries on the right side (one in each), two had one accessory renal artery on the left side (one in each), and one patient had one accessory renal artery on each side. Nephrectomy was performed in 18 of 20 patients at the time of this submission. In all these patients, CTA had an accuracy of 100% for diagnosing the number of renal arteries on the side of nephrectomy.
A total of 16 accessory arteries were identified in 10 of the 24 patients in the 100-kVp group: Five patients had accessory renal arteries on the left side (one accessory renal artery in each), two patients had accessory renal arteries on the right side (one accessory renal artery in one patient, two in the other), and three patients had accessory renal arteries on both sides (one accessory renal artery on either side in one patient; two right accessory renal arteries and one on the left in one patient; two accessory left renal arteries and one on the right in the other). Nephrectomy was performed in 20 patients at the time this article was submitted for publication. In all of these patients, CTA had an accuracy of 100% for diagnosing the number of renal arteries on the side of nephrectomy.
Radiation Dose
The mean radiation dose in CTDIvol was 25 ± 3 mGy in the
140-kVp group, 17 ± 4 mGy in the 120-kVp group, and 12 ± 3 mGy
in the 100-kVp group (Table 1).
Based on one-way analysis of variance, a statistically significant difference
was observed in radiation dose among the three groups (p < 0.01).
Tukey's multiple comparisons test showed a significant difference in the
radiation dose between the 140- and 120-kVp groups, between the 140- and
120-kVp groups, and between the 120- and 100-kVp groups. The radiation dose
decreased to almost 50% with selection of a kVp of 100 in comparison with a
kVp of 140.
Among the patients in the 120- and 100-kVp groups, the maximum tube current-time product (in milliampere-seconds) achieved at the level of the SMA was 210 mAs (mean, 200 mAs in the 120-kVp group and 208 mAs in the 100-kVp group); and among the patients in the 140-kVp group, the maximum tube current-time product achieved was 190 mAs (mean, 177 mAs). There was a significant difference in mAs delivered among the three groups (p < 0.01). There was no statistically significant difference in mAs delivered between the 120- and 100-kVp groups. However, there was a significant difference in milliampere-seconds delivered between the 120- and 140-kVp groups and also between the 100- and 140-kVp groups.
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The impact of kVp on image quality is complex because the kVp affects both the quantum noise due to decreased photon fluence and the tissue contrast due to a change in the mean energy of the X-ray beam [7]. The resultant change in the radiation dose is approximately proportional to the square of the tube voltage change (i.e., square of the ratio of final and initial peak voltages) [9], and it is significantly more than that achieved by changing tube current alone. However, the resultant change in image noise is approximately inversely proportional to the tube voltage change [9].
When high-contrast objects, such as air and soft tissue in the chest, are being studied, higher background noise can be tolerated without affecting the diagnostic quality of the images. In a study comparing low-kVp and standard-kVp CT examinations of the chest, the background noise increased when a low-kVp technique was used without affecting the qualitative analysis [13]. A substantial difference in the contrast between enhancing arteries and the background tissues exists during CTA, and these differences can be further accentuated by decreasing the kVp. However, when the kVp is decreased, mAs should be appropriately increased to maintain a constant image noise. This can be achieved by selecting a higher mAs or, when available, by selecting an ACTM option on the scanner.
When the ACTM option is used, the CT system alters the amount of mAs delivered through the X-ray tube to achieve a constant image noise as set by the operator [14]. However, the consequent increase in mAs to maintain a constant noise will reduce the benefit of reduced radiation exposure achieved by decreasing the kVp and may actually increase the absorbed dose because the mean energy of the X-ray beam is now reduced. To avoid this issue, we chose an upper limit on the mAs delivered by the tube even when the ACTM option was adopted. This approach offers two theoretic advantages; First, for the given noise index selection, in areas where the required mAs is still below the set upper limit, the radiation dose will decrease. Second, in areas where the maximum mAs required is beyond the selected upper limit, the tube delivers only the maximum mAs chosen (in our case, 210 mAs). However, the background noise in the image at these regions will increase.
We observed higher image noise and relatively lower image quality at 100 kVp compared with 120 and 140 kVp in our study. However, because of the substantial differences in contrast between the vessels and the background, neither the quality of 2D and 3D reformations nor the visibility of small branches of the SMA and main renal artery was affected by lower kVp. Although the image quality obtained using the 100-kVp technique was considered relatively inferior to that obtained using either 120 or 140 kVp, it was still considered diagnostically acceptable. In addition, the ACTM option was useful around the domes of the diaphragms to reduce radiation dose. For these reasons, the total radiation dose (CTDIvol) delivered to the patient was significantly lower at 100 and 120 kVp in our study despite a minimal increase in mAs delivered at 100 and 120 kVp as compared with 140 kVp.
Data from a recent study performed in the brain for the evaluation of MDCT angiography at various kVp settings have also shown that image quality with the use of 140 kVp is superior to that obtained using 120 and 100 kVp and that the mean attenuation of vessels is substantially higher with the use of the 100-kVp technique [5]. The authors found that images obtained using the 140-kVp technique were most useful in differentiating enhancing arteries and bones at the skull base, thus suggesting better utility of higher kVp for visualization of low-contrast structures. Images obtained using the 100-kVp technique were found to be of sufficient quality to study the intracerebral arteries. Because of the inherent contrast from the mesenteric and retroperitoneal fat surrounding the vessels in the abdomen, differentiating bone from enhancing arteries is usually not an issue.
Likewise, the higher tissue contrast achieved with the lower kVp technique was also considered beneficial in CT perfusion studies of the brain performed using 80 and 120 kVp [15]. The authors observed a 2.8 times decrease in radiation dose and a significant increase in contrast enhancement for the 80-kVp technique compared with the 120-kVp technique, and they found that image noise using the 80-kVp technique was comparable to that of the 120-kVp technique [15]. This principle has also been extended to digital subtraction angiography. Gkanatsios et al. [16] observed that lowering the kVp resulted in an increase in the contrast between arteries and surrounding structures.
There were several limitations in our study. First, the impact of patient body weight and dimensions on the image quality was not studied. These patient-related factors are known to independently affect image quality [17]. In addition, 94% of the patients in our study weighed less than 200 lb (90 kg). It is conceivable that in patients weighing more than 200 lb (90 kg), selecting a kVp of 100 or lower may degrade the image quality due to excessive background noise. Likewise, images of patients with a metallic prosthesis in the spine or pelvis may have similar issues in terms of higher image noise. We used an upper limit of 210 mAs while the ACTM option was used, and this resulted in higher noise at 100 kVp. If the upper limit of mAs had been increased to a higher level (e.g., 250 or 300 mAs), the observed noise in the images obtained at 100 kVp would have been comparable to that observed in images obtained at 120 or 140 kVp. However, this would result in a higher radiation dose to patients than was achieved in our study.
Additional studies are required to assess the optimal mAs when a lower kVp is used to optimize image quality and decrease radiation dose. We have not studied the impact of a low-kVp technique in a clinical setting wherein the evaluation of solid organs for pathologic processes is also needed along with vascular mapping. Likewise, the diagnostic accuracy of CTA for renal vascular mapping could be confirmed only in kidneys removed for transplantation. Therefore, the potential of missing a small accessory renal artery because of increased image noise with a low-kVp technique cannot be ruled out.
We assessed the CTA portion of the kidney donor workup and not the renal parenchymal, ureteral, and renal calculus portions of the renal donor CT assessment. This was done to focus the attention of the study on vascular imaging at various kVp settings. We have not attempted to evaluate the ability of CTA at varying kVp settings to assess for fibromuscular dysplasia, which is an important abnormality to detect in renal donors. Also, the retrospective nature of our study introduces an inadvertent bias; a prospective randomized study would be more useful in confirming the results. Finally, we have not evaluated the performance of techniques using different kVp settings in the same patients due to obvious reasons of additional radiation dose to a healthy adult.
In conclusion, our initial observations suggest that the image quality of 16-MDCT renal angiography in adult kidney donors performed at 120 kVp is similar to that of CTA performed at 140 kVp but with a significant radiation dose reduction. CTA at 100 kVp results in higher image noise, but the images are diagnostically acceptable and significant radiation dose reduction is achieved compared with CTA at 120 or 140 kVp.
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