MDCT angiography assessment of renal artery in-stent restenosis: Can we reduce the radiation exposure burden? A feasibility study

European Journal of Radiology 79 (2011) 224–231

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MDCT angiography assessment of renal artery in-stent restenosis: Can we reduce the radiation exposure burden? A feasibility study Eirini Manousaki a,1 , Kostas Perisinakis b,2 , Apostolos Karantanas a,3 , Dimitrios Tsetis a,∗ a b

Department of Radiology, Faculty of Medicine, University of Crete, Greece Department of Medical Physics, Faculty of Medicine, University of Crete, Greece

a r t i c l e

i n f o

Article history: Received 17 November 2009 Received in revised form 6 March 2010 Accepted 17 March 2010 Keywords: MDCT/diagnosis Low dose CT Renal artery angiography Renal artery in-stent restenosis

a b s t r a c t Aim: We explored the feasibility of renal artery multidetector computed tomography (MDCT) and detection of in-stent restenosis at low exposure settings. Patients/methods: Sixteen patients with 19 renal artery stents underwent CT angiography. A biphasic protocol was performed including arteriographic acquisition at standard 120 kVp and a late-arterial scan at 100 kVp (n = 9) or 80 kVp (n = 7). Images were reconstructed under various algorithms. Signal-to-noise and contrast-to-noise ratios (SNR, CNR) were determined within stent, aorta and renal arteries. Image quality and the presence of restenosis were assessed. Volume CT dose-index was recorded and dose reduction (DR%) between phases was calculated. Results: Ten patients presented with Hounsfield values >250 HU in all segments, phases and reconstructions and were further evaluated. The 120 kVp protocol performed better in all vessels and reconstruction algorithms. SNR at 120 kVp (B31f) did not differ significantly compared to 100 kVp (B31f). CNR within stent was borderline compromised at 100 kVp (p = 0.042). All but two image sets (at 80 kVp) were considered diagnostic. Minor loss of subjective image quality was noticed at 100 kVp. No difference in assessment of restenosis was observed between 120 kVp and the diagnostic low-exposure scans. Mean DR% was estimated 45% at 100 kVp and 77% at 80 kVp. Conclusions: Renal MDCT angiography and stent-restenosis assessment are feasible at 100 kVp with minor loss of image quality and almost half radiation exposure. © 2010 Elsevier Ireland Ltd. All rights reserved.

1. Introduction Renovascular hypertension (RH) associated with atherosclerosis is a common disorder in the adult population and if left untreated can lead to renal insufficiency and permanent kidney loss. The incidence is estimated 6.8% among patients >65 yrs [1]. Individuals in the 65–69 yrs age group tend to be affected more often [1]. Interestingly, younger adults or even children with theoretically longer life expectancy may present with clinically significant renal artery stenosis, either due to atherosclerotic disease, fibromuscular dysplasia (FMD) or other vasculopathy [2,3]. Percutaneous transluminal revascularization of the renal artery has been recognized as an efficient therapeutic approach to treat

∗ Corresponding author at: University Hospital of Heraklion, Department of Radiology, Voutes, 71110 Heraklion Crete, Greece. Tel.: +30 2810392542033; fax: +30 2810542095. E-mail address: [email protected] (D. Tsetis). 1 Tel.: +30 2810392542033; fax: +30 2810542095/28001. 2 Tel.: +30 2810392542564; fax: +30 2810542095. 3 Tel.: +30 2810392542072; fax: +30 2810542095. 0720-048X/$ – see front matter © 2010 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2010.03.017

RH and preserve renal function. Although there appears to be just a slight clinical benefit compared to medical treatment alone [4], it has become common practice in many centers. Compared to conventional percutaneous transluminal angioplasty, renal artery stenting has not only proven technically superior, but also reported longer patency rates [5]. Even in FMD, stent-assisted angioplasty proves successful [6]. Unfortunately, the unavoidable response of the vascular wall to the mechanical trauma caused by the implanted stent carries the potential of in-stent restenosis (ISR) that can become haemodynamically significant in approximately 17% of cases, limiting the kidney arterial inflow and thus compromising the renal function [5]. Currently there is no consensus on the best approach to detect restenosis after stent-assisted revascularization. Digital subtraction angiography (DSA), with all the associated risks of an invasive procedure, remains the reference technique to identify and estimate ISR. The clinical follow-up may not be able to detect asymptomatic restenosis before it becomes symptomatic or it progresses to kidney loss; hypertension may relapse in the absence of restenosis, or critical ISR may exist silently [7–10]. Color Doppler ultrasonography, apart from the limitations associated with operator’s skills and patient’s body habitus that may hinder direct

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visualization and the quantification of flow parameters within the stent, often fails to compensate for downstream parameters related to vessel compliance that may produce either false positive or negative results [11–14]. The major drawback of MR angiography is associated with the magnetic susceptibility artifacts induced by the metallic stents [15]. Multidetector computed tomography (MDCT) appears promising in the noninvasive detection of ISR with reported sensitivity rates and negative predictive value (NPV) of 78% and 100% respectively in the coronary arteries, assessed with 16-row multidetector systems [16–19]. In larger-diameter stented arteries sensitivity seems to further improve [20] and thus MDCT could be an alternative imaging method for the follow-up of patients with peripheral stents [21]. The MDCT induced ionizing radiation raises great concern about its use as a screening and follow-up tool [22,23]. Current literature has showed that low radiation exposure settings on MDCT angiography could serve as a safe and clinically acceptable imaging protocol [24–29]. In our study, we explore the feasibility of performing MDCT renal angiography and detecting renal ISR at low radiation exposure settings. 2. Materials and methods This is a prospective feasibility study that took place between October 2007 and September 2008. The study protocol was designed according to recent experimental data from our institute and received the approval of the Ethics Committee of our Hospital and the Faculty of Medicine of the local University. Patients in regular follow-up protocols enrolled under informed consent. 2.1. Patients Sixteen patients (11 males, 5 females) with 19 renal stents enrolled in this study. All patients were >60 years old (mean age 64.29 yrs) and had been under casual combined clinical and ultrasonographic follow-up protocols for a mean of 18.8 months. All subjects had serum Cr levels <1.5 mg/dl in resent blood samples and no contraindication to iodine contrast material administration. Stent diameter and material (6 mm, stainless steel; JoStent Renal, Abbott) were uniform for the whole group. 2.2. CT protocols All scans were performed at a 16-row MDCT unit (Somatom Sensation 16, Siemens, Erlangen, Germany) with the patients lying supine. Acquisition followed craniocaudal direction, during breathhold. A series of unenhanced images of the upper abdomen was taken to map the renal region, specify the level of the origin of the renal arteries and identify focal calcification. One hundred milliliters of non-ionic contrast media at a concentration of 370 mg/ml (Ultravist, Shering) were administered through an automatic injector at a flow rate of 3.3–3.5 ml/s via an 18-G catheter placed at a superficial vein in the antecubital fossa. Arteriographic scan followed a standard protocol (P120 : 120 kVp, 160 effective mAs) assisted by Care Bolus technique that triggered arteriographic acquisition of the renal artery region. The latter was applied at the attenuation threshold of 120 HU with a circular 1 cm2 region of interest (ROI) placed within the lumen of the abdominal aorta at the level of renal arteries origin. A second late arterial scan of the same area was performed during another breathhold after the minimally allowed devise-delay (6 s) following the arteriographic scan. Tube voltage was set to 80 kVp for the first 7 patients examined (protocol P80 ) and 100 kVp for the following 9 (protocol P100 ). During acquisition of P80 and

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P100 the automatic exposure control system CARE Dose 4d was utilized to ensure diagnostic image quality. For all contrast-enhanced scans detector collimation (16 × 0.75 mm), gantry rotation time (0.5 s), pitch (0.9) and matrix (512 × 512) remained constant. 2.3. Image reconstruction For both scanning protocols, both sets of contrast-enhanced images were further reconstructed in a restricted Field of View (10–12 cm) limited to the aorta, renal arteries and branches and the renal hilum with a section thickness of 1 mm, an increment of 0.7 mm, using a smooth and a medium convolution kernel (B31f and B40f). For the patients of the P100 protocol, sharp B46f was additionally used. Final quantitative image analysis was based upon axial scans. Multiplanar and curved planar reformations were additionally used for qualitative assessment. 2.4. Quantitative assessment Signal intensity (SI) measurements by means of Hounsfield Units recording, were performed for each patient in all contrastenhanced image sets. Measurements were performed at a workstation (Leonardo, Siemens) in approximately identical positions within the center of the lumen of the (i) abdominal aorta at the level of the origin of the renal arteries, (ii) renal artery of interest 1 cm distal to the stent, (iii) stented segment and (iv) contralateral renal artery trunk approximately 2 cm distal to the ostium. For the two subjects with metallic stents in both renal arteries, measurements were performed distal to both stents and within the stents. Background noise (BN) was measured as the standard deviation of the Hounsfield value of the surrounding air anterior to the patient. In addition, the Hounsfield value in the middle part of the psoas muscle (SImuscle) was recorded. The ROIs used within the aorta and the other vascular segments were circular with a minimum area of 1 cm2 and 0.25 cm2 respectively. Signal intensity within the stented segment was determined from polygonal ROI restricted to the contrast-enhanced lumen to avoid apparent hyperplasia; instent sampled area was minimum 0.2 cm2 . Air attenuation and muscle signal intensity were determined within ROIs of 1.5 cm2 . Signal intensity within the left renal vein was measured in P80 and P100 image sets to document the post-arterial scan. Based on these measurements, aortic enhancement and enhancement of the renal arteries and the stented lumen was documented for all scans. The limit of 250 HU of lumen enhancement was set to identify the arteriographic quality of scans; patients with enhancement less than 250 HU in at least one of the sampled regions in any data set were excluded from further analysis. For subjects fulfilling the above criterion, signal-to-noise ratio (SNR) was calculated separately for each of the four arterial segments to quantify aortic, in-stent and renal angiography, according to the equation: SNR arterial segment =

SI within lumen . BN

Contrast-to-noise ratio (CNR) was determined according to the equation: CNR =

(SI within lumen − SI muscle) . BN

2.5. Qualitative assessment All images were assessed by consensus by two senior radiologists (one experienced in vascular interventional radiology and one

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Table 1 SNR and CNR for the three scanning protocols, in the different reconstruction algorithms for all sampled arterial segments. Values are presented as mean (SD). SNR

CNR

P120

P100

P80

P120

P100

P80

Aorta

B31f B40f B46f

26.44(9.544) 24.17(6.563) 16.98(4.354)

20.75(5.797) 16.66(4.054) 11.00(2.868)

15.35(3.708) 8.706(1.746)

22.95(8.511) 20.93(5.859) 14.64(4.069)

17.44(5.266) 13.98(3.710) 9.190(2.509)

12.92(3.384) 7.317(1.602)

RA distal to stent

B31f B40f B46f

24.81(9.071) 23.26(6.727) 15.82(3.575)

18.99(5.401) 15.77(4.175) 10.13(2.692)

14.61(3.664) 8.765(2.171)

21.19(8.013) 19.97(6.006) 13.45(3.340)

15.51(4.943) 12.95(3.902) 8.159(2.410)

12.18(3.257) 7.376(1.947)

Contralateral RA

B31f B40f B46f

24.39(7.126) 22.75(4.990) 15.40(2.153)

19.02(3.983) 15.68(2.769) 10.28(2.177)

14.52(2.898) 8.742(1.457)

20.77(6.086) 19.45(4.277) 13.03(1.886)

15.54(3.531) 12.85(2.512) 8.304(1.914)

12.08(2.506) 7.353(1.245)

Within stent

B31f B40f B46f

27.48(10.02) 24.36(6.739) 17.35(4.445)

21.90(5.051) 17.22(3.502) 11.61(2.711)

16.70(4.775) 8.921(1.851)

23.86(8.977) 21.07(6.058) 14.98(4.199)

18.42(4.511) 14.39(3.120) 9.636(2.214)

14.27(4.504) 7.532(1.713)

in MDCT) for the delineation of vascular segments (aorta and renal arteries) and the visibility of the stented lumen. Both observers were blinded to the different protocols of each patient and assessed P120 , P80 and P100 sets independently. We adopted flexibility in windowing and level settings, and the observers adjusted the images arbitrarily. A 4 scale quality score [24] was adopted for both requirements as follows—0: poor, non-diagnostic quality, 1: adequate, unsatisfactory image quality with acceptable information provided, 2: good, quality satisfactory enough for diagnosis, 3: excellent, image quality optimal for diagnosis. Images scored 1–3 were also assessed for the presence of ISR, categorized as A—no apparent hyperplasia, B—moderate hyperplasia causing <50% diameter reduction, C—severe hyperplasia causing >50% diameter reduction and D—total lumen occlusion.

2.7. Statistical analysis

2.6. Radiation dose assessment

3. Results

Volume CT dose index (CTDI) provided by the scanner was recorded for each patient and each contrast-enhanced scan. Percentage dose reduction (DR%) was calculated as:

Five patients of P100 protocol and five patients of P80 had vessel Hounsfield values >250 HU in all vessels of interest in all reconstructed sets of the two scans, and were further evaluated. One of them had bilateral stents. All other subjects were excluded from further study. Calculated SNR and CNR values for the aorta, renal arteries and the stented segment for the different reconstruction kernels in the standard arteriographic and the two low dose protocols are shown in Table 1. For both the standard arteriographic protocol (P120 ) and P100 , B31f performed better with higher SNR and CNR mean values in all sampled vessels; both B31f and B40f differed significantly in terms

DR% =

CTDIstandard − CTDIlow-dose × 100% CTDIstandard

where CTDIstandard is volume CTDI for the 120 kVp standard protocol and CTDIlow-dose is the volume CTDI of either the 100 kVp or 80 kVp low dose protocols.

Statistical analysis was performed using Graph Pad Prism 4, Graph Pad Software Inc. for Windows XP (Microsoft). Two-tailed unpaired t-test with Welsh’s correction was used to analyze SNR and CNR values of each of the four vascular segments for all reconstruction kernels and all scanning protocols. The Wilcoxon Signed rank test was used to analyze differences in qualitative scoring of the various scans. 2.8. Further evaluation and management Patients with CT images consistent with significant ISR and with clinical necessity of reintervention underwent DSA and angioplasty.

Table 2 p Values of the two-tailed unpaired t-test with Welsh correction for SNR, CNR in the different scanning protocols and reconstruction algorithms. ns: not significant, p > 0.05. p=

SNR

CNR

Aorta

RA distal to stent

Contra lateral RA

Within stent

Aorta

RA distal to stent

Contra lateral RA

Within stent

P120: B31f vs B40f P120: B31f vs B46f P120: B40f vs B46f P100: B31f vs B40f P100: B31f vs B46f P100: B40f vs B46f P80: B31f vs B40f

ns 0.0218 0.0281 ns 0.0199 0.038 0.0151

ns 0.0117 0.01 ns 0.0088 0.0238 0.0219

ns 0.0022 0.0009 ns 0.0022 0.0045 0.0105

ns 0.0123 0.022 ns 0.0032 0.0127 0.0193

ns 0.0251 0.0338 ns 0.025 0.048 0.0204

ns 0.0144 0.0123 ns 0.0136 0.0339 0.0299

ns 0.015 0.0303 ns 0.0036 0.014 0.0261

ns 0.0019 0.0008 ns 0.0031 0.0065 0.0129

P120/B31f vs P100/B31f P120/B40f vs P100/B31f P120/B40f vs P100/B40f P100/B31f vs P80/B31f P120/B31f vs P80/B31f P120/B40f vs P80/B31f

ns ns 0.0185 ns 0.0073 0.0061

ns ns 0.0134 ns 0.007 0.0056

ns ns 0.0021 ns 0.0017 0.0014

ns ns 0.0122 ns 0.0121 0.0265

ns ns 0.0175 ns 0.007 0.0058

ns ns 0.0114 ns 0.007 0.0052

ns ns 0.0096 ns 0.0139 0.0314

0.0417 ns 0.0013 ns 0.0014 0.001

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of CNR and SNR compared to B46f (p < 0.05) though no significant difference was noted between them. In P80 , B31f performed significantly better compared to B40f. In terms of SNR, B31f at 100 kVp did not differ significantly compared to best performing B31f and B40f at 120 kVp. CNR of B31f at 100 kVp did not differ significantly compared to B40f at 120 kVp; the stented arterial segment produced CNR values at B31f (100 kVp) that were marginally outperformed compared to B31f at 120 kVp (p = 0.042), though no significant difference was noticed compared to B40f at 120 kVp. In 80 kVp, both CNR and SNR of B31f were significantly lower compared either to B31f at 120 kVp, B40f at 120 kVp or B31f at 100 kVp (Table 2). Nine of the 10 cases fulfilling the angiographic quality criterion (i.e. Hounsfield values >250 HU in all studied segments in all acquisition protocols and reconstructions) were regarded of optimal image quality at 120 kVp (score = 3). A single case was assigned score = 2, satisfactory enough for diagnostic purposes. In the P100 group, 2 out of the 5 image sets at 100 kVp remained score = 3, while image quality was minimally downgraded in three cases. The overall quality assessment at 100 kVp did not differ significantly in reference with the optimal (score = 3) image quality. When comparing the degree of in-stent restenosis at 120 kVp and 100 kVp, no difference was noticed. In the P80 group, 2 image sets were characterized as non-diagnostic (score 0) and did not allow assessment of the presence of ISR. Regarding the 3 remaining cases, all of them were significantly downgraded to score 1, without any difference in the visual evaluation of ISR at 120 kVp and 80 kVp. Score results and relevant images are provided in Tables 3–5 and Figs. 1–3.

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Table 3 Qualitative image assessment and presence of ISR in the P100 patient group. Patient no.

1. 2. 3. 4. 5.

Image quality score

ISR

P120

P100

P120

P100

3 3 3 3 3

2 2 3 2 3

Not apparent >50% <50% >50% <50%

Not apparent >50% <50% >50% <50%

Table 4 Qualitative image assessment and presence of ISR in the P80 patient group. Patient no.

Image quality score ISR P120

P80

P120

P80

1. 2. 3. 4. 5.

3 3 3 3 2

1 1 1 0 0

Not apparent Not apparent <50% Not apparent <50%

Not apparent Not apparent <50% Impossible to assess Impossible to assess

For the P80 and P100 low dose protocols under CARE Dose 4d technique, the tube load range was 98–122 mAs and 124–149 mAs, respectively. CTDIvol value (mean, SD) was 12.5 (0) mGy, 6.9 (0.56) mGy and 2.9 (0.29) mGy for the P120 , P100 and P80 respectively. Mean DR% was estimated 44.5(4.5)% and 76.7(2.3)% for the 100 kV and 80 kV groups respectively.

Fig. 1. Reconstructed images of the same patient at 120 kVp (a, b) and 100 kVp (c, d). Almost circumferential hyperplasia within the middle and distal part of the stent is shown (arrows). The image quality score 3 and a degree of ISR of <50%, was assigned in both image sets.

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Fig. 2. Reconstructed images of the same patient at 120 kVp (a, b) and 100 kVp (c, d). Focal eccentric hyperplasia is shown (arrows) causing >50% lumen stenosis. The image quality downgrades from 3 (120 kVp) to 2 (100 kVp).

One patient from the P100 group with significant ISR (>50%) underwent DSA and reintervention. Both approaches concluded on severe ISR. 4. Discussion The aim of our study was to assess the feasibility of MDCT angiography and the detection of renal artery ISR at low exposure settings. Current literature investigates low radiation MDCT angiographic protocols, reporting safer, acceptable and interpretable angiographic imaging [24–29]. Our data conclude that we can obtain diagnostic angiographic images of the aorta and the renal arteries at tube voltage as low as 100 kVp. In addition, our data suggest that detection and assessment of renal artery ISR is feasible, with a significant dose reduction. Tube voltage setting is the major determinant of photon energy and certainly affects image quality [30]. At low tube voltage settings, the photon energy is reduced, resulting in an unbalanced Table 5 Wilcoxon signed rank test for the differences in qualitative scoring of the various exposure scans. Scanning protocol

P120

P100

P80

n Optimal diagnostic image score Actual median image score p value (two-tailed) Significant (˛ = 0.05)

10 3 3 0.5 No

5 3 2 0.125 No

5 3 1 0.0313 Yes

contrast increase and noise induction, total CNR reduction and amplification of streak artifacts that may downgrade the overall image quality. Interestingly, at low tube voltage settings, the attenuation value of iodine increases; due to the difference in the atomic number between iodine and soft tissue, the disproportionate increase in image contrast between iodine and soft tissue results in a higher iodine enhancement and allows delineation of contrastenhanced vascular structures within a more noisy background. The relationship between iodine concentration and attenuation appears to follow a linear relationship at the various tube voltage settings and has been estimated that attenuation at 90 kVp is 43% and 74% higher than that at 120 kVp and 140 kVp respectively [31]. We exploited the above phenomenon to reproduce double angiographic imaging for our patients with a single contrast injection and be able to compare low-exposure scans in reference to standard 120 kVp protocol on a patient-basis. We chose a relatively low injection flow-rate of contrast media, to prolong arterial enhancement. The generally applied criterion of signal intensity of 250 HU was used for a technically acceptable angiographic imaging. Failure of couple imaging occurred in 6 out of 16 patients, 4 in the P100 and 2 in the P80 groups. This is probably explained on the basis of individual circulation status and variations in hemodynamics which modified the contrast pass and clearance. We excluded these subjects as a whole, regardless of possible signal intensity values >250 HU in separate sampled arterial segments or separate reconstruction algorithms in the aim to produce uniform angiographic data sets for further evaluation. Calculated CNR and SNR of the late arterial

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Fig. 3. Reconstructed images of the same patient at 120 kVp (a, b) and 80 kVp (c, d). Eccentric restenosis at the proximal part of stent is shown, causing <50% lumen stenosis. The image quality score downgrades from 3 (120 kVp) to 1 (80 kVp).

scan are definitely deficient per se. This could have been overcome by increasing the volume of the contrast medium injected which would prolong intense lumen enhancement; we chose not to do so, in order not to jeopardize renal function. We found that the utilized B31f kernel performed better in terms of CNR and SNR in all protocols. An increase in image noise resulting in statistically significant reduction of CNR and SNR was noticed with the sharp B46f algorithm at both 120 kVp and 100 kVp, a finding in accordance with previous reports comparing a similar kernel (B30f) with B46f at 120 kVp [32]. The fact that no statistically significant difference was noticed with B31f at 120 kVp and 100 kVp in SNR and CNR values (with the exception of CNR within the stented arterial segment) supports the use of 100 kVp protocols for CT angiography of the renal artery region. The borderline (p = 0.042) difference of CNR values within the stent at 100 kVp can be explained with the presence of artifacts due to the metallic properties of stainless steel stents, which is more prominent in lower kVp settings, and the lower lumen attenuation at the late arterial phase. It could also be related to the small number of subjects. In any case, we consider this difference as negligible. Despite the lower lumen attenuation in the late arterial phase at 100 kVp, the SNR proved sufficient. The equation to calculate CNR is influenced by SNR and the related BN; if the study protocol would enable greater lumen attenuation values at the low exposure scan, this could easily overcome the increase of BN, and increase SNR along with CNR to a level of no statistical significant difference. Besides, compared to the second best performing recon-

struction algorithm (B40f at 120 kVp) – that showed no significant difference compared to B31f (120 kVp) – B31f at 100 kVp produced comparable CNR values. Finally, it appears that 80 kVp protocols produce such noise that images become of unacceptable quality, even in medium reconstruction algorithms. In this concept, the sharp B46f algorithm was not utilized or studied for the 80 kVp group. In the current study we estimated that reducing tube voltage to 100 kVp resulted in 44.5% dose reduction. Image quality was moderately downgraded (one score scale) compared to 120 kVp image sets in three out of five cases, but remained satisfactory and adequate for diagnosis without assessing any statistical difference regarding the image quality. No change in characterizing the degree of ISR was noticed either. Tube voltage set to 80 kVp resulted in 76.7% dose reduction and significantly lower quality of images in three out of five cases (two score scales), without divergence in ISR assessment. The fact that 2 out of 5 image sets were considered non-diagnostic suggests that 80 kVp protocols cannot be applied in renal artery or renal stent evaluation in clinical practice. Various parameters have been associated with the presence of artifacts and image quality reduction in the evaluation of renal artery stents. These include the specific clinical setting of renal arteries orientated almost perpendicular to the z-axis, the metallic properties of the implanted stainless steel stents and the deep location within radiation-absorbing soft tissue [20,33–34]. All these factors must have influenced our results. In any case, it has been shown that visualization of the stented lumen in the renal ter-

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ritory can be achieved regardless of the artifacts associated with stainless steel stents with dedicated image-data manipulation [35]. In this concept, we visually evaluated the quality of extended data sets including multiplanar reformations, variable windowlevel settings and different reconstruction algorithms for each scan, before we conclude for image quality and assess the extent of ISR. Thus, we managed to correlate our results in our standard and lowexposure protocols without significant divergence. Assuming that image quality in CT often exceeds the diagnostic needs [36], the moderate deficiency of CNR with adequate SNR within stents at 100 kVp did not hinder our visual qualitative evaluation. The P120 protocol we used as reference for comparison was based on the study by Raza et al. [21] who reported 100% sensitivity and 100% negative predictive value for detection of renal artery ISR in stainless steel stents under 120 kVp/270 mAs compared to gold standard DSA. This has been the most conservative protocol reported in terms of radiation exposure, compared to former studies which utilized 140 kVp tube voltages, with comparable results [37,38]. The 16-row MDCT scanner we used definitely performs greater isotropic acquisition than the 4-row unit used by Raza et al. and enables greater visualization of the stented lumen [39]. The main limitation of our study is the small number of subjects participating; however, this was unavoidable in the aim firstly to exploit a homogenous group in terms of the implanted stent type and size and secondly not to expose young individuals to ionizing radiation for the needs of this feasibility study. Moreover, the angiographic exclusion criterion utilized reduced the number of patients even further. We could have overcome the obstacle of non-diagnostic low exposure scans by performing the low exposure scan at the true arterial phase; though this would necessitate a second contrast injection for the standard protocol for clinical diagnosis. Another limitation is that the same exposure settings for the scan during true arterial phase were used for all patients, without taking into account their body size. It is common practice to use higher exposure settings for large body patients. In the current study, however, there were small variations in the somatometric characteristics of the participants and the standard protocol exposure settings for abdominal CT scans were applicable to all. Further studies with higher number of subjects, including analysis of the somatometric characteristics of patients and different stent types and size may be required to confirm the feasibility of low-dose MDCT protocols for the assessment of renal artery stents. In the current study we did not correlate our findings with DSA imaging. Definitely, DSA correlation would be advantageous, but we decided to perform DSA only on clinical necessity for reintervention. Besides, our aim was to assess the feasibility of low exposure CT angiography assuming that CT angiography at 120 kVp is adequate for safe diagnosis [21].

5. Conclusions We conclude that renal artery angiography and assessment of renal artery stents for restenosis are feasible at 100 kVp with minor loss of image quality compared to 120 kVp. Radiation exposure can be significantly reduced to about half and this could allow for regular follow-up, when indicated. This would appear beneficial for young patients with a long life expectancy. Our results might stimulate further studies on the use of contrast material with lower concentration of iodine that would produce sufficient lumen attenuation at lower kVp.

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