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1 Department of Radiology, Duke University Medical Center, Box 3808, Rm. 1532,
Erwin Rd., Durham, NC 27710.
2 Department of Medicine, Duke University Medical Center, Durham, NC
27710.
Received July 13, 2001;
accepted after revision November 6, 2001.
Address correspondence to R. C. Nelson.
Abstract
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SUBJECTS AND METHODS. CT angiography was performed within 24-48 hr of renal artery stent placement in 15 patients. Two patients had bilateral stens, resulting in a total of 17 stents. CT angiography was performed using a multidetector scanner and a bolus of IV contrast material with the scanning delay determined by a small-volume timing bolus. A volumetric data set was acquired through the stented arteries in the axial plane using a 4.0 x 1.25 mm detector configuration and a pitch of 3:1. The stent lumen diameter, as measured on direct CT angiography and curved multiplanar reformations in both the axial and coronal planes, was compared with that measured on catheter angiography.
RESULTS. The lumina of all 17 stents were well visualized and patent on both CT angiography and catheter angiography. Anatomic definition, including stent position and wall apposition in the renal artery, correlated well with catheter angiography. The diameter of the renal artery stent lumen measured on catheter angiography (mean, 5.9 ± 1.3 mm) was greater than that on CT angiography (mean stent lumen diameter for direct axial plane was 4.6 ± 1.0 mm, for curved multiplanar reformations in the axial plane was 4.3 ± 1.0 mm, and for curved multiplanar reformations in the coronal plane was 4.4 ± 1.0 mm) in 14 (82%) of 17 stents.
CONCLUSION. CT angiography produced interpretable multiplanar images of the renal artery, even with a metallic stent in place, and was adequate for determining stent patency. Compared with catheter angiography, the intrastent luminal diameter was underestimated in most patients who underwent CT angiography.
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Sonography has been used to measure arterial velocity even with a metallic stent in place; however, patient habitus, shadowing from bowel gas, aorta pulsatility, and operator inexperience severely limit the evaluation [8]. MR angiography correlates well with catheter angiography, with a high sensitivity and specificity for determining renal artery stenosis [9,10,11]. However, MR angiography tends to overestimate the degree or severity of stenosis [9]. More important, after metallic stent placement, MR imaging is limited by susceptibility artifacts and flow dephasing that leads signal voids on spin-echo images and flow voids or dropout on gradient-echo images [12]. In addition, although the presence of a stent does not preclude performing MR imaging, the metallic scaffolding may conduct electricity and lead to local tissue heating [12].
New multidetector CT scanners allow the rapid acquisition of images with thin collimation, thereby generating data sets that yield excellent three-dimensional and multiplanar renderings that have been shown to be accurate in depicting renal artery stenosis [13,14,15,16]. However, to our knowledge, the issue of stent artifacts on CT has not been adequately addressed in the literature. The purpose of our study is to assess the renal artery lumen after stent placement using multidetector CT angiography and to compare the results with those of catheter angiography.
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A total of 131 patients were referred for treatment of renovascular hypertension or renal insufficiency between June 2, 1999 and November 16, 2000. Seventy-five patients met the study criteria, and of these patients, 23 were approached regarding study participation. Nineteen patients agreed to participate, although four withdrew before CT was performed. The final patient population consisted of eight men and seven women with an age range of 43-83 years (mean age, 65 years). Two patients underwent bilateral stent placement. One of these patients underwent unilateral stenting; however, the contralateral renal artery had been treated in this patient by stent placement 9 months earlier. Therefore, the study cohort consisted of 17 renal artery stents evaluated with both catheter angiography and CT angiography.
Catheter Angiography
Patients were chosen to undergo renal artery stenting on the basis of
clinical and angiographic evaluation independent of the study design. Stent
selection was left to the discretion of the interventionalist. All procedures
were performed with a digital image acquisition (Integris V3000; Philips
Medical Systems, Seattle, WA) and a maximum field of view of 38 cm.
Balloon-expandable stents were deployed using standard techniques. Images of
the final stent position were obtained with a selective renal artery catheter
in place (Fig. 1A). Stent
diameters were measured either by the Philips Quantitative Analysis program or
by an external ruler using the selective catheter as an internal standard.
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Of the 17 renal artery stents placed for the treatment of hypertension caused by renal artery atherosclerosis, 14 were ostial in location, and three were truncal. Bilateral stents were present in two patients, one of whom had a stent placed approximately 1 year previously. Five different balloon-mounted stents were used during the course of the study, including a bridge stent (n = 4; Medtronic AVE; Peripheral Technologies, Santa Rosa, CA), Herculink 14 stent system (n = 4; Advanced Cardiovascular Systems, Temecula, CA), DoubleStrut endoprosthesis (n = 1; Intratherapeutics, St. Paul, MN), Megalink SDS stent system (n = 2; Advanced Cardiovascular Systems), and Palmaz balloon-expandable stent (n = 6; Cordis Endovascular, Miami, FL). Stent placement was complicated in one patient by a small peristent pseudoaneurysm (Fig. 2A,2B,2C). One additional complication occurred that consisted of shearing a stent from the balloon, thereby requiring deployment in the iliac artery (n = 1). A second stent was placed successfully.
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CT Angiography
CT angiography was performed using one of two multidetector scanners
(LightSpeed QX/i or OX/i Plus; General Electric Medical Systems, Milwaukee,
WI). The stent was localized using frontal and sagittal scout images and axial
unenhanced images as follows: detector configuration, 4 x 5 mm;
reconstruction thickness, 5 mm; reconstruction interval, 5 mm; pitch, 3:1;
table speed, 15 mm per gantry rotation; rotation speed, 0.8 sec; field of
view, 20 cm; kVp, 140; and mA, 80. The scan delay was determined by placing a
region of interest on the aorta at the level of the renal artery stent,
followed by a 15 mL bolus of 76% of iopamidol (Isovue 370 mg I/mL; Bracco
Diagnostics, Princeton, NJ) administered at 5 mL/sec. The time to reach a
target of 50 H based on a time—attenuation curve was used as the
scanning delay. The scanning delay ranged from 11 to 27 sec (mean, 19 ±
5 sec). The mean acquisition time for CT angiography was 12.0 ± 2.5
sec. No respiratory motion artifacts were encountered in any of the patients.
For the first two patients, an additional 10-sec delay was arbitrarily added
to the calculated delay. Although the images produced were diagnostic, the
resulting 27-sec scanning delay produced enhancement of the aorta as follows:
cephalad to the superior mesenteric artery, 170 H; at the level of the renal
arteries, 179 H; and below the renal arteries, 170 H. With experience, we
found that arterial opacification was superior using only the calculated
timing delay, which resulted in arterial enhancement as follows: cephalad to
the superior mesenteric artery, 294 ± 105 H; at the level of the renal
arteries, 351 ± 91 H; and inferior to the renal arteries, 367 ±
104 H.
CT angiography was performed after IV administration of 76% of iopamidol using a volume of 135-175 mL at a rate of 5 mL/sec. Initially, we used a larger volume of contrast material; however, as our technique improved, we decreased the volume while maintaining excellent arterial opacification. A volumetric data set was acquired through the renal arteries in the axial plane using the following parameters: detector configuration, 4.0 x 1.25 mm; pitch, 3:1; table speed, 3.75 mm per gantry rotation; rotation speed, 0.8 sec; field of view, 20 cm; kVp, 140; and mA, 170-220. The patient was scanned from 3 cm above the stent to 3 cm below the stent. Axial sections were reconstructed at a thickness of 1.25 mm and an interval of 0.5 mm (60% overlap) and were evaluated with three-dimensional and curved multiplanar reformation using a separate workstation with appropriate three-dimensional software (Vitrea II; Vital Images, Minneapolis, MN) (Fig. 1B).
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Fourteen patients underwent CT angiography within 24 hr of stent placement. For logistic reasons, one patient underwent CT angiography on the second day (within 48 hr) after stent placement. All examinations required less than 30 min of table time, and the postprocessing time ranged from 10 to 15 min. All patients tolerated the procedure without experiencing an adverse reaction or a delay in discharge. One examination was performed as an outpatient procedure.
Analysis
The stent lumen was evaluated using direct axial images, lighted
volume-rendered images, and curved multiplanar reformations in the axial and
coronal planes. All CT angiograms were evaluated in a nonblinded fashion and
were compared directly with the angiographic images. Direct comparison of
stent luminal diameter on CT angiography was made with the catheter angiogram
after stent placement. Statistical analysis was applied to the diameter
measurements and included comparison of means by a signed rank test and of
confidence intervals based on the Student's t distribution.
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Stent Lumen Comparison
The stent luminal diameter, as determined by catheter angiography, averaged
5.9 ± 1.3 mm (range, 2.9-7.2 mm). The stent lumen that was visualized
on CT angiography was measured by three imaging techniques, and the average
luminal diameters were determined as follows: direct axial images, 4.6
± 1.0 mm (range, 3.1-6.6 mm); curved multiplanar reformatted axial
images, 4.3 ± 1.0 mm (range, 2.6-6.4 mm); and curved multiplanar
reformatted coronal images, 4.4 ± 1.0 mm (range, 2.9-6.2 mm). Fourteen
(82%) of 17 stents measured larger using catheter angiography than with all
three imaging modes on CT angiography. The remaining three stents were larger
on CT angiography in one of three techniques used in a single patient and on
two of three techniques used in two patients. Comparing CT angiography with
catheter angiography showed a nonzero mean for each CT angiographic technique
for measuring luminal diameter (-0.79, -1.1, and -0.94 mm) with significant
p values as determined by the signed rank test (0.001, 0.0001, and
0.0001, respectively). The 95% confidence limits for the differences in means
were -1.2 and -0.4 mm for the direct axial technique, -1.5 and -0.7 mm for the
curved multiplanar reformatted axial technique, and -1.3 and -0.6 mm for the
curved multiplanar reformatted coronal technique. The difference in mean
diameter between the direct axial and curved axial reformations (0.31 mm) was
significant (p < 0.01), with a 95% confidence interval for a mean
difference of 0.14-0.44 mm.
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Our study revealed that anatomic images of the renal artery, including the intrastent lumen, can be confidently obtained using CT angiography. Reconstruction of thin sections and using a low pitch at narrow intervals allow improved z-axis resolution in the stent. An intrastent stenosis was shown in one patient that was appreciable on both CT angiography and catheter angiography. The combination of axial images and curved multiplanar reformations allows detailed evaluation of the stent position in relation to the ostium and easy visualization of the entire stent. Curved multiplanar reformations are particularly helpful for evaluating tortuous vessels. The lighted volume renderings are excellent for the evaluation of arterial anatomy and the presence of branch vessels. CT angiography showed patency of a branch vessel covered by the stent, which was more difficult to evaluate on catheter angiography (Fig. 4A,4B).
The ability of CT angiography to depict the lumen in a renal artery metallic stent was directly compared with the traditional gold standard of catheter angiography. A small but significant discrepancy was noted between the luminal sizes when angiography was compared with CT angiography. The latter tended to underestimate the luminal dimension regardless of the method used for rendering the data set. The exact reason for this discrepancy is unknown. Elastic recoil of vessels even after stenting could account for some of the discrepancy, although this phenomenon could not be determined from our study. Beam hardening due to the metallic nature of the stent most likely accounts for the discrepancy. All stents used were manufactured of stainless steel. Stents composed of other materials, such as nitinol, may show lesser degrees of beam hardening, but this has yet to be determined. More important, visualization of the arterial lumen in the stent was not limited by beam hardening artifacts in this small series. Finally, it is possible that the algorithm for measuring the stent on catheter angiography is inaccurate for vessels of this caliber.
The renal artery stent was easily localized on the anteroposterior and lateral CT scout digital radiographs. For timing of the contrast material, our protocol used a timing bolus (15 mL at 5 mL/sec) and a time—attenuation curve to determine the scanning delay. Initially, 10 sec was arbitrarily added to the delay; however, we found less than optimal opacification of the arteries and excessive venous enhancement. We also used a faster injection rate (5 mL/sec) than that used in previous studies and a higher iodine concentration (370 mg I/mL vs 300 mg I/mL) to maximize arterial opacification. Future refinement of our protocol should be directed toward decreasing the iodine load by decreasing the contrast volume and the implementation of automated triggering. The use of iodinated contrast material severely limits the use of CT angiography in patients with renal insufficiency. However, because the stents are mainly constructed of stainless steel, MR imaging with a gadolinium chelate is not an option.
This feasibility study is limited predominately by population size and lack of pathology studies. The latter was the result of limiting the study exclusively to patients who had just received arterial stenting. No patients had immediate thrombosis, stenosis, or other untoward clinical events after stent placement. A small but measurable difference in stent diameter, as determined on CT angiography compared with catheter angiography, was present in this population. The cause of this difference may be a result of either a discrepancy between the modalities or inaccuracies in our ability to repeatedly measure small distances. A larger patient population and longer follow-up interval will be necessary to determine the clinical significance of such small differences.
Even in light of the small sample size, CT angiography appears to provide diagnostic images of the lumen of a renal artery stent and warrants further study. Because of the small sample size, a particular grading scheme for image quality was not used for the images. For subsequent larger studies evaluating arterial abnormalities and degree of stenosis, such observer bias would have to be eliminated by separate blinded interpretations of catheter angiography and CT angiography. Our study was designed only as a pilot investigation to evaluate the technical feasibility and adequacy of multidetector CT angiography. The technique appears to be adequate in determining the anatomy in the lumen, although one must remember the sizing discrepancies between catheter angiography and CT angiography.
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