Pediatric Imaging Original Research

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1 Pediatric Imaging Original Research Eichhorn et al. CT of Pediatric Vascular Stents Pediatric Imaging Original Research Joachim G. Eichhorn 1,2 Claudia Jourdan 1,3 Sharon L. Hill 4 Subha V. Raman 5 John P. Cheatham 4 Frederick R. Long 6 Eichhorn JG, Jourdan C, Hill SL, Raman SV, Cheatham JP, Long FR Keywords: congenital heart disease, MDCT, pediatrics, phantom, radiation exposure, stents DOI: /AJR Received September 21, 2007; accepted after revision November 12, J. G. Eichhorn was supported by a postdoctoral research grant from the Max Kade Foundation, Inc., New York, NY. 1 Department of Radiology, Ohio State University College of Medicine, Columbus, OH. 2 Department of Pediatric Cardiology, University Children s Hospital, Im Neuenheimer Feld 153, Heidelberg, Germany. Address correspondence to J. G. Eichhorn (eichhorn.12@osu.edu). 3 Department of Pediatric Radiology, University Children s Hospital, Heidelberg, Germany. 4 Heart Center, Columbus Children s Hospital, Columbus, OH. 5 Division of Cardiology, Davis Heart and Lung Research Institute, Ohio State University College of Medicine, Columbus, OH. 6 Children s Radiological Institute, Columbus Children s Hospital, Columbus, OH. AJR 2008; 190: X/08/ American Roentgen Ray Society CT of Pediatric Vascular Stents Used to Treat Congenital Heart Disease OBJECTIVE. The purpose of our study was to assess the visibility of lumen narrowing of pediatric vascular stents using various CT dose parameters in an in vitro model. MATERIALS AND METHODS. Ten steel stents of varying designs and sizes commonly used in the treatment of congenital heart disease were implanted in polyvinyl chloride (PVC) tubes and three of the 10 stents were partially obstructed with wax by filling 25% (mild) to 60% (moderate) of the lumen with contrast material. On a 64-MDCT scanner, the stents were scanned at tube voltages (kvp) of 80, 100, and 120 and at tube currents (ma) of 40, 80, 120, and 160. CT measurements of inner-stent diameter, strut thickness, and percent lumen (in-stent) stenoses were compared with biplane fluoroscopy of digital angiography. RESULTS. The stent diameter and percent stenosis on all CT images were consistently smaller than measured on digital angiography but were highly correlated (r = 0.97; p < ) with improvement as stent diameter increased (93% agreement with digital angiography for 4-mm stent, up to 99% for 25-mm stent; p = 0.001). Moderate stenosis could be assessed better than mild stenosis (99% vs 91% agreement with digital angiography; p = 0.003). Increasing exposure settings improved CT correlation of all measurements for ma up to 120 and kvp up to 100 (98.1% agreement). Higher settings did not improve accuracy (93.9% for 160 ma at 120 kvp; p = 0.03). CONCLUSION. CT is feasible to assess lumen narrowing of pediatric vascular stents at a wide range of tube settings. The study suggests that it is possible to lower the radiation exposure settings without loss in image quality or accuracy in detecting in-stent stenoses. T ranscatheter cardiac interventional procedures using balloon angioplasty with stent placement have become a common nonoperative way to treat stenoses of the pulmonary and systemic arteries and veins in children with congenital heart disease (CHD) [1, 2]. A frequent problem after stent placement is the development of in-stent luminal restenoses attributable to intima hyperplasia and thrombosis, stent fracture, or stent-associated vascular narrowing [3, 4], which require repeat interventional procedures [5, 6]. MDCT is helpful in planning optimal stent placement by showing the orientation and extent of stenoses, and it is also helpful in the follow-up evaluation of stent patency [6, 7]. In contrast to sonography or MRI, MDCT is surprisingly free of artifacts from the metallic component of stents [8, 9], which allows the lumen of the stent to be assessed for in-stent stenosis [6]. The ability to diagnose in-stent stenosis on MDCT was evalu ated in a first study of children with pulmonary artery and aortic stents who also underwent conventional digital angiography [9]. MDCT correlated well with digital angiography for all grades of stenosis. Using a threshold of approximately 20% stenosis, the sensitivity and specificity of MDCT exceeded 95% [9]. One of the main limitations of CT is exposure to ionizing radiation. In infants and children, this is of particular concern because of their greater sensitivity to radiation and their longer life span, which increases the risk of developing radiation-induced cancer [10, 11]. The optimal radiation dose settings for performing cardiac CT in infants and pediatric patients are still being established. Preliminary work indicates that diagnostically adequate cardiac CT scans can be obtained at much lower radiation doses than commonly quoted in the literature [9, 12 14]. The purpose of this study was to evaluate the effect of varying tube voltages and currents on the assessment of stent lumen diameter and simulated in-stent stenosis in comparison AJR:190, May

2 Eichhorn et al. with digital radiographic angiography in an in vitro model. Materials and Methods Stent Phantom Ten commercially available peripheral vascular stainless steel stents (Bx Velocity, Palmaz Genesis, Aviator SDS, Palmaz Genesis XD, and Palmaz XL, Cordis Corporation; and Intrastent Double Strut XS, Intrastent Double Strut LD, Intrastent Mega LD, and Intrastent MAX LD, ev3, Inc.) of varying designs and different sizes used in the treatment of CHD with vascular obstructions were studied. Details regarding the type and size of the stents are summarized in Table 1. The stents were implanted in polyvinyl chloride (PVC) tubes with inner diameters of 4, 6, 7, 10, 13, 15, 17, 20, and 25 mm (Fig. 1). Three of these tubes (inner diameters of 7, 15, and 20 mm) were filled with wax to partially obstruct the stent lumen, simulating mild and moderate in-stent stenoses. This filling resulted in two mild (25%) and one moderate (60%) stenoses. The tubes were filled with contrast material ([ioversol 320], Optiray 320, Mallinckrodt Imaging) diluted to a density of 330 H, which correlates to in vivo results in pediatric patients [9]. The prepared tubes were placed between two layers of sponges to simulate surrounding lung. Imaging The phantom was imaged in the helical mode with a 64-MDCT scanner (Sensation Cardiac 64, Siemens Medical Solutions) with the following settings: 1-mm slice thickness, 319-mm field of view, mm pixel spacing, helical pitch TABLE 1: Stent Types and Sizes Tube Diameter (mm) Stent Type of 1.0, 0.36 s 1 rotation speed, and B31f (soft) and B70f (sharp) reconstruction kernels in a nearly axial orientation. Dose settings were 80, 100, and 120 kvp at tube currents of 40, 80, and 120 ma for each kvp. An additional scan at 120 and 160 ma was performed. Altogether, the phantom was scanned 10 times with 10 varying tube settings. The phantom was also imaged using angled bi plane fluoroscopy as would be performed in con ventional digital angiography, with a 1,024 1,024 pixel matrix (anteroposterior and 90 lateral projections). Quantitative Analysis With software for 3D image display and analysis (VGStudio Max version 1.2, VolumeGraphics), transverse and longitudinal (perpendicular to each other) reformations were reconstructed from the CT data at slice thicknesses of 0.5 or 0.6 mm. To improve delineation of the stents, the images were displayed in a zoom mode with a window width of 1,020 H and window level of 320 H. Using electronic calipers, two observers measured independently the cross-sectional stent (single) strut diameters (n = 100 for each observer; 10 stents scanned 10 times with varying tube settings), the inner stent size (n = 100), and the residual lumen diameter in cases of simulated instent stenosis (n = 30; three in-stent stenoses were scanned 10 times). The diameters were measured from cross-sectional images obtained perpendicular to the long axis of the stent (Fig. 2). The measurements of the two observers were averaged for statistical analysis and graphical display of the data. Diameter Stent Size (mm) Unexpanded Length Strut Thickness 4 Bx Velocity Palmaz Genesis Intrastent Double Strut XS NA 7 Aviator SDS 7 24 NA 10 Intrastent Double Strut LD NA 13 Palmaz Genesis XD Intrastent Mega LD NA 17 Intrastent MAX LD NA 20 Palmaz XL Intrastent MAX LD NA Note Bx Velocity, Palmaz Genesis, Aviator SDS, Palmaz Genesis XD, and Palmaz XL are manufactured by Cordis. Intrastent Double Strut XS, Intrastent, Double Strut LD, Intrastent Mega LD, and Intrastent MAX LD are manufactured by ev3. NA indicates no information about strut thickness was available; according to the radiographic measurements, the ev3 stents were in the same range as the Cordis stents. The same measurements of the phantom were made fluoroscopically using a quantitative angiog raphy system by two observers. The system was calibrated using the known diameter of a metal ball (20 mm). Measurements were made in anteroposterior and lateral projections of the stent strut diameter, the original lumen, and residual lumen of the stents. Statistical Analysis Descriptive statistics were presented with means and SDs. Pearson s correlation coefficients and simple linear regression models were fitted to study the linear relationships between the MDCT and digital angiography variables. In comparing the results of a wide range of stent sizes and tube settings, the assessed diameters (stent strut, original, and residual stent lumen) were lined out as the ratio between MDCT and digital angio g raphy measurements and were given as percent agreement. A two-sided paired Student s t test was used to compare the stent sizes measured using CT images (interobserver variability and variability between the varying technical settings and CT scanners). A value of p < 0.05 was considered statistically significant. Results There was a high correlation between the measurements of the two observers. The mean difference of diameter measurements was 0.2 mm (95% CI, mm; correlation coefficient, 0.98; p < ) and the mean difference of percent in-stent stenosis was 0.3% (95% CI, 1.4 to 1.1%; correlation coefficient, 0.94; p < 0.001). The diameters (inner stent size and residual stent lumen) measured by MDCT and digital angiography showed a high correlation with a linear relationship (r = 0.97; p < ). The inner stent diameters measured on MDCT images were consistently and significantly smaller than those measured on digital angiography (mean agreement, 96.5%; SD, 3.4%). Correspondently, the stent strut was significantly thicker on CT (mean, 200.7%; SD, 11.1%) than measured on digital angiography. There was improvement in agreement between CT and digital angiography measurements as the stent diameter increased (92.6%, SD of 2.1% for the 4-mm stent up to 99.3%, SD of 1.2% for the 25-mm stent; p = 0.001). The moderate in-stent stenoses (98.7% mean agreement with digital angiography, SD of 2.8%) could be assessed better on CT than mild stenoses (mean, 90.7%; SD, 5.7%; p = 0.003) (Fig. 3). The sharp reconstruction kernel had 1242 AJR:190, May 2008

3 CT of Pediatric Vascular Stents better agreement for all measurements than the soft kernel: inner stent diameters (soft vs sharp kernel: 93.1% vs 96.5%; p = 0.04) and stent strut thickness (218.3% vs 200.7%; p = 0.01). A Fig. 1 Four stents after placement in polyvinyl chloride (PVC) tubes. A D, Three-dimensional MDCT reconstructions and conventional photographs (insets) of Palmaz Genesis (Cordis) in 13-mm tube (A), Intrastent MAX LD (ev3) in 17-mm tube (B), Palmaz XL (Cordis) in 20-mm tube (C), and Intrastent MAX LD in 25-mm tube (D). See also Table 1. Fig. 2 Measurements of cross-sectional stent strut (double arrow 1) and stent lumen diameter (double arrow 2) and contrast-enhanced in-stent residual lumen (double arrow 3) perpendicular to long axis of stent. Diameter ratio between residual stent lumen and original stent lumen formed basis for grading instent stenosis. B Comparing the different tube settings (Fig. 4), there was an improvement in agreement with an increase of tube current up to 120 ma and of tube voltage up to 100 kvp (agreement with digital angiography of 98.1%; SD, 2.4% (Fig. 5). Increasing tube current to 160 ma showed no further improvement (93.9% agreement, SD of 1.7% for 160 ma combined with 120 kvp (Fig. 5) but resulted in an increase of artificial lumen narrowing caused by an increase of strut thickness up to 10%. These results correlated with a better assessment of mild stenoses (Fig. 3) using 100 kvp (94.6% agreement; SD, 3.8%) than 80 kvp (89%; SD, 5.6%; p = 0.03) or 120 kvp (mean, 87.5%; SD, 5.3%; p = 0.003) using varying tube currents. Furthermore, the improvement in agreement of 100 kvp (97.7%; SD, 3.1%) versus 80 (96.1%; SD, 2.4%; p = 0.05) and 120 kvp (mean, 94.7%; SD, 3.6%; p = 0.03) could also be shown using a fixed tube current of 80 ma for measuring inner stent and residual lumen diameter. C Discussion The benefit of MDCT has rapidly expanded the use of CT in children, especially for imaging the thoracic systemic and pulmonary vasculature [12, 15 17]. This report is the first to our knowledge that examines CT angiography of vascular (noncoronary) pediatric stents at a variety of dose settings. The assessment of stent lumen diameters and in-stent stenoses using MDCT was highly correlated with digital angiography for all stent diameters (4 25 mm). Both mild and moderate simulated stenoses were identified accurately (with more than 90% for the mild and up to 99% agreement for moderate stenosis). Better differentiation between mild and moderate stenosis has potential therapeutic importance whether there is a subsequent interventional or surgical approach or not. The difficulty in detecting mild stenoses can be explained by partial volume averaging artifact from the metallic vascular stent struts that can partially (or totally) obscure the stented vessel lumen on MDCT [8, 9]. Previous in vitro and in vivo studies showed that the stent lumen could be visualized and stent patency assessed. However, due to artifacts arising from metallic stent struts, nonocclusive instent stenoses were obscured [18, 19]. In a study of children with CHD, MDCT had a low sensitivity for detecting mild (0 30%) in-stent D AJR:190, May

4 Eichhorn et al. In-Stent Stenosis in Relation to Digital Angiography Measurements 100% 90% 80% 70% Mild stenoses but detected all moderate and higher stenoses seen by digital angiography [9]. The sensitivity reached more than 98% for stenoses 20% without any differences between higher grades, which means that 50% of stenoses could be assessed as well as 70% with a sensitivity of nearly 100% for pediatric vascular stents with 7-mm median diameter [9]. Theoretically, the artificial lumen narrowing is approximately 60% for coronary stents at 4-MDCT [18], decreases to 25% at 16- MDCT [20], and further with later-generation MDCT scanners [21 23]. Nevertheless, small-diameter stents (< 3 mm) are not assessable very well with 64-MDCT and stent type is very important in the ability to visualize the lumen diameter. For stents with a diameter more than 3 mm, assessment appears to be feasible, and in-stent stenosis was correctly identified with an acceptable sensitivity (42 99%) and good specificity (88 98%) [24, 25]. In previous studies of stent imaging concerning mostly coronary stents in the adult population, radiation exposure and tube settings were not a major issue. Consequently, the commonly used tube voltage was 120 kvp or higher and the tube currents had a wide range of ma (effective tube current time of 680 mas). The radiation exposures to patients from cardiac CT in adults at these settings are reported to be between 9 and 16 msv for examinations without dose modulation [26]. Grade of In-Stent Stenosis Moderate Fig. 3 Scatter graph of agreement between MDCT and digital angiography for assessment of in-stent stenosis for mild (n = 40: two stents were scanned, 10 tube settings; data of two observers) and moderate instent stenoses (n = 20: one stent). Data are shown in three groups according to tube voltage: = 80, = 100, and = 120 kvp. For moderate stenoses, there were no significant differences found between kvp groups (p values between 0.6 and 0.9). For mild stenoses, there was significant improvement for 100 kvp versus 80 (p = 0.03) and also versus 120 kvp (p = 0.003). Mean and SD of all tube voltages are added. Fig. 4 Multiplanar 2D MDCT reconstruction images of two stents of different sizes, one with moderate (upper row) and one with mild (lower row) in-stent stenosis show effect of varying tube voltages and currents on image quality. The results of our study suggest that higher radiation exposure settings do not correlate automatically with better image quality. In fact, a tube voltage of 120 kvp resulted in an increase in artifacts arising from the stents. This resulted in decreased accuracy compared with digital angiography that we hypothesize is due to greater beam-hardening artifact and greater partial volume averaging. We found that lower kvp of 80 to 100 and mas of 120 yielded results as accurate as those of higher radiation exposure settings. Of course, higher radiation exposure settings may be necessary in adult patients to penetrate the soft tissues overlying the stent, not just the lungs as in our in vitro model. Because of the reduced X-ray attenuation secondary to lower body mass in children [13, 14], the data from our model would be expected to simulate more closely actual scanning conditions. Recently described cardiac CT protocols for children use a kvp of either 80 or 120 with ma values between 25 and 140 related to the patient s weight [27]. In patients with smaller body habitus (< 50 kg), the studies could be performed at 80 kvp [12]. Such protocols result in a considerable reduction in radiation dose of approximately 30% at 4-MDCT [12] and 60% at 16-MDCT (80 kvp approach compared with the standard 120 kvp protocol) in children [28]. Siegel et al. [13] showed that there is no appreciable difference in image noise in the infant-sized phantoms at the 80- and 120-kVp settings. The image contrast increased with reduction in tube voltage (this effect was greatest in smaller phantoms). Different Methods of Stent Imaging At present, conventional digital angiography performed during cardiac catheterization remains the reference standard for luminal assessment after stent placement. The disadvantages of digital angiography are that it is a 2D technique, more invasive, requires longer sedation times, and exposes the patient to higher doses of ionizing radiation [9, 29]. The dose range for pediatric procedures is obviously wide and varies from approximately 5 msv to more than 20 msv for complex interventional approach [28, 29]. Doppler echocardiography is limited by field of view, which especially affects visualization of extracardiac structures and is susceptible to metallic artifacts. The 1244 AJR:190, May 2008

5 CT of Pediatric Vascular Stents Mean Stent Diameter in Relation to Digital Angiography 100% 95% 90% 0 20 chief advantage of MDCT over MRI in stent evaluation is the significant signal dephasing due to inhomogeneities induced in the magnetic field by the metallic struts [8, 30]. In comparison with studies using MRI for evaluating in-stent stenoses, MDCT is superior in most types of stent materials, comparable to MR angiography in nitinol stents, and inferior only in the new tantalum stents [8]. Furthermore, the spatial resolution capabilities of MRI are limited by the strength of gradients. Current MRI scanners are already operating at the fastest gradients possible, whereas further improvements with MDCT have been anticipated with the arrival of more slice detectors. Study Limitations Our phantom was designed to simulate pulmonary arteries, where many of the stents are implanted to treat children with CHD. Nevertheless, there are limitations to our phantom model. The effects of cardiac and respiratory motion and the surrounding chest wall soft tissues were not simulated during the experiments. In addition, the stents were orientated parallel to the z-axis of the MDCT scanners. This orientation would be expected to result in less partial volume averaging and improved results than would occur in vivo in obliquely oriented stents [22]. In conclusion, this study supports the feasibility of using MDCT to image intravascular stents in pediatric patients because of a remarkable lack of artifacts at a wide range of kvp and ma settings with accuracies up to 99% of digital angiography, the reference standard, in determining stent diameters Tube Current (ma) Fig. 5 Graph shows agreement between MDCT and digital angiography assessment of innerstent diameters in relation to dose (each data point represents mean of measurements for 10 stents). 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