Vascular and Interventional Radiology Original Research

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1 Vascular and Interventional Radiology Original Research Apfaltrer et al. High-Pitch Versus Standard-Pitch CTA of the Aorta Vascular and Interventional Radiology Original Research Paul Apfaltrer 1,2 E. Lexworth Hanna 1 U. Joseph Schoepf 1,3 J. Reid Spears 1 Stefan O. Schoenberg 2 Christian Fink 2 Rozemarijn Vliegenthart 1,4 Apfaltrer P, Hanna EL, Schoepf UJ, et al. Keywords: aorta, CT angiography, high-pitch CT angiography, radiation dose DOI: /AJR Received January 22, 2012; accepted after revision March 26, U. J. Schoepf is a consultant to and receives research support from Bayer-Schering, Bracco, GE Healthcare, Medrad, and Siemens Healthcare. 1 Department of Radiology and Radiological Science, Medical University of South Carolina, Ashley River Tower, 25 Courtenay Dr, MSC 226, Charleston, SC Address correspondence to U. J. Schoepf (schoepf@musc.edu). 2 Institute of Clinical Radiology and Nuclear Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany. 3 Department of Medicine, Division of Cardiology, Medical University of South Carolina, Charleston, SC. 4 Department of Radiology, Center for Medical Imaging, North East Netherlands, University Medical Center Groningen/University of Groningen, Groningen, The Netherlands. CME This article is available for CME credit. AJR 2012; 199: X/12/ American Roentgen Ray Society Radiation Dose and Image Quality at High-Pitch CT Angiography of the Aorta: Intraindividual and Interindividual Comparisons With Conventional CT Angiography OBJECTIVE. The objective of our study was to evaluate radiation dose and quantitative image quality parameters at high-pitch CT angiography (CTA) of the aorta compared with conventional CTA. MATERIALS AND METHODS. We studied the examinations of 110 patients (65 men and 45 women; mean age ± SD, 64 ± 15 years) who had undergone CTA of the entire aorta on a second-generation dual-source CT system; 50 examinations were performed in highpitch mode. The mean arterial attenuation, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and figure of merit (FOM) were calculated for the high-pitch CTA and conventional CTA groups. Radiation exposures were compared. RESULTS. All studies were considered of diagnostic quality. At high-pitch CTA, the mean tube voltage and tube current exposure time product were 118 ± 7 kv (SD) and 197 ± 78 mas compared with 120 ± 1 kv and 258 ± 78 mas, respectively, at conventional CTA (p < 0.05). The mean volume CT dose index, dose-length product, and effective dose were 8.1 ± 2.4 mgy, ± mgy cm, and 9.6 ± 3.0 msv at high-pitch CTA and 18.3 ± 7.7 mgy, ± mgy cm, and 19.8 ± 8.2 msv at conventional CTA (p < 0.001). Attenuation was similar for both protocols, whereas significantly less contrast medium was injected for high-pitch CTA than for standard-pitch CTA (87.3 ± 16 ml vs 97.9 ± 16 ml, respectively; p < 0.01). The SNR and CNR were significantly lower in the high-pitch CTA examinations (p < 0.01), whereas the FOM was nonsignificantly higher. Twenty patients underwent both high-pitch CTA and conventional CTA, with a 45% reduction in radiation dose (p < 0.001). CONCLUSION. High-pitch CTA of the aorta yields 45 50% reduction of radiation exposure as well as contrast medium savings with maintained vessel attenuation. T he technical advances of MDCT technology have enabled acquisition of image data about the entire torso within seconds. The clinical success of CT angiography (CTA) is based on precise synchronization of image acquisition with optimal vascular contrast medium attenuation, high spatial resolution, and fast image acquisition times [1, 2]. More detectors, shorter CT rotation times, and narrower section profiles have resulted in progressively shorter image acquisition times, improved volume coverage, and substantial improvements in image quality. Thus, MDCT studies can be performed 3 20 times faster than studies performed using the fastest single-detector CT systems [3, 4]. Speed is essential to limit motion artifacts by reducing the time required for the patient to hold his or her breath. The recently introduced second-generation dual-source CT (DSCT) system is equipped with a high-pitch data acquisition mode with pitch values of up to 3.4 [5]. With a highpitch dual-helical CTA technique, datasets of the thorax and abdomen can be acquired in less than 2 seconds [6] with low radiation dose and gapless coverage of the entire patient volume. However, imaging patients at high pitch values has raised questions about the subsequent increase in CT image noise [7] and the potential to outrun the contrast medium bolus, resulting in insufficient and therefore nondiagnostic vascular attenuation. In prior studies, investigators have reported the feasibility of high-pitch acquisitions of the thorax [6, 8] and abdomen [9]. To date, a direct comparison of thoracoabdominal CTA performed with a high-speed, high-pitch technique and CTA performed with conventional 1402 AJR:199, December 2012

2 High-Pitch Versus Standard-Pitch CTA of the Aorta techniques is limited to an initial report that did not correct for differences in patient-related factors [10]. We aimed to investigate with interindividual and intraindividual comparisons the quantitative image quality, imaging parameters, and potential radiation dose reduction of high-pitch CTA of the aorta compared with conventional CTA. Materials and Methods Subject Population In a retrospective search of the institution s radiology information system (RIS), 110 patients (65 men, 45 women; mean age ± SD, 64 ± 15 years) who had undergone thoracoabdominal CTA using second-generation DSCT between September 2009 (installation date) and March 2011 were identified. During the study period, 50 patients underwent high-pitch CTA with an identical imaging protocol. The remaining 60 patients underwent standard-pitch CTA, which was performed using an identical standardized imaging protocol. For patients who had undergone high-pitch CTA, the RIS was searched for prior CTA studies performed using a standard-pitch technique for intraindividual comparisons. If a patient had undergone more than one standard-pitch CTA examination, the study performed closest to the date of the highpitch CTA study was selected for comparison. In total, 20 of the 50 patients had previously (January 2008 March 2011) undergone standard-pitch CTA. The indications for thoracoabdominal CTA included suspected aortic syndrome (n = 12), postoperative follow-up after thoracoabdominal vascular surgery or endovascular aneurysm repair (n = 35), and follow-up of aneurysm (n = 37) or dissection (n = 20). Age, height, weight, and body mass index (BMI) were recorded for all patients. Because of the retrospective nature of the study protocol, the institutional ethics review board waived the need for informed consent. CT Procedures All studies were acquired using a second-generation DSCT system (Somatom Definition Flash, Siemens Healthcare). This system uses two tubedetector pairs with a mechanical offset of nearly 90 ; one detector covers a 50-cm FOV and the second detector covers a smaller, 33-cm, FOV. The following image acquisition parameters were used: two physical mm channel detector arrays, effective slice collimation of mm by using a z-flying focal spot technique, and 0.28-second gantry rotation time. The pitch was 3.4 for the high-pitch CTA examinations and 0.8 for the standard-pitch CTA examinations. Selection of the high-pitch versus the standard-pitch image acquisition protocol was at the discretion of the attending radiologist of the day. The acquired range covered the area above the aortic arch to the common iliac arteries. All patients had a cannula placed in the antecubital fossa. A dual-syringe injection system (Stellant D, Medrad) was used to administer 350 mg I/mL of iohexol (Omnipaque, GE Healthcare). Personalized contrast medium injection protocol software (P3T version 1.0, Medrad), customizing a triphasic injection protocol to each patient and procedure, was used for all 110 subjects. A test bolus was used to determine the circulation time of the contrast material. The test bolus was a 20-mL bolus of contrast medium followed by a 40-mL chaser bolus of saline solution administered at 5 ml/s. The personalized contrast medium injection protocol software uses patient weight, study duration, and con- Fig. 1 Examples of aortoiliac CT attenuation measurements in 55-year-old woman with suspected aortic syndrome. A, CT angiography images show exact locations of measurements in ascending aorta (white arrow), thoracic descending aorta at level of pulmonary trunk (blue arrow) and at level of diaphragm (yellow arrow), abdominal aorta at level of renal arteries (black arrow), and right iliac artery (red arrow). B, Three-dimensional volume-rendered image reconstruction shows excellent image quality at all described levels. AJR:199, December

3 Apfaltrer et al. trast medium concentration and accounts for the attenuation peak and time-to-peak of a timing bolus acquisition with an ROI placed in the ascending aorta. Studies were reconstructed at a section thickness of 1.5 mm with a 1-mm overlap using a vascular image reconstruction algorithm. The total amount of iodine injected and the iodine delivery rate were calculated using the contrast medium volume, contrast concentration, and injection rate. Radiation Dose Volume CT dose index (CTDI vol ) values were indicated in the dose report of the CT system provided for each CT study, and individual radiation dose was estimated using the dose-length product (DLP) given by the CT system. The effective radiation dose delivered at thoracoabdominal CTA was calculated with a method proposed by the European Working Group for Guidelines on Quality Criteria for CT [11]. Because a combination of chest, abdominal, and pelvic acquisitions was performed for thoracoabdominal CTA, the mean of these region-specific conversion coefficients (k = msv / mgy cm) was used as previously described [6, 12]. Qualitative and Quantitative Evaluations of Arterial Contrast Medium Attenuation A radiologist with 5 years of experience in CTA assessed the image quality for evaluating the aortoiliac system as diagnostic or nondiagnostic. The quantitative image analysis was performed on reconstructed transverse sections. Aortoiliac CT attenuation was calculated in Hounsfield units by measuring a circular region of interest (ROI) in the TABLE 1: Characteristics of Patients in the High-Pitch CT Angiography (CTA) and Standard-Pitch CTA Groups Characteristic High-Pitch CTA Group (n = 50) Standard-Pitch CTA Group (n = 60) p Sex, no. (%) of patients Male 29 (58) 36 (60) Female 21 (42) 24 (40) Age (y) a 61.1 ± ± 11.8 < 0.05 Height (cm) a ± ± 10.1 NS Weight (kg) a 81.0 ± ± 28.0 NS Body mass index (kg/m 2 ) a 26.9 ± ± 6.5 NS Indication for CTA, no. (%) of patients Suspected aortic syndrome 3 (6) 9 (15) Known aneurysm 14 (28) 23 (38) Known dissection 13 (26) 7 (12) Follow-up imaging after aortic repair or grafting 18 (36) 17 (28) Stenosis or atherosclerosis 1 (2) 3 (5) Other 1 (2) 1 (2) Note NS = not significant. a Mean ± SD. center of the vessel at the following seven different levels within the arterial system (Fig. 1): ascending aorta, aortic arch, thoracic descending aorta at the level of pulmonary trunk, thoracic descending aorta at the diaphragm, abdominal aorta at the level of the renal arteries, abdominal aorta above the bifurcation, and left and right common iliac arteries (both measurements averaged to one measurement, IlA). Each ROI was drawn as large as the vessel lumen diameter allowed while carefully avoiding atherosclerotic plaques of the vessel wall as previously described [13] (Fig. 1). Attempts were made to select ROI areas of at least 40 mm 2 for the aorta and at least 20 mm 2 for the iliac arteries. Mean arterial attenuation was calculated as the average of the mean attenuation values of the ROIs. At each of the described levels, an ROI was also placed in an adjacent muscle (Fig. 1D). Image noise was defined as the SD of the CT attenuation value of the adjacent muscle ROI [14]. Signal-to-noise ratio (SNR) was calculated as the mean attenuation of the artery divided by the image noise per level. Furthermore, the contrast-to-noise ratio (CNR) was calculated as the mean attenuation of the artery minus the mean attenuation of the muscle divided by image noise (mean HU artery mean HU muscle / image noise). SNR and CNR were calculated similar to previously published studies [15, 16] as follows: and SNR = HU vessel / noise CNR = (HU vessel HU muscle ) / noise. To account for differences in the radiation exposure of the two protocols, the CNR of each artery was normalized by effective dose (ED) using the figure of merit (FOM) as previously described [14]: FOM = CNR 2 / ED. Statistical Analysis Acquisition parameters, contrast medium parameters, quantitative image quality parameters, and radiation dose estimations were compared for high-pitch CTA and standard-pitch CTA. Continuous variables were expressed as means ± SD and categoric variables, as frequencies or percentages. Statistical significance of differences in continuous variables was assessed by an independent-samples Student t test for interindividual comparison and by a paired-samples Student t test for intraindividual comparison. For categoric variables, the chi-square test was used. The influence of BMI on image noise was evaluated by linear regression analysis. Statistical analyses were performed using commercially available software (SPSS, release 17, SPSS). All p values were two-sided and a p value of < 0.05 was considered statistically significant. Results Table 1 shows the characteristics of the study population. The majority of the study population was male (58%). Patients who had undergone high-pitch CTA were significantly younger than those who had undergone standard-pitch CTA (mean age ± SD, 61.1 ± 15.3 vs 67.1 ± 11.8 years, p < 0.05), but their BMI was nonsignificantly lower (26.9 ± 4.5 vs 29.1 ± 6.5 kg/m 2, respectively; p = not significant). The CT indications for both groups were very similar (p = not significant). The NS NS 1404 AJR:199, December 2012

4 High-Pitch Versus Standard-Pitch CTA of the Aorta TABLE 2: Scanning Parameters for High-Pitch CT Angiography (CTA) and Standard-Pitch CTA Scanning Parameter High-Pitch CTA Group (n = 50) Standard-Pitch CTA Group (n = 60) p Tube voltage setting, no. (%) of patients NS 80 kv 1 (2) 100 kv 2 (4) 120 kv 47 (94) 60 (100) mas ± 80.7 a ± 76.3 a < CTDI vol (mgy) 8.1 ± 2.4 a 18.3 ± 7.7 a < Scan length (cm) 69.0 ± 6.6 a 63.6 ± 5.8 a < DLP (mgy cm) ± a ± a < Effective dose (msv) 9.6 ± 3.0 a 19.8 ± 8.2 a < Contrast volume (ml) 87.3 ± 15.7 a 97.9 ± 15.6 a < 0.01 Iodine delivery rate (g/s) 1.67 ± 0.22 a 1.68 ± 0.19 a NS Iodine amount (g) 31.4 ± 6.2 a 34.3 ± 5.9 a < 0.05 Note NS = not significant, CTDI vol = CT dose index, DLP = dose-length product. a Mean ± SD. Fig. 2 Interindividual comparison: 67-yearold woman (A) who underwent CT angiography (CTA) for evaluation of aortic aneurysm and 58-yearold woman (B) who underwent CTA for evaluation of aortic dissection. A and B, Both conventional (A) and high-pitch (B) CTA images show excellent image quality. However, patient undergoing conventional CTA technique (A) received greater volume of contrast medium volume and radiation dose compared with patient undergoing high-pitch technique (B) (100 vs 85 ml, respectively; 202 vs 154 mas). tube current exposure time product and contrast medium volume were significantly lower in the high-pitch CTA cohort and the coverage length was somewhat longer (Table 2). The iodine delivery rate was similar for both cohorts. The CTDI vol, DLP, and ED were all significantly lower in the high-pitch CTA cohort than in the standard-pitch cohort. The ED in the high-pitch CTA group was 9.6 ± 3.0 msv (mean ± SD) versus 19.8 ± 8.2 msv in the standard-pitch CTA group (p < 0.001). All studies were considered of diagnostic quality for their respective indication. An example illustrating the comparative image quality of studies obtained in the high-pitch mode and those obtained in the standard-pitch mode is shown in Figure 2. The minimum arterial attenuation differed by level and ranged from 168 HU (for Ila) to 229 HU (for the thoracic descending aorta at the level of the pulmonary trunk) in high-pitch CTA studies and from 89 HU (for the abdominal aorta above the bifurcation) to 134 HU (for the aortic arch) in standardpitch CTA studies. The mean arterial attenuation at high-pitch CTA was, overall, similar to that of standard-pitch CTA (354 ± 95 vs 332 ± 61 HU, respectively; p > 0.05), although there was a trend toward higher attenuation of highpitch CTA studies (not significant). The noise level was significantly higher in the high-pitch CTA group (23.2 ± 6.6 vs 16.7 ± 2.9 HU; p < 0.001). The SNR and CNR were significantly lower at high-pitch CTA than standard-pitch CTA (16.4 ± 5.6 vs 20.7 ± 5.5 [p < 0.001] and 14.4 ± 5.0 vs 17.8 ± 5.2 [p < 0.01], respectively). However, the FOM, a measure of image quality independent of radiation dose, tended to be higher for high-pitch CTA studies than for the standard-pitch CTA studies (32.1 ± 50.3 vs 21.4 ± 15.2), although this difference did not reach statistical significance. The arterial attenuation, noise, SNR, CNR, and FOM are provided by level in Table 3. Of the patients who had undergone highpitch CTA, 20 had also undergone at least one standard-pitch CTA examination using DSCT (Table 4). The mean interval between the two studies was 353 ± 257 days (range, days). The use of the high-pitch acquisition technique reduced ED by 45 50% (p < 0.001) within the same patient. Although on average 20 ml less iodinated contrast medium and a significantly lower amount of iodine were injected during high-pitch CTA (30.0 ± 6.1 vs 36.4 ± 4.2 g; p < 0.01), differences in arterial attenuation were nonsignificant. However, noise levels were higher for high-pitch CTA than for standard-pitch CTA. An intraindividual comparison of a patient who underwent both high-pitch and standard-pitch CTA examinations is shown in Figure 3. Discussion The purpose of this study was to compare image quality and radiation dose estimates AJR:199, December

5 Apfaltrer et al. TABLE 3: Quantitative Image Quality Parameters of the High-Pitch CT Angiography (CTA) and Standard-Pitch CTA Groups, by Level Level Arterial Attenuation (HU) Noise (HU) SNR CNR FOM Ascending aorta High-pitch CTA 349 ± ± ± ± ± 33.6 Standard-pitch CTA 337 ± ± 3.4 a 20.8 ± 5.6 a 17.9 ± 5.1 a 20.5 ± 14.9 Aortic arch High-pitch CTA 366 ± ± ± ± ± Standard-pitch CTA 333 ± 60 b 14.8 ± 3.0 a 23.7 ± 7.3 a 20.4 ± 6.7 c 27.7 ± 23.1 Thoracic descending aorta at the level of the pulmonary trunk High-pitch CTA 351 ± ± ± ± ± 39.0 Standard-pitch CTA 332 ± ± 3.5 a 20.7 ± 6.1 a 17.8 ± 5.5 c 20.8 ± 15.2 Thoracic descending aorta at diaphragm High-pitch CTA 348 ± ± ± ± ± 70.6 Standard-pitch CTA 326 ± ± 3.7 a 21.4 ± 6.7 c 18.6 ± 6.1 b 23.2 ± 19.5 Abdominal aorta at the level of the renal arteries High-pitch CTA 352 ± ± ± ± ± 31.1 Standard-pitch CTA 329 ± ± 3.9 a 19.8 ± 6.0 a 16.9 ± 5.8 c 20.1 ± 21.3 Abdominal aorta above the bifurcation High-pitch CTA 356 ± ± ± ± ± 35.7 Standard-pitch CTA 333 ± ± 4.5 a 19.3 ± 6.4 c 16.2 ± 6.2 c 18.2 ± 13.6 Left and right common iliac arteries High-pitch CTA 358 ± ± ± ± ± 42.7 Standard-pitch CTA 334 ± ± 4.7 a 19.7 ± 6.7 c 16.6 ± 6.4 b 19.4 ± 16.1 Note Values are means ± SD. SNR = signal-to-noise ratio, CNR = contrast-to-noise ratio, FOM = figure of merit. a p < b p < c p < 0.01 of high-pitch CTA performed on a DSCT system of the entire aorta with standard-pitch CTA. Our results show that in comparison with the conventional technique, high-pitch thoracoabdominal CTA yields a radiation dose reduction of 45 50%, at maintained diagnostic quality. Also, a considerably lower amount of contrast medium was needed for the same arterial attenuation. Second-generation DSCT systems, which have double the number of detectors (and, hence, double the volume coverage) of firstgeneration DSCT systems, offer high-pitch or Flash mode acquisition at pitch values of up to 3.4 [5]. Increasing the pitch decreases the time necessary to cover the entire patient volume. This high-pitch mode has primarily been investigated for coronary CTA. In addition to limiting radiation exposure to a single cardiac cycle, the high-pitch mode eliminates overlapping volume coverage of sequential transverse sections, enabling coronary CTA examinations at EDs of less than 1 msv in suitable patients [17, 18]. However, the principle can be readily applied to thoracoabdominal CTA studies, where data are still limited. The potential application of this technique for the evaluation of the thoracoabdominal aorta is attractive because of the potential radiation dose reduction that can be achieved in patients with aortic disease who often need to undergo repeated CTA at regular intervals. In the current investigation all imaging studies were considered of diagnostic quality for their indication even though the noise level with high-pitch CTA was found to be higher. The maintained diagnostic quality of high-pitch CTA despite lower radiation dose observed here is in agreement with recent investigations evaluating high-pitch acquisition of the thorax [6] and abdomen [9] with diagnostic image quality in 83% and 100% of cases, respectively. Recently, Beeres et al. [10] showed significantly better subjective image quality using the high-pitch acquisition technique compared with conventional CTA. The investigators focused especially on the ability of CTA in imaging the aortic annulus, coronary arteries, and ascending aorta without motion artifacts. A higher aortic attenuation value was achieved with lower contrast medium injection volumes in the high-pitch CTA group than with the standard-pitch CTA group. However, the iodine delivery rates in grams per second were almost identical. One potential explanation for the differences in aortic attenuation favoring the high-pitch mode could be that the patients undergoing highpitch CTA had a lower body weight. However, a difference in body weight was not found in our study. The BMI of the patients who had undergone high-pitch CTA was only nonsignificantly lower than that of patients who had undergone standard-pitch CTA (26.9 ± 4.5 vs 29.1 ± 6.5 kg/m 2, respectively). Additionally, we specifically investigated a group of patients who had undergone both high-pitch and standard-pitch CTA to account for patient-related differences that could potentially (partly) explain differences in CT protocol, amount of 1406 AJR:199, December 2012

6 High-Pitch Versus Standard-Pitch CTA of the Aorta TABLE 4: Results of 20 Patients Who Underwent Both High-Pitch and Standard-Pitch CT Angiography (CTA) Parameter High-Pitch CTA Standard-Pitch CTA p Indication, no. (%) of patients < 0.05 Suspected aortic syndrome 1 (5) Known aneurysm 4 (20) 5 (25) Known dissection 8 (40) 7 (35) Follow-up imaging after aortic repair or grafting 8 (40) 7 (35) 120 kv 20 (100) 20 (100) NS Tube current exposure time product (mas) ± 61.7 a ± 56.3 a < 0.01 CTDI (mgy) 8.4 ± 1.6 a 16.5 ± 4.1 a < Scan length (cm) 69.0 ± 5.1 a 63.6 ± 6.5 a < 0.01 DLP (mgy cm) ± a ± a < Effective dose (msv) 10.0 ± 2.0 a 18.1 ± 5.1 a < Contrast volume (ml) 83.6 ± 15.3 a ± 12.5 a < 0.01 Iodine amount (g) 30.0 ± 6.1 a 36.4 ± 4.2 a < 0.01 Averaged arterial attenuation (HU) b 338 ± 57 a 316 ± 73 a NS Averaged image noise (HU) b 21.9 ± 5.7 a 15.8 ± 3.7 a < Averaged SNR b 16.4 ± 4.9 a 21.3 ± 6.8 a < 0.01 Averaged CNR b 14.2 ± 4.3 a 18.1 ± 6.5 a < 0.05 Averaged FOM b 24.2 ± 15.5 a 23.7 ± 16.2 a NS Note NS = not significant, CTDI = CT dose index, DLP = dose-length product, SNR = signal-to-noise ratio, CNR = contrast-to-noise ratio, FOM = figure of merit. a Mean ± SD. b Averaged over the seven measurement levels. Fig. 3 Intraindividual comparison of initial CT angiography (CTA) and repeated CTA for followup of ascending aortic aneurysm in 41-year-old woman. A and B, Both conventional standardpitch (A) and high-pitch (B) CTA images show excellent image quality. However, for standardpitch CTA, amount of contrast medium and radiation dose were higher than for highpitch CTA (130 vs 97 ml of contrast medium and 245 vs 152 mas, respectively). iodinated contrast material, and CT attenuation. The results for that group were in agreement with the findings for the overall cohort. Thus, we do not believe that differences in patient weight explain the difference in aortic CT attenuation in our study. Rather, one might speculate that shorter acquisition times combined with appropriate contrast medium bolus timing enable capturing the vascular system during a phase of higher, more concentrated intraluminal contrast medium attenuation, despite lower contrast medium volumes, than with slower image acquisition and possible greater dilution of intravascular contrast medium. In patients undergoing abdominal, pelvic, or abdominopelvic imaging, Sahani et al. [19] reported similar image quality using high-pitch or low-pitch technique. Factors affected by the pitch include effective section thickness, image noise, beam-hardening artifacts, image acquisition speed, and radiation dose [19, 20]. Lowering the pitch reduces the slice-broadening effect, minimizes helical artifacts, and lowers image noise [21, 22]. These reported results correlate with our results because we also found significantly higher SNR and CNR in the conventional low-pitch CTA group (p < 0.01). However, lowering the pitch also increases image acquisition time and radiation dose [19]. To account for differences in the radiation exposure of the two protocols, the CNR of each arterial segment and the overall CNR were normalized by the ED using the FOM as previously described [14]. Although the CNR with highpitch CTA was significantly lower than that with standard-pitch CTA, the FOM was higher with high-pitch CTA at all arterial levels but these differences did not reach statistical significance. Thus, one might argue that high-pitch AJR:199, December

7 Apfaltrer et al. CTA has the tendency to yield higher image quality per unit of radiation exposure. A slight increase in image noise may be acceptable as long as diagnostic information is not compromised, in keeping with the as low as reasonably achievable (ALARA) principle. The high-pitch acquisition technique resulted in significant reductions of CTDI vol, DLP, and ED compared with the conventional CTA technique (p < 0.001); these results are in line with recently published data on the high-pitch acquisition technique [10]. This was true although no significant difference in mean tube voltage and mean BMI of both investigated groups was found (both p > 0.05). Despite these similar patient cohorts, the mean tube current exposure time product for the high-pitch CTA studies was significantly lower compared with that of the conventional CTA studies. However, we think that the effect of this reduction in tube current time product does not solely explain the radiation dose reduction of up to 45 50% achieved with the high-pitch technique; rather, we believe that an interaction of various attributes of this new acquisition technique is responsible for the dose reduction. Effective reduction of patient exposure of ionized radiation is of particular importance in patients with aortic disease who tend to undergo regular follow-up CT studies for disease surveillance and often incur high cumulative radiation doses. In our patient cohort, 20 individuals had undergone at least one high-pitch CTA and one conventional CTA examination within a mean of about 1 year for surveillance of aneurysms, dissections, or postaortic repairs. By using the high-pitch acquisition techniques, radiation doses could be reduced by up to 45% (p < 0.001) within the same patient while differences in arterial attenuation were nonsignificant. Because the potential detrimental effects of radiation are assumed to be linearly related to both dose and frequency of exposure [23, 24], low radiation techniques such as the high-pitch CTA method investigated here should be performed whenever feasible especially in patients who require frequent follow-up. In addition, high-pitch CTA allows an average contrast medium savings of ml, which may be relevant in patients with decreased renal function [25, 26] and patients with cardiovascular compromise who are at risk for volume overload. Equal CT attenuation values for high-pitch CTA and standard-pitch CTA could potentially have been obtained by further lowering the amount of iodinated contrast medium injected. However, because the noise level is higher in high-pitch CTA due to the reduced radiation dose, suboptimal diagnostic quality could potentially have occurred. Thus, in the absence of further prospective investigations in this field, we do not advocate lowering the amount of iodinated contrast medium even further for high-pitch CTA. In a previous study of abdominopelvic CT, investigators described image quality for highpitch acquisitions in patients weighing more than 90 kg as unacceptable [19]. Nevertheless, in our study, a quarter of the patients who underwent high-pitch CTA had a BMI of greater than 30 kg/m 2 and nearly a third weighed more than 90 kg. This divergence in observations is likely related to the use of more mature current technology in our study. If, however, these obese patients had been excluded from our analysis, the difference in radiation dose would have been even more pronounced. Our study is limited by its retrospective nature. Also, we decided not to perform a subjective image quality evaluation but chose to rely on objective parameters of image quality. However, a radiologist considered all 110 studies of diagnostic quality for their indication. Finally, in this study we did not investigate the effect of more recent iterative image reconstruction techniques, which may improve the image noise characteristics as well as subjective and objective image quality. In conclusion, the use of the high-pitch technique for thoracoabdominal CTA substantially reduces radiation dose and contrast medium volume while maintaining arterial contrast medium attenuation. Given the recent focus on the importance to image wisely, we have since adjusted our clinical practice and we advocate the preferential use of this high-pitch acquisition technique especially in patients who undergo frequent follow-up CT studies for aortic disease surveillance. References 1. Holalkere NS, Matthes K, Kalva SP, et al. 64-Slice multidetector row CT angiography of the abdomen: comparison of low versus high concentration iodinated contrast media in a porcine model. 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