Original Research
Vascular and Interventional Radiology
October 5, 2017

Whole-Body High-Pitch CT Angiography: Strategies to Reduce Radiation Dose and Contrast Volume

Abstract

OBJECTIVE. The purpose of this study was to assess the noninferiority of dual-source high-pitch CT angiography (CTA) performed with high-concentration (iopamidol 370) low-volume (60 mL) iodinated contrast material at low voltage (100 kVp) in comparison with dual-source high-pitch CTA with standard-of-care low-concentration (iopamidol 300) standard-volume (75 mL) iodinated contrast material at high voltage (120 kVp) to determine whether use of the high-concentration low-volume method would afford a reduction in radiation dose and contrast volume without negatively affecting vascular opacification.
SUBJECTS AND METHODS. This study had three arms. A phantom was used to assess vascular contrast enhancement at different iodine and saline solution dilutions with iopamidol 300 or 370 to compare lower-iodination (iopamidol 300) high-voltage (120 kVp) high-pitch (120 kVp, 250 mAs) imaging with higher-iodination (iopamidol 370) low-voltage (100 kVp) high-pitch (100 kVp, 100–240 mAs) acquisition. Metal-oxide-semiconductor field-effect transistors were placed in an anthropomorphic phantom to extract organ-based radiation profiles, and ANOVA was performed. The study prospectively enrolled 150 patients: 50 patients received 75 mL iopamidol 300, and image acquisition was performed at 120 kVp and 250 mAs; 50 patients received 75 mL iopamidol 370, and acquisition was performed at 100 kVp and 240 mAs; and 50 patients received 60 mL iopamidol, and acquisition was performed at 370 at 100 kVp and 240 mAs. Vascular signal-to-noise ratio was evaluated at 18 anatomic locations. Longitudinal signal-to-noise ratio was used to assess homogeneity of contrast enhancement. Size-specific dose estimates were calculated. Statistical analyses were performed by ANOVA.
RESULTS. Noninferiority of high-concentration (iopamidol 370) low-voltage (100 kVp) high-pitch acquisitions compared with low-concentration (iopamidol 300) high-voltage (120 kVp) high-pitch acquisition was achieved at 170 mAs in vitro. Radiation assessment showed significant decreases in radiation dose for the 100-kVp 240-mAs protocol (p < 0.0001). Noninferior vascular contrast (p > 0.280) and luminal homogeneity (p > 0.191) were found for all high-pitch protocols. Significantly decreased radiation dose was observed for the two groups that received 60 and 75 mL of iopamidol 370 at 100 kVp and 240 mAs (p < 0.0001).
CONCLUSION. Dual-source high-pitch CTA with high-concentration (iopamidol 370) low-volume (60 mL) iodinated contrast medium and low-voltage acquisition (100 kVp) is noninferior to dual-source high-pitch CTA with low-concentration (iopamidol 300) standard-volume (75 mL) iodinated contrast material at high voltage (120 kVp) and affords simultaneous reduction in radiation dose and contrast volume without negatively affecting vascular contrast enhancement.
CT angiography (CTA) is well known to be an excellent method of assessment of a wide spectrum of diseases involving the vascular system. Many treatment decisions are based on imaging results [1, 2]. Guided by the core idea of personalized medicine, the aim of which is to adapt the needs of clinical examinations to the characteristics of individual patients, custom-tailored imaging protocols have become the clinical hallmark of a patient-centered approach in radiologic practice [3]. The many options for adjusting the imaging parameters of CTA studies are based on patient characteristics, such as injected iodine dose, injection rate, and type of injection protocol; x-ray tube voltage and amperage combination; and the CT acquisition technique itself [4]. Particularly important for patients who need serial examinations to evaluate vascular diseases and treatment successes longitudinally over time are reproducibility and adequate quality of vascular opacification, which have to be balanced with patient safety, specifically minimal radiation exposure and contrast administration.
The use of a dual-source high-pitch acquisition scheme has been found to reduce motion artifact without a radiation dose penalty compared with single-source standard-pitch and ECG-gated acquisitions [4, 5]. The challenge for dual-source imaging is the rapidity of acquisition, in particular the resulting limited total imparted x-ray photon energy, specifically representing an issue in imaging of larger patients or use of low peak kilovoltage levels [4]. Because of the rapid acquisition scheme, not only delivery of sufficient photon counts at the detector level but also precise shaping and timing of the contrast bolus for optimal vascular contrast enhancement remains a substantial challenge. Although the injection rate predominantly determines the level of vascular opacification, the volume of injected contrast material has to be sufficient to homogeneously opacify the entire vascular lumen [6]. These challenges are of particular interest because previous studies have shown that use of a higher iodine concentration of contrast media in combination with low x-ray tube voltage acquisition techniques represents a valid method of reducing both the amount of contrast medium and radiation exposure without negatively affecting image quality in CTA for limited z-axis coverage [7, 8]. To prevent escalation of injected iodine through use of higher-iodination contrast materials, a reduction in overall contrast volume has to be considered. The optimization of dual-source high-pitch CTA performed with lower x-ray voltages and different concentrations and volumes of contrast material is the focus of this investigation.
In this study we sought to assess opacification of the entire thoracoabdominal vasculature by implementing a dual-source high-pitch CTA technique entailing a high-iodine concentration (iopamidol 370) and low contrast volume (60 mL) with image acquisition at low x-ray tube voltage (100 kVp) over extended z-axis coverage. The study comprised an in vitro contrast phantom arm, an anthropomorphic radiation phantom arm, and an in vivo prospective protocol implementation arm with three patient subpopulations. The study was designed to test the hypothesis that dual-source high-pitch CTA with high-concentration (iopamidol 370) low-volume (60 mL) iodinated contrast media at low voltage (100 kVp) is noninferior to dual-source high-pitch standard-of-care low-concentration (iopamidol 300) standard-volume (75 mL) iodinated contrast media at high voltage (120 kVp), potentially affording simultaneous reduction in radiation exposure and contrast volume without escalating the injected iodine dose or negatively affecting vascular contrast enhancement.

Subjects and Methods

This single-center HIPAA-compliant prospective study received institutional review board approval.

In Vitro Contrast Phantom

A custom-built cylindric phantom designed to simulate adult patient sizes was used. The phantom was made of homogeneous polymer acrylic materials mimicking the absorption and scatter characteristics of nonenhancing soft tissues at diagnostic x-ray energies and measured 15 cm in cross-sectional diameter. It contained cylindric tunnels for the placement of air-tight sealed vials positioned 7 cm radially from the isocenter of the phantom [9]. Two sets of dilution series containing 20 dilution steps each, mixing iodine (iopamidol 300 mg I/mL in the first set and iopamidol 370 mg I/mL in the second set [Isovue, Bracco Diagnostics]) with saline solution (0.9% sodium chloride), covered a mixing range of 1:4–1:100. This titration reproduced in vivo levels of vascular iodine concentration and resultant CT attenuation of the aorta in conventional 120-kVp contrast-enhanced CT examinations with iopamidol 300 mg I/mL (range, 50–400 HU) [9]. The iodine–to–contrast agent dilution series were injected into air-tight sealed vials with the following specifications: diameter, 10 mm; wall thickness, 0.75 mm; luminal diameter, 8.5 mm; maximum capacity, 5 mL. Residual air was flushed from the vials with an air-cushion pipette. Vials were placed sequentially into the tunnels of the phantom for imaging.
Initially, the phantom was loaded with the dilution series of standard-of-care iopamidol 300 mg I/mL and positioned at the isocenter of the gantry with its cross-sections perpendicular to the z-axis. Anteroposterior and lateral digital scout radiographs were obtained to define an adequate scan range. All dual-source high-pitch MDCT examinations for this study were performed with a dual-source MDCT scanner (Somatom Definition Flash, Siemens Healthcare). The system incorporated two x-ray tubes and two corresponding 128-MDCT arrays mounted in a perpendicular configuration. For the dual-source high-pitch MDCT acquisitions, a clinical imaging protocol with an x-ray tube voltage of 120 kVp and effective reference x-ray tube current of 250 mAs, gantry rotation time of 0.5 seconds and pitch of 3.0 were applied. For phantom imaging, the real-time angular dose modulation for the x-ray tube current (Care Dose4D, Siemens Healthcare) was deactivated. All craniocaudal MDCT studies were reconstructed at 1-mm thickness with 0.5-mm reconstruction increment by means of hybrid iterative reconstruction (Sinogram Affirmed Iterative Reconstruction [SAFIRE], Siemens Healthcare) and soft-tissue kernel I31f.
The phantom was loaded with the dilution series with the identical mixing ratio but based on the iopamidol 370 mg I/mL contrast agent, and imaging was repeated. For imaging with high-concentration iopamidol, the x-ray tube voltage was decreased to 100 kVp and the x-ray tube current sequentially increased from 100 to 240 mAs in 20-mAs increments. The other imaging and reconstruction parameters remained identical to the low-concentration iopamidol acquisition.
Cloned 0.5-cm2 circular ROIs were placed over each vial cross-section, and resultant signal-to-noise ratios (SNRs) were calculated. Each ROI encompassed three-fourths of the lumen of the vial and was carefully drawn to avoid the wall of the vial. To ensure consistency, all measurements were performed on three consecutive images along the long axis of the vial, and SNR values were averaged. A board-certified radiologist with more than 4 years of experience in image postprocessing performed all measurements [4]. All SNR assessment in this study was performed with the cardiovascular application of commercially available postprocessing software (syngo.via version 3.0, Siemens Healthcare).

Anthropomorphic Radiation Phantom

The radiation dose assessment was performed before in vivo study initiation comparing organ-based radiation dose profiles of dual-source high-pitch CTA at high-voltage (120 kVp) imaging settings with the new acquisition technique at low-voltage (100 kVp) imaging settings. Metal-oxide-semiconductor field-effect transistor (MOSFET) sensors were placed in an anthropomorphic phantom, which can be configured as either male or female, depending on the location of the gonad radiation sensors and installation of breast tissue with corresponding radiation sensors. Subsequent dual-source high-pitch MDCT acquisitions were performed with imaging parameters identical to those in the in vitro contrast phantom with the single difference that the real-time angular dose modulation for the x-ray tube current (Care Dose4D) was and remained activated for imaging of the anthropomorphic radiation phantom and the in vivo protocol implementation. Activating the automatic exposure control (AEC), the scanner adapted the tube current in real time according to the detector-level attenuation at any projection angle, the anatomic region, and patient size. To ensure comparable image contrast across the patient population, a fixed quality reference tube current–time setting was selected according to imaging protocol, which subsequently was adapted to actual projection angle, imaged anatomic region, and patient size. The quality reference tube current–time setting equaled the tube current–time product divided by the spiral pitch value. Depending on the strength of the AEC, the actual applied tube current was either increased or decreased. For the AEC strength of average used in this study, a reduction or amplification of approximately ± 30% of selected quality reference tube current–time setting dependent on projection angle, imaged anatomic region, and patient size can occur [10].
MOSFET detectors were placed to analyze organ absorbed doses in the thyroid, lungs, esophagus, liver, stomach, colon, red bone marrow within vertebral and pelvic skeleton, and gonads and at multiple locations at the skin level. The female anthropomorphic phantom also included organ dose measurements of breast tissue. The data were measured a total of three times and measurement results averaged.

In Vivo Protocol Implementation

Between June 2014 and June 2015, 150 patients referred from the department of cardiothoracic surgery for contrast-enhanced MDCTA of the thoracoabdominal aorta were enrolled in this prospective cohort study. All patients provided written informed consent. Patients were not eligible for this prospective study if they were allergic to iodinated contrast material, pregnant or breast-feeding, younger than 18 years, or had a serum creatine level of 2.0 mg/dL or greater. No exclusion parameters were based on habitus.
Enrolled patients were randomly distributed into three cohorts of 50 patients each with a predefined distribution key. The cohort sizes were determined by Multiple-Sample Non-Inferiority power analysis. Previous studies of differences in arterial vascular enhancement of various anatomic regions in MDCT examinations between low iodine content and higher iodine content contrast media showed increased vascular contrast enhancement of 5–14% [1113]. Under the assumption that the lowest level of enhancement was 5% (at least 80% accuracy) by iopamidol 370 compared with the standard-of-care iopamidol 300, the minimum sample size was determined to be 50 and power greater than 0.99; sample size calculations were based on type II error of α = 0.05.
The three patient cohorts characterized by demographics were imaged with the following acquisition and contrast administration protocols. Group A included 50 patients (31 men, 19 women; mean age, 67.5 ± 9.9 [SD] years; range, 50–87 years; body mass index, 26.2 ± 4.7) who received 75 mL of iopamidol 300 (300 mg I/mL) and were imaged at 120 kVp, 250 mAs, and a pitch of 3.0. Group B included 50 patients (31 men, 19 women; mean age, 69.8 ± 11.3 years; range, 40–95 years; BMI, 26.2 ± 3) who received 75 mL of iopamidol 370 (370 mg I/mL) and were imaged at 100 kVp and a pitch of 3.0. The quality reference tube current–time setting (240 mAs) in this group was based on contrast analysis of the in vitro contrast phantom plus a 30% tube current increase to counteract potential effects from the AEC to ensure that the predetermined tube current–time floor was not undercut. Group C included 50 patients (23 men, 27 women; mean age, 68.4 ± 13.5 years; range, 24–90 years; BMI, 26.0 ± 3.1) who received 60 mL of iopamidol 370 (370 mg I/mL) and were imaged at 100 kVp and a pitch of 3.0. The quality reference tube current–time setting (240 mAs) was determined analogously and was identical to that of group B.
For all patients, individual contrast bolus tracking was performed during repetitive low-dose acquisitions at 120 kVp and 40 mAs. A threshold ROI was placed within the descending aorta at the level of the diaphragm. Attenuation and contrast wash-in were plotted to a level of 100 HU after administration of the group-specific contrast agent and volume injected at 4 mL/s chased by 50 mL of normal saline solution injected at the same rate into a right antecubital vein with a CTA dual injector after a preprogrammed constant diagnostic delay of 9 seconds. The diagnostic craniocaudal MDCT study with a collimation of 128 × 0.6 mm and a gantry rotation time of 0.5 second was automatically initiated with group-specific acquisition parameters. Real-time angular dose modulation for the x-ray tube current (Care Dose4D) was used. Volume CT dose index (CTDIvol) and dose-length product were documented, and size-specific dose estimates (SSDEs) were calculated.
All craniocaudal MDCT studies were reconstructed at 1-mm thickness with 0.5-mm reconstruction increment by use of a hybrid iterative reconstruction SAFIRE (level 2) soft-tissue kernel I31f.
The mean segmental vascular SNR for assessing vascular enhancement was evaluated by placement of luminal ROIs on axis-corrected axial reformations based on automatically defined vessel center-lines at these distinct levels. Placement encompassed the entire inner vessel lumen but carefully avoided the following vessel wall and potential atherosclerotic calcifications [4]: aortic annulus; sinuses of Valsalva (aortic sinuses); ascending aorta 4 cm distal to the aortic annulus; aortic arch at the level of the left subclavian artery takeoff; descending aorta at the level of the left atrium; descending aorta at the level of the diaphragmatic hiatus; infrarenal aorta at the level of the lowest renal artery takeoff; infrarenal aorta just above the aortic bifurcation; and the main branches (celiac trunk 1 cm distal to the orifice, superior mesenteric artery 1 cm distal to the orifice, common iliac arteries bilaterally 1 cm distal to the aortic bifurcation, external iliac arteries bilaterally 1 cm distal to the iliac bifurcations, and common femoral arteries bilaterally 1 cm distal to their takeoff).
The mean longitudinal vascular SNR for assessment of homogeneity of contrast distribution along the aortic, pelvic, and femoral vasculature was measured by calculation of the coefficient of variance by extracting attenuation values along the previously generated vessel centerlines and forming the ratio of SD to the mean [4, 14].
Analogously to in vitro assessments, all in vivo measurements were performed three times, and SNR and coefficient of variance values were averaged. All in vivo measurements were performed by a radiology fellow.

Statistical Analysis

In vitro contrast phantom—SNR values achieved with the standard-of-care iopamidol 300 mg I/mL dilution series at 120 kVp and 250 mAs were sequentially compared with the SNR values based on iopamidol 370 mg I/mL dilution series with the identical mixing ratio at 100 kVp and various x-ray tube currents ranging from 100 to 240 mAs. For every increment in x-ray tube current, the difference in SNR between the iopamidol 370 dilution series and the standard-of-care iopamidol 300 dilution series was calculated. The x-ray tube current that resulted in no measurable SNR difference between 100-kVp imaging of iopamidol 370 mg compared with the standard-of-care 120-kVp imaging at 250 mAs with iopamidol 300 mg I/mL was defined as the lowest x-ray tube current that ensured noninferior vascular contrast enhancement.
Anthropomorphic phantom—Organ absorbed radiation dose was assessed by univariate ANOVA with Bonferroni correction. Organ absorbed radiation dose (in milligray) was the dependent factor, and imaging protocol (120 kVp at 250 mAs versus 100 kVp at 240 mAs) was the independent factor. Specific organ was the cofactor, and sex was the fixed factor. Bonferroni correction was performed for the fixed factor only.
In vivo protocol implementation—Statistical analyses of segmental vascular contrast, longitudinal homogeneity, and actually applied radiation dose expressed as SSDE and CTDIvol were performed by ANOVA with Bonferroni correction. Vascular contrast (SNR), luminal homogeneity (coefficient of variance), and radiation dose estimate (SSDE) and CTDIvol were the dependent factors, and imaging protocol (75 mL of iopamidol 300 mg I/mL at 120 kVp and 250 mAs vs 75 and 60 mL of iopamidol 370 mg I/mL at 100 kVp) was the independent factor. For statistical assessment of vascular contrast and luminal homogeneity, the specific organ (segmental cross-sectional location and longitudinal vascular location) was the additional cofactor [15].

Results

In Vitro Contrast Phantom

Noninferiority of the high-pitch high-concentration (iopamidol 370) low-voltage (100 kVp) acquisition protocol under investigation compared with the high-pitch standard-of-care low-concentration (iopamidol 300) high-voltage (120 kVp) imaging protocol across the entire dilution series was realized at 170 mAs in the in vitro contrast phantom (Fig. 1). For phantom imaging, the real-time angular dose modulation of the x-ray tube current was deactivated. To ensure that this determined tube current–time setting floor was not undercut in the protocol implementation with activated angular dose modulation, the reference tube current–time product for the in vivo component was set to 240 mAs (240 mAs − 30% ≈ 170 mAs) (Fig. 1).
Fig. 1 —Signal-to-noise ratio (SNR) comparison of protocols. Graph on left shows SNR achieved with iopamidol 300 at 120 kVp and 250 mAs (dashed line) at different mixing ratios of contrast medium to saline solution in comparison with iopamidol 370 at 100 kVp and 100–240 mAs (solid lines) at same mixing ratios. Graph on right shows difference in SNR between imaging with iopamidol 300 and iopamidol 370 (iopamidol 370 at 120 kVp and 250 mAs) for every increment in x-ray tube current of 100-kVp protocol. Noninferiority of high-pitch high-concentration low-voltage protocol compared with high-pitch low-concentration high-voltage protocol across entire dilution series was realized at 170 mAs.

Anthropomorphic Radiation Phantom

Organ absorbed radiation dose (in milligray) assessed by ANOVA with Bonferroni correction decreased significantly with the dual-source high-pitch 100-kVp 240-mAs imaging protocol compared with the standard-of-care dual-source high-pitch 120-kVp 250-mAs acquisition (p < 0.0001). Sex and the organ gonads were significant cofactors (p < 0.0001) (Fig. 2).
Fig. 2 —Chart shows results of organ absorbed dose assessment performed in anthropomorphic radiation phantom with metal-oxide-semiconductor field-effect transistor (MOSFET) sensors, located throughout phantom. Substantial decrease in organ absorbed radiation dose for dual-source high-pitch 100-kVp 240-mAs imaging protocol compared with standard-of-care dual-source high-pitch 120-kVp 250-mAs acquisition was found.

In Vivo Protocol Implementation

In the assessment of SNR, noninferior vascular contrast (p > 0.280) and noninferior luminal contrast homogeneity (p > 0.191) were detected for both lowkilovoltage protocols compared with the standard 120-kVp protocol, and the organ cofactor was found not to be statistically significant (Tables 13 and Fig. 3). Patients who received 75 mL of iopamidol 300 and were imaged at 120 kVp and 250 mAs had a mean vascular CT attenuation of 289.7 ± 23.8 HU. Patients who were imaged at 100 kVp and 240 mAs and received either 75 or 60 mL of iopamidol 370 had a mean vascular CT contrast enhancement of 400.9 ± 31.1 HU for 75 mL and 393.9 ± 28.1 HU for 60 mL.
TABLE 1: Demographic Characteristics and Radiation Dose Measurements
CharacteristicGroup AGroup BGroup C
Population size (no. of patients)505050
Age (y)67.5 ± 9.969.8 ± 11.368.4 ± 13.5
Sex (no.)   
 Men313123
 Women191927
Body mass indexa26.2 ± 4.726.2 ± 3.326.± 3.1
Protocol75 mL of 300 mg I/mL contrast agent; 120 kVp; 250 mAs; Flash scanner (Siemens Healthcare); pitch, 3.075 mL of 370 mg I/mL contrast agent; 100 kVp; 240 mAs; Flash scanner (Siemens Healthcare); pitch, 3.060 mL of 370 mg I/mL contrast agent; 100 kVp; 240 mAs; Flash scanner (Siemens Healthcare); pitch, 3.0
DLP (mGy × cm)1217.2 ± 230.1878.8 ± 190.5876.3 ± 242.0
CTDIvol (mGy)20.3 ± 3.814.6 ± 3.214.7 ± 4.1
SSDE (mGy)35.1 ± 6.225.4 ± 5.325.5 ± 7.0

Note—Except where otherwise indicated, data are mean ± SD. DLP = dose length product, CTDIvol = volume CT dose index, SSDE = size-specific dose estimate.

a
Weight in kilograms divided by the square of height in meters.
TABLE 2: Cross-Sectional Contrast Enhancement: Results of Signal-to-Noise Assessment
Anatomic LocationGroup AGroup BGroup C
Signal-to-Noise RatioArea (mm2)Signal-to-Noise RatioArea (mm2)Signal-to-Noise RatioArea (mm2)
MeanSDMinimumMaximumMeanSDMeanSDMinimumMaximumMeanSDMeanSDMaximumMeanMeanSD
Annulus12.24.74.527.74.52.713.93.83.826.53.11.314.15.64.030.22.80.9
Sinus of Valsalva (aortic sinus)15.14.66.728.07.32.515.93.94.227.26.82.315.45.54.431.05.92.2
Aorta                  
 Ascending17.05.86.531.97.82.817.64.55.029.56.82.317.66.15.432.07.2.7
 Subclavian artery17.06.07.035.65.52.218.35.44.437.45.11.919.97.68.146.74.82.3
 Descending13.74.54.525.24.41.813.84.23.624.65.2.115.85.86.433.44.22.1
 Hiatus12.04.84.323.53.51.911.83.54.321.24.1.913.86.34.237.23.51.6
  Celiac trunk13.06.23.929.80.30.214.16.23.445.50.30.114.06.53.629.30.30.4
  Superior mesenteric artery13.36.03.832.40.30.114.46.14.138.20.30.115.17.35.649.80.20.1
 Infrarenal12.35.13.825.32.1.212.24.15.527.22.11.114.35.95.127.11.91.
 Bifurcation13.55.93.732.61.51.113.04.86.029.41.50.715.46.65.332.71.51.1
Right iliac artery13.25.64.125.40.60.513.04.75.031.30.50.314.76.65.032.90.50.3
Right external iliac artery13.15.94.631.80.70.512.94.45.428.80.60.315.05.83.928.90.40.2
Right femoral artery11.84.74.522.40.40.212.94.24.623.00.30.214.95.14.430.00.30.2
Left iliac artery12.16.23.732.70.40.212.95.24.131.00.30.214.46.13.936.40.30.2
Left external iliac artery13.16.14.929.80.40.213.54.53.626.00.30.116.76.96.239.20.30.2
Left femoral artery13.56.05.439.70.30.213.34.63.823.50.30.217.08.44.246.60.30.2
TABLE 3: Longitudinal Contrast Enhancement: Coefficient of Variation
LocationGroup AGroup BGroup C
Aorta0.095 ± 0.0470.093 ± 0.0420.100 ± 0.037
Iliac arteries   
 Right0.117 ± 0.0660.136 ± 0.0670.116 ± 0.062
 Left0.126 ± 0.0960.145 ± 0.0730.106 ± 0.054
Entire vascular system0.113 ± 0.0730.124 ± 0.0630.107 ± 0.059

Note—Values are mean ± SD.

Fig. 3 —75-year-old man who underwent imaging with all three imaging and contrast administration protocols. Signal-to-noise ratio (SNR) comparison of low-iodine-concentration high-tube-voltage acquisition versus high-iodine-concentration low-tube voltage high-pitch acquisition with additional reduction in contrast volume. Chart and MDCT angiographic images show vascular contrast enhancement in thoracoabdominal vasculature. Throughout patient population, noninferior vascular contrast enhancement was observed for two protocols with decreased radiation dose and with decreased radiation dose and decreased contrast volume. In this example, opposite was noted: increase in vascular SNR transitioning from standard-of-care protocol to decreased radiation dose protocol (left) and no substantial decrease in vascular contrast enhancement after reduction in contrast volume (right). Group A = 75 mL of 300 mg I/mL contrast agent, 120 kVp, 250 mAs; group B = 75 mL of 370 mg I/mL contrast agent, 100 kVp, 240 mAs; group C = 60 mL of 370 mg I/mL, 100 kVp, 240 mAs. SoV = sinus of Valsalva (aortic sinus), Asc = ascending, Desc = descending.
As in the anthropomorphic phantom assessment, a significant decrease in in vivo estimated radiation dose expressed as SSDE and CTDIvol was observed for the newly incorporated imaging protocols of 75 mL of iopamidol 370 at 100 kVp and 240 mAs (25.4 ± 5.3 mGy SSDE and 14.6 ± 3.2 mGy CTDIvol) or 60 mL of iopamidol 370 imaged at 100 kVp and 240 mAs (25.5 ± 7.0 mGy SSDE and 14.7 ± 4.1 mGy CTDIvol) compared with the standard-of-care protocol of 120 kVp and 250 mAs, resulting in 35.1 ± 6.2 mGy SSDE and 20.3 ± 3.8 mGy CTDIvol (p < 0.0001).

Discussion

In routine clinical practice CTA has become a commonly performed examination to detect causes of acute chest pain, because a wide spectrum of severe and life-threatening abnormalities of the aorta mandate early and rapid diagnosis [1]. Accurate characterization of the extent of dissection or intramural hematoma and precise axis-corrected diametric and craniocaudal extension measurements are imaging biomarkers of paramount importance because they can alter conservative or surgical management decisions [16]. The main concerns about the technique, in particular for serial examinations, are the radiation dose used and eventually the accumulated radiation dose and the amount of contrast medium administered, given that the overall amount of injected iodine is a contributing risk factor in the development of contrast media–induced nephropathy [17].
Dual-source MDCT technology allows high-pitch scanning owing to the unique perpendicular configuration of the two detector banks within the gantry [18]. For single-source CT, the pitch factor—defined as the distance of table travel per rotation divided by the collimation of the x-ray beam—is effectively limited to less than 1.5 because higher pitch values would result in substantial data acquisition gaps [19, 20]. Dual-source CT technology essentially overcomes this limitation because a second x-ray source and detector array are used to acquire data with a quarter-rotation offset. After the rebinning of source data, which principally converts the x-ray beam configuration from a fan-beam to parallel-beam geometry, virtualization of the two individual detector banks into one larger detector bank, which results in a single raw dataset, is performed by filling the data sampling gaps resulting from the high-pitch acquisition of the first detector bank with data from the perpendicularly located second detector bank.
Dual-source high-pitch technique allows scanning at a pitch of up to 3.4 on second-generation dual-source CT scanners with interspersed data acquisition and reconstruction. Use of the dual-source high-pitch technique may decrease cardiac and respiratory motion, particularly in imaging of the thoracic aorta, because of improved temporal resolution and faster overall acquisition times [21, 22]. In addition to reduced scan times, it has been found that dual-source high-pitch technology may also result in lower radiation dose to patients than occurs with use of single-source scanners [23].
The results of our study show that with high-pitch acquisition with high-concentration (iopamidol 370) low-volume (60 mL) iodinated contrast medium at low voltage (100 kVp), the entire thoracic, abdominal, and pelvic arterial vasculature can be visualized without escalating the administered iodine dose. This effect occurs through reduction in administered contrast volume (≈ 11% reduction in contrast volume, similar iodine dosage) and lower radiation exposure (≈ 28% reduction in dose-length product, CTDIvol, and SSDE) without deterioration in image quality compared with dual-source high-pitch low-concentration (iopamidol 300) standard-volume (75 mL) imaging with iodinated contrast medium at high voltage (120 kVp).
Previous studies have shown individually that, on the one hand, the dual-source high-pitch technique has the potential to improve image quality by reducing cardiac and respiratory motion artifacts and to lower the radiation dose to patients compared with results achieved with single-source scanners. They have also shown that use of higher-iodination contrast material is beneficial for vascular contrast opacification. Our study, however, showed in a prospective manner that the performance of the dual-source high-pitch mode of scanning in combination with administration of higher-iodination contrast material improves patient safety through reduction of radiation dose without escalating overall iodine dose [2123].
Homogeneous contrast enhancement of the thoracoabdominal aorta and its major branches can be challenging, partly because of the craniocaudal extent of the vasculature and substantially differing cross-sectional diameters within a patient and across patient populations. Fast acquisitions of approximately 3 seconds with a dual-source high-pitch protocol require careful timing and thoughtful use of contrast volume and injection rate to ensure consistent image quality, particularly vascular contrast enhancement across patient populations with various cardiovascular conditions and body sizes and across operators implementing the protocol.
One proven approach to optimizing vascular enhancement, in particular for smaller vessels, is to use contrast agents with higher iodine concentration [11, 24]. The concept of a directly proportional relation between iodine content and vascular enhancement in general holds true and has been verified in studies examining different contrast agents. However, additional factors influencing vascular enhancement are at play. In previous studies it was postulated that the viscosity of a contrast agent in general and independent of the chelated iodine concentration may be at least partially responsible for the tightness, or compactness, of the contrast bolus through hemodynamic mechanisms that lead to a lesser degree of contrast dilution [13].
The results of our study show that a reduced overall volume of iopamidol 370 led to no statistically significant variation in vascular enhancement compared with a standard-of-care volume of iopamidol 300 as measured by the SNR of all assessed aortic segments and main abdominal branches. Homogeneous enhancement of the entire aorta and iliac vasculature was achieved with all three methods despite the accelerated method of data acquisition entailing dual-source high-pitch technology with an acquisition time of less than 2 seconds and reduced bolus volume in group C, even with a reported slight probability regarding the impact of mixing artifacts [25, 26]. Lowering the x-ray tube peak voltage led to previously described increases in attenuation (in Hounsfield units) as the mean photon energy of the x-ray spectrum converged toward the k-edge of iodine [4, 27, 28].
Owing to the complex and interdependent cross-play of imaging parameters, in particular during high-pitch imaging, protocol implementation in an in vivo patient environment should be based on previous assessments of vascular contrast, depending on delivered iodine concentrations and actually applied organ radiation dose, because many imaging and contrast administration parameters cannot be changed individually on the basis of previous regular-pitch CT experiences but only in concert with all relevant variables.
This study had limitations. The foremost criticism has to be based on the study design, which compared three independent patient populations and did not assess the three imaging protocols within the same patient groups. Because of the high degree of variability in vascular anatomy, cardiac function, and resultant vascular contrast enhancement, a certain selection bias might have been present. The reduction in contrast volume without a negative effect on the homogeneity and level of vascular contrast enhancement, however, suggests that additional decreases in contrast material may be possible. These decreases would depend on, for instance, patient size and cardiac output to actually reduce the administrated iodine dose. Second, the size of the three assessed patient populations was determined to ensure noninferiority. For analysis of a statistically significant increase in vascular contrast enhancement, larger randomized study populations may be necessary to overcome interindividual and intraindividual variability. Last, no quantitative analysis was performed because interindividual preferences of radiologists would add complexity to the general comparison. In general, better contrast allows more reliable and automated postprocessing, which is essential during image dataset assessment by radiologists.
In summary, in CTA of the aortoiliac vasculature, dual-source high-pitch technique with high-concentration low-volume iodinated contrast medium and low-voltage acquisition was found to be noninferior to dual-source high-pitch technique with low-concentration standard-volume iodinated contrast medium and high-voltage imaging settings. The technique affords simultaneous reduction in radiation exposure and injected contrast volume without negative impact on vascular contrast enhancement.

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 1396 - 1403
PubMed: 28981350

History

Submitted: November 15, 2016
Accepted: May 5, 2017
Version of record online: October 5, 2017

Keywords

  1. contrast media
  2. CT angiography
  3. dual-source high-pitch acquisition
  4. radiation dose

Authors

Affiliations

Sebastian Manneck
Department of Radiology, University Hospital of Basel, Basel, Switzerland.
Lynne M. Hurwitz
Department of Radiology, Duke University Medical Center, Erwin Rd, Box 3808, Durham, NC 27710.
Danielle M. Seaman
Department of Radiology, Duke University Medical Center, Erwin Rd, Box 3808, Durham, NC 27710.
Tobias Heye
Department of Radiology, University Hospital of Basel, Basel, Switzerland.
Daniel T. Boll
Department of Radiology, University Hospital of Basel, Basel, Switzerland.
Department of Radiology, Duke University Medical Center, Erwin Rd, Box 3808, Durham, NC 27710.

Notes

Address correspondence to D. T. Boll ([email protected]; [email protected]).

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