Original Research
Multispecialty Articles
October 14, 2020

Split-Bolus, Single-Acquisition, Dual-Phase Abdominopelvic CT Angiography for the Evaluation of Lung Transplant Candidates: Image Quality and Resource Utilization

Abstract

OBJECTIVE. The purpose of this study was to assess the image quality and resource utilization of single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (hereafter referred to as dual-enhancement CTA) performed for combined vascular and solid organ assessment compared with those of single-injection, single-enhancement abdominopelvic CT angiography (hereafter referred to as single-enhancement CTA) for vascular assessment in combination with additional examinations (CT, MRI, and US) performed to assess for malignancy in lung transplant candidates.
MATERIALS AND METHODS. We retrospectively reviewed 100 patients who underwent abdominopelvic CTA examinations before lung transplant. Cohort A (n = 50) underwent dual-enhancement CTA and cohort B (n = 50) underwent single-enhancement CTA. Contrast opacification of the vasculature was assessed along the abdominal aorta through the right femoral artery. Solid organ enhancement was assessed in the right lobe of the liver and the right renal cortex. Measurements of mean radiation dose, contrast exposure, and cost of the studies (in U.S. dollars) were compared.
RESULTS. Mean (± SD) vascular enhancement on dual-enhancement CTA and single-enhancement CTA was 334.2 ± 26.5 HU (coefficient of variation, 8.3%) and 340.0 ± 21.6 HU (coefficient of variation, 6.5%) (p = 0.23), respectively. For dual-enhancement CTA and single-enhancement CTA, mean liver enhancement was 125.8 ± 30.5 HU and 60.4 ± 6.9 HU (p < 0.01), respectively, whereas mean renal cortical enhancement was 260.3 ± 62.2 HU and 133.4 ± 38.6 HU (p < 0.01), respectively. The mean IV contrast volume was 150 mL for dual-enhancement CTA and 75 mL for single-enhancement CTA. Cohort A underwent six additional imaging studies (one of which was a CT colonography study with an effective dose of 19.0 mSv) at a total cost of $9840 per patient. Cohort B underwent 44 additional imaging studies (mean effective dose, 12.7 ± 6.5 mSv) at a total cost of $12,846 per patient (resulting in a 30.6% reduction in cost for dual-enhancement CTA studies; p < 0.0001).
CONCLUSION. Dual-enhancement abdominopelvic CTA allows combined vascular and abdominopelvic solid organ assessment with improved image quality and a lower cost compared with traditional imaging pathways,
Pretransplant assessment of lung transplant candidates for evidence of malignancy is performed because such individuals are at higher risk of malignancy than the general population as a result of the use of long-term immunosuppressive therapy after transplant [1, 2]. Patients with advanced cardiovascular disease who undergo lung transplant have a higher risk of requiring advanced cardiovascular support, including intraaortic balloon pump placement, during transplant [3]. Appropriate surgical risk stratification of patients with atherosclerotic disease includes understanding the feasibility of placement of support devices in the aortoiliac vessels as well as the risk for mesenteric ischemia, both of which will be greater for patients with advanced atherosclerotic disease [4].
Traditional imaging workflows most commonly include MDCT studies involving two separate imaging examinations: arterial phase CT angiography (CTA) of the abdomen and pelvis, which is performed to assess the vasculature for cardiovascular support placement and the risk for mesenteric ischemia, and a portal venous phase abdominopelvic CT, ultrasound, or MRI examination, which is conducted to evaluate the solid organs of the abdomen and pelvis [5]. When independently acquired, these studies can require the patient to receive two separate contrast injections for each examination in addition to two radiation exposures, and they usually necessitate two separate visits to the imaging department.
Prior studies have described the use of a dual-injection MDCT protocol, also referred to as a split-bolus protocol, to obtain multiple phases of enhancement in a single acquisition [6, 7]; however, to our knowledge, the studies published to date have assessed the use of this protocol in the evaluation of single-organ systems only. A study by Chen et al. [6] reported high image quality and lower radiation doses as well as an overall reduction in cost with the use of dual-injection protocols compared with multiple single-injection protocols in the simultaneous evaluation of the renal vasculature, parenchyma, and urinary tract. Hur et al. [7] focused on the evaluation of left atrial thrombus and pulmonary vein anatomy before radiofrequency catheter ablation.
The purpose of the present study is to assess the image quality and resource utilization of single-injection, split-bolus, dual-enhancement abdominopelvic CTA (hereafter referred to as dual-enhancement CTA) for combined vascular and solid organ assessment compared with those of single-injection, single-enhancement abdominopelvic CTA (hereafter referred to as single-enhancement CTA) for vascular assessment in combination with additional examinations (CT, MRI, and ultrasound) performed to assess for solid organ malignancy in lung transplant candidates.

Materials and Methods

This retrospective single-center study was HIPAA compliant and received institutional review board approval.

Patient Population

A total of 100 patients who were lung transplant candidates and had undergone a clinically indicated CTA examination of the abdomen and pelvis were included in this study. A cohort of 50 patients (cohort A) was generated after review of 62 consecutive patients who underwent dual-enhancement CTA between January 1, 2017, and July 31, 2017. Twelve of the 62 patients were excluded for the following reasons: two patients had incomplete information in the PACS regarding contrast injection site; two had undergone prior aortic or iliac stent placement, which could have produced inconsistencies in attenuation values and coefficient of variance evaluation; and eight had metal artifact from implants that obscured the aorta or iliac arteries (Fig. 1). A historical cohort of 50 patients (cohort B) was generated after review of 58 consecutive patients who underwent single-enhancement CTA of the abdomen and pelvis from May 1, 2014, through May 31, 2015. Eight of the 58 patients were excluded from cohort B for the following reasons: two patients had missing patient information in the PACS (one had incomplete information on the contrast injection site and one had incomplete patient dose information), and six patients had metal artifact from implants that was obscuring the aorta or iliac arteries. No patients in cohort B were excluded because of prior stent placement. For both cohorts, data on patient age, sex, body mass index, contrast exposure, radiation dose, and total number of imaging examinations performed were compiled from clinical and radiology records.
Fig. 1 —Flow diagram of patient population with exclusion criteria specified. Missing CT information was defined as no documentation of patient radiation dose, contrast injection site, protocol, or any combination of such information. CT scans were excluded from analysis because of metal artifact from spinal implants or hip replacements affecting aorta or iliac arteries, because of incomplete, thin (< 1 mm) axial dataset, or because of both scenarios occurring.

CT Protocols

All CT examinations were performed using a single-source 64-MDCT scanner (Discovery CT750HD, GE Healthcare) or a dual-source 128-or 192-MDCT scanner (Somatom Flash or Force, respectively; Siemens Healthineers). During the CT examination, z-axis coverage was from the level of the diaphragm through the femoral heads, with scanning performed in the craniocaudal direction. CT images were reconstructed at a slice thickness of 1.0–1.25 mm with a 0.6-mm overlap. Tube current modulation was used for all examinations, and tube voltage was set at 120 kVp for two scanners (Discovery CT750HD and Somatom Flash), whereas variable tube voltage (100–120 kVp) was chosen using automatic tube voltage selection software (Care kVp, Syngo VA50/Somaris 7, Siemens Healthcare) for the other MDCT scanner (Somatom Force). The FOV was set to 50 cm2, and the displayed FOV was set for patient body size to include the entire abdomen.
Dual-enhancement technique—Iopamidol (150 mg I/mL, Isovue 370, Bracco Diagnostics) was administered in two steps, consistent with information published by Bae [8], with step 1 involving initial administration of 100 mL of contrast medium at a rate of 3 mL/s and, with step 2 involving, after a 10-second delay, administration of 50 mL of contrast medium at a rate of 4 mL/s, followed by 50 mL of saline administered at 4 mL/s. CT image acquisition (i.e., abdominopelvic CTA) was initiated after the second administration of contrast medium, and initiation of scanning was timed to begin 9 seconds after blood pooling in the supra-renal abdominal aorta reached an attenuation of 250 HU during fluoroscopic monitoring. Overall, this resulted in acquisition of the portal venous phase occurring at approximately 65–70 seconds after initial contrast injection. The combination of using a standard delay for contrast administration in the first phase and using a timing bolus for the second phase, with appropriate timing for the portal venous and arterial phases, allows dual-phase acquisition. Use of a saline push, a fast rate of injection, and a high iodine concentration optimizes arterial phase images and reduces venous contamination [9, 10].
Single-enhancement technique—A single injection of iopamidol (75 mg I/mL, Isovue 370, Bracco Diagnostics) was administered at a rate of 4 mL/s and was followed by administration of 50 mL of saline at 4 mL/s. CT image acquisition (i.e., abdominopelvic CTA) was timed to begin 9 seconds after blood pooling in the suprarenal abdominal aorta reached an attenuation of 150 HU during fluoroscopic monitoring.
Portal venous phase imaging was considered sufficient for lesion detection when screening for occult malignancy.

Quantitative Image Analysis

Quantitative assessment of enhancement of the vasculature and solid organs was performed by a radiology fellow with 6 years of experience in diagnostic radiology. Images were analyzed on commercially available viewing software (Aquarius iNtuition, version 4.4.12, TeraRecon). The CT images of all patients were displayed with a preset soft-tissue window (window, 400 HU; level, 40 HU). Axial reconstructions performed at 1-mm increments with 0.6-mm overlap were used for all vascular and solid organ measurements.
Vascular assessment—A curved planar reformatted image of the aortoiliac vasculature was created using automated segmentation tools, and the centerline was confirmed as being centrally located within the lumen of the vessel. The mean attenuation of the abdominal aorta through the right common femoral artery was measured by extracting attenuation values from the straightened curved planar reformatted image of the vessel at every 1-mm increment, with use of open access software (ImageJ, version 1.52, U.S. National Institutes of Health) (Fig. 2). The mean attenuation along the length of the vessels was calculated as the mean of all measurements. The coefficient of variance of the attenuation measurements along the vessels was obtained to determine the change in attenuation along the z-axis.
Fig. 2A —55-year-old female lung transplant candidate.
A, Three-dimensional volumetric rendering of dual-injection abdominopelvic CT angiography image shows centerline through systemic vasculature. Runoff r refers to centerline through abdominal aorta and right iliac and femoral arteries.
Fig. 2B —55-year-old female lung transplant candidate.
B, Curved (B) and straightened (C) curved planar reconstructed images, each obtained with window settings adjusted to width of 900 HU and level of 200 HU, show area along abdominal aorta through right common femoral artery. Enhancement of liver (asterisk, B), inferior vena cava (arrowhead, B), and kidney (arrow, C) is noted.
Fig. 2C —55-year-old female lung transplant candidate.
C, Curved (B) and straightened (C) curved planar reconstructed images, each obtained with window settings adjusted to width of 900 HU and level of 200 HU, show area along abdominal aorta through right common femoral artery. Enhancement of liver (asterisk, B), inferior vena cava (arrowhead, B), and kidney (arrow, C) is noted.
Solid organ assessment—Solid organ parenchymal enhancement was assessed in the liver and renal parenchyma by drawing on axial images the largest possible circular ROIs within the liver parenchyma (i.e., the right hepatic lobe at the level of the porta hepatis; mean ROI size, 3.1 cm2 [range, 3.0–3.3 cm2] for both cohorts) and distant from vessels, biliary ducts, and hypo- or hyperenhancing lesions and in the right renal cortex (i.e., at the level of the renal hilum; mean ROI size, 0.3 cm2 [range, 0.13–0.33 cm2] for both cohorts). The renal cortex was chosen as the measurement location, in accordance with the existing literature [1114], to ascertain that sufficient cortical enhancement was achieved for detection of renal lesions. A single-plane circular ROI of the same size was propagated along three consecutive slices for each individual CTA image, and mean (± SD) values were calculated for each group of three measurements.
Image noise—The largest possible circular ROI (mean ROI size, 1.6 cm2; range, 0.22–9.73 cm2) was placed in the subcutaneous fat at the level of the porta hepatis and was propagated through three consecutive axial slices. The SD of the pixel values was measured on each slice, and the mean value was obtained. The signal-to-noise ratio (SNR) of the liver parenchyma (SNRliver parenchyma) was calculated by dividing the attenuation value (expressed as Hounsfield units) of the liver parenchyma (HUliver parenchyma) by the SD of the liver parenchyma (SDliver parenchyma): SNRliver parenchyma = HUliver parenchyma / SDliver parenchyma. The contrast-to-noise ratio (CNR) of the liver parenchyma (CNRliver parenchyma) was calculated by subtracting the attenuation value (expressed as Hounsfield units) of fat (HUfat) from the HUliver parenchyma and then dividing the difference by the SD of fat (SDfat): CNRliver parenchyma = (HUliver parenchymaHUfat) / SDfat [15, 16].

Radiation Dose

The volume CT dose index and dose-length-product were recorded for each CT examination. The mean and SD for the dose parameters were determined for both patient cohorts. For all patients, effective dose (expressed in millisieverts) was determined by multiplying the dose-length-product by a k-factor (for the abdomen) of 0.015 [17]. For all patients, radiation exposure resulting from additional imaging examinations was recorded. The mean and SD of the total effective dose were determined for cohort A and cohort B.

Contrast Volume

IV contrast volume was recorded for all patients. As per institutional protocol, a set volume of 150 mL was used for dual-enhancement CTA, and 75 mL was used for single-enhancement CTA. For the patients who underwent additional CT examinations, contrast volume was documented for each examination, and the mean total contrast volume was determined for each patient and cohort.

Cost

Cost was defined as institutional charges because reimbursement amount could vary based on insurance coverage. The same charge for each type of examination was kept constant to reduce change related to variation in charges over time. The total charge per patient (i.e., the cost of all imaging examinations performed to complete abdominopelvic vascular and solid organ assessment) was determined for patients in cohorts A and B.
The number of imaging examinations performed to assess both solid organs and the vasculature was noted for patients in each cohort through review of the electronic health records (Epic, version 2015, Epic Systems). Time spent in the radiology department and patient time for completing all examinations were not calculated; however, the number of visits to the radiology department was derived from the number of days that different imaging studies were performed.

Statistical Analysis

Statistical analysis was performed using spreadsheet software (Microsoft Office Excel 2016 for Windows, Microsoft) and SPSS software (version 20, IBM SPSS) along with descriptive statistics and a t test. A p < 0.05 was considered statistically significant.

Results

The mean age and body mass index of patients in cohorts A and B were not significantly different, and sex distribution was similar for both cohorts (Table 1). For most patients in cohorts A and B, the location of IV contrast administration for CTA was the right upper extremity (72% and 86% of patients, respectively). The mean (± SD) tubevoltage for CTA was significantly lower (p < 0.0001) in cohort A (113 ± 12 kVp; range, 90–120 kVp; 33 examinations performed at 120 kVp) compared with cohort B (120 ± 3 kVp; range, 100–120 kVp; 49 examinations performed at 120 kVp). The mean tube current was significantly higher in cohort A compared with cohort B (188 ± 62 mAs and 163 ± 49 mAs; p = 0.0276). Mean vascular enhancement on dual-enhancement CTA and single-enhancement CTA was 334.2 ± 26.5 HU (coefficient of variation, 8.3%) and 340.0 ± 21.6 HU (coefficient of variation, 6.5%) (p = 0.23), respectively. For dual-enhancement CTA and single-enhancement CTA, mean liver enhancement was 125.8 ± 30.5 HU and 60.4 ± 6.9 HU (p < 0.01), respectively, whereas mean renal cortical enhancement was 260.3 ± 62.2 HU and 133.4 ± 38.6 HU (p < 0.01), respectively.
TABLE 1: Patient Demographic Characteristics
CharacteristicCohort A (n = 50)aCohort B (n = 50)bp
Age (y), mean (range)63.3 (20–76)64.5 (41–78)0.5162
Female sex (%)34300.6618
BMI, mean ± SD25.2 ± 3.226.5 ± 3.90.0579

Note—BMI = body mass index (weight in kilograms divided by the square of height in meters).

a
Patients who underwent single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography.
b
Patients who underwent single-injection, single-enhancement abdominopelvic CT angiography.
For both cohorts, the mean attenuation for vascular enhancement of the abdominal aorta through the right femoral artery reached more than 300 HU. The difference of approximately 6 HU in mean vascular attenuation was not statistically significantly different between the cohorts (p = 0.23). The coefficient of variance was 1.8% higher for dual-enhancement CTA than for single-enhancement CTA, but this finding was not statistically significant (p = 0.09) (Table 2).
TABLE 2: Image Quality of Abdominopelvic CT Angiography (CTA) Images of Cohort A Versus Cohort B
Image QualityCohort A (n = 50)aCohort B (n = 50)bp
Attenuation of area from aorta to right common femoral artery (HU), mean ± SD334.2 ± 26.5340.0 ± 21.60.23
Aorta to right common femoral artery, COV (%)8.36.50.09
Liver attenuation (HU), mean ± SD125.8 ± 30.560.4 ± 6.9< 0.01
Kidney attenuation (HU), mean ± SD260.3 ± 62.2133.4 ± 38.6< 0.01
SNR3.772.230.65
CNR11.911.270.92

Note—COV = coefficient of variance, SNR = signal-to-noise ratio, CNR = contrast-to-noise ratio.

a
Patients who underwent single-injection, split-bolus, dual-enhancement abdominopelvic CTA.
b
Patients who underwent single-injection, single-enhancement abdominopelvic CTA.
The mean attenuation value for liver and renal cortical enhancement was higher with dual-enhancement CTA than with single-enhancement CTA (Table 2). When split dual injection was compared with a single arterial phase injection, the mean absolute improvement in contrast opacification was 65.4 HU (a 52% increase) for liver and 126.9 HU (a 49% increase) for kidney parenchyma (Fig. 3). The SNR and CNR were not statistically significantly different between the cohorts (p = 0.65 for SNR and 0.92 for CNR) (Table 2).
Fig. 3A —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
A, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (A) shows overview of whole dual-enhancement CT examination. Dual-enhancement CTA images show ROIs (circles, B and C) in right hepatic lobe (B) and renal cortex (C), which had mean (± SD) attenuation of 140.0 ± 25.0 HU and 305.0 ± 14.3 HU, respectively.
Fig. 3B —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
B, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (A) shows overview of whole dual-enhancement CT examination. Dual-enhancement CTA images show ROIs (circles, B and C) in right hepatic lobe (B) and renal cortex (C), which had mean (± SD) attenuation of 140.0 ± 25.0 HU and 305.0 ± 14.3 HU, respectively.
Fig. 3C —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
C, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (A) shows overview of whole dual-enhancement CT examination. Dual-enhancement CTA images show ROIs (circles, B and C) in right hepatic lobe (B) and renal cortex (C), which had mean (± SD) attenuation of 140.0 ± 25.0 HU and 305.0 ± 14.3 HU, respectively.
Fig. 3D —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
D, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (D) shows overview of arterial enhancement CT study as part of single-enhancement CT examination. Single-enhancement CTA images of vasculature show ROIs (circles, E and F) in right hepatic lobe (E) and renal cortex (F), which had mean attenuation of 66.0 ± 16.6 HU and 182.0 ± 12.1 HU, respectively. Three-dimensional rendering (G) shows overview of portal venous enhancement CT study as part of single-enhancement CT examination. Additional abdominopelvic CT images (H and I) obtained for solid organ assessment show ROIs (circles) in right hepatic lobe (H) and renal cortex (I), which had mean attenuation of 99.0 ± 37.3 HU and 204.0 ± 34.7 HU, respectively.
Fig. 3E —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
E, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (D) shows overview of arterial enhancement CT study as part of single-enhancement CT examination. Single-enhancement CTA images of vasculature show ROIs (circles, E and F) in right hepatic lobe (E) and renal cortex (F), which had mean attenuation of 66.0 ± 16.6 HU and 182.0 ± 12.1 HU, respectively. Three-dimensional rendering (G) shows overview of portal venous enhancement CT study as part of single-enhancement CT examination. Additional abdominopelvic CT images (H and I) obtained for solid organ assessment show ROIs (circles) in right hepatic lobe (H) and renal cortex (I), which had mean attenuation of 99.0 ± 37.3 HU and 204.0 ± 34.7 HU, respectively.
Fig. 3F —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
F, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (D) shows overview of arterial enhancement CT study as part of single-enhancement CT examination. Single-enhancement CTA images of vasculature show ROIs (circles, E and F) in right hepatic lobe (E) and renal cortex (F), which had mean attenuation of 66.0 ± 16.6 HU and 182.0 ± 12.1 HU, respectively. Three-dimensional rendering (G) shows overview of portal venous enhancement CT study as part of single-enhancement CT examination. Additional abdominopelvic CT images (H and I) obtained for solid organ assessment show ROIs (circles) in right hepatic lobe (H) and renal cortex (I), which had mean attenuation of 99.0 ± 37.3 HU and 204.0 ± 34.7 HU, respectively.
Fig. 3G —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
G, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (D) shows overview of arterial enhancement CT study as part of single-enhancement CT examination. Single-enhancement CTA images of vasculature show ROIs (circles, E and F) in right hepatic lobe (E) and renal cortex (F), which had mean attenuation of 66.0 ± 16.6 HU and 182.0 ± 12.1 HU, respectively. Three-dimensional rendering (G) shows overview of portal venous enhancement CT study as part of single-enhancement CT examination. Additional abdominopelvic CT images (H and I) obtained for solid organ assessment show ROIs (circles) in right hepatic lobe (H) and renal cortex (I), which had mean attenuation of 99.0 ± 37.3 HU and 204.0 ± 34.7 HU, respectively.
Fig. 3H —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
H, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (D) shows overview of arterial enhancement CT study as part of single-enhancement CT examination. Single-enhancement CTA images of vasculature show ROIs (circles, E and F) in right hepatic lobe (E) and renal cortex (F), which had mean attenuation of 66.0 ± 16.6 HU and 182.0 ± 12.1 HU, respectively. Three-dimensional rendering (G) shows overview of portal venous enhancement CT study as part of single-enhancement CT examination. Additional abdominopelvic CT images (H and I) obtained for solid organ assessment show ROIs (circles) in right hepatic lobe (H) and renal cortex (I), which had mean attenuation of 99.0 ± 37.3 HU and 204.0 ± 34.7 HU, respectively.
Fig. 3I —Value of protocol for simultaneous evaluation of solid organ structures and general vascular anatomy. Coronal maximum-intensity-projection images show hepatic and renal cortical parenchymal enhancement (window settings for all images: width, 400 HU; level, 40 HU) on single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography (i.e., dual-enhancement CTA) and single-injection, single-enhancement abdominopelvic CT angiography (i.e., single-enhancement CTA) examinations.
I, 65-year-old man (body weight, 66 kg) who was lung transplant candidate. Three-dimensional rendering (D) shows overview of arterial enhancement CT study as part of single-enhancement CT examination. Single-enhancement CTA images of vasculature show ROIs (circles, E and F) in right hepatic lobe (E) and renal cortex (F), which had mean attenuation of 66.0 ± 16.6 HU and 182.0 ± 12.1 HU, respectively. Three-dimensional rendering (G) shows overview of portal venous enhancement CT study as part of single-enhancement CT examination. Additional abdominopelvic CT images (H and I) obtained for solid organ assessment show ROIs (circles) in right hepatic lobe (H) and renal cortex (I), which had mean attenuation of 99.0 ± 37.3 HU and 204.0 ± 34.7 HU, respectively.
The mean volume CT dose index for dual-enhancement and single-enhancement CTA examinations, without inclusion of additional examinations, was lower in cohort A (5.5 mGy lower than that in cohort B; p < 0.01), whereas the dose-length product was similar between the cohorts (with a mean increase of 15 mGy/cm seen in cohort B) (Table 3). Between patients in cohort A and cohort B, there was no statistically significant difference in the overall mean effective dose from dual-enhancement CTA versus single-enhancement CTA; however, the mean effective dose was 1.7 mSv higher for patients in cohort B when all imaging studies (i.e., CTA examinations plus additional examinations) were taken into account (p = 0.1).
TABLE 3: Dose Parameters for Cohort A and Cohort B With and Without Inclusion of Additional Imaging Examinations
Dose ParameterCohort A (n = 50)aCohort B (n = 50)bp
CTDIvol (mGy), mean ± SD (range)c8.2 ± 2.9 (3.6–13.6)13.7 ± 7.4 (5.3–20.2)0.0006
DLP (mGy/cm), mean ± SDc603 ± 276618 ± 3220.8167
Effective dose (mSv), mean ± SD   
 Patients who underwent CTA only10.9 ± 5.011 ± 60.928
 Patients who underwent CTA plus additional examinations11 ± 512.7 ± 6.50.1189

Note—CTDIvol = volume CT dose index, DLP = dose-length product, CTA = CT angiography.

a
Patients who underwent single-injection, split-bolus dual-enhancement abdominopelvic CTA.
b
Patients who underwent single-injection, single-enhancement abdominopelvic CTA.
c
For patients who underwent CTA only.
The number of patients who underwent two or more imaging examinations in cohort A and cohort B was six patients (12%) and 33 patients (66%), respectively. For cohort A, one CT colonography examination and five liver ultrasound examinations were performed, because of patient-specific risk factors and in accordance with the standard clinical protocol. None of these examinations were triggered by findings on dual-enhancement CTA scans. For cohort B, 18 liver ultrasound examinations, 12 abdominopelvic CT examinations, 11 CT colonography examinations, and three MRI examinations were performed. Of these examinations, seven were ordered because of findings seen on single-enhancement CTA, and the remaining 37 examinations were obtained because of patient-specific risk factors and in accordance with the standard clinical protocol (Table 4). Eight of 50 patients (16%) in cohort B underwent two or more imaging examinations in addition to single-enhancement CTA. For patients who underwent single-enhancement CTA and additional cross-sectional imaging, the mean additional amount of iodinated contrast medium administered was 139 ± 25 mL (total amount of iodinated contrast medium, 214 mL versus 150 mL for dual-enhancement CTA alone), and the mean amount of MRI contrast medium administered was 18 mL.
TABLE 4: Supplemental Nonvascular Examinations Performed
Additional Examination(s)Cohort ACohort B
Dual-Enhancement CTA Examinations (n = 50)Follow-Up ExaminationsaStandard Clinical Protocol ExaminationsbSingle-Enhancement CTA Examinations (n = 50)Follow-Up ExaminationscStandard Clinical Protocol Examinationsd
Total60644737
CT colonography10111011
Abdominal CT with contrast enhancement0001239
Abdominal MRI000330
Abdominal US50518117

Note—Data are no. of examinations. Dual-enhancement CTA = single-injection, split-bolus, dual-enhancement abdominopelvic CT angiography, single-enhancement CTA = single-injection, single-enhancement CT angiography, US = ultrasound.

a
Performed to evaluate findings seen on dual-enhancement CTA.
b
In cohort A, six patients underwent a total of six additional examinations, all of which were performed in accordance with the standard clinical protocol.
c
Performed to evaluate findings seen on single-enhancement CTA.
d
In cohort B, 33 patients underwent a total of 44 additional examinations, 35 (79.6%) of which were performed in accordance with the standard clinical protocol.
For cohort A, the mean per-patient cost to complete imaging evaluation of both the solid organs and the vasculature (including all examinations to complete workup; total number of examinations, 56) was $9840 ± $629. In cohort B, the mean per-patient cost to complete imaging evaluation (including all examinations to complete workup; total number of examinations, 94) was $12,846 ± $3550, which represents a 30.6% increase in costs compared with cohort A. For the subgroup of six patients in cohort A who underwent more than one examination, the mean per-patient cost was $11,031 ± 1394. For the subgroup of 33 patients in cohort B who underwent more than one examination, the mean per-patient cost was $14,478 ± 3387 (a 31.3% increase in costs compared with cohort A), which was $4800 more than the mean cost for all patients in cohort B who underwent CTA only (Table 5).
TABLE 5: Costs for Cohorts A and B to Complete Imaging Assessment of the Vasculature and Solid Organs of the Abdomen and Pelvis
Examination(s)Costs (U.S. $)p
Cohort A (n = 50)aCohort B (n = 50)b
CTA only967896781.0000
CTA plus 1 additional examination9840 ± 62912,846 ± 3550< 0.0001
CTA plus > 1 additional examination11,031 ± 139414,478 ± 33870.0195

Note—Except where otherwise indicated, data are mean or mean ± SD. CTA = CT angiography.

a
Patients who underwent single-injection, split-bolus dual-enhancement abdominopelvic CTA. Six of 50 patients (six total examinations) in cohort A underwent additional examinations to assess for solid organ malignancy before lung transplant.
b
Patients who underwent single-injection, single-enhancement abdominopelvic CTA. Thirty-three of 50 patients (44 total examinations) in cohort B underwent additional examinations to assess for solid organ malignancy before lung transplant.
The mean number of days that patients were present in the imaging department was 1.14 ± 0.4 days (range, 1–2 days) for cohort A and 1.86 ± 0.8 days (range, 1–3 days) for cohort B (p < 0.0001).

Discussion

A dual-enhancement abdominopelvic CTA protocol for assessment of the vasculature and the solid organs can result in similar vascular enhancement (attenuation, > 300 HU), optimal portal venous liver parenchymal opacification (a 52% increase in attenuation values) and improved renal cortical opacification (a 49% increase in attenuation values) compared with a single-enhancement CTA protocol.
Vascular enhancement with the dual-enhancement CTA protocol was comparable to that seen with use of a dedicated vascular or single-enhancement CTA protocol, even though the threshold for initiating the scan was higher at 250 HU for dual-enhancement CTA compared with 150 HU for single-enhancement CTA. This threshold vascular enhancement value was similar to that reported in a recent study by Leung et al. [18], which found mean attenuation of 326.2 ± 22.4 HU after a single injection of 100 mL of iopamidol (340 mg I/mL). The maintenance of high vascular enhancement with both protocols may result from the timing of image acquisition being closely associated with the second injection and from the use of a split bolus providing the additive impact of two contrast injections.
Solid organ enhancement achieved using the dual-enhancement CTA protocol is similar to that reported in previous studies of the use of dedicated abdominopelvic CTA, with mean portal venous phase hepatic enhancement of 112.0 ± 15.3 HU and mean arterial phase renal enhancement of 211.0 ± 20.0 HU [19, 20]. Improved solid organ enhancement with dual-enhancement CTA compared with the single-enhancement CTA was achieved through a combination of a higher total contrast volume and a delayed acquisition time (65–70 seconds after the first contrast infusion).
Objective image quality, as measured using SNR and CNR, was similar between the cohorts, likely reflecting similar cohort characteristics regarding body mass index, IV location, and comparable CT techniques (tube voltage, tube current, and the use of MDCT scanners with ≥ 64 detectors).
For both cohorts, the total radiation dose from CTA was not significantly different. In the subanalysis comparing patients in cohorts A and B who underwent additional imaging, the difference was more pronounced because of the additional number of CT examinations performed for cohort B, even though this finding did not reach the level of statistical significance. Use of the dual-enhancement CTA protocol minimizes the number of additional CT examinations performed and thus reduces the mean total radiation dose on a per-patient basis.
The split-bolus dual-enhancement CTA protocol requires a higher (≈ 50% higher) single dose of contrast medium than does the single-injection protocol, resulting in a total dose more similar to that commonly used for routine solid organ assessment (i.e., 100 mL) [8]. For patients undergoing an additional CT examination, the amount of contrast media used would be higher, as evidenced by the subgroup in cohort B that underwent two CT examinations. In this subgroup, contrast exposure was increased by 42.7% (mean, 214 mL) compared with the amount used in cohort A (mean 150 mL). The amount of contrast medium used might be further reduced with the application of evolving techniques such as dual-energy acquisition with monoenergetic reconstructions, which has been shown to provide better CNR and allows the use of smaller amounts of contrast material [15, 21, 22]. Dual-energy acquisition could also help distinguish between possible renal lesions on the basis of vascularization and iodine thresholds, as recently shown by Patel et al. [23]. This potential may be explored in future studies.
Utilization of a dual-enhancement CTA protocol resulted in reductions in the number of patients who underwent more than one imaging study (12% vs 66%), the number of days that patients were present in the imaging department (≈ 1 day less), and overall cost (a 30.6% lower cost). An even larger reduction (31.3%) in cost was seen when the patients who underwent more than one imaging study were considered. An added indirect benefit of the dual-enhancement CTA protocol is a reduction in travel time for the patient and improved utilization of radiology scanner time in a radiology department if only one examination is performed.
Although this study focused on patients who were candidates for lung transplant, the same principle of dual-injection protocols could be applied to patients with other clinically important conditions, such as mesenteric ischemia (for which both the mesenteric vasculature and the bowel wall need to be evaluated), or patients who are renal transplant candidates (for whom both the vascular anatomy and the renal parenchyma need to be evaluated). It may also be applicable to the growing field of oncology because many patients need follow-up oncologic examinations as well as vascular assessments.
One limitation of the present study is its relatively small sample size. Although all patients were evaluated at one referral site, it is possible that additional imaging examinations were performed at other sites. These examinations could not be included in the study, but they would likely show greater reduction in radiation dose, contrast exposure, and cost with the use of a dual-enhancement CTA protocol. There is the possibility for selection bias because of the retrospective study design. To help mitigate selection bias, we included consecutive patients in each cohort.
The radiology fellow who made the quantitative measurements (i.e., attenuation values in the aorta and solid organs) was not blinded to the time of acquisition but made no qualitative judgments for either cohort. The ROI size in the renal cortex was relatively small, to decrease the likelihood of partial volume effects from the renal pyramids and medulla or perirenal fat, which may have led to higher SDs for dual-enhancement CTA than for single-enhancement CTA. This method is in accordance with that used in other studies that measured renal cortex attenuation separately from that of the medulla with the use of small ROI sizes (mean, 3 mm) [14].
An additional limitation is that direct comparison of lesion detection between dual-enhancement CTA and single-enhancement CTA was not performed. This was outside the scope of the present study and may have to be evaluated further in the future.
Future studies may be performed to evaluate a similar approach that reduces the overall contrast dose by decreasing the amount of contrast medium administered in step 1 or by increasing the amount of contrast medium administered in step 2, to widen the differential between the arterial and portal venous phases.

Conclusion

A dual-enhancement CT protocol results in similar vascular enhancement and improved portal venous liver and renal cortical opacification for solid organ assessment compared with single-enhancement CTA. The dual-enhancement CTA protocol results in reductions in total patient radiation exposure, the amount of IV contrast medium used, and overall cost, as a result of the need for a reduced number of additional examinations.

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 1520 - 1527
PubMed: 33052735

History

Submitted: September 19, 2019
Accepted: March 5, 2020
Version of record online: October 14, 2020

Keywords

  1. cost reduction
  2. dual-enhancement CT protocol
  3. radiology workflow improvement
  4. transplant patient evaluation

Authors

Affiliations

Fides Regina Schwartz
Department of Radiology, Duke University, Box 3808 DUMC, Durham, NC 27710.
Ranish Deedar Ali Khawaja
Department of Radiology, Duke University, Box 3808 DUMC, Durham, NC 27710.
Daniele Marin
Department of Radiology, Duke University, Box 3808 DUMC, Durham, NC 27710.
Bhavik N. Patel
Department of Radiology, Stanford University, Stanford, CA.
Alice L. Gray
Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado Anschutz Medical Center, Aurora, CO.
John M. Reynolds
Department of Pulmonary Medicine, Duke University, Durham, NC.
Lynne Hurwitz Koweek
Department of Radiology, Duke University, Box 3808 DUMC, Durham, NC 27710.

Notes

Address correspondence to F. R. Schwartz ([email protected]).

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