This retrospective study was approved by our institutional review board, and informed consent was waived for the BW-tailored conventional TAVI-CTA protocol (group B). Furthermore, this prospective study was approved by our institutional review board, and written informed consent was obtained from all patients in the new p-COP-tailored TAVI-CTA protocol (group A).
Simulation software for contrast enhancement
Bae et al. proposed a representative physiology-based pharmacokinetic (PBPK) model for contrast enhancement [14–16]. We used p-COP and Nemoto–Kyorindo, which uses PBPK. The p-COP outputs the CMs volume, injection rate, and injection duration for the optimal contrast enhancement of the target organs. The input parameters required to calculate the optimal CMs injection protocol include the target organ, target CT value, sustained time, admissible maximal CMs dose, patient factors [17], CMs factors, and CT scanning factors (Fig. 1). We defined the target CT value as the minimum CT value necessary for diagnosing the target organ, and sustained time as the duration at which the CT value of the target organ is higher than the target CT value.
Study population
Our study retrospectively considered 30 consecutive patients who underwent TAVI-CTA to evaluate the preoperative TAVI in April 2020 with the conventional TAVI-CTA protocol using BW (group B), and prospectively considered 34 patients who underwent TAVI-CTA to evaluate the preoperative TAVI in January 2021 with the new TAVI-CTA protocol for the p-COP (group A). The exclusion criteria were as follows: (1) hemodialysis (n = 2) and (2) severe renal failure (estimated glomerular filtration rate < 40 mL/min/1.73 m2 and n = 2). Thus, the final study population comprised 60 patients (median age, 72 years; range, 42–89 years; 30 males and 30 females). The BW, height, and CO of the patients were recorded immediately before the TAVI-CTA examination. The CO was measured using a non-invasive cardiovascular monitor (Aesculon mini; Peace Bussan). The COs obtained with the electrical velocimeter were recorded and displayed continuously on the monitor at the average of more than 10 valid cardiac cycles.
CT scanner
We used a 64-detector row CT scanner (LightSpeed VCT; GE Healthcare, Milwaukee, WI, USA) to scan all the patients. Retrospective electrocardiogram-gating helical scans were performed to assess the aortic valve and coronary arteries. Scanning parameters were as follows: 100 kVp, 300 to 770 mA, 0.35 s rotation, 0.625 mm detector row width, 0.20–0.22 helical pitch (beam pitch), 8.0 mm table movement and 50 cm scan field of view with cardiac filter, and adaptive statistical iterative reconstruction (30% standard). After table migration, helical scans were performed to assess the entire aortic vessels from the subclavian artery to the femoral artery. Scanning parameters were as follows: 100 kVp, 200 to 770 mA (noise index: 10) with an automatic tube current modulation, 0.4 s rotation time, 5-mm detector row width, 1.375 helical pitch, and 50-cm scan field of view. Image reconstruction was performed with a slice thickness of 0.625 mm and slice spacing of 0.625 mm.
Contrast material injection protocol
With a power injector (Dual Shot, Nemoto–Kyorindo, Tokyo, Japan), wedelivered a CM (Omnipaque-300; Daiichi Sankyo, Tokyo, Japan) via a22-gauge catheter into the antecubital vein. In both groups, the CMs volume were delivered during 22 s, followed by the 20 mL saline chaser for the same injection rates. In group A, the CMs dose was determined by p-COP based on CO, BW, and height. Patient characteristics were recorded before the TAVI-CTA examination, and CO was measured using a cardiovascular monitor [18, 19]. The abdominal aorta at the celiac artery level was the target of CE. The target CT value was 280 HU, and sustained time were 20 s for simulated 100 kVp.
For group B, we used standard injection protocol for TAVI-CTA: An iodine dose for 450 mgI/kg was injected during 22 s, followed by 20 mL of saline chaser. The minimum CT value of the abdominal aorta was 280 HU [9, 10]. As in group A, the patient characteristics were recorded before the CT examination, and CO was measured using cardiovascular monitor.
Data analysis
Intravascular CT value were measured for all patients by using workstation (Advantage Workstation ver. 4.4; GE Healthcare). On TAVI-CTA scans, we recorded the CT value in the abdominal aorta within an approximately 1.0 cm2 circular region of interest. The CT value of the abdominal aorta at the level of the celiac artery was compared between the two groups based on the optimal CT value of TAVI-CTA scans [9, 10].
Statistical analysis
We used the Mann–Whitney U-test, to compare the patient characteristics to assess the interpatient variability of subjects exposed to both groups. To compare the male/female ratio and the number of patients in both groups whose CT value of the abdominal aorta at the celiac artery level was acceptable (≥ 280 HU) or unacceptable (less than 280 HU), we used the chi-square test. Differences were considered statistically significant at P < 0.05. Statistical analyses were performed using free statistical software (R version 3.0.2, R Project for Statistical Computing; http://www.r-project.org/).