This prospective study was approved by the Institutional Review Board of our Hospital. Written informed consent for study participation was obtained from all involved patients.

During the period of December 2023 to April 2024, a total of 110 consecutive patients who were scheduled to undergo CTC examination with clinically suspected CRC in our hospital were prospectively enrolled in this study. The exclusion criteria were as follows: patients with poor bowel preparation, unable to tolerate gas injection, metal artifacts from lumbar implants or total hip replacement, and patients who were recommended to use 120 kVp tube voltage by Auto prescription technique. Among the 110 patients, 11 patients who had no pathological report, 5 patients who had maximum tumor diameter less than 10 mm, and another 4 patients who underwent chemotherapy for treatment were further excluded from analysis. Thus, 90 patients finally made up of our study population (Fig. 1).

Study pipeline. CRC colorectal cancer; CTC CT colonography; SDCTC standard dose CT colonography; ULDCTC ultra-low dose CT colonography

Prior to the CTC, a clear liquid diet was restricted to all patients three days before the examination. A standard bowel preparation was conducted on all patients with one sachet of soluble polyethylene glycol electrolyte powder (Hygecon, Jiangxi Hygecon Pharmecutical) dissolved in one litre of warm water as laxative, and without fecal tagging. In addition, patients were instructed to consume 30 millilitres (mls) of Simethicone dissolved in 100 mls of warm water to alleviate possible abdominal bloating. On the day of CTC examination, all patients had fasted overnight and lied in the left lateral decubitus position on the CT examination table. A senior radiologist assisted by a dedicated nurse manually inflated CO2 into the colon via a flexible rectal tube, until the patient experienced abdominal discomfort. The CTC scanning started with patients in prone position with care taken to avoid abdominal compression by putting a pillow under the chest. After the patients rotated into the supine position, the colon distension was checked based on the supine scout image. If the residual air was inadequate, further inflation would be performed before the second scan to ensure the CTC image quality. To avoid repeated bowel preparation, the CTC examinations were conducted on the same day with colonoscopy.

All CTC examinations were performed with a 256-row spectral CT scanner (Revolution CT, GE HealthCare, Milwaukee, WI, USA) using the following parameters: detector width, 80 mm; pitch, 0.992; rotation time, 0.5 s; slice thickness, 5 mm; tube voltage, 120 kVp in the prone position [standard dose CTC (SDCTC)] and Auto-prescription kVp selection in the supine position [ultra-low dose CTC (ULDCTC)] with subgroups of 80 kVp or 100 kVp; tube current, Smart mA mode (range, 20–450 mA); preset ASIR-V, 30% for the SDCTC and 60% for the ULDCTC. Radiation doses were controlled by using noise index (NI) setting: 13 for the 120 kVp group and 80 kVp subgroup and 15 for the 100 kVp subgroup. All raw data were reconstructed using a STND kernel with 30% and 80% post-set ASIR-V for the SDCTC and the ULDCTC, respectively. The reconstruction thickness and increment were both 0.625 mm.

The volume CT dose index (CTDIvol) and dose-length product (DLP) were recorded for comparison. Moreover, the scan length was obtained from the starting and ending points for comparison between the prone and supine positions. Furthermore, the effective dose (ED) was calculated from DLP using 0.015 mSv/ (mGy·cm) as the conversion factor in abdomen, recommended by the European guidelines on quality criteria [15].

All axial images were transferred to an AW 4.7 (GE HealthCare, Milwaukee, WI, USA) workstation for the following reformatted images: two-dimensional (2D) Multi-planar Reformation (MPR), three-dimensional (3D) endoluminal view from CT Virtual Colonoscopy (CTVC) and 3D Raysum. On the axial images, the circular regions of interest (ROIs) were drawn on the homogenous area of the colorectal tumor and intraluminal air to measure the CT value and the standard deviation (SD) values, in order to calculate the signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR) of tumors. The formulas were as follows: SNR=CT tumor/SD tumor, CNR=(CT tumor-CT intraluminal air)/SD intraluminal air. The tumor ROIs were depicted on the slice of maximum diameter, and were manually placed on the prone and supine images, keeping the ROI size accounting for 70-80% of the tumor. The measurements on three consecutive slices were recorded and averaged to avoid measuring bias, and the ROI sizes ranged from 50 to 100 mm2.

Two abdominal radiologists (one junior radiologist with 5-year experience, and one senior radiologist with 8-year experience in abdominal imaging), who were blinded to the group division, independently performed subjective image quality assessment on the workstation. The 2D MPR images and 3D CTVC images were graded using a five-point scale. The images scored greater than or equal to 3 points were considered diagnostically acceptable. The marking criteria were demonstrated as follows:

5 points: clear image, virtually no image noise, excellent confidence to detect colorectal lesions;

4 points: relatively clear image, low image noise, good confidence to detect colorectal lesions;

3 points: fair image, moderate image noise, fair confidence to detect colorectal lesions;

2 points: slightly blurry image, large image noise, low confidence to detect colorectal lesions;

1 point: blurry image, heavy image noise, uninterpretable.

Different criteria were applied to 3D images:

5 points: smooth endoluminal wall, clear lesion morphology;

4 points: relatively smooth endoluminal wall, distinguishable lesion morphology;

3 points: irregular endoluminal wall, relatively distinguishable lesion morphology;

2 points: rough endoluminal wall, barely distinguishable lesion morphology;

1 point: highly rough endoluminal wall, indistinguishable lesion morphology.

The colonic and rectal tumor locations were recorded by the same radiologists mentioned above while assessing the image quality. The colon and rectum were divided into nine segments [16]: ileocecal junction, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon, rectosigmoid junction, and rectum. If divergent opinions appeared, a consensus reading was performed to determine a final conclusion. An intergroup comparison was made between the tumor locations reported by the two radiologists and by a surgeon, using the operation result as the reference standard.

Data analysis was performed with SPSS 26.0 statistical software. For the normally distributed continuous variables, expressed as mean ± SD, the Paired sample t test was used, whereas for the non-normally distributed variables, expressed as median (Interquartile range, IQR), Wilcoxon sign rank test was used. The comparison of subjective scores between the control group and Auto-kVp groups used Wilcoxon sign rank test. The Kappa test was used to compare the inter-reader consistency and tumor location consistency between CTC and surgery: Kappa value ≥ 0.75, good consistency, 0.75 > Kappa value > 0.4, moderate consistency, Kappa value ≤ 0.4, poor consistency. P < 0.05 represented a statistically significant difference.