Optimizing computed tomography enterography with virtual monoenergetic imaging of second-generation dual-layer spectral CT in inflammatory bowel disease

Introduction

Inflammatory bowel disease (IBD) represents a group of chronic and clinically burdensome gastrointestinal disorders characterized by a relapsing-remitting pattern, primarily encompassing Crohn’s disease (CD) and ulcerative colitis (UC) (1,2). UC manifests as a predominantly mucosal inflammatory process, with 30–50% of cases presenting as distal colitis localized to the rectosigmoid region, whereas CD represents a more aggressive and transmural pathological process (3).

Although ileocolonoscopy demonstrates 74–100% sensitivity for ileocolic CD detection (4) and capsule endoscopy achieves 90% sensitivity for small bowel CD assessment (5), cross-sectional imaging reveals transmural pathology in 50% of CD patients, notwithstanding normal endoscopic findings (6), due to the inherent limitation of mucosal surface evaluation. This discrepancy establishes cross-sectional imaging as an essential complement to endoscopic evaluation.

CD is pathologically defined by transmural inflammation and associated extraintestinal manifestations, including penetrating complications (fistulae/abscesses) and mesenteric adipose tissue hyperplasia (“creeping fat”). This pathobiology underscores the clinical imperative for developing personalized stratification protocols to optimize therapeutic decision-making and mitigate treatment risks (5). Cross-sectional enterography demonstrates variable diagnostic performance for penetrating lesions (sensitivity: 20–100%; specificity: 91–100%) across intestinal segments (7). Beyond diagnostic confirmation, quantitative assessment of inflammatory activity is critical for disease monitoring. Meta-analytic evidence confirms high diagnostic accuracy of magnetic resonance enterography (MRE) and computed tomography enterography (CTE) in activity grading (per-patient accuracy: 86% vs. 84%, respectively) (8). CTE’s superior spatial resolution and multiplanar reconstruction capabilities enable precise characterization of the following: segmental mural hyperenhancement; bowel wall thickening/structuring; mesenteric vascular engorgement (“comb sign”); and perivisceral fat stranding. These imaging biomarkers collectively serve as quantifiable indicators of active inflammation (9).

Dual-energy computed tomography (DECT) is an imaging technique based on data acquisition at two different energy settings, which can be used to diagnose lesions with higher quality and a more accurate quantity compared with a routine CT (10). The dual-energy computed tomography enterography (DECTE) used in this study employs an innovative dual-layer detector architecture, where a single X-ray source simultaneously captures spectral data through differential photon absorption: low-energy photons (70–100 keV) are detected in the superficial detector layer, whereas high-energy photons (130–150 keV) penetrate to the deeper layer (11). This configuration enables simultaneous dual-energy data acquisition during routine scanning protocols, facilitating advanced post-processing workflows. A key advantage of DECTE lies in its capacity for direct quantitative comparison between reconstructed virtual monoenergetic images (VMIs) and conventional polyenergetic imaging (PEI). The VMI algorithm, particularly at low photon energies approximating the iodine K-edge (33 keV), enhances iodine contrast-to-noise ratios (CNRs) by 40–60% compared to PEI, serving as a critical tool for improving lesion conspicuity in abdominal malignancies and inflammatory conditions (12-14).

So far, spectral CT VMI has achieved good results in esophageal cancer, gastric cancer, and colorectal cancer (15-17); however, there are few reports on intestinal inflammatory lesions. Therefore, the purpose of this study was to investigate the optimal VMI for the diagnosis of active IBD, especially active CD, in continuous DECTE (40–100 keV in 10 keV increments) and to investigate the diagnostic efficacy of the optimal monoenergetic keV for specific signs of CTE in patients with CD. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1192/rc).

Methods

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of Zhejiang Provincial Hospital of Traditional Chinese Medicine (No. 2025-KLS-234-01) and the requirement for written informed consent was waived due to the retrospective nature of the study.

Study population

The inclusion criteria were as follows: (I) IBD diagnosis confirmed through consensus of endoscopic, histopathological, radiological, and clinical–laboratory criteria; (II) CTE with complete image sequences of the three sets of raw data in the dual-energy spectral CT scan, arterial and venous phases; and (III) lesion region of interest (ROI) selection not affected by artefacts. The exclusion criteria were as follows: (I) CTE spectral raw data image sequence was incomplete; and (II) intra-abdominal artefacts severely affected lesion ROI measurement results.

This retrospective analysis of prospective data identified 181 consecutive patients who underwent spectral DECTE at Zhejiang Provincial Hospital of Traditional Chinese Medicine between December 2023 and November 2024, as queried through the Picture Archiving and Communications System (PACS). Of 181 patients, 103 cases without endoscopic and pathological findings were excluded, leaving 78 cases with endoscopic and pathological findings. Based on the inclusion and exclusion criteria, 28 patients with small bowel ulcers, 1 case of unqualified inflammation, and 1 patient with missing images of spectral CT raw data were excluded. The diagnosis of IBD was confirmed in 48 cases. After further exclusion of 4 cases with endoscopic evidence of remission, a total of 44 patients with IBD confirmed as active by endoscopy and pathology were finally included in this study, including 40 cases of CD and 4 cases of UC (Figure 1).

Figure 1 Flowchart of study enrollment. CT, computed tomography; CTE, computed tomography enterography; IBD, inflammatory bowel disease.

DECTE acquisition and image reconstruction

All examinations were conducted on a dual-layer spectral detector CT system (Spectral CT 7500; Philips Healthcare, Amsterdam, The Netherlands) using a triphasic protocol: non-contrast, arterial (35±5 s post-injection), and venous phases (70±10 s). Technical parameters included 120 kVp tube voltage with automated tube current modulation. The patients were asked to adopt a low-residue diet during the day before the examination and fast for over 10 hours before the DECTE examination. At 45 minutes before scanning, the patients were required to drink a mixture of 20% mannitol solution 250 and 2,000 mL of water at 15-minute intervals (45, 30, 15 minutes before scanning). Immediately after drinking, an intramuscular injection of 1 mL of scopolamine was given for about 10–15 minutes, and the patient was asked to wait until they felt thirsty before the CT scan was performed, which would keep the small bowel in a hypoventilated state, ensure that the proximal segment of the small bowel was filled and dilated, and reduce small bowel peristalsis. A 1.2–1.3 mL/kg dose of iodine contrast medium was injected via antecubital vein at a flow rate of 3–4 mL/s using a power injector, followed by a 20 mL saline flush at the same injection rate.

A standardized bolus-tracking protocol positioned the ROI within the abdominal aortic lumen at the celiac trunk level, triggering arterial phase acquisition 9 seconds after reaching the predefined 150 HU enhancement threshold. Venous phase imaging was initiated 35 seconds post-arterial phase completion. The scanning parameters were as follows: tube voltage, 120 kVp; tube current, 50–400 mAs (automated exposure control, with an arterial phase of 160 mAs); scan type, helical; helical pitch, 0.9, detector collimation 112×0.625 mm, smooth filter (filter A), and reconstruction thickness 1.5 mm, and slice spacing 5 mm; thin slice thickness 1 mm, slice spacing 0.8 mm, smooth filter (filter A). The post-acquisition workflow involved transferring spectral base images (SBI) to the IntelliSpace Portal v12.1 platform (Philips) via PACS, where virtual monoenergetic reconstructions were generated across energy levels from 40 to 100 keV in 10 keV increments. Volume CT dose index (CTDIvol) and dose length product (DLP) were recorded for each patient to assess radiation dose for DECTE. Traditional image reconstruction algorithms are not universally applicable. In spectral CT devices, traditional images can be reconstructed through iterative methods such as iDose, but spectral data have dedicated reconstruction algorithms (Philips Spectral: level 0). The window width and window level are set to default, firstly to control variables and ensure that the window width and window level remain unchanged while assessing image quality. Secondly, the low keV mainly increases the CT value of iodine, while the changes in the CT values of other tissues are not particularly significant.

Quantitative image analysis

Objective quantification was independently performed by two board-certified radiologists with more than 8 years of experience in abdominal imaging, and the averaged values were used for analysis. CT values and virtual single-energy CT values were measured in the post-processing workstation on three-phase images of the same patient by selecting the lesion area and the subcutaneous fat ROI, respectively, using copy and paste to ensure that the size of the ROI remained constant (area, 0.4–0.5 cm²). The ROI was placed in the area of the bowel lesion with the most pronounced enhancement and in the subcutaneous fat region at the same level to avoid areas of blood vessels, intramuscular fat, or calcifications, and so on, as much as possible. The attenuation values (AV) acquired using two distinct ROIs inside the gut wall were used to compute a mean bowel attenuation. The noise was represented by the standard deviation (SD) of the mean AV in the ROI. For every dataset, the signal-to-noise ratio (SNR) and CNR were computed at each VMI between 40 and 110 keV. SNR was computed by dividing the mean AV of the affected intestinal wall by the mean background noise. The CNR was found by dividing the difference in mean HU between the muscle and the diseased tissue by the SD of the noise in the image.

Subjective image quality was assessed independently by two radiologists of different expertise, all with more than 8 years of experience in abdominal imaging, who were blinded to the final diagnosis. They rated the overall image quality on a five-point scale, where 5= excellent, no significant artefacts and noise, anatomical details are visible; 4= good, minor artefacts and noise, and most anatomical structures are visible for assessment; 3= fair, noise continues to increase, detailed anatomical structures are not very clear, but meets assessment requirements; 2= poor, obvious noise, detailed anatomical structures are not clear and cannot be identified; and 1= unreadable. An image with a score >3 meets the diagnostic requirements. The average scores of the two readers were used for statistical analysis. The analysis of imaging lesions was derived from a discussion between two radiologists, and in cases of disagreement, a third senior physician participated in the discussion and reached a consensus when the qualitative subjective assessment results were inconsistent.

Statistical analysis

Statistical analysis was performed using the software SPSS 25.0 (IBM Corp., Armonk, NY, USA). Initial data normality assessment employed the Shapiro-Wilk test, categorizing quantitative variables into parametric (mean ± SD) or nonparametric {median [interquartile range (IQR)]} distributions. Differences in quantitative measures were tested by analysis of variance (ANOVA) followed by the Bonferroni-adjusted post-hoc comparisons, and results were corrected using the Greenhouse-Geisser method when Mauchly’s test for sphericity was not satisfied. Pairwise comparisons with Bonferroni corrections were carried out for significant results, and the Friedman test was applied for multi-group comparisons of measures that did not follow a normal distribution and for ordered categorical data. A difference was deemed statistically significant at P<0.05.

Results

Participant characteristics

The study cohort comprised 44 IBD patients [27 males (61.4%), 17 females (38.6%)] undergoing comprehensive diagnostic evaluation, including ileocolonoscopy and spectral CT enterography. Imaging biomarkers characteristic of active IBD pathophysiology—including bowel wall thickening (mean 5.2±1.3 mm), stratified enhancement patterns, luminal stricturing, and mesenteric comb sign manifestations—were confirmed through consensus review by dual board-certified radiologists (8+ years’ experience). Demographic and clinicoradiological characteristics are systematically tabulated in Tables 1,2.

Quantitative image analysis

Quantitative metrics across VMIs (40–100 keV) and conventional PEIs are detailed in Table 3. The CT AV of inflamed bowel walls demonstrated an inverse relationship with energy levels, showing a progressive decrease from 40 to 100 keV in all phases (P<0.001). Notably, 40 keV VMI achieved the highest CT values across non-contrast (57.7±13.2 HU), arterial (172.1±34.6 HU), and venous phases (200.2±46.0 HU), exceeding PEIs by 42.8%, 115.1%, and 125.3%, respectively.

Image noise exhibited a distinct energy-dependent pattern. The noise of VMI 40 keV [non-contrast: 13.1±4.8; arterial: 13.4±4.2; venous: 11.5 (9.1–16.8)] was higher than that of PEI (P>0.05), but VMI 60–100 keV showed significantly reduced noise and was lower than PEI (P<0.001). The lowest noise was observed at 100 keV [non-contrast: 8.4 (6.9–11.3); arterial: 8.5 (7.2–11.2); venous: 9.3±2.8], with adjacent energy groups showing no inter-group differences (adjusted P>0.05).

SNR and CNR followed similar energy-dependent trends (Figure 2). Both metrics peaked at 40 keV across all phases [non-contrast SNR: 4.9 (3.5–7.1), CNR: 17.2 (14.2–25.2); arterial SNR: 12.5±6.1, CNR: 27.4±9.8; venous SNR: 10.8±4.3, CNR: 29.9 (20.0–43.4)], with progressive declines at higher energy levels (P<0.001). VMIs ≤70 keV consistently outperformed PEIs in SNR/CNR (P<0.001), whereas ≥80 keV VMIs showed no significant differences versus PEIs (P>0.05). The specific data are shown in Table 3.

Figure 2 Box-and-Whisker plots showing the SD (A-C) and CNR (D-F) of DLCTE-VMI and PEI during the non-contrast, arterial phase, and portal venous phase. Each box displays the upper and lower quartiles, with the median value indicated by the horizontal line within each box. CNR, contrast-to-noise ratio; DLCTE, dual-layer spectral computed tomography enterography; PEI, polyenergetic imaging; SD, standard deviation; VMI, virtual monoenergetic image.

Qualitative image analysis

Comparative analysis of diagnostic image quality across virtual monoenergetic (VMI 40–100 keV) and conventional PEI revealed significant differential performance (Table 4, Figure 3). VMI reconstructions at 50–70 keV demonstrated superior diagnostic efficacy compared to PEI, with 60 keV achieving peak subjective ratings across all metrics (overall quality: 4.9±0.3 vs. 4.1±0.4, P<0.001; contrast resolution: 4.9±0.3 vs. 4.1±0.3, P<0.001; noise profile: 5.0±0.2 vs. 4.0±0.4, P<0.001). However, the statistical significance was minimal (P>0.05). Overall, the consistency of subjective image quality assessment between the two physicians was relatively good. During the non-contrast phase, only the PEI exhibited moderate consistency, whereas the others demonstrated high consistency. In the arterial phase and portal venous phase, 60 keV showed high consistency, as illustrated in Table 5.

Figure 3 A 47-year-old female diagnosed with CD. Venous imaging revealed PEI (A) and 40–100 keV (B-H, respectively), indicating thickening of the ileus and terminal ileus, complex fistula formation, stratified enhancement of the intestinal wall, and surrounding exudation. The images at 40 keV (B) and 50 keV (C) exhibit greater enhancement accompanied by diminished exudation. The 60 keV (D) image preserves comprehensive lesion information and enhances image quality. The 70 keV (E) image exhibits contrast comparable to that of PEI. High-keV (F-H) reconstructed images reduce contrast enhancement, which is detrimental to the delineation of lesion extent. CD, Crohn’s disease; PEI, polyenergetic imaging.

Radiation dose

The mean CTDIvol, DLP, and ED for precontrast images were 2.59 mGy [95% confidence interval (CI): 2.38–2.80 mGy], 139.13 mGy·cm (126.11–152.14 mGy·cm), and 2.11 mSv (1.17–3.77 mSv), respectively. The mean CTDIvol, DLP, and ED for the arterial phase and portal venous phase were 9.89 mGy (9.03–10.74 mGy), 534.13 mGy·cm (480.91–587.35 mGy·cm), and 8.00 mSv (1.18–14.37 mSv), respectively. Therefore, the mean ED for all three phases was 18.11 mSv (16.45–20.04 mSv).

Discussion

Our study demonstrated that VMI 40 obtained the highest CNR and SNR of the lesion. However, at the same time, the noise of VMI 40 keV is significant, so the diagnostic usability is restricted. Previous studies by Lu et al. (18) using spectral CT on conventional images of CD and VMI of 40, 55, and 70 keV also yielded the same results, and Chen et al. (15) demonstrated that 40 keV VMI significantly improved the accuracy of T-staging of gastric cancer, and Lee et al. (16) reported that 40 keV VMI improved the qualitative assessment of esophageal cancer before treatment. Lee et al. (16) reported that 40 keV VMI could improve the qualitative assessment of esophageal cancer before treatment. In addition, according to our study results, there was no significant difference between the CT values, SNR, and CNR between PEI and VMI 80–100 keV, which is the same as the results reported by Lee et al. (16) and the results of the study by Allocca et al. (17) on the representation of abdominal venous vessels, which may be explained by the fact that the spectral center of a conventional PEI at 120 kVp is 65–70 keV (19).

The noise level of the PEI in this study was significantly higher than that of the VMI_50–100 keV images because the stereo double-layer detector design and the noise reduction algorithm of the spectral CT greatly reduced the noise in the VMI images. The stereo double-layer detector consists of two layers: the upper layer absorbs low-energy photons, whereas the lower layer absorbs high-energy photons. The anti-correlated noise model amplifies the noise in the spectral images through all decomposition functions, with the unique property that the noise is “anti-correlated” between the two basis images; when the noise is positive in one basis image, it tends to be negative in the other. In each VMI image group, the noise level increases slightly as the energy level decreases, but the difference is not significant and remains at a stable level. This finding is consistent with the results of Tao et al. (20) on noise in pancreatic neuroendocrine tumors.

In terms of subjective evaluation, the overall image quality score for VMI 40 keV was comparatively lower, and the 60 keV VMI received the highest overall image quality score. Overall, the consistency of subjective image quality assessment between the two physicians is relatively good. During the non-contrast phase, arterial phase, and portal venous phase, 60 keV showed high consistency. Affected by the best single-level image quality score of 5 points, the variation in PEI scores was greater, resulting in poorer consistency differences. In combination with an objective indicator, it may be more recommended to use VMI 60 keV for image diagnostics.

Our findings established an inverse correlation between VMI energy levels and CNR, with 40 keV reconstructions demonstrating optimal CNR values for both pathological and normal small bowel wall characterization in DECTE (P<0.001 vs. conventional imaging). This CNR enhancement at lower photon energies (40–70 keV) must be balanced against inherent quantum mottle effects that cause heterogeneous signal distribution—particularly in ultralow-energy reconstructions—a finding consistent with the results reported by Arico’ et al. (21). Nagayama et al.’s study (22) demonstrated that the spectral CT in the experimental group showed significantly lower SSDE (17.3 vs. 19.0 mGy) and iodine load (17.4 vs. 35.5 mg/mL) compared to the control group at 120 kVp, with both P values <0.01 when reducing the loading by 50% and radiation dose (SSDE). At 40 keV, the active arterial and hepatic parenchymal CT values were highest, whereas the CNR was optimal; the contrast between images was comparable to that of the 120 kVp group at 55–60 keV.

This study compared single-energy images with PEI and found that 60 keV VMI improved inter-physician agreement and confidence in the diagnosis of active IBD. Chen et al. (15) used dual-energy CTE and found that 60 VMI keV was the best single-energy image and that the best single-energy image had higher inter-physician agreement than the mixed-energy image. This is in line with research showing that the optimal single energy level maximizes image quality and boosts diagnostic confidence in IBD activity.

Our study has several limitations. First, the classification analysis of IBD subtypes was omitted due to limited cases, despite that these subtypes may require distinct optimal energy levels. Future studies with larger cohorts should address this. Second, although we compared VMIs’ effects on image quality, diagnostic accuracy was not statistically analyzed—an assumption requiring validation. Third, endoscopic activity-based stratification was not performed. Given that increased vascular permeability enhances iodine uptake in active lesions (23), and prior evidence supports iodine concentration’s value in CD assessment (24), future work should quantify VMI diagnostic performance across disease severities. Fourth, we excluded patients with extra-intestinal complications (e.g., fistulas/abscesses). As chronic inflammation alters bowel wall microstructure (e.g., edema/fibrosis) and affects CT parameters (25), subsequent studies require systematic subgroup analyses (26). Finally, this study concludes that VMI 60 keV can optimize image diagnosis, but this result is limited to the use of a specific device or configuration, as different types of post-processing can greatly affect image quality.

Conclusions

In summary, the DECTE-derived low keV VMI significantly enhances both the subjective and objective image quality of CTE for its evaluation. When combined with subjective and objective assessment criteria, the images obtained at 60 keV VMI not only effectively minimize noise within a limited range but also deliver a high-quality SNR for the affected intestinal wall. Furthermore, it demonstrates that optimal image quality is advantageous for detecting focal lesions and significantly enhances radiologists’ diagnostic capabilities in identifying active inflammatory intestinal diseases.

Acknowledgments

Philips Healthcare provided technical support for spectral CT reconstruction but had no role in data analysis or interpretation.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1192/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1192/dss

Funding: This research was supported by the Zhejiang Provincial Administration of Traditional Chinese Medicine (No. 2016ZA091) and Zhejiang Provincial Health Science and Technology Program of Zhejiang Provincial Health Commission (No. 2022KY914).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1192/coif). All authors report that Philips Healthcare provided technical support for spectral CT reconstruction but had no role in data analysis or interpretation. This research was supported by the Zhejiang Provincial Administration of Traditional Chinese Medicine (No. 2016ZA091) and Zhejiang Provincial Health Science and Technology Program of Zhejiang Provincial Health Commission (No. 2022KY914). The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by ethics Committee of Zhejiang Provincial Hospital of Traditional Chinese Medicine (No. 2025-KLS-234-01) and because the retrospective study waived the requirement for written informed consent.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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