Large bowel diseases are highly prevalent and contribute significantly to morbidity and mortality worldwide (Arnold et al., 2016)¹. These include a variety of conditions such as colorectal carcinomas, adenomas, polyps, inflammatory bowel diseases, and tubercular infections. In India, tuberculosis remains a common cause of large bowel pathology, adding to the disease burden in this region (Dhali et al., 2021) ². Despite advances in diagnostic modalities, there remains a need for methods that are both accurate and patient-friendly. Conventional radiology has played a pivotal role in diagnosing large bowel diseases for many years. Currently, colonoscopy is regarded as the "gold standard" for diagnosing conditions like colorectal polyps, inflammatory lesions, and neoplastic masses. It allows for direct visualization of the mucosa and enables biopsy or polypectomy during the procedure (Lieberman et al., 2012) ³. However, colonoscopy has limitations, including its invasive nature, the discomfort experienced by patients, and the requirement for extensive bowel preparation. de Wijkerslooth et al. (2012)⁴ reported that patient discomfort and unpleasantness associated with colonoscopy were common reasons for refusal to undergo the procedure. Additionally, colonoscopy cannot visualize extraintestinal structures, and it is contraindicated in many cases, especially among elderly or frail patients (Rex et al., 2015) ⁵.

Computed tomographic colonography (CTC), also known as virtual colonoscopy, offers an alternative diagnostic modality, especially for patients with failed or refused colonoscopy, or when assessing regions proximal to strictures (Johnson et al., 2003) ⁶. CTC provides a less invasive approach with high accuracy in detecting polyps and carcinomas. A critical factor influencing the diagnostic accuracy of CT colonography is adequate luminal distension. Incomplete distension can mimic inflammatory conditions or obscure lesions, leading to false-negative or false-positive results (Johnson et al., 2003) ⁶. Traditionally, luminal distension is achieved by insufflating air or water rectally; however, this process can be uncomfortable and poorly tolerated by patients (Baron et al., 2005) ⁷. Bowel preparation remains an essential component of CT colonography, as optimal cleansing improves visualization and diagnostic accuracy. Nevertheless, bowel preparation is often regarded as the most unpleasant part of the procedure, affecting patient compliance. Consequently, research efforts are directed toward developing less invasive or prep-less methods, including reduced cathartic regimens or eliminating bowel cleansing altogether (Pickhardt et al., 2010) ⁸. Studies comparing various contrast agents for CT enterography have demonstrated that mannitol, a nonabsorbable osmotic agent, can facilitate adequate distension of the small bowel and, in some cases, the proximal large bowel. Its high osmolarity and delayed absorption contribute to its effectiveness in distension without significant absorption or adverse effects (Prakasini et al., 2016) ⁹.

The present study aims to evaluate the feasibility of using mannitol as a neutral oral contrast agent in per oral MDCT colonography. If sufficient distension and mucosal visualization can be achieved with minimal preparation, this approach could minimize the need for rectal agents and improve patient comfort, potentially revolutionizing large bowel imaging without compromising diagnostic quality.

MATERIALS AND METHOD

Ethical Approval and Study Setting 

This study was conducted after obtaining ethical clearance from the Institutional Ethical Committee. It was performed in the Department of Radiodiagnosis and Department of Surgery at Govt. Medical College, Azamgarh, Uttar Pradesh.

Study Population and Criteria 

Patients aged 18 to 70 years, referred for MDCT of the abdomen and pelvis for non-bowel-related indications, and with no prior bowel surgery, were included. Exclusion criteria included intra-abdominal masses causing large bowel compression, perioperative or emergency cases, contraindications to contrast media (such as allergy, renal impairment, or risk factors for contrast nephropathy), and pregnancy. Due to the absence of prior related studies, a sample size of 100 patients was selected for this pilot study, conducted from November 2023 to April 2025.

Preparation Protocol 

Patients followed a low-residue diet for 3 days before the procedure and received oral laxatives (Dulcolax tablets x 2 and Charcoal tablets x 4) for two days prior. On the day of the scan, they fasted overnight and consumed 1500–2000 mL of 3% mannitol solution over 20 minutes. Imaging was scheduled approximately 120–145 minutes after starting contrast ingestion, as most patients could not complete the full volume within 90 minutes. To reduce bowel motility, intravenous Buscopan was administered in most cases.

Imaging Procedure 

Patients were positioned on the CT table, and 100–120 mL of non-ionic iodinated contrast was injected at 3 mL/sec using a power injector, followed by 40 mL of saline. Scans were acquired about 55 seconds after contrast injection with parameters including 120 kVp, variable mAs based on body habitus, collimation of 64 x 0.6 mm, a reconstruction interval of 2 mm, and slice thickness of 1.5 mm.

Image Acquisition and Analysis 

Images were transferred to a workstation for multiplanar reconstruction in axial, coronal, and sagittal planes. Two observers independently evaluated the images for bowel distension, mucosal fold visibility, mural wall visualization, and intraluminal contents. Bowel distension was scored qualitatively on a 4-point scale and quantitatively by measuring the maximum inner diameter of each segment. The large bowel was divided into six segments—rectum, sigmoid, descending, transverse, ascending colon, and cecum—based on established criteria. Presence of fecal matter in sigmoid and rectal segments was documented across different planes.

Outcome Measures 

Primary outcomes included the percentage of segments with complete collapse, partial distension, suboptimal distension, and optimal distension, along with the distribution of maximum diameters categorized into four groups. Secondary outcomes assessed the visibility of mucosal folds and mural walls. Data analysis involved descriptive statistics, and interobserver agreement was evaluated using kappa or intra-class correlation coefficients, with a p-value <0.05 indicating statistical significance.

 RESULTS

Table 1: Age Distribution of Patients Who Underwent Per Oral CT Colonography (N=100)

A total of 100 patients (62 males, 38 females) aged between 18 and 70 years (mean age: 36.04 ± 14.22 years; median age: 35 years) were included. The majority of patients were in the age groups 21–30 and 41–50 years (25% each). Most patients (70%) were aged 21–50 years. The distribution across age groups was as follows: <20 years (15%), 21–30 years (25%), 31–40 years (20%), 41–50 years (25%), 51–60 years (11%), and 61–70 years (4%).

Table 2: Indications For MDCT Abdomen in PA ITNETS Who Underwent Per Oral CT Colonography (CTC)

The primary indications for CT were to exclude abdominal tuberculosis (17 cases), evaluate complications of acute pancreatitis (14 cases), and assess gall bladder or hepatic lesions.

Table 3: Qualitative Analysis of Colonic Distension or Per Oral CT Colonography by Reviewer-1 And Reviewer-2 (N=100)

Both reviewers independently evaluated colonic distension, assigning scores from 1 (complete collapse) to 4 (optimal distension). The majority of segments showed optimal distension (Score 4), with the highest rates in the ascending (R1: 89%, R2: 89%) and descending colon (R1: 85%, R2: 85%). The rectum had the lowest optimal distension rate (R1: 62%, R2: 58%).

Partial collapse (Score 2) was observed in 2–10% of segments, while complete collapse (Score 1) was rare, seen in 4–5% of rectal segments and in isolated cases of sigmoid, ascending, and caecal segments. Complete rectal collapse was associated with patient discomfort during the procedure.

Inter-Reviewer Agreement

The agreement between reviewers was excellent for all segments: near perfect agreement with weighted kappa values ranging from 0.820 (descending colon) to 0.979 (ascending colon), all statistically significant (p<0.001).

Table 4: Quantitative Analysis: Range of Maximum Diameter of Colon in Per Oral CT Colonography

SD- Standard Deviation

IQR-interquartile range

Maximum colonic diameters ranged from 1.0 cm to 7.5 cm. The ascending colon exhibited the greatest distension (mean: 5.15 ± 0.9 cm), followed by the caecum (mean: 4.57 ± 1.01 cm). The sigmoid (mean: 3.38 ± 0.67 cm) and descending colon (mean: 3.43 ± 0.51 cm) showed comparatively less distension.

Table 5: Grouping Of Quantitative Analysi of Colonic Distension On Per Oral CT Colonography (CTC)

Diameters were categorized into four groups (Table 5):

Group A (2.00–3.50 cm): most common in sigmoid and descending colon (>50% of segments).

Group B (3.51–5.00 cm): predominant in the ascending colon and caecum, representing 33–53% of segments.

Group C (5.01–6.50 cm): observed mainly in the ascending colon (R1: 51.5%, R2: 52.5%).

Group D (6.51–8.00 cm): least common, observed in the rectum and ascending colon (7–8%).

Correlation analysis demonstrated very strong agreement between reviewers (interclass correlation coefficients >0.95; p<0.001), with over 92% of measurements within limits of agreement.

Maximum colonic diameters were measured in most patients, with some segments (sigmoid, ascending colon, caecum) excluded due to complete collapse. The most common diameter range was 3.51–5.00 cm (Group B), indicating moderate distension. Over half of the segments in the sigmoid and descending colon had diameters in the 2.00–3.50 cm range (Group A), reflecting poor distensibility. The ascending colon showed the best distensibility, with many segments falling into the 5.01–6.50 cm range (Group C) and some into over-distended Group D (6.51–8.00 cm), especially in the ascending colon and rectum. Overall, the ascending colon demonstrated the highest distension capacity, while the sigmoid and descending colon showed limited distensibility in this study.

Table 6: Mucosal Fold and Mural Visualization in Various Segments of Distended Colon in Per CT Colonography: R-1 And R-2(N = 100)

Mural and Mucosal Fold Visualization (Table 6)

Good visualization of mucosal folds was achieved in most segments. Mural visualization was hampered primarily in segments with fecal residue or complete lumen collapse. Fecal residues were most commonly observed in the rectum (50%), followed by the sigmoid colon (33%).

Procedural Tolerability and Complications

Pain abdomen: 20%, Vomiting: 11%, Nausea: 9% and Loose motions: 5%.

The mean time to consume 1500–2000 ml of 3% mannitol was 45.85 minutes (range: 40–80 minutes). The interval from initiation of mannitol intake to CT acquisition ranged from 90 to 145 minutes, with a mean of 117.3 minutes (~2 hours). The majority (54%) underwent imaging between 120–140 minutes after starting mannitol.

Table 7: Presence Of Fecal Residue in Various Segments of Colon in Per Oral Ct Colonography (N = 100)

The length of each large bowel segment was divided into three equal parts; fecal residue in all three parts was labeled 3+, in two parts 2+, and in one part 1+.

Fecal residue was most commonly observed in the rectum (50%), followed by the sigmoid colon (33%) and descending colon (11%). In the ascending colon and caecum, 98–99% of patients had no fecal matter.

Table 8: Patient Problems During Per Oral Ct Colonography (n=100)

Mild pain abdomen was the most common complaint during Per oral CT Colonography (while consuming 1500-2000 ml of 3% mannitol solution orally), in 20% patients. Other common complaints were vomiting, nausea, and loose stools in 11%, 9% and 5% cases respectively.

Qualitative Assessment:

Agreement between two reviewers on colonic distension was evaluated using Weighted Kappa statistics. Near-perfect agreement was observed across all segments: rectum (Kappa=0.920, p<0.001), sigmoid (0.878, p<0.001), descending (0.820, p<0.001), transverse (0.901, p<0.001), ascending (0.979, p<0.001), and caecum (0.941, p<0.001). Agreement percentages ranged from 89% to 99%.

Quantitative Assessment:

Maximum colonic diameters measured by both reviewers were analyzed using scatter plots (R1 vs. R2) and Bland-Altman plots. Scatter diagrams visualized measurement correlation, while Bland-Altman plots assessed agreement by plotting the mean of R1 and R2 measurements against their differences for each colon segment

DISCUSSION

This pilot study evaluated peroral CT colonography using 1500–2000 ml of 3% mannitol, optimized with appropriate CT delay to achieve adequate colonic distension and clear visualization of the bowel wall. In a cohort of 100 patients aged 18–70, most colonic segments demonstrated optimal or excellent distension, particularly in the cecum, ascending, and transverse colon. Qualitative assessment revealed clear mural and mucosal fold visualization, although occasional issues with fecal residue and collapse, especially in the rectum and sigmoid colon, impacted image quality. Quantitative measurements indicated the highest distension in the ascending colon (~5.13 cm), with most segments falling within the good distension range. Minor adverse effects such as nausea and loose stools were noted during mannitol intake. Overall, this approach effectively provides good colonic distension and visualization, with potential benefits in patient comfort by reducing the need for rectal preparation and insufflation.

Our findings align with previous research demonstrating the efficacy of oral contrast agents like mannitol in achieving adequate colonic distension for CT colonography. For example, Rathi et al. (10) reported optimal colonic distension (score 4) in 58–89% of cases, with high inter-observer agreement (weighted kappa 0.820–0.979, p<0.001). Their study observed that most colonic segments attained good mural and mucosal fold visualization, particularly in proximal segments, similar to our results. Quantitative analysis in their research showed mean maximum diameters ranging from 3.4 to 5.2 cm, closely matching our measurements, especially in the ascending colon and cecum.

Similarly, Zheng et al. (11) conducted a randomized comparison between mannitol and polyethylene glycol (PEG) in 70 patients. They found no significant difference in bowel distension or wall visualization between the two agents (p>0.05). Notably, patients in the mannitol group reported significantly lower nausea scores and higher satisfaction levels (p<0.001 for nausea; p<0.05 for tolerability), supporting our observations that mannitol offers comparable imaging quality with improved patient tolerability.

Further supporting evidence comes from Khan et al. (12), who evaluated six bowel preparation regimens—including water, 1%, 2%, and 3% mannitol, and combinations with psyllium. They concluded that 3% mannitol combined with psyllium provided optimal bowel distension and mucosal visualization, with enhanced tolerability compared to mannitol alone. Their study emphasized the synergistic benefits of adding psyllium to improve distension while minimizing adverse effects—a finding that our study further supports in a randomized, double-blind setting with extended adverse event monitoring.

In summary, these studies corroborate our findings that oral mannitol at 1500–2000 ml effectively achieves adequate colonic distension and mucosal visualization, with high patient acceptability. Notably, Prakashini et al. (9) demonstrated the utility of water, mannitol, and iodine-based contrast agents in bowel analysis, highlighting the importance of optimal contrast for diagnostic accuracy. Additionally, Wong et al. (13) compared mannitol versus VoLumen for CT enterography, concluding that mannitol provides comparable distension and visualization with favorable tolerability profiles.

Additional relevant studies include Meiklejohn et al. (14), who explored minimal preparation colonography for detecting colorectal cancer among elderly and frail patients, emphasizing the need for patient-friendly protocols. Patil et al. (15) conducted an audit of colorectal cancer cases in India, providing insights into disease prevalence and screening challenges in low-prevalence areas, underscoring the importance of effective, minimally invasive diagnostic methods. Furthermore, Issa and Noureddine (16) reviewed current options for colorectal cancer screening, highlighting recent advancements and the importance of tailored strategies.

Advances in imaging techniques, such as CT colonography, have been evaluated for their efficacy and patient tolerability. For example, Nagata et al. (17) conducted a randomized study comparing colonic distention using IV hyoscine butylbromide and automated CO₂ insufflation, both proving effective for optimal visualization. The choice of bowel preparation and insufflation technique remains vital for diagnostic accuracy and patient comfort, underscoring the need for tailored protocols in CRC screening.

Despite some limitations, including variability in mannitol consumption time, patient tolerance, and the presence of fecal residues, the method shows promise—particularly for right-sided colon evaluation, with excellent distension observed in the cecum and ascending colon in most cases.

CONCLUSION

Peroral CTC with 1500–2000 ml of mannitol is a feasible, patient-friendly alternative for colon evaluation, particularly effective for right-sided colonic pathology. Optimizing timing and patient selection may further improve distension and diagnostic quality.

Recommendations:

1) This procedure is suitable for outpatients with suspected colonic diseases, as most patients can tolerate the large volume of mannitol without significant discomfort, pain, or vomiting.

2) Given the excellent distension observed in the right colon—89% in the ascending colon and 73% in the caecum—peroral CT colonography is particularly useful for evaluating diseases of the right side of the colon.

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