Purpose

There is a lack of consensus regarding the necessity or appropriateness of intra-rectal gel administration for rectal cancer MRI.¹ Recently, one point of concern was that poor suppression of the gel on diffusion-weighting imaging (DWI) sequences might result in T2 shine through and significantly effect a radiologist’s ability to evaluate the presence and extent of disease.¹ However, this remains largely untested. Adequacy of DWI evaluation is important to the accurate assessment of rectal cancer, especially in the evaluation of possible treatment response.^2-5 Thus, the purpose of this study was retrospectively to investigate the effectiveness of intra-rectal gel suppression on rectal cancer MR diffusion-weighted imaging (DWI) and its effect on diagnostic adequacy of the study for tumor evaluation.

Methods

Patients who underwent rectal cancer MRI for initial staging, treatment response assessment, or surveillance from March 29-August 20, 2021 were included. Standard preparation included rectal enemas, administration of rectal gel (100 mL), and intravenous glucagon. Reduced field of view DWI, using field-of-view optimized and constrained undistorted single-shot (FOCUS) acquisition, was obtained (b=0, 1000 s/mm²). Apparent diffusion coefficient maps and a computed synthetic high b-value series (b=1500 s/mm²) were created. GE Healthcare (Waukesha, WI USA) MR scanners were utilized. Fifteen MR examinations were completed on a 3 tesla scanner (GE Signa Premier, n=14; Signa Architect, n=1) and 35 examinations on a 1.5 tesla scanner (GE Signa Artist, n=34; GE 450w, n=1). DWI acquisition parameters were as follows: slice thickness/gap 4.8/0.2 mm, field of view 32 x 16 cm, 9 signal averages, repetition time of 4500 ms, echo time of 52.0-66.9 ms.

Rectal gel suppression was graded independently by 2 board certified, abdominal radiologists, with 10 and 4 years of rectal MR experience respectively. Suppression was graded on a 5 point Likert scale with a score of 4-5 considered fully diagnostic and scores of 1-3 as demonstrating limitations causing diagnostic uncertainty due to incomplete signal suppression from gel (5, fully suppressed; 4, minimal non-suppression, but diagnostic; 3, mild diagnostic uncertainty due to suppression; 2, major diagnostic limitation; 1, no suppression). Luminal air and associated susceptibility artifact were also evaluated. Wilcoxon signed rank test was used to test the distribution of rectal gel suppression scores.

Results

Sixty patients were included (age 59.3±11.4 years; 26 female, 34 male). Study indications included initial staging (n=26), treatment response (n=23), and surveillance (n=11). Fully diagnostic suppression achieved in 57/60 (95.0%) and 53/60 (88.3%) cases for reader 1 and 2, respectively, and the median suppression score was significantly greater than 3 for both (p<0.05). Minor limitations but leading to an element of diagnostic uncertainty (score of 3) were present in 3 and 7 studies, respectively. No cases were given a score of 1 or 2 (i.e., marked incomplete suppression) by either reader. Gel completely filled the lumen at the level of interest in approximately half of cases, 32/60 and 30/60 respectively. If present, luminal gas typically caused a minor artifact involving <90⁰ circumference (16/28 and 20/30, respectively).

Conclusions

Rectal gel suppression was fully diagnostic in the vast majority of cases for both readers. When limitations were present due to incomplete suppression they were only minor in degree. These results suggest that gel does not impede reader evaluation for tumor restriction at diffusion weighted imaging.

Acknowledgements

No acknowledgement found.