Gas-induced susceptibility artefacts on diffusion-weighted MRI of the rectum at 1.5 T – Effect of applying a micro-enema to improve image quality

Abstract

Purpose

Assess whether application of a micro-enema can reduce gas-induced susceptibility artefacts in Single-shot Echo Planar Imaging (EPI) Diffusion-weighted imaging of the rectum at 1.5 T.

Materials and methods

Retrospective analysis of n = 50 rectal cancer patients who each underwent multiple DWI-MRIs (1.5 T) from 2012 to 2016 as part of routine follow-up during a watch-and-wait approach after chemoradiotherapy. From March 2014 DWI-MRIs were routinely acquired after application of a preparatory micro-enema (Microlax^®; 5 ml; self-administered shortly before acquisition); before March 2014 no bowel preparation was given. In total, 335 scans were scored by an experienced reader for the presence/severity of gas-artefacts (on b1000 DWI), ranging from 0 (no artefact) to 5 (severe artefact). A score ≥3 (moderate-severe) was considered a clinically relevant artefact. A random sample of 100 scans was re-assessed by a second independent reader to study inter-observer effects. Scores were compared between the scans performed without and with a preparatory micro-enema using univariable and multivariable logistic regression taking into account potential confounding factors (age/gender, acquisition parameters, MRI-hardware, rectoscopy prior to MRI).

Results

Clinically relevant gas-artefacts were seen in 24.3% (no micro-enema) vs. 3.7% (micro-enema), odds ratios were 0.118 in univariable and 0.230 in multivariable regression (P = 0.0005 and 0.0291). Mean severity score (±SD) was 1.19 ± 1.71 (no-enema) vs 0.32 ± 0.77 (micro-enema), odds ratios were 0.321 (P < 0.0001) and 0.489 (P = 0.0461) in uni- and multivariable regression, respectively. Inter-observer agreement was excellent (κ0.85).

Conclusion

Use of a preparatory micro-enema shortly before rectal EPI-DWI examinations performed at 1.5 T MRI significantly reduces both the incidence and severity of gas-induced artefacts, compared to examinations performed without bowel preparation.

Introduction

Diffusion-weighted imaging (DWI) is nowadays increasingly adopted as an integral part of oncologic imaging protocols. In rectal cancer, DWI has mainly shown its value for response evaluation and follow-up of rectal tumors after chemoradiotherapy, specifically for the discrimination of viable tumor within areas of post-radiation fibrosis [[1], [2], [3], [4], [5]].

The most commonly used sequence for abdominal diffusion imaging is a single-shot Echo Planar Imaging (EPI) sequence. The main benefit from an EPI approach is its short acquisition time, which minimizes the risk of motion artefacts. However, an important drawback is that EPI sequences are prone to susceptibility artefacts, particularly at higher field strengths [6,7]. Susceptibility artefacts are changes or distortions in image signal caused by local magnetic field inhomogeneities, for example due to the presence of metal objects (e.g. hip replacements or surgical clips). In bowel imaging, these artefacts are mainly caused by the presence of gas in the rectal lumen. In a study by Caglic et al. (in prostate MRIs) it was reported that increased rectal gas-distension correlates significantly with reduced DWI image quality and increased DWI artefacts [8]. Particularly when the bowel itself is the organ under investigation, gas-induced susceptibility artefacts can severely reduce the diagnostic image quality, in some cases even rendering the images non-diagnostic. In published reports on bowel DWI 4–11% of patients had to be excluded from analyses due to poor DWI scan quality [[9], [10], [11], [12]].

To reduce the influence of these artefacts on image quality, two main strategies can be employed: 1) change the acquisition parameters (i.e. type of DWI sequence) or 2) remove the cause of the artefact.

So far, most published studies have focused on the first approach and tested alternative ways of DWI image acquisition such as parallel imaging [13], smaller Field of View (FOV) [14] or bipolar DWI acquisition [15]. A potential solution to remove the cause of the artefact is to reduce the amount of air in the rectal lumen by rectal filling, where the rectum is filled with a liquid (such as ultrasound gel) prior to image acquisition, replacing the gas. However, a potential downside of this approach is that it causes distension of rectum and compression of the surrounding mesorectal fat [16,17], potentially hampering correct assessment of the relation between the tumor and mesorectal fascia [18]. Use of endorectal filling is therefore not routinely recommended [19].

An alternative potential solution is the application of a preparatory micro-enema shortly prior to image acquisition. A micro-enema can typically be self-administered by the patient to reduce the amount of gas (and stool) in the rectum.

The aim of this study was to test this hypothesis and investigate to what extend the use of a micro-enema can reduce the amount of gas-induced susceptibility artefacts on EPI-DWI of the rectum.