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
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.
Section snippets
Materials and methods
The study was approved by the local institutional review board. Due to the retrospective nature of the study, informed consent was not required.
Baseline characteristics
Table 2 shows the baseline study characteristics. The 50 study patients together underwent a total of 364 DWI-MRI examinations. Twenty-nine scans from the transit period after the introduction of the micro-enema (March 2014–June 2014) were excluded. This left a total of 335 scans for analysis (mean 6.7 examinations per patient, range 2–10), of which 226 (67.5%) were acquired without a micro-enema (before March 2014) and 109 (32.5%) were acquired with a micro-enema (after June 2014).
Intersobserver agreement
Agreement
Discussion
The results of our study show that the application of a preparatory micro-enema shortly before image acquisition significantly reduces both the incidence as well as the severity of gas-induced susceptibility on rectal DWI performed at 1.5 T. The use of a micro-enema reduces the proportion of rectal DWI scans suffering from clinically relevant artefacts – i.e. artefacts that hamper clinical image interpretation – from 1 in every 4 scans to 1 in every 20 scans, thereby offering a substantial
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. No funding was received for this study.
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