Basic research for validation of MRI
Several initial studies have validated MRI techniques for studying gastric physiology.
In 1999, the applicability of MRI to assess gastric emptying and motility of liquid and solid meals was studied by Kunz et al. [5]. Gastric emptying and motility of a liquid and a solid meal were studied in eight volunteers. Gastric emptying of the liquid meal was faster than emptying of the solid meal when considering half-times of emptying. Hereby, this study showed that with MRI it was possible to evaluate gastric motility and gastric emptying, and at the same time it provided an important new insight into the physiology of gastric emptying of different gastric contents.
In 2002, MRI was validated for evaluation of gastric accomodation with the barostat method, which is considered the gold standard for evaluation of postprandial accommodation of the gastric wall. In the barostat method, the subject ingests an intragastric balloon that is connected to a device that maintains the pressure in the barostat balloon at a certain level. When the stomach wall relaxes by postprandial relaxation, the system starts to sufflate air into the balloon to maintain the pressure inside the balloon to the preselected level. Measurements of gastric volume and motility with MRI were compared with simultaneously performed measurements with a barostat. MR images and barostat measurements were obtained both at rest and after infusion of glucagon and erythromycin, which alter gastric volume and motility. Volume measurements with MRI followed volume changes of the barostat balloon. That study showed that MRI is as accurate as barostat measurement in determining changes in gastric volume [10]. The same study raised questions about the influence of the barostat balloon on gastric physiology. To address this issue, gastric accommodation, motility and emptying have been studied twice in 14 healthy subjects with MRI once in the presence of a barostat bag and once when the barostat bag was not present. Fasting and postprandial intragastric volumes were significantly higher in the experiment with a barostat vs. without a barostat. No significant differences were found in gastric emptying and contraction frequency between both experiments. The accommodation response observed in the presence of the barostat bag was not observed in the absence of the barostat bag. The presence of an intragastric barostat bag did not interfere with gastric emptying or motility, but the accommodation response measured with the barostat in situ is not observed without the barostat bag in situ [23]. Postprandial gastric accomodation shown as gastric wall distension is solely an effect caused by the barostat device. The intragastric meal alone does not cause the stomach to distend to the same proportions as the barostat balloon, which distends the stomach more pronouncedly than the intragastric meal volume.
Demonstrating that MRI is able to detect changes in gastric physiology is the basis of the validation of MRI for evaluation of gastric motility and emptying disorders. These changes in emptying and motility can, of course, be caused by the most natural way, i.e., food that acts as both a mechanical and a chemical stimulus to gastric function, but also more artificially by infusion of pharmacological substances. The evaluation of the effect of these pharmacological stimuli in itself forms the basis of development and evaluation of future pharmacological therapy for disturbances of gastric function disorders.
Lauenstein et al. assessed the effect of intravenously administered erythromycin on gastric emptying and subsequent small-bowel filling using three-dimensional (3D) MRI in both healthy subjects and patients with functional dyspepsia [17]. Six healthy volunteers and six patients with symptoms of functional dyspepsia ingested 500 ml of a gadolinium-labeled, fluid meal. In healthy volunteers, gastric volumes decreased significantly more after the administration of erythromycin. In three patients with functional dyspepsia, MRI revealed reduced rates of gastric emptying. The administration of erythromycin resulted in a significantly faster rate of gastric emptying in two of those three patients, indicating the possible therapeutic effect of this drug.
Ajaj et al. determined the practicality of MRI for the assessment of gastric motion, and tried to quantify the effects of metoclopramide and scopolamine [22]. The intravenous application of these substances resulted in significant changes in the motility index. The administration of metoclopramide resulted in an average increase of the index by a factor of 1.5, whereas the application of scopolamine led to a decrease of the index by a factor of 3.0 [22].
One could criticize MRI because in most MRI scanners, subjects are studied in a non-physiological body position, i.e., supine instead of upright or seated, which might cause a different meal distribution inside the stomach, which might influence gastric physiology (Fig. 5). Several studies were aimed at evaluation of these possible disadvantages of MRI as compared to other validated examination methods. Treier et al. determined the effect of the right decubitus lying body position on relevant parameters of human gastric motor function in healthy volunteers (see Fig. 6) [18]. In this study, postprandial gastric function after ingestion of a solid/liquid meal was assessed in volunteers in the right decubitus position and seated position. Stomach and intragastric air volume, intragastric meal distribution, gastric emptying and gastric peristalsis were compared between the right decubitus position and seated position. It was shown that body position did not affect gastric relaxation and initial gastric volumes. Postprandial stomach volume and gastric activity were also similar. Meal emptying showed different characteristics, resulting in a significant but small difference in meal volume (see Fig. 6) that could have been induced by posture-dependent vagal activity. This study is valuable because it confirmed that MRI in the right decubitus position is a reliable imaging technique for assessing gastrointestinal physiology and that results from MRI studies in the right decubitus position can be compared to studies performed in the sitting position. This is a solution to the fact that MRI in the sitting position is not widely possible to perform.
Fig. 5  a Left: Volunteer in RP inside the 1.5-T whole-body MRI system. Rectangular surface coils were fixed around the abdomen. Right: Three sagittal MR image slices (two proximal and one distal) of a volume scan in RP with outlined stomach wall. Air and meal are indicated in the MR images by arrows (F: feet, H: head, A: anterior, P: posterior). b Left: Volunteer sitting inside the 0.5-T open-configuration MRI system. A send-receive coil was fixed around the abdomen. Right: Three sagittal MR image slices (two proximal and one distal) of a volume scan in SP with outlined stomach wall. Air and meal are indicated in the MR images by arrows (F: feet, H: head, A: anterior, P: posterior)
Fig. 6 Normalized gastric emptying curves (percentage of remaining meal volume in the stomach) for RP (black) and SP (gray). Data are expressed as the median (interquartile range). A divergence of the emptying curves is observed, especially during the first 30 min after meal ingestion