Data analysis
Gastric emptying
As part of the post-processing, in most studies, the outline of the stomach is drawn manually and special software (such as MASS© [21]) is used. Slices encompassing the entire stomach are then summed up to measure momentary gastric volume. As a function in time, the percentage in decrease of the gastric volume represents gastric emptying. As gastric secretions mix with the meal and thereby influence gastric volume, Kunz et al. designed a robust method to correct gastric meal volume for progressive dilution due to gastric secretion [4]. First, the relationship between dilution of gastric content by gastric secretions and signal intensity for meals was experimentally determined by filling small bottles with defined quantities of the meal and mixing them with increasing quantities of hydrochloric acid. Then, the bottles were imaged simultaneously in a single plane. With liquid meals, an exponential correlation was found between the concentration of hydrochloric acid and the signal intensity. During gastric emptying studies, an external standard (containing the same meal that the subject had just ingested consisting of a mixture of fluid or solid food mixed with contrast medium) was placed in a polyethylene tube beside the subject and imaged simultaneously. The image intensity of the tube content served as a reference to correct for gastric secretion. An increase in gastric secretion resulted in a decrease in the signal intensity of the gastric contents [8]. (2) Gastric motility
When gastric motility is studied, most attention is given to gastric peristalsis. In most studies, gastric diameters were measured at equally distributed points perpendicular to the stomach axis. On the basis of this diameter calculation, peristaltic contractions are detected, and their frequency is calculated [4, 15]. Other authors advocate the use of dark parallel tagging lines that can be applied in the images in the cranio-caudal direction in a manner similar to that described for cardiac MRI. The gap between two tagging lines can be measured, and the distance that a peristaltic wave had passed, by counting the number of tagging lines, can be quantified (Figs. 3 and 4) [3, 29].
Fig. 3 For comparison purposes, T2-weighted and T1-weighted hydro-MR images of subject 1 are shown. a T2-weighted hydro-MR coronal projection image of subject 1: The stomach is loaded with 500 ml of 10% dextrose solution. b T1-weighted coronal projection image of subject 1 24 h later. The stomach is loaded with 500 ml of 10% dextrose solution and 1.0 mM of the contrast agent Gd-DOTA
Fig. 4  a T2-weighted coronal slice-selective images of subject 1 depicting the stomach in its full J-shaped form. For dynamic study, respiratory gating determined data acquisition at times 0, 3.2, 6.5, 9.5 and 13.3 s. The contractions started in the proximal part of the body and moved aborally toward the antrum. The mean velocity was approximately 49.5 cm/min. b Transaxial scan through the middle part of the stomach. Nearly symmetrically and concentrically configured constrictor rings can be assessed. The in-plane resolution of 1.4 mm × 1.4 mm was sufficient to identify the longitudinal fissure of the gastric wall