Contrast-Enhanced Magnetic Resonance Imaging of Gastric Emptying and Motility in Rats

Abstract

The assessment of gastric emptying and motility in humans and animals typically requires radioactive imaging or invasive measurements. Here, we developed a robust strategy to image and characterize gastric emptying and motility in rats based on contrast-enhanced magnetic resonance imaging (MRI) and computer-assisted image processing. The animals were trained to naturally consume a gadolinium-labeled dietgel while bypassing any need for oral gavage. Following this test meal, the animals were scanned under low-dose anesthesia for high-resolution T1-weighted MRI in 7 Tesla, visualizing the time-varying distribution of the meal with greatly enhanced contrast against non-gastrointestinal (GI) tissues. Such contrast-enhanced images not only depicted the gastric anatomy, but also captured and quantified stomach emptying, intestinal filling, antral contraction, and intestinal absorption with fully automated image processing. Over four postingestion hours, the stomach emptied by 27%, largely attributed to the emptying of the forestomach rather than the corpus and the antrum, and most notable during the first 30 min. Stomach emptying was accompanied by intestinal filling for the first 2 h, whereas afterward intestinal absorption was observable as cumulative contrast enhancement in the renal medulla. The antral contraction was captured as a peristaltic wave propagating from the proximal to distal antrum. The frequency, velocity, and amplitude of the antral contraction were on average 6.34 ± 0.07 contractions per minute, 0.67 ± 0.01 mm/s, and 30.58 ± 1.03%, respectively. These results demonstrate an optimized MRI-based strategy to assess gastric emptying and motility in healthy rats, paving the way for using this technique to understand GI diseases, or test new therapeutics in rat models.The assessment of gastric emptying and motility in humans and animals typically requires radioactive imaging or invasive measurements. Here, we developed a robust strategy to image and characterize gastric emptying and motility in rats based on contrast-enhanced magnetic resonance imaging (MRI) and computer-assisted image processing. The animals were trained to naturally consume a gadolinium-labeled dietgel while bypassing any need for oral gavage. Following this test meal, the animals were scanned under low-dose anesthesia for high-resolution T1-weighted MRI in 7 Tesla, visualizing the time-varying distribution of the meal with greatly enhanced contrast against non-gastrointestinal (GI) tissues. Such contrast-enhanced images not only depicted the gastric anatomy, but also captured and quantified stomach emptying, intestinal filling, antral contraction, and intestinal absorption with fully automated image processing. Over four postingestion hours, the stomach emptied by 27%, largely attributed to the emptying of the forestomach rather than the corpus and the antrum, and most notable during the first 30 min. Stomach emptying was accompanied by intestinal filling for the first 2 h, whereas afterward intestinal absorption was observable as cumulative contrast enhancement in the renal medulla. The antral contraction was captured as a peristaltic wave propagating from the proximal to distal antrum. The frequency, velocity, and amplitude of the antral contraction were on average 6.34 ± 0.07 contractions per minute, 0.67 ± 0.01 mm/s, and 30.58 ± 1.03%, respectively. These results demonstrate an optimized MRI-based strategy to assess gastric emptying and motility in healthy rats, paving the way for using this technique to understand GI diseases, or test new therapeutics in rat models.

Keywords: Image segmentation; Magnetic resonance imaging; Rats; Rodents; Stomach.

Fig. 1.

Imaging sequence and slice package selection. A. 2D multi-slice volumetric scan (denoted as “Volumetric”) and fast antral motility scan (denoted as “Fast”) were performed interleaved throughout the experiment. B. Two example T2-weighted images (left: coronal view, right: sagittal view) acquired from a multi-slice localizer sequence. An oblique 30-slice package was placed along the long axis of the stomach from the sagittal image. C. Example semi-coronal view images from the 2D multi-slice volumetric scan. D. Fast scan of the antrum with four slices. The position of the slice package was determined from images acquired from the volumetric scan.

Fig. 2.

Image segmentation and classification of the GI tract. A. Contrast-enhanced gastric MRI with volumetric scans. B. Preliminary segmentation of the GI tract with 3D fuzzy c-means clustering algorithm. C. Refinement of the segmentation result with 2D localized active contour method. D. Partition of the stomach and the intestine. E. Maximum intensity projection (MIP) of the stomach. F. A stomach mask obtained from all non-zero voxels from the MIP of the stomach. The forestomach and the corpus were separated along the red dotted line, and the corpus and the antrum was separated along the blue dotted line. G. Partition of the forestomach, the corpus, and the antrum on an example slice. H. Partition of the forestomach, the corpus, the antrum, and the intestine on 8 example slices.

Fig. 3.

Image analysis of antral motility. A. Contrast-enhanced gastric MRI with fast sampling. B. Segmentation of the antrum. C. Temporal progression of the 3-D antral volume reveals the peristaltic wave. D. Changes in the cross-sectional area at different locations along the long axis of the antrum (from the proximal to the distal) are shown as time series or an image, both characterizing the antral motility. The red arrows mark the occurrence and progression of an antral contraction. E. The frequency and amplitude of the antral motility time series reveals the contraction frequency and amplitude. The distensions are marked with * and the contractions are marked with o. F. The spatial gradient of the phase of the motility time series reports the velocity of the peristaltic wave.

Fig. 4.

Total and compartmental gastric emptying profiles. A. 3D volume rendering of the GI tract. B. Change of the total GI volume. C. Stomach emptying profile. D. Intestinal filling profile. E. Forestomach emptying profile. F. Corpus emptying profile. G. Antrum emptying profile. All volumes were normalized against the total GI volume at time 0. Values are mean±standard error of the mean.

Fig. 5.

Representative slices of temporal progression of gastric emptying in different stomach compartments. A. Forestomach emptying. B. Corpus emptying. C. Antrum emptying.

Fig. 6.

Correlates of gastric emptying rate and antral motility. A. Correlation between volume change in the stomach (%) and the antral contraction frequency (r = 0.0056). B. Correlation between volume change in the stomach (%) and the antral contraction velocity (r = 0.0555). C. Correlation between volume change in the stomach (%) and the antral contraction amplitude (r = 0.2709).

Fig. 7.

The extent of absorption of nutrients measured in terms of kidney handling of systemically circulating gadolinium. Representative slices of temporal progression of signal intensity enhancement in the renal medulla.