Gastric emptying scintigraphy

Initial gastric accommodation

Gastric accommodation is a postprandial reflex resulting in reduced proximal gastric tone that occurs with eating a meal . With gastric accommodation, the fundus acts to provide a reservoir function for ingested foods without significantly increasing intragastric pressure. The accommodation reflex has two principal components. Receptive relaxation occurs within seconds of eating and is triggered by both oropharyngeal and gastric stimulation. This response involves relaxation of both the lower esophageal sphincter and proximal stomach. Adaptive relaxation is a slower process triggered by gastric or duodenal distension and likely also modified by specific nutrients . The accommodation reflex is vagally mediated and represents the balance between cholinergic excitatory drive and non-adrenergic, non-cholinergic (NANC) inhibitory input. The afferent signal is generated by activation of stretch-sensitive mechanoreceptors in the stomach wall and by activation of osmolar and chemoreceptors in the stomach and duodenum . The efferent NANC signal involves nitric oxide (NO) as the principal neurotransmitter . A role for vasoactive intestinal polypeptide (VIP) has also been suggested . Gastric tone is also modulated by sympathetic inputs acting directly through post-junctional α1-adrenoceptors, and indirectly on cholinergic nerve terminals mediated by pre-junctional α2-adrenoceptors .

With normal fundic accommodation, the food initially localizes in the proximal portion of the stomach ( Fig. 11.1 ). Impaired gastric accommodation (GA) to a meal may cause postprandial symptoms . With impaired GA, there is increased pressure in the upper stomach compromising the ability of the upper stomach to act as a reservoir for ingested food. This can result in rapid transit into the distal stomach. Impaired accommodation has been shown to be associated with early satiety and weight loss in both functional dyspepsia and idiopathic gastroparesis .

The accommodation reflex can be recorded using an intragastric barostatically controlled balloon which directly records intragastric volume and pressure. This methodology is however invasive, not widely available, and not well tolerated by patients. As a result, alternate and less invasive imaging methods have been sought to measure gastric accommodation.

Single photon emission computed tomography (SPECT) utilizes 3-dimensional nuclear medicine imaging following an injection of technetium-99m pertechnetate which outlines the outer volume of the gastric wall by imaging gastric mucosal uptake similar to its use for visualizing Meckel diverticula ( Fig. 11.2 ). SPECT measurements have been well correlated with the intragastric balloon and have demonstrated that improvement in functional dyspepsia symptoms with low dose antidepressants are associated with increased SPECT measured accommodation .

As the intragastric balloon and SPECT technologies are only available at a limited number of specialized clinical laboratories, there have been recent studies showing that one can obtain an index of gastric accommodation as a part of the more widely available, conventional 2-dimensional imaging of solid-meal GES. This is done by evaluating intragastric meal distribution (IMD) from the images of the solid meal distribution immediately post meal ingestion . ( Fig. 11.3A and B ). GES, as most commonly performed, measures total gastric retention and subsequent emptying of the ingested meal over time. A distinct advantage of scintigraphy for measurement of regional GE is its ability to divide the stomach for quantification into functional physiologic and anatomic segments. GES images can be segmented to measure intragastric meal distribution (IMD) and how this changes as the meal progresses from the proximal to distal stomach over time. This provides information regarding fundic accommodation and antral function. Thus, the commonly available GES study can be used to measure both total and regional GE. Both visual inspection and quantification of separate fundic and antral GE are helpful for defining abnormal physiology and to correlate with dyspeptic symptoms especially when total GE values may be normal .

Recent studies suggest that 2-dimensional measures of IMD immediately post-meal ingestion may not correlate with gastric accomodation measurement by SPECT . This is however not unexpected as one measures the total volume expansion (solid and liquid) of the stomach (SPECT) while the other only measures the volume distribution of the radiolabeled solids.

Use of 2-dimensional GES to assess proximal gastric accommodation was originally suggested in observational studies by Troncon, et al . Piessevaux et al using scintigraphy in patients with functional dyspepsia quantitated the ratio of the proximal gastric counts to the distal gastric counts . Forty five percent of patients had distal redistribution of the solid phase of the meal consistent with impaired fundic accommodation.

Another recent study has shown that fundic accommodation can be assessed visually during routine 2-dimensional GES with high consistency among trained nuclear medicine physicians or radiologists . A simple, visual assessment of normal IMD was defined as visualization of the majority of gastric activity (e.g. >50%) being present in the upper portion of the stomach in the first set of post meal ingestion (t=0 minutes) images ( Fig. 11.3 ). In this study, semi-automated software quantified IMD with an optimum cutoff of IMD <0.57 was used to define abnormal IMD . Abnormal IMD was associated with early satiety, loss of weight, and low body mass index (BMI); and predominantly seen in nondiabetic patients. Assessment of IMD during GES correlates with patient symptoms and may lead to therapy directed at improving fundic accommodation. Studies using the IMD measurement of gastric accommodation to assess symptom response to treatment are currently underway.

Technical improvements have been made to improve regional assessment of proximal gastric emptying and measurement of IMD using standard GES. To quantitate the retention dynamics of the proximal stomach, Friedman et al rotate the anterior gastric images so that the axis of the body of the stomach is vertically oriented and determine the maximum body axis length (BAL). With this method, a proximal gastric region of interest (ROI) is defined as a rectangle of fixed height, BAL/2, positioned at the most superior aspect of the stomach on the immediate (t=0 min) postprandial image ( Fig. 11.4A ). Time-activity curves were derived from the proximal ROIs ( Fig. 11.4B ) to evaluate proximal retention in healthy volunteers with normal gastric emptying; upper and lower limits of normal proximal retention were established at each acquisition time point.

Gastric emptying of a meal

Normal gastric emptying reflects a coordinated effort between the fundus, antrum, pyloric sphincter, and duodenum . Coordination of these motor events are carefully regulated and governed by gastrointestinal electrical activity through the interstitial cells of Cajal (ICCs) and neural connectivity through enteric nerves and vagal efferent nerves from the central nervous system. Feedback from nutrients and volume in the stomach and small bowel impact on the process of gastric emptying and is conveyed though local enteric sensory nerves, vagal afferent nerves, and hormones.

Fundic and antral smooth muscle contractions are primarily cholinergically mediated. Rhythmic antral contractions, typically at approximately 3 cycles per minute, triturate large food particles into smaller 2–3 mm particles which are then permitted to pass through the pylorus when they are an appropriate size for intestinal digestion. The rate of these contractions is governed by the electrical pacemaker of the stomach involving the interstitial cells of Cajal.

Pyloric sphincter relaxation synchronized with antral contractions, allows the smaller food particles and chyme to pass out of the stomach into the duodenum . Pyloric relaxation is mediated through release of inhibitory nerves, especially nitric oxide and possibly vasoactive intestinal polypeptide (VIP) .

Solid and liquid foods empty from the stomach at different rates . Liquids empty from the stomach at a monoexponential rate as their emptying depends primarily on the gastric–duodenal pressure gradient with less reliance on coordinated antral contractions and pyloric opening ( Fig. 11.5 ). As discussed above, solids are initially retained selectively within the stomach until particles have been triturated to a size smaller than 2–3 mm. The time from meal ingestion to onset of solid emptying from the stomach is often referred to as the “lag phase” ( Fig. 11.3B ). This lag phase includes the time for fundal accommodation and initial trituration. The solid emptying curve is therefore more complex than that for liquids. Once the small solid particles are created and suspended with the gastric liquid content, the solids and liquids empty at the same monoexponential rate .