# Gastric Emptying Scintigraphy

## Overview

Gastric emptying scintigraphy is the gold standard test for quantifying the rate at which the stomach empties its contents. It is the primary diagnostic study for gastroparesis and can also identify rapid gastric emptying, known as dumping syndrome. A standardized solid-meal protocol is critical for producing reproducible and comparable results across institutions. Common indications include evaluation of diabetic gastroparesis, post-surgical dysmotility, and unexplained nausea or vomiting.

## Standardized Protocol (SNMMI/ANMS Consensus 2008)

### Meal Composition

The standardized meal consists of 120 mL of egg-white substitute (such as Egg Beaters), mixed with 18.5 to 37 MBq (0.5 to 1.0 mCi) of Tc-99m sulfur colloid, and cooked on a non-stick skillet as scrambled eggs or an omelet. The eggs are served on two slices of white bread with strawberry jam, accompanied by 120 mL (4 ounces) of water. The total caloric content is approximately 255 kcal. The patient must consume the entire meal within 10 minutes.

### Imaging Protocol

Anterior and posterior images are acquired at 0, 1, 2, and 4 hours after meal ingestion. Gastric retention is quantified using the geometric mean calculation, which takes the square root of the product of anterior and posterior counts to correct for depth-dependent attenuation. Decay correction is applied to all time points. Between imaging time points, the patient should remain upright or seated, as lying down significantly slows gastric emptying and produces falsely abnormal results.

### Normal Values (Consensus)

At 1 hour, at least 10% of the meal should be emptied, meaning that greater than 90% retained is abnormal. At 2 hours, at least 20% should be emptied. At 4 hours, at least 90% should be emptied, and less than 90% emptied at 4 hours defines gastroparesis. The 4-hour value is the single most important data point. Studies truncated at 1 to 2 hours have unacceptably high false-normal rates and should be avoided.

| Time Point | Normal Retention | Abnormal (Delayed) | Rapid Emptying |
|---|---|---|---|
| 1 hour | ≤90% retained | >90% retained | <50% retained (dumping) |
| 2 hours | ≤60% retained | >60% retained | — |
| 4 hours | ≤10% retained | >10% retained (gastroparesis) | — |

| Severity | 4-Hour Retention | Clinical Implication |
|---|---|---|
| Mild gastroparesis | 10–20% | Dietary modification, prokinetics |
| Moderate gastroparesis | 20–35% | Prokinetics, further workup |
| Severe gastroparesis | >35% | Aggressive management, possible device/surgery |

<image>Gastric emptying scintigraphy showing serial anterior images at 0, 1, 2, and 4 hours with time-activity curve demonstrating delayed emptying (only 45% emptied at 4 hours) consistent with gastroparesis</image>

## Severity Classification

Gastroparesis is graded by the degree of retention at 4 hours. Mild gastroparesis corresponds to 10 to 20% retention at 4 hours (80 to 90% emptied). Moderate gastroparesis is defined as 20 to 35% retention. Severe gastroparesis is greater than 35% retention at 4 hours.

## Medication Management

### Medications That Slow Gastric Emptying (Hold Before Study)

Several medication classes delay gastric emptying and should be held before the study. Opioids should be withheld for 48 to 72 hours if possible. Anticholinergics should be held for 48 hours. GLP-1 receptor agonists such as semaglutide and liraglutide may need to be held for days to weeks given their long half-lives, and the prescribing physician should be consulted. Other medications to hold include calcium channel blockers, progesterone, octreotide, and cannabis or cannabinoids.

### Medications That Accelerate Gastric Emptying (Hold Before Study)

Prokinetic agents including metoclopramide, domperidone, and erythromycin should be held for 48 hours before the study. Cisapride, where available, should also be discontinued.

### Medications That Can Continue

Proton pump inhibitors and H2 blockers have minimal effect on gastric emptying and can be continued. Beta-blockers similarly have minimal impact and do not need to be held.

## Common Causes of Gastroparesis

Diabetic gastroparesis is the most common identifiable cause, associated with autonomic neuropathy in patients with long-standing type 1 or poorly controlled type 2 diabetes. Post-surgical causes include vagotomy, whether intentional or inadvertent during fundoplication or bariatric surgery, and partial gastrectomy. Idiopathic gastroparesis, for which no identifiable cause is found, is often thought to follow a viral illness and represents the most common overall category. Opioids are the most common iatrogenic cause of delayed emptying. Connective tissue diseases including scleroderma, systemic lupus erythematosus, and amyloidosis can impair gastric motility. Neuromuscular conditions such as Parkinson disease and myotonic dystrophy are additional causes. Critical illness can also produce gastroparesis in the ICU setting.

## Rapid Gastric Emptying (Dumping Syndrome)

Rapid gastric emptying, or dumping syndrome, is defined as greater than 40 to 50% of the meal emptied at 1 hour. It most commonly occurs after gastric surgery, including Roux-en-Y gastric bypass, sleeve gastrectomy, Billroth procedures, and vagotomy. Early dumping develops within 30 minutes of eating and presents with cramping, diarrhea, and vasomotor symptoms. Late dumping occurs 1 to 3 hours after eating and is caused by reactive hypoglycemia. The gastric emptying study can confirm the diagnosis when clinical suspicion is high.

<image>Comparison of normal gastric emptying curve, delayed emptying (gastroparesis), and rapid emptying (dumping syndrome) showing the characteristic time-activity curve patterns</image>

## Liquid Gastric Emptying

Liquid emptying can be assessed using water or clear liquid labeled with In-111 DTPA or, if the solid study uses In-111, with Tc-99m sulfur colloid. Liquid emptying is primarily controlled by fundal tone, a different mechanism from solid emptying, which depends more on antral contractions. Normal liquid emptying has a half-time of approximately 20 to 30 minutes. Abnormal liquid emptying may occur even when solid emptying is normal, and vice versa. Liquid studies are less commonly performed and are generally reserved for patients with symptoms but a normal solid emptying study.

## Technical Pitfalls and Quality Issues

### Protocol Non-Compliance

Common protocol errors include failure to consume the entire standardized meal within 10 minutes, substituting a non-standard meal (which many institutions still do), allowing the patient to lie down between imaging time points (which delays emptying by up to 30%), and truncating the study at 1 to 2 hours rather than completing the full 4-hour protocol.

### Imaging Errors

Failure to obtain both anterior and posterior views undermines the geometric mean correction, which is essential for accurate quantification. Forgetting to apply decay correction introduces systematic error. Incorrect region-of-interest placement, such as excluding the antrum or inadvertently including bowel activity, affects the results. Activity that moves into the duodenum and then refluxes back into the stomach can also confuse the measurements.

### Physiologic Factors

Hyperglycemia above 200 mg/dL delays gastric emptying, so blood glucose should be checked before starting the study. The menstrual cycle phase affects emptying, with the luteal phase being slower. Smoking accelerates emptying. Emotional stress can alter the emptying rate in either direction.

## Whole Gut Transit Scintigraphy

The whole gut transit study extends the standard gastric emptying protocol using the same Tc-99m sulfur colloid meal, with additional imaging at 24 and 48 hours to assess small bowel and colonic transit. Small bowel transit is evaluated by calculating the geometric center of activity at 6 hours. Colonic transit is assessed by the geometric center at 24 and 48 hours. This extended study is useful for evaluating patients with chronic constipation or diffuse dysmotility, such as intestinal pseudo-obstruction.

<image>Whole gut transit scintigraphy showing images at 0, 2, 4, 6, 24, and 48 hours to assess gastric, small bowel, and colonic transit with geometric center calculations</image>

## GLP-1 Receptor Agonist Considerations

GLP-1 receptor agonists are an increasingly recognized cause of delayed gastric emptying. Semaglutide (marketed as Ozempic and Wegovy) and other agents in this class slow gastric emptying as a core part of their mechanism of action. It is important to document GLP-1 agonist use and consider holding the medication before the study, in consultation with the prescribing physician. For long-acting formulations, a washout period of 2 to 5 half-lives may be needed, which can span several weeks. There is rising clinical concern for patients presenting with unexplained gastroparesis who are taking these medications.

## Clinical Pearls

The 4-hour imaging point is the most important data point in the study, and the study should never be truncated at 1 to 2 hours.

Always use the standardized egg-white meal. Non-standard meals produce unreliable results that cannot be compared to published reference values.

Blood glucose should be checked before starting the study. Hyperglycemia above 200 mg/dL delays emptying and should be corrected first.

Opioids are the most common medication cause of delayed gastric emptying, and all current medications should be documented.

The geometric mean calculation using anterior and posterior counts is mandatory for accurate quantification.

Patients should remain upright between imaging time points. Supine positioning artificially delays emptying.

GLP-1 agonists are an increasingly important cause of medication-induced gastroparesis.

## References

- Abell, T. L., et al. "SNMMI/ANMS Consensus Recommendations for Gastric Emptying Scintigraphy." *Journal of Nuclear Medicine Technology*, 2008.
- Camilleri, M., et al. "Gastroparesis." *New England Journal of Medicine*, 2018.
- Defined Role of Gastric Emptying Study in Clinical Practice. *Seminars in Nuclear Medicine*, 2012.
- SNMMI Procedure Standard for Gastrointestinal Transit Scintigraphy.