Nuclear medicine evaluation of gastric motility in adults is an extremely useful but often underutilized procedure. The purpose of this article is to make gastric emptying studies user-friendly for patients and radiologists. The review addresses the relevant issues of anatomy and physiology; factors affecting gastric emptying; the radiolabled meal; imaging technique; quantitative analysis; and reporting.
is an Associate Professor of Radiology and the Director of
Nuclear Medicine at the University of Virginia Health System,
Charlottesville, VA. She is also a member of the Editorial Board
of this journal.
Nuclear medicine evaluation of gastric motility in adults is an
extremely useful but often underutilized procedure.
Underutilization undoubtedly results from a number of factors and
misunderstandings. The purpose of this article is to change
attitudes and make gastric emptying studies user-friendly for
patients and radiologists. The images, lacking the anatomic detail
so appealing to radiologists, could be considered relatively "ugly"
and featureless even when compared with other scintigraphic
examinations, such as bone or hepatobiliary scans. Perhaps equally
unappealing to radiologists and patients is the idea of a breakfast
meal of radioactive liver eaten after an overnight fast. It is
possible that the number of imaging protocols and variety of
radiolabeled meals appear challenging. The different protocols have
different "normal values" that are dependent on the scan technique
and meal, which can make it difficult to compare results between
institutions. In the case of gastric emptying studies, too many
options can be a disadvantage.
Evaluation of gastric emptying becomes much simpler when one
looks at it as a diagnostic study that is clinically useful for
diagnosis and for monitoring the effects of therapy. The goal is to
provide an accurate and reproducible quantification of gastric
motility. The examination is noninvasive, accepted by patients, and
performed with equipment that is widely available. From this
standpoint, much of the variation in methodology and all the
esoteric processing is relegated to the realm of research. A gamma
camera with a computer and a radiolabeled meal are all that is
required. From the scintigraphic images, a time-activity curve is
generated that displays the amount of radioisotope in the stomach
(defined by a region of interest [ROI]) over time (Figure 1).
A variety of techniques have been used to evaluate gastric
emptying, but none fills the niche occupied by scintigraphy.
Mechanical obstruction should be excluded by contrast radiography
or esophagogastroduodenoscopy first. However, these procedures are
relatively insensitive for detection of gastric dysmotility unless
it is severe. Radiographic studies are associated with
significantly greater radiation than the nuclear examination, and
do not allow for quantification. The radiation dose to the patient
for a gastric emptying study varies with the specific radionuclide,
the dose administered, and bowel transit-time, but it is small.
Whole body exposure from a 1.0 mCi (37.0 MBq) Tc-99m sulfur colloid
solid meal is approximately equal to 22 days of environmental
background radiation in Charlottesville, VA.
The procedure is useful to establish the presence of dysmotility
but does not determine an etiology. It is the appropriate study
when chronic dysmotility is suspected (Table 1). The clinical
diagnosis is difficult because the patient's presenting clinical
signs and symptoms are often vague and may overlap with other
conditions. Most patients referred for the examination have
diabetes mellitus and are being evaluated for gastroparesis
diabeticorum, with autonomic neuropathy as the suspected etiology.
However, there are numerous, less common etiologies of
gastroparesis (Table 2). The study also can be performed to
evaluate for rapid gastric emptying, which most commonly occurs
after vagotomy. Rapid emptying may be associated with other
conditions, and occasionally with diabetes (Table 3).
After the diagnosis of dysmotility is established, the examination
is also useful to evaluate the effectiveness of medical therapy.
Scintigraphic gastric emptying studies are not usually employed in
the evaluation of acute gastric obstruction for which other
diagnostic studies are more appropriate.
Anatomy and physiology
For purposes of gastric emptying, one can view gastric anatomy
and physiology in the simplest fashion. The proximal stomach is
represented by the cardia and gastric body and serves as a
reservoir. Following a meal, neural refluxes allow it to distend to
contain the ingested volume without a significant increase in
intra-gastric pressure. When this neural reflex pathway is
disrupted (for example, after vagotomy), the ingested meal results
in increased intragastric pressure. The dumping syndrome,
characterized by sweating, tachycardia, dizziness, and weakness,
may result. The distal stomach, composed of the antrum, serves as
the grinder of the ingested contents through cycles of rhythmic
contractions at a rate of 3 per minute initiated by a gastric
pacemaker located on the greater curvature at the junction of the
proximal and distal stomach. The pylorus controls outflow to
particles <1 mm. Solids begin to empty after a short period
during which no emptying occurs, referred to as the lag phase.
During the lag phase, the contents of the stomach are broken down
to small particles that can pass through the pylorus. The duration
of the lag phase may vary in length (Figure 2). Compared with the
overall results of the gastric emptying study, no clinical
significance has been determined for the lag phase alone.
Between meals, nondigestible materials (for example, a swallowed
foreign body) are cleared by strong antral contractions.
Factors affecting gastric emptying
The rate of gastric emptying is dependent on what is consumed.
Liquid and solid contents empty from the stomach at different
rates. Non-nutrient liquids, such as water, empty in an exponential
fashion in 10 to 20 minutes. Nutrient liquids empty more slowly.
Liquids ingested in conjunction with solids empty more slowly than
liquids ingested alone. All solids empty in a linear fashion.
However, the emptying rate is influenced by the contents of the
meal, such as the amount of carbohydrate, fat, and protein as well
as the total caloric content and volume of the meal. Patient
position and patient activity also affect the rate of gastric
emptying. Emptying is faster when the patient is sitting than when
lying supine. Emptying also increases if the patient is allowed to
ambulate during breaks in the scanning procedure.
Therefore, it is important that the entire gastric emptying
procedure be standardized, not just the radiolabeled meal.
Measurement of gastric emptying using a liquid phase marker is
appropriate for infants, but is a topic beyond the scope of this
article. An examination using a solid test meal is appropriate for
adults. Although simultaneous measurement of liquid and solid
emptying can be performed in a single, dual-isotope gastric
emptying study, in the vast majority of cases assessment of solid
emptying alone is adequate and preferred. Gastric emptying
evaluation using a solid meal is more sensitive for detecting
abnormal motility than a liquid meal. In normal patients, both
liquid emptying and solid emptying will be normal. In patients with
gastroparesis, liquid emptying may be delayed or normal. Because
the test's sensitivity for detecting abnormalities lies in the
assessment of solid emptying, simultaneous liquid and solid gastric
emptying studies (using In-111 DTPA and Tc-99m sulfur colloid,
respectively, as labels) are rarely necessary in clinical practice.
A number of authors have recommended that each clinic validate its
own procedure and establish its own standard values. However, a
more practical approach is to rigorously adopt a validated
procedure from an academic center and "do it like the experts."
The only things required for the study are a gamma camera with
computer capability and a radionuclide-labeled meal. It is critical
that the radionuclide remains attached to the solid component of
the meal to reflect the physiology of solid emptying accurately. If
the radionuclide dissociates from the solid, it will leave the
stomach in the liquid phase; the resulting scan and calculations
will result in misinformation and an overestimation of gastric
emptying. A variety of food substances of varying palatability and
availability (oatmeal, fiber, etc) have been labeled with a number
of radiopharmaceuticals. However, Tc-99m sulfur colloid labeling of
chicken liver or egg has been found superior. The egg sandwich and
the chicken-liver stew meals both contain fat, carbohydrate, and
protein and mimic a normal meal. However, there is no consensus as
to the optimal meal.
Both the Tc-99m sulfur colloid chicken-liver stew meal and the
egg sandwich meal are relatively easy to prepare. Historically, in
vivo labeling of the chicken liver was achieved by injecting Tc-99m
sulfur colloid intravenously into a chicken that was subsequently
slaughtered. The in-vivo labeled chicken liver then was cooked and
incorporated in a can of prepared beef stew. Fortunately, it was
subsequently shown that in-vivo labeling of the liver was not
necessary. Further investigations proved that in-vitro labeling of
the liver or in-vitro labeling of eggs or egg whites performed as
well as in-vivo labeling of liver, with no significant elution of
the Tc-99m sulfur colloid from the solid component. Because of the
ready availability of eggs and the general patient preference of
eggs over liver, a labeled egg meal is widely used (Tables 4 and
5). A standard meal is two large eggs or 3 egg whites beaten and
mixed with 0.5 to 1.0 mCi (18.5 to 37.0 MBq) of Tc-99m sulfur
colloid. The eggs or egg whites are cooked in omelet fashion in a
nonstick frying pan and placed between two slices of standard white
bread. Using an egg meal provides similar information regarding
gastric emptying as using a liver meal. However, purists will note
that an egg or egg-white meal is correctly considered a semisolid
(rather than solid) meal. Because the egg meal has a shorter
emptying time than a stew meal, a shorter examination is possible,
which is an advantage in a busy clinic. Because the meal must
consist of an adequate volume to reflect a typical meal, the egg is
typically administered as an egg sandwich with fluid to help with
ingestion. Patients are occasionally allergic to eggs, which may
require utilization of a liver meal. It is important that
appropriate normal values be applied for each standardized
The patient should eat the entire meal within 10 minutes. If a
longer period is allowed, some gastric emptying may occur before
the patient has finished the meal, and before the imaging and data
acquisition has begun. Any standardized technique must take into
account the type of meal, patient position during imaging,
specifics of the scan acquisition, and methods of quantitation.
The scanning period should be at least as long as the time
needed for half of the meal to empty, based on an average normal
subject. A meal of stew with liver empties more slowly than eggs,
so a 120-minute scan is recommended. A 90-minute study is
appropriate for an egg-sandwich meal.
Most people eat in the sitting position and remain in the
sitting or standing position following a meal. Patient positioning
in the semi-upright or upright position mimics the normal
physiologic state. Therefore, imaging in these positions would seem
preferable. However, gamma camera logistics may determine patient
positioning and scan protocol. The supine position can be used, and
may be necessary for some dual-headed cameras to utilize the
geometric mean technique. Patient comfort is always critical and at
times may be the limiting factor for positioning.
More frequent image acquisition will result in more data points
and a more representative time-activity curve. Modern nuclear
medicine camera-computer systems allow frequent images to be
obtained at regular intervals with minimal technologist effort.
However, some clinics employ infrequent static images, rather than
continuous dynamic imaging as the basis for generation of the
Quantitative analysis: Corrections for background, decay
On the computer, an ROI is drawn around the stomach using either
an individual image or a summed image. Counts in the ROIs must be
corrected for radioactive decay to allow comparison with the start
of the study. If decay correction is not done, gastric emptying
will be overestimated. For example, assume a patient has no gastric
emptying. Decay alone will result in a decrease in counts in the
gastric ROI, indicating apparent emptying of 16% at 90 minutes and
21% at 120 minutes if decay correction is not performed.
Attenuation due to tissue between the camera and the stomach is
also a factor affecting quantitation. From the perspective of a
gamma camera placed anterior to the patient, as the radionuclide
moves from the more posterior gastric fundus to the more anterior
gastric body and antrum, the thickness of tissues between the
radioisotope and the camera gradually decreases. Therefore, the
attenuation of photons arising from the labeled meal
. Due to this geometry, the gamma camera will record counts from
the gastric ROI that
as the meal moves from the fundus to the antrum. Imaging in the
anterior or posterior view only can result in errors up to 35% in
determination of emptying if attenuation is not taken into account.
Direct measurement of attenuation is the most accurate method to
correct for attenuation, as is done using a transmission scan in
positron emission tomography (PET). However, that complex technique
is impractical and unnecessary for gastric emptying studies. An
alternative method to compensate for attenuation incorporates a
mathematical correction known as the geometric mean method (Figure
This technique requires two opposed views of the area of interest,
typically anterior and posterior, closely in time. Both views
should be obtained nearly simultaneously if a single-headed camera
is used, or can be obtained simultaneously if a dual-headed camera
is used. The geometric mean value for the gastric ROI counts is the
square root of the product of the counts in the stomach ROI on the
anterior and on the posterior views [geometric mean = (anterior *
]. The geometric mean technique compensates for the variable
attenuation of the labeled meal during movement from the fundus to
the pylorus. The geometric mean value for gastric counts is
decay-corrected and then recorded on the TAC for each time
An alternative method that allows for attenuation compensation
employs the left anterior oblique (LAO) method of acquisition.
It is particularly valuable when a dual-headed camera is not
available, and is well suited to using a single-headed camera. It
eliminates the need for the technologist to switch a single camera
head back and forth between the anterior and posterior positions at
each time point, as needed for the geometric mean method. By
positioning a single camera head roughly parallel to the axis of
the stomach, the effect of attenuation is relatively constant and
there is no need for a mathematical correction for attenuation. The
LAO value for gastric counts is decay-corrected and then recorded
on the TAC for each time point. In the LAO projection, there may be
superimposition of the antrum on the first portion of the duodenum.
Nevertheless, the method is adequate for clinical purposes, is
technologist friendly, and is more accurate than using the anterior
or posterior view only (Figures 4 and 5).
Patient movement during the scan should be discouraged.
Occasionally, patient motion can be a problem and lead to spurious
results. Basic gastric emptying software supplied with the gamma
camera commonly applies the same ROI to every image in a dynamic
scan. If there is significant patient motion, the stomach will not
remain within the ROI and the emptying determination will be
incorrect. Patient motion is easily identified if the images are
viewed in cinematic mode as a movie. Remedies include motion
correction, which is available on some cameras, or manual placement
of the ROI around the stomach on each frame to adjust for the
change in patient position during the scan.
Reporting can be done using a variety of parameters that include
the percent emptying at the end of the study, the emptying rate
(%/minute) or the half time of emptying (T1/2). Because reporting a
T1/2 value implies an exponential rate of emptying, that parameter
is not truly appropriate, although it is often used (incorrectly).
Either the percent emptying at the end of the study or the emptying
rate are more appropriate values to report. Normal values must be
based on the meal used as well as the specific imaging and
processing protocol. In the vast majority of clinics, adoption of a
standardized protocol that has been documented in the literature
and application of its normal values is preferable to establishing
values for the normal range on-site.
Gastric emptying studies using either Tm-99m
sulfur-colloidlabeled egg or stew meals can provide accurate and
reproducible quantification of gastric motility. This clinically
useful test can be performed using a single- or dual-headed gamma
camera equipped with a computer, but the meal and scan procedure
should be standardized. Factors critical for accurate determination
include decay correction and compensation for attenuation, using
either the left anterior oblique view or the geometric mean
technique. In some cases, correction for patient motion is
necessary. Quantitative determination of normal and abnormal
emptying values must be appropriate to the labeled meal used as
well as the specific imaging and processing protocol.