The stomach
normally empties its contents into the duodenum within about two to
six hours. A variety of pathologic conditions can alter the normal
emptying rate of the stomach's contents. These disorders can be
conveniently divided into two broad categories:
- those that
increase the rate of gastric emptying;
- those that
decrease the rate of gastric emptying.
Increased rates of
gastric emptying (i.e.: a decreased GET½) occur with:
Ingestion of
stimulants such as coffee or nicotine from smoking cigarettes prior to
a gastric emptying study may increase the rate of emptying.
Most disorders and
diseases encountered when performing GET½ studies result in
impairment of gastric motility and a subsequent decrease in the rate
of gastric emptying (i.e.: an increased GET½). These conditions
include:
Return to
Top of Page
The determination
of a gastric emptying half-time (GET½) is based upon monitoring
the emptying of a radioactively labelled standard meal from a
patient's stomach into the duodenum. After the patient ingests a
standard meal, the activity remaining within a region of interest
(ROI) placed over the stomach is monitored over a period of time
(typically ~ 1 to 2 hours) and the time for the activity to decrease
to one-half its original level is determined and reported.
Standard meals may
have one of three general forms:
- liquid;
- semi-solid;
- solid.
Normal gastric
emptying rates vary significantly with the type and composition of the
standard meal used. It is important that once the characteristics of
the standard meal are defined and the protocol is established, they be
adhered to.
Return to
Top of Page
A gastric emptying
half-time determination is indicated in patients with gastric stasis
disorders or dumping syndromes. The procedure will evaluate the rate
of gastric emptying but is unable to identify a cause.
The test also has a
role in evaluating patient response to prokinetic drug therapy or
surgery.
Return to
Top of Page
Patient Preparation
Gastric emptying
half - time studies are best performed in the morning on patients who
have been NPO for at least eight hours. The patients must not take any
medications that are likely to alter gastric emptying rates. They must
also be advised not to drink coffee or to smoke cigarettes prior to
the test.
Patients must not
have had an upper GI series within the last 72 hours.
Diabetic patients
should take their normal morning insulin dose.
Pharmacological
Interventions
No specific
pharmacological interventional techniques are commonly used with this
study.
Radiopharmaceuticals
Characteristics of
an ideal radiopharmaceutical for incorporation into a standard meal
for GET½ determination include:
- must not alter
the gastric emptying rate;
- must not be
absorbed across the gastrointestinal tract;
- must not adhere
to the stomach wall or to the other phase in a dual phase
liquid/solid study;
- must be stable
in gastric juice;
- must bind
strongly to the standard meal and not separate out;
- must mix evenly
with the standard meal and remain intimately mixed during the
emptying process;
- must have a
short T½ and good imaging characteristics;
- must provide an
acceptable radiation dose to the patient.
Because of their
ability to satisfy these requirements, Tc-99m sulfur colloid and
In-111 pentetate tend to be the radiopharmaceuticals of choice.
The standard
meal used in a gastric emptying half-time determination has a
significant influence on the GET½ value obtained and on the
sensitivity of the procedure for aiding in the diagnosis of certain
disorders. Factors influencing the rate of gastric emptying of a
particular standard meal include:
- volume - as
volume increases, rate of emptying increases;
- composition -
carbohydrates empty more rapidly than proteins which empty more
rapidly than fats;
- physical state -
liquids empty more rapidly than semi-solids which empty more rapidly
than solids;
- caloric content
- as caloric content increases, rate of emptying tends to decrease;
- nutrient content
- as nutrient content increases, rate of emptying tends to decrease;
- viscosity - as
viscosity increases, rate of emptying tends to decrease;
- acidity - as
acidity increases, rate of emptying tends to decrease;
- osmolality - as
osmolality increases, rate of emptying tends to decrease.
Liquids empty very
rapidly because they do not have to be broken down in size to pass
through the pyloric sphincter. Nutrient and calorie free liquids, such
as pure water, empty most rapidly and in a mono- exponential manner.
As the nutrient and caloric content of the liquid increases, the rate
of emptying decreases and the emptying pattern shifts proportionally
toward a linear pattern.
Standard liquid
meals that have been successfully used for GET½ determinations
include:
- Tc-99m sulfur
colloid (10 to 50 MBq) in water and/or orange juice;
- Tc-99m pentetate
(10 to 50 MBq) in water and/or orange juice;
- In-111 pentetate
(3 to 5 MBq) in water and/or orange juice.
Because of the
emptying kinetics of liquid meals, they tend to be much less sensitive
than solids and their use offers little clinical information in most
gastric emptying studies. They are not generally required for routine
studies. The use of liquid meals by themselves, in the form of Tc-99m
pentetate or sulfur colloid in water, is reserved only for patients
who cannot tolerate solid meals. If a liquid meal is indicated, it is
most commonly labelled with 3 to 5 MBq of In-111 pentetate and used in
conjunction with Tc-99m labelled solid meals in a dual nuclide / dual
phase GET½ determination.
Standard semi-solid
meals labelled with 10 to 50 MBq of Tc-99m sulfur colloid that have
been used for GET½ determinations include:
- Tc-99m sulfur
colloid labelled cornflakes (~20 to 25g) with milk (~150 mL) and
sugar (~10g);
- Tc-99m sulfur
colloid labelled porridge;
- Tc-99m sulfur
colloid labelled pudding.
Standard semi-solid
meals are currently used in some departments.
The best and most
sensitive standard meals are solids labelled with 10 to 50 MBq of
Tc-99m sulfur colloid. Among solids, foods rich in carbohydrates empty
most rapidly while protein rich foods empty somewhat more slowly and
meals containing significant amounts of fats empty most slowly. After
an initial lag phase as the stomach contents move from the fundus to
the antrum, solid meals tend to empty from the stomach is a relatively
linear manner.
Solid standard
meals labelled with 10 to 50 MBq of Tc-99m sulfur colloid that have
been successfully used include:
- Tc-99m sulfur
colloid labelled chicken liver
- in vivo
labelled (the gold standard)
- in vitro
labelled;
- Tc-99m sulfur
colloid labelled liver paté in beef/chicken stew;
- Tc-99m sulfur
colloid labelled hamburger;
- Tc-99m sulfur
colloid labelled fried/scrambled eggs.
Some variation of
Tc-99m sulfur colloid labelled fried/scrambled eggs (one or two large
eggs), two slices of white bread (generally toasted), a "pat"
of butter or margarine (one or the other consistently) and 150 to 200
mL of tap water or orange juice (to assist in swallowing the solid
meal) is a very commonly used standard meal. The eggs are broken into
a clean beaker (~400mL) and the sulfur colloid is injected into the
whites (albumin) at several locations. After an incubation period of
five minutes, the eggs are scrambled, poured into an electric frying
pan, and cooked until firm. The eggs may be placed between the
toasted, lightly buttered bread slices and served as a sandwich or
they may be eaten separately along with the bread. The patient should
ingest the standard meal as quickly as possible; ideally within 5
minutes.
The most important
consideration is that once the composition and preparation of a
standard meal is decided upon, it should not be randomly altered.
If a dual phase
liquid-solid GET½ study is being performed, the liquid tracer
(In-111 pentetate) is added to the tap water or orange juice.
Equipment
The equipment
required will vary depending on weather just Tc-99m is being used or
if In-111 is also being used in a dual nuclide, liquid/solid study.
Single Phase Solid
Meal Study (Tc-99m)
A LFOV gamma camera
equipped with a low energy, general purpose, parallel hole collimator
and interfaced to a nuclear medicine computer is preferred. The
camera's analyzer is set at 140 keV with a 15% to 20% window. The
computer acquisition uses a 64 x 64 or 128 x 128 matrix.
Dual Phase
Liquid/Solid Meal Study (In-111 and Tc-99m)
A LFOV gamma camera
with selectable dual channel capabilities and equipped with a medium
energy, parallel hole collimator and interfaced to a nuclear medicine
computer is preferred. The camera's analyzers are set at 140 keV and
247 keV with 15% to 20% windows. The percentage of downscatter of
In-111 into the Tc-99m window should be determined so the Tc-99m data
can be corrected. The acquisitions at both energies use a 64 x 64 or
128 x 128 matrix.
Image / Data
Acquisition Parameters
There are almost as
many protocols for performing gastric emptying half-time
determinations as there are nuclear medicine departments. As well as
the possible variations in the standard meals, there are also many
potential variations in the procedure itself.
Patient Positioning
Patient positioning
is yet another factor that must be standardized in the gastric
emptying protocol. Having the patient stand or sit in front of the
gamma camera permits gravity to be a factor and more closely reflects
physiological conditions. If the protocol calls for the patient to lie
down, then gravity is less of a factor unless the patient is permitted
or encouraged to walk about between images. Once again, the procedure
must be standardized and adhered to.
The patient is
initially positioned facing the gamma camera (upright or supine) with
the stomach located towards the upper left of the field of view. If "cinescintigraphy"
on a dual head camera is not being used and the patient will be moved
between images, then markers (Tc-99m or Co-57) securely taped to the
xiphoid process and the left iliac crest will assist in repositioning
the patient for each view. The locations of the markers are noted on
the p-scope with a water soluble pen. An alternative is to outline the
location of the stomach activity on the p-scope using a water soluble
pen during the first few images and to reposition within the outlines.
Newer nuclear medicine image processing software contains
re-registration capabilities to permit alignment of images.
After the standard
meal is swallowed, it is initially located in the fundus of the
stomach. Because the fundus is located more posteriorly than the
antrum, as the digestion process begins and the food moves more
anteriorly, the count rate for an anterior view may initially increase
as the food moves closer to the collimator. Because of this, a study
using only anterior views is prone to considerable error in the
calculation of the GET½ value.
Several
techniques have been advocated to correct for this phenomenon.
The generally accepted method is to obtain both anterior and posterior
images and to calculate the geometric mean.
This technique is especially easy if a dual head camera system is
being used because both images can be collected at the same time. For
a single head camera, the patient or camera head must be moved to
collect the two views. A second correction method is to image the
patient using a left anterior oblique view. This method is not as
satisfactory as the geometric mean method.
Single Phase Solid
Meal Study (Tc-99m)
Once the patient
has finished the standard meal, he is appropriately positioned for an
anterior view. If using a single head gamma camera, a 1 or 2 minute
image is taken after which the patient is quickly repositioned for a
posterior view and a corresponding 1 or 2 minute image is collected.
Obviously, when a dual head camera is being employed, both anterior
and posterior images can be collected simultaneously. This first image
pair is considered to be "time = 0 minutes" for the study.
Imaging is repeated at 5 to 10 minute intervals for about the first 60
minutes after which the interval may be lengthened to 15 to 20
minutes. The study continues for up to 2 hours or until gastric
emptying exceeds 50% (i.e.. less than 50% of the meal remains in the
stomach). The increased frequency of imaging during the first hour
assists in defining the length of the lag phase and provides more
reliable data for calculating the GET½ value. A "cinescintigraphy"
variation employs a dual head camera to simultaneously collect
continuous 1 minute anterior and posterior images for the 2 hour
duration of the study.
Dual Phase Liquid /
Solid Meal Study (In-111 and Tc-99m)
The procedure for a
dual phase study is very similar to that of the single phase study
except with the modifications required to accommodate data acquisition
in two energy windows -- 140 keV +/- 15/20% for Tc-99m and 247 keV +/-
15/20% for In-111. After the patient ingests the solid meal, initial 1
or 2 minute anterior and posterior images are acquired using the
Tc-99m window and with either a single or dual head gamma camera as
described above. This first image pair is taken to be "time = 0
minutes" for the solid phase portion of the study. The patient
then quickly ingests the In-111 pentetate labelled liquid phase and a
second pair (ant & post) of 1 or 2 minute images are acquired
using the Tc-99m window. The increase in counts compared to the
initial images permits calculation of the downscatter of In-111 into
the Tc-99m window. The gamma camera's analyzer is then adjusted to the
In-111 window and a third pair of 1 or 2 minute images are acquired.
This pair of images is considered to be "time = 0 minutes for the
liquid phase of the study. Imaging in the Tc-99m and In-111 windows is
repeated at 5 to 10 minute intervals for about the first 60 minutes
after which the intervals may be lengthened to 15 to 20 minutes.
Because liquids empty so rapidly, frequent imaging during the first
hour is recommended. The study continues for up to 2 hours or until
gastric emptying of the solid meal exceeds 50%.
Data Analysis
Gastric Emptying
Half-Times (GET½) are determined by developing a time - activity
curve for gastric emptying and calculating the time required for half
the activity to leave the stomach. After producing composite anterior
and posterior images, regions of interest (ROIs) are drawn to
encompass the stomach for each projection. Care must be taken to
ensure that the region of interest passes through the pylorus and
fairly snugly along the greater curvature of the stomach so as to
exclude intestinal activity. If the patient was carefully
re-positioned during each image acquisition, the same ROIs should
function for all individual frames. Display the anterior and posterior
images in cine / dynamic mode with their ROIs overlaid to determine if
the ROIs are valid for all frames. If some images fall outside the
ROIs, then either redraw the ROI, develop individual ROIs for those
particular frames, or realign the individual frames back into the
boundaries of the ROI.
The total counts
within the ROIs for each anterior / posterior image pair are decay
corrected and used to calculate a geometric mean count value. The
geometric mean values are then plotted as a function of the time of
acquisition. Alternatively, the geometric mean values can be
normalized to the maximum value and the percentage of activity
remaining in the stomach versus study time (minutes) is plotted. The
times required for the stomach to empty half its contents (GET½
values) are then determined for both the liquid and solid phases.
An alternative
quantitation method is to determine the percentage emptying at a
particular time interval after ingestion of the standard meal (e.g..
45 or 60 minutes) and compare the patient's result to normal emptying
values for the same time.
Return to
Top of Page
As discussed above,
a large number of technical factors can affect the rate of gastric
emptying:
- composition of
the meal
- caloric and
nutrient content
- amount and
physical state
- acidity,
viscosity and osmolality
- etc.,
- positioning of
patient during imaging
- patient
supine versus upright during the study
- patient
walks versus does not walk between images.
Because of the
number of influencing factors, it is imperative that each nuclear
medicine facility develop its own normal range of GET½ values
based upon its own particular standard meal and imaging protocol. The
protocol must be strictly adhered to in the performance of the study.
The literature
suggests that there may be a sufficiently significant difference
between the GET½ values for males and females to warrant the
determination of separate normal ranges.
Emptying of Solid
Standard Meals
Standard solid
meals tend to empty in an approximately sigmoid pattern (figure 1).
There is an initial lag phase as the solid meal moves from the fundus
to the antrum and the process of digestion begins. This lag phase is
normally about 5 to 20 minutes long. After the lag phase, the solid
meal begins to empty from the stomach in an approximately linear
fashion until almost all of it has emptied. A final slower phase
represents emptying of the last remnants of the meal (figure 1).
As discussed above,
it is necessary that each facility develop its own range of normal
values based upon the particular standard meal and methodology
employed. An approximate GET½ value for the commonly used egg
sandwich meal is 60 to 100 minutes. When evaluating a gastric emptying
study, the overall shape of the emptying curve should also be
considered.
Emptying of Liquid
Phase of Dual Phase Standard Meals
Liquids tend to
empty in an approximately monoexponential pattern without a lag phase
(figure 1). Normal GET½ values for In-111 pentetate in water /
orange juice ingested at the end of an egg sandwich solid meal are
approximately 10 to 20 minutes.
Emptying of Semi-Solid
Standard Meals
Semi-solids tend to
have very short lag phases and empty with GET values of about 30 to 40
minutes.
Return to
Top of Page
Perhaps the most
significant technical problem relating to gastric emptying studies is
the lack of standardization of the procedure. Because so many factors
can influence the results of a GET½ study, it cannot be
emphasized enough that a department's meal and procedure be evaluated,
standardized and adhered to.
Many standard meals
have been evaluated in the literature. It is recommended that
departments use one of the tested standard meals that have been
determined to fulfill the requirements of an ideal standard meal.
Non-adherence of the radioactive tag to the meal can result in falsely
increased rates of gastric emptying.
Return to
Top of Page
Gastric Intubation
After intubation of
the patient, a standard liquid meal of known volume, to which a
nonabsorbable marker has been added, is instilled into the patients
stomach. Gastric contents are sampled at various times post ingestion
and the rate of gastric emptying is calculated by measuring the volume
and concentration of the marker in the aspirate. The procedure is
somewhat uncomfortable for the patient and the presence of the tubing
may influence the rate of gastric emptying.
Radiographic
Procedures
Radiographic
monitoring of gastric emptying of a barium "meal" will
provide a gross indication of whether or not the stomach is emptying
its contents but the barium does not constitute a physiological meal
and the technique is not readily amenable to quantitation. The patient
also receives a relatively high radiation exposure.
Return to
Top of Page
Return to
Top of Page
Pathology
Texts
Procedure
Texts
Journals
Videos
Internet URL's
Return to
Top of Page
|
lgoodin@idirect.com
or
