A
Meckel's
diverticulum is a
pathologic
sacculation of the ileum, usually about 2 to 6 cm in length,
derived from the persistence of the omphalomesenteric (embryonic
vitelline) duct at the point where it joined the ileum, about 50 to
100 cm from the ileocecal valve. It is a relatively common congenital
anomaly, occurring in about 1% to 3% of the population. Most
Meckel's
diverticula are lined with ileal mucosa, but a little less than
half are lined with an ectopic mucosa. The ectopic tissue most
commonly present is gastric mucosa (~80%) but duodenal mucosa, jejunal
mucosa, colonic mucosa, or pancreatic tissue may also be present.
Most Meckel's
diverticula are asymptomatic but a small proportion (~4%) will produce
complications sometime during the patient's lifetime. Generally just
over half of these (~2% of total) present before the age of two. In
children, the most common presentation is painless rectal bleeding
which results from ulceration of the adjacent bowel by the pepsin and
hydrochloric acid secreted by the Meckel's ectopic gastric mucosa.
Bleeding with abdominal pain occurs less frequently. In adults
obstruction, intussusception and infection are also common
presentations.
A
convenient way to remember the statistical information related
to a Meckel's diverticulum is to remember the "Rules of
2":
- occurs in ~2% of
the population;
- male to female
ratio ~2:1;
- is ~2 inches
long;
- is located on
the ileum within ~2 feet of the ileocecal valve;
- ~2% become
symptomatic by age 2;
- 2 major types of
ectopic tissue - gastric and pancreatic;
- 2 major
complications - bleeding and obstruction.
Once identified as
being present, the treatment for a Meckel's diverticulum is surgical
removal.
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The nuclear
medicine procedure to detect the presence of a Meckel's diverticulum
that contains functioning gastric mucosa is based upon the observation
that Tc-99m pertechnetate is concentrated and secreted by the mucus
secretory cells of gastric mucosa, both normal and ectopic. The uptake
of Tc-99m pertechnetate in a Meckel's should have the same temporal
characteristics as its concentration in the stomach.
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To detect the
presence of a Meckel's diverticulum that contains functioning gastric
mucosa.
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Patient Preparation
The patient should
be NPO for at least four hours prior to beginning the study. Fasting
reduces the secretion by the mucosal cells of the stomach of Tc-99m
pertechnetate into the lumen of the bowel where it may produce a false
positive interpretation. Fasting also improves the visibility of a
Meckel's diverticulum by decreasing the release of the Tc-99m
pertechnetate concentrated by its ectopic gastric mucosa.
Pharmacologic
agents may also be used during patient preparation to enhance the
probability of observing the ectopic gastric mucosa in a Meckel's
diverticulum.
Cimetidine:
Cimetidine
is a competitive, reversible histamine H2 - receptor antagonist which
results in a decreased production and release of hydrochloric acid by
the stomach. Along with this, it inhibits Tc-99m pertechnetate
secretion into the gastric lumen allowing for the continued
accumulation of the pertechnetate in the gastric mucosa, both normal
and ectopic. For pediatrics, the cimetidine dose is 20 mg / kg / day
orally for two days prior to the study. For adults, 300 mg is
administered orally four times per day for two days prior to the
study.
Ranitidine:
Ranitidine
is also a histamine H2 - receptor antagonist and exhibits actions
similar to cimetidine. Ranitidine is administered by intravenous
infusion over a period of 15 to 20 minutes at a dose of 1 mg/kg to a
maximum of 50 mg. Imaging for ectopic gastric mucosa should not begin
until one hour after the completion of ranitidine infusion.
Pentagastrin /
Glucagon:
Pentagastrin is a
synthetic analogue of gastrin, the hormone that stimulates gastric
acid secretion. When administered, it increases the uptake of Tc-99m
pertechnetate by both normal and ectopic gastric mucosa thus enhancing
the visualization of a Meckel's diverticulum. However, pentagastrin
also stimulates gastrointestinal motility and this latter action may
both move Tc-99m activity away from the ectopic site making it
difficult to locate and speed the movement of stomach activity into
the bowel which may hinder interpretation. Pentagastrin is
administered subcutaneously at a dose of 6 µg / kg fifteen
minutes before the administration of Tc-99m pertechnetate.
Glucagon is the
pancreatic hormone that stimulates conversion of glycogen to glucose
in the liver. It also has a relaxing effect on the smooth muscle of
the GI tract which inhibits the movement of Tc-99m pertechnetate
secreted by both the stomach and a Meckel's diverticulum thus
enhancing the ability to interpret a Meckel's study. Glucagon is also
antagonistic to pentagastrin in that it decreases the uptake of Tc-99m
pertechnetate by gastric mucosa. Glucagon is administered
intravenously at a dose of 50 µg / kg ten minutes after the
administration of Tc-99m pertechnetate.
Pentagastrin and
glucagon are often used together.
Pharmacologic
Interventions
See Patient
Preparation above. Potassium perchlorate administered orally at a dose
of 6 mg / kg of body weight after completion of the study will purge
Tc-99m pertechnetate from the thyroid and reduce the radiation dose to
the gland. The perchlorate must not be given prior to the procedure.
Radiopharmaceutical(s)
Technetium (Tc-99m)
pertechnetate is used to image ectopic gastric mucosa. The
radiopharmaceutical is administered intravenously at a dose of about
7.0 to 7.5 MBq / kg (190 to 200 µCi / kg) body weight for
pediatrics and about 350 to 400 MBq (9.5 to 10.8 mCi) for adults.
Equipment
A LFOV gamma camera
with a low energy, all purpose or high resolution parallel hole
collimator interfaced to a nuclear medicine computer system is
preferred. The camera's analyzer is set at 140 keV with a 15% or 20%
window. The computer acquisition uses a 64 x 64 x byte matrix.
Image / Data
Acquisition Parameters
The patient should
be placed supine in the field of view with both the xiphoid and the
symphysis pubis within the field of view.
The computer is set
to acquire an anterior flow at a rate of 1 or 2 seconds / frame for 60
seconds followed by a stage two dynamic acquisition at 15 to 30
seconds / frame for 29 minutes.
For films, the
gamma camera is set to acquire anterior flow images at a rate of 2 to
5 seconds / frame for 60 seconds. This is followed by 300k to 500k
count static anterior images taken every 5 minutes for 30 minutes. The
first image is taken for the determined count (e.g.: 500k) and the
remainder are taken for the same time. At the end of the 30 minute
period, right lateral and posterior images are acquired. If the
presence of a Meckel's diverticulum is not immediately obvious on the
30 minute images, the patient is instructed to void and the anterior
and right lateral (and possibly the posterior) images are repeated.
Static images may also be obtained at 45 and 60 minutes if indicated.
An example of
clinical protocols for the detection of Meckel's diverticulum have
been posted by
The
University Hospital in London, ON and by
The Society of Nuclear
Medicine.
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In a normal study,
the Tc-99m pertechnetate is observed during the flow phase in the
large abdominal vessels, kidneys, spleen and liver. Vascular activity
remains visible is these organs throughout the study. After about 5
minutes, activity is seen to increasingly accumulate in the stomach.
Bladder activity increases as the pertechnetate is eliminated by the
kidneys. Toward the end of the study, activity released from the
stomach may be observed within the proximal small bowel.
The
classic
characteristic of a positive result for a Meckel's diverticulum
is a small, persistent focal area of Tc-99m pertechnetate uptake in
the right lower abdominal quadrant that is not consistent with the
organs normally visualized. A lateral view will confirm its presence
within the peritoneal cavity, generally located anteriorly. The level
of activity in the Meckel's diverticulum is not normally visualized on
the flow or blood pool images but increases in parallel with the
stomach activity to become very prominent.
Although the right
lower abdominal quadrant is the most common location for a Meckel's
diverticulum, it
can
be located almost anywhere in the abdomen. Lateral, oblique and
posterior views must be used to investigate any suspicious
concentration of Tc-99m pertechnetate. because it is attached to the
ileum, the location of a Meckel's diverticulum will often change with
a change in patient position, unless it is also attached to the
umbilicus by a fibrous cord. Post voiding images are necessary to
ensure that a Meckel's diverticulum occurring in close proximity to
the bladder is not missed.
The sensitivity
(~90%) and specificity (~95%) of nuclear medicine imaging to locate a
Meckel's diverticulum are very good but false positive and false
negative interpretations may be made for a variety of reasons.
False positive
interpretations of a Meckel's diverticulum can result from:
- activity
released from the stomach into the small bowel;
- hyperemia and
increased blood pooling in inflammatory lesions such as
intussusception, appendicitis, Crohn's disease, ulcerative colitis
or peptic ulcer;
- visualization
of the
uterus
in a menstruating female;
- vascular
disorders such as hemangiomas, arteriovenous malformations or
abdominal vessel aneurysms;
-
hypervascularity associated with vascular neoplasms;
- urinary tract
abnormalities such as ectopic kidney, extrarenal pelvis,
hydronephrosis or vesicoureteral reflux;
- residual
activity in the ureters or renal pelvis;
- intestinal
duplication (cyst) containing ectopic gastric mucosa.
Unlike a Meckel's
diverticulum, vascular abnormalities tend to have higher activity on
the flow phase which then decreases with time. Location, as determined
from lateral, oblique and posterior views, and the activity - time
sequence compared to that of the stomach also provide valuable
information to reduce the incidence of false positives.
False negative
interpretation can occur due to:
- the
diverticulum being too small and / or having insufficient ectopic
gastric mucosa to be detectable;
- washout of the
Tc-99m pertechnetate from the ectopic gastric mucosa;
- the
diverticulum being obscured by normally visualized structures such
as the bladder;
- necrosis of the
ectopic gastric mucosa;
- poor
methodology.
Despite the
significant number of conditions that have the potential to contribute
to a false interpretation, the actual number of false reports is less
than 10%.
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Colonic purging or
barium enemas can induce an inflammatory response in the colon that
can potentially result in a false positive interpretation. Nuclear
medicine localization of a Meckel's diverticulum is best performed
before either of these procedures or several days after them. Also,
for abdominal radiographic studies requiring barium contrast agents
the possibility exists that residual contrast agent may attenuate the
radiation released by the activity present in the diverticula and thus
hinder its detection.
Potassium
perchlorate will block the uptake of Tc-99m pertechnetate by gastric
mucosa. It must not be administered before the study but may be given
after completion of the study to reduce the radiation dose to the
thyroid gland which also concentrates pertechnetate.
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Radiographic
procedures using barium contrast are generally not satisfactory for
detecting a Meckel's diverticulum. Barium may fill a Meckel's that has
a wide opening to the ileum but the sensitivity of the procedure as a
diagnostic technique for detecting a Meckel's diverticulum is very
low.
Plain film
radiography may detect enteroliths that may occasionally be present
within the diverticulum.
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Case
#1 has been posted by the Mallinckrodt Institute of
Radiology at Washington University Medical Center. A 2½-year-old
boy who presents with nausea, vomiting and melena.
Case
#2 has been posted by the Mallinckrodt Institute of
Radiology at Washington University Medical Center. A 2½-year-old
male presents with a one day history of bloody stool. Sequential 3
minute anterior using Tc-99m pertechnetate are provided.
Case
#3 is hosted by Brigham and Women's Hospital's Department of
Radiology. A 20-year-old man described an acute episode of rectal
bleeding. One minute images using Tc-99m pertechnetate are provided.
Case
#4 is hosted by the Harvard University School of Medicine. A
27-year-old male presented with gastrointestinal bleeding.
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Pathology
Texts
Netter F, >>>
InterNet URL's
The Merck Manual of Diagnosis and Therapy. This manual is posted by
Merck & Co. Choose the publications option. (http://www.merck.com)
Procedure
Texts
Bernier DR, Christian PE, Langan JK. Nuclear Medicine: Technology
and Techniques. 3rd ed. St. Louis: Mosby - Year Book, 1994: 332 &
433.
Datz FL. Handbook of Nuclear Medicine. 2nd ed. St. Louis: Mosby -
Year Book, 1993: 128 - 131.
Early PJ, Sodee DB. Principles and Practice of Nuclear Medicine. 2nd
ed. St. Louis: Mosby - Year Book, 1995: 514 - 516.
Thrall JH, Ziessman HA. Nuclear Medicine: The Requisites. St. Louis:
Mosby - Year Book, 1995: 248 - 252.
Wagner H, : 929 - 930, 932 & 1159.
Journals
Goel V. Meckel's Diverticulum in Seminars in Nuclear Medicine, Vol.
XII #1, Jan. 1982: pages 97 & 98.
Park H-M, Duncan K. Nonradioactive Pharmaceuticals in Nuclear
Medicine published in The Journal of Nuclear Medicine Technology, Vol.
22 #4, Dec. 1994: pages 240 - 249.
InterNet URL's
Review
and Reference Notes on Nuclear Medicine. These notes are
posted by Dr. S.C. Williams from the Madigan Army Medical Center,
Tacoma WA.
<http://www.mamc.amedd.army.mil/WILLIAMS/NucMed/GI07.htm#RTFToC31>
Gastrointestinal
Bleeding - Meckel's Diverticulum. A very brief overview
posted by the Division of Nuclear Medicine within the Department of
Radiology at The Children's Hospital associated with the Harvard
University School of Medicine. The information on Meckel's
diverticulum is located close to the bottom of the document. <http://nucmedweb.tch.harvard.edu/Patient/Gastrointestinal/>
Meckel's
Scanning. An overview posted by the University of Iowa
within their Virtual Hospital project. <http://indy.radiology.uiowa.edu/Providers/Textbooks/ElectricGiNucs/Text/Meckels.html>
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