Nuclear Medicine
CLINICAL DECISION SUPPORT
99mTc-labelled erythrocytes are administered to patients with gastrointestinal tract bleeding of unknown location in order to obtain images of the extravasation of the radiopharmaceutical into the bowel. The aim of this examination is to determine whether the patient is actively bleeding at the time of the test, and, in the event of a positive scintigram, to provide information required for arteriography or surgical intervention. In some patients, the bleeding site is identified with sufficient accuracy using scintigraphy alone. Localization is only possible if the extravasate itself and its subsequent dissemination in the bowel are visualized by imaging. Theoretically, 111In-labelled erythrocytes can also be used, but this results in poorer images. Technetium-99m sulphur colloid can be used if injected during bleeding, but this is rarely the case. Also, late imaging of colloid is not possible.
Recurrent intermittent gastrointestinal tract bleeding of unknown origin in which endoscopy or other investigations are negative.
A successful investigation is most likely in patients needing a blood transfusion of at least 500 mL in the 24 h prior to the examination. It is not usually possible to see lighter bleeding of less than 0.5 mL/min or if the patient requires 2-3 transfusions per week.
Gastrointestinal Bleeding Scintigraphy (GIBS) is commonly indicated for identifying an active gastrointestinal bleeding site in patients with overt gastrointestinal bleeding. GIBS should not be performed on patients with chronic occult gastrointestinal bleeding, because the guaiac faecal occult blood test may detect bleeding at rates well below those necessary to be identified on GIBS.
GIBS is indicated primarily for overt mid or lower gastrointestinal bleeding, specifically when an upper gastrointestinal bleed has been excluded by nasogastric lavage. In this scenario, GIBS can be used as an early diagnostic study for gastrointestinal bleeding especially for hospitalized patients or patients in the emergency department. GIBS can be beneficial when other studies require lengthy patient preparation or are contraindicated. Although GIBS can also identify overt upper gastrointestinal bleeding, usually the first examinations performed to confirm upper gastrointestinal bleeding are nasogastric lavage followed by esophagogastroduodenoscopy to identify and treat suspected overt upper gastrointestinal bleeding.
GIBS is also indicated to help identify the source of obscure overt gastrointestinal bleeding. Two standards/guidelines have removed GIBS from the diagnostic algorithm for obscure overt gastrointestinal bleeding. However, most studies have shown that GIBS can help localize the obscure overt bleeding site in these patients.
Among some of the other common clinical indications for GIBS are stratifying risk in patients with gastrointestinal bleeding, directing timely diagnostic angiography, and assisting in plans for surgical or other interventional procedures [103].
For adults, the recommended administered activity is 550-1110 MBq [103].
In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/). The minimum recommended activity to administer is 20 MBq.
The effective dose per administered activity is [3]:
The effective dose for the suggested activity (normal liver condition) is 2,8 mSv.
Caveat
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."
If the scintigram is positive, this is usually seen within 4 h following injection of labelled erythrocytes. The examination is positive if abdominal accumulation is seen which increases in intensity over time and is transported intraluminally in antegrade or retrograde direction.
The investigation must be evaluated in cine format, because the source of bleeding can be missed on the individual images due to a rapid peristalsis in both the large and small intestine.
Recurrent intermittent gastrointestinal bleeding for which no cause has been found using other modalities is usually due to angiodysplasia or possibly a large bowel tumour. Bleeding could also originate in the small intestine, although this occurs less frequently.
Information on individual bowel anatomy gained from previous radiological examinations is very useful to report, because significant individual differences exist in the anatomical course of the bowel.
Excretion of activity into the kidneys and the ureters must be taken into account. It can be useful to ask the patient to empty his/her bladder, since urine activity can mask rectal bleeding.
Since scintigraphic detection of bleeding is an acute examination, patient preparation is generally not possible.
As with angiography, this examination is hindered by the presence of contrast media from radiological examination of the colon, so this should, therefore, be avoided in these patients.
Heparinization can be used to provoke bleeding in certain circumstances (long-term medical history, several previous negative centigrams and angiography). This does require hospitalization.
It is important that patients with active bleeding are observed during the investigation (1-8 h).
The detailed recommendations are available in the SNMMI Procedure Standar/EANM Practice Guideline for Gastrointestinal Bleeding Scintigraphy 2.0 [103].