GI Bleeding Scintigraphy

Gastrointestinal bleeding scintigraphy finds active bleeding by following technetium-99m labelled red blood cells over a long period. Labelled cells stay in the circulation, so when a vessel bleeds into the gut lumen, activity collects at the bleeding site and moves along the bowel.

It detects slow bleeding at rates of 0.1 to 0.5 mL/min, lower than the rate needed for CT angiography (about 0.3 to 0.5 mL/min) or catheter angiography (about 0.5 to 1 mL/min).

Red cells are labelled in vitro with a commercial kit, or in vivo with stannous pyrophosphate followed by pertechnetate. The dose is 740 to 1100 MBq intravenously.

Continuous dynamic images of the abdomen and pelvis are taken at 1 frame each minute for 60 to 90 minutes, extended to 24 hours if needed. The effective dose is about 6 to 8 mSv.

Gastrointestinal bleeding causes over 1 in 1000 hospital admissions each year, with lower tract bleeding in 20 to 30%, and stops without intervention in 80% but recurs in a proportion.

Scintigraphy has a sensitivity of 80 to 95% for active bleeding and can localise the site to a region of bowel in 70 to 90%. It cannot give a precise location, and the finding is used to direct angiography, endoscopy or surgery.

Indications.

• Lower gastrointestinal bleeding that is intermittent, slow, or not found at colonoscopy.
• Localisation of active bleeding before catheter angiography and embolisation, or surgery.
• Upper gastrointestinal bleeding with negative endoscopy.
• Obscure bleeding in the small bowel.
• Recurrent bleeding with negative investigations, as a repeat study.
• Haemodynamically stable patients with suspected ongoing bleeding when CT angiography is not possible or is negative.
• Bleeding in children when other tests are negative.

Contraindications and precautions.

• Pregnancy: avoid unless essential. Breast feeding: a short interruption as the local rule requires.
• The patient must be reasonably stable for a long study, with resuscitation equipment available. Unstable patients go directly to CT angiography or angiography.
• Recent barium or contrast studies in the bowel obscure interpretation.
• Heparin, some antibiotics, doxorubicin, hydralazine and iodinated contrast reduce in vivo labelling efficiency. The in vitro method is preferred.
• Blood transfusion does not prevent the test, but labelling is done in the patient's own cells.
• The patient lies still for up to 90 minutes. Sedation is not needed.
• Anticoagulants are continued unless the clinician decides otherwise.
• Allergy to the label is very rare.

Preparation and technique.

No fasting is needed. A labelled red cell preparation is checked by assessing blood pool activity, and activity in the stomach, thyroid and bladder is noted as it shows free pertechnetate. The tracer is injected, and imaging begins at once.

The camera covers the abdomen and pelvis in the anterior view, with the heart and femoral vessels in the field to follow the blood pool.

Dynamic images are collected for 60 to 90 minutes, then delayed views are taken at intervals up to 24 hours if there is no bleeding. Images are reviewed in cine mode and with a display that enhances low level activity.

Bleeding is confirmed when focal activity appears, increases in intensity and moves along the bowel in an antegrade or retrograde direction. SPECT-CT at the end of the study improves localisation.

Normal appearances.

The blood pool shows the heart, aorta, iliac vessels, liver, spleen and kidneys, and decreases slowly with time. There is a faint, stable image of the major vessels in the abdomen and pelvis.

The bowel contains no activity, apart from a small amount of free pertechnetate in the stomach if labelling is poor. The bladder shows activity from free pertechnetate or labelled cells breaking down.

Interpretation.

• Positive: a focal area of activity that appears in the lumen of the bowel, increases in intensity with time and moves along the bowel. Its first appearance gives the site. The time of appearance is recorded.
• Small bowel and right colon bleeding: movement is often to the right lower quadrant, and retrograde movement is common, so the earliest frame is used for localisation.
• Left colon and rectal bleeding: activity in the left abdomen or pelvis that moves toward the rectum.
• Upper gastrointestinal bleeding: early activity in the upper abdomen that moves to the small bowel.
• Negative study: no abnormal activity at 90 minutes and at delayed images. It does not exclude bleeding that has stopped or is under the detection rate.
• Fixed activity that does not move with the bowel is vascular: haemangioma, varices, aneurysm, arteriovenous malformation, or hyperaemic bowel in inflammation. Activity in the bladder, kidney, genitalia, spleen and uterus is normal or physiological.
• Delayed positive images (after 2 to 4 hours) have a poor localisation value because of bowel transit, and the location is stated as uncertain.
• Early positive images (within 10 to 30 minutes) indicate rapid, probably arterial bleeding, and the patient should proceed to angiography promptly.
• Free pertechnetate in the stomach, thyroid and bladder is recognised and is not bleeding.
• Report states the first appearance, the site of origin, the direction of movement and the level of confidence.

Critical values and findings needing immediate action.

• Rapid bleeding with haemodynamic instability: resuscitation, urgent CT angiography or catheter angiography with embolisation, and surgical review.
• Active bleeding localised to a segment: direct endoscopy, angiography or surgery to that segment.
• Bleeding from varices: endoscopic treatment and portal pressure management.
• Large drop in haemoglobin, shock or ongoing transfusion requirement: critical care.
• Anaphylaxis (rare): resuscitation.

Limitations and artefacts.

• Intermittent bleeding may stop during the scan and give a false negative. Repeat or delayed imaging, and a repeat study if bleeding recurs, are used.
• Rapid bowel transit and retrograde movement can place the apparent source away from the true site.
• Free pertechnetate in the stomach, thyroid and bladder, and activity in the genitalia, can mimic bleeding or hide it.
• Vascular structures (aneurysm, haemangioma, varices) and hyperaemic bowel give fixed activity that is mistaken for bleeding.
• Poor labelling with high free pertechnetate leads to errors. The in vitro method is more reliable.
• Patient motion, a full bladder, and urinary tract activity hide pelvic bleeding. Voiding or a catheter helps.
• The method gives a region, not an anatomical site. CT angiography gives more precise localisation.
• The long study delays treatment in a patient who is unstable.
• The test does not find the cause of bleeding.
• Radiation dose is moderate.

Special groups.

• Children: used when other tests are negative, with weight based dose and a Meckel diverticulum study when appropriate.
• Older patients: diverticular disease and angiodysplasia are the commonest causes of lower bleeding.
• Patients on anticoagulants or antiplatelet drugs: bleeding may be brisker and the clinical team decides on reversal.
• Pregnant patients: avoid.
• Patients with renal failure: bladder and kidney activity hides the pelvis, and a catheter may be used.
• Regions: lower gastrointestinal bleeding from infection, haemorrhoids, hookworm, schistosomiasis, typhoid and cancer is common. Scintigraphy is available in few centres, so endoscopy, surgery and clinical judgement are used.

Follow up tests.

• CT angiography of the abdomen and pelvis.
• Catheter angiography with embolisation.
• Colonoscopy and upper gastrointestinal endoscopy.
• Capsule endoscopy and double balloon enteroscopy for small bowel bleeding.
• Meckel diverticulum scan in young patients.
• Full blood count, coagulation, group and crossmatch, urea and creatinine.
• Surgical consultation for recurrent or life threatening bleeding.
• Referral to gastroenterology, interventional radiology or surgery.