Gastrointestinal Bleed

Gastrointestinal Bleed

GI Bleed Imaging Overview

  • Purpose of Imaging:

    • Detection and localization of active GI bleeding.

    • Assists in diagnosis and management decisions.

    • Guides interventional radiology and surgical therapy.

What are GI Bleeds?

Significance

  • Public Health Burden:

    • A major cause of death in the United States.

    • Mortality rates range from 10% to 30%.

    • Requires rapid diagnosis and evaluation for potential emergency intervention.

Types of GI Bleeding
Upper Gastrointestinal Bleeding (UGIB)

  • Definition: Originates above the duodenal flexure.

  • Incidence: Affects 50–150 per 100,000 adults annually.

  • Statistics: Responsible for approximately 20,000 deaths in the U.S. each year.

Lower Gastrointestinal Bleeding (LGIB)

  • Definition: Originates distal to the duodenal flexure.

  • Statistics: Accounts for roughly 21% of all GI bleeding cases.

    • Commonly self-limited but hospitalizes approximately 21 per 100,000 adults annually.

  • Age-Related Risk: The prevalence increases over 200-fold between ages 30 and 90.

Clinical Signs of GI Bleeds

  • Hematemesis:

    • Vomiting bright red blood or "coffee-ground" material.

  • Melena:

    • Black, tarry, foul-smelling stools, suggesting digested blood.

  • Hematochezia:

    • Passage of bright red or maroon blood per rectum.

  • Self-Limited Nature:

    • Most cases resolve without immediate surgery, though hospitalization may be required.

Key Terms

  • Overt Bleeding: Visible bleeding (blood in stool or vomit).

  • Occult Bleeding: Microscopic bleeding that cannot be seen but is detected by lab tests (FOBT).

  • Acute Bleeding: Sudden, severe bleeding that may cause shock (dizziness, rapid pulse, drop in blood pressure).

  • Chronic Bleeding: Slow, long-term bleeding that often leads to anemia (fatigue, shortness of breath).

Indications, Contraindications, and Patient Prep

Purpose & Rationale

  • GI Bleed Scintigraphy is performed to:

    • Confirm active GI bleeding.

    • Localize the bleeding site using radiotracer to detect extravasation into bowel lumen.

    • Prolonged imaging can detect intermittent bleeding.

Clinical Indications

  • Bright red or maroon blood in stool.

  • Low red blood cell count.

  • Dark/tarry stools (earlier or chronic bleed).

Contraindications

  • Not actively bleeding patients.

  • Recent radionuclide administration.

  • Recent barium studies.

Causes of GI Bleeds
Upper GI Bleeds

  • Gastric/duodenal ulcers.

  • Gastritis.

  • Esophagitis.

  • Neoplasm.

Lower GI Bleeds

  • Diverticula.

  • Neoplasm and inflammation.

  • Meckel’s diverticulitis.

Patient Preparation & Education

  • No special preparation required (eating discouraged).

  • Encourage patients to void immediately before imaging.

  • Educate about the duration, positioning, motion, and potential delays during the procedure.

Tagging and Radiation Exposure

Radiopharmaceutical

  • Technetium labeled RBCs (Tc labeled RBCS):

    • Dosage: 20–25 mCi.

    1. In vitro:

    • With Kit – Ultra-tag (95% labeling efficiency).

    1. In vivo:

    • (80-90% labeling efficiency).

    1. Modified In vivo:

    • (90-95% labeling efficiency).

99mTc-RBC Technique: Overview

  • Preferred Technique: 99mTc-labeled RBCs.

  • Sensitivity: Highly sensitive, detects bleeding rates of approximately 0.05–0.1 mL/min.

  • Advantages:

    • Requires strict blood product safety procedures.

  • Disadvantages: Not specified in the transcript.

Radiation Exposure (99mTc-RBC)

Adult Male

  • Dose: 925 MBq / 25 mCi 99mTc-RBCs.

  • Effective Dose: Approximately 6.5 mSv (0.65 rem).

  • Critical Organ Exposure: Heart wall ~21.3 mGy (2.13 rad).

Adult Female

  • Dose: 925 MBq / 25 mCi 99mTc-RBCs.

  • Effective Dose: Approximately 8.2 mSv (0.82 rem).

  • Critical Organ Exposure: Heart wall ~26.8 mGy (2.68 rad).

Imaging Procedure

Patient Positioning & Camera Setup

  • Patient should be supine with arms at sides.

  • Use a large-field-of-view gamma camera (either single or dual head).

  • Collimator: Low-energy, parallel-hole.

  • Field: From inferior liver margin through the pelvis.

99mTc-RBC Injection & Dynamic Imaging

  • Dose: 20–30 mCi (740–1010 MBq) of 99mTc-RBCs administered intravenously.

  • Dynamic Serial Images: 10–60 sec/frame with a 128 × 128 matrix.

    • Optional initial flow imaging for 2–3 minutes.

    • Continuous dynamic imaging can last up to 60–90 minutes.

    • Static images (1–2 million counts) taken in oblique/lateral/posterior views if bleeding is observed.

Rapid SPECT/CT for GI Bleed

  • Improvement: Rapid SPECT/CT can enhance specificity.

  • Typical SPECT Protocol:

    • 10 sec/stop with 45–60 stops.

    • 90–120 projections (dual-head).

    • Expected total time is approximately 9–12 minutes.

  • CT should follow immediately per manufacturer’s protocol.

Extended Delayed Imaging

  • Delayed imaging possible up to 24–36 hours to help find intermittent bleeding.

  • Limitations: Severely limits accurate localization and thus is used rarely.

Tc99m Sulfur Colloid

99mTc–Sulfur Colloid Technique: Overview

  • Historically used; now rarely utilized.

  • Advantages:

    • Minimizes background noise.

    • Provides high contrast ratios.

    • Allows for repeat injections.

  • Disadvantages:

    • Lower sensitivity compared to RBC;

    • Active bleeding must be present at injection time.

99mTc–Sulfur Colloid Protocol

  • Dose: 7–10 mCi of freshly prepared 99mTc–SC.

  • Dynamic Flow: 2–3 min at 1–2 sec/frame in a 128 × 128 matrix.

  • Static Images: Take 500,000–750,000 counts every 1–2 min for 20–30 min.

  • Limit the intensity so that the bone marrow is visible.

Additional Sulfur Colloid Imaging

  • After around 30 min, no further extravasation is expected.

  • Use oblique, lateral, posterior images to improve visualization.

  • Enhanced images with 1,000,000 counts for upper abdomen can showcase hepatic/splenic flexures.

  • Delayed 40–60 min lower abdominal images may reveal previously obscured activity.

  • Consider repeat injection and imaging if a new bleed is suspected after 60–90 min.

Image Processing: Dynamic Data

  • Motion Correction: May be required for dynamic images.

  • Cine Format: View dynamic data to assess flow.

  • Reformatting: Extend viewing intervals to enhance visualization.

  • Background Subtraction and Contrast Enhancement: Optional but avoid if motion or biodistribution changes are noted.

SPECT/CT Processing

  • Process SPECT/CT per manufacturer and physician preferences, which typically involve:

    1. Preprocessing.

    2. Reconstruction (transverse, sagittal, coronal).

    3. Filter selection.

    4. Image display.

    5. Iterative reconstruction is recommended.

Normal and Abnormal Findings

Normal 99mTc-RBC Study

  • Normal Distribution: Blood pool distribution in:

    • Large blood vessels.

    • Liver.

    • Spleen.

    • Kidneys.

  • No extravasation seen into GI lumen.

  • Small free 99mTc-pertechnetate may show:

    • Gastric lining uptake,

    • Activity in urinary tract (kidneys, ureters, bladder).

Positive (Abnormal) GI Bleed Study

  • SNMMI Guideline Criteria for Positive Study:

    1. Activity found outside expected anatomic blood pool structures.

    2. Change in intensity observed over consecutive images.

    3. Movement of activity is consistent with bowel pattern—may move forward or backward in the GI tract.

Positive RBC Study

  • Observation: Dynamic anterior images show progressive activity accumulation in the left lower quadrant.

  • Other normal structures include the liver, spleen, abdominal vessels, kidneys, and bladder while showing progressive focal activity consistent with the GI bleeding site.

Patterns of Movement

  • Small Bowel: Rapid curvilinear movement located centrally.

  • Large Bowel: Linear pattern, peripherally identified, typically an S shape indicative of rectosigmoid distribution.

Artifacts: Free 99mTc-Pertechnetate

  • Sources of free 99mTc-pertechnetate:

    • Swallowed salivary activity.

    • Excreted gastric mucosal activity.

  • Pathway from stomach to small bowel can mimic upper GI bleeding.

  • Assessment: Free pertechnetate should be evaluated by neck imaging (thyroid, salivary glands).

  • Renal pelvis, ureters, bladder show urinary excretion but not bowel bleed.

Artifacts: Reproductive and Pelvic Activity

  • Increased RBC activity may be observed in reproductive organs:

    • Penis (males).

    • Uterus (variable flow with menstrual cycle).

  • Lateral views may help differentiate penile activity from rectal GI bleeding.

Artifacts: Vascular Variants & Tumors

  • Vascular Abnormalities: May alter blood pool distribution and include:

    • Vessel aneurysms.

    • Vascular tumors.

  • Important for the recognition of false-positive GI bleed diagnoses and can uncover unrecognized vascular abnormalities.

Sensitivity, Specificity, Accuracy

  • During the first 90 minutes, over 80% of active GI bleeding sites are identified.

  • Only 2-3 mL of extravasated blood is needed for detection.

  • Overall Accuracy: Approximately 94%.

    • Sensitivity: ~95%

    • Specificity: ~93%

  • Caveat: Accuracy can be reduced by poor methodology (e.g., infrequent acquisitions).

Summary

Clinical Role of GI Bleed Scintigraphy

  • GI bleed scintigraphy is a key tool for suspected GI bleeding.

  • It detects and localizes active bleeding, including low-rate and intermittent instances.

  • Complements angiography and endoscopy.

  • Guides interventional radiology and surgical management.