6.3 Tc-99m GI and Hepatobiliary Agents
Key Takeaways
- Tc-99m mebrofenin (Choletec®) is an IDA (HIDA) agent taken up by hepatocytes and excreted into bile—used for acute cholecystitis, biliary patency, and bile leaks
- HIDA patients are typically NPO about 4–6 hours (not prolonged days of fasting); morphine and CCK (sincalide) are common adjuncts
- Tc-99m sulfur colloid is phagocytized by RES (liver, spleen, bone marrow); uses include liver-spleen scan, gastric emptying (meal labeling), and lymphoscintigraphy (filtered)
- Tc-99m pertechnetate is trapped by gastric mucosa (Meckel’s diverticulum) and thyroid/salivary tissue; not an organified thyroid hormone analog like iodine
- Know particle size: unfiltered sulfur colloid for RES liver-spleen; filtered smaller particles for lymph studies
6.3 Tc-99m GI and Hepatobiliary Agents
Quick Answer: Mebrofenin (HIDA) → hepatocyte uptake and biliary excretion (NPO ~4–6 h; morphine/CCK adjuncts). Sulfur colloid → RES phagocytosis (liver-spleen, gastric emptying meals, filtered lymph). Pertechnetate → trapping in gastric mucosa (Meckel’s) and thyroid. Match prep and particle size to the study.
Hepatobiliary Imaging: Tc-99m Mebrofenin (Choletec®)
Mechanism
Tc-99m mebrofenin is an iminodiacetic acid (IDA) analog—often still called a “HIDA scan” generically. After IV injection it is extracted from blood by hepatocytes and excreted into bile canaliculi, filling ducts, gallbladder (if cystic duct is patent), and bowel. It is not a RES colloid and should not light up spleen like sulfur colloid.
Exam language may say “IDA/HIDA agent” interchangeably with mebrofenin; mebrofenin performs relatively well at higher bilirubin levels.
Indications
Acute cholecystitis (cystic duct obstruction—nonvisualized gallbladder with adequate hepatic excretion), biliary dyskinesia with ejection fraction (CCK), bile leak, and pediatric biliary atresia vs neonatal hepatitis workups.
Dose, Route, Imaging
| Item | Typical practice |
|---|---|
| Activity | ~3–10 mCi (111–370 MBq) IV (adult; higher end sometimes if bilirubin elevated—follow protocol) |
| Route | Intravenous |
| Dynamic imaging | Continuous or serial images 0–60 min; delayed views as needed |
| Normal pattern | Liver → ducts → gallbladder → bowel within ~60 min (lab-specific criteria) |
Patient Prep — NPO Rules
Patients are usually NPO about 4–6 hours before injection. A recent meal leaves the gallbladder contracted from endogenous CCK and causes false nonvisualization of an otherwise patent cystic duct. Prolonged fasting (>24 h) or TPN packs the gallbladder with viscous bile that may not fill either—some protocols pretreat with CCK to empty it first. Document opioids and other sphincter of Oddi / gallbladder-active medications; they change filling kinetics and morphine-augmentation decisions.
Adjuncts (Intro-Level)
| Adjunct | Role |
|---|---|
| Morphine | Contracts sphincter of Oddi; diverts bile into GB if cystic duct patent when GB unseen but duct/bowel activity present |
| CCK (sincalide) | GB contraction for ejection fraction; may empty GB after prolonged fasting |
| Phenobarbital (peds) | Enzyme induction in neonatal cholestasis protocols |
Know why morphine or CCK is given—not every milligram schedule.
HIDA Traps
Do not confuse nonvisualization from poor hepatic function with cystic duct obstruction. Do not expect a hot spleen (wrong agent class). Recent fatty meal invalidates prep.
Tc-99m Sulfur Colloid
Mechanism
Tc-99m sulfur colloid particles are phagocytized by reticuloendothelial (RES) cells: Kupffer cells (liver), spleen macrophages, and bone marrow. Biodistribution is liver > spleen > marrow in normal adults. Particle size determines which application:
| Preparation | Particle size concept | Primary uses |
|---|---|---|
| Unfiltered sulfur colloid | Larger particles | Liver-spleen scan; bone marrow imaging; GI bleeding (historical/alternate); gastric emptying meal label |
| Filtered sulfur colloid | Smaller particles | Lymphoscintigraphy / sentinel node mapping |
Indications and Notes
Liver-spleen scan assesses size, colloid shift (increased spleen/marrow uptake in cirrhosis), and focal defects (nonspecific without correlation). Gastric emptying labels a standardized solid meal (commonly egg) with sulfur colloid and reports percent retention or emptied at timed points; solid and liquid protocols differ. Filtered colloid is injected intradermally or interstitially for lymphoscintigraphy / sentinel-node mapping (compare with tilmanocept). Some labs also use liquid sulfur colloid for reflux or aspiration studies. Adult liver-spleen activity is often ~4–6 mCi IV; meal doses are lower and meal-specific. Prep traps: boiling/kit errors that change particle size; free Tc to thyroid/stomach; never administer an interstitial lymph preparation as an IV liver-spleen dose.
Tc-99m Pertechnetate (TcO₄⁻)
Mechanism
Sodium pertechnetate is the eluate chemical form. It is handled like iodide by the sodium-iodide symporter for trapping in thyroid and salivary glands but is not organified—so it is not a true long-term thyroid hormone pathway agent like I-123/I-131. Gastric mucosa also concentrates pertechnetate—basis of Meckel’s scanning for ectopic gastric mucosa.
Indications
| Study | Role of pertechnetate | Typical adult activity (approx.) |
|---|---|---|
| Meckel’s diverticulum | Hot focus in RLQ/ectopic gastric mucosa | ~8–12 mCi IV |
| Thyroid imaging / uptake adjunct | Trapping map; less ideal than I-123 for many diagnoses | ~2–10 mCi (protocol) |
| Salivary / lachrymal | Gland function/patency studies | Protocol-dependent |
| Testicular (historical) | Blood flow/perfusion patterns | Protocol-dependent |
| RBC labeling | Source of Tc for UltraTag/in vivo methods | As required for kit |
Meckel’s and Thyroid Traps
Premedication protocols (cimetidine to reduce washout, pentagastrin to stimulate uptake, glucagon to decrease bowel motility—lab-specific combinations) aim to improve sensitivity for ectopic gastric mucosa. Time sequential images carefully; do not confuse bladder activity or normal stomach with a Meckel’s focus—use lateral/oblique views when needed. Recent barium studies or active bleeding can confound. Pertechnetate shows thyroid trapping only, not organification; discordance with radioiodine can occur in organification defects. Many cancer/met workups still prefer iodine isotopes, but CNMT items still test pertechnetate’s gastric and thyroid trapping behavior.
Side-by-Side Comparison
| Agent | Key mechanism | Flagship indication |
|---|---|---|
| Mebrofenin | Hepatocyte uptake → bile excretion | Acute cholecystitis / biliary leak |
| Sulfur colloid | RES phagocytosis | Liver-spleen; gastric emptying; filtered lymph |
| Pertechnetate | NIS trapping / gastric mucosa | Meckel’s; thyroid trapping |
NPO + nonvisualized GB → HIDA. Egg meal emptying → sulfur colloid. Child with painless bleeding / ectopic gastric mucosa → pertechnetate Meckel’s.
What is the correct patient-preparation principle for a routine adult Tc-99m mebrofenin hepatobiliary study?
Filtered Tc-99m sulfur colloid is most appropriately chosen over unfiltered colloid for which procedure?
A pediatric patient is injected with Tc-99m pertechnetate to evaluate painless lower GI bleeding. Which localization principle is being exploited?