7.1 Iodine and Indium Diagnostic Agents
Key Takeaways
- I-123 NaI (t½ ≈ 13.2 h, 159 keV) is preferred for diagnostic thyroid imaging; I-131 NaI (t½ ≈ 8.0 d, 364 keV + β−) is used for uptake and whole-body thyroid-cancer surveys at diagnostic activities
- I-123 MIBG (AdreView®) is a norepinephrine analog for pheochromocytoma/neuroblastoma imaging; give thyroid blockade and hold interfering catecholamine-active meds per protocol
- I-123 ioflupane (DaTscan®) binds striatal dopamine transporters for Parkinsonian syndrome evaluation; require thyroid blockade and hold cocaine, amphetamines, and selected dopaminergic drugs
- In-111 (t½ ≈ 2.8 d; principal gammas ≈ 171 and 245 keV) labels WBCs (oxine), supports cisternography (DTPA), and forms In-111 pentetreotide (Octreoscan®) for somatostatin-receptor imaging
- Key traps: confusing diagnostic vs therapeutic iodine activities; skipping thyroid blockade for free iodide/MIBG/DaTscan; wrong med holds that produce false-negative DaTscan or MIBG studies
7.1 Iodine and Indium Diagnostic Agents
Quick Answer: I-123 NaI (13.2 h, 159 keV) → diagnostic thyroid imaging. I-131 NaI (8 d, 364 keV + β−) → uptake and diagnostic whole-body surveys (therapy is higher activity). I-123 MIBG → NET/sympathetic tumors with thyroid blockade. I-123 ioflupane (DaTscan®) → striatal DAT SPECT with med holds. In-111 (2.8 d; 171/245 keV) → WBCs, DTPA cisternography, pentetreotide (Octreoscan®).
Non-technetium single-photon agents still fill high-yield CNMT slots: thyroid physiology, neuroendocrine/sympathetic imaging, infection cell labeling, CSF studies, and receptor SPECT. Learn each agent as half-life + energy + mechanism + indication + prep trap.
Radioiodine Sodium Iodide: I-123 vs I-131 (Diagnostic)
Both chemical forms are NaI. The sodium-iodide symporter traps iodide in thyroid follicular cells; organification incorporates iodine into thyroid hormone pathways. That is why free iodide also concentrates in salivary glands and can appear in stomach.
| Feature | I-123 sodium iodide | I-131 sodium iodide (diagnostic use) |
|---|---|---|
| Half-life | ≈ 13.2 hours | ≈ 8.0 days |
| Primary photons | 159 keV gamma | 364 keV gamma (+ β− emission) |
| Decay | Electron capture | β− decay |
| Typical diagnostic roles | Thyroid imaging and uptake | Uptake measurements; diagnostic whole-body scan after thyroidectomy for cancer staging/follow-up |
| Why choose | Better imaging energy for gamma cameras; lower patient dose for morphology | Longer half-life suits delayed whole-body surveys; β component matters more for therapy |
Diagnostic adult activities (order-of-magnitude teaching values; follow package insert/lab protocol):
| Study | Agent | Approximate activity |
|---|---|---|
| Thyroid scan / uptake (imaging preference) | I-123 | Often ~200–400 µCi (≈7–15 MBq) oral for scan (uptake doses may be lower) |
| Thyroid uptake only | I-123 or low-activity I-131 | I-131 classic uptake often tens of µCi |
| Diagnostic whole-body scan (thyroid cancer) | I-131 | Often 1–5 mCi range (protocol/regulatory dependent)—not a therapy dose |
Therapy I-131 activities (ablation/treatment) are much higher (tens to hundreds of mCi) and belong with written-directive rules—do not mix “scan dose” with “treatment dose” on exam stems.
Patient prep (thyroid radioiodine): hold antithyroid drugs, levothyroxine/liothyronine, and recent iodinated contrast per protocol; low-iodine diet may be ordered before cancer surveys; document pregnancy status; counsel on radiation precautions proportional to activity. Recent IV contrast or amiodarone can suppress uptake for weeks to months.
I-123 MIBG (Iobenguane, AdreView®)
Mechanism: MIBG is a guanethidine/norepinephrine analog. It is taken up by the norepinephrine transporter (NET) into adrenergic tissues and stored in neurosecretory granules—mapping pheochromocytoma, paraganglioma, neuroblastoma, and other NET-positive sympathetic tumors (and selected cardiac sympathetic innervation protocols).
| Item | Teaching points |
|---|---|
| Nuclide | I-123 (13.2 h, 159 keV) |
| Admin | IV (slow; follow insert for infusion precautions) |
| Typical adult activity | Often ~10 mCi (370 MBq) order of magnitude |
| Imaging | Planar ± SPECT at ~24 h (sometimes earlier/later views) |
| Thyroid blockade | Required—SSKI, Lugol’s, or potassium perchlorate per protocol to block free iodide uptake in thyroid |
Medication holds: many catecholamine-active drugs reduce MIBG uptake (classic lists include certain tricyclic antidepressants, labetalol, some sympathomimetics, cocaine, and others—use the current package insert list). Missing a hold → false-negative tumor study. Free iodide without blockade → unwanted thyroid dose and confusing neck activity.
I-123 Ioflupane (DaTscan®)
Mechanism: Ioflupane binds presynaptic dopamine transporters (DAT) in the striatum (caudate/putamen). SPECT distinguishes neurodegenerative Parkinsonian syndromes (reduced striatal binding) from essential tremor or drug-induced Parkinsonism patterns when clinically appropriate.
| Item | Teaching points |
|---|---|
| Nuclide / energy | I-123; 159 keV |
| Admin | IV |
| Typical adult activity | ~3–5 mCi (111–185 MBq) common teaching range |
| Imaging | SPECT typically 3–6 hours after injection |
| Thyroid blockade | Yes—block free iodide before injection |
Critical med holds: drugs that bind DAT or alter dopamine terminals can invalidate interpretation—classically cocaine, amphetamines, methylphenidate, and selected other agents per insert. Do not tell patients that all Parkinson medications must always be stopped—follow the ordered hold list; many levodopa regimens continue. Prep traps: no blockade (thyroid visualization), motion on SPECT, and interpreting without clinical correlation.
Indium-111 Diagnostic Family
In-111 decays by electron capture with principal gammas near 171 keV and 245 keV and a physical half-life of about 2.8 days—ideal for 24–72 hour delayed imaging.
| Product | Mechanism | Classic indications | Notes |
|---|---|---|---|
| In-111 chloride | Source material / labeling precursor | Component for labeling workflows | Not a routine free “scan agent” by itself in most clinics |
| In-111 oxine WBCs | Lipophilic oxine labels leukocytes; cells migrate to infection | Osteomyelitis, abscess, FUO, prosthesis infection workups | Compare with Tc-99m HMPAO WBCs; longer t½ favors delayed imaging; dual-isotope with sulfur colloid for marrow |
| In-111 DTPA | CSF tracer after intrathecal injection | Cisternography, CSF leak, shunt patency (protocol-specific) | Never confuse with IV renal DTPA (Tc-99m); route is critical |
| In-111 pentetreotide (Octreoscan®) | Somatostatin receptor (SSTR) binding (mainly subtype 2) | Neuroendocrine tumors historically | Increasingly displaced by Ga-68 DOTA-peptide PET, but still tested |
WBC Labeling Safety (In-111 oxine)
Same non-negotiables as Tc-labeled cells: positive patient ID, aseptic technique, adequate leukocyte harvest, remove unbound indium, assay labeling efficiency, and reinject only the donor. Image often at ~4 h and 24 h (sometimes later). Damaged cells sequester in lungs/liver/spleen and create false patterns.
Octreoscan Prep Notes
Hold short-acting somatostatin analogs per protocol (long-acting depot timing is specially scheduled). Hydrate; laxatives sometimes used to clear gut activity. Bowel activity is a classic interpretation trap—delayed imaging and SPECT help.
Comparison Snapshot
| Clinical goal | Preferred agent (this section) |
|---|---|
| Thyroid morphology (diagnostic imaging) | I-123 NaI |
| Post-thyroidectomy diagnostic whole-body survey | I-131 NaI (diagnostic activity) |
| Pheo/neuroblastoma sympathetic imaging | I-123 MIBG + thyroid blockade |
| DAT SPECT (Parkinsonian workup) | I-123 ioflupane (DaTscan®) |
| Labeled WBC infection (In product) | In-111 oxine WBCs |
| Cisternography / CSF | In-111 DTPA (intrathecal) |
| SSTR SPECT (legacy/available) | In-111 pentetreotide |
Memorize 159 keV / 13 h for I-123 products, 364 keV / 8 d for I-131, and 171+245 keV / 2.8 d for In-111—then attach mechanism and prep.
Which statement correctly matches a diagnostic radioiodine product with its primary imaging advantage?
A patient is scheduled for I-123 ioflupane (DaTscan®) SPECT. Which preparation step is essential?
In-111 DTPA used for cisternography is correctly described by which pairing of route and purpose?