14.2 Central Nervous System Procedures
Key Takeaways
- Brain death flow studies demonstrate absence of intracranial arterial perfusion (hot-nose pattern teaching) using Tc-99m agents with bolus technique and correct FOV from carotids through skull vertex
- Brain perfusion SPECT uses Tc-99m HMPAO (exametazime) or ECD (bicisate); acetazolamide (Diamox®) challenge assesses cerebrovascular reserve versus baseline
- DaTscan® (I-123 ioflupane) images presynaptic dopamine transporters; hold interfering meds, block thyroid with iodine/perchlorate per protocol, and image at the correct delay
- Cisternography, CSF leak, and shunt-patency studies use In-111 DTPA (or lab-specific agents) with timed planar/SPECT imaging along CSF pathways
- Amyloid PET and ictal/interictal SPECT or PET support dementia and epilepsy workups—timing relative to seizure onset and tracer choice is critical
14.2 Central Nervous System Procedures
Quick Answer: Brain death = no intracranial flow. HMPAO/ECD SPECT ± acetazolamide = perfusion and reserve. DaTscan = dopamine transporter imaging with med holds. In-111 DTPA = cisternogram / leak / shunt. Amyloid PET and ictal/interictal SPECT-PET = dementia and epilepsy pathways. Prep and timing dominate pass-rate items.
Brain Flow and Brain Death Scintigraphy
Nuclear brain-death flow is an ancillary test when ordered. Inject a bolus of a lipophilic Tc-99m agent retained in viable brain (HMPAO or ECD), or use a flow-only approach per lab (some use DTPA). Acquire dynamic anterior flow from carotids through the calvarium, then statics (anterior, lateral, sometimes posterior).
| Pattern (teaching) | Meaning |
|---|---|
| No intracranial arterial phase | Consistent with absent cerebral perfusion when technically adequate |
| “Hot nose” | External carotid flow without intracranial perfusion—classic teaching sign, not standalone criteria |
| Clear hemispheric HMPAO/ECD uptake | Perfusion present—does not support brain death |
Tech non-negotiables: true bolus, FOV including neck vessels and whole brain, no infiltrate, adequate counts. A weak bolus can falsely suggest no flow. Follow institutional checklist exactly.
Brain Perfusion SPECT (HMPAO / ECD)
Tc-99m HMPAO (exametazime) and Tc-99m ECD (bicisate) cross the blood–brain barrier proportional to regional blood flow and are retained for delayed SPECT. Adult activity often ~15–30 mCi class. Quiet, dim environment before/after injection reduces activation artifacts; start SPECT after agent-specific uptake (~30–90 min class—SOP).
| Topic | Teaching points |
|---|---|
| Indications | Dementia patterns, stroke/TIA, trauma, epilepsy (ictal timing) |
| SPECT/CT | AC and localization; check co-registration |
| Tumor / viability | Brain FDG PET or perfusion SPECT may help viable tumor vs necrosis—FOV/prep differ from whole-body FDG |
Minimize head motion (chin strap, cushion); reconstruct per lab filters.
Acetazolamide (Diamox®) Challenge
Acetazolamide is a carbonic anhydrase inhibitor that normally increases cerebral blood flow. After baseline SPECT (or in a two-day protocol), inject acetazolamide (classic adult teaching ~1 g IV over minutes—follow order), then inject the perfusion tracer at the protocol interval and reimage.
| Result concept | Teaching |
|---|---|
| Global increase | Intact cerebrovascular reserve |
| Regions that fail to augment or relatively worsen | Compromised reserve (e.g., severe stenosis/Moyamoya teaching) |
Contraindications/cautions: sulfa allergy discussion, severe renal/hepatic disease, electrolyte issues—defer to the authorized user/protocol. Hydrate; warn about transient flushing/paresthesias.
DaTscan® (I-123 Ioflupane)
I-123 ioflupane binds presynaptic dopamine transporters in the striatum. Used to help distinguish essential tremor from Parkinsonian syndromes (not to stage all movement disorders alone).
| Protocol element | Tech action |
|---|---|
| Thyroid blockade | Stable iodine or perchlorate per package insert/SOP before dose |
| Activity | Often ~3–5 mCi class IV (follow insert) |
| Uptake delay | Image commonly 3–6 hours post-injection |
| Acquisition | SPECT (often with CT) centered on striata; immobilization critical |
| Medication holds | Many stimulants, certain antidepressants, and other drugs interfere—pharmacy/neurology med review days before (lab list) |
Trap: imaging too early, motion, and unblocked thyroid (though primary issue is med interference and positioning). Normal “comma” vs abnormal “period” striatal shape is physician pattern language—you deliver motion-free, timed, med-reconciled data.
CSF Studies: Cisternogram, Leak, and Shunt Patency
In-111 DTPA (common) is injected intrathecally (usually lumbar) by a qualified physician under sterile technique—not a standard peripheral IV skill for this indication. Image at timed points (e.g., 2–4–6–24–48–72 h per protocol) of spine and head.
| Study goal | What you image / look for (tech level) |
|---|---|
| Cisternogram (NPH workup teaching) | Ascent over convexities vs early ventricular reflux/persistence patterns per physician criteria |
| CSF leak | Pledgets in nasal cavity ± imaging; assay pledgets in well counter; compare to plasma |
| Shunt patency | Inject into reservoir/shunt system as directed; image pathway of flow into peritoneum or distal site |
Tech points: absolute asepsis, correct energy windows for In-111 (171 and 245 keV), medium-energy collimators, and meticulous labeling of pledgets (left/right, site). Leak studies fail if pledgets are mishandled or timing is wrong.
Amyloid PET
Florbetapir, florbetaben, flutemetamol (F-18 amyloid agents) bind cortical β-amyloid plaques. Prep is agent-specific (often less fasting than FDG). Inject, wait the prescribed uptake (commonly ~30–90 min class), then PET/CT brain. Patient must hold still; motion ruins cortical-to-cerebellar interpretation. Know this is not FDG—different biology and read criteria (binary positive/negative frameworks in labeling).
Epilepsy: Ictal vs Interictal
| Mode | Tracer examples | Timing concept |
|---|---|---|
| Ictal SPECT | HMPAO/ECD injected at seizure onset | Hyperperfusion of epileptogenic zone if injection is truly ictal |
| Interictal SPECT/PET | HMPAO/ECD or FDG PET | Hypoperfusion/hypometabolism in chronic focus |
Team sport: EEG monitoring unit, rapid injection kit at bedside, stopwatch documentation of injection vs clinical/EEG onset. Late injection becomes post-ictal and can mislocalize. Transport for SPECT after stabilization; co-register to MRI when available.
CNS Prep and Timing Snapshot
| Procedure | Critical prep / timing |
|---|---|
| Brain death flow | Bolus + full head/neck FOV |
| Perfusion SPECT | Quiet room; agent-specific delay |
| Diamox | Baseline vs challenge schedule; med history |
| DaTscan | Med holds; thyroid block; 3–6 h image |
| CSF studies | Intrathecal sterility; multi-day imaging |
| Amyloid PET | Agent delay; still head |
| Ictal SPECT | Inject at onset—document seconds |
Master which tracer, which route, which clock. That triad covers most CNMT CNS clinical items.
Which technical factor most threatens the validity of a nuclear brain-death flow study?
What is the primary purpose of an acetazolamide (Diamox®) challenge paired with brain perfusion SPECT?
Before I-123 ioflupane (DaTscan®) imaging, which technologist responsibility is most specific to this exam?