Vascular & Stroke
Diamox CT Perfusion: A Stress Test for Flow Reserve
Fellowship-level neuroradiology teaching by E. Brooke Schrickel, MD. Open this topic in the interactive reading room →
Watch · concise explainerWhat rest hides, the challenge reveals
Core clinical idea
Diamox CTP is a stress test, not an acute-infarct study: image at rest, give the vasodilator, image again, and ask whether the chronically narrowed territory can augment flow. Read the augmentation (augment / flat / steal) as a cerebrovascular-reserve statement — the near-normal resting CBF is the trap, not the reassurance.
Bottom line
Diamox CTP is a reserve test — the resting map sets the stage, but augment-vs-steal is the answer, and it's decision-support for a selected subgroup, not a mandate to bypass.
Core workstation questions
- Is the resting abnormality delay-only (prolonged MTT/TTP, preserved CBF) or has CBF already fallen?
- After Diamox, did the affected territory augment, stay flat, or get worse (steal phenomenon)?
- Which territory is affected, and is the contralateral/posterior circulation a clean internal control?
- Is this a pre-op baseline or a post-bypass follow-up — and did reserve improve?
- Will this answer change management (bypass candidacy in a selected subgroup), or is the doubled radiation dose unjustified?
What changes reporting / management
- Report CVR explicitly per named territory: preserved vs. impaired vs. absent augmentation after acetazolamide — that phrase, not the raw numbers, is what surgery acts on.
- Call out paradoxical worsening of MTT/T-max with falling CBF as a steal phenomenon indicating severely impaired reserve and elevated stroke risk.
- Impaired/absent CVR is the imaging argument for EC-IC (STA-MCA) bypass candidacy and for pre-/post-operative comparison; a successful bypass regains augmentation and resolves ischemic/TIA symptoms.
- Reserve me to a selected subgroup: the carotid occlusion surgery trial was negative for atherosclerotic ICA occlusion despite high graft patency; adult Moyamoya was not represented and is where some centers still bypass.
Practical traps
- Being falsely reassured by a near-normal resting CBF in chronically exhausted brain — delay (MTT/TTP) is the early tell, falling CBF is the late tell.
- Reading the resting map only and not comparing post- to pre-Diamox augmentation.
- Presenting bypass as settled therapy rather than decision-support for a selected subgroup (negative surgical trial outside Moyamoya).
- Ordering the doubled-dose challenge when the reserve answer won't change management.
Teaching pearls
- Rest can lie; the challenge tells the truth — read the augmentation.
- Delay comes first, falling CBF comes last — a near-normal resting CBF is not reassurance.
- If the diseased side worsens while the normal side improves, that's steal — reserve is gone.
- Spared posterior circulation is your built-in normal control.
- Impaired CVR argues for bypass candidacy, but the surgery trial was negative outside selected subgroups — read it as evidence, not a verdict.
Teaching visuals
Source lectures
- CT Perfusion with Diamox Challenge
Educational material for radiology residents and neuroradiology fellows. Nothing here drives individual patient care, and it contains no patient data.