MRA Circle of Willis — 3D time-of-flight
Distal vertebral and petrous/cavernous internal carotid arteries through the circle of Willis and proximal major branches.
Typical, not policy
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Screening for intracranial aneurysm in familial aneurysmal disease or polycystic kidney disease.
- Surveillance of a known unruptured aneurysm or of a previously treated aneurysm.
- Suspected large-vessel steno-occlusive disease, moyamoya or intracranial vasculitis.
- Assessment of a vascular abnormality suggested by the parenchymal brain sequences.
- Suspected vascular loop or arterial cause of a cranial neuropathy, alongside the high-resolution T2 study.
Technique
- Three-dimensional time-of-flight with submillimetre through-plane resolution, a short repetition time and a moderate flip angle; magnetisation transfer and a superior saturation band improve background and venous suppression.
- Multiple overlapping thin slabs improve inflow through the volume at the cost of slab boundary artefact.
- Maximum intensity projections are generated but the source images remain the primary data.
- Reported parameters vary widely by field strength and vendor; local optimisation matters more than any published number.
Where it goes wrong
- Saturation of slow or in-plane flow producing apparent stenosis or apparent aneurysm non-filling.
- Susceptibility from coils, clips and stents causing local signal loss that is misread as occlusion — this is the main reason time-of-flight surveillance after treatment must be interpreted against the treatment record.
- Intrinsic T1-bright material such as subacute thrombus or fat appearing on the projections and mimicking flow.
- Motion during a long acquisition degrading exactly the small vessels the study exists to assess.
- Reading projections only: small aneurysms and residual necks are seen on source images.
Clinical questions that reach this study
Contrast
None
- Time-of-flight relies on inflowing unsaturated spins, not on injected contrast. No IV access is needed unless a parenchymal contrast-enhanced series is also requested.
Acquisition
- Breathing
- Free breathing; head immobilisation is critical because the acquisition is long relative to its voxel size.
- Preparation
- State the territory: intracranial, cervical, or both — they are different acquisitions with different contrast requirements. For aneurysm surveillance, supply prior imaging and any coil or clip history; hardware degrades time-of-flight locally. IV access is only needed if a contrast-enhanced neck acquisition is planned.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- MR safety screening for implants and foreign bodies· radiographer at scan