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Neuroimaging modality choice matters because each scan trades off speed, resolution, radiation exposure, and what it actually measures. For example, imagine a patient who has just fallen and hit their head: a CT (computed tomography) scan is chosen because it is fast and shows bleeding and bone injury well. An MRI (magnetic resonance imaging) shows soft tissue detail better, but a bleed cannot wait for the slower scan. In practice, the clinical question, is this urgent bleeding, or a slower structural or functional problem, decides which scan to order.

CT (computed tomography) is fast, widely available, and shows bleeding and bone well, but it uses ionising radiation and shows soft tissue relatively poorly. MRI (magnetic resonance imaging) shows soft tissue, including grey and white matterBrain tissue made mostly of the insulated, myelin-covered wires connecting nerve cells.Read more →, in much finer detail without radiation. This is why MRI is preferred over CT for the structural work-up of suspected dementia. fMRI (functional MRI) measures brain activity indirectly through the BOLD (blood-oxygen-level-dependent) signal. It has good spatial resolution, meaning it locates activity precisely, but relatively poor temporal resolution, meaning it is slow to track rapid changes over time. PET (positron emission tomography) and SPECT (single-photon emission computed tomography) use radioactive tracers to image specific molecules. For example, a DaTscan, a SPECT-based dopamine transporter scan, helps distinguish Lewy body dementia from Alzheimer's disease, while amyloid PET detects amyloid plaques in suspected Alzheimer's disease. EEG (electroencephalogramA recording of the brain's electrical activity, usually shortened to EEG.Read more →) has excellent temporal resolution, catching changes over milliseconds, but poor spatial resolution, since it cannot precisely localise deep brain sources. In practice, this is why EEG is the investigation of choice for epilepsy or delirium rather than for structural localisation. DTI (diffusion tensor imaging), an MRI-based technique, maps white matter tracts, the brain's connecting fibre bundles, useful for research into connectivity but not yet routine clinical practice.

• An older man has fluctuating cognition, visual hallucinationsSeeing, hearing, or otherwise sensing things that are not really there.Read more → and parkinsonism, and the team wishes to image dopamine nerve terminals to support the diagnosis. Which scan is most…
• A man due for magnetic resonance imaging mentions several previous operations and implants. Which of these would most strongly contraindicate the scan?
• A researcher wants to measure concentrations of brain chemicals such as N-acetylaspartate and choline within a defined region, without injecting any tracer. Which technique does this?
• A confused elderly man attends at night with a suspected bleed after a fall, and imaging is needed urgently. Which combination of advantages makes computed tomography preferred here?

• CT, fast and shows bleeding/bone well, unlike MRI's superior soft-tissue detail and lack of radiation.
• fMRI, good spatial but poor temporal resolution, whereas EEG has excellent temporal but poor spatial resolution.
• DaTscan (SPECT), distinguishes Lewy body dementia from Alzheimer's disease, rather than amyloid PET which detects amyloid plaques.
• DTI, maps white matterBrain tissue made mostly of the insulated, myelin-covered wires connecting nerve cells.Read more → tract connectivity, unlike standard structural MRI which shows anatomy rather than connections.

• That technique maps task-related activity rather than dopamine terminals.
• That technique maps white matterBrain tissue made mostly of the insulated, myelin-covered wires connecting nerve cells.Read more → tracts rather than dopamine terminals.
• That scan shows structure and cannot assess dopamine terminals.
• A plastic lens implant poses no magnetic hazard.
• A healed scar poses no hazard during scanning.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.