4.3.3.2 Imaging of the nervous system

Structural and functional imaging

Imaging answers a defined clinical question, such as tumour, haemorrhage, stroke, hydrocephalus, demyelination or encephalitis. Group findings in psychiatric disorders are research averages that overlap between groups and cannot diagnose an individual.

• Diffusion tensor imaging is structural, although its colourful tract maps make it easy to mistake for a functional technique.

• PET and SPECT depend on an administered radioactive tracer, whereas MRI-based techniques use magnetic fields and the body's own signal and need no radiopharmaceutical.

• CT, PET and SPECT use ionising radiation, so exposure must be justified and possible pregnancy discussed; MRI may be used in pregnancy, but gadolinium contrast needs a specific benefit-risk decision.

Computed tomography

The CT grey scale reflects X-ray attenuation.

• Unenhanced CT is the first imaging when acute intracranial haemorrhage or acute head injury is suspected, because it is fast, always available and tolerated by an unwell patient.

• MRI is more sensitive than CT for small or old bleeds and for small, early ischaemic or posterior fossa lesions.

• Calcification is the one finding CT shows better than MRI, because calcium gives weak or variable signal on conventional MRI.

• MRI is better for soft tissue, grey-white contrast and posterior fossa detail.

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Magnetic resonance imaging sequences

MRI uses magnetic fields and radiofrequency pulses without ionising radiation, and its contrast depends on the sequence.

• Herpes simplex encephalitis is detected earliest by MRI, which shows medial temporal and insular signal change on FLAIR and diffusion sequences within days, when CT is often still normal.

• Implants, devices and metal fragments need formal screening before MRI; some are MR-conditional rather than absolutely incompatible.

Functional MRI

• Functional MRI infers neuronal activity indirectly from the blood-oxygen-level-dependent (BOLD) signal and uses no ionising radiation.

• The BOLD signal rises because local activation brings an oversupply of oxygenated blood, so paramagnetic deoxyhaemoglobin falls.

• Finger tapping in a block design gives its largest, most reproducible BOLD response in the contralateral primary motor cortex (precentral gyrus).

• Task-based fMRI examines activation and resting-state fMRI examines connectivity; results depend on vascular responses, motion and analysis choices.

Perfusion and spectroscopy

• Arterial spin labelling is an MRI perfusion method that quantifies cerebral blood flow using magnetically labelled arterial blood water as an endogenous tracer, so it needs no contrast agent or radiation.

• Magnetic resonance spectroscopy quantifies metabolites in living brain tissue and can show biochemical change before structural change.

Spectroscopy can be tuned to different nuclei.

Diffusion tensor imaging

• Diffusion tensor imaging quantifies white matter microstructure through fractional anisotropy, the degree and directionality of water diffusion.

• Fractional anisotropy is affected by several tissue and geometric properties, not by myelination alone.

• Tractography follows directional water diffusion along axons, so it is the method of choice for visualising a specific tract and detecting tract-specific damage.

• Tractography is an inference, not a photograph of individual axons.

PET and SPECT tracers

Perfusion, metabolism and receptor binding are different processes, and the tracer decides which is measured.

Dopamine transporter imaging

• Ioflupane SPECT shows reduced striatal dopamine transporter uptake, lost first from the putamen, in dementia with Lewy bodies.

• Reduced uptake is an indicative biomarker of dementia with Lewy bodies and separates it from Alzheimer's disease when the two cannot be told apart clinically.

• The National Institute for Health and Care Excellence (NICE) recommends the scan when dementia with Lewy bodies is suspected and uncertain, not for every hallucinating patient.

• Reduced uptake is common to all Lewy body disorders, so it does not separate dementia with Lewy bodies from Parkinson's disease, nor reliably from progressive supranuclear palsy or multiple system atrophy.

• Antipsychotic-induced parkinsonism leaves the scan normal, because antipsychotics block postsynaptic D2 receptors and spare presynaptic terminals; an abnormal scan points to degenerative parkinsonism.

• A normal scan also supports essential tremor but does not exclude every degenerative case, and structural lesions and interfering medicines complicate reading.

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Dementia and neurodegeneration

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• Alzheimer's disease shows a reduction in temporoparietal perfusion on SPECT, never an increase.

• Vascular lesion burden alone does not establish vascular dementia.

• The hummingbird sign on midsagittal MRI is midbrain atrophy with relative pontine sparing in progressive supranuclear palsy.

• Normal pressure hydrocephalus shows disproportionate ventriculomegaly with a compatible clinical syndrome.

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Diffusion abnormalities in prion disease vary by subtype.

Schizophrenia and psychosis

• Enlarged lateral ventricles are the best-replicated structural finding.

• The insula is the cortical region with the most consistently replicated grey matter loss across pooled studies; hippocampal loss is real but shared with mood disorders.

• Hypofrontality is reduced dorsolateral prefrontal activation during executive and working memory tasks, with hippocampal recruitment also reduced.

• Auditory verbal hallucinations come with increased activity in the superior temporal (auditory) cortex, even though superior temporal grey matter is structurally reduced.

• DOPA PET shows raised presynaptic dopamine synthesis capacity in the striatum, most marked in the associative striatum and present before any antipsychotic exposure.

• Ultra-high-risk young people who convert to psychosis lose grey matter faster over time, most consistently in the prefrontal cortex.

• NICE does not recommend routine structural imaging solely for first-episode psychosis.

Mood, anxiety and developmental disorders

• Deep (subcortical) white matter hyperintensities are in excess in older depressed patients, with enlarged ventricles and sulci, and underpin the vascular depression hypothesis.

• Periventricular hyperintensities are common with ageing and no greater in depressed patients than in controls.

• Uncomplicated anxiety or depression does not by itself justify scanning.

Electroencephalography

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• Routine interictal EEG is fairly specific (about 78 to 98%) but insensitive (about 25 to 56%) for epilepsy, so a normal trace cannot rule epilepsy out.

• Non-convulsive status epilepticus can be shown only by EEG, as continuous or recurrent ictal discharges, and EEG is needed urgently because its treatment differs from that of delirium.

Indications and interpretation

• A normal scan does not exclude neurological disease, and an abnormality, including an incidental one, may be unrelated to the presentation.

• Motion, metal, susceptibility and partial volume artefacts can mimic disease.