🔬 MRI Brain

MRI Brain is a high-resolution imaging technique used to visualise the structure and function of the central nervous system. It provides superior soft-tissue contrast compared to CT and is the modality of choice for complex neurological diagnosis, though it is limited by contraindications and availability.

Indications include investigation of chronic headache with red flags, suspected Multiple Sclerosis, primary or metastatic brain tumours, and neurodegenerative disorders like Alzheimer's. It is also used for the evaluation of pituitary disorders, cranial nerve palsies, and unexplained seizures. In the acute setting, it is used for suspected stroke when CT is inconclusive or when 'wake-up' stroke protocols are being followed.

MRI uses strong magnetic fields and radiofrequency pulses to align hydrogen protons in the body. The patient lies within a bore (tube) while sequences are acquired. Gadolinium-based contrast may be injected to highlight areas of blood-brain barrier breakdown, such as in tumours or active MS plaques. No ionising radiation is used.

A normal MRI Brain shows symmetrical cerebral hemispheres with normal grey-white matter differentiation. Ventricles and sulci should be appropriate for the patient's age. There should be no abnormal signal intensities within the brain parenchyma, no evidence of restricted diffusion (ischaemia), and the midline should be central. Cranial nerves and major vessels (flow voids) should appear normal.

The interpretation relies on comparing different 'sequences'. T1-weighted images show anatomy well (fluid is dark); T2-weighted images show pathology and fluid well (fluid is bright). FLAIR suppresses normal CSF signal to highlight periventricular lesions. Diffusion-Weighted Imaging (DWI) and the Apparent Diffusion Coefficient (ADC) map are critical for identifying 'cytotoxic oedema' in acute ischaemia.

Pathological findings include hyperintense lesions on T2/FLAIR images in Multiple Sclerosis (often periventricular), space-occupying lesions with associated vasogenic oedema (neoplasms), or restricted diffusion on DWI representing acute ischaemic stroke. Chronic findings may include global or focal atrophy (dementias) and microvascular leucoencephalopathy. Midline shift or herniation are critical findings in trauma or mass effect.

MRI is the definitive modality for most non-emergent neurological conditions. It is superior to CT for assessing the posterior fossa, pituitary gland, and white matter diseases. In stroke medicine, 'DWI' sequences can identify ischaemia within minutes of onset, far earlier than CT. It is essential for the specialist management of epilepsy, neuro-oncology, and neuro-inflammatory conditions.

Movement artefact is common and can render the scan uninterpretable. Interpretation can be complicated by 'incidental' findings like benign meningiomas or arachnoid cysts which may not relate to the patient's symptoms. Small metallic objects or dental work can cause 'susceptibility artefacts' that distort the image around the face and jaw.

The exam is time-consuming (20-45 minutes) and requires the patient to remain very still, making it difficult for confused or claustrophobic patients. It is contraindicated in patients with non-MRI compatible metalwork (e.g., certain old pacemakers, metallic orbital foreign bodies, or some aneurysm clips). It is much less available than CT in the UK, often leading to longer waiting times.

Know the 'T1 vs T2' rule: T1 = CSF is dark (Water is black); T2 = CSF is bright (Water is white). Remember that DWI is the most sensitive sequence for acute stroke. Recognize the classic 'Dawson's fingers' appearance of MS lesions on sagittal views.