Brain

Methods of imaging the brain

Imaging the brain’s structure and examining its physiology, both in the acute and elective setting, are now the domain of multiplanar, computer-assisted imaging. The imaging modalities in use today include the following:

1. Computed tomography (CT). This is the technique of choice for the investigation of serious head injury; for suspected intracranial haemorrhage, stroke, infection and other acute neurological emergencies. CT is quick, efficient and safer to use in the emergency situation than MRI.

2. Magnetic resonance imaging (MRI). This is the best and most versatile imaging modality for the brain, constrained only by availability, patient acceptability, and the logistics and safety of patient handling in emergency situations. New protocols and higher field strength magnets have raised the sensitivity of MRI in epilepsy imaging, acute stroke, aneurysm detection and follow-up post treatment of neoplastic and vascular disorders. It is the only effective way of diagnosing multiple sclerosis.

3. Angiography. This is very important in intracranial haemorrhage (ICH), especially subarachnoid haemorrhage (SAH) and, increasingly, in intra-arterial management of ischaemic stroke. However, with the widespread availability of multi-detector CT scanners, CT angiography (CTA) is now preferentially used in ischaemic stroke, SAH and ICH. Angiography is still requested for pre-operative assessment of tumours, vascular malformations and angiographic expertise is vital for the performance of many neurointerventional procedures.

4. Radionuclide imaging. There are two principal methods. The first is regional cerebral blood flow scanning, still more used in research than in clinical management, especially in the dementias and in movement disorders such as Parkinsonism; second is positron emission tomography (PET). By this method focal hyper-metabolism may be shown using ¹⁸F fluorodeoxyglucose (¹⁸FDG), for example in epilepsy, and cell turnover may be shown using ¹¹C-methionine, for example in tumour studies.

5. Ultrasound (US). This is particularly helpful in neonates and during the first year of life to image haemorrhagic and ischaemic syndromes, developmental malformations, and hydrocephalus using the fontanelles as acoustic windows. In adults, transcranial Doppler may be used for intracerebral arterial velocity studies to assess the severity of vasospasm.

6. Plain films of the skull. These are of little value except in head injury.

Computed tomography of the brain

Indications

CT is the imaging modality most commonly used in triaging acute neurological disease. For non-emergency indications CT is second best to MR, but is still widely used, often because it is more broadly available and simpler to interpret. The indications include the following:

1. Following major head injury (if the patient has lost consciousness, has impaired consciousness, or has a neurological deficit). The presence of a skull fracture also justifies the use of CT. NICE (National Institute for Health and Care Excellence) guidance has been issued on the use of imaging for head injuries for adults and children, specifically CT, listing the criteria for assessment based on best relevant data and consensus recommendations.

2. In suspected intracranial infection (the use of contrast enhancement is recommended).

3. For suspected intracranial haemorrhage and cases of ischaemic and haemorrhagic stroke.

4. In suspected raised intracranial pressure, and as a precaution before lumbar puncture once certain criteria are fulfilled. These would include reduced consciousness (a Glasgow coma score of less than 15), definite papilloedema, focal neurological deficit, immune suppression and bleeding dyscrasias.

5. In other situations, such as epilepsy, migraine, suspected tumour, demyelination, dementia and psychosis, CT is a poorer-quality tool. If imaging can be justified, MRI is greatly preferable and is recommended by NICE in these situations except for the first episode of psychosis.

Technique

1. Most clinical indications are adequately covered by 3-mm sections parallel to the floor of the anterior cranial fossa, from the foramen magnum to the midbrain, with 7-mm sections to the vertex (or contigious 3-mm slices throughout). In all trauma cases, window width and level should be adjusted to examine bone and any haemorrhagic, space-occupying lesions. Review of all trauma studies should be done on brain windows, bone and ‘blood windows’ (i.e. W175 L75).

2. In suspected infection, tumours, vascular malformations and subacute infarctions, the sections should be repeated following intravenous (i.v.) contrast enhancement, if MR is not available. Standard precautions with regard to possible adverse reactions to contrast medium should be taken.

3. Dynamic studies using iodinated contrast are increasingly being used as a routine in high-velocity head trauma, the assessment of intracerebral bleeding in young patients, aneurysmal SAH, ruptured arteriovenous shunts and dural venous sinus thrombosis. CT angiography (CTA) on a typical 64-slice multidetector scanner is performed using 70–100 ml of contrast and 50 ml saline chaser, injected at 4 ml s⁻¹ with a delay of 15 s or triggered by bolus tracking with ROI in the aortic arch. Overlapping slices of 0.75–1.25 mm are reconstructed. CT venography (CTV) involves injecting 90–100 ml of contrast with a delay of 40 s. Images are usually reviewed both as three-dimensional rendered data and multiplanar reformats (MPRs).

Indications

MRI is indicated in all cases of suspected intracranial pathology. Techniques in use change with the development of new sequences and higher field strength magnets. The techniques described below are, therefore, only basic indications as to sequences in use. The greatest advantages in the use of MRI are the improved contrast resolution between grey and white matter, brain and cerebrospinal fluid (CSF); the removal of artifact due to bone close to the skull base and in the posterior fossa; and in obtaining multiplanar images for lesion localization.

Technique

1. Long TR sequences. The whole brain can be examined with 4-mm sections with 1-mm interspaces using T2-weighted turbo spin echo imaging. Proton density sequences, long TR and short TE, are used mostly for the assessment of demyelination and intra-articular disc changes in the temporomandibular joints.

2. Short TR sequences. T1-weighted sequences are used for the demonstration of detailed anatomy but gadolinium chelate contrast agents are required to view pathology. Common practice is to obtain a sagittal or coronal T1-weighted sequence as part of a standard brain study. Volumetric sequences pre and post contrast are used for image guidance software interfaces for epilepsy imaging, insertion of deep brain stimulators for movement disorders and the removal of intra- and extra-axial tumours.

3. Gradient-echo T2-weighted sequences and susceptibility weighted imaging. Although suffering from various artifacts, the sensitivity of these sequences to susceptibility effects makes them very sensitive to the presence of blood products, as in cases of previous head injury, SAH and cavernomas. Haemosiderin produces marked focal loss of signal in such cases, and all patients with a history of head injury or other causes of haemorrhage should be imaged with this sequence. These sequences identify abnormal mineral deposition and can be used in deposition disorders.

4. FLAIR sequences (FLuid Attenuated Inversion Recovery) provide very good contrast resolution in the detection of demyelinating plaques and infarcts, and have the advantage that juxta-ventricular pathology contrasts with dark CSF, and is not lost by proximity to the intense brightness of the ventricular CSF, as in spin-echo T2-weighted studies. There are usually obtained in the sagittal or coronal plane.

5. Angiographic sequences. There are many methods, of which ‘time-of flight’ is one of the more commonly used. This is a very short TR, T1-weighted gradient echo three-dimensional sequence, with sequential presaturation of each partition so that only non-presaturated inflowing blood gives a high signal. Image display is by so-called ‘MIP’ or maximum intensity projection, giving a three-dimensional model of the intracranial vessels. As it uses the T1 properties, high signal from blood products in the subarachnoid space may reduce the sensitivity to aneurysms. Phase contrast MR angiography uses velocity encoding flow and is useful to detect flow in small and tortuous vessels. Contrast-enhanced MR angiography requires a pump injector and is less susceptible to flow artifacts. Timing of image acquisition is crucial and it is very useful in neck vessel imaging.

6. Echoplanar or diffusion weighted imaging (DWI). This sequence is becoming widely available on scanners. Most units perform DWI on all patients with suspected stroke, vasculitis, encephalitis, abscesses and in the workup of intracranial tumours. DWI examines the free movement, or Brownian motion, of water molecules at a cellular level. In acute infarcts cytotoxic oedema prevents free movement of water, whereas in tumours there is no restriction. All DWI should be reviewed together with conventional sequences and apparent diffusion coefficient (ADC) maps. Acute infarcts are hyperintense on DWI and hypointense on ADC. DWI is a useful technique in assessment of cholesteatoma both in detecting cholesteatoma if CT is equivocal and is ideal in evaluation of recurrent disease.

Computed tomography

A conventional study consists of 3-mm sections through the brainstem and posterior fossa, and 7-mm sections through the cerebrum. This is the basic multi-detector CT protocol for brain imaging. This is performed without contrast to avoid diagnostic uncertainty in deciding whether a parenchymal lesion is due to enhancement or blood. Acute blood is typically hyperdense on CT. An exhaustive differential diagnosis for bleeding in different compartments of the brain can be sourced elsewhere but, in general, bleeding can be extra-axial (i.e. epidural, subdural, subarachnoid, intraventricular) or intra-axial. Intra-axial bleeding can be due to head trauma, ruptured aneurysms or arteriovenous malformations, bleeding tumours (either primary disease or secondaries), hypertensive haemorrhages (cortical or striatal) or haemorrhagic transformation of venous or arterial infarcts. In the assessment of subarachnoid haemorrhage and ischaemic stroke CTA is becoming increasingly used as the screening modality for deciding further intervention. Neurosurgeons are increasingly using CTA as the sole modality for planning microsurgical clipping, particularly in the cases where haematoma exerting mass effect needs to be evacuated immediately adjacent to a freshly ruptured intracranial aneurysm. In ischaemic stroke CTA can localize an acute embolus and its source. CT perfusion imaging can demonstrate the ischaemic core (irreversibly damaged brain) by calculating the relative cerebral blood volume and the ischaemic penumbra (recoverable brain parenchyma) by evaluating the relative cerebral blood flow (rCBF).

Magnetic resonance imaging

MRI is predominantly used to exclude the presence of an underlying tumour or a cavernoma at an interval after the initial haemorrhage when there would be less perilesional brain swelling and obscuration of the anatomy due to blood degradation products. It is also used in the setting of subarachnoid bleeding where no aneurysm or arteriovenous malformation is found on CTA or catheter angiography. In these cases the entire neuraxis must be examined to exclude an ‘occult’ source of the haemorrhage. Diffuse axonal shear injuries, in patients with depressed coma scores post head injury, in light of a normal-appearing CT scan, are best demonstrated on MRI with gradient echo imaging or susceptibility weighted imaging looking for susceptibility artifact due to ‘microbleeds’. Where resources are optimal, and MRI is used as part of the initial imaging pathway in ischaemic stroke, MR will help to determine the volumes of brain that can be recovered as well as the presence of early haemorrhage that is not visible on CT which would contraindicate thrombolysis.