Imaging Of The Central Nervous System
Imaging of the central nervous system (CNS) is essentially designed to look either at structure (computed tomography [CT], magnetic resonance imaging [MRI], angiography) or function (functional MRI [fMRI], positron emission tomography [PET], and single photon emission CT [SPECT]). In clinical practice it is the former that is the mainstay of practice, with the latter tests reserved for patients being investigated for specific problems or as part of a research project.
· In general structural scanning is undertaken to determine if there is any abnormality in the CNS on imaging.
· If there is an abnormality, then where is it and does it fit with the history and clinical examination?
· What is the likely nature of that abnormality pathologically based on its radiological appearance?
This information can be used for the future investigation and management of patients with neurological disorders. This chapter outlines the major imaging modalities used in clinical practice, their indications, value and drawbacks.
• Basic principle: this technique uses X-rays to scan the brain or spine (typically lumbar) and then reconstruct an image of that structure; it can be performed with or without a contrast agent, the latter being used to better define blood vessels and abnormalities in the blood–brain barrier.
• Use: imaging of the brain looking for major abnormalities, in particular stroke, head trauma, hydrocephalus or tumour, especially in the acute medical situation. It can also be used to look for skull fractures and prolapsed intervertebral discs in the lumbar spine, and in some cases to look for cerebral aneurysms.
· Advantages: widely available, and often gives useful and vital information especially in acute situations. It is well tolerated by nearly all patients, even those who cannot fully cooperate, and if general anaesthesia is needed, this is more easily performed with CT than MRI.
· Disadvantage: it has poor contrast resolution compared with MRI and as such is not so good at identifying lesions in the posterior fossa and cervicothoracic spine. This is because dental fillings can often result in several artefacts on scans of the posterior fossa. It also involves radiation, which can be an issue in some situations – e.g. pregnancy.
· Basic principle: this technique places the patient in a strong magnetic field which is then subject to a series of magnetic perturbations (scan sequence), which alter the orientation of hydrogen ions, such that their change and subsequent shift back to normal position is detected. Thus, it does not use X-rays and is very sensitive to subtle changes in water content, which makes it a highly sensitive scan.
· Use: most patients with neurological problems should have an MRI scan, given its superior spatial resolution compared with CT scanning and the fact that any part of the neural axis can be scanned with it. Thus, it is employed in patients with chronic neurological problems (e.g. multiple sclerosis) as well as those with evolving acute disorders (e.g. herpes encephalitis). It can also be used with a contrast agent (gadolinium) and to image blood vessels both on the arterial side (magnetic resonance angiography [MRA]) looking for carotid artery disease or intracerebral aneurysms and on the venous side (magnetic resonance venography [MRV]), especially to look for major venous sinus thromboses.
· Advantages: high spatial resolution and the fact that any part of the neural axis can be imaged, along with the major vessels, without recourse to X-ray exposure or invasive procedures.
· Disadvantage: it is a noisy, claustrophobic experience and requires the patient to be cooperative to some extent. Some patients cannot cope with the claustrophobia while agitated patients will move in the scanner causing major artefacts on the images. It also cannot be used in patients with metallic magnetic materials such as a cardiac pacemaker.
· Basic principle: this is the imaging of blood vessels and it can be carried out using CT and MRI, but in some cases it requires the direct visualization of blood vessels using a radiolucent contrast agent injected into an artery with video fluoroscopy to follow its course. Thus, the flow of the dye can be followed through the vasculature and X-rays taken to capture various different phases of the injection; this can identify problems on the arterial and venous sides of the circulation.
• Use: its main value is the identification of vascular abnormalities such as aneurysms, arteriovenous malformations and venous sinus disease. In all cases angiography is either performed to confirm an equivocal MRA/MRV result or as a prelude to a more invasive procedure to deal with the underlying abnormality such as the obliteration of vascular malformations through intravascular occlusion techniques (gluing or coiling).
• Advantage: it is the most high-resolution scan for identifying vascular abnormalities and is essential if intravascular interventional therapies are being considered.
• Disadvantage: it is an invasive procedure with a small but nevertheless real complication rate of stroke and local haemorrhage/haematoma at the site at which the catheter is passed into the artery (typically the femoral artery in the groin).
This embraces SPECT, PET and fMRI. Although there are a number of different types of scan, they can be thought of as looking at either:
• Blood flow/metabolism, using glucose and oxygen markers to reflect neuronal activity and pathology, such that a loss of activity reflects an area that contains dysfunctional or dead neurones. So, for example, in Alzheimer’s disease, there will be hypoperfusion in the parietotemporal cortices. Such ‘metabolic’ scans can be under- taken for diagnostic and therapeutic purposes in some patients in routine clinical practice. Another related approach, which is currently only used in research, relies on looking at oxygen extraction in areas of the brain while the patient is being tested on a particular task while being imaged in the MRI scanner. The resultant scan will show which areas of the brain are activated by that task. This is called fMRI, and has been used, for example, to see which brain areas are activated by specific types of cognitive or visual processing tasks.
• Specific neurochemical markers which are used to identify and label particular aspects of a neurotransmitter pathway. In Parkinson’s disease this may involve looking at the dopamine transporter (e.g. DAT scans) or certain types of dopamine receptors (e.g. 11C-raclopride labelling of D2 receptors in PET). The former types of scan are found in many nuclear medicine departments and are widely available, while PET scanning is still only an experimental tool and found in a few research centres. However, [18F]2-fluoro- 2-deoxy-D-glucose (FDG)-PET scanning is being increasingly used to find small tumours in patients with suspected paraneoplastic syndromes (see Chapter 62). This is because they can detect small metabolically active tumours that cannot be seen using traditional imaging modalities.