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Imaging in neurological diseases 

Imaging in neurological diseases

Chapter:
Imaging in neurological diseases
Author(s):

Andrew J. Molyneux

, Shelley Renowden

, and Marcus Bradley

DOI:
10.1093/med/9780199204854.003.02433_update_001

August 28, 2014: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

Update:

Additional information on the use of MRI in patients presenting with a seizure.

Updated on 28 Nov 2012. The previous version of this content can be found here.
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date: 27 March 2017

Computed tomography (CT) and magnetic resonance imaging (MRI) are the most important imaging techniques in the diagnosis of neurological disease.

CT

During exposure to a series of narrow X-ray beams, a detector array spins around the patient and measures the absorption coefficients of tissues within the beam, the different coefficients providing image contrast. Helical and multidetector CT now allow analysis of up to 256 slices at a time (and with the prospect of more), with the patient moving continuously through the machine. This enables very rapid scanning and the ability to acquire angiographic (CT angiography and venography) and functional information (CT perfusion). A series of cross-sectional images are produced, usually in the axial plane. Multiplanar reconstructions can be obtained in the sagittal, coronal, and oblique planes, as required. Iodinated contrast agents, as employed in general vascular imaging, are commonly used for image enhancement.

MRI

When the body is placed in a magnetic field, a small number of the tissue protons align themselves with the main magnetic field. They are subsequently displaced from their alignment by application of a radiofrequency gradient, and when this radiofrequency pulse terminates, the protons realign themselves with the main magnetic field, releasing a small pulse of energy as a radio signal that is detected, localized and processed by a computer to produce a cross-sectional anatomic image.

Many different and complex radio-pulse sequences are used in MRI, each of which is designed and used to answer particular clinical questions. They detect different aspects of tissue properties known as the ‘relaxation times’ of the protons; times that will vary according to the proton-containing tissue and the relative mobility of the protons. Gadolinium-labelled compounds, which shorten the T1 relaxation time, are commonly used for image enhancement.

Choice of imaging modality

The choice between CT or MRI depends on a number of factors. CT is usually more readily available, is quicker to do, and is used in most acute situations, particularly in stroke and subarachnoid haemorrhage, intracranial infection, trauma, and suspected intracranial masses. MRI is now increasingly available and is the imaging modality of choice in suspected spinal pathology and also in the detailed investigation of cranial neurological diseases, particularly those affecting the white matter, epilepsy, stroke, tumours, and congenital anomalies.

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