Diffusion imaging in neurological disease
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TECHNIQUES IN CLINICAL SCIENCE
Diffusion imaging in neurological disease V. F. J. Newcombe • T. Das • J. J. Cross
Received: 12 November 2012 / Accepted: 15 November 2012 / Published online: 2 December 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract Diffusion-weighted imaging can be used to assess the microscopic properties of measured tissues, providing insights into the architecture of neural tissues and how they change in physiological and pathological states. Such imaging data can be readily quantified to provide numerical values of parameters that have clinical relevance, particularly in regard to cellular constituents of tumours, chemical contents of cysts, the integrity of neuronal cell bodies and myelin sheaths. Diffusion based techniques have proven to be particularly useful in investigating cerebral infarction, cerebral infections, epidermoid and other cysts, cerebral tumours, and white matter disorders. The purpose of this review is to introduce key concepts in diffusion imaging and illustrate how it may be applied to clinical practice, with particular reference to head injury. Keywords MRI Diffusion Tensor Anisotropy Tractography Head injury
What is diffusion-weighted imaging? Diffusion-weighted imaging (DWI) is based on the microscopic motion of water [1]. In DWI a radiofrequency pulse and a magnetic field gradient are used to dephase protons. After a short interval, an exactly opposite pulse V. F. J. Newcombe Department of Anaesthesia, Addenbrooke’s Hospital, Cambridge University Biomedical Research Centre, Cambridge, UK T. Das J. J. Cross (&) Department of Radiology, Addenbrooke’s Hospital, Cambridge University Biomedical Research Centre, Cambridge, UK e-mail: [email protected]
and gradient are applied which return the protons into phase. Protons that have moved in the interval do not generate a signal, so that DWI signal intensity is inversely proportional to the amount that protons have moved (diffused) during the imaging process (Fig. 1). The rate of proton (and therefore water) diffusion is calculated by comparing the signal when diffusion weighting is applied to the signal at the same location when no diffusion weighting is applied (b = 0). The b value describes the strength and duration of the diffusion gradients, and in the clinical setting the high b value is typically about 1,000 s/mm2. Although it is the most simple diffusion based technique, DWI is a useful clinical tool and is available in most MRI units. It is a rapid sequence (typically 60–120 s acquisition time on a 1.5 T MRI scanner). Figure 2 illustrates raw DWI as unweighted (b = 0), antero-posterior gradient, left–right gradient, supero-inferior gradient and combined diffusion-weighted (b = 1,000) images. The raw images are a good way of seeing areas of restricted diffusion, which are seen as conspicuous areas of high intensity. Regions with higher diffusion lead to larger signal loss on DWI and are seen as darker areas. Diffusion-weighted images need to be interpreted with some caution as they d
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