Diffusion-weighted MRI: a new functional clinical technique for tumour imaging

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One of the key aims of oncological imaging is to differentiate between malignant and non-malignant tissues at all stages of the patient’s cancer care. Accurate staging and precise delineation of the extent of malignancy influences therapeutic decisions, therapy outcomes and, ultimately, patient prognosis. Conventional imaging using ultrasound, CT or MRI detects cancer by identifying anatomical distortion or altered tissue appearances. Tumour tissue conspicuity may be increased after the administration of intravenous contrast medium, thus enhancing detection and delineation. However, identification of small volume active tumour, either at presentation or at early disease relapse remains challenging because small volume disease may not result in detectable structural or morphological change on conventional imaging. Furthermore, the effects of therapy and complications thereof may obscure or mimic recurrent disease. Functional imaging techniques using CT, MRI and positron emission tomography (PET) are increasingly being applied to the evaluation of tumours. These techniques exploit as their contrast mechanism unique pathophysiological changes that occur within tumours; such as altered blood flow, increased glucose metabolism, hypoxia and cellularity. Such functional techniques are increasingly used for tumour detection, for the monitoring of treatment response and to detect relapsed disease. Clinical experience has shown that functional techniques have their own unique strengths and limitations. A new, emerging functional technique that is now finding a role in cancer imaging is diffusion-weighted MRI (DWI or DW-MRI), which produces information about tissue cellularity and the integrity of cellular membranes. This technique may not be well appreciated by general radiologists. DWI can be performed on most modern MRI machines with relative ease, in a short period of time and without the need for contrast medium administration. The potential for this technique to evaluate the larynx for tumour recurrence after prior radiotherapy is demonstrated in a short communication from Vandecaveye et al in this issue [1]. At a fundamental level, DWI provides information on the random (Brownian) motion of water molecules in tissues. The Brownian displacements of millions of water molecules over time are normally distributed with a mean final value of zero for all time periods measured, but with a standard deviation that is proportional to the diffusion coefficient and time measured. This was the basis for Einstein’s diffusion equation published in 1905, which subsequently helped to earn him the 1921 Physics Nobel Prize. In tissues, DWI probes the movement of water molecules, which occurs largely in the extracellular space. However, the movement of water molecules in the extracellular space is not entirely free, but is modified by interactions with hydrophobic cellular membranes and macromolecules. Hence, diffusion in biological tissue is often referred to as ‘‘apparent diffusion’’. By comparing differences in the apparent diffusion between tissues, tissue characterization becomes possible. For example, a tumour would exhibit more restricted apparent diffusion compared with a cyst because intact cellular membranes in a tumour would hinder the free movement of water molecules. One of the simplest methods of obtaining DWI images is to apply pairs of opposing and balanced magnetic field gradients (but of differing durations and amplitudes) around a spin-echo refocusing pulse of a T2 weighted sequence. Stationary water molecules are unaffected by the paired gradients, and thus retain their signal. Nonstationary water molecules acquire phase information The British Journal of Radiology, 79 (2006), 633–635