REVIE\N ARTICLE Diffusion weightedMR • •Imaging Abstract BRamsing MBChB, FCRad Diffusion vveighted imaging (OWl) al lovvs the measu rement of molecular motion in tissue. This technique has significant clinical appl ications. Recent technological developments in fast MR imaging have brought diffusion imaging into clinical practice. This revievv vvill explain the physical principles, and cu rrent and futu re potential appl ications of diffusion imaging in medicine. PCorr MBChB, FFRad Basic theory Diffusion is the physical property of molecules which leads them to move randomly at a rate in propor- tion to their thermal energy.' A use- ful analogy is if we drop some ink into a glass of water, the ink will slowly distribute itself throughout the water and change its colour. This movement of ink molecules from a region of higher concentration to a region of 4 SAJOURNAL OF RADIOLOGY· August 1998 lower concentration along a gradient is called diffusion. However, even when the ink has distributed itself throughout the water and there are no concentration gradients, there are still random movements of ink mole- cules. This intrinsic kinetic energy of molecules is called Brownian motion. Water within tissues displays Brownian motion. The diffusion pro- cess is three dimensional and is dis- tributed in all directions perpendicu- lar to its origin. This is called isotropic distribution. However, in tissues wa- ter molecule movement is determined by the local molecular environment and cannot distribute itself in an iso- tropic manner. This distribution is called anisotropy. An example is wa- ter molecules within neurones, in which the molecules move parallel to the axons rather than at right angles to them. Because of anisotropy, a three dimensional data acquisition is per- formed during imaging. Using conventional spin echo imaging, diffusion causes minimal sig- nal reduction, but with very strong gradients it is possible to obtain dif- fusion weighted images (OWl). OWl is very sensitive to motion artefacts such as breathing and cardiac pulsa- tion. Fast imaging using echo planar techniques will reduce motion arte- facts and improve image quality. The images have a poor signal-to-noise ratio compared to conventional spin echo sequences of the brain, with sus- ceptibility artefacts adjacent to the skull and sinuses. Regions of high dif- fusion have a low intensity and ap- pear dark on the scan, while regions of low diffusion have a relatively in- creased intensity and appear bright on the scan. The apparent diffusion co- efficient (AOC) is a quantitative to page 5 Diffusion weighted MR inlaging frompage4 measure of the degree of diffusion in a tissue or organ. The average value for the brain is 2 100XI06 mmvs.' The other important concept is the b value.This is a measure of the strength and duration of the MR gradient's pulses, as well as the time between gradient pulses and the gyromagnetic ratio. The higher the b value, the bet- ter the differentiation between tissues of different diffusion coefficients and hence lesion conspicuity. OWl is best between a b value of 600 to 1000 mm//sec.' OWl is essentially a cellular energy test. OWl indicates cell osmolality which is dependent on the integrity of the cell membrane pump and cell energy levels of ATP. With depletion of cell energy supplies of ATP, the cellular pump fails and the cell swells resulting in cytotoxic oedema. This results in a marked decrease in diffu- sion (AOC) and a focal bright signal on the scan. Clinical applications Acute stroke Wi th acute stroke, cytotoxic oedema develops in the neurones and the diffusion of water molecules de- creases. This is thought to be due to decreased Na K-ATPase enzyme ac- tivity in the cell membrane, and there- fore decreased water transport. Wa- ter becomes trapped within the cell. Water viscosity is increased due to the dissociation of large macromolecules and the cellular fluid becomes a gel. This also decreases water diffusion. In animal models, the decreased diffu- sion is detected as a high intensity focal lesion within 45 minutes of ex- perimental vascular occlusion? After two days, diffusion returns to normal and then increases. In humans OWl detects changes as early as 38 min- utes and diffusion decreases over 24 hours to 4 days (Figure 1). Diffusion Figure 1: Diffusion MR scan shows a focal hyperintense infarct in the posterior limb of the L internal capsule (arrow) in a patient with a 6 hour history of R hemiparesis. T2 MR and CT scans were both normal. returns to normal at 5 to 10 days post ictus. The slow return to normal (M- fusion levels is due to persistent cyto- toxic oedema as well as vasogenic oedema. This is a consequence of cell membrane disruption and resultant extracellular oedema which increases diffusion levels. Reversibility of dif- fusion to normal levels can be con- sistently detected in the animal model, with a threshold AOC above which no permanent tissue injury is detected. In humans this has yet to be achieved.' CT and MR cannot detect cerebral infarction before 8 hours when thrombolytic therapy is most effec- tive; however OWl is able to provide this information." A lesion with de- creased diffusion correlates strongly with cytotoxic oedema and infarction. The absence of decreased diffusion correlates with the absence of infarc- tion, and rather suggests stroke mimics 5 SA JOURNAL OF RADIOLOGY· August 1998 such as transient ischaemie attacks, migraine, and metabolic causes. Subacute stroke OWl is useful in differentiating white matter infarcts from the T2 hyperintensities associated with aging. On OWl, infarcts are hyperintense while the aging changes demonstrate no diffusion change. OWl is also use- ful to distinguish subacute from chronic infarction, which is impossi- ble with spin echo T2 imaging. Sub- acute infarcts are hyperintense on OWl due to decreased diffusion from cytotoxic oedema, while chronic infarcts are hypointense as a result of increased diffusion from vasogenic oedema and increased extracellular water. 5 T2* susceptibility effects are more readily detected with OWl as opposed to spin echo imaging, so that haemorrhagic infarction is more eas- ily identified. Neonatal ischaemia OWl is useful in imaging hypoxic ischaemie encephalopathy in neonates. The white matter is unmye- linated and CT and MR detection of infarction are difficult. OWl easily de- tects infarcted brain where diffusion is decreased compared to normal brain. OWl can differentiate between embolic infarcts and cortical laminar necrosis. This is important clinically as neonates with embolic infarcts have a better prognosis.' Venous infarct OWl distinguishes between ve- nous infarction and extracellular oedema resulting from occluded cor- tical veins and venous sinuses. Infarc- tion is hyperintense while the sur- rounding extracellular vasogenic oedema is hypointense. topage6 [)iffusioll \Ncighteci MR inl