Magnetic resonance imaging MRI

Diffusion weighted imaging (DWI) is a technique that can be used to probe the microenvironment of water. Contrast in DWI is derived from the translational motion of water molecules. Quantitative assessment of the (apparent) diffusion coefficient (ADCw) is a unique method of examining tissue status.

In closed head injuries, focal lesions such as contusions resulting from mechanical distortion of tissue, and haematoma, may be detected on conventional MR and CT images. Diffuse axonal injury, including axonal shearing and hypoxic brain damage, are less identifiable using such modalities. Diffusion weighted and magnetisation transfer imaging sequences may

Figure 2.3 Diffusion image of patient with normal CT scan of brain but persistently abnormal neurology.

The history of the mechanism of injury suggests diffuse axonal injury and there is a history of hypoxia and hypotension. Encircled area = area with reduced ADC suggesting cellular oedema, not apparent on T2 and CT imaging

Figure 2.3 Diffusion image of patient with normal CT scan of brain but persistently abnormal neurology.

The history of the mechanism of injury suggests diffuse axonal injury and there is a history of hypoxia and hypotension. Encircled area = area with reduced ADC suggesting cellular oedema, not apparent on T2 and CT imaging prove to be useful in highlighting axonal structural changes not obvious on T2 weighted images (Figure 2.3). With the capability of highlighting the chemical changes that accompany such diffuse head injuries, MR spectroscopy has the potential to detect such disorders in vivo.69 Of particular interest is proton MR spectroscopy at long echo times in the metabolite N-acetyl aspartate, an amino acid found exclusively in neurons.70 Using single slice two dimensional spectroscopic imaging, nine acute head injury patients and six controls have been successfully scanned. The problems presented by the need for ICU monitoring of these patients during MR scanning were overcome using MR compatible monitoring equipment.71 In previous studies of head injury which used proton spectroscopy, single voxel localisation procedures have meant that the spatial extent of the spectral data has been limited, but with spectral data from a whole axial slice they have been able to identify N-acetyl aspartate abnormalities in regions remote from any T2 visible lesions. This observation suggests that spectroscopic imaging (of N-acetyl aspartate in particular) will be useful for the diagnosis of diffuse axonal injury. It may be possible to use this technique to guide therapy, monitor recovery, and aid outcome prediction.

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