Diffusion Tensor Imaging of Brain Tumours at 3 T: A Potential Tool for Assessing White Matter Tract Invasion?

Abstract

AIM: To determine whether diffusion tensor imaging (DTI) of brain tumours can demonstrate abnormalities distal to hyperintensities on T2-weighted images, and possibly relate these to tumour grade.

MATERIALS AND METHODS: Twenty patients with histologically confirmed supratentorial tumours, both gliomas (high and low grade) and metastases, were imaged at 3 T using T2-weighted and DTI sequences. Regions of interest (ROI) were drawn within the tumour, in white matter at various distances from the tumour and in areas of abnormality on DTI that appeared normal on T2-weighted images. The relative anisotropy index (RAI)—a measure of white matter organization, was calculated for these ROI.

RESULTS: The abnormality on DTI was larger than that seen on T2-weighted images in 10/13 patients (77%) with high-grade gliomas. New abnormalities were seen in the contralateral white matter in 4/13 (30%) of these cases. In these high-grade tumours the RAI in areas of white matter disruption with normal appearance on T2-weighted images was reduced (0.19±0.04). Even excluding patients with previous radiotherapy this difference remains significant. In all non high-grade tumours (WHO grade II gliomas and metastases) the tumour extent on DTI was identical to the abnormalities shown on T2-weighted imaging and RAI measurements were not reduced (0.3±0.04).

CONCLUSIONS: Subtle white matter disruption can be identified using DTI in patients with high-grade gliomas. Such disruption is not identified in association with metastases or low-grade gliomas despite these tumours producing significant mass effect and oedema. We suggest the changes in DTI may be due to tumour infiltration and that the DTI may provide a useful method of detecting occult white matter invasion by gliomas.

Introduction

Gliomas are the most common type of primary brain tumour in adults, glioblastoma (WHO grade IV) being the most common. Although accounting for only 2–3% of all cancers, the impact on average years of life lost is greater than with more common tumours [1]. Despite recent advances, survival remains poor, with median survival rates varying between 16 and 53 weeks [2]. Radiotherapy is the most important non-surgical treatment option for malignant gliomas and improves survival [3]. A radiotherapy dose–response relationship also exists; increasing dose providing a moderate improvement in survival 4, 5. Conventional doses are determined by the tolerance of surrounding normal brain, but an unfortunate consequence of sparing this tissue is that not all tumour cells are destroyed and therefore tumour recurs in virtually all patients.

Very high doses of radiotherapy, up to 90 Gy, have been shown to sterilize gliomas, but at the cost of normal brain necrosis [6], with tumour recurrence commonly occurring in the zone that received 60–70 Gy. One method of reducing radiation necrosis to normal brain, while delivering a higher dose, would be to target a smaller volume [7]. The difficulty lies with determining the margins of the brain that are infiltrated by glioma. Most tumours recur within 2 cm from the enhancing edge of the original tumour and only 4% are multicentric [8]. Gliomas have no capsule and spread diffusely through the brain [9], preferentially infiltrating along white matter tracts 9, 10, 11, 12. Such infiltration is very different to the margin of a cerebral metastasis, which typically has a well-defined margin and preferentially infiltrates along vascular planes [13].

Diffusion-weighted imaging (DWI) is a magnetic resonance (MR) technique that is sensitive to the movement of water. Diffusion tensor imaging (DTI) is a modification of DWI that is sensitive to the preferential diffusion of brain water along axonal fibres, a property called anisotropic diffusion; this technique can demonstrate white matter tract anatomy (Fig. 1) and can detect subtle changes in white matter tracts in disease [14]. We have used this method to investigate whether DTI can detect more extensive abnormalities compared with conventional imaging in patients with high-grade gliomas. Our aim, in this preliminary study, was to assess the possible use of DTI as a method of detecting occult white matter invasion in cerebral gliomas.