Assessment of the tumour viability by diffusion-weighted MR imaging

We aimed to compare the capability of conventional MR imaging and diffusion-weighted imaging to delineate components of malignant neoplasm and to utilize a diffusion-weighted fast spin echo (DWFSE) imaging technique for differentiation of viable from necrotic tumour tissues. A total of eleven patients with malignant tumours (chest-4, abdomen-5, pelvis-2) were included in this study. Multislice axial and sagittal DWFSE scans covering the area of interest were performed. For each slice, four images were acquired (one T2-weighted image and three diffusion- weighted images with diffusion gradient applied to x, y, and z directions, respectively). A quantitative apparent diffusion coefficient (ADC) map was calculated using the diffusion and T2-weighted images to assist the evaluation. In the ADC maps where necrotic tumours showed low signal intensity and high ADC (1.6±0.3 × 10^-3 mm/sec), viable malignant tumours revealed high signal intensity and low ADC values (0.7±0.2 × 10^-3 mm/sec). A very high ADC component (2.3±0.3 × 10^-3 mm/sec) in the peritumoural edema was also observed. ADCs of the adjacent or contralateral normal tissues were significantly greater than those of malignant viable. Apparent diffusion coefficients (ADC) appear to be more accurate in differentiating between viable malignant and necrotic tumour than relaxation times. Diffusion-weighted MR imaging depicts differences in diffusion and in membrane integrity between viable and necrotic tumour and may be used to monitor viability during treatment. The diffusion- weighted MR imaging in the diagnosis of acute cerebral infarction is widely accepted. The reduced diffusion typical of acute stroke is thought to be related to the cytotoxic edema and shrinking of the extracelluar space. It might be possible to differentiate various tumour components and to distinguish tumoural invasion from normal tissue or edema. This distinction would be helpful for planning surgical resection, biopsies and evaluation the tumour management and treatment response. It is critical to monitor the response to chemotherapy or radiation therapy and to determine whether the chosen treatment is effective. It is necessary to determine the efficacy of the therapy, so that ineffective treatment can be changed. Histologic assessment of tumour necrosis during the course of chemotherapy or radiation therapy would require repeated surgical biopsy which is traumatic and subject to sampling error. Bruerg, et al reported that the apparent diffusion coefficient (ADC) values and index of diffusion anisotropy (index of diffusion aniosotropy = ADC maximum - ADC minimum/ADC mean) distinguished normal white matter areas from necrosis and cyst formation, edema, and solid enhancing tumour. MR imaging has evolved as a method for the staging of so tissue tumours and contrast-enhanced MR imaging has improved tissue contrast. However, since the contrast agents that are clinically available are of small molecular size, they are distributed in the interstitial space and can diffuse into necrotic tissue, thereby obscuring tumour necrosis. Dynamic MR imaging (MRI) including arterial, capillary and venous phases may be more accurate in assessing tumor necrosis, and it is becoming available. Tumour necrosis is characterized by increased membrane permeability and breakdown of the cell membrane and intracellular membrane structures. Lang et al, reported that viable tumour can be differentiated from tumour necrosis with diffusion-weighted MRI. Viable tumour produces intermediate to high signal intensity. Necrotic tumour, in contrast, corresponds to an increase in the mean free path length of the diffusing molecules, leading to a loss of signal or decreased signal intensity.