Magnetic Resonance Imaging of Human Melanoma Xenografts in Vivo: Proton Spin—lattice and Spin—spin Relaxation Times Versus Fractional Tumour Water Content and Fraction of Necrotic Tumour Tissue

Abstract

Proton nuclear magnetic resonance (¹H-nmr) imaging is used routinely in clinical oncology to provide macroscopic anatomical information, whereas its potential to provide physiological information about tumours is not well explored. To evaluate the potential usefulness of ¹H-nmr imaging in the prediction of tumour treatment resistance caused by unfavourable microenvironmental conditions, possible correlations between proton spin—lattice and spin-spin relaxation times (T₁ and T₂) and physiological parameters of the tumour microenvironment were investigated. Tumours from six human melanoma xenograft lines were included in the study. ¹H-nmr imaging was performed at 1·5T using spin—echo pulse sequences. T₁- and T₂-distributions were generated from the images. Fractional tumour water content and the fraction of necrotic tumour tissue were measured immediately after ¹H-nmr imaging. Significant correlations across tumour lines were found for T₁ and T₂ versus fractional tumour water content (p < 0·001) as well as for T₁ and T₂ versus fraction of necrotic tumour tissue (p < 0·05). Tumours with high fractional water contents had high values of T₁ and T₂, probably caused by free water in the tumour interstitium. Fractional water content is correlated to interstitial fluid pressure in tumours, high interstitial fluid pressure being indicative of high vascular resistance. Tumours with high fractional water contents are thus expected to show regions with radiobiologically hypoxic cells as well as poor intravascular and interstitial transport of many therapeutic agents. T₁ and T₂ decreased with increasing fraction of necrotic tumour tissue, perhaps because complexed paramagnetic ions were released during development of necrosis. Viable tumour cells adjacent to necrotic regions are usually chronically hypoxic. Tumours with high fractions of necrotic tissue are thus expected to contain significant proportions of radiobiologically hypoxic cells. Consequently, quantitative ¹H-nmr imaging has the potential to be developed as an efficient clinical tool in prediction of tumour treatment resistance caused by hypoxia and/or transport barriers for therapeutic agents. However, much work remains to be done before this potential can be adequately evaluated. One problem is that high fractional tumour water contents result in longer T₁ and T₂ whereas high fractions of necrotic tumour tissue result in shorter T₁ and T₂; i.e. the two parameters which are indicative of treatment resistance contribute in opposite directions. Another problem is that the correlations for T₁ and T₂ versus fraction of necrotic tumour tissue are not particularly strong.