[Role of magnetic resonance imaging in the tissue characterization of tumors].

Aim of this article is to examine the possibilities and the limits of MR imaging in oncology, with special regard to the tissue characterization of tumors. Basic information is given on the fundamental parameters of MR, and the concept of image contrast is discussed, with a comparison between the image in conventional radiology and in MR. Factors affecting MR parameters and influencing the image contrast are analyzed; they are classified as extrinsic, or connected to the patient's conditions and to technical modalities, and as intrinsic or inherent to biological factors. Intrinsic factors are scheduled in three groups: a) tissue physico-chemical structure; b) functions or physiological conditions; c) pathological aspects. One of the most important factors related with the first group is water content. The possible influences of water content on the measurement of the proton MR relaxation times are numerous and complex. As a first approximation, relaxation times are directly related to water content; nevertheless many experimental results show that this statement is not sufficient to explain the observed variations of relaxation times in different tissues and organs. Some other data are accounted for the presence of different relaxation times of water in the same sample, and for the microtrabecular structure of the cell. In normal tissues and structures, further elements are explained on the basis of simple physico-chemical characteristics. In tumors the increase of T1 and T2 observed in experimental evaluations performed on samples of tissues, was largely confirmed in the images. To an equal extent failure of specificity of the increase of relaxation times of tumors is well documented in experimental research and in clinical experiences. In experimental research a strict comparison was made between the tumor and the tissue of origin. Increase of relaxation parameters appears related to the presence of larger fluid-filled areas, to the diminution of average cell size, and to reduced amount of rough and smooth endoplasmic reticulum. In clinical experiences good results are obtained in the ascertainment of the tumors localization and extension; macroscopic aspects connected to the peritumoral structures, inhomogeneities, and necrosis are well evaluated. New data are acquired in the study of early and late effects of radiation in patients submitted to radiotherapy for various types of tumors.