Inverse correlation between T1 relaxation times before and after administration of contrast media in malignant brain tumors

T he assessment of contrast enhancement on Tl-weighted images is an essential element of MR imaging studies on the diseases of the central nervous system [1, 2]. Inversion recovery sequences would be a particularly sensitive method for studying T1 changes, but their use in clinical practice has been limited [3]. The main reason for this lack of popularity has been the long imaging times required. Fast image-acquisition methods, such as echoplanar imaging, provide a means of obtaining inversion recovery images more rapidly [4, 5]. In addition to their selective T1 sensitivity, the inversion recovery sequences are an effective tool for measuring in vivo actual T1 relaxation times of tissues [6]. With single-shot techniques, a series of inversion recovery images with several inversion times (TIs) can be obtained within minutes [7]. This fast data collection permits estimations of the changes in the longitudinal relaxation rates after the injection of contrast agents. The extent of contrast enhancement in tissues depends on the gadolinium content within each voxel. Because the major distribution volume of small paramagnetic chelates is the interstitial space, the gadolinium content in tissues should be related directly to the size of the extracellular compartment. We measured T1 in brain tumors before and after administration of a contrast agent using echoplanar inversion recovery sequences. Our purpose was to assess whether the magnitude of changes in the T1 relaxation rate induced with gadolinium chelates would be related to the initial T1 relaxation rates before contrast material was administered 9