Magnetic Resonance Imaging of Short T 2 Components in Tissue

The most widely used clinical magnetic resonance imaging techniques for the diagnosis of parenchymal disease employ heavily T2-weighted sequences to detect an increase or decrease in the signal from long T2 components in tissue. Tissues also contain short T2 components that are not detected or only poorly detected with conventional sequences. These components are the majority species in tendons, ligaments, menisci, periosteum, cortical bone and other related tissues, and the minority in many other tissues that have predominantly long T2 components. The development and clinical application of techniques to detect short T2 components are just beginning. Such techniques include magic angle imaging, as well as short echo time (TE), and ultrashort TE (Ute) pulse sequences. Magic angle imaging increases the T2 of highly ordered, collagen-rich tissues such as tendons and ligaments so signal can be detected from them with conventional pulse sequences. Ute sequences detect short T2 components before they have decayed, both in tissues with a majority of short T2 components and those with a minority. In the latter case steps usually need to be taken to suppress the signal from the majority of long T2 components. Fat suppression of different types may also be helpful. Once signal from short T2 components has been detected, different pulse sequences can be used to determine increases or decreases in T1 and T2 and study contrast enhancement. Using these approaches, signals have been detected from normal tissues with a majority of short T2 components such as tendons, ligaments, menisci, periosteum, cortical bone, dentine and enamel (the latter four tissues for the first time) as well as from the other tissues in which short T2 components are a minority. Some diseases such as chronic fibrosis, gliosis, haemorrhage and calcification may increase the signal from short T2 components while others such as loss of tissue, loss of order in tissue and an increase in water content may decrease them. Changes of these types have been demonstrated in tendonopathy, intervertebral disc disease, ligament injury, haemachromatosis, pituitary perivascular fibrosis, gliomas, multiple sclerosis and angiomas. Use of these techniques has reduced the limit of clinical detectability of short T2 components by about two orders of magnitude from about 10 ms to about 100 ms. As a consequence it is now possible to study tissues that have a majority of short T2 components with both “bright” and “dark” approaches, with the bright (high signal) approach offering options for developing tissue contrast of different types, as well as the potential for tissue characterization. In addition, tissues with a minority of short T2 components may demonstrate changes in disease that are not apparent with conventional heavily T2-weighted sequences. Gatehouse, P. D. and Bydder, G. M. (2003). Clinical Radiology 58, 1–19. q 2003 The Royal College of Radiologists. Published by Elsevier Science Ltd. All rights reserved.