Magnetic resonance imaging of short T2 components in tissue.

The most widely used clinical magnetic resonance imaging techniques for the diagnosis of parenchymal disease employ heavily T(2)-weighted sequences to detect an increase or decrease in the signal from long T(2) components in tissue. Tissues also contain short T(2) 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 T(2) components.The development and clinical application of techniques to detect short T(2) 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 T(2) 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 T(2) components before they have decayed, both in tissues with a majority of short T(2) 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 T(2) components. Fat suppression of different types may also be helpful. Once signal from short T(2) components has been detected, different pulse sequences can be used to determine increases or decreases in T(1) and T(2) and study contrast enhancement. Using these approaches, signals have been detected from normal tissues with a majority of short T(2) 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 T(2) components are a minority. Some diseases such as chronic fibrosis, gliosis, haemorrhage and calcification may increase the signal from short T(2) 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 T(2) components by about two orders of magnitude from about 10 ms to about 100 micros. As a consequence it is now possible to study tissues that have a majority of short T(2) 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 T(2) components may demonstrate changes in disease that are not apparent with conventional heavily T(2)-weighted sequences.