Techniques and analyses for ultrashort echo-time magnetic resonance imaging [electronic resource]
Keywords: Electrical Engineering.

'Ultrashort echo time' (UTE) sequences are a class of techniques for acquiring magnetic resonance image (MRI) signals with rapid transverse (R2) relaxation rates. They reorganize the image encoding process to begin signal detection very quickly after excitation. This requires changing both the excitation and acquisition phases of MRI. These aspects are well-understood, and they are also widely employed in MR contexts other than UTE imaging. Originally described for MRI of lung, UTE sequences have been variously applied for imaging tendons, meniscii, scar tissue, dental enamel and cortical bone. The general unifying theme for applications of UTE is a high R2 rate in the anatomy of interest. However, a challenge of distinguishing low R2 signals from high R2 ones—or of developing conspicuous image contrast in high R2 image components—is posed by the nature of the relaxation phenomenon. Variations on the basic UTE sequence have incorporated preparatory pulses, known as 'long-T2 suppression' pulses, that are designed to attenuate signal specifically from low-R2 components and leave unchanged high-R2 ones. Alternative or complementary variations acquire additional images at non-ultrashort echo times and use subtraction to display images with the desired high-R2 emphasis. Of the varied anatomy detectable by UTE MRI, cortical bone imaging is perhaps the most motivating—and extensively studied—application, so that R2 relaxation rates salient for MR of hard bone have been characterized in detail. A confluence of the particular R2 rates for cortical bone with the physical performance capabilities of standard clinical MR machines present opportunities to balance the excitation and relaxation effects of nuclear magnetic resonance, such that subtle differences in signal amplitudes, depending upon the applied excitation, can be generated for high-R2 signals while little change is observed for low-R2 signals. This is the regime of operation for long-T2 suppression preparatory pulse techniques. Here, several innovations are described that can expand the range of functionality and application in UTE imaging. The flexibility of MRI affords opportunities to realize improved imaging performance by modifying the acquisition structure, so that ideas of sampling incoherence can be used to relax sample-count requirements for UTE sequences or similarly-organized ones, such as 'zero echo time' or ZTE imaging. This reformed acquisition structure draws from concepts of Poisson disc random sampling to develop desirable imaging characteristics. When UTE and later-echo images are acquired, image subtraction can spuriously emphasize phenomenon other than rapid relaxation. To address this, a UTE-specific model is stated for the temporal evolution of signals received, enabling decomposition of a time-series of MR images into components of high R2 or of other common in vivo anatomy. The extensive literature of water-fat Dixon separation in MRI is the basis for this approach. The mechanism and effects of long-T2 suppression preparatory pulses in MRI are explicable through the Bloch description of MR. This is done so here. However, application of the same principle can be incorporated directly into excitation rather than as preparation. The result is that cortical bone signal can be emphasized in MR images through a practicable MRI sequence, and it is demonstrated as developing CT-similar contrast in MRI. As a collection, the technical developments for UTE MRI presented here advance the frontier of its feasible applications.