Abstract
Ultrashort echo time (UTE) imaging is a well-known technique used in medical MRI, however, the implementation of the sequence remains non-trivial. This paper introduces UTE for non-medical applications and outlines a method for the implementation of UTE to enable accurate slice selection and short acquisition times. Slice selection in UTE requires fast, accurate switching of the gradient and r.f. pulses. Here a gradient “pre-equalization” technique is used to optimize the gradient switching and achieve an effective echo time of 10μs. In order to minimize the echo time, k-space is sampled radially. A compressed sensing approach is used to minimize the total acquisition time. Using the corrections for slice selection and acquisition along with novel image reconstruction techniques, UTE is shown to be a viable method to study samples of cork and rubber with a shorter signal lifetime than can typically be measured. Further, the compressed sensing image reconstruction algorithm is shown to provide accurate images of the samples with as little as 12.5% of the full k-space data set, potentially permitting real time imaging of short T2* materials.
Highlights
Ultrashort echo time (UTE) [1] imaging is a valuable technique for imaging short T2 and T2* samples, its implementation is challenging and acquisition times can be long
UTE is first simulated using the Bloch equations to demonstrate the concept and illustrate the artifacts that commonly arise during slice selection
UTE has been shown as an efficient method of imaging short T2 and T2* systems
Summary
Ultrashort echo time (UTE) [1] imaging is a valuable technique for imaging short T2 and T2* samples, its implementation is challenging and acquisition times can be long. UTE uses a soft excitation pulse, typically of a half Gaussian shape, to minimize the echo time (TE) [23]. Slice selection is achieved by applying a gradient at the same time as the soft pulse. The duration of the refocusing gradient limits the minimum TE for slice selective excitations. As the excitation ends at the zero phase point, the refocusing gradient is not needed and the acquisition can begin as soon as the r.f. pulse ends. The two acquisitions are identical except that the slice select gradient has opposite sign. The sum of these two acquisitions produces an identical slice to that produced by a full Gaussian and refocusing gradient as the imaginary signals, i.e
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