Signal and contrast effects due to T2 decay during k-space readout of UTE (ultrashort TE) sequences

Jing-Tzyh Alan Chiang,Michael Carl,Jiang Du

doi:10.1016/j.mri.2013.11.002

Jing-Tzyh Alan Chiang, Michael Carl + Show 1 more

Open Access

https://doi.org/10.1016/j.mri.2013.11.002

Copy DOI

Abstract

In ultrashort TE (UTE) imaging, the short T2 values of the tissues of interest are comparable to the k-space readout duration, which result in significant T2 decay during k-space readout. This decay consequently causes significant effects on signal and contrast in UTE sequences, which we evaluate in this paper using models that incorporate the gradient slew rate slew and maximal constant gradient strength gmax, in conjunction with objects of diameter L. The resulting signal and contrast relationships demonstrate steep signal changes between T2 values of ~50–500μs, corresponding to high T2 weighted contrast in this range. When γ⋅gmax2/(4π⋅slew)>1/(2L), termed the “ramp only” regime, gmax has no significant effect whereas decreasing slew leads to decreases in signal amplitude and shifts the contrast peak to higher T2 values. When γ⋅gmax2/(4π⋅slew)<1/(2L), termed the “mixed gradient” regime, both gmax and slew have significant effects, where decreases in either gmax or slew lead to lower signal amplitudes and shifts the contrast peak to higher T2 values. Under typical scan settings, the “ramp only” regime is usually dominant. Further, we demonstrate an unusual dependence of T2 weighted signal and contrast on object size, whereby objects with smaller values of L demonstrate lower signal amplitudes and peak contrast at higher T2 values, compared to otherwise identical objects with larger L. These results improve understanding of T2 weighted signal and contrast properties in short T2 tissue imaging with UTE.

Highlights

In recent years, there has been significant improvement in the ability of MR to directly image bone, the deep layers of cartilage, and other tissues with T2 values of milliseconds or less, that normally produce negligible signal due to rapid T2 decay when imaging with standard pulse sequences [1,2,3,4,5]
Appreciable decay can occur during RF excitation [6] and k-space readout [7] in ultrashort TE (UTE) sequences, as the duration of these intervals may be comparable to the T2 values of the tissues of interest
To examine whether the “ramp only” or the “mixed gradient” regime is more commonly encountered in typical scanning conditions, we evaluated a range of slew and gmax values in figure 5 to see which regime they gave rise to

Summary

Introduction

There has been significant improvement in the ability of MR to directly image bone, the deep layers of cartilage, and other tissues with T2 values of milliseconds or less, that normally produce negligible signal due to rapid T2 decay when imaging with standard pulse sequences [1,2,3,4,5]. The ultrashort TE (UTE) pulse sequence is one technique where acquisition of the MR signal occurs as soon after the end of the radiofrequency (RF) excitation pulse as possible, with data sampling beginning immediately at k-space center (gradients off) and proceeding along a radial center-out trajectory for each k-space line with data sampling occurring both during the gradient ramp as well as after the maximal gradient value is attained This approach allows the bulk of the signal in kspace center to be sampled rapidly after RF excitation, with readout of each radial k-space line typically lasting several hundred microseconds. Depending on the RF duration, T2 decay during k-space readout can be the primary and dominant source of T2 weighted contrast in short T2 tissue imaging with UTE

Methods

Results

Conclusion