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

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.

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“…Fourth, different severities of arrhythmia have variable effects on the correct acquisition time. Fifth, partial blurring due to T2-decay may occur during the rapid FSE echo train [ 29 ]. The most spatial blurring occurs when TE is short because the higher order phase encoding steps that provide edge detail are being filled with late echoes in the train.…”

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“…For example, while dataprocessing methods like bicompartmental T 2 * mapping is easily available, there may be variation in results secondary to the choice of echo times, the SNR, and the relaxation model, which may not fully describe the entire UTE signal. 74 IR methods do not require multiple echoes or as high of an SNR as biexponential analysis, but signal variation from B11 field inhomogeneity could affect results. The suppression ratio and porosity index are methods which may help overcome some of these limitations.…”

“…This will permit researchers and clinicians to determine the best data acquisition schemes. For example, while data‐processing methods like bicompartmental T 2 * mapping is easily available, there may be variation in results secondary to the choice of echo times, the SNR, and the relaxation model, which may not fully describe the entire UTE signal . IR methods do not require multiple echoes or as high of an SNR as biexponential analysis, but signal variation from B1+ field inhomogeneity could affect results.…”