T2* quantification using multi-echo gradient echo sequences: a comparative study of different readout gradients
Seonyeong Shin, Seong Dae Yun and N. Jon Shah
23rd January 2023
Changes in T2* relaxation can be used in imaging as a marker of brain activity, water content or to map the effects of paramagnetic entities, such as iron. This is particularly useful in the diagnosis and treatment of various diseases as well as in helping us understand more about the function of the human brain.
T2* is typically quantified by acquiring multiple echoes with a multi-echo gradient echo sequence that useseither monopolar or bipolar readout gradients. A sequence using bipolar readout, termed QUTE, achieves a shorter echo spacing time, enabling the acquisition of a larger number of echoes in the same scan time. However, despite their relative time efficiency and the potential for more accurate quantification, sequences using monopolar readout are generally more widely favoured due to the challenges associated with implementing artefact-free and robust bipolar readout.
This work compares the performance of monopolar and bipolar readout gradients for T2* quantification. The differences in readout gradients were theoretically investigated with a Cramér-Rao lower bound and validated with computer simulations with respect to the various imaging parameters. The readout gradients were then compared at 3T MRI using phantom and in vivo experiments.
The results demonstrate that the bipolar readout gradients provided higher precision than monopolar readout gradients in both computer simulations and experimental results. The difference between the two readout gradients increased for a lower SNR (signal-to-noise ratio) and smaller TE range, consistent with the prediction made using Cramér-Rao lower bound.
Consequently, the use of bipolar readout gradients is advantageous for regions or situations where a lower SNR is expected or a shorter acquisition time is required.
The image below shows T2* maps from a healthy volunteer obtained using monopolar and bipolar readout gradients with amplitude modulation correction. Each column and row display the results for a different set of (a) α and TE range and (b) R and TE range. Imaging parameters: BW/px = 801, ∆TE = 2.92 ms for monopolar readout and 1.47 ms for bipolar readout. The number in parentheses is the maximum number of echoes achieved in the given TE range. As indicated by the yellow arrows in the magnified view, anatomical structures are clearly distinguished in the T2* maps from bipolar readouts. The figures in (c) show the results for a different set of ∆TEs.
Original publication: T2* quantification using multi-echo gradient echo sequences: a comparative study of different readout gradients