Oxygen extraction fraction based on 10-echo GE-SE EPIK

The oxygen extraction fraction (OEF) is a valuable biomarker for brain health and metabolism and can give valuable insights into the prediction and therapy of stroke as well as tumour heterogeneity. As the current gold standard for OEF quantification are 15O-PET measurements that require radioactive tracers, MR methods are desirable. For this purpose, the quantitative BOLD approach directly relates OEF to R2‘ [1], where the latter one is defined by the two relaxation times T2 and T2*. To simultaneously quantify T2 and T2* we have developed a novel 10-echo GE-SE EPIK sequence. Based on the combined acquisition of GE and SE contrasts (Fig. 1a), the sequence overcomes previously used sequences by an increased matrix size, higher number of acquired echoes and two pure SEs with echo times below 114ms. The sequence acquires 20 slices in 57 seconds and in addition, the use of EPIK readouts [2] offers a good temporal resolution and stability [3,4]. The quantified T2 and T2* values have been validated against reference methods in different phantoms and 15 in vivo subjects (Fig. 1b) [5]. The observed baseline OEF values of the GE-SE EPIK approach are in good agreement with the literature and the sensitivity of the method has been investigated in breath-hold experiments. An alternating setup of normal breathing and breath-holding has yielded a decreasing OEF during breath-holding with the following recovery during normal breathing (Fig. 2). The proposed sequence offers fast access to simultaneous T2 and T2* maps which can be related to OEF information. Thus, the GE-SE EPIK method provides a promising basis for future clinical applications in stroke and tumours which are the focus of the future work.

Oxygen Extraction Fraction based on 10-echo GE-SE EPIK
10-echo overview of a representative in vivo slice acquired by the GE-SE EPIK sequence.
T2 and T2* distributions for 15 healthy in vivo studies obtained by the GE-SE EPIK sequence and reference methods.
(a) OEF time envelope during a breath-hold experiment for three volunteers. The acquisition consists of 6 blocks of alternating breathing and breath-hold. (b) For each task block, a linear regression is fitted to the OEF time envelope and the obtained slopes are summarized in the box plot. A significant difference between both states is observed.

REFERENCES

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  5. Küppers F, Yun SD, Shah NJ. Development of a novel 10-echo multi-contrast sequence based on EPIK to deliver simultaneous quantification of T2 and T2*. Magn Reson Med. 2022. In press.

Last Modified: 06.03.2023