SCMR 22nd Annual Scientific Sessions
Cardiac magnetic resonance (CMR) offers several tools to determine tissue characteristics and can even assess tissue oxygenation by making use of the difference between the paramagnetic properties of oxygenated and deoxygenated hemoglobin1,2. Measuring of myocardial oxygenation could indicate tissue viability after coronary artery stenosis and also the presence of microvascular dysfunction. A number of T2 and T2* relaxation-based sequences have been introduced to measure blood oxygen level dependence (BOLD) changes in the heart3-5, however there remains a trade-off between quantitative or dynamic measurements. This work shows that a dynamic signal intensity (SI) based sequence could be prone to heartrate changes and proposes a 5-echoes modified gradient-echo-spin-echo EPI (GESE-EPI) sequence to mitigate this issue. This 5-echo-GESE-EPI provides T2* and T2 maps of the myocardium per heartbeat and offers a dynamic and direct readout of oxygenation during a breath-hold.
A 2-echo-GESE-EPI and 5-echo-GESE-EPI were acquired in 7 healthy volunteers on a 3T Siemens Magnetom Prisma using the Body-18 and Spine-32 coil. The (shared) scan parameters were: End-diastole ECG-triggering, res=2.77x2.77x6mm, GRAPPA=2, FOVPE=37.5%(FOV=127x350). Other parameters for 2-echo-GESE-EPI were TEs=9.8/38.7ms and TR=120ms, and for 5-echo-GESE-EPI were TEs=9.5/23.6/37.8/51.9/66.1ms and TR=120ms (Fig1a). For T2* and T2 mapping all 5 echoes were included in a previously described 4-parameters fit6. Simulations of the heartrate and TR changes during a breath-hold, which result in different relaxation times and therefore different magnitudes for each acquisition, were performed by using the Bloch equations.
The simulations showed that the TR can increase up to 33% over the timespan of a single breath-hold (Fig1b), which results in a magnitude change of 20% (Fig1c). When acquiring a 2-echo-GESE-EPI over the time of a breath-hold the RRpeak distance indeed increased (Fig2a) which would result in a simulated magnitude increase of 17% solely due to the heartrate. The acquired GE and SE images also show an increased SI over time, reflecting a mixture of the heartrate effects and oxygenation during the breath-hold (Fig2b). The 5-echo-GESE-EPI provides a T2* and T2 map per heartbeat (Fig3a), which is insensitive to the heartrate changes. These T2* and T2 values still show an increase of 19% over the time of a breath-hold. All the heartbeat-to-heartbeat T2* and T2 values were comparable with previously described healthy values7.
The 5-echo-GESE-EPI sequences offers heartbeat-to-heartbeat T2* and T2 maps that give a direct measure of myocardial oxygenation during a single breath-hold. This approach could offer a potential alternative for current myocardial viability MR methods.