Sophie Schmid1,2, Leonie Petitclerc1,2, André Paschoal3, Suzanne Franklin1,4, Merlijn Van der Plas1,2, and Matthias Van Osch1,2
1Radiology, Leiden University Medical Center, Leiden, Netherlands, 2Leiden Institute of Brain and Cognition (LIBC), Leiden, Netherlands, 3Physics, InBrain Lab, University of Sao Paulo, Ribeirao Preto, Brazil, 4University Medical Center Utrecht, Center for Image Sciences, Utrecht, Netherlands
Synopsis
In
this study the variation in the measured apparent T2 was assessed over
different slices for venous
velocity selective inversion (vVSI) with 2D EPI readout in the sagittal sinus and we
compared vVSI with 2D EPI and 3D multi-echo spin echo EPI (mTE-SE-EPI) readout
to the gold standard TRUST. With the 2D EPI readout the T2 increases significantly over the slices.
Introduction
Last year, venous velocity selective
inversion (vVSI) was introduced to spatially non-selectively label venous blood
and measure the T2.1 This method benefits from the
improved SNR of Velocity Selective Inversion2 (VSI) and could be
used to measure the oxygen extraction fraction from venous and arterial blood
separately, also in small vessels closer to the tissue, within a single scan. However,
the type of readout influences the measured T2 values. With a multi-slice
2D-readout the time between T2-preparation and image readout increases for each
consecutively acquired slice, during which T1-relaxation occurs,
thereby influencing the measured apparent T2.
In this study the variation in the measured
apparent T2 was assessed over different slices for vVSI with 2D EPI readout in
the sagittal sinus and we compared vVSI with 2D EPI and 3D multi-echo spin echo
EPI (mTE-SE-EPI) readout to the gold standard TRUST3.Methods
In eight healthy volunteers (23-59yrs, 6f/4m) the vVSI scan with a 2D EPI readout was scanned using a 32ch-head coil at a 3T Philips Achieva scanner, whereas in
three of these subjects also two types of mTE-SE-EPI readout was acquired. All
volunteers provided written informed consent and the study was approved by the
local IRB.
The vVSI-method employs a pulsed ASL module
T1,blood*ln(2) sec before VSI-labeling to null the arterial pool and
cycles through 4 different labeling combinations. When using the label condition
of the pulsed ASL (PASL label, Pl) in the post-processing arterial signal will
be nulled and thus eliminated. Subtraction of VSI label (Vl) and control (Vc)
images will subsequently provide a map of venous signal. Similarly, when using
the control condition of VSI and subtracting PASL control (Pc) and label, only
arterial signal is obtained.
Just before the 2D EPI read out a T2-preparation
module was performed.The results were compared with a 3D mTE-SE-EPI readout acquired
with 6 or 12 echoes (Details in figure 1 and table 1).
A single slice EPI-readout TRUST
measurement was used as gold standard for the T2 measurement. The T2 was
calculated with a mono-exponential fit in the voxel with the highest signal
intensity at the first echo time in the superior sagittal sinus per slice to minimize partial voluming effects. Results
Examples of the venous VSI maps (at the first
effective echo time) for the different readouts in a single volunteer are shown
in figure 2, showing the venous vasculature. As can be seen in figure 3, an slice-wise
increase of almost 2.5ms in T2 is measured when using the 2D EPI readout (n=8)).
In figure 4 is shown what the average and individual T2-values for the subjects (n=3) is for the vVSI scans with
the different readouts and TRUST scan. For both 3D readouts a slice-wise decrease in T2 was found up till slice 7 and increase for the higher slices. The T2-values of the 3D readout with the 6 eTEs was on average slightly lower than with the 12 eTEs. The positioning of the TRUST scan was located at the lowest slice of the other scans and had T2-values that were higher than acquired with the lowest slice of vVSI scan with the 3D readouts and lower than the 2D readout. Discussion and conclusion
In
this study we compared vVSI with 2D and 3D readouts and evaluated the T2 over
the slices. For the 2D EPI readout scans a steady slice-wise increase was found for
higher slices. However, for the 3D readout scans the T2-values show a decrease
in the lower slices, followed by an increase in the higher slices.
A
correction for the slice-wise effect on the T2 is not straightforward and requires
additional scans and/or assumptions have to be made about the local M0 and T1
of venous blood.
Limiting
the scan to a single slice, such as TRUST, would circumvent this problem, but also
limits the T2 measurements to a (few) large veins. vVSI could provide the oxygen extraction fraction at tissue level in a single scan, but to fully benefit from this, full brain coverage would be required.
Another option would be to precede a 3D readout with a separate T2-preparation module, like performed in the 2D readout. Drawback of this approach would be the four fold increase in scantime. From the current dataset it is difficult to conclude which of the 3D readouts would be preferable, due to the difference in scan duration and it can be expected the
mono-exponential T2-fit with 6 eTEs is slightly less accurate than with 12 eTEs, if it is not compensated in scan duration.
In conclusion, further optimization of the vVSI readout is necessary to get more accurate T2-values, before the method can be applied to acquire reliable full brain oxygen extraction fraction measurements.
Acknowledgements
This work is
part of the research programme Innovational Research Incentives Scheme Vici
with project number 016.160.351, which is financed by the Netherlands
Organisation for Scientific Research (NWO). This study is supported by the Out
of the Box challenge 2018 from the ISMRM Benelux Chapter. We thank Qin Qin for
his help on implementing the VSI sequence.References
1. S Schmid et al. Proc. Intl. Soc. Mag. Reson. Med. 27 (2019) 4968
2. Q Qin and PCM van Zijl. Magn Reson Med. 2016;76(4):1136-48
3. H Lu and Y Ge. Magn Reson Med. 2008;60(2):357-63