Synopsis
The relaxation parameters of CSF may have
potential as imaging markers in several diseases. In this work T2
mapping of CSF in the lateral ventricles and in the fourth ventricle was performed
in six volunteers at 7T, with comparison to 3T. The sensitivity for B1
was assessed by comparing the T2s in both regions, with equal B1
at 3T and different B1 at 7T. T2 values were
significantly lower at 7T compared to 3T. No significant difference was found
between the lateral and the fourth ventricular T2s at 7T (and 3T),
indicating negligible B1-sensitivity for the used T2-mapping
sequences.Purpose
The relaxation parameters T
1 and T
2 of cerebrospinal fluid (CSF) depend on its content and on the MRI field strength [1] and may, thus, have potential as an imaging marker in several diseases. The T
1 of CSF at 7T is known [2]. The T
2 of CSF has been reported as part of a relaxation parameter mapping study at 7T, but the same study considered the reported CSF parameters not reliable because the used method was not robust against the pulsatile flow of CSF [3]. In this work, a flow insensitive T
2 mapping method was used to measure the T
2 of CSF at 7T and compare it to 3T. The sensitivity to B
1 was assessed by comparing the T
2 in two regions with different B
1 at 7T, relative to the T
2 in the same regions at 3T (where the B
1 was not significantly different).
Methods
An MLEV-based pulse sequence for T
2
mapping of CSF was used as described by Qin (2011) [4],
using non-selective preparation pulses, which is intrinsically flow-insensitive.
Effective echo times of 600, 1200, 2400, 4800ms were used in all experiments.
Additionally, a plain, single echo SE-EPI sequence was used for comparison,
using the following echo times: 240, 285, 420, 645, 960, 1365, 1860, 2445,
3120, 3885, 4740ms. In all scans two single slice scans were acquired with
1x1x4 mm
3 and 3x3x6mm
3 resolution, SENSE 2.3, TR 15s, FOV
240x240x4mm
3, using a single shot 2D SE-EPI readout with TE 344ms
and 53ms for the higher and lower resolution scans, respectively. For all
volunteers B
1 maps were acquired [5],
with 1x1x4mm
3 and 3x3x6mm
3 resolution.
Six volunteers (aged 21-45 years, 3 male) were scanned with both T
2
mapping sequences, at 3T (Philips Healthcare) with an 8ch head coil and at 7T
(Philips Healthcare) with a 32ch head coil (Nova Medical). The scans planning
is shown in Figure 1A. Conservative region-of-interest (ROI) masks of the
lateral ventricles and fourth ventricle were obtained, avoiding partial volume
effects with tissue (Figure 1BC). Peripheral CSF was not studied, as these
values suffer from partial volume effects with tissue (data not shown).
In order to minimize the effect of noise, signal intensities within the ROIs were
averaged before estimating the T
2 values from a single exponential
decay model. Also mean B
1 in the ROIs was determined. Paired t-tests
were used to compare T
2- and B
1-values.
Results
T
2 values and B
1
values are summarized in Table 1. For both sequences and for both resolutions
the measured T
2s are significantly shorter at 7T compared to 3T. At
both field strengths and for both resolutions, the SE-EPI sequence results in a shorter T
2
than the MLEV sequence. No significant differences were found between the T
2
values of the lateral ventricles and the fourth ventricle at 7T or 3T. At 3T,
the B
1 values in the lateral ventricles and the fourth ventricle
were similar, but at 7T the B
1 was significantly lower (by approximately
20%) in the fourth ventricle compared to the lateral ventricles. For the
MLEV-based sequence the T
2 values of the high- and low-resolution
scans were similar, but for the SE-EPI scans significant differences were found
between the resolutions.
Discussion
The measured T
2 of CSF is
significantly lower at 7T compared to 3T, which may be caused by a field
strength dependent relaxivity of the content (proteins and/or O
2) of
CSF [6].
These results seem to show a stronger field strength dependency for T
2
than for T
1 [2],
though a side-by-side comparison is still needed.
The SE-EPI sequence resulted in shorter T
2s than the MLEV-based
sequence, which is probably due to diffusion and flow effects: the MLEV-based
sequence is less diffusion- and flow sensitive due to the use of multiple,
non-selective refocusing pulses. Since similar T
2 values were found
for the high- and low-resolution MLEV scans, despite the longer readout TE in
the higher resolution scan, this method seems indeed robust for flow effects. With
the current parameters the SE-EPI sequence is more vulnerable for partial volume
effects compared to the MLEV sequence (data not shown), which likely explains the
significantly lower T
2 values for the low resolution SE-EPI scans.
No significantly different T
2 values were found in the lateral
ventricles and the fourth ventricle, at both field strengths. Since the B
1
at 7T was different for the fourth ventricle, these results show that both T
2-mapping
sequences are relatively insensitive to B
1 variation.
Conclusions
The measured T
2 values of CSF were significantly
lower at 7T compared to 3T, suggesting a field strength dependent relaxivity of
the content of CSF.
Acknowledgements
This work was supported by the
European Research Council, ERC grant agreement n°337333.References
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