Synopsis
The
oxygen tension in CSF and tissue is coupled, but may be easier to measure in
CSF. This work investigated the effect of O2 administration on the T2
of CSF. In five subjects, T2s were mapped in the lateral ventricles
and the peripheral CSF during administration of a hyperoxic gas mixture
(end-tidal O2 level: 500mmHg). In the lateral ventricles no T2
change was observed, in the periphery a significant T2 reduction was
found under O2 administration (T2=1.54s) compared to
baseline (T2=1.61s), showing the feasibility to use CSF relaxation
parameter mapping to detect oxygen tension changes.Purpose
When
a subject is exposed to hyperoxic breathing, the O
2 concentration in
blood, and potentially also in tissue, increases [1, 2]. The oxygen tension in CSF and tissue is
coupled, but may be easier to measure in CSF due to the absence of e.g.
exchange between extracellular and intracellular fluid [2, 3]. In Zaharchuk et al. [4] it is shown that administration of a hyperoxic
gas mixture to a subject results in a T
1 change in the third
ventricle, the cortical sulci and the basilar cisterns. In this research it was
investigated if this effect can also be measured using T
2 mapping. If
so, future work could investigate if T
2 changes of CSF precede brain
atrophy, since it is known that brain perfusion, and thus brain oxygenation,
decreases in these patients [5]. In this case, T
2 mapping of CSF
could serve as an early marker for Alzheimer’s disease.
Methods
For
this study five volunteers were included (24-34 years, 3
male), MR imaging was performed on a 3T Philips system, using an 8ch receive head
coil. For T
2 mapping of CSF an
MLEV-based pulse sequence, described by Qin [6],
was used. Effective echo times of 400, 800, 1600, 3200ms were used in all
experiments for quantitative T
2 mapping. Five repetitions of T
2
mapping were performed: the first repetition was performed at baseline
breathing after which a hyperoxic gas mixture was administered to the subject
using a RespirAct
TM system (Thornhill
Research Inc, Toronto, Canada), the target end-tidal O
2 level was
set at 500mmHg [7]
(Figure 1). The acquisition duration of one T
2 map was 1:22min,
meaning that at the start of the final repetition, diffusion of O
2 towards
CSF had occurred during 4:06min.
In all scans a single slice was acquired with 3x3x6mm
3 resolution,
SENSE 2.3, TR 15s, FOV 240x240x6mm
3 and a single shot 2D SE-EPI
readout (TE = 53ms). Slice planning is shown in Figure 2A.
Two regions of interest (ROIs) were selected: one
for the lateral ventricles and one for the peripheral CSF (Figure 2). For all scans
the mean signal in each of the two ROIs was determined, and T
2s were
fitted over this mean signal, using a plain single exponential decay model. The
image analysis was performed using Matlab (version 2015A, Mathworks).
Statistical paired t-tests were used to compare the T
2 values
(p<0.05 was regarded significant).
Results
In Table
1 the mean fitted T
2s of the 5 volunteers for the different MLEV repetitions
are shown. For the lateral ventricles, no
significant differences in T
2 were found between the baseline and
the administration of hyperoxic gas. In peripheral CSF, the T
2
values in the last three repetitions (during hyperoxic gas administration) were
significantly different from the baseline and the second MLEV repetition (at
the start of hyperoxic gas administration).
Overall lower T
2 values were measured in the periphery, compared to
the lateral ventricles.
Discussion
In
the lateral ventricles no significant T
2 changes were found during the
administration of hyperoxic gas, but in peripheral CSF a significant T
2
decay was found. These results are compliant with the work by Zaharchuk et al. [4]. This indicates that the oxygen tension of CSF
in the lateral ventricles may be relatively independent of the blood oxygen
tension, and that there might have been more O
2 diffusion from blood
and/or tissue towards peripheral CSF, compared to CSF in the lateral ventricles.
In this research no data was available of the third and fourth ventricle.
Therefore, additional work is necessary to study possible T
2 changes
in these regions as well. Further work is also needed to obtain the relaxivity
of O
2 in CSF in order to be able to translate T
2 changes to changes
in oxygen tension.
The overall lower T
2 values of peripheral CSF compared to that of
the ventricles corresponds to previous work [6, 8], and is probably due to partial volume effects
with brain tissue and its interstitial fluid (data not shown).
Conclusions
During
hyperoxia a decrease in T
2 of peripheral CSF was found while the T
2
of ventricular CSF remained constant.
Acknowledgements
This work was supported by the
European Research Council, ERC grant agreement n°337333.References
1. Derdeyn, et al., Variability of cerebral blood volume and
oxygen extraction: stages of cerebral haemodynamic impairment revisited.
Brain, 2002. 125: p. 595-607.
2. Jarnum, et al., Cisternal fluid oxygen tension in man.
Neurology, 1964(14): p. 703-707.
3. Bloor, et al., A study of cerebrospinal fluid oxygen tension preliminary experimental
and clinical observation. Arch Neurol, 1961. 4(1): p. 37-46.
4. Zaharchuk, et al., Measurement of cerebrospinal fluid oxygen
partial pressure in humans using MRI. Magn Reson Med, 2005. 54(1): p. 113-21.
5. Binnewijzend, et al., Distinct perfusion patterns in Alzheimer's
disease, frontotemporal dementia and dementia with Lewy bodies. Eur Radiol,
2014. 24(9): p. 2326-33.
6. Qin, A simple approach for three-dimensional mapping of baseline
cerebrospinal fluid volume fraction. Magn Reson Med, 2011. 65(2): p. 385-91.
7. Ito, et al., Non-invasive prospective targeting of arterial PCO2 in
subjects at rest. J Physiol, 2008. 586(15):
p. 3675-3682.
8. de Vis, et al., Cerebrospinal fluid volumetric MRI mapping
as a simple measurement for evaluating brain atrophy. Eur Radiol, 2015.