Marcel Warntjes1,2, Anders Tisell1,3, Irene Håkansson4, and Peter Lundberg1
1Center for Medical Imaging Science and Visualization, Linköping, Sweden, 2SyntheticMR AB, Linköping, Sweden, 3Department of Medical and Health Sciences, Radiation Physics, Linköping, Sweden, 4Department of Clinical and Experimental Medicine, Neurology, Linköping, Sweden
Synopsis
The rate of brain atrophy
in neuro-degenerative diseases is monitored using the brain parenchymal
fraction (BPF, the ratio of brain volume and intracranial volume). The true
atrophy, however, may be obscured by the simultaneous brain swelling due to
inflammatory processes, disease activity and medication. Measurement of the
average relaxation rates and proton density of the brain allows correction for
the presence of edemic water. The edema-corrected BPF showed a higher rate of
atrophy, 0.495%/year (p = 0.003), in comparison to the uncorrected BPF,
0.175%/year (p = 0.12), in a group of early-onset Multiple Sclerosis patients.Purpose
To correct brain volumetric
measurements, determined by the brain parenchymal fraction (BPF), for the
presence of edema. The brain may be more or less swollen due to inflammatory processes, disease activity and
medication. Volume changes of the brain due to edemic water may obscure the true rate of atrophy or lead to a large
variation in BPF values during follow-up.
Methods
In the prospective study 10
relatively young patients were included after a first relapse, suggestive of Multiple
Sclerosis (Clinically Isolated Syndrome, CIS). Their mean age was 32±10 years
at inclusion. The mean longitudinal R1 rate, the mean transverse R2
rate and the mean proton density PD of the entire brain was measured using the
Qrapmaster method [1]. Measurement of brain volume BPV, intracranial volume ICV
and the ratio BPF was performed by SyMRI (SyntheticMR,Linköping, Sweden). The
patients were measured at base-line, and at 1, 2 and 4 years follow-up. Also 10
healthy controls were included (mean age 38±5) to determine a (single) reference
brain R1ref, R2ref and PDref values. The
presence of edema was estimated using an edema fraction EF
$$EF=\frac{1}{3}(\frac{R1ref-R1brain}{R1ref-R1CSF}+\frac{R2ref-R2brain}{R2ref-R2CSF}+\frac{PDref-PDbrain}{PDref-PDCSF})$$
where CSF values were taken from Ref. 1. The
edema-corrected EC-BPF was then calculated as
$$ECBPF=\frac{BPV}{ICV}(1-EF)$$
All scans were performed with a Philips 1.5T Achiva
system (Philips Healthcare, Best, the Netherlands).
Results
The reference brain values were determined at R1ref
= 1.125 s-1, R2ref = 11.091 s-1 and PDref
= 76.393%. The average BPF and EC-BPF of all patients at baseline and follow-up
is shown in Table 1. All data points are shown in Fig. 2, as the relative
difference compared to baseline. Linear regression showed that uncorrected BPF
decreased with a non-significant 0.175% per year (95% confidence interval
[-0.373, 0.043], p = 0.12), whereas the EC-BPF decreased with a significant
0.495% per year (95%CI [-0.805, -0.184]
(p = 0.003).
Discussion and conclusion
The results show that BPF may vary substantially in
the first years after the onset of MS, which may be due to the presence of
edema, partial recovery, difference in disease actively (Relapse-Remitting) and
effects of medication. The variation may be strongest for relatively young
patients, early their disease. Edema may obscure the true rate of atrophy in
the BPF measurements. Edema correction of BPF may produce a more reliable and
robust method of monitoring brain atrophy in neuro-degenerative diseases.
Acknowledgements
No acknowledgement found.References
[1] Warntjes et
al. Magn Reson Med 60; 320-329(2008)