Synopsis

We evaluated the signal intensity of the CSF on low b-value (b=500 s/mm²) diffusion weighted images (DWI) in cases with ventricular dilatation versus controls by a scoring method. Although low b-value DWI cannot quantify the absolute flow speed, it may be possible to evaluate the distribution of altered CSF dynamics within the cranium in the cases of ventricular dilatation. We also evaluated the characteristic signal void findings adjacent to the septum pellucidum in the cases with ventricular dilatation, which was speculated to be due to a standing wave in a thinned septum pellucidum.

Purpose

In routine clinical practice, diffusion-weighted imaging (DWI) with a b-value of 1000 s/mm² is preferred. In contrast, usually we do not use the b=500 s/mm² image for the clinical diagnosis because the interpretation of the signal intensity is complicated by a larger effect of T2 shine through, and incomplete and inhomogeneous signal suppression of cerebrospinal fluid (CSF). We considered that the incomplete and inhomogeneous signal suppression of the CSF might contain information regarding CSF dynamics.

The main purpose for this study is to investigate if an alteration of CSF dynamics can be visualized on the b=500 s/mm² image. In addition, we found characteristic signal voids adjacent to the septum pellucidum, and also made an evaluation of this finding.

Subjects and Methods

The subjects of this retrospective study consisted of controls (Figure 1: n=50) and patients with ventricular dilatation (Figure 2: n = 50). We acquired DWI by 1.5T clinical scanner (MAGNETOM Avanto) with the following parameters: TR=3500ms, TE=94ms, b-value=0, 500, 1000s/mm², no flow compensation, bandwidth=1132Hx/Px., FOV=200mm, matrix=192x144.

The signal intensity of the CSF on the b=500 s/mm² DWI were evaluated qualitatively in the lateral, 3rd and 4th ventricles by the score shown in Figure 3. Signal void in the Sylvian fissure and the fronto-parietal sulci were also evaluated. The scores were compared between the ventricular dilatation group and controls using a Pearson’s chi-square test. Scores for the signal void adjacent to the septum pellucidum were also compared between the ventricular dilatation and control groups for the thickness, the length and the shape of the signal void.

Results

Figure 3 show the results. The CSF signal intensities on the b=500 s/mm² DWI were significantly lower in the ventricular dilatation group in the anterior horn of the lateral ventricle (p<0.01), the body of the lateral ventricle (p<0.05), the atrium of the lateral ventricle (p<0.01), the temporal horn of the lateral ventricle (p<0.001) and in the 3rd ventricle (p<0.001). However, there was no significant difference in the 4th ventricle. The scores for the signal void in the Sylvian fissure were significantly higher in the ventricular dilatation group (p<0.05), indicating that the signal void occupied a larger area.

For the signal void adjacent to the septum pellucidum on the b=500 s/mm² DWI between the ventricular dilatation and control groups. The ventricular dilatation group had significantly higher scores compared to controls in the thickness (p<0.001), the length (p<0.001) and the shape of the signal void (p<0.001).

Discussion

The findings of the current study indicate that CSF within the lateral ventricle on the b=500 s/mm² DWI had a higher signal intensity in the ventricle dilatation group. The signal intensity of a diffusion-weighted image depends mainly on the relaxation time, proton density, intra-voxel diffusion and perfusion. In the CSF, the perfusion fraction may correspond to CSF flow. When the MPG is lower such as b=500 s/mm², the signal of the CSF is not suppressed completely. Therefore, our finding that the signal intensity of the CSF within the ventricle was higher in the cases with ventricular dilatation means that the motion of the water molecules is slower compared to that of controls (1,2).

The bead-like signal void was more frequently seen in the cases with a greater ventricular dilatation. In addition, in the cases with perforation of the septum pellucidum, the signal void could only be seen in the area where septum present (Figure 4). Thus we speculate that the septum pellucidum becomes thinner with ventricular dilatation and may behave as a drum membrane, where exposure to vibration may cause standing waves (3). A source of the vibration could be arterial pulsation, however, the septum pellucidum does not contain a large artery. We speculated that the vibration of the MR scanner while scanning may be a source, and performed a phantom experiment with a plastic membrane. Figure 5 shows an image of the phantom with the same imaging sequence as applied in the current study. On the image, there is a bead-like signal void along the plastic membrane from which the vibration was only due to the scanning of the DWI.

Conclusion

Acknowledgements

References

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2. Le Bihan D, Breton E, Lallemand D, Aubin ML, Vignaud J, Laval-Jeantet M. Separation of diffusion and perfusion in intravoxel incoherent motion MR imaging. Radiology 1988;168(2):497-505

3. Sarwar M. The septum pellucidum: normal and abnormal. AJNR Am J Neuroradiol 1989;10(5):989-1005.