What is the relative distribution of spinal subarachnoid space volumetric compliance in healthy subjects?

Streamlines representing CSF flow in the neck (C3-C5 sections). Eight planes are placed axially 5 mm apart from each other and are chosen as emitters for the streamlines. Image view is oblique sagittal/coronal approximately 45° in the left/posterior direction (frames a-c). Anteroposterior and head-feet directions are represented by the letters A-P and H-F, respectively(Santini, Wetzel et al. 2009)

Personel

• Bryn Martin
• Francesco Santini
• Francis Loth
• Nikolaos Stergiopulos
• Matthias Stuber

The problem

Craniospinal compliance has been thought to be an important indicator of craniospinal health. However, a non-invasive technique for assessment of craniospinal compliance is not available. The proposed pilot study aims to investigate the feasibility of a novel non-invasive 4D MR flow quantification technique (Santini, Wetzel et al. 2009) to measure the relative distribution of volumetric compliance of the spinal subarachnoid space (SSS) (Figure 1).

The volumetric compensation of SSS to the cerebrospinal fluid pulsations (CSF) can be measured by determining the instantaneous difference of flow into and out of a SSS control volume. If the entire flow field in the SSS is known (Santini, Wetzel et al. 2009), then it is possible to obtain the relative distribution of compliance along the spinal canal.

Hypothesis and research objectives

We hypothesize that in healthy persons most of the volumetric compensation of the CSF pulsation takes place in the cervical subarachnoid space. The goal of this research is thus to measure the distribution of spinal SAS compliance in healthy subjects using two novel MR protocols.

Methods and study outline

A. In vivo MR measurements.

Our approach is to obtain CSF flow velocity measurements on three age-matched healthy subjects using the following protocols.

1. For SA1b, high-resolution balanced-steady-state free procession cine (bSSFP) images gated to the cardiac cycle will be obtained from a healthy subject at the various vertebrae levels (i.e. C2, C8, and T4 as in Figure 10). These cine loops will be acquired for at least 25 frames over the cardiac cycle and be used to investigate the distribution of spinal SAS compliance.

2. For SA1c, and SA1d, flow measurements in cranial (c) and spinal (d) SAS will be conducted using the protocol developed by Santini et al. (Figure 10) (Santini, Wetzel et al. 2009). Subjects will be asked to lie in the supine position in the scanner bed with a standard 12-channel head coil and neck coil (Siemens Medical Solutions). CSF flow measurements will be performed in healthy volunteers in the spinal SAS (C2 to lumbar if possible), hindbrain, and ventricles with the following imaging parameters: venc = 10 cm/s, TE = 6 ms, TR = 12 ms, flip angle = 70°. Total scan time for each subject will be approximately 20 min.

3. Repeatability will be examined by obtaining measurements in 1) and 2) on the same subject three times.

B. Processing of MR data.

The relative distribution of spinal canal compliance will be calculated based on the CSF flow velocity measurements

1. CSF flow waveforms in the spinal SAS will be obtained from the recorded velocity for each pixel from pcMR images. Pixel velocity will be integrated over a manually selected cross-sectional ROI for an entire cardiac cycle as detailed by Kalata (Kalata 2002). The basic equation for this process is given by , where Apixel is the area of one MRI pixel, Vpixel is the velocity for the corresponding pixel, and Q(t) is the summation of the flow for each pixel of interest.

2. Using a technique similar to Martin et al. (Kalata 2002; Martin, Kalata et al. 2005), the volume change of a section of the spinal SAS will be computed as follows. A control volume will be defined by two parallel planes that are each orthogonal to CSF flow separated by a distance (L). The volume flux of the ROI will be computed by integration of the flow difference into and out-of the ROI.

3. Thus, If the entire 4D flow field in the SAS is known (Santini, Wetzel et al. 2009), then it is possible to obtain the relative distribution of compliance along the entire spinal canal using the provided techniques.

Expected results and potential impact

The proposed work will determine if the non-invasive MR protocol is capable of measuring the distribution of compliance in the spinal canal in healthy volunteers. If proven efficacious, the test could be further explored with patients, and has the potential to provide a clinically useful tool for assessment of relative distribution in craniospinal compliance.

Preliminary results

To be posted

References

Kalata, W. (2002). "Numerical simulation of cerebrospinal fluid motion within the spinal canal." Masters Thesis Chicago, University of Illinois at Chicago.

Martin, B. A., W. Kalata, et al. (2005). "Syringomyelia hydrodynamics: an in vitro study based on in vivo measurements." J Biomech Eng 127(7): 1110-20.

Santini, F., S. G. Wetzel, et al. (2009). "Time-resolved three-dimensional (3D) phase-contrast (PC) balanced steady-state free precession (bSSFP)." Magn Reson Med.