What is the relative distribution of spinal subarachnoid space volumetric compliance in healthy subjects?
Craniospinal compliance has been thought to be an important indicator of craniospinal and cerebrovascular health. However, a non-invasive technique for assessment of craniospinal compliance is not available. The proposed pilot study aims to investigate the spinal subarachnoid space (SSS) compliance distribution in three healthy volunteers using 1) a thru-plane phase contrast MRI flow scan obtained at various locations along the SSS (Figure 1) and 2) a novel non-invasive 4D MR flow quantification technique (Santini, Wetzel et al. 2009) (Figure 2).
Volumetric compensation of a region of interest (ROI) of the SSS can be determined by measuring the instantaneous difference of cerebrospinal fluid (CSF) flow into and out of a control volume in the SSS. If the entire flow field in the SSS is known using the 4D MR flow scan (Santini, Wetzel et al. 2009), then it is possible to obtain the relative distribution of SSS compliance in the ROI.
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. 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. 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 could have impact by providing a new tool to help quantitatively assess the cerebrospinal fluid system. The results will quantify and compare the SSS compliance distribution in the three healthy volunteers using the two MR protocols. The same MR protocols could be used in a larger number of healthy people and specific patient populations such as those with craniospinal and cerebrovascular disorders (i.e. hydrocephalus, Chiari malformation, hypertension, and chronic venous insufficiency). The protocols could also be used to quantify changes in SSS compliance distribution which may occur due to various stimuli such as continuous positive airway pressure (CPAP) and queckenstedt’s test.
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.