Assessment of axonal fiber tract architecture in excised rat spinal cord by localized NMR q‐space imaging: Simulations and experimental studies

Abstract

NMR q-space imaging is a method designed to obtain information from porous materials where diffusion-diffraction phenomena were observed from which pore size was derived. Recently, the technique has been applied to the study of biological structures as well. Although diffusive diffraction has so far not been observed in multicellular systems, displacement profiles have been used with some success as a means to estimate structure size. However, there have been no quantitative correlations of the retrieved structure sizes with histology. Clearly, the complexity of tissue architecture poses significant challenges to the interpretation of q-space data. In this work, simulations were first performed on a two-compartment model to demonstrate the effects of interference of the diffraction patterns arising from intra and extra-axonal compartments and finite boundary permeability on q-space data. Second, q-space echo attenuation was simulated on the basis of histologic images of various rat spinal cord fiber tracts and the information obtained from the displacement profiles were compared with structural parameters computed from the histologic images. The results show that calculated mean displacements and kurtosis parallel mean axon size and axonal density.

Indirect measurement of regional axon diameter in excised mouse spinal cord with q-space imaging: Simulation and experimental studies

“…Simulations did not include signal from myelin. The T 2 values were chosen to match the long T 2 component of neural tissues reported in literature (Chin et al, 2004). The diffusion coefficients were chosen to match those of water at room temperature as it was assumed that it would be an upper limit for the diffusion coefficients found in axonal tissue.…”

“…The following T 2 , permeability and diffusion coefficients were chosen from literature values (Chin et al, 2004). The ECS, ICS and myelin diffusion coefficients were assigned values of 1.65, 1.12 and 1.12 µm 2 /s, respectively.…”

“…Imposition of the SGPA demands very high gradient amplitudes not commercially available even on micro-imaging systems. As a result, the QSI studies cited above either violated the SGPA or their displacement profile resolutions (ranging from 2 (Chin et al, 2004) to 20 (King et al, 1997) µm) exceeded typical axon diameters (1-2 µm). Mitra et al (Mitra and Halperin, 1995) predicted that as the gradient duration increased in relation to diffusion time, the displacement profile would artifactually narrow, which has been observed experimentally (Assaf et al, 2002a).…”

“…QSI allows assessment of microarchitecture by exploiting the regularity of restrictions to molecular diffusion in porous systems as applied to determine pore geometry in porous solids (Callaghan et al, 1991), the study of packed erythrocytes (Torres et al, 1998), and the assessment of axonal architecture (Chin et al, 2004;Cohen and Assaf, 2002;Malmborg et al, 2006). In WM tracts, the porous system consists of water diffusion restricted by axon membranes and myelin sheaths (Beaulieu, 2002).…”

“…Other metrics that are used to characterize the displacement profile include the peak value, otherwise known as the zero-displacement probability, which is inversely related to FWHM and reflects the fraction of molecules that diffuse short distance or not at all, and kurtosis, i.e. the fourth moment of the displacement profile, which is a measure of the degree of deviation from a Gaussian distribution and therefore reflects the degree of restriction to diffusion (Chin et al, 2004). Kurtosis may correlate with axon density and may provide insight to long-range diffusion behavior (Jensen et al, 2005).…”