Limits...
An exploration into diffusion tensor imaging in the bovine ocular lens.

Vaghefi E, Donaldson PJ - Front Physiol (2013)

Bottom Line: Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions.The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens.This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.

View Article: PubMed Central - PubMed

Affiliation: Auckland Bioengineering Institute, University of Auckland Auckland, New Zealand ; Department of Optometry and Vision Sciences, University of Auckland Auckland, New Zealand.

ABSTRACT
We describe our development of the diffusion tensor imaging modality for the bovine ocular lens. Diffusion gradients were added to a spin-echo pulse sequence and the relevant parameters of the sequence were refined to achieve good diffusion weighting in the lens tissue, which demonstrated heterogeneous regions of diffusive signal attenuation. Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions. The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens. Up to 30 diffusion gradient directions, and 8 signal acquisition averages, were applied to lenses in culture in order to improve maps of diffusion tensor eigenvalues, equivalent to ADC, across the lens. From these maps, fractional anisotropy maps were calculated and compared to known spatial distributions of anisotropic molecular fluxes in the lens. This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.

No MeSH data available.


Related in: MedlinePlus

Effects of the number of diffusion gradient directions, and signal averages, used in estimating the diffusion tensor. Grayscale maps of λmax, λmid, and λmin, passing through the visual axis of the lens (see Figures 1B,C), were calculated from lens image slices acquired using the four different scanning regimes described in Table 1 results from each regime are represented in one horizontal row of image maps shown above. In each row, one bovine lens has been scanned using a number of diffusion gradient directions (labeled at left) and a maximum corresponding number of signal averages (labeled at right) possible within the lens culture time limit (see Text). The eigenvalue maps are shown masked (manually) from the surround for ease of comparison. The very bright lens edge seen on some of the panels is due to an edge effect and thresholding.
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Figure 8: Effects of the number of diffusion gradient directions, and signal averages, used in estimating the diffusion tensor. Grayscale maps of λmax, λmid, and λmin, passing through the visual axis of the lens (see Figures 1B,C), were calculated from lens image slices acquired using the four different scanning regimes described in Table 1 results from each regime are represented in one horizontal row of image maps shown above. In each row, one bovine lens has been scanned using a number of diffusion gradient directions (labeled at left) and a maximum corresponding number of signal averages (labeled at right) possible within the lens culture time limit (see Text). The eigenvalue maps are shown masked (manually) from the surround for ease of comparison. The very bright lens edge seen on some of the panels is due to an edge effect and thresholding.

Mentions: After choosing the b-value and TE above for diffusion-weighted imaging, we investigated the effects of using multiple diffusion gradient directions, and signal averaging, for DTI. The numbers of diffusion gradient directions and signal averages that could be used in combination in these experiments, were constrained by the maximum total imaging time allowed for cultured lenses (up to 12 h; see Materials and Methods). The scanning regimes used within this time limit are listed in Table 1. The effects of the different regimes on estimates of local diffusivity in the lens, can be observed in Figure 8, which shows maps of the maximum, minimum, and intermediate eigenvalues of the estimated diffusion tensor, calculated from lens image slices acquired under each regime. The overall map quality was greatly improved as the number of diffusion gradient directions was increased and (owing to the time constraint) the number of signal averages was decreased. It was clear from the eigenvalue maps that the effect of increasing the number of directions dominated over increasing the number of signal averages, in producing smoother, more radially symmetric eigenvalue maps.


An exploration into diffusion tensor imaging in the bovine ocular lens.

Vaghefi E, Donaldson PJ - Front Physiol (2013)

Effects of the number of diffusion gradient directions, and signal averages, used in estimating the diffusion tensor. Grayscale maps of λmax, λmid, and λmin, passing through the visual axis of the lens (see Figures 1B,C), were calculated from lens image slices acquired using the four different scanning regimes described in Table 1 results from each regime are represented in one horizontal row of image maps shown above. In each row, one bovine lens has been scanned using a number of diffusion gradient directions (labeled at left) and a maximum corresponding number of signal averages (labeled at right) possible within the lens culture time limit (see Text). The eigenvalue maps are shown masked (manually) from the surround for ease of comparison. The very bright lens edge seen on some of the panels is due to an edge effect and thresholding.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3585442&req=5

Figure 8: Effects of the number of diffusion gradient directions, and signal averages, used in estimating the diffusion tensor. Grayscale maps of λmax, λmid, and λmin, passing through the visual axis of the lens (see Figures 1B,C), were calculated from lens image slices acquired using the four different scanning regimes described in Table 1 results from each regime are represented in one horizontal row of image maps shown above. In each row, one bovine lens has been scanned using a number of diffusion gradient directions (labeled at left) and a maximum corresponding number of signal averages (labeled at right) possible within the lens culture time limit (see Text). The eigenvalue maps are shown masked (manually) from the surround for ease of comparison. The very bright lens edge seen on some of the panels is due to an edge effect and thresholding.
Mentions: After choosing the b-value and TE above for diffusion-weighted imaging, we investigated the effects of using multiple diffusion gradient directions, and signal averaging, for DTI. The numbers of diffusion gradient directions and signal averages that could be used in combination in these experiments, were constrained by the maximum total imaging time allowed for cultured lenses (up to 12 h; see Materials and Methods). The scanning regimes used within this time limit are listed in Table 1. The effects of the different regimes on estimates of local diffusivity in the lens, can be observed in Figure 8, which shows maps of the maximum, minimum, and intermediate eigenvalues of the estimated diffusion tensor, calculated from lens image slices acquired under each regime. The overall map quality was greatly improved as the number of diffusion gradient directions was increased and (owing to the time constraint) the number of signal averages was decreased. It was clear from the eigenvalue maps that the effect of increasing the number of directions dominated over increasing the number of signal averages, in producing smoother, more radially symmetric eigenvalue maps.

Bottom Line: Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions.The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens.This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.

View Article: PubMed Central - PubMed

Affiliation: Auckland Bioengineering Institute, University of Auckland Auckland, New Zealand ; Department of Optometry and Vision Sciences, University of Auckland Auckland, New Zealand.

ABSTRACT
We describe our development of the diffusion tensor imaging modality for the bovine ocular lens. Diffusion gradients were added to a spin-echo pulse sequence and the relevant parameters of the sequence were refined to achieve good diffusion weighting in the lens tissue, which demonstrated heterogeneous regions of diffusive signal attenuation. Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions. The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens. Up to 30 diffusion gradient directions, and 8 signal acquisition averages, were applied to lenses in culture in order to improve maps of diffusion tensor eigenvalues, equivalent to ADC, across the lens. From these maps, fractional anisotropy maps were calculated and compared to known spatial distributions of anisotropic molecular fluxes in the lens. This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.

No MeSH data available.


Related in: MedlinePlus