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

Ocular lens imaged using a diffusion weighting pulse sequence, varying b-value. Ten different b-values were tested in the first set of diffusion-weighted experiments (see Materials and Methods). Image slices were positioned to pass through the visual axis of the lens (see Figures 1B,C). The direction of the diffusion gradients (see Figure 2) was from bottom-left to top-right in the plane of the images. Improvement in the diffusion weightings of the images appeared as greater signal differentiation around the lens cortex, with diffusive attenuation in the upper-left and lower-right regions visibly stronger as b-value was increased from 214 to 1340 s/mm2. Further increase of the b-value resulted in imaging artifacts (see Text). Lenses were oriented with anterior pole facing down and posterior pole facing up.
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Figure 4: Ocular lens imaged using a diffusion weighting pulse sequence, varying b-value. Ten different b-values were tested in the first set of diffusion-weighted experiments (see Materials and Methods). Image slices were positioned to pass through the visual axis of the lens (see Figures 1B,C). The direction of the diffusion gradients (see Figure 2) was from bottom-left to top-right in the plane of the images. Improvement in the diffusion weightings of the images appeared as greater signal differentiation around the lens cortex, with diffusive attenuation in the upper-left and lower-right regions visibly stronger as b-value was increased from 214 to 1340 s/mm2. Further increase of the b-value resulted in imaging artifacts (see Text). Lenses were oriented with anterior pole facing down and posterior pole facing up.

Mentions: To obtain diffusion-weighted images, a bi-polar gradient was added on both sides of the 180° pulse of the spin-echo sequence (see Materials and Methods; Figure 2). In the first set of experiments, the effects of the b-value on diffusion weighting (see Eq. 1) were investigated. The TE was kept as short as possible (again 12 ms) while the b-value was varied from 214 to 2264 s/mm2. Selected images resulting from the diffusion-weighted scans are shown in Figure 4. In these images it was apparent that with increasing b-value, the signal in the upper-left and lower-right areas of the lens cortex was attenuated as expected from Eq. 1. This attenuation was especially visible at b-values > 600 s/mm2. Since the direction of the diffusion gradients was from bottom-left to top-right in these images (i.e., at ∼45° to the visual axis; see Materials and Methods), the observed attenuation suggested measurable diffusivity in this direction. Toward the center of the lens the overall signal decreased. At b-values > 1620 s/mm2 however, significant blurring artifacts were observed, possibly due to vibration from rapid switching of large magnetic field gradients, as has been reported in other studies (e.g., Gallichan et al., 2010).


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

Vaghefi E, Donaldson PJ - Front Physiol (2013)

Ocular lens imaged using a diffusion weighting pulse sequence, varying b-value. Ten different b-values were tested in the first set of diffusion-weighted experiments (see Materials and Methods). Image slices were positioned to pass through the visual axis of the lens (see Figures 1B,C). The direction of the diffusion gradients (see Figure 2) was from bottom-left to top-right in the plane of the images. Improvement in the diffusion weightings of the images appeared as greater signal differentiation around the lens cortex, with diffusive attenuation in the upper-left and lower-right regions visibly stronger as b-value was increased from 214 to 1340 s/mm2. Further increase of the b-value resulted in imaging artifacts (see Text). Lenses were oriented with anterior pole facing down and posterior pole facing up.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 4: Ocular lens imaged using a diffusion weighting pulse sequence, varying b-value. Ten different b-values were tested in the first set of diffusion-weighted experiments (see Materials and Methods). Image slices were positioned to pass through the visual axis of the lens (see Figures 1B,C). The direction of the diffusion gradients (see Figure 2) was from bottom-left to top-right in the plane of the images. Improvement in the diffusion weightings of the images appeared as greater signal differentiation around the lens cortex, with diffusive attenuation in the upper-left and lower-right regions visibly stronger as b-value was increased from 214 to 1340 s/mm2. Further increase of the b-value resulted in imaging artifacts (see Text). Lenses were oriented with anterior pole facing down and posterior pole facing up.
Mentions: To obtain diffusion-weighted images, a bi-polar gradient was added on both sides of the 180° pulse of the spin-echo sequence (see Materials and Methods; Figure 2). In the first set of experiments, the effects of the b-value on diffusion weighting (see Eq. 1) were investigated. The TE was kept as short as possible (again 12 ms) while the b-value was varied from 214 to 2264 s/mm2. Selected images resulting from the diffusion-weighted scans are shown in Figure 4. In these images it was apparent that with increasing b-value, the signal in the upper-left and lower-right areas of the lens cortex was attenuated as expected from Eq. 1. This attenuation was especially visible at b-values > 600 s/mm2. Since the direction of the diffusion gradients was from bottom-left to top-right in these images (i.e., at ∼45° to the visual axis; see Materials and Methods), the observed attenuation suggested measurable diffusivity in this direction. Toward the center of the lens the overall signal decreased. At b-values > 1620 s/mm2 however, significant blurring artifacts were observed, possibly due to vibration from rapid switching of large magnetic field gradients, as has been reported in other studies (e.g., Gallichan et al., 2010).

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