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Comparison of diffusion tensor imaging by cardiovascular magnetic resonance and gadolinium enhanced 3D image intensity approaches to investigation of structural anisotropy in explanted rat hearts.

Bernus O, Radjenovic A, Trew ML, LeGrice IJ, Sands GB, Magee DR, Smaill BH, Gilbert SH - J Cardiovasc Magn Reson (2015)

Bottom Line: Both FLASH (v3(ST)) and DTI (e3(DTI)) where compared to directly measured laminar arrays in the FLASH images.We show that ST analysis of FLASH is a useful and accurate tool in the measurement of cardiac microstructure.While both FLASH and the DTI approaches appear promising for mapping of the alignments of myocytes throughout myocardium, marked discrepancies between the cross myocyte anisotropies deduced from each method call for consideration of their respective limitations.

View Article: PubMed Central - PubMed

Affiliation: Inserm U1045 - Centre de Recherche Cardio-Thoracique, L'Institut de rythmologie et modélisation cardiaque LIRYC, Université de Bordeaux, PTIB - campus Xavier Arnozan, Avenue du Haut Leveque, 33604, Pessac, France. olivier.bernus@u-bordeaux.fr.

ABSTRACT

Background: Cardiovascular magnetic resonance (CMR) can through the two methods 3D FLASH and diffusion tensor imaging (DTI) give complementary information on the local orientations of cardiomyocytes and their laminar arrays.

Methods: Eight explanted rat hearts were perfused with Gd-DTPA contrast agent and fixative and imaged in a 9.4T magnet by two types of acquisition: 3D fast low angle shot (FLASH) imaging, voxels 50 × 50 × 50 μm, and 3D spin echo DTI with monopolar diffusion gradients of 3.6 ms duration at 11.5 ms separation, voxels 200 × 200 × 200 μm. The sensitivity of each approach to imaging parameters was explored.

Results: The FLASH data showed laminar alignments of voxels with high signal, in keeping with the presumed predominance of contrast in the interstices between sheetlets. It was analysed, using structure-tensor (ST) analysis, to determine the most (v1(ST)), intermediate (v2(ST)) and least (v3(ST)) extended orthogonal directions of signal continuity. The DTI data was analysed to determine the most (e1(DTI)), intermediate (e2(DTI)) and least (e3(DTI)) orthogonal eigenvectors of extent of diffusion. The correspondence between the FLASH and DTI methods was measured and appraised. The most extended direction of FLASH signal (v1(ST)) agreed well with that of diffusion (e1(DTI)) throughout the left ventricle (representative discrepancy in the septum of 13.3 ± 6.7°: median ± absolute deviation) and both were in keeping with the expected local orientations of the long-axis of cardiomyocytes. However, the orientation of the least directions of FLASH signal continuity (v3(ST)) and diffusion (e3(ST)) showed greater discrepancies of up to 27.9 ± 17.4°. Both FLASH (v3(ST)) and DTI (e3(DTI)) where compared to directly measured laminar arrays in the FLASH images. For FLASH the discrepancy between the structure-tensor calculated v3(ST) and the directly measured FLASH laminar array normal was of 9 ± 7° for the lateral wall and 7 ± 9° for the septum (median ± inter quartile range), and for DTI the discrepancy between the calculated v3(DTI) and the directly measured FLASH laminar array normal was 22 ± 14° and 61 ± 53.4°. DTI was relatively insensitive to the number of diffusion directions and to time up to 72 hours post fixation, but was moderately affected by b-value (which was scaled by modifying diffusion gradient pulse strength with fixed gradient pulse separation). Optimal DTI parameters were b = 1000 mm/s(2) and 12 diffusion directions. FLASH acquisitions were relatively insensitive to the image processing parameters explored.

Conclusions: We show that ST analysis of FLASH is a useful and accurate tool in the measurement of cardiac microstructure. While both FLASH and the DTI approaches appear promising for mapping of the alignments of myocytes throughout myocardium, marked discrepancies between the cross myocyte anisotropies deduced from each method call for consideration of their respective limitations.

No MeSH data available.


Related in: MedlinePlus

Exploration of the relative magnitudes of the putative myocyte orientation eigenvalues in the lateral ROI. In order to assess for DTI and for ST whether meaningful sorting of the putative myocyte eigenvector from the intermediate-eigenvector is possible the magnitudes of the putative myocyte orientation eigenvalue was compared to the λ2 (i.e. for ST λ3 was compared to λ2 and for DTI λ1, was compared to λ2). In each case the smaller eigenvalue was expressed as a percentage of the larger eigenvalue. 100% indicates identity and that there is no confidence in sorting the putative myocyte orientation from the intermediate-eigenvector orientation, and approaching 0% the confidence in sorting is high. DTI: Scan #2, 12-direction, b = 1000 s/mm2; ST: Scan #8, DTW = 3, STW = 3. Data in this figure is from the lateral ROI which was visualized and compared to FI laminar orientations in Figures 3 and 7. FLASH: fast low angle shot; ST: structure tensor of FLASH data; DTI: diffusion tensor magnetic resonance imaging; DTW: derivative template width STW: smoothing template width; ROI: region(s) of interest. The symbols for vectors and derived angles are defined in Table 2.
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Fig12: Exploration of the relative magnitudes of the putative myocyte orientation eigenvalues in the lateral ROI. In order to assess for DTI and for ST whether meaningful sorting of the putative myocyte eigenvector from the intermediate-eigenvector is possible the magnitudes of the putative myocyte orientation eigenvalue was compared to the λ2 (i.e. for ST λ3 was compared to λ2 and for DTI λ1, was compared to λ2). In each case the smaller eigenvalue was expressed as a percentage of the larger eigenvalue. 100% indicates identity and that there is no confidence in sorting the putative myocyte orientation from the intermediate-eigenvector orientation, and approaching 0% the confidence in sorting is high. DTI: Scan #2, 12-direction, b = 1000 s/mm2; ST: Scan #8, DTW = 3, STW = 3. Data in this figure is from the lateral ROI which was visualized and compared to FI laminar orientations in Figures 3 and 7. FLASH: fast low angle shot; ST: structure tensor of FLASH data; DTI: diffusion tensor magnetic resonance imaging; DTW: derivative template width STW: smoothing template width; ROI: region(s) of interest. The symbols for vectors and derived angles are defined in Table 2.

Mentions: Unlike the case of myolaminar orientation, there is no method to directly determine local myocyte orientation from the FLASH data against which e1DTI and v1ST can be compared. This is because it is not possible for FLASH to resolve individual cardiac myocytes at 50 × 50 × 50 μm3 resolution. Therefore the putative myocyte orientation vectors e1DTI and v1ST are compared to each other. The first part of this comparison is to evaluate the basis for eigenvector assignment (in the same manner as was carried out for laminar eigenvectors in section ‘Quantification of the confidence in the sorting of laminar eigenvectors’ above. The relative magnitudes of the putative local myocyte eigenvalue are compared to the next closest eigenvalue (λ2) in Figure 12. It might be expected that there would be a stronger basis for sorting of e1DTI from the other DTI eigenvectors than in sorting v1ST from the other ST eigenvectors, as e1DTI is a primary eigenvector. This is not the case as there is greater difference between λ2ST and λ3ST than between λ2DTI and λ1DTI. The median difference between λ2ST and λ3ST is 61.1% ± 28.1%. This compares to a median difference of 23.9% ± 11.5%. As discussed above, the sets of eigenvalues which are relevant to assignment of the local myocyte orientation are not the same for DTI and ST (for DTI: λ1 and λ2, for ST: λ2 and λ3). An implication of this greater separation of the eigenvalues relevant to v1ST assignment (λ2ST and λ3ST) than the eigenvalues relevant to e1DTI assignment (λ2DTI and λ1DTI), is that misclassification of e1DTI and e2DTI is more probable than misclassification of v1ST and v2ST. There is no absolute cut-off for the median difference between eigenvalues which is acceptable to allow confidence in the DTI or ST assignment of the putative local myocyte orientation (m). The order of the eigenvalue sets from greatest difference to least difference is: (i) λ2ST with λ1ST; (ii) λ3ST with λ2ST; (iii) λ2DTI with λ1DTI; and, (iv) λ2DTI with λ3DTI.Figure 12


Comparison of diffusion tensor imaging by cardiovascular magnetic resonance and gadolinium enhanced 3D image intensity approaches to investigation of structural anisotropy in explanted rat hearts.

Bernus O, Radjenovic A, Trew ML, LeGrice IJ, Sands GB, Magee DR, Smaill BH, Gilbert SH - J Cardiovasc Magn Reson (2015)

Exploration of the relative magnitudes of the putative myocyte orientation eigenvalues in the lateral ROI. In order to assess for DTI and for ST whether meaningful sorting of the putative myocyte eigenvector from the intermediate-eigenvector is possible the magnitudes of the putative myocyte orientation eigenvalue was compared to the λ2 (i.e. for ST λ3 was compared to λ2 and for DTI λ1, was compared to λ2). In each case the smaller eigenvalue was expressed as a percentage of the larger eigenvalue. 100% indicates identity and that there is no confidence in sorting the putative myocyte orientation from the intermediate-eigenvector orientation, and approaching 0% the confidence in sorting is high. DTI: Scan #2, 12-direction, b = 1000 s/mm2; ST: Scan #8, DTW = 3, STW = 3. Data in this figure is from the lateral ROI which was visualized and compared to FI laminar orientations in Figures 3 and 7. FLASH: fast low angle shot; ST: structure tensor of FLASH data; DTI: diffusion tensor magnetic resonance imaging; DTW: derivative template width STW: smoothing template width; ROI: region(s) of interest. The symbols for vectors and derived angles are defined in Table 2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4414435&req=5

Fig12: Exploration of the relative magnitudes of the putative myocyte orientation eigenvalues in the lateral ROI. In order to assess for DTI and for ST whether meaningful sorting of the putative myocyte eigenvector from the intermediate-eigenvector is possible the magnitudes of the putative myocyte orientation eigenvalue was compared to the λ2 (i.e. for ST λ3 was compared to λ2 and for DTI λ1, was compared to λ2). In each case the smaller eigenvalue was expressed as a percentage of the larger eigenvalue. 100% indicates identity and that there is no confidence in sorting the putative myocyte orientation from the intermediate-eigenvector orientation, and approaching 0% the confidence in sorting is high. DTI: Scan #2, 12-direction, b = 1000 s/mm2; ST: Scan #8, DTW = 3, STW = 3. Data in this figure is from the lateral ROI which was visualized and compared to FI laminar orientations in Figures 3 and 7. FLASH: fast low angle shot; ST: structure tensor of FLASH data; DTI: diffusion tensor magnetic resonance imaging; DTW: derivative template width STW: smoothing template width; ROI: region(s) of interest. The symbols for vectors and derived angles are defined in Table 2.
Mentions: Unlike the case of myolaminar orientation, there is no method to directly determine local myocyte orientation from the FLASH data against which e1DTI and v1ST can be compared. This is because it is not possible for FLASH to resolve individual cardiac myocytes at 50 × 50 × 50 μm3 resolution. Therefore the putative myocyte orientation vectors e1DTI and v1ST are compared to each other. The first part of this comparison is to evaluate the basis for eigenvector assignment (in the same manner as was carried out for laminar eigenvectors in section ‘Quantification of the confidence in the sorting of laminar eigenvectors’ above. The relative magnitudes of the putative local myocyte eigenvalue are compared to the next closest eigenvalue (λ2) in Figure 12. It might be expected that there would be a stronger basis for sorting of e1DTI from the other DTI eigenvectors than in sorting v1ST from the other ST eigenvectors, as e1DTI is a primary eigenvector. This is not the case as there is greater difference between λ2ST and λ3ST than between λ2DTI and λ1DTI. The median difference between λ2ST and λ3ST is 61.1% ± 28.1%. This compares to a median difference of 23.9% ± 11.5%. As discussed above, the sets of eigenvalues which are relevant to assignment of the local myocyte orientation are not the same for DTI and ST (for DTI: λ1 and λ2, for ST: λ2 and λ3). An implication of this greater separation of the eigenvalues relevant to v1ST assignment (λ2ST and λ3ST) than the eigenvalues relevant to e1DTI assignment (λ2DTI and λ1DTI), is that misclassification of e1DTI and e2DTI is more probable than misclassification of v1ST and v2ST. There is no absolute cut-off for the median difference between eigenvalues which is acceptable to allow confidence in the DTI or ST assignment of the putative local myocyte orientation (m). The order of the eigenvalue sets from greatest difference to least difference is: (i) λ2ST with λ1ST; (ii) λ3ST with λ2ST; (iii) λ2DTI with λ1DTI; and, (iv) λ2DTI with λ3DTI.Figure 12

Bottom Line: Both FLASH (v3(ST)) and DTI (e3(DTI)) where compared to directly measured laminar arrays in the FLASH images.We show that ST analysis of FLASH is a useful and accurate tool in the measurement of cardiac microstructure.While both FLASH and the DTI approaches appear promising for mapping of the alignments of myocytes throughout myocardium, marked discrepancies between the cross myocyte anisotropies deduced from each method call for consideration of their respective limitations.

View Article: PubMed Central - PubMed

Affiliation: Inserm U1045 - Centre de Recherche Cardio-Thoracique, L'Institut de rythmologie et modélisation cardiaque LIRYC, Université de Bordeaux, PTIB - campus Xavier Arnozan, Avenue du Haut Leveque, 33604, Pessac, France. olivier.bernus@u-bordeaux.fr.

ABSTRACT

Background: Cardiovascular magnetic resonance (CMR) can through the two methods 3D FLASH and diffusion tensor imaging (DTI) give complementary information on the local orientations of cardiomyocytes and their laminar arrays.

Methods: Eight explanted rat hearts were perfused with Gd-DTPA contrast agent and fixative and imaged in a 9.4T magnet by two types of acquisition: 3D fast low angle shot (FLASH) imaging, voxels 50 × 50 × 50 μm, and 3D spin echo DTI with monopolar diffusion gradients of 3.6 ms duration at 11.5 ms separation, voxels 200 × 200 × 200 μm. The sensitivity of each approach to imaging parameters was explored.

Results: The FLASH data showed laminar alignments of voxels with high signal, in keeping with the presumed predominance of contrast in the interstices between sheetlets. It was analysed, using structure-tensor (ST) analysis, to determine the most (v1(ST)), intermediate (v2(ST)) and least (v3(ST)) extended orthogonal directions of signal continuity. The DTI data was analysed to determine the most (e1(DTI)), intermediate (e2(DTI)) and least (e3(DTI)) orthogonal eigenvectors of extent of diffusion. The correspondence between the FLASH and DTI methods was measured and appraised. The most extended direction of FLASH signal (v1(ST)) agreed well with that of diffusion (e1(DTI)) throughout the left ventricle (representative discrepancy in the septum of 13.3 ± 6.7°: median ± absolute deviation) and both were in keeping with the expected local orientations of the long-axis of cardiomyocytes. However, the orientation of the least directions of FLASH signal continuity (v3(ST)) and diffusion (e3(ST)) showed greater discrepancies of up to 27.9 ± 17.4°. Both FLASH (v3(ST)) and DTI (e3(DTI)) where compared to directly measured laminar arrays in the FLASH images. For FLASH the discrepancy between the structure-tensor calculated v3(ST) and the directly measured FLASH laminar array normal was of 9 ± 7° for the lateral wall and 7 ± 9° for the septum (median ± inter quartile range), and for DTI the discrepancy between the calculated v3(DTI) and the directly measured FLASH laminar array normal was 22 ± 14° and 61 ± 53.4°. DTI was relatively insensitive to the number of diffusion directions and to time up to 72 hours post fixation, but was moderately affected by b-value (which was scaled by modifying diffusion gradient pulse strength with fixed gradient pulse separation). Optimal DTI parameters were b = 1000 mm/s(2) and 12 diffusion directions. FLASH acquisitions were relatively insensitive to the image processing parameters explored.

Conclusions: We show that ST analysis of FLASH is a useful and accurate tool in the measurement of cardiac microstructure. While both FLASH and the DTI approaches appear promising for mapping of the alignments of myocytes throughout myocardium, marked discrepancies between the cross myocyte anisotropies deduced from each method call for consideration of their respective limitations.

No MeSH data available.


Related in: MedlinePlus