David Lohr1, Maxim Terekhov1, Andreas Max Weng2, Anja Schroeder3,4, Heike Walles3,4, and Laura Maria Schreiber1
1Comprehensive Heart Failure Center, University Hospital Wuerzburg, Wuerzburg, Germany, 2Department of Diagnostic and Interventional Radiology, University Hospital of Wuerzburg, Wuerzburg, Germany, 3Department Tissue Engineering and Regenerative Medicine, University Hospital Wuerzburg, Wuerzburg, Germany, 4Translational Center Wuerzburg 'Regenerative therapies', Wuerzburg branch of the Fraunhofer IGB, Wuerzburg, Germany
Synopsis
A whole heart,
high resolution diffusion tensor data set with 1.3 mm isotropic voxels was
acquired in 15 ex vivo pig hearts using a Stejskal-Tanner sequence at 3T. ADC,
FA and HA values were calculated and analyzed for the whole heart. Purpose was
to create a reliable statistical reference of diffusion parameters. Sharp modes
for median and interquartile range of the ADC and median and mean values of the
helix angle indicate similar distributions of those values for the individual
hearts. This provides a statistically compelling reference for future cardiac
DTI pig studies in vivo and at higher field strengths.
Introduction
Diffusion MRI
provides information on the structural and functional integrity of tissue.
Measuring directional parameters and fiber tracking requires both high spatial
resolution and SNR, which is difficult to provide simultaneously at 1.5T and
even 3T. Going to higher field strengths has shown to provide improved
diffusion imaging quality in neurological DTI. However higher field strengths
also lead to increased B0 inhomogeneity and therefore signal loss,
distortions in the EPI readout, and reduced available diffusion encoding time.
We have recently shown that B0 inhomogeneity leads to essential bias
in diffusion parameters measured with a stimulated echo sequence. Purpose of
this study was to create a reliable statistical reference of diffusion parameters
of the ex vivo pig heart based on diffusion images with high 3D spatial
resolution and isotropic voxels for validation of future measurements in vivo
and at ultra high field strengths.Methods
All
measurements were performed on a 3T whole-body scanner (Siemens MAGNETOM
Prisma, Erlangen, Germany) with a four channel head coil. Fifteen ex vivo pig
hearts were placed in a 0.9% sodium chloride solution and imaged within 10
hours after excision. A Stejskal-Tanner preparation was used to cover the whole
organ with an isotropic resolution of 1.3x1.3x1.3mm3. Further
measurement parameters were TE: 55ms, TR: 15000ms (65
slices without gaps), matrix size: 84x128, FOV: 170mm2 and 5/8
partial-Fourier acceleration. A set of 30 diffusion directions after Skare1
and 5 b0 images were acquired in a total scan time of 9:00 minutes.
All post processing was done using DSI studio2 and MATLAB (MathWorks,
Natick, USA). All hearts were segmented according to the American Heart
Association (AHA) model3 defining the apical cap by papillary
muscle rather than left ventricular lumen. Manual segmentation of the
myocardium was done using directional information of the main eigenvector of
diffusion (e1). ADC, FA and helix angle4 (HA) values were
analyzed for all slices and the 17 segments of all hearts.Results
Figure 1 shows
an example of the HA distribution mapped to the shown fibers reconstructed by
tractography. On average 7±2
slices of apical cap, 9±1
apical, 9±1
mid-cavity, and 9±1
basal slices were analyzed for each heart. Figure 2 shows the distribution of the
median (2a) and interquartile range (IQR) (2b) of ADC values for the left ventricle
of all 15 hearts. The highest count for the median ADC value was found at 0.5x10-3
mm2/s. The IQR histogram has a sharp mode at 0.1x10-3
mm2/s. Histograms for the median and mean HA distributions
over the left ventricle of all hearts (Figure 3) also display sharp modes with
a difference of 1% for a helix angle range of 120°. Figure 4 shows mean FA
values (4a) and the mean HA range from epicardium to endocardium (4b) of all
hearts mapped to the 17 segments of the AHA model. HA values span a
range of about 120° to 140°. The mean FA value varies only slightly (~0.05)
between apical and basal segments. In Figure 5 the components of the main
eigenvector of diffusion (e1) are shown for each of the 17 segments.
A negative correlation between the contributions of e1(1) and e1(2)
to the fiber direction was observed. The overall directional contribution of e1(1)
and e1(2) is larger than the one of e1(3).Discussion
Sharp modes
for median and IQR of the ADC value indicate a similar distribution of those
for the individual hearts and allows the generation of a stable statistical reference.
The same applies to median and mean values of the helix angle. Values of about 120°-140°
and a mean span of 128° over all segments for the helix angle is in agreement
with the value of 120° found in literature5. The slight skew in the
direction of positive values could be a result of the manual segmentation. The negative
correlation of e1(1) and e1(2) values and therefore the
variation of fiber orientation for different segments corresponds to the morphology
of the left ventricle. Lower values for e1(3) are expected, since
papillary muscles were excluded in the segmentation and fiber orientation in
the mid-myocardium is circumferential.Conclusion
Conventional
spatial resolution used for in vivo measurements5 at 3T ranges from
2x2x4 mm3 to 2.7x.2.7x8 mm3. With spatial resolution
of 1.3x1.3x1.3 mm3 the analyzed data set has significantly higher
resolution of both the helix angle and diffusion tensor parameters. This should
provide a statistically sound reference for cardiac DTI measurements in
vivo at 3T and both ex vivo and in vivo at higher field strengths such as 7T,
where higher SNR allows for significantly increased spatial resolution.Acknowledgements
We acknowledge financial support of German
Ministry of Education and Research (BMBF, grants: 01EO1004, 01E1O1504).
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