2012 Proceedings - International Tissue Elasticity Conference

081 MR ELASTOGRAPHY OF IN–VIVO AND EX–VIVO PROSTATE CANCER AND CORRELATION
WITH HISTOLOGY: INITIAL RESULTS.
Ramin S. Sahebjavaher 1,2 , Guy Nir 1 , Omid Mohareri 1 , Louis O. Gagnon 3 , Edward C. Jones 3 ,
Larry Goldenberg 3 , Philippe Garteiser 2 , Ralph Sinkus 2 , Piotr Kozlowski 3 , Septimiu E. Salcudean 1 .
1 University of British Columbia (UBC), Vancouver, BC, CANADA; 2 Centre de Recherche Biomedicale
Bichat–Beaujon, Paris, FRANCE; 3 UBC Faculty of Medicine, Vancouver, BC, CANADA.
Background: Mechanical properties of tissue are important indicators of disease potential. Viscoelastic
properties of the prostate have been shown to correlate with prostate cancer for ex–vivo specimens using MR
elastography (MRE) [1], and several in–vivo studies using ultrasound elastography techniques [2]. One feasibility
study compared results from in–vivo prostate cancer MRE with biopsy [3]. In this study, we performed MRE
in–vivo (on a 3T MRI) and ex–vivo (on a 7T MRI) for one patient. To the best of our knowledge, this is the first
time, that both in–vivo and ex–vivo MRE images are compared together and with histopathology.
Aims: To identify the cancerous tumors in the elasticity images in both in–vivo and ex–vivo cases and
correlate them to the whole mount histopathology marked with the Gleason score.
Methods: In–vivo: The MRE images were acquired using a novel gradient echo sequence named eXpresso [4].
The wave images were acquired on a 128×128×24 matrix with 2mm isotropic voxel size. Eight vibration
phases were encoded the mechanical excitation of 70Hz (trans–perineal). The entire MRE acquisition lasted
about 8 min for the 3D wave field. Images were processed offline similarly to the approach described in [5].
Ex–vivo: A conventional motion–sensitized spin–echo pulse sequence was used for MRE. The 3D wave field
was acquired in 9 slices for a 64x64 matrix with 1mm isotropic voxel of size. The mechanical excitation was
applied externally using a passive electromagnetic driver positioned in the fringe field of the scanner at 800Hz.
The total imaging time was 22 min. Histology: Following the surgery, the whole prostatectomy specimen was
immersed in formalin for 72 hours prior to ex–vivo MRE. The outline of the tumor along with its Gleason score
were delineated manually on the histology slide by an experienced pathologist.
Results: In–vivo: The peak amplitude of the mechanical wave was 130μm with a mean of 25μm in the
prostate. No patient discomfort was reported when specifically asked. In the axial plane, the prostate
gland is outlined in Figure 1(a) T2W and the reconstructed shear modulus (b) G’ and (c) loss modulus G”.
The histology is shown in (d) where the outline of the large (Gleason score of 4+3) and smaller (3+3) tumors are
shown. The mean values of G’ were {3.0 and 0.8kPa} for Gleason scores of {4+3 and healthy tissue}, respectively.
Also, the mean values for G” were {1.7 and 0.4kPa} for the same regions of interest, respectively. Ex–vivo: Similar
to the in–vivo case, the tumor can be clearly identified in Figure 1 (b) and (c). The mean values of G’ were {120
and 47.2kPa}, and G” were {146 and 57.8kPa} for Gleason scores of {4+3 and healthy tissue}, respectively.
Conclusions: A very promising correspondence between reconstructed shear and loss moduli G’ and G”,
and the matching histology slide can be observed in both in–vivo and ex–vivo. The data show differences
in shear modulus, and especially in loss modulus, between normal and cancerous tissue in the prostate.
The stiffness of the prostate dramatically increases after the fixation as described previously [6]. One of
the limitations of this initial study was registration was not performed. Also, stiffer structures within the
prostate may be mistaken for tumors. We are presently accruing patients to obtain sufficient images to
carry out a meaningful statistical analysis. These results further strengthen the case for MRE techniques
which promise to improve staging of prostate cancer.
Acknowledgements: This work was supported by NSERC, CIHR and Philips Healthcare.
References:
[1] Dresner et al.: Proc. 11th ISMRM, p. 578, 2003.
[2] Krouskop et al.: Ultrasonic Imaging, 20(4), p. 260–274, 1998.
[3] Li et al.: Acta Radiologica, 52(3), p. 354–358, 2011.
[4] Sahebjavaher et al.: Proc 23rd ESMRMB, p. 124, 2011.
[5] Sinkus et al.: PMB, 45, p. 1649, 2000.
[6] McGrath et al.: Proc. 19th ISMRM, p. 1478, 2011.
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