2012 Proceedings - International Tissue Elasticity Conference

066 COMBINED ULTRASOUND ELASTICITY AND THERMAL STRAIN IMAGING FOR
COMPREHENSIVE ASSESSMENT OF ATHEROSCLEROTIC PLAQUE IN A RABBIT MODEL.
A Mahmoud 1,2 , D Dutta 1 , L Lavery 1 , K Kim 1,3,4 – Presented by JM Rubin .
1 Center for Ultrasound Molecular Imaging and Therapeutics, Heart and Vascular Institute,
University of Pittsburgh Medical Center, Pittsburgh, PA, USA; 2 Bioengineering Department, Cairo
University, Giza, EGYPT; 3 Bioengineering Department, University of Pittsburgh, Pittsburgh, PA,
USA; 4 McGowan Institute for Regenerative Medicine, University of Pittsburgh, Pittsburgh, PA, USA.
Background: Atherosclerosis imposes both mechanical and compositional changes in the vascular wall
leading to the formation of atherosclerotic plaque (AP) [1]. We hypothesize that ultrasound (US) elasticity
imaging (UEI) is able to assess the vascular wall mechanical property changes via measuring local strain
distributions, while US thermal strain imaging (TSI) can identify lipids in the vascular wall, and combining
these two techniques along with US B–mode morphology imaging can therefore provide more comprehensive
information for diagnosis and monitoring of atherosclerosis development and AP vulnerability.
Aims: The aims of this study are: 1) demonstrate in vivo feasibility of the combined UEI–TSI technique
assessing the mechanical and compositional characteristics of AP using atherosclerotic rabbit model, and
2) correlate the UEI–TSI findings with histology.
Methods: AP was induced to a group of New Zealand white rabbits (n=6) by introducing a balloon injury
to the femoral artery while administering high–fat high–cholesterol diet. Optimal US beamforming,
imaging–heating sequence, heating US transducer and electronics were designed and integrated to
commercial US scanners including a clinical system (SonixTOUCH, Ultrasonix Medical Corp., Canada) and a
high–resolution small animal imaging system (Vevo2100, VisualSonics Inc., Canada). US duplex was used to
localize atherosclerotic lesions. US radiofrequency (RF) frames were acquired for approximately 2 seconds for
UEI, followed by 8 second RF capturing for US–TSI while heating transducer was operated to induce a slight
temperature rise ( ≤2ºC) within the safe range according to the American Institute of Ultrasound in Medicine.
ECG was used to trigger TSI frame acquisition per the QRS complex and assure eliminating the mechanical
strain effect. A 2D phase-sensitive speckle tracking was applied to estimate mechanical strains for UEI due to
inter–cardiac pulsation and thermal strains for TSI due to sound speed changes by temperature rise. UEI and
TSI strain maps, co–registered to B–mode images, were compared to Oil–Red–O histology.
Results: Lipid–rich lesions exhibited average total compressional strain of approximately -0.44% over the
systolic phase, higher than that observed in normal areas in vascular wall. That correlates well with the
known elastic characteristics of soft lipid–rich lesions. Whereas in TSI, lipids showed a distinct positive
strains (≈+0.18%) in AP upon the slight temperature increase, and negative, or zero, strains in th e
surrounding water–based tissues. UEI and TSI findings showed good agreement with histology.
Conclusions: The combined UEI–TSI imaging technique was able to assess successfully both mechanical
and compositional characteristics of AP in the femoral artery of atherosclerotic rabbit model in vivo. The
co–registered complete set of morphological, mechanical, and compositional information obtained by the
UEI–TSI imaging correlated well with histological findings. This combined UEI–TSI can be relatively easily
implemented into commercial US systems with minimal modification to complement other modalities in
diagnosing and assessing AP vulnerability in clinics in the future.
Acknowledgements: This study was supported in part by NIH 1R01HL098230–01A1 (PI: Kim). Small animal imaging
system (Vevo2100) was supported through NIH 1S10RR027383–01 (PI: Kim).
References:
[1] Virmani R, Narula J, Leon MB, Willerson JT: The Vulnerable Atherosclerotic Plaque: Strategies for Diagnosis and
Management. First edition, Wiley–Blackwell, 2006.
Figure 1: Ultrasound in vivo images including Doppler, UEI and TSI compared well with Oil–Red–O histology in assessing
atherosclerotic plaque (red arrows) developed in rabbit femoral artery 10 weeks after balloon injury.
indicates Presenter 109