2012 Proceedings - International Tissue Elasticity Conference

061 SINGLE-HEARTBEAT 2–D MYOCARDIAL ELASTOGRAPHY USING AN UNFOCUSED TRANSMIT
SEQUENCE: AN IN VIVO FEASIBILITY STUDY.
SJ Okrasinski 1 , J Provost 1 , D Legrand 1 , EE Konofagou 1,2 .
1 Biomedical Engineering Department, 2 Radiology Department, Columbia University, 622 W 168th
St, New York, NY, 10025, USA.
Background: Myocardial Elastography (ME) is a radiofrequency (RF) based modality for myocardial strain
imaging. However, the methods previously used required spatial and temporal resolution that could be
accomplished through electrocardiogram (ECG) gating over multiple cardiac cycles and long breath–holding
times.
Aims: For this study, imaging sequences were developed and applied in an in vivo canine model based on
a virtual source sequence to image the strain in the entire left ventricle at very high frame rates (2000 fps)
during free breathing and over a single heartbeat, in both open– and closed–chest configurations.
Methods: In this study, approved by the Columbia Institutional Animal Care and Use Committee, two
male mongrel dogs were anesthetized. Transthoracic images in the short–axis, mid–level view were
acquired using a virtual source sequence with a virtual source located behind the transducer with a
Verasonics system (Verasonics, Redmond, WA) and a 64–element phased array (ATL P4–2). Images were
acquired at 2000fps during 2s, immediately followed by the acquisition of 128–line, 30fps, B–mode
frames during 1.5s. ECGs were acquired simultaneously. Their chests were then opened by lateral
thoracotomy and images using the same protocol were repeated. RF signals were reconstructed from the
element data in a pixel–wise fashion as previously reported [1]. Inter–frame axial and lateral
displacements were estimated at 500Hz motion–estimation rate and at 2000Hz motion–sampling rate
using normalized cross–correlation (window size: 7mm, 90% overlap) and were then accumulated during
systole. Axial and lateral strains were estimated using a least–squares estimator on the axial and lateral
displacements (window size: 10.7mm) and then converted to radial and circumferential strains with a
previously described principal strain method [2].
Results: The maximum axial and lateral cumulative strains during systole in the open–chest
configuration were 35±7% and 30±8%, respectively. The average radial and circumferential strains were
found to be 30±10% and -32±9% respectively, which is consistent with previous studies in normal canine
hearts using conventional beamforming [3]. In the closed–chest (transthoracic) configuration, the
maximum axial and lateral cumulative strains during systole from the open–chest canines were 33±10%
and 35±10%, respectively. The average radial and circumferential strains (see Figure) were found to be
33±11% and –36±12%, respectively.
Conclusions: Myocardial Elastography was concluded to be capable of mapping axial and lateral
displacement and strain in both transthoracic and closed–chest regimes using unfocused transmit beams
over the entire canine left ventricle in vivo at very high frame rates during free breathing in a single
heartbeat.
Acknowledgements: This study was funded in part by NIH–NHLBI R01EB006042.
References:
[1] Provost et al.: Phys. Med. Biol., 57, pp. 1095–1112. Feb 2012.
[2] Zervantonakis et al.: Phys. Med. Biol., 52, pp. 4063–4080. July 2007.
[3] Lee et al.: Phys. Med. Biol., 56, pp. 1155–1172. Feb 2011.
Figure 1: Radial and circumferential strain in a normal, open–chest canine. Strains were estimated using a virtual
source sequence with a virtual source located behind the transducer face.
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