2012 Proceedings - International Tissue Elasticity Conference

093 VISCOELASTIC STRESS RELAXATION (VISR) INDEPENDENCE FROM MUSCLE FIBER
ORIENTATION WITH LARGE FOCAL CONFIGURATION.
Mallory R. Scola 1 , Caterina M. Gallippi 1 .
1 Joint Department of Biomedical Engineering, The University of North Carolina at Chapel Hill,
Chapel Hill, NC, USA.
Background: Viscoelastic Stress Relaxation (ViSR) ultrasound is a new method for quantitatively
assessing the viscoelastic properties of tissue [1]. Using two successive acoustic radiation force (ARF)
impulses and monitoring induced displacements in the region of excitation, ViSR measures the stress
relaxation time constant in viscoelastic materials, τσ, which equals viscosity coefficient over relaxed
elastic modulus in the Voigt model. The key advantages of ViSR ultrasound are that τσ measurements are
quantitative and independent from the applied ARF magnitude, and measurements are made only in the
ARF region of excitation making it less dependent on tissue homogeneity than methods that rely on shear
wave propagation. Moreover, we hypothesize that ViSR measurements of τσ are independent of muscle
fiber orientation by using a large focal configuration.
Aims: The purpose of this work is to evaluate the impact of ARF impulse beamwidth on the variance of τσ
measurements due to fiber orientation.
Methods: Four different focal configurations were implemented and included F/1.5, F/3, F/4 and F/5.
The lateral and elevational point spread functions (PSF) of each focal configuration were characterized in
a water bath with a hydrophone. ViSR ultrasound was performed in excised porcine muscle using each
focal configuration. The angle of rotation between the transducer and the muscle fibers was varied from
0º (transducer parallel to fibers) to 90º (transducer perpendicular to fibers), with 15º steps. Imaging was
performed using a Siemens ACUSON AntaresTM imaging system specially equipped for research
purposes and a VF7–3 linear array transducer. ViSR was implemented using two 300–cycle ARF
excitations administered to the same region of excitation and separated by 0.35ms in time.
Results: As expected, increasing the F/# of the beam broadened the beam laterally and reduced the
difference between the lateral and elevational beamwidths. The measured full width at half maximum
values of the 2D PSFs at the focal depth are given in Table 1 and show that for F/5, the lateral and
elevational beamwidths are nearly equivalent. Mean ViSR measurements of τσ for the four focal
configurations across fiber angle are depicted in Figure 1. For F/1.5, τσ was smallest (0.39 ± 0.03 ms)
when the transducer was at 0º (lateral axis parallel to muscle fibers, elevational axis perpendicular to
muscle fibers) and largest (0.73 ± 0.08 ms) when the transducer was at 90º (lateral axis perpendicular to
muscle fibers, elevational axis parallel to muscle fibers), the mean τσ across all angles of rotation had a
variance of 0.012. For F/3, τσ varied from 0.44±0.05ms (0º) to 0.64±0.06ms (90º), with a variance of
mean τσ across all angles of 0.005. For F/4, τσ varied from 0.50±0.03ms (0º) to 0.60±0.04ms (90º), with a
variance of mean τσ across all angles of 0.002. For F/5, τσ varied from 0.57±0.01ms º) (0 t o 0.59±0.02ms
(15º), with a variance of mean τσ across all angles of 7×10-5.
Conclusions: Results showed a decrease in variance of mean τσ values across fiber orientation as the
lateral beamwidth approached the elevational beamwidth suggesting that ViSR is independent of fiber
orientation when using a large focal configuration.
Acknowledgements: This work was supported by National Institutes of Health Grant R01NS074057.
References:
[1] MR Scola and CM Gallippi: Multipush (MP) ARF Assessment of Viscoelastic Properties in Tissue–Mimicking
Phantoms. Proc. of the 37th Intl. Symp. on Ultra. Imag and Tiss. Charac., Arlington, VA, June 11–13, 2012.
Elevational Lateral Elevational/
F/# Beamwidth Beamwidth Lateral
F/1.5 2.87 mm 1.33 mm 2.16
F/3 2.91 mm 1.80 mm 1.62
F/4 2.84 mm 2.00 mm 1.42
F/5 2.79 mm 2.93 mm 0.95
Table 1: Elevational and Lateral Beamwidths of Focal
Configurations.
Figure 1: Stress relaxation (τσ) as a function of angle of rotation
between the transducer and muscle fibers.
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