ASA Lay Language Papers

2nd Pan-American/Iberian Meeting on Acoustics

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Shear Wave Elastography for Detecting Blunt Force Trauma Liver Injuries

 

 

Jiao Yu -  jiaoy@u.washington.edu

Peter Kaczkowski, Lawrence Crum, and Stuart Mitchell

Center for Industrial and Medical Ultrasound, Applied Physics Lab, University of Washington

Seattle, WA 98105

 

Popular version of paper 3aBB1

Presented Wednesday morning, November 17, 2010

2nd Pan-American/Iberian Meeting on Acoustics, Cancun, Mexico

 

 

The liver, the largest organ inside the body, can be injured by falling, or an impact during a car accident or a sports related incident, due to its bulky size and relatively fixed position in the abdominal cavity. Liver injuries can be severe enough to be life-threatening because the liver has a large blood supply and capacity; fractures of the liver or tears in the major hepatic blood vessels present a serious risk for shock and even exsanguination. Currently, a fast and robust way of visualizing hepatic fractures due to blunt force trauma does not exist; hence, there is a need to develop better imaging modalities of hepatic injuries to assist in clinical assessments in an emergency room. In this study, we investigated the feasibility of using shear wave elastography for detecting fractures of liver due to blunt force trauma.

 

Shear wave elastography is a new method to image and characterize tissue structures based on the use of shear waves induced by the focused ultrasound beams inside the tissue. Shear waves are different from longitudinal waves (the waves that constitute sound). For shear waves, the motion of the medium is perpendicular to the direction the wave is traveling. Shear waves travel at a speed of 1-10 m/s, much lower than the typical longitudinal waves (1540 m/s in tissue), and shear wave speed changes uniformly with the elasticity of the local tissue region. Compared to the conventional ultrasound elastography, shear wave elastography has mainly two advantages: It is more sensitive because the shear modulus ranges over more orders of magnitude than the bulk modulus, which characterizes longitudinal elastography; it is more localized and less affected from tissue boundaries. Shear wave elastography has been found to be useful in characterizing breast lesions and assessing liver fibrosis.

 

We also expect the shear wave elastography to be useful in the detection of bleeding; for example, the shear wave can only propagate in an elastic medium and thus it cannot propagate in fluids. When the shear wave travels to the edge of a liver fracture, say, we expect to see a contrast at the boundary of the fracture, where typically blood will present. To test our hypothesis, an ultrasound beam was focused at different depths from 1 cm to 3.5 cm consecutively with a duration of 100 microseconds each to create the displacement and to initialize the motion. The motion was tracked with Doppler pulses at a PRF of 5000 Hz. We processed the data using a phase shift algorithm in real time.

 

Figure 1 displays plane shear waves created within a homogeneous PVA phantom (Fig. 1a) and are propagating in opposite directions (Fig. 1b) with no fracture in the phantom. Figure 2 displays plane shear waves created in the presence of a split (2/3 depth long from the bottom, located at 1/5 width from the left). These wave encounter a scattering effect at the split tip (Fig. 2a) and the shear wave propagating towards the left encounters a phase change when it arrives at the split edge.

 

yu01.jpg

 

Figure 1.  Plane shear waves are created within a homogeneous PVA phantom (Fig. 1a) and are propagating in opposite directions (Fig. 1b) with no split in the phantom.

 

yu02.jpg

 

Figure 2.  Plane shear waves created in the presence of a split (2/3 depth long from the bottom, located at 1/5 width from the left) encounter a scattering effect at the split tip (Fig. 2a) and the shear wave propagating towards the left encounters a phase change when it arrives at the split edge (Fig. 2b). Figure 2 provides support to our hypothesis that shear wave elastography can image liver fractures.