Shear wave Dispersion Ultrasound Vibrometry (SDUV) on an ultrasound system: In vivo measurement of liver viscoelasticity in healthy animals

Shear wave Dispersion Ultrasound Vibrometry (SDUV), is an acoustic radiation force based technique that measures tissue shear viscoelasticity by characterizing shear wave speed dispersion. One of the potential applications of this technique is the non-invasive measurement of liver viscoelasticity to stage liver fibrosis. In this study, a Philips' iU22 ultrasound system was modified to allow for the same transducer to generate and track propagating shear waves. The pulse sequence was implemented on a C5–1 curvilinear transducer, with RF data acquisition and off-line processing. Liver scans of three healthy pigs were conducted under in vivo and ex vivo conditions. In vivo data were captured during inspiration or expiration of the breathing cycle with breath hold. Shear modulus and viscosity were estimated in a 4 × 4 mm² area close to the pushing focus. For pig #3, the in vivo measurement revealed a shear modulus µ<inf>1</inf> = 2.64 ± 0.48 kPa and a shear viscosity µ<inf>2</inf> = 1.09 ± 0.30 Pa·s. The ex vivo measurement made directly on the exposed liver after euthanasia resulted in µ<inf>1</inf> = 2.63 ± 0.39 kPa and µ<inf>2</inf> = 0.98 ± 0.18 Pa·s. For pigs #1 and #2, the in vivo results are µ<inf>1</inf> = 2.41 ± 0.65 kPa, µ<inf>2</inf> = 2.01 ± 1.00 Pa·s; and µ<inf>1</inf> = 1.74 ± 0.56 kPa, µ<inf>2</inf> = 1.25 ± 0.87 Pa·s. Their corresponding ex vivo results are µ<inf>1</inf> = 2.87 ± 0.32 kPa, µ<inf>2</inf> = 1.06 ± 0.18 Pa·s; and µ<inf>1</inf> = 1.84 ± 0.33 kPa, µ<inf>2</inf> = 0.77 ± 0.15 Pa·s, respectively. Higher variance in in vivo measurements of pigs #1 and 2 may be attributed to phase aberration as these two animals had thicker abdominal fat and muscle layers than pig #3. To study phase aberration effects, SDUV measurements were made subsequently on the ex vivo liver under different conditions: from intact to sequentially removing the overlying tissue layers until exposing liver. Results from this study demonstrate the feasibility of conducting SDUV measurement using an ultrasound system and a single transducer. The reconstructed viscoelasticity for normal porcine liver agrees well with the values reported in the literature. Further in vivo evaluation in animals and humans is required to validate these results.