Experimental performance comparison between CMUT and piezoelectric probes in measuring backscatter anisotropy

Quantitative ultrasound (QUS) techniques based on the parameterization of the backscatter coefficient (BSC) generally assume that the tissue under consideration is homogeneous and isotropic. However, tissues such as flowing blood aggregates and biceps muscles exhibit anisotropy in the BSC and/or attenuation coefficient. Recent works have proposed the measurement of anisotropic QUS parameters by using the reference phantom method when performing conventional imaging modalities, i.e. beamsteering or plane wave, with linear array transducers. One limitation remains the maximal angle to which the BSC can be measured, which is determined by the probe steering capability that depends on element directivity (array design) and crosstalk (transducer technology). The aim of this work is to compare the performance of a Capacitive Micromachined Ultrasonic Transducer (CMUT) and a piezoelectric probe in measuring backscatter anisotropy. Ultrasonic experiments were conducted on two comparable probes, i.e. the commercial piezoelectric probe LA435 (ESAOTE, Florence, Italy) (f
_c
=12.5 MHz, 80% FBW, pitch=0.2 mm), and the HF3 CMUT prototype probe (f
_c
=12.5 MHz, 100% FBW, pitch=0.2 mm), using the ULA-OP open ultrasonic system. In vitro measurements of the BSC using the focused beam steering imaging strategy were performed on an anisotropic tissue-mimicking phantom at different insonification angles ranging from 0° to 20°. The results show how the limited steering capabilities of linear probes could affect the measurement of BSC, and in general the anisotropic QUS parameters, bringing into discussion their consideration in the development of experimental strategies for the assessment of tissue anisotropy.