On factors impacting subharmonic aided pressure estimation (SHAPE)

Subharmonic aided pressure estimation (SHAPE) uses ultrasound contrast agents (UCAs) to estimate hydrostatic pressure by transmitting at one frequency, receiving at its subharmonic frequency and then monitoring the subharmonic amplitude variations. The subharmonic response of the UCAs has an inverse linear relationship with the ambient pressure. In order to optimize SHAPE, we studied the impact of varying input acoustic output (IAO), UCA concentration and hematocrit concentrations. The current IAO selection algorithm is prone to noise, due to motion. A modified Logiq 9 ultrasound scanner with a 4C curvi-linear probe (GE, Milwaukee, WI) was used to acquire SHAPE data from Sonazoid (GE Healthcare, Oslo, Norway) transmitting at 2.5 MHz and receiving subharmonic signals at 1.25 MHz. The new selection algorithm provided improved or similar correlation between the SHAPE signal and hydrostatic pressure for all setups (r ranging from -0.85 to -0.95 vs -0.39 to -0.98). Also, it is important to study the effect of hematocrit, since the clinical population presents with a wide range of values. Additionally, the effect on SHAPE of varying UCA concentration was studied in vitro. The reduction in subharmonic amplitude as the pressure increased from 10 to 40 mmHg remained almost the same (Δ0.00-0.01 dB, p=0.18) with no significant change as the hematocrit concentration was tripled (from 1.8 to 4.5 ml/l). Likewise, the SHAPE gradient changed only slightly (Δ 0.02-0.05 dB, p=0.75) as the UCA concentration was increased from 0.2 to 1.2 ml/l. In conclusion the relative change in the subharmonic signal is independent of hematocrit and UCA concentration. An improved algorithm for identifying optimum IAO levels correctly should make SHAPE more sensitive and accurate.