Hydrophone Spatial Averaging Artifacts for ARFI Beams from Array Transducers.

Current clinical ultrasound systems rely on pressure measurements to calculate safety indices like Mechanical Index (MI) and Thermal Index (TI). These indices are essential for monitoring patient exposure during procedures. However, ARFI beams from linear and phased arrays differ from circularly-symmetric beams, for which correction methods exist. No correction method is available for rectangularly-symmetric ARFI beams, leading to uncorrected pressure measurements. This creates a risk of inaccurate MI and TI values. Prior research has shown that hydrophone spatial averaging affects pressure readings, but its impact on ARFI beams remains understudied. This gap motivated an investigation into how hydrophone size influences ARFI pressure measurement errors. Understanding this relationship is critical for improving safety index accuracy in clinical settings.

This study aimed to quantify how hydrophone spatial averaging affects ARFI pressure measurements from clinical array transducers. ARFI beams differ from traditional beams due to their rectangular symmetry, which complicates correction methods. The researchers sought to determine if current uncorrected measurements lead to significant errors in pressure parameters like peak compressional pressure (p_c), peak rarefactional pressure (p_r), and pulse intensity integral (pii). By measuring these parameters across various hydrophone sizes, the study aimed to assess the magnitude of spatial averaging errors. The goal was to establish whether typical membrane hydrophones introduce substantial underestimation of ARFI pressures. This could help in developing correction strategies for clinical use. The study also aimed to evaluate how these errors affect MI and TI calculations. The findings could inform better safety index reporting during ultrasound procedures.

The researchers used three clinical linear array transducers to generate ARFI beams. They measured output pressure waveforms using five hydrophones with sensitive element sizes ranging from 85 to 1000 micrometers. The hydrophones included typical membrane types with sizes around 500 micrometers. The study focused on ARFI beams with a frequency of 2.25 MHz and an F/# of 1.5. The team analyzed how spatial averaging errors changed with hydrophone size. They compared peak compressional, peak rarefactional pressures, and pulse intensity integrals across all hydrophones. The measurements were conducted under controlled conditions to isolate spatial averaging effects. The study did not introduce new correction methods but evaluated existing ones for rectangularly-symmetric beams. The results were used to estimate average errors in pressure parameters.

The study found that spatial averaging errors increased with hydrophone sensitive element size. At 500 micrometers, typical for membrane hydrophones, the average errors were approximately -20% for peak compressional pressure (p_c), -10% for peak rarefactional pressure (p_r), and -25% for pulse intensity integral (pii). These errors suggest significant underestimation of ARFI pressure measurements. The largest errors occurred in p_c and pii, which are critical for calculating MI and TI. The smallest hydrophone (85 micrometers) showed the least error, indicating that smaller elements reduce spatial averaging effects. The results were consistent across all three transducers tested. The findings highlight the limitations of current uncorrected pressure measurements in clinical settings. The study did not find evidence that correction methods for circularly-symmetric beams apply to rectangularly-symmetric ARFI beams.

The authors conclude that spatial averaging errors from typical membrane hydrophones lead to significant underestimation of ARFI pressure parameters. These errors can compromise the accuracy of Mechanical Index (MI) and Thermal Index (TI) calculations used in clinical ultrasound. The study shows that current correction methods are not applicable to rectangularly-symmetric ARFI beams from linear and phased arrays. The findings suggest that pressure measurements from these transducers may not reflect true values without correction. The researchers emphasize the need for new correction strategies tailored to ARFI beams. They propose that uncorrected measurements may lead to unsafe exposure levels if MI and TI are used as primary safety indicators. The results support the development of hydrophones with smaller sensitive elements to reduce spatial averaging errors. The study does not propose new correction methods but highlights the limitations of current approaches.