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Variability in Effective Radiating Area at 1 MHz Affects Ultrasound Treatment Intensity

SJ Straub, PhD, ATC, is Associate Professor, Department of Physical Therapy, Quinnipiac University, 275 Mt Carmel Ave FOTRN, Hamden, CT 06489 (USA)
LD Johns, PhD, ATC, is Professor, Department of Physical Therapy, Quinnipiac University
SM Howard, PhD, is Chief Technology Officer, Onda Corporation, Sunnyvale, Calif

Address all correspondence to Dr Straub at: stephen.straub{at}quinnipiac.edu

Background and Purpose: Previous research has indicated that not all ultrasound transducers heat at equal rates; however, the cause of this disparity is unclear. Variability in spatial average intensity (SAI) has been implicated in this disparity at 3 MHz. This variability has not been explored at 1 MHz.

Methods: Sixty-six 5-cm² ultrasound transducers were purchased from 6 different manufacturers. Transducers were calibrated and assessed for effective radiating area (ERA), total output power, and SAI using standardized measurement techniques.

Results: Total output power values fell within US Food and Drug Administration guidelines, but there were large variations in ERA. The resulting SAI values showed large deviations (–43% to +61%) from the digitally displayed value. Intra-manufacturer SAI values varied up to 53%.

Discussion and Conclusion: Spatial average intensity can vary largely from the values displayed on these ultrasound generators; in a calibrated cohort, this difference is primarily attributable to differences in measured ERA. Patterns of SAI variability within the manufacturer at 1 MHz do not follow previous reports of variability at 3 MHz.