Variability in effective radiating area and output power of new ultrasound transducers at 3 MHz.

CONTEXT

Spatial average intensity (SAI) is often used by clinicians to gauge therapeutic ultrasound dosage, yet SAI measures are not directly regulated by US Food and Drug Administration (FDA) standards. Current FDA guidelines permit a possible 50% to 150% minimum to maximum range of SAI values, potentially contributing to variability in clinical outcomes.

OBJECTIVE

To measure clinical values that describe ultrasound transducers and to determine the degree of intramanufacturer and intermanufacturer variability in effective radiating area, power, and SAI when the transducer is functioning at 3 MHz.

DESIGN

A descriptive and interferential approach was taken to this quasi-experimental design.

SETTING

Measurement laboratory.

PATIENTS OR OTHER PARTICIPANTS

Sixty-six 5-cm(2) ultrasound transducers were purchased from 6 different manufacturers.

INTERVENTION(S): All transducers were calibrated and then assessed using standardized measurement techniques; SAI was normalized to account for variability in effective radiating area, resulting in an nSAI.

MAIN OUTCOME MEASURE(S): Effective radiating area, power, and nSAI.

RESULTS

All manufacturers with the exception of Omnisound (P = .534) showed a difference between the reported and measured effective radiating area values (P < .001). All transducers were within FDA guidelines for power (+/-20%). Chattanooga (0.85 +/- 0.05 W/cm(2)) had a lower nSAI (P < .05) than all other manufacturers functioning at 3 MHz. Intramanufacturer variability in SAI ranged from 16% to 35%, and intermanufacturer variability ranged from 22% to 61%.

CONCLUSIONS

Clinicians should consider treatment values of each individual transducer, regardless of the manufacturer. In addition, clinicians should scrutinize the power calibration and recalibration record of the transducer and adjust clinical settings as needed for the desired level of heating. Our data may aid in explaining the reported heating differences among transducers from different manufacturers. Stricter FDA standards regarding effective radiating area and total power are needed, and standards regulating SAI should be established.