Ultrasound device calibration.

To the Editor:

I am writing concerning "A Calibration Study of Therapeutic Ultrasound Units" in the March 2002 issue of Physical Therapy. This article addresses a number of important areas of concern in the use of therapeutic ultrasound in physical therapy. I was pleased to see this matter addressed in a major physical therapy journal.

In the past I have noticed that the terms "ultrasonic intensity" and "power" sometimes have been used interchangeably in the physical therapy community, and this appears to be the case in this article. In addition, some of the information in this article is not correct, and this may have led to some incorrect conclusions.

First, contrary to what was stated in the article, the standards for ultrasound therapy devices do not require an ultrasonic output accuracy of [+ or -] 20% of the intensity set in the United States and of [+ or -] 30% of the intensity set in Canada and by the International Electrotechnical Commission (IEC). The current standards for ultrasonic therapy devices in the United States (1) and in Canada (2) and as set by the IEC (3) all require an accuracy of [+ or -] 20% on the indicated ultrasonic output power. The standards of the IEC and the ones used in Canada also require a [+ or -] 20% accuracy for the effective radiating area (ERA). The US standard requires only that the error in the ERA be indicated.

My second point concerns the formula described in the article for calculating the error in the power output indicator of the ultrasonic devices that were tested.

The authors stated, "The difference between the power output registered on the UW-2 [Bio-Tek Digital Ultrasound Wattmeter, model UW-2 *] (measured power output) and the intensity output indicated on the US [ultrasound] unit (indicated power output) was expressed as a percentage of error using the formula: [(measured power output--indicated intensity output)/indicated intensity output] x 100. This percentage of error was calculated at each intensity setting and recorded for each US unit tested."

The terms "ultrasonic intensity" (ultrasonic power/ERA) and "ultrasonic power" appear to be used interchangeably here; this perpetuates the use of incorrect terminology. Furthermore, the formula used for percentage of error also is incorrect.

The correct formula to determine the percentage of error in a power indicator is: [(indicated power - true power) / true power] x 100. This approach is in agreement with the Institute of Electrical and Electronics Engineers definitions. (4)

The power in this case was measured with a Bio-Tek UW-2 power meter specified as having a measurement accuracy of [+ or -] 10%. I contend, therefore, that the correct formula should include this tolerance in the measured value. This would mean that the power measured by the Bio-Tek UW-2 must differ by more than 30% from the power indicated by the ultrasound therapy device before one can conclude that the device being tested does not meet the requirements of a standard with a [+ or -] 20% tolerance on the power indicator.

I think that this article is an excellent idea to promote the calibration and safe use of ultrasound therapy equipment in physical therapy. I take issue with some of the authors' points in the interests of promoting consistent and up-to-date evaluation of ultrasound therapy device calibrations.

Robert Hussey
Senior Inspector
Acoustics Division
Consumer and Clinical Radiation
  Protection Bureau
Product Safety Directorate
Health Canada
775 Brookfield Rd
PL 6301B
Ottawa, Ontario, Canada K1A1C1

* Bio-Tek Instruments Inc, Highland Park, Box 998, Winooski, VT 05404.

References

(1) US Department of Health and Human Services, Food and Drug Administration. Ultrasonic Therapy Products. 21 CFR [section] 1050.10 (2001).

(2) Health Canada. Radiation Emitting Devices Regulations. CRC, c 1370, sch II, pt XIII, as am. SOR/84-930, ss 1-5.

(3) Particular Requirements for Ultrasonic Physiotherapy Equipment. 2nd ed. Geneva, Switzerland: International Electrotechnical Commission; 2000. International Electrotechnical Commission publication 60601-2-5 (2000-07).

(4) The New IEEE Standard Dictionary of Electrical and Electronic Terms. New York, NY: Institute of Electrical and Electronics Engineers Inc; 1992. IEEE Standard 100-1992.

Author Response:

We appreciate Mr Hussey's insightful comments regarding our article on the calibration of therapeutic ultrasound (US) devices. We thank Mr Hussey for clarifying the standards used in Canada and those used by the International Electrotechnical Commission (IEC) and for providing the updated references. Our reference to the radiation-emitting devices regulations was dated 1981, and Mr Hussey's reference was dated 1984. It is valuable information for American, Canadian, and international physical therapists, trainers, health care providers, technicians, and engineers dealing with the application and calibration of therapeutic ultrasound devices to know that the units are considered out of calibration for power output when the percentage of error of power output is beyond [+ or -] 20%. This did not affect the results of our study, as we used the 20% calibration standard for the accuracy of US emission. Ultrasound devices that displayed an output of more than 20% above or less than 20% below the measured output by the Bio-Tek Digital Ultrasound Wattmeter (model UW-2) * were considered out of calibration.

Regarding the use of the formula for calculation of the percentage of error of US output, we agree with Mr Hussey's comments and we will address each issue raised by Mr Hussey and explain the reason for using intensity instead of power in the formula in our study. Our formula needs clarification, as we used the term "indicated intensity" in the numerator of the formula to refer to the ultrasound intensity indicated on the US device, whereas we used the term "indicated intensity" in the denominator of the formula to refer to the intensity calculated from the UW-2 measurement. So, to clarify this issue, we want to take this opportunity to submit an erratum to the March 2002 publication of our article. We used the following formula to identify the US devices considered out of calibration for output (erratum: Phys Ther. 2002;82;260, lines 19 and 20--the formula should read):

[(measured UW-2 true intensity - indicated intensity) / measured UW-2 true intensity] x 100

Although the formula we used was not identical to that conveyed by Mr Hussey, we anticipate that similar results would have been found. Mr Hussey suggests using the following formula: [(indicated power - true power) / true power] x 100. The formula would lead to a correct absolute value, but the sign + or - for the error would have to be changed to obtain the true correct percentage of error measurement of power. For example, if a US machine displays a power of 10 W and the power meter measures a true power of 8 W only, according to Mr Hussey's formula, the results would be: [(10W - 8W)/8W] x 100 = +25%. This is a correct absolute value, but it does not represent a true value of the percentage of error. A more appropriate formula when using power is: [(true power-indicated power) / true power] x 100. This formula, in the present example, would lead to the following value: [(8W - 10W)/8W] x 100 = -25%.In the present example, the US machine true (measured) power output is 25% below the power output indicated by the US device.

We used for our calculations the intensity instead of the power measurements used for the calibration standard. (1,2) Hekkenberg et al (3) and Guirro et al (4) also used intensity in their evaluation of the calibration of US devices. We followed this approach for 2 reasons. The first reason was that several older US devices that we tested indicated the intensity readings only, so the power was not directly measurable without multiplying the intensity by the effective radiating area (ERA). Thus, we converted the power readings provided by the UW-2 wattmeter from watts to watts per square centimeter. This process involves a potential source of error because we were not equipped to measure the true ERA and referred to the value given by the manufacturer. We feel confident that similar results would have been found had power been used instead of intensity in our formula. Hekkenberg and associates (3) tested the calibration of 28 US devices and found 67% of them to be out of calibration for intensity and 71% of them to be out of calibration for power output.

The second reason for using intensity instead of power in the formula is that intensity has greater clinical relevance. It is not very useful for a clinician to know that a device is out of calibration for power output with a percentage of error of +50% at a power of 10 W. This means that when the device indicates 10 W, its true power output is 20 W [(20 W - 10 W) / 20 W] x 100 = +50%. Using a transducer with a 10-[cm.sup.2] ERA, a power of 10 W results in an intensity of 1 W/[cm.sup.2] and a power of 20 W results in an intensity of 2 W/[cm.sup.2]. Using the same power parameters with the same calibration error with a 5-[cm.sup.2] transducer results in intensities of 2 W/[cm.sup.2] and 4 W/[cm.sup.2], respectively. Clinically, the same power value results in intensities that are within a therapeutic range with a 10-[cm.sup.2] ERA transducer, whereas with a 5-[cm.sup.2] ERA transducer, the intensity of 4 W/[cm.sup.2] is beyond the therapeutic range, with a potential deleterious effect on tissue. (5,6) Thus, we felt that by using intensity in the equation in place of power, the information would be more pertinent to clinicians.

It might appear that we used the terms "power" and "intensity" interchangeably in our article. The reason is that the raw data were in watts for the UW-2 wattmeter (reason for reporting "power" in our original formula in the article) and in watts or watts per square centimeter for most of the US devices, and for some US devices only in watts per square centimeter (reason for reporting "intensity" in our original formula in the article). Thus, the raw data had different units, but they were all converted to watts per square centimeter (intensity) in our formula for the calculation of the percentage of error of US output.

Regarding the need to increase the tolerance in the measured power value due to the [+ or -] 10% measurement accuracy of the Bio-Tek UW-2 wattmeter, we do not concur with Mr Hussey that the power measured by the Bio-Tek UW-2 wattmeter should have differed by more than 30% from the power indicated by the US device to be considered out of calibration for power. The UW-2 instrument used in our study was calibrated just prior to the beginning of testing and was guaranteed by the manufacturer for accuracy within 10% for a period of 1 year.

When the UW-2 instrument was sent to the manufacturer for calibration 2 weeks' prior to the beginning of data collection, the manufacturer tested it at 3 different times within 1 week for measurements of reliability at a frequency of 1 MHz and powers of 0 to 20 W. In fact, the UW-2 instrument we used was found to be accurate within a 2% range and did not require any adjustments from the manufacturer. Although the UW-2 instrument was guaranteed for an accuracy of 10%, it was found to be accurate within a 2% range during its calibration testing. (7)

Our study design further enhanced the accuracy of the application of the wattmeter because one investigator (who had a perfect correlation coefficient of 1.0, with zero variance during test-retest protocol of the pilot study) took all of the measurements and because optimum conditions such as degassed water with oxygen content of less than 2 ppm was used and a clamp attached to a ring stand was used to eliminate any motion of the transducer during testing. Therefore, we do not concur with Mr Hussey that our results should be revised to allow a [+ or -] 30% error acceptance. By allowing a [+ or -] 22% error acceptance, we would have had an additional 3 machines considered within the standard for calibration. Thus, instead of 32 machines (39%) out of calibration for intensity output, only 29 machines (33%) would be considered out of calibration in our study.

Again, we thank Mr Hussey for sharing his expertise in the area of therapeutic US and taking the time to bring very interesting issues to the table in an area in need of continued research and evaluation of the calibration of the devices for the safety and benefit of the patient population.

Jean-Michel Brismee, PT, MS, OCS
Assistant Professor
Physical Therapy Program
Texas Tech University Health
  Sciences Center
Lubbock, TX 79430
([e-mail])

Steven Sawyer, PT, PhD
Associate Professor
Texas Tech University Health
  Sciences Center

Neal S Latman, PhD
Associate Professor
West Texas A&M University
Canyon, Tex

Barry P Warring PT, MPT
Physical Therapist
Wichita Falls, Tex

Chris D Willis PT, MPT
Physical Therapist
Kilgore, Tex

Paul A Artho PT, MPT
Physical Therapist
Amarillo, Tex

* Bio-Tek Instruments Inc, Highland Park, Box 998, Winooski, VT 05404.

References

(1) US Department of Health and Human Services, Food and Drug Administration, Center for Devices and Radiological Health. Performance Standards for Sonic, Infrasonic, and Ultrasonic Radiation-Emitting Products. 21 CFR 1050.10. Revised 04/01/2001. Available at: http://www.accessdata.fda.gov/ scripts/cdrh/cfdocs/cfPCD/ShowCFR.cfm?F R=1050.10.

(2) The New IEEE Standard Dictionary of Electrical and Electronic Terms. New York, NY: Institute of Electrical and Electronics Engineers Inc; 1992. IEEE Standard 100-1992.

(3) Hekkenberg RT, Oosterbaan WA, van Beekum WT. Evaluation of ultrasound therapy devices, TNO test: radiation safety and dose accuracy often leave something to be desired. Physiotherapy. 1986;72:390-395.

(4) Guirro R, Serrao F, Elias D, Bucalon AJ. Calibration of therapeutic ultrasound equipment. Physiotherapy. 1997;83:419-422.

(5) Payton OD, Lamb RL, Kasey ME. Effect of therapeutic ultrasound on bone marrow in dogs. Phys Ther. 1975;55:20-27.

(6) Kitchen SS, Partridge CJ. A review of therapeutic ultrasound: background and physiological aspects. Physiotherapy. 1990;76:593-595.

(7) Brismee JM, Latman N, Artho P, Sawyer S. Author response to letters to the editor by Ken Coffey and Denes Roveti. Phys Ther. 2002;82:617-618.