GB2323496A - Measurement of effective radiating area of an ultrasonic transducer - Google Patents

Abstract

A system for measuring the effective radiating area of an ultrasonic transducer includes an ultrasonic mask 10 which is provided with a plurality of spatial ultrasonic filters 12 of different see and/or shape. The mask 10 is intended to be placed in an anaechoic chamber between the transducer to be measured and a sensor. The mask is made from an acoustically absorbing material and is moved during a test so as to present varying sizes and/or shapes of apertures through the mask to detect transmission by a selected portion of the transducer. In this manner, "hot spots" and other transmission anomalies can be detected.

Description

Measurement of effective radiating area of a transducer The present invention relates to a system and method for measuring the effective radiating area of a transducer of the type used, for example, in physiotherapy treatment heads.

Physiotherapy ultrasound is widely used for the treatment of muscular injuries, where ultrasonic power is applied to tissue to accelerate healing. In order that tissue damage be avoided, a requirement exists that the effective acoustic intensity of a physiotherapy system must be less than 3 W cam~2. Because it is too difficult to determine this parameter directly with any degree of accuracy, the effective intensity is derived as the quotient of the ultrasonic power and an area, known as the effective radiating area (ERA), over which the majority of the ultrasonic power is distributed.

In recent years, considerable progress has been made in standardising test methods for determining ERA and this has been made possible by an EC collaborative project aimed at validating protocols. These have recently been embodied in a new International Standard IEC 1689 which specifies the use of miniature hydrophones to derive the ERA through a spatial mapping of the acoustic pressure distribution. This requires a relatively sophisticated computerised beam-plotting and associated acquisition and signal analysis system.

As a result of this, measurement of ERA, which is clearly important for equipment acceptance testing as well as testing on a routine basis, is beyond the capabilities of the majority of Hospital Physics Departments. Even for manufacturers, scanning in this way is exceptionally time-consuming, adding significantly to the product cost, and can only realistically be applied on a typetesting or batch sampling basis, a testing arrangement which is permissible under IEC 1689. Treatment heads of low ERA which fall through the manufacturers testing net will produce higher values of the effective intensity thus increasing the risk of thermal injury to the patient. There is therefore a well recognised need for an ERA measurement method which is rapid, reliable and can easily be applied on a routine testing basis.

The preferred embodiment of this invention seeks to provide a method which is based fundamentally on measuring the total ultrasonic output power of a physiotherapy treatment head using, for example, a radiation force balance. The strength of this approach lies in the fact that good quality commercial radiation force balances are now available. Their use has become widespread throughout hospitals as they are relatively quick and easy to use.

According to an aspect of the present invention, there is provided an ultrasonic mask for use in measuring the output ultrasonic beam characteristics of a treatment head. This mask forms a spatial ultrasonic filter consisting of a body member in which one or more spatial ultrasonic filters are provided, at least one being of a size smaller than the radiating surface of the treatment head under test.

According to another aspect of the present invention, there is provided a spatial ultrasonic filter which can be applied to the output ultrasonic beam of a physiotherapy treatment head which does not adversely perturb the output ultrasonic beam characteristics to a significant degree.

The body member defining the area of the spatial ultrasonic filter consists of a material with high ultrasonic attenuation and low ultrasonic reflection.

The spatial ultrasonic filter can be positioned at different distances from the radiating surface of the physiotherapy treatment head.

The defined shape and/or area of the spatial ultrasonic filter is preferably variable.

The spatial ultrasonic filter can be used to determine the area of the output ultrasonic beam at a specified distance from the face of the ultrasonic treatment head through which a defined percentage of the total ultrasonic power is being transmitted.

According to another aspect of the present invention, there is provided a method of measuring the radiation characteristics of a treatment head including the step of filtering radiation from the treatment head to allow only a selected portion thereof to be sensed by a detector.

The principle behind the preferred measurement method involves gradually reducing the effective radiating aperture of the treatment head under test until the acoustic power passing through the spatial ultrasonic filter, as determined using a therapy-level force balance, falls below a specified level e.g. 75%. In the preferred embodiment, this gradual reduction is effected by inserting collimating masks in front of the treatment head which consists of spatial ultrasonic filters covering the diameter range 5 mm to 30 mm. The ERA can then be derived from a knowledge of the diameter of the spatial ultrasonic filter which produces the specified reduction in total ultrasonic power.

The preferred embodiment can provide a measurement technique in its own right for determining the effective radiating area; and a rapid end-of-production-line monitoring method for determining the quality of treatment head manufacture, one possible implementation being to quickly test whether a particular physiotherapy treatment head comes within a pre-determined acceptance band, using a limited number of spatial ultrasonic filters.

This method can be a rapid, relatively low-cost, means of measuring treatment head ERA which will be applicable in both manufacturing and hospital environments.

An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a plan view of an embodiment of collimating mask; Figure 2 is a cross-sectional view of the body member forming the spatial ultrasonic filter taken along line 2-2 of Figure 1; and Figures 3 to 12 are graphs showing the treatment head radiation following collimation by the spatial ultrasonic filter.

Referring to Figure 1, the example of collimating spatial ultrasonic filter 10 shown is in the form of a disc suitable for location in an anechoic chamber of round axial cross-section. The collimating spatial ultrasonic filter 10 is made of a non-reflective material described in detail below. Within the disc 10 there is provided a plurality of spatial ultrasonic filters 12 of varying diameter. In the preferred embodiment, the diameter of the largest spatial ultrasonic filter 12 must be sufficiently greater than the geometrical diameter of the physiotherapy treatment head to enable the total ultrasonic power of the physiotherapy treatment head to be measured. The smallest spatial ultrasonic filter 12 is intended to cover the smallest area of the physiotherapy treatment head which it is desired to test.

Referring to Figure 2, the preferred embodiment of spatial ultrasonic filter 10 is formed of two layers 14, 16 of acoustically absorbing material, described in detail below. The layer 16, intended to be the more remote layer relative to a treatment head under test, is provided with an array of scatter cones 18 to prevent reflection of radiation back into the test chamber between the spatial ultrasonic filter 10 and treatment head under test. The scatter cones 18 may not be necessary in some embodiments.

The disc 10 is rotatable about an axis thereof (not shown). In use, it is located in an anechoic chamber in which a treatment head to be tested is placed. The spatial ultrasonic filter 10 is located between the treatment head and a radiation sensor. The location of the treatment head is such that each spatial ultrasonic filter 12 is, in this embodiment, locatable such that the geometrical centre of each spatial ultrasonic filter is coaxial with the geometrical centre of the physiotherapy treatment head, and typically separated from it by a distance of a few mm. As the disc 10 is incrementally rotated, for example from the largest aperture 12 towards the smallest aperture 12, successive annuli are excluded from measurement. Thus, measurements of ultrasonic power are made as a function of spatial ultrasonic filter diameter, one in respect of each spatial ultrasonic filter 12.

The difference in the magnitude of a power measurement made using one spatial ultrasonic filter compared to that made using an adjacent spatial ultrasonic filter represents the power contained in an annulus whose area is given by the difference in areas of the two spatial ultrasonic filters. Thus, a pattern of annular radiation characteristics can be generated for the physiotherapy treatment head (see Figures 3-12).

The distribution of power in the output ultrasonic beam from the physiotherapy treatment head may also be investigated using spatial ultrasonic filters of different structure. An example would be an aperture in the form of a slot which is rotated about the centre point of the treatment head. A combination of annular and radial patterns may provide information about the output ultrasonic beam produced by the physiotherapy treatment head.

SPATIAL ULTRASONIC FILTER The preferred spatial ultrasonic filter is manufactured from two types of special polyurethane based acoustically absorbing material (high attenuation material or HAM) referred to as HAM A and HAM B. The respective properties of the preferred HAMs are as follows: HAM A is a low front face reflection material comprising two layers.

The front layer (which faces the treatment head during spatial ultrasonic filter measurements) is a base polyurethane with an acoustic impedance well matched to that of water, the reflection loss at 3 MHz being approximately -40 dB. The second layer comprises an Expancel (air-filled micro-balloons) filled material of high transmission loss. The interface of the two materials consists of a pyramidal scattering structure 18. The one-way transmission loss of HAM A at 1 Mllz has been measured at 25 dB, the overall thickness of the HAM being 14 mm.

HAM B again consists of two layers and is designed specifically to produce an intermediate front face reflection coupled with a very high transmission loss. The reflection loss at 3 MHz is -17 dB, the transmission loss at 1 MHz being 43 dB. The overall thickness of the HAM is 11 mm.

The use of two different HAMs was employed specifically to investigate the influence of reflections from the spatial ultrasonic filter 10 on the treatment head radiation conductance (and therefore power output).

Concerning the manufacture of the preferred spatial ultrasonic filter 10: These were manufactured using a high speed router. Nominal spatial ultrasonic filter apertures manufactured were: 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 20, 21, 22, 24, 26 and 30 mm. At 20 and 22 mm HAM A and HAM B spatial ultrasonic filters were manufactured.

Actual spatial ultrasonic filter diameters were determined using a micrometer. Average measured diameters were generally within 2% to 3 % of the nominal values, although some spatial ultrasonic filters differed by up to 5 % . For all spatial ultrasonic filters, asymmetry in the spatial ultrasonic filter aperture was less than +1.5% (semi-range).

ULTRASOUND PHYSIOTHERAPY SYSTEMS The two ultrasound systems and eight treatment heads on which measurements were carried out are shown in Table 1. Particular points of note regarding Table 1 are as follows: The EMS system was a dual frequency system, with two different sized treatment heads each being able to operate at both 1 and 3 MHz; in addition to the treatment heads identified above, spatial ultrasonic filter measurements were made on a defective Sonacel 1.5 MHz treatment head which was known to exhibit a local high intensity acoustic "hot-spot".

Table 1 Tabulation of the ultrasound physiotherapy systems and treatment heads used in this study.

System Treatment head Frequency Nominal identifier (MHz) diameter (mm) Electro-Medical 40591 Crystal 2 1 26 Supplies (EMS) Therasonic 1032 40591 Crystal 2 3 26 Model 50 Serial number: 36783 1 13 37381(11/88) 36783 3 13 Sonacel Kept with unit 0.75 26 Multiphon Plus 1 ring) (JPM Products) Kept with unit 1.5 26 2 rings) Serial number: RT-2945 3 26 4040 (9/89) Kept with unit 3 10 (probe) BEAM-PLOTTING MEASUREMENTS Measurements on each of the treatment heads were undertaken using hydrophone scanning to the Committee Draft (August 1994) of IEC 1689. The specific requirements of IEC 1689 were followed, principally the need to undertake measurements of the beam cross-sectional area (BCSA) at various distances from the treatment head. Additionally, beam plots and raster scans were undertaken close to the face of the treatment head (separation < 5 mm) to aid in the validation of the spatial ultrasonic filter derived results.

SPATIAL ULTRASONIC FILTER MEASUREMENT PROTOCOL After initial investigations, a measurement protocol is identified, the important points of which are as follows: Generally, for large treatment heads, measurements are undertaken at a nominal power setting of 5 watts; for the smaller treatment heads the power is set to its maximum value which is typically 1 to 2 watts; measurements of power with the spatial ultrasonic filters in place are preferably derived after 5 seconds, thus minimizing the heating of the spatial ultrasonic filter surface; in setting up, it is important that the cone, spatial ultrasonic filter aperture and treatment head are co-axial. In particular, the misalignment of the cone below the spatial ultrasonic filter can give rise to errors in the measurements. These two are aligned first, by eye, and then the treatment head positioned centrally over the spatial ultrasonic filter aperture using markings made on the spatial ultrasonic filter; for measurements, the position of the physiotherapy treatment head should be altered (moving in a plane parallel to the face of the spatial ultrasonic filter) in order that the measured power determined for that spatial ultrasonic filter is maximised. This is to allow for situations in which the centre of the output ultrasonic beam is not coaxial with the geometrical centre of the physiotherapy treatment head.

SPATIAL ULTRASONIC FILTER RESULTS ANALYSIS (RADIATION FORCE BALANCE) The data derived in experiments consisted of a set of powers corresponding to each of the spatial ultrasonic filter apertures. Deriving the 75 % beam crosssectional area requires some analysis, but which basically consists of the following steps: by subtracting the powers derived from adjacent size spatial ultrasonic filters, and dividing this by the corresponding annular area, it is possible to derive an averaged intensity over the annulus; these intensity values are placed in descending order (with regard to intensity) and are tabulated along with the corresponding areas and power contributions of the annuli; starting from the maximum intensity value, a cumulative sum of powers and corresponding areas is calculated.

This method of analysis is consistent with that used in IEC 1689 to analyse raster-scan data. The area corresponding to 75 % of the total summed power then represents the minimum area which contains 75 % of the radiated power. The spatial ultrasonic filter derived beam cross-sectional area is then multiplied by 1.33333 to derive the ERA of the treatment head.

RESULTS - SPATIAL ULTRASONIC FILTER MEASUREMENTS Only a selection of measurements will be presented here. It should be noted that for the treatment heads and physiotherapy systems studied here there was no difference between measurements made using the two different types of HAM spatial ultrasonic filter. Therefore, in the following presentation of results, no differentiation is made between the two sets of results.

DEPENDENCE ON POWER Figure 3 shows measurements made on a treatment head giving the power normalised to the uncollimated case for applied powers of 2 W and 5 W. The results are independent of applied power, indicating that, subject to sensitivity of the force balance, measurements may be made at any power. Effective radiating area values derived from these two data sets were 330 mm2 and 350 mm2 for the 5 W and 2 W measurements respectively.

SPECIFIC ANALYSIS EXAMPLE The objective in this section is to illustrate the series of steps used to derive the effective radiating area for one specific treatment head: the large diameter 3 MHz Sonacel. Figure 4 shows the variation of measured power as a function of the spatial ultrasonic filter diameter (note that in this particular diagram nominal diameters are used).

Figure 5 shows the power contributions made by the annuli derived by taking (consecutive) pairs of spatial ultrasonic filters derived in the way described above. These power contributions have been converted into intensities in Figure 6 and ordered. The ordered data set is used to derive the cumulative power against cumulative area plot shown in Figure 7, which is used to derive the 75 % beam cross-sectional area. The ERA derived using actual spatial ultrasonic filter diameters is 350 mm2.

'HOT-SPOT' TREATMENT HEAD The defective treatment head was a Sonacel treatment head of frequency 1.5 MHz. Measurements of collimated power for this treatment head were undertaken and compared with measurements made on the treatment head which was nominally operating correctly. These results are shown in Figure 8, where normalised measurements made by two different operators (BZ and PAB) are presented alongside the DE-04 results. The graph illustrates the good agreement between the two operators for the correctly operating treatment head, indicating the reproducibility of the method. There is also some evidence of higher power values for DE-04 obtained using the smaller spatial ultrasonic filters. This is more pronounced in Figure 9, where the intensity (derived by dividing the normalised power in Figure 8 by the spatial ultrasonic filter area) for the defective treatment head is 50% higher.

COMPARISON WITH BEAM-PLOT DATA DERIVED CLOSE TO THE FACE OF THE TREATMENT HEAD In order to 'validate' the spatial ultrasonic filter measurements, two orthogonal beam-plots were made close to the treatment head face using the beam-plotting facility and used to simulate the expected response of the spatial ultrasonic filter power measurements. The beam-plot data was analysed in terms of four individual quadrants. The analysis was carried out at both frequencies (1 and 3 MHz). Figures 10 and 11 show the comparison for the 1 MHz treatment head, where modelled and actual data have been normalised. Figure 11 represents an expanded view for the smaller spatial ultrasonic filters. The fall off in the measured power as the spatial ultrasonic filter size is reduced is greater than that predicted.

The corresponding data for the treatment head operated at 3 MHz is shown in Figure 12. Although there is some uncertainty in the normalisation of the two plots, the observed falling off of the measured power is well modelled by the beam-plot data.

COMPARISON OF SPATIAL ULTRASONIC FILTER RESULTS WITH REFERENCE ERA VALUES The results for the effective radiating area derived using the spatial ultrasonic filter and beam-plotting methods are shown in Table 2. The final column provides values of the ERA derived from beam-plotting measurements made very close to the face.

Table 2: Compilation of the results of this feasibility study. ERA values are presented for each of the treatment heads studied. These have been derived using the spatial ultrasonic filter method being investigated, the reference method based on IEC 1689 as well as beam-plotting measurements made close to the face of the treatment head.

System Treatment head Effective Radiating Area (ERA) mm Collimator IEC 1689 Beam method plotting derived close to face EMS 1MHz40591 340 370 # 30 365 Therasonic 3 MHz 40591 227 285 + 20 247 1032 1 MHz 36783 60 56 + 7 43 3 MHz 36783 58 52 l 3 51 Sonacel 0.75 MHz1 159 185 + 63 171 Multiphon 1.5 MHz 419 374 l 26 373 Plus 3 MHz (RT- 350 327 l 23 320 2945) 3 MHz small 48 40 + 3 41 probe IEC 1689 determination of beam cross-sectional area against distance showed strong nonlinear dependence, hence the relatively large uncertainty, which arises predominantly from the uncertainty in the intercept.

2 Uncertainties are at the 95% confidence level and are all expressed in mum2.

Experiments to date have demonstrated the potential use of the techniques as a rapid means of determining the effective radiating area of a physiotherapy treatment head or other radiating treatment head. A number of conclusions may be drawn regarding the results derived: the level of agreement between the reference measurements made using IEC 1689 and the investigated method is encouraging; the rms difference being +12.5%. For comparison, uncertainties in the ERA measurements made using IEC 1689 are estimated to be typically in the range +7% to +9%; there is some evidence that the method will trap 'hot-spot' treatment heads; the spatial ultrasonic filter technique, as implemented in the example, is sensitive and reproducible enough to consider a restricted checking of treatment heads against some reference value data set, perhaps using only two or three spatial ultrasonic filters.

KEY TO FIGURES 3 TO 12 Figure 3. Variation of the normalised measured power with the diameter of the collimator applied for the EMS large diameter transducer (40591) operating at 1 Mhz. Measurements are presented for the treatment head operating at uncollimated powers of 2 W (+ ) and 5 W (A).

Figure 4. Variation of the measured power with the diameter of the applied collimator for the large 3 Mhz Sonacel treatment head (RT-2945). The uncollimated power reading was 5.45 W.

Figure 5. Variation of the power contribution calculated between successive collimator diameters. On the x-axis, the collimator pairing is identified, such that "20 to 18" refers to difference in powers measured using the nominal 20 mm and 18 mm diameter collimators. Data are for the Sonacel treatment head RT-2945 and have been derived from Figure 2.

Figure 6. Intensity values derived from Figure 3 for treatment head RT-2945 sorted in descending order. The intensities have been calculated by taking the annular power contributions in Figure 3 and dividing them by the respective annular areas.

Figure 7. 75 % power plot for the treatment head RT-2945, derived by plotting the cumulative power against the cumulative area. The beam cross sectional area corresponds to the area of the collimator which reduces the power to 75% of its uncollimated value (for the example given, this corresponds to 0.75*5.45 W or 4.09 W).

Figure 8. Variation of the measured power with the applied collimator diameter for the Sonacel 1.5 Mhz treatment head. Measurements made on a treatment head nominally operating correctly by two operators, BZ ( ) and PAB (ll1l)are presented. Also shown are the results of measurements made on a transducer of the same generic type (DE-04) but which was know to produce a high local field intensity or 'hot-spot' (A).

Figure 9. Intensity values derived for the Sonacel 1.5 Mhz treatment head. Values have been calculated by taking the normalised power values shown in Figure 6 and dividing them by the area of the collimating mask. Again, results are also presented for the 'hot-spot' treatment head DE-04.

Figure 10. Variation of the normalised measured power against the applied collimator diameter for the EMS small l Mhz treatment head (36783). Experimental results (t) are presented alongside the dependence calculated from the acoustic beam-profile derived by hydrophone-scanning close to the treatment head (+).

Figure 11. Same as figure 8, but presenting the collimator diameter range 5 mm to 12 mm in more detail.

Figure 12. Variation of the normalised measured power against the applied collimator diameter for the EMS small 3 Mhz treatment head (36783). Experimental results (.)are presented alongside the dependence calculated from the acoustic bam-prnfile derived by hydrophone-scanning close to the treatment head (+).

Claims (14)

1. Claims

   l. An ultrasonic mask for use in measuring the output ultrasonic beam characteristics of a treatment head including a body member in which one or more spatial ultrasonic filters is provided.
2. 2. A mask according to claim 1, wherein the body member is formed of a material with high ultrasonic attenuation and low ultrasonic reflection.
3. 3. A mask according to claim 1 or 2, wherein the filter or filters provides a defined shape and/or area which is variable.
4. 4. A mask according to claim 1, wherein a plurality of filters are provided of different size and/or area.
5. 5. A mask according to any preceding claim, wherein the body member is in the form of a disc.
6. 6. A mask according to any preceding claim, wherein the body member is formed of two layers of acoustically absorbing material
7. 7. A mask according to claim 6, wherein the first layer is provided with an array of scatter cones operable to prevent reflection of radiation.
8. 8. A mask according to claim 6 or 7, wherein the body member is formed from polytrethane based acoustically absorbing material.
9. 9. A method of measuring the radiation characteristics of a treatment head including the step of filtering radiation from the treatment head to allow only a selected portion thereof to be sensed by a detector.
10. 10. A method according to claim 9, including the step of gradually reducing the effective radiating aperture of the treatment head under test until the acoustic power passing through the spatial ultrasonic filter falls below a specified level.
11. II. A method according to claim 10, wherein level is 75%.
12. 12. A method according to any one of claims 9 to 11, wherein the step of reducing the effective radiating aperture includes inserting a collimating mask in front of the treatment head which includes spatial ultrasonic filters of different or variable diameter.
13. 13. An ultrasonic mask substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
14. 14. A method of measuring the radiation characteristics of a treatment head substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.