- Email: [email protected]
Determination of physiotherapy ultrasound beam quality parameters from images derived using thermochromic material
Ultrasonics 99 (2019) 105943
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Determination of physiotherapy ultrasound beam quality parameters from images derived using thermochromic material
T
Gordana Žauhara,b, , Ðeni Smilović Radojčićc, Zoran Kalimanb, Tea Schnurrer-Luke-Vrbanićc, Slaven Jurkovićc,a ⁎
a
University of Rijeka, Faculty of Medicine, Department of Medical Physics and Biophysics, Braće Branchetta 20, 51 000 Rijeka, Croatia University of Rijeka, Department of Physics, Radmile Matejčić 4, 51 000 Rijeka, Croatia c Clinical Hospital Center Rijeka, Krešimirova 42, 51 000 Rijeka, Croatia b
ARTICLE INFO
ABSTRACT
Keywords: Ultrasound therapy Thermochromic material Effective radiating area Beam non - uniformity ratio
The evaluation of the performance of nine physiotherapy ultrasound transducers used clinically was performed in the hospital environment using an acoustically absorbing thermocromic tile developed at the National Physical Laboratory (UK). The method consists of exposing an acoustic absorber tile, part of which contains a thermochromic pigment, to the ultrasonic beam, thereby forming an image of the intensity profile of the transducer. Images acquired using thermochromic materials were postprocessed in order to estimate effective radiating area (ERA) and beam nonuniformity ratio (BNR) for ultrasound transducers operating within the frequency range from 1.0 to 3.3 MHz, and nominal applied intensities in the range of 1–2 W/cm2 . Results of our measurements have shown that thermocromic tile can be used for quality control of ultrasound transducers in the hospital environment. Experimental results show that proposed method can be used to distinguish highly non - uniform ultrasound beams with high value of BNR. Influence of exposure duration on obtained ERA and BNR values was also analysed. Our results show that values for ERA increase with insonation time, while BNR values decrease. In order to compare our results with theory we have estimated temperature rise in thermochromic material experimentally and compare it with theoretical prediction.
1. Introduction The ability of ultrasound to interact with tissue to produce local heating has been known for a long time and because of that ultrasound is widely used in physiotherapy to treat tissue injuries, to reduce pain and to stimulate soft tissue healing. Regarding to the safe exposure of human tissue with either ionizing or non-ionizing radiations, it is crucial to determine the spatial distribution (location) of this deposited energy. This is usually described by the applied ultrasound intensity beam profile, which is commonly assessed using small hydrophones immersed in water combined with scanning procedures that are exceedingly time-consuming. This method involves precise measurement systems and controlled (laboratory) conditions and as such is not an appropriate tool for quality control procedures to be performed in busy clinical environments. Thus, an alternative method based on the use of thermochromic materials was proposed first by Cook and Werchan [1]. After that some other studies have also reported the use of thermal
imaging techniques for ultrasound field mapping by direct radiation on a thermochromic material [2–4]. Ultrasound transducers with intensities of the order of a few W/cm2 can produce a thermal image on thermochromic tile exposed to ultrasound beam. Namely, when ultrasonic wave propagates through biological tissue, it is attenuated due to absorption and scattering. Absorption results from the irreversible conversion of acoustic energy to local heat, and it is the primary mode of attenuation of ultrasound in tissue. The method proposed by Butterworth et al. [3] has proved to be suitable as a characterisation tool appropriate for use in hospital environment [5]. The method consists of coupling the emitting transducer to a specially-designed acoustic absorbing tile parts of which contain a thermochromic pigment. These thermochromic pigments become colourless at locations where temperature is elevated above a certain threshold level (called the switch temperature). The thermochromic tile used in the original work contained thermochromic pigments with
Corresponding author at: University of Rijeka, Faculty of Medicine, Department of Medical Physics and Biophysics, Braće Branchetta 20, 51 000 Rijeka, Croatia. E-mail addresses: [email protected] (G. Žauhar), [email protected] (Ð.S. Radojčić), [email protected] (Z. Kaliman), [email protected] (T. Schnurrer-Luke-Vrbanić), [email protected] (S. Jurković). ⁎
https://doi.org/10.1016/j.ultras.2019.06.005 Received 1 October 2018; Received in revised form 31 May 2019; Accepted 9 June 2019 Available online 15 July 2019 0041-624X/ © 2019 Elsevier B.V. All rights reserved.
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
switching temperature around 30 °C. Energy deposition within the tile results in a temperature distribution which is closely related to the applied acoustic intensity distribution. The method provides information about whole thermal pattern and this close relationship to the intensity distribution provides the opportunity of a quantitative assessment of beam parameters. Due to possible harmful effects of respective ultrasound beams on biological matter requirements for physiotherapy machines should be well defined [6]. Standard for physiotherapy equipment includes two limits for parameters which are related with the patient safety. Those parameters are effective intensity and beam non-uniformity ratio. The effective intensity is defined as the quotient of the effective ultrasound power and the effective radiating area (ERA). According to IEC standard 60601 [7] the effective intensity must not exceed 3 W/cm2 . Another important parameter concerning patient safety is the ultrasonic beam distribution generated by the treatment head. Beam homogeneity can be quantified by the parameter called beam non - uniformity ratio (BNR), which represents the ratio of the highest intensity in the field to the intensity averaged over the effective radiating area. Sometimes, distribution can be non-uniform and can potentially generate regions of high local pressure, also called hot spots [8]. These regions may produce excessive heating in small regions of the tissue. According to IEC standard 61689 [9] transducers with BNR > 8 are considered unsafe. Since the first introduction of standards and limits for output parameters of the devices used in physiotherapy a numerous surveys has been published [10–12]. These papers show that lot of physiotherapy devices have significant differences between the indicated and actual output power. Also, some cases of excessive heating as a result of equipment failure have been described [12]. All these publications have emphasized the need for measurement and calibration equipment used in physiotherapy. This paper is successor of earlier papers which involved a method for the quality assurance of physiotherapy transducer heads using a thermochromics material [3,5]. The aim of our work was an addendum to our previous work related with the origin of the systematic underestimation of the ERA values which has been observed in our previous work [5]. Besides, this study investigated whether this method could be employed for the verification of BNR of physiotherapy ultrasound treatment heads within a clinical environment.
Fig. 1. Schematic presentation of experimental set – up showing the cross section of thermochromic tile, transducer and the direction of application of the ultrasonic beam.
measurements can be found in Butterworth et al. [3], where it is described as a Type B tile. The top layer, with thickness of 3 mm, is optically transparent with relatively low ultrasonic absorption and water matched acoustic impedance. The thermochromatic pigment whose switching temperature is around 30 °C is added to intermediate layer. The backing layer is white and strongly absorbing to ultrasound and gradually becomes visible through the thermochromic – loaded intermediate layer as it changes colour from blue to colourless. The thermochromic tile was removed from the refrigerator and maintained at room temperature for at least 1 h before measurement to allow temperature equilibration. The image capture protocol defined in [5] has been applied. Firstly, a reference image of the tile next to a ruler was acquired immediately prior to ultrasonic exposure. After that, the ultrasound transducer was placed on the tile with coupling gel as used in clinical conditions. After ultrasonic exposure, the transducer was removed and the tile wiped clean of coupling gel. Approximately 10–20 s after insonation of the tile, optical images were acquired using a digital camera Optio WG-1 (Pentax Ricoh imaging Co., Ltd., Indonesia). All images were imported and postprocessed in order to calculate effective radiating area (ERA) and beam non - uniformity ratio (BNR) for transducers of interest. The calculation was performed using the algorithm developed for postprocessing optical images for the purpose of this work. Once the reference and beam profile images were imported into the program, they are converted from colour to grey scale. Afterwards, a difference image is produced by subtracting the reference image from the beam profile image. The millimetre scale on the ruler, was also photographed, and was used to enable dimensional scaling leading to the calculation of the ERA and BNR.
2. Material and methods In this work a non–standardized method developed at National Physical Laboratory (NPL) was used for quality assurance of ultrasound therapy beams. The method consists of exposing the thermochromic tile to the ultrasonic beam thereby forming a thermal image of the intensity profile of transducer. The image is in turn photographed and analysed using software developed for that purpose. Nine physiotherapy ultrasound treatment heads (able to function at frequencies close to either 1 MHz or 3 MHz) used for physiotherapy at Clinical Hospital Center Rijeka were measured again by using thermochromic tile in order to obtain new ERA values for comparison with the four years old data and to determine BNR values. The measurements for each ultrasound treatment head were repeated three times for respective measuring conditions. The thermochromic tile used in measurements was obtained from National Physical Laboratory, UK and was manufactured by Acoustic Polymers Ltd. (Mitcheldean, Gloucestershire, UK). It consists of three discrete layers sandwiched together. A simple schematic of the cross section of thermochromic tile is presented on Fig. 1. These layers are based on a polyurethane material, previously developed and applied as a material for an absorbing target of a radiation force balance [13]. Since exact values for material properties are not known, for theoretical prediction it was assumed that thermocromic tile is made from anechoic material for medical ultrasonic applications described in reference [13]. A detailed description of the thermochromic tile used for our
2.1. Calculation of effective radiating area The effective radiating area (ERA) denotes the area close to the face of transducer over which the majority of ultrasonic power is distributed. There are two specification standards which describe methods for determination of the ERA. The International Electrotechnical Commission (IEC) and the United States Food and Drug Administration (FDA) specification standards describe methods for determining ERA. However, the IEC and FDA specification standards define effective radiating area in different ways. Both approaches were used in this paper. According to standard IEC 61689 [9], ERA is evaluated through the derivation of an intermediate quantity called the beam cross-sectional area (ABCS ) . The ABCS is defined as the minimum area which contains 75% of the total mean square acoustic pressure. It is determined by sorting analysis of the acquired data. ERA of the treatment head is 2
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
calculated by multiplying the beam cross-sectional area determined at a distance of 0.3 cm from the treatment head’s face, by a dimensionless factor FAC given by 1.333 [9]. The procedure was based on the assumption [2] that the degree of whiteness at each point bears a simple relationship to the local time-average intensity. In order to determine the beam cross-sectional area, first sorting of all the pixel values into descending order was performed. In this procedure the gray values of the pixels were used. Afterward, a plot of cumulative power against cumulative area is produced. The beam cross-sectional area is then derived by reading from the plot cumulative area corresponding to 75% of the transmitted power. According to standard FDA [14] the ERA was calculated as the area over which the intensity was larger than 5% of the peak intensity. The ERA was determined for various clinically used treatment heads with frequencies in the range from 1 MHz to 3.3 MHz, and intensities range from 1 W/cm2 to 2 W/cm2 . They were electrically driven with their respective physiotherapy systems, in a way that they would be used in clinical practice.
Table 1 The ERA and BNR values for physiotherapy ultrasound transducers clinically used in Clinical Hospital Center Rijeka. Comparison of ERA determined using the IEC and FDA methods, explained in Section 2. Values are presented as mean ± standard deviation (SD). ERA/cm2 Transducer Cosmogama F120 Sonoplus 992 ENRAF NONIUS Sonoplus 992 ENRAF NONIUS Gymna Combi 200 Gymna Combi 200 Gymna Pulson 100 Cosmogamma US10 Cosmogamma Mixing10 Cosmogamma F230
2.2. Calculation of beam non-uniformity ratio
2 pmax ERA
pmst A0
,
(1)
2.3. Estimation of temperature The relationship between temperature rise and gray – scale change was obtained with parallel thermal and visual monitoring which was conducted with combination of infra-red and imaging cameras [3]. We estimate temperature from the obtained gray intensity on thermochromic beam image. For experimental estimation of temperature we use [3, Figure 13] which present dependence of gray – scale intensity on temperature. We compare experimentally obtained temperature with theoretical prediction described by Nyborg [15]. For the cylindrical source with radius x and height z* temperature on ring axis at distance z from the transducer is given by [15, eq:16]
{
qv x E [2 r
1 4c v
erfc(t *
R)] +
}
1 erfc(t * + R) dx dz *. E (2a)
where
E = exp
r
, t* =
t
, R=
r . 4 t
Specification
IEC
FDA
IEC
1 1
5 5
3.1 ± 0.4 3.8 ± 0.1
4.0 ± 0.3 5.4 ± 0.2
3.8 ± 0.4 3.1 ± 0.1
3
5
4.0 ± 0.4
5.7 ± 0.6
6.9 ± 0.2
1.1 3.3 1 1 1
4 4.7 4 3.6 3.6
3.3 ± 4.1 ± 3.6 ± 2.9 ± 3.1 ±
0.6 0.6 0.7 0.2 0.5
3.8 ± 0.8 5.4 ± 1.2 4.3 ± 0.8 3.9 ± 0.4 3.7 ± 0.8
5.6 ± 7.4 ± 3.3 ± 2.9 ± 3.2 ±
1
5
2.7 ± 0.6
3.1 ± 0.7
9.8 ± 1.1
1.0 0.8 0.6 0.7 1.1
The evaluation of the performance of physiotherapy ultrasound transducers was performed in the hospital environment using thermochromics tile. Thermal images were postprocessed in order to estimate effective radiating area (ERA) and beam non-uniformity ratio (BNR) for tested transducers. Obtained results for the calculated ERA values, and respective Type A (random) uncertainties for nine transducers were presented previously [5]. In this work BNR values were calculated from re – measurements of the same transducers and results are presented in Table 1. Results for ERA values are different from results presented previously in reference [5]. Differences between new and four years old data varied between +1.8% and −20.8%, except for the defective transducer Cosmogamma F230 whose ERA value varied even more (−41.9%). The root-mean-squared difference calculated over the nine treatment heads is −21.6% indicating that generally new data for ERA are smaller than old data. In order to find out the origin of the systematic underestimation of the ERA values, which has been observed in [5] the ERA values for the each transducer were calculated according to both, IEC and FDA, specification standards. Results are also presented in Table 1. Results suggested that ERA values derived according to FDA method are larger than those derived by IEC 61689 methodology. In our opinion, this could be the reason for the underestimation of the ERA values, which has been observed in our previous work [5]. Namely, the ERA values for treatment heads used in our previous work was determined according to current IEC methodology. Thermocromic images for three different ultrasound treatment heads are shown on Fig. 2. These specific devices were selected because obtained images on thermochromic material show three completely different shapes of ultrasonic beam and thus show how this method can be used for visualization of the beam and testing of ultrasound physiotherapy treatment heads. From obtained results for BNR and from thermal images presented on Fig. 2 it can be seen that treatment head presented on Fig. 2(a) has non-uniform intensity distribution. Calculated BNR for that treatment head was 9.76 which is too much for regular application because according to IEC 61689 transducers with BNR > 8 are considered unsafe for medical application [9]. The beam intensity map shown in Fig. 2(b) shows the symmetry and relatively uniform nature of the intensity distribution which is confirmed with calculated value for BNR of 3.09. Fig. 2(c) shows the intensity map and thermal image from a 3.3 MHz transducer which shows a small region with high intensity in the centre of the beam surrounded by wide region of much lower intensity and then one ring with higher intensity like a donut. Calculated BNR for
where A0 is the unit area of the raster scan and pmst is the total mean square acoustic pressure. To acquire this data hydrophone scans over a number of planes perpendicular to the beam axis are required. In this work the beam non - uniformity ratio was expressed as ratio of the maximum intensity in the beam to the spatial average of intensity, where the spatial average is taken over the effective radiating area. The maximum intensity in the beam is proportional to the square 2 / A0 ) and the spatial of the maximum r.m.s acoustic pressure (pmax average intensity is proportional to spatial average of the square r.m.s. acoustic pressure (pmst / ERA) .
T=
f/MHz
3. Results
The beam non-uniformity ratio (BNR) is defined as the ratio of the square of the maximum r.m.s. acoustic pressure ( pmax ) to the spatial average of the square of the r.m.s. acoustic pressure [9], where the spatial average is taken over the effective radiating area (ERA). BNR is calculated according to following expression:
BNR =
BNR
(2b)
Here r is the distance from any point on the ring to the axial point at z, i.e. r = x 2 + (z z *)2 . qv is the heat source function, T the temperature rise above the ambient level, the thermal diffusivity, the time constant for perfusion and c v the volume specific heat. 3
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
Fig. 2. Beam uniformity analysis of thermocromic images for three transducer heads: (a) Transducer Cosmogamma F230, f = 1 MHz, BNR = 9.76 ; (b) Transducer Sonoplus 992 ENRAF NONIUS, f = 1 MHz, BNR = 3.09 ; (c) Tranducer Gymna Combi 200, f = 3.3 MHz, BNR = 7.35.
that treatment head was 7.35. In order to determine the optimal exposure time for all nine devices testing was conducted with exposure periods: 3, 5 and 7 s while the beam intensity was always same and it was I = 2 W/cm2 . This range of exposure durations was chosen to provide suitable temperature rise in the absorber which will cause visible colour change. Beam profiles for different ultrasound exposure durations for transducer Cosmogamma US10 with f = 1 MHz , and intensity I = 2 W/cm2 are shown on Fig. 3. It is visible that difference between beam profiles for intensity I = 2 W/cm2 obtained after insonation time of 5 and 7 s is small. We conclude that 5 s insonation is good enough for obtaining reliable data for determination of ERA and BNR if average beam intensity is 2 W/cm2 .
This exposure time was found long enough to trigger thermochromic change and short enough to avoid thermal saturation as well as to prevent damage of the thermochromic tile. For 3 s insonation temperature rise above 30 °C only in the small area around the middle of profile, which is visible on the bottom panel. It means that 3 s insonation time was too short for obtaining good quality thermal image. The beam profiles for different intensities and for the same transducer were also measured (Fig. 4) to explore influence of changing intensity on beam profile and determination of ERA and BNR. In order to investigate the changing of optical image of the tile “whiteness” and corresponding beam profiles with time after insonation, optical image of the tile “whiteness” were photographed
Fig. 3. Beam profiles for ultrasound exposure durations of 3 s (stars), 5 s (circles) and 7 s (boxes) for transducer Cosmogamma US10 with f = 1 MHz , and intensity I = 2 W/cm2 . The bottom image shows a temperature profiles. We estimate temperature from the obtained gray intensity as described in Section 2.3.
Fig. 4. Gray intensity and temperature profiles measured using different nominal intensities of 1 W/cm2 (cross, dashed) and 2 W/cm2 (boxes, full line) for the same transducer with frequency f = 1 MHz and 7 s ultrasound exposure. 4
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
4. Discussion and conclusion For quick visualisation of ultrasound beam it is interesting to see thermal images and corresponding beam profiles for examined treatment head. Beam parameters for three different physiotherapy ultrasound treatment heads were analysed in more details since they exhibited significantly different beam intensity patterns. The purpose was to investigate the potential of the thermochromic tile to distinguish highly non - uniform beams with high value of BNR. From thermal images obtained for three transducer heads presented in Fig. 2 it was observed that thermal image shown in Fig. 2(a) and (c) have non uniform intensity distribution while thermal image shown in Fig. 2(b) has uniform intensity distribution. An equivalent analysis arises for BNR. The smallest value of BNR were calculated for thermal image presented in Fig. 2(b), while thermal image presented in Fig. 2(a) yields the largest BNR value. Moreover, despite the non – circular heating pattern, the value of BNR for this thermal image is too large and thus this transducer head can be considered unsafe for regular application. As previously mentioned [5], this thermal image was produced with an old transducer head which is decommissioned consequently. From results presented in this paper it can be seen that although the thermocromic method was developed primarily for qualitative analysis and characterization of ultrasonic physiotherapy equipment, it also offers the potential for quantitative measurements. The effective radiated area (ERA) values obtained in our previous work [5] for seven of nine physiotherapy transducers are lower than the respective ERA values reported by manufacturer. It is important to note that generally it is not clear how the ERA values declared by the manufacturer were determined. For some transducers, such as transducers Sonoplus 992 ENRAF NONIUS, in the transducer specification is written that ERA value is derived according FDA methodology. Unfortunately, for some transducers methodology which is used for determination of ERA and BNR is unknown. In our previous work ERA was determined, using the methodology described in IEC 61689 [9]. Comparison of the recently determined ERA values with four years old data showed that for some transducers a new data are significantly smaller than the old ones. It is anticipated that one of the reasons for such a difference might be degradation of some transducers with time. However, for most transducers the difference in calculated values for ERA is rather small. This indicates the long – time constancy of the physiotherapeutic ultrasound devices. In this work the FDA methodology [14] for comparison purposes was used. The results in Table 1 indicate that ERA values obtained according the FDA methodology are larger than ERA values determined according to IEC 61689. Results of our measurements have shown that this could be one of the reasons for underestimation of ERA values obtained previously. The differences in the ERA values determined by the two standard methods are also discussed in [6] and noticed that the ERA values determined using the FDA method could be up to 69% larger than those produced using IEC 61689. Costa et al. [4] also reported that the heated area values obtained with thermochromic phantom are lower than the respective effective radiating area determined as described in IEC 61689. The results presented in Fig. 3 show dependence of the beam and temperature profiles on time of insonation. It can be seen, that exposure duration of 3 s was too short for obtaining good quality thermal image. Energy deposition within the tile results in a temperature rise and it is obvious that for 3 s exposure duration the temperature rise was not large enough to cause a colour change in thermochromics material. Exposure duration of 5 s seems optimum in order to obtain image appropriate for determination of ERA. When duration of exposure was 7 s thermochromics image is only slightly changed. From the results shown in Table 2 it can be seen the influence of increase of the exposure time on respective beam parameters: ERA and BNR. The values for ERA slightly increase with insonation time, while BNR value decrease with insonation time. It is probably due to spreading of heat through the material causing thermal imaging
Fig. 5. Changing of beam profiles with time passed after insonation for treatment head Sonoplus 992 with frequency 3 MHz at intensity 2 W/cm2 after 5 s ultrasound exposure.
successively many times within 300 s after the exposure of the thermochromic tile was ceased. Obtained beam profiles for various times passed after insonation are presented in Fig. 5. Within 1 min the shape of beam profile doesn’t change significantly. Because of cooling and spreading of heat in the thermocromic material after five minutes beam profile is getting less visible and slightly spreading. In order to predict temperature rise in thermochromic material we have used bio-heat transfer equation for a circular source transducer Eq. (16) proposed by Nyborg [15]. In our calculation it was assumed that material has following characteristics: the absorption coefficient at f = 1 MHz is = 27 dB/cm = 622 Np/m , time constant = 250s , thermal diffusivity = 1.9 × 10 7m2 /s . Tile thickness is 10 mm. The volume rate of heat generation is given by [15, Eq. (18)] (3)
qv = 2 I0exp( 2 z *),
where intensity is I0 = 1W/cm2 . Results of calculation and comparison with experimental results are presented in Table 2. From Table 2 we can conclude that ERA values increase with exposure time as expected. In contrast, BNR values decreases. Using the theoretical prediction described by Nyborg [15] we have calculated the expected temperature rise in the material and with assumption that the initial temperature of thermochromic tile was equal to room temperature (20.3 °C), we calculated the expected temperature in the material. From results presented in Table 2 it can be seen that experimentally determined temperature and theoretical prediction of temperature in thermochromic material are in good agreement.
Table 2 The ERA and BNR values for different ultrasound exposure durations for treatment head Gymna PULSON 100 with f = 1 MHz , and intensity I = 1 W/cm2 . To obtain temperature from gray – scale intensity we used fitting curve from reference [3] which has uncertainty of about 0.5 °C. Theoretical prediction of temperatures (T) are compared with experimental ones. Experimental results Exposure/s
ERA/cm2
3 5 7 9
1.17 2.74 3.12 3.51
T/°C mean ± SD 28.8 ± 30.6 ± 32.1 ± 33.4 ±
1.0 0.4 0.3 0.2
Theoretical prediction BNR
T /°C
T/°C
5.84 4.53 4.06 3.97
8.4 10.4 12.1 13.5
28.7 30.7 32.1 33.8
5
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
expansion and increasing the ERA parameter value. At the same time, the colour of the tile becomes more and more uniform which results in decreasing of BNR value. Results of our measurements have shown that proposed procedure could be used for estimation of beam parameters such as ERA and BNR. In our opinion the method is suitable for quality assurance in clinical routine. Namely, it can be used for monthly checks, performed by technician with purpose of finding large deviations in transducer head performance. Furthermore, it is expected that the proposed methodology could be used as a quick test to qualitatively assess possible changes in transducer head performance during time. In order to achieve this, it would be necessary to make a thermal image of transducer head during the acceptance test of the ultrasound physiotherapy device which could be used as a reference for recurrent tests procedures. The results presented in Fig. 4 indicate that lower intensity shows smaller resulting colour change and consequently calculated ERA values were smaller. However, the results obtained for BNR shows an opposite trend. For higher intensity the obtained value of BNR is smaller. This indicates that thermal spreading may be the cause of it. Namely, the white of the backing layer is only seen after the majority of intermediate layer containing the thermochromic pigment has reached the transition temperature. When this happened significant lateral heat diffusion already occurred. The information on the structure is blurred, which causes the BNR parameter to decrease. Analysis of successive images obtained for various timed passed after insonation (Fig. 5) showed almost constant ERA and BNR value during first minute. From the comparison of the successive beam profiles it is apparent that the increase of the time passed after the insonation results in the lateral heat spread which is visible as widening of beam profile and lowering of beam profile due to cooling. Results of our measurements have shown that thermocromic tile can be used for check of ultrasound transducers in the hospital environment. Furthermore, our work has also shown that it is possible to postprocess thermal images in order to estimate important beam parameters such as effective radiating area (ERA) and beam non-uniformity ratio (BNR). The limitation of this method is that thermocromatic tile will only enable the determination of BNR in a single plane at approximate distance of 5 mm from the transducers front face. If the ‘hot – spot’ is at greater depths it may not show up on the derived beam non-uniformity ratio (BNR) using thermochromic tile. The methodology presented in this paper clearly needs further validation to compare with existing methods. Unfortunately, this requires detailed beam-plotting using a hydrophone. This is a proof–of–concept study, requiring more extensive validation in future work.
Acknowledgments Authors would like to thank to Dr. Bajram Zeqiri at the United Kingdom National Physical Laboratory for the donation of thermocromic tile without which this work would not be possible. This work was supported by the University of Rijeka under the project number: uniri-prirod-18-75. References [1] B.D. Cook, R.E. Werchan, Mapping ultrasound fields with cholesteric liquid crystals, Ultrasonics 9 (1971) 88–94, https://doi.org/10.1016/0041-624X(71)90126-0. [2] K. Martin, R. Fernandez, A thermal beam-shape phantom for ultrasound physiotherapy transducers, Ultrasound Med. Biol. 23 (1997) 1267–1274, https://doi. org/10.1016/S0301-5629(97)00109-9. [3] I. Butterworth, J. Barrie, B. Zeqiri, G. Žauhar, B. Parisot, Exploiting thermochromic materials for the rapid quality assurance of physiotherapy ultrasound treatment heads, Ultrasound Med. Biol. 38 (2012) 767–776, https://doi.org/10.1016/j. ultrasmedbio.2012.01.021. [4] R.M. Costa, A. Alvarenga, R.P.B. Costa-Felix, T.P. Omena, M.A. von Krüger, W.C.A. Pereira, Thermochromic phantom and measurement protocol for qualitative analysis of ultrasound physiotherapy systems, Ultrasound Med. Biol. 42 (2016) 299–307, https://doi.org/10.1016/j.ultrasmedbio.2015.08.017. [5] G. Žauhar, D. Smilović Radojčić, D. Dobravac, S. Jurković, Quantitative testing of physiotherapy ultrasound beam patterns within a clinical environment using a thermochromic tile, Ultrasonics 58 (2015) 6–10, https://doi.org/10.1016/j.ultras. 2015.01.006. [6] A. Shaw, M. Hodnett, Calibration and measurement issues for therapeutic ultrasound, Ultrasonics 48 (2008) 234–252, https://doi.org/10.1016/j.ultras.2007.10. 010. [7] IEC 60601 part 2-5: Medical Electrical Equipment: Particular Requirements for the Safety of Ultrasound Physiotherapy Equipment, 2009. [8] M. Gutiérrez, H. Calás, A. Ramos, A. Vera, L. Leija, Acoustic field modeling for physiotherapy ultrasound applicators by using approximated functions of measured non-uniform radiation distributions, Ultrasonics 5 (2012) 767–777, https://doi. org/10.1016/j.ultras.2012.02.006. [9] IEC 61689: Ultrasonics – Physiotherapy Systems – Field Specifications And Methods of Measurement In the Frequency Range 0.5 MHz to 5 MHz, 2013. [10] S.D. Pye, C. Milford, The performance of ultrasound physiotherapy machines In Lothian Region, Scotland, Ultrasond Med. Biol. 20 (4) (1994) 347–359, https://doi. org/10.1016/0301-5629(94)90003-5. [11] R. Hekkenberg, W. Oosterban, W.V. Beekum, Evaluation of ultrasound therapy devices, Physiotherapy 72 (1986) 390–395. [12] S.D. Pye, Ultrasound therapy equipment – does it perform? Physiotherapy 82 (1) (1996) 39–44, https://doi.org/10.1016/S0031-9406(05)66996-9. [13] B. Zeqiri, C. Bickley, A new anechoic material for medical ultrasonic applications, Ultrasound Med. Biol. 26 (2000) 481–485, https://doi.org/10.1016/S03015629(99)00147-7. [14] US Food and Drug Administration (FDA), Radiation Safety Performance Standard. Ultrasound Therapy Products, title 21, Part 1050.10, Rules and Regulations, February 1979. [15] W.L. Nyborg, Solutions of the bio-heat transfer equation, Phys. Med. Biol. 33 (7) (1988) 785–792.
6
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Determination of physiotherapy ultrasound beam quality parameters from images derived using thermochromic material
T
Gordana Žauhara,b, , Ðeni Smilović Radojčićc, Zoran Kalimanb, Tea Schnurrer-Luke-Vrbanićc, Slaven Jurkovićc,a ⁎
a
University of Rijeka, Faculty of Medicine, Department of Medical Physics and Biophysics, Braće Branchetta 20, 51 000 Rijeka, Croatia University of Rijeka, Department of Physics, Radmile Matejčić 4, 51 000 Rijeka, Croatia c Clinical Hospital Center Rijeka, Krešimirova 42, 51 000 Rijeka, Croatia b
ARTICLE INFO
ABSTRACT
Keywords: Ultrasound therapy Thermochromic material Effective radiating area Beam non - uniformity ratio
The evaluation of the performance of nine physiotherapy ultrasound transducers used clinically was performed in the hospital environment using an acoustically absorbing thermocromic tile developed at the National Physical Laboratory (UK). The method consists of exposing an acoustic absorber tile, part of which contains a thermochromic pigment, to the ultrasonic beam, thereby forming an image of the intensity profile of the transducer. Images acquired using thermochromic materials were postprocessed in order to estimate effective radiating area (ERA) and beam nonuniformity ratio (BNR) for ultrasound transducers operating within the frequency range from 1.0 to 3.3 MHz, and nominal applied intensities in the range of 1–2 W/cm2 . Results of our measurements have shown that thermocromic tile can be used for quality control of ultrasound transducers in the hospital environment. Experimental results show that proposed method can be used to distinguish highly non - uniform ultrasound beams with high value of BNR. Influence of exposure duration on obtained ERA and BNR values was also analysed. Our results show that values for ERA increase with insonation time, while BNR values decrease. In order to compare our results with theory we have estimated temperature rise in thermochromic material experimentally and compare it with theoretical prediction.
1. Introduction The ability of ultrasound to interact with tissue to produce local heating has been known for a long time and because of that ultrasound is widely used in physiotherapy to treat tissue injuries, to reduce pain and to stimulate soft tissue healing. Regarding to the safe exposure of human tissue with either ionizing or non-ionizing radiations, it is crucial to determine the spatial distribution (location) of this deposited energy. This is usually described by the applied ultrasound intensity beam profile, which is commonly assessed using small hydrophones immersed in water combined with scanning procedures that are exceedingly time-consuming. This method involves precise measurement systems and controlled (laboratory) conditions and as such is not an appropriate tool for quality control procedures to be performed in busy clinical environments. Thus, an alternative method based on the use of thermochromic materials was proposed first by Cook and Werchan [1]. After that some other studies have also reported the use of thermal
imaging techniques for ultrasound field mapping by direct radiation on a thermochromic material [2–4]. Ultrasound transducers with intensities of the order of a few W/cm2 can produce a thermal image on thermochromic tile exposed to ultrasound beam. Namely, when ultrasonic wave propagates through biological tissue, it is attenuated due to absorption and scattering. Absorption results from the irreversible conversion of acoustic energy to local heat, and it is the primary mode of attenuation of ultrasound in tissue. The method proposed by Butterworth et al. [3] has proved to be suitable as a characterisation tool appropriate for use in hospital environment [5]. The method consists of coupling the emitting transducer to a specially-designed acoustic absorbing tile parts of which contain a thermochromic pigment. These thermochromic pigments become colourless at locations where temperature is elevated above a certain threshold level (called the switch temperature). The thermochromic tile used in the original work contained thermochromic pigments with
Corresponding author at: University of Rijeka, Faculty of Medicine, Department of Medical Physics and Biophysics, Braće Branchetta 20, 51 000 Rijeka, Croatia. E-mail addresses: [email protected] (G. Žauhar), [email protected] (Ð.S. Radojčić), [email protected] (Z. Kaliman), [email protected] (T. Schnurrer-Luke-Vrbanić), [email protected] (S. Jurković). ⁎
https://doi.org/10.1016/j.ultras.2019.06.005 Received 1 October 2018; Received in revised form 31 May 2019; Accepted 9 June 2019 Available online 15 July 2019 0041-624X/ © 2019 Elsevier B.V. All rights reserved.
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
switching temperature around 30 °C. Energy deposition within the tile results in a temperature distribution which is closely related to the applied acoustic intensity distribution. The method provides information about whole thermal pattern and this close relationship to the intensity distribution provides the opportunity of a quantitative assessment of beam parameters. Due to possible harmful effects of respective ultrasound beams on biological matter requirements for physiotherapy machines should be well defined [6]. Standard for physiotherapy equipment includes two limits for parameters which are related with the patient safety. Those parameters are effective intensity and beam non-uniformity ratio. The effective intensity is defined as the quotient of the effective ultrasound power and the effective radiating area (ERA). According to IEC standard 60601 [7] the effective intensity must not exceed 3 W/cm2 . Another important parameter concerning patient safety is the ultrasonic beam distribution generated by the treatment head. Beam homogeneity can be quantified by the parameter called beam non - uniformity ratio (BNR), which represents the ratio of the highest intensity in the field to the intensity averaged over the effective radiating area. Sometimes, distribution can be non-uniform and can potentially generate regions of high local pressure, also called hot spots [8]. These regions may produce excessive heating in small regions of the tissue. According to IEC standard 61689 [9] transducers with BNR > 8 are considered unsafe. Since the first introduction of standards and limits for output parameters of the devices used in physiotherapy a numerous surveys has been published [10–12]. These papers show that lot of physiotherapy devices have significant differences between the indicated and actual output power. Also, some cases of excessive heating as a result of equipment failure have been described [12]. All these publications have emphasized the need for measurement and calibration equipment used in physiotherapy. This paper is successor of earlier papers which involved a method for the quality assurance of physiotherapy transducer heads using a thermochromics material [3,5]. The aim of our work was an addendum to our previous work related with the origin of the systematic underestimation of the ERA values which has been observed in our previous work [5]. Besides, this study investigated whether this method could be employed for the verification of BNR of physiotherapy ultrasound treatment heads within a clinical environment.
Fig. 1. Schematic presentation of experimental set – up showing the cross section of thermochromic tile, transducer and the direction of application of the ultrasonic beam.
measurements can be found in Butterworth et al. [3], where it is described as a Type B tile. The top layer, with thickness of 3 mm, is optically transparent with relatively low ultrasonic absorption and water matched acoustic impedance. The thermochromatic pigment whose switching temperature is around 30 °C is added to intermediate layer. The backing layer is white and strongly absorbing to ultrasound and gradually becomes visible through the thermochromic – loaded intermediate layer as it changes colour from blue to colourless. The thermochromic tile was removed from the refrigerator and maintained at room temperature for at least 1 h before measurement to allow temperature equilibration. The image capture protocol defined in [5] has been applied. Firstly, a reference image of the tile next to a ruler was acquired immediately prior to ultrasonic exposure. After that, the ultrasound transducer was placed on the tile with coupling gel as used in clinical conditions. After ultrasonic exposure, the transducer was removed and the tile wiped clean of coupling gel. Approximately 10–20 s after insonation of the tile, optical images were acquired using a digital camera Optio WG-1 (Pentax Ricoh imaging Co., Ltd., Indonesia). All images were imported and postprocessed in order to calculate effective radiating area (ERA) and beam non - uniformity ratio (BNR) for transducers of interest. The calculation was performed using the algorithm developed for postprocessing optical images for the purpose of this work. Once the reference and beam profile images were imported into the program, they are converted from colour to grey scale. Afterwards, a difference image is produced by subtracting the reference image from the beam profile image. The millimetre scale on the ruler, was also photographed, and was used to enable dimensional scaling leading to the calculation of the ERA and BNR.
2. Material and methods In this work a non–standardized method developed at National Physical Laboratory (NPL) was used for quality assurance of ultrasound therapy beams. The method consists of exposing the thermochromic tile to the ultrasonic beam thereby forming a thermal image of the intensity profile of transducer. The image is in turn photographed and analysed using software developed for that purpose. Nine physiotherapy ultrasound treatment heads (able to function at frequencies close to either 1 MHz or 3 MHz) used for physiotherapy at Clinical Hospital Center Rijeka were measured again by using thermochromic tile in order to obtain new ERA values for comparison with the four years old data and to determine BNR values. The measurements for each ultrasound treatment head were repeated three times for respective measuring conditions. The thermochromic tile used in measurements was obtained from National Physical Laboratory, UK and was manufactured by Acoustic Polymers Ltd. (Mitcheldean, Gloucestershire, UK). It consists of three discrete layers sandwiched together. A simple schematic of the cross section of thermochromic tile is presented on Fig. 1. These layers are based on a polyurethane material, previously developed and applied as a material for an absorbing target of a radiation force balance [13]. Since exact values for material properties are not known, for theoretical prediction it was assumed that thermocromic tile is made from anechoic material for medical ultrasonic applications described in reference [13]. A detailed description of the thermochromic tile used for our
2.1. Calculation of effective radiating area The effective radiating area (ERA) denotes the area close to the face of transducer over which the majority of ultrasonic power is distributed. There are two specification standards which describe methods for determination of the ERA. The International Electrotechnical Commission (IEC) and the United States Food and Drug Administration (FDA) specification standards describe methods for determining ERA. However, the IEC and FDA specification standards define effective radiating area in different ways. Both approaches were used in this paper. According to standard IEC 61689 [9], ERA is evaluated through the derivation of an intermediate quantity called the beam cross-sectional area (ABCS ) . The ABCS is defined as the minimum area which contains 75% of the total mean square acoustic pressure. It is determined by sorting analysis of the acquired data. ERA of the treatment head is 2
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
calculated by multiplying the beam cross-sectional area determined at a distance of 0.3 cm from the treatment head’s face, by a dimensionless factor FAC given by 1.333 [9]. The procedure was based on the assumption [2] that the degree of whiteness at each point bears a simple relationship to the local time-average intensity. In order to determine the beam cross-sectional area, first sorting of all the pixel values into descending order was performed. In this procedure the gray values of the pixels were used. Afterward, a plot of cumulative power against cumulative area is produced. The beam cross-sectional area is then derived by reading from the plot cumulative area corresponding to 75% of the transmitted power. According to standard FDA [14] the ERA was calculated as the area over which the intensity was larger than 5% of the peak intensity. The ERA was determined for various clinically used treatment heads with frequencies in the range from 1 MHz to 3.3 MHz, and intensities range from 1 W/cm2 to 2 W/cm2 . They were electrically driven with their respective physiotherapy systems, in a way that they would be used in clinical practice.
Table 1 The ERA and BNR values for physiotherapy ultrasound transducers clinically used in Clinical Hospital Center Rijeka. Comparison of ERA determined using the IEC and FDA methods, explained in Section 2. Values are presented as mean ± standard deviation (SD). ERA/cm2 Transducer Cosmogama F120 Sonoplus 992 ENRAF NONIUS Sonoplus 992 ENRAF NONIUS Gymna Combi 200 Gymna Combi 200 Gymna Pulson 100 Cosmogamma US10 Cosmogamma Mixing10 Cosmogamma F230
2.2. Calculation of beam non-uniformity ratio
2 pmax ERA
pmst A0
,
(1)
2.3. Estimation of temperature The relationship between temperature rise and gray – scale change was obtained with parallel thermal and visual monitoring which was conducted with combination of infra-red and imaging cameras [3]. We estimate temperature from the obtained gray intensity on thermochromic beam image. For experimental estimation of temperature we use [3, Figure 13] which present dependence of gray – scale intensity on temperature. We compare experimentally obtained temperature with theoretical prediction described by Nyborg [15]. For the cylindrical source with radius x and height z* temperature on ring axis at distance z from the transducer is given by [15, eq:16]
{
qv x E [2 r
1 4c v
erfc(t *
R)] +
}
1 erfc(t * + R) dx dz *. E (2a)
where
E = exp
r
, t* =
t
, R=
r . 4 t
Specification
IEC
FDA
IEC
1 1
5 5
3.1 ± 0.4 3.8 ± 0.1
4.0 ± 0.3 5.4 ± 0.2
3.8 ± 0.4 3.1 ± 0.1
3
5
4.0 ± 0.4
5.7 ± 0.6
6.9 ± 0.2
1.1 3.3 1 1 1
4 4.7 4 3.6 3.6
3.3 ± 4.1 ± 3.6 ± 2.9 ± 3.1 ±
0.6 0.6 0.7 0.2 0.5
3.8 ± 0.8 5.4 ± 1.2 4.3 ± 0.8 3.9 ± 0.4 3.7 ± 0.8
5.6 ± 7.4 ± 3.3 ± 2.9 ± 3.2 ±
1
5
2.7 ± 0.6
3.1 ± 0.7
9.8 ± 1.1
1.0 0.8 0.6 0.7 1.1
The evaluation of the performance of physiotherapy ultrasound transducers was performed in the hospital environment using thermochromics tile. Thermal images were postprocessed in order to estimate effective radiating area (ERA) and beam non-uniformity ratio (BNR) for tested transducers. Obtained results for the calculated ERA values, and respective Type A (random) uncertainties for nine transducers were presented previously [5]. In this work BNR values were calculated from re – measurements of the same transducers and results are presented in Table 1. Results for ERA values are different from results presented previously in reference [5]. Differences between new and four years old data varied between +1.8% and −20.8%, except for the defective transducer Cosmogamma F230 whose ERA value varied even more (−41.9%). The root-mean-squared difference calculated over the nine treatment heads is −21.6% indicating that generally new data for ERA are smaller than old data. In order to find out the origin of the systematic underestimation of the ERA values, which has been observed in [5] the ERA values for the each transducer were calculated according to both, IEC and FDA, specification standards. Results are also presented in Table 1. Results suggested that ERA values derived according to FDA method are larger than those derived by IEC 61689 methodology. In our opinion, this could be the reason for the underestimation of the ERA values, which has been observed in our previous work [5]. Namely, the ERA values for treatment heads used in our previous work was determined according to current IEC methodology. Thermocromic images for three different ultrasound treatment heads are shown on Fig. 2. These specific devices were selected because obtained images on thermochromic material show three completely different shapes of ultrasonic beam and thus show how this method can be used for visualization of the beam and testing of ultrasound physiotherapy treatment heads. From obtained results for BNR and from thermal images presented on Fig. 2 it can be seen that treatment head presented on Fig. 2(a) has non-uniform intensity distribution. Calculated BNR for that treatment head was 9.76 which is too much for regular application because according to IEC 61689 transducers with BNR > 8 are considered unsafe for medical application [9]. The beam intensity map shown in Fig. 2(b) shows the symmetry and relatively uniform nature of the intensity distribution which is confirmed with calculated value for BNR of 3.09. Fig. 2(c) shows the intensity map and thermal image from a 3.3 MHz transducer which shows a small region with high intensity in the centre of the beam surrounded by wide region of much lower intensity and then one ring with higher intensity like a donut. Calculated BNR for
where A0 is the unit area of the raster scan and pmst is the total mean square acoustic pressure. To acquire this data hydrophone scans over a number of planes perpendicular to the beam axis are required. In this work the beam non - uniformity ratio was expressed as ratio of the maximum intensity in the beam to the spatial average of intensity, where the spatial average is taken over the effective radiating area. The maximum intensity in the beam is proportional to the square 2 / A0 ) and the spatial of the maximum r.m.s acoustic pressure (pmax average intensity is proportional to spatial average of the square r.m.s. acoustic pressure (pmst / ERA) .
T=
f/MHz
3. Results
The beam non-uniformity ratio (BNR) is defined as the ratio of the square of the maximum r.m.s. acoustic pressure ( pmax ) to the spatial average of the square of the r.m.s. acoustic pressure [9], where the spatial average is taken over the effective radiating area (ERA). BNR is calculated according to following expression:
BNR =
BNR
(2b)
Here r is the distance from any point on the ring to the axial point at z, i.e. r = x 2 + (z z *)2 . qv is the heat source function, T the temperature rise above the ambient level, the thermal diffusivity, the time constant for perfusion and c v the volume specific heat. 3
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
Fig. 2. Beam uniformity analysis of thermocromic images for three transducer heads: (a) Transducer Cosmogamma F230, f = 1 MHz, BNR = 9.76 ; (b) Transducer Sonoplus 992 ENRAF NONIUS, f = 1 MHz, BNR = 3.09 ; (c) Tranducer Gymna Combi 200, f = 3.3 MHz, BNR = 7.35.
that treatment head was 7.35. In order to determine the optimal exposure time for all nine devices testing was conducted with exposure periods: 3, 5 and 7 s while the beam intensity was always same and it was I = 2 W/cm2 . This range of exposure durations was chosen to provide suitable temperature rise in the absorber which will cause visible colour change. Beam profiles for different ultrasound exposure durations for transducer Cosmogamma US10 with f = 1 MHz , and intensity I = 2 W/cm2 are shown on Fig. 3. It is visible that difference between beam profiles for intensity I = 2 W/cm2 obtained after insonation time of 5 and 7 s is small. We conclude that 5 s insonation is good enough for obtaining reliable data for determination of ERA and BNR if average beam intensity is 2 W/cm2 .
This exposure time was found long enough to trigger thermochromic change and short enough to avoid thermal saturation as well as to prevent damage of the thermochromic tile. For 3 s insonation temperature rise above 30 °C only in the small area around the middle of profile, which is visible on the bottom panel. It means that 3 s insonation time was too short for obtaining good quality thermal image. The beam profiles for different intensities and for the same transducer were also measured (Fig. 4) to explore influence of changing intensity on beam profile and determination of ERA and BNR. In order to investigate the changing of optical image of the tile “whiteness” and corresponding beam profiles with time after insonation, optical image of the tile “whiteness” were photographed
Fig. 3. Beam profiles for ultrasound exposure durations of 3 s (stars), 5 s (circles) and 7 s (boxes) for transducer Cosmogamma US10 with f = 1 MHz , and intensity I = 2 W/cm2 . The bottom image shows a temperature profiles. We estimate temperature from the obtained gray intensity as described in Section 2.3.
Fig. 4. Gray intensity and temperature profiles measured using different nominal intensities of 1 W/cm2 (cross, dashed) and 2 W/cm2 (boxes, full line) for the same transducer with frequency f = 1 MHz and 7 s ultrasound exposure. 4
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
4. Discussion and conclusion For quick visualisation of ultrasound beam it is interesting to see thermal images and corresponding beam profiles for examined treatment head. Beam parameters for three different physiotherapy ultrasound treatment heads were analysed in more details since they exhibited significantly different beam intensity patterns. The purpose was to investigate the potential of the thermochromic tile to distinguish highly non - uniform beams with high value of BNR. From thermal images obtained for three transducer heads presented in Fig. 2 it was observed that thermal image shown in Fig. 2(a) and (c) have non uniform intensity distribution while thermal image shown in Fig. 2(b) has uniform intensity distribution. An equivalent analysis arises for BNR. The smallest value of BNR were calculated for thermal image presented in Fig. 2(b), while thermal image presented in Fig. 2(a) yields the largest BNR value. Moreover, despite the non – circular heating pattern, the value of BNR for this thermal image is too large and thus this transducer head can be considered unsafe for regular application. As previously mentioned [5], this thermal image was produced with an old transducer head which is decommissioned consequently. From results presented in this paper it can be seen that although the thermocromic method was developed primarily for qualitative analysis and characterization of ultrasonic physiotherapy equipment, it also offers the potential for quantitative measurements. The effective radiated area (ERA) values obtained in our previous work [5] for seven of nine physiotherapy transducers are lower than the respective ERA values reported by manufacturer. It is important to note that generally it is not clear how the ERA values declared by the manufacturer were determined. For some transducers, such as transducers Sonoplus 992 ENRAF NONIUS, in the transducer specification is written that ERA value is derived according FDA methodology. Unfortunately, for some transducers methodology which is used for determination of ERA and BNR is unknown. In our previous work ERA was determined, using the methodology described in IEC 61689 [9]. Comparison of the recently determined ERA values with four years old data showed that for some transducers a new data are significantly smaller than the old ones. It is anticipated that one of the reasons for such a difference might be degradation of some transducers with time. However, for most transducers the difference in calculated values for ERA is rather small. This indicates the long – time constancy of the physiotherapeutic ultrasound devices. In this work the FDA methodology [14] for comparison purposes was used. The results in Table 1 indicate that ERA values obtained according the FDA methodology are larger than ERA values determined according to IEC 61689. Results of our measurements have shown that this could be one of the reasons for underestimation of ERA values obtained previously. The differences in the ERA values determined by the two standard methods are also discussed in [6] and noticed that the ERA values determined using the FDA method could be up to 69% larger than those produced using IEC 61689. Costa et al. [4] also reported that the heated area values obtained with thermochromic phantom are lower than the respective effective radiating area determined as described in IEC 61689. The results presented in Fig. 3 show dependence of the beam and temperature profiles on time of insonation. It can be seen, that exposure duration of 3 s was too short for obtaining good quality thermal image. Energy deposition within the tile results in a temperature rise and it is obvious that for 3 s exposure duration the temperature rise was not large enough to cause a colour change in thermochromics material. Exposure duration of 5 s seems optimum in order to obtain image appropriate for determination of ERA. When duration of exposure was 7 s thermochromics image is only slightly changed. From the results shown in Table 2 it can be seen the influence of increase of the exposure time on respective beam parameters: ERA and BNR. The values for ERA slightly increase with insonation time, while BNR value decrease with insonation time. It is probably due to spreading of heat through the material causing thermal imaging
Fig. 5. Changing of beam profiles with time passed after insonation for treatment head Sonoplus 992 with frequency 3 MHz at intensity 2 W/cm2 after 5 s ultrasound exposure.
successively many times within 300 s after the exposure of the thermochromic tile was ceased. Obtained beam profiles for various times passed after insonation are presented in Fig. 5. Within 1 min the shape of beam profile doesn’t change significantly. Because of cooling and spreading of heat in the thermocromic material after five minutes beam profile is getting less visible and slightly spreading. In order to predict temperature rise in thermochromic material we have used bio-heat transfer equation for a circular source transducer Eq. (16) proposed by Nyborg [15]. In our calculation it was assumed that material has following characteristics: the absorption coefficient at f = 1 MHz is = 27 dB/cm = 622 Np/m , time constant = 250s , thermal diffusivity = 1.9 × 10 7m2 /s . Tile thickness is 10 mm. The volume rate of heat generation is given by [15, Eq. (18)] (3)
qv = 2 I0exp( 2 z *),
where intensity is I0 = 1W/cm2 . Results of calculation and comparison with experimental results are presented in Table 2. From Table 2 we can conclude that ERA values increase with exposure time as expected. In contrast, BNR values decreases. Using the theoretical prediction described by Nyborg [15] we have calculated the expected temperature rise in the material and with assumption that the initial temperature of thermochromic tile was equal to room temperature (20.3 °C), we calculated the expected temperature in the material. From results presented in Table 2 it can be seen that experimentally determined temperature and theoretical prediction of temperature in thermochromic material are in good agreement.
Table 2 The ERA and BNR values for different ultrasound exposure durations for treatment head Gymna PULSON 100 with f = 1 MHz , and intensity I = 1 W/cm2 . To obtain temperature from gray – scale intensity we used fitting curve from reference [3] which has uncertainty of about 0.5 °C. Theoretical prediction of temperatures (T) are compared with experimental ones. Experimental results Exposure/s
ERA/cm2
3 5 7 9
1.17 2.74 3.12 3.51
T/°C mean ± SD 28.8 ± 30.6 ± 32.1 ± 33.4 ±
1.0 0.4 0.3 0.2
Theoretical prediction BNR
T /°C
T/°C
5.84 4.53 4.06 3.97
8.4 10.4 12.1 13.5
28.7 30.7 32.1 33.8
5
Ultrasonics 99 (2019) 105943
G. Žauhar, et al.
expansion and increasing the ERA parameter value. At the same time, the colour of the tile becomes more and more uniform which results in decreasing of BNR value. Results of our measurements have shown that proposed procedure could be used for estimation of beam parameters such as ERA and BNR. In our opinion the method is suitable for quality assurance in clinical routine. Namely, it can be used for monthly checks, performed by technician with purpose of finding large deviations in transducer head performance. Furthermore, it is expected that the proposed methodology could be used as a quick test to qualitatively assess possible changes in transducer head performance during time. In order to achieve this, it would be necessary to make a thermal image of transducer head during the acceptance test of the ultrasound physiotherapy device which could be used as a reference for recurrent tests procedures. The results presented in Fig. 4 indicate that lower intensity shows smaller resulting colour change and consequently calculated ERA values were smaller. However, the results obtained for BNR shows an opposite trend. For higher intensity the obtained value of BNR is smaller. This indicates that thermal spreading may be the cause of it. Namely, the white of the backing layer is only seen after the majority of intermediate layer containing the thermochromic pigment has reached the transition temperature. When this happened significant lateral heat diffusion already occurred. The information on the structure is blurred, which causes the BNR parameter to decrease. Analysis of successive images obtained for various timed passed after insonation (Fig. 5) showed almost constant ERA and BNR value during first minute. From the comparison of the successive beam profiles it is apparent that the increase of the time passed after the insonation results in the lateral heat spread which is visible as widening of beam profile and lowering of beam profile due to cooling. Results of our measurements have shown that thermocromic tile can be used for check of ultrasound transducers in the hospital environment. Furthermore, our work has also shown that it is possible to postprocess thermal images in order to estimate important beam parameters such as effective radiating area (ERA) and beam non-uniformity ratio (BNR). The limitation of this method is that thermocromatic tile will only enable the determination of BNR in a single plane at approximate distance of 5 mm from the transducers front face. If the ‘hot – spot’ is at greater depths it may not show up on the derived beam non-uniformity ratio (BNR) using thermochromic tile. The methodology presented in this paper clearly needs further validation to compare with existing methods. Unfortunately, this requires detailed beam-plotting using a hydrophone. This is a proof–of–concept study, requiring more extensive validation in future work.
Acknowledgments Authors would like to thank to Dr. Bajram Zeqiri at the United Kingdom National Physical Laboratory for the donation of thermocromic tile without which this work would not be possible. This work was supported by the University of Rijeka under the project number: uniri-prirod-18-75. References [1] B.D. Cook, R.E. Werchan, Mapping ultrasound fields with cholesteric liquid crystals, Ultrasonics 9 (1971) 88–94, https://doi.org/10.1016/0041-624X(71)90126-0. [2] K. Martin, R. Fernandez, A thermal beam-shape phantom for ultrasound physiotherapy transducers, Ultrasound Med. Biol. 23 (1997) 1267–1274, https://doi. org/10.1016/S0301-5629(97)00109-9. [3] I. Butterworth, J. Barrie, B. Zeqiri, G. Žauhar, B. Parisot, Exploiting thermochromic materials for the rapid quality assurance of physiotherapy ultrasound treatment heads, Ultrasound Med. Biol. 38 (2012) 767–776, https://doi.org/10.1016/j. ultrasmedbio.2012.01.021. [4] R.M. Costa, A. Alvarenga, R.P.B. Costa-Felix, T.P. Omena, M.A. von Krüger, W.C.A. Pereira, Thermochromic phantom and measurement protocol for qualitative analysis of ultrasound physiotherapy systems, Ultrasound Med. Biol. 42 (2016) 299–307, https://doi.org/10.1016/j.ultrasmedbio.2015.08.017. [5] G. Žauhar, D. Smilović Radojčić, D. Dobravac, S. Jurković, Quantitative testing of physiotherapy ultrasound beam patterns within a clinical environment using a thermochromic tile, Ultrasonics 58 (2015) 6–10, https://doi.org/10.1016/j.ultras. 2015.01.006. [6] A. Shaw, M. Hodnett, Calibration and measurement issues for therapeutic ultrasound, Ultrasonics 48 (2008) 234–252, https://doi.org/10.1016/j.ultras.2007.10. 010. [7] IEC 60601 part 2-5: Medical Electrical Equipment: Particular Requirements for the Safety of Ultrasound Physiotherapy Equipment, 2009. [8] M. Gutiérrez, H. Calás, A. Ramos, A. Vera, L. Leija, Acoustic field modeling for physiotherapy ultrasound applicators by using approximated functions of measured non-uniform radiation distributions, Ultrasonics 5 (2012) 767–777, https://doi. org/10.1016/j.ultras.2012.02.006. [9] IEC 61689: Ultrasonics – Physiotherapy Systems – Field Specifications And Methods of Measurement In the Frequency Range 0.5 MHz to 5 MHz, 2013. [10] S.D. Pye, C. Milford, The performance of ultrasound physiotherapy machines In Lothian Region, Scotland, Ultrasond Med. Biol. 20 (4) (1994) 347–359, https://doi. org/10.1016/0301-5629(94)90003-5. [11] R. Hekkenberg, W. Oosterban, W.V. Beekum, Evaluation of ultrasound therapy devices, Physiotherapy 72 (1986) 390–395. [12] S.D. Pye, Ultrasound therapy equipment – does it perform? Physiotherapy 82 (1) (1996) 39–44, https://doi.org/10.1016/S0031-9406(05)66996-9. [13] B. Zeqiri, C. Bickley, A new anechoic material for medical ultrasonic applications, Ultrasound Med. Biol. 26 (2000) 481–485, https://doi.org/10.1016/S03015629(99)00147-7. [14] US Food and Drug Administration (FDA), Radiation Safety Performance Standard. Ultrasound Therapy Products, title 21, Part 1050.10, Rules and Regulations, February 1979. [15] W.L. Nyborg, Solutions of the bio-heat transfer equation, Phys. Med. Biol. 33 (7) (1988) 785–792.
6