# Low-Cost Thermochromic Quality Assurance Phantom for Therapeutic Ultrasound Devices: A Proof of Concept

Matt Eames, Zachary Larrabee, Arik Hananel, Frederic Padilla, Jean-François Aubry

► To cite this version:

Matt Eames, Zachary Larrabee, Arik Hananel, Frederic Padilla, Jean-François Aubry. Low-Cost Thermochromic Quality Assurance Phantom for Therapeutic Ultrasound Devices: A Proof of Concept. Ultrasound in Medicine & Biology, 2023, 49 (1), pp.269-277. 10.1016/j.ultrasmedbio.2022.09.001. hal-04296818

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Low-Cost Thermochromic Quality Assurance Phantom for Therapeutic

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Ultrasound Devices: a proof of concept

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Matt Eames¹,², Zachary Larrabee¹, Arik Hananel¹,², Frederic Padilla¹,³, Jean-François Aubry³,⁴

1 Focused Ultrasound Foundation, Charlottesville, VA, USA,

2 Department of Radiation Oncology, University of Virginia, Charlottesville, VA, USA.

3 Department of Radiology, University of Virginia School of Medicine, Charlottesville, VA, USA.

4 Physics for Medicine Paris, CNRS UMR8063, INSERM U979, ESPCI Paris, PSL Research University, Paris,
France

Corresponding author:

Jean-Francois Aubry (e-mail: jean-francois.aubry@espci.fr)
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**Abstract**

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High intensity focused ultrasound (HIFU) transducer acoustic output can vary over time due to an inconsistent power supply, damage to the transducer, or deterioration over time. Therefore, easy implementation of a daily quality assurance (DQA) method is of great importance for preclinical research and clinical applications.

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We present here a thermochromic material-based phantom, validated by thermal simulations, and shown to provide repeatable visual power output assessments in fewer than 15 seconds that are accurate to within 10%. Whereas current available methods such as radiation force balance measurements provide an estimate of the total acoustic power, we demonstrate here that the thermochromic phantom is sensitive to the shape of the acoustic field at focus, by changing the aperture of a multielement transducer with a fixed acoustic power. The proposed phantom allows the end-user to visually assess the transducer's functionality without resorting to expensive, time-consuming hydrophone measurements or image analysis.

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**Keywords**

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Therapeutics, System & device design, General Physical Acoustics, Medical Transducers

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INTRODUCTION

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With the increasing use of focused ultrasound (FUS) for research and development applications and for the treatment of medical conditions in the clinical arena, there is a growing need for a simple, expeditious, and inexpensive method for assessing constant and safe device output. Literature reports that, within a sample of 64 physiotherapy therapeutic ultrasound systems, the measured output power of 59% of the systems falls outside a +/- 20% deviation from prescribed power output (Schabrun et al. 2008). Patient safety may be compromised by an unexpected higher dosage. On the contrary, the efficacy of the treatment would be affected by a lower dosage.

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Existing methods for conducting quality assurance on ultrasound systems require complex set-ups and specific expertise. The most comprehensive method is the measurement of the 3D pressure field generated by the transducer with a hydrophone. Due to the intensities involved in therapeutic ultrasound, an appropriate hydrophone should be used, such as fiber-optic displacement sensors, commercial HIFU needle hydrophone, or membrane hydrophone with a protective coating against cavitation effects (Haller et al. 2012). Three-dimensional measurements are time consuming. The 3D pressure field can be inferred from pre-focal 2D planar field acquisition using back- and forward-projection (Clement and Hynynen 2000; Sapozhnikov et al. 2015; Schafer and Lewin 1989). The 2D projection of the HIFU beam can be quickly visualized by Schlieren imaging (Kudo et al. 2004). This technique takes advantage of acoustic-induced changes in optical refractive index. Nevertheless, quantitative characterization of the 3D pressure field requires several projections at varying angles coupled with tomographic backprojection, and is therefore not straightforward to extract (Civale et al. 2015). Infrared cameras have been shown efficient for mapping the 3D heating distribution induced by the transducer in an absorbing material (Khokhlova et al. 2013; Shaw and Nunn 2010). It provides excellent spatial and thermal resolutions but the main disadvantages are the cost of the camera and the inability of infrared radiation to travel through water.

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Radiation force balance measurements can be used to quantify the total acoustic power emitted by the transducer (Payne et al. 2021). In these measurements, the transducer is aimed at a target (an ultrasound absorber) suspended in water tank from precision balance in a draft-free environment (Aubry 2012). The momentum carried by the ultrasonic wave is measured as a weight shift on the balance after reflection or absorption on the target (Gélat and Shaw 2015; Maruvada et al. 2007).
Non-invasive quantitative thermometry can also be achieved with ultrasound, based on detection and estimation of minute temperature-induced echo shifts, which can be processed to generate the 2D maps of temperature change. Important challenges remain however, requiring development of dedicated software architecture and signal processing, to compensate artifacts arising from tissues or samples deformation leading to echo decorrelation and ultrasound beam distortion (Ebbini et al. 2018). For Magnetic Resonance guided Focused Ultrasound Treatment (MRgFUS), calibration can be achieved with the sole use of a tissue-mimicking hydrogel in conjunction with the MR. The thermal rise induced by the ultrasound beam inside the hydrogel can indeed be quantified and mapped with MR thermometry (Aubry 2012; McDannold and Hynynen 2006). Permanent thermochromic phantoms have been proposed to counteract the need for MR to image the thermal rise: the color of the 3D phantom changes permanently when the temperature reaches a threshold. The cross section can be assessed by cutting the phantom open (Negussie et al. 2016). Thermochromic paper has been proposed as the most rapid quality assurance technique (Butterworth et al. 2012). Nevertheless, it relies on the analysis of images captured by a digital camera and on automatic fitting of the beam profile. Good and consistent lighting conditions are critical. Thermochromic paper approaches thus concentrated on improving the precision of the estimation of the spatial dimension of the color change. We propose here to use a thermochromic liquid crystal affixed to a highly absorbing material to record the time for inducing a color change, after the heat radiates through the absorbing material. We hypothesize that the time needed for the heat to diffuse from the transducer side to the thermochromic side is a function of power and that this measurement can be used to assess the stability of the transducer. We additionally investigate the sensitivity of the thermochromic phantom to the shape of the acoustic field at focus, by changing the aperture of a multielement transducer with a fixed acoustic power.

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# MATERIALS AND METHODS

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## Phantom

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The phantom is a 2.8 mm thick highly attenuating ultrasound absorber (AptFlex F28, Precision Acoustics Ltd, Dorchester, UK) (Fig. 1 and Table 1), with a surface layer of thermochromic liquid crystals (TLCs) (R35C5B, LCR Hallcrest LLC, Glenview, IL, USA). The TLC material is black in color until it transitions to red at 35 °C followed by green at 36 °C and blue at 40 °C, allowing an observer to visually detect when temperature reaches and exceed these thresholds (Fig 1). The time to see color change, denoted as changing time, is nonzero because the absorbed heat must diffuse from the non-TLC side of the absorber to the TLC surface. Thus, the changing time for TLC of a given thickness is a function of baseline temperature, acoustic power and center frequency.

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The phantom can be designed to be positioned either at the surface or at the acoustic focus of the transducer of interest. Fig. 2 depicts the two transducers and phantom geometries tested: one with a flat 11 mm aperture transducer (Model XDR058, Sonic Concepts, Bothel, WA) to which the phantom was designed to be coupled with acoustic gel directly to the face of the transducer; and another the Insightec Neuro device, a 30 cm-diameter hemispherical transducer, to which the phantom was mounted at the acoustic focus using a 3D printed holder. In both cases, the phantom measures 2.8 mm in thickness.

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## Numerical simulations

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A time-domain finite difference simulation of ultrasound-induced thermal rise in the acoustic absorbing material was performed.

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The pressure field was first measured experimentally. The pressure was then used as an input to the bioheat equation.

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## Pressure Field measurements to characterize the acoustical output of the transducers

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The pressure field of a high-power 1.1 MHz transducer (Model XDR058, Sonic Concepts, Bothell, WA, USA) was measured in water with a 400 µm needle hydrophone (HNA-0400, Onda Corp., Sunnyvale, CA, USA) mounted on a three-axis motor gantry (AIMS III, Onda Corp., Sunnyvale, CA, USA). The active surface of the transducer is an 11 mm diameter disc. An x-z plane scan was performed, z being the axisymmetric axis of the

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transducer. The transducer was mounted on a mechanical holder to position it along the z-gantry axis. The scan was 30 mm along the x axis and 40 mm along the z axis with a 0.25 mm step for each axis. Assuming symmetry in the x and y directions, this scan was then radially projected to obtain a three dimensional pressure field, P(x,y,z).

The 3D pressure field $p_{Aptflex}$ within the 2.8 mm-thick absorbing layer was deduced from the measured field in water $p_{water}$ by adding an exponential attenuation as a function of distance:

$$p_{Aptflex}(x, y, z) = p_{water}(x, y, z) \cdot e^{-\alpha z} \quad (1)$$

where $\alpha$ is the attenuation coefficient in Np/m at the (x,y,z) coordinate. The pressure field in the Aptflex was then used as an input for the thermal simulations (see next section).

Table 1 provides material properties used in the simulation.

The pressure field of the Neuro system (Insightec, Tirat Carmel, Israel) was also measured with the same setup. The transducer was operated at 670 kHz, with the central elements activated only (256 active elements out of the 1024-elements). The z axis corresponds to the axisymmetric axis of the transducer. Two planes were acquired: the transverse XY plane and the longitudinal XZ plane. Each plane was 10x10 mm² with a 0.25 mm step in each direction.

### Estimation of the thermal rise and time to color change by solving the bioheat equation

The physical heat transfer is modeled by a Matlab (Mathworks, Natick, MA, USA) simulation of the bio-heat equation throughout the experiment volume. The equation is solved with a finite-difference scheme in which the derivatives of the equation are replaced by second order polynomial approximation:

$$\rho c_p \frac{\partial T}{\partial t} = k \nabla^2 T + \frac{\alpha P^2}{\rho c} \quad (2)$$

Where T is temperature, k is thermal conductivity, $\alpha$ is the absorption coefficient, c is speed of sound, $\rho$ is density, $c_p$ is specific heat, and P is the acoustic pressure amplitude field. The pressure profile from one scan is assumed to be constant for all simulations. This pressure is normalized and then modulated by the square root of the electrical power to model thermal change for escalating power values. The simulations are performed in a
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61x61x44 grid with an isotropic resolution (0.3215 mm) and a 0.05 s time step. All voxels corresponding to water are set to a fixed temperature (17 °C) at each time step to account for its cooling effect. Dirichlet conditions are applied at each boundary planes except for the air interface where a Neumann condition is applied to account for the air thermal insulation, as compared to water.

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The minimum time required to achieve a peak temperature of 35 °C in a single voxel at the distal plane of the absorbing medium is recorded, and the simulation is re-run across several acoustic output intensities.

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## Time to color change measurements

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Measurements of the time until first-observed TLC material color change (T=35 °C) were recorded using a stopwatch on the two different setups.

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## Single element transducer

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Measurements were performed across a [0.4-12] W range of electric power. All power levels were measured 8 times within six months.

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Two DQA phantoms were assembled from different sections of single sheets of AptFlex and TLC material. Coupling gel (Aquasonic 100, Parker Laboratories) was placed between the transducer and the phantom.

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Two identical SonicConcepts transducers (Model XDR058, Sonic Concepts, Bothel, WA) with 1.1 MHz acoustic working frequency were used to sonicate the phantoms.

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A waveform generator (DG1022, Rigol, Beaverton, OR, USA) supplied a 1.1 MHz sinusoid to a power amplifier (100A100, Amplifier Research Corp., Souderton, PA, USA). The amplified signal was monitored by an oscilloscope (TDS2001C, Tektronix, Beaverton, OR, USA) and connected to the input of a Sonic Concepts matching network and ultrasound transducer. The impedance of the transducer with matching network was measured with a network analyzer (4194A Impedance/Gain-Phase Analyzer, Yokogawa Hewlett Packard, Tokyo, Japan) and was used to calculate the electric power applied to the transducer based on the measured voltage.

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The time until first visible TLC color change was recorded for electrical power in the range 0.4-12 W. Temperature was monitored by Copper-Constantan thermocouples (Type T, 0.010 in diameter, Omega, Stamford,

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CT, USA) attached to the side of the transducers housing in order to ensure the temperature returned to baseline before performing a new measurement. Recording was achieved by an acquisition board (TC-08, Pico Technology, Cambridgeshire, UK) connected to a PC. An example of the longest cool-down period (for the highest power) can be seen in Fig. 3.

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## Multielement transducer

Measurements were performed using the central 256 elements of the 1024-element hemi-spherical, 30 cm diameter, 15 cm focal length Exablate Neuro high frequency transducer (Insightec Inc., Tirat Carmel, Israel), across a [0.2-1.5] W range of electric power. Over the course of 6 months, all power levels have been measured 6 times with 4 repetitions. The system was operated at 670 kHz The dedicated Insightec user interface and hardware were used to power the transducer in continuous-wave mode for up to 45 s in duration, as necessary, to measure time to first TLC material color change. The Insightec logfile measures and records the actual total electrical energy delivered by the system during the sonication. This value was divided by sonication time to estimate the delivered average power.

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Two DQA phantoms were assembled to suit the multielement transducer, and the same 3D-printed holder was used to ensure reliable repositioning of the phantom at the geometric focus across all measurements. A three minute cool down period was used between measurements, and the phantom, which was large enough to permit it to be placed 2 cm off-center in the plane of the focal spot, was rotated 90° between sonications such that more than 12 minutes elapsed between sonications at the same region of the phantom. Four measurements (one at each rotation angle) were recorded per power setting, per experiment session.

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192 **RESULTS**

193 <u>Pressure field measurements</u>

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The peak negative pressure is displayed, in decibels, in Fig. 4 in the transverse and longitudinal planes for both
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the single-element (a) and Insightec Neuro (b) transducers.

196 <u>Thermochromic phantom measurements</u>

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The time to first color change is plotted for the single-element transducers (Fig. 5). During the course of testing,
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one transducer used for these measurements was damaged, as is evident in the collected data.

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The time to first color change for the Insightec Neuro transducer is plotted in (Fig. 6). In order to reproduce the
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effect of reduced acoustic output, data were collected with the central 256 elements (labeled "Apodized") and
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with the full array, which had the effect of distributing the requested power across more elements by energizing
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the outer ring of 768 elements in addition to the central 256 (labeled "Unapodized"). This served to produce a
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shorter focal spot in the propagation direction. As with the single element case, the effect of this change is evident
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in the data. Of note, one set of measurements (blue, Fig 6) were supposed to be performed without apodization.
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As this set of measurement seems to align with the apodized data, we double checked the logs of the Exablate
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Neuro system and noticed that the time of acquisition corresponded to apodized sonications. The have been labeled
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as "Apodized unintent.".

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Single-element data and Insightec Neuro data are plotted against the associated bioheat simulation results in the
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respective Figures.

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DISCUSSION

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The single-element simulation reproduces the general trend of the experimental results (Fig. 5) but at high electric powers the simulation indicates a time-to-color-change that is 2 seconds faster than experimental results.

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This could be due to both latency in the TLC material and observer reaction time.

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Two different behaviors, both for simulations and experiments, were observed for each of the two apodization scenario tested with the Insightec Neuro (Fig. 6): the time to see a color change was much shorter without apodization (using only 256 central elements out of the 1024) that with apodization (using the full 1024 elements) due to the longer focal spot in the propagation direction without apodization. Nevertheless, at lower electric powers, the simulation indicates a time-to-color-change that is faster than experimental measurements. This is likely due primarily to observer reaction time at longer times to first color change, in which the color change is more gradual than those that occur at higher powers.

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For optimal sensitivity, it is recommended to select an electrical power of 1.5 – 2.0 W in the single-element case and an electrical power of 0.3 – 0.5 W in the case of the Insightec Neuro. At these powers, DQA assessments are complete within 20 seconds, the color change is relatively sudden, and the typical user (with reaction time of 0.5 s) can detect a 10 % deviation in acoustic power output.

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It must be mentioned here that at the starting point of the study, the idea was to measure the diameter of the color change (as can be seen on Fig 1) as a function of power. A color change can indeed be seen when TLC material is placed in front of a transducer, even without any absorbing material. Nevertheless, the first experiments showed that the diameter of the color change depends primarily on the size of the pressure field generated by the transducer at the surface of the TLC. For this reason, we added an absorbing material between the TLC and the transducer in order to take advantage of the time needed for the temperature to build up in the absorbing material. The time until first visible TLC color change was found to depend more significantly on the acoustic power output of the transducer, as presented in this paper.

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It is noted that one of the single-element transducers used in this study was damaged during the course of the project, and its reduced performance was easily detected by the thermochromic DQA. The thermochromic phantom thus appears to be an effective way to check the proper functioning of focused ultrasound transducers.

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To explore the phantom's sensitivity to transducer deficiencies during measurements of the Insightec Neuro transducer, sonications were performed with the central 256 elements ("Apodized" data in Fig. 6) and will the full 1024 elements in the hemispherical array ("Unapodized" data in Fig. 6). The Insightec software is designed to distribute the requested power across the available elements in the array. By enabling the outer 768 array elements (and keeping power fixed), the acoustic focus becomes shorter in the axis of propagation and therefore extends a shorter distance into the absorber, having the effect of increasing the distance of thermal diffusion until the thermochromic liquid crystal is heated. The data are in broad agreement with bioheat simulation of this scenario, both indicating a perceived reduction in the thermal output in the lengthened time to first color change. Interestingly, our measurements highlighted a set of odd data. A set of measurements (Fig 6, in blue) were supposed to be performed without apodisation but appeared to align with the apodised data. The thermochromic setup appeared to be sensitive enough to the shape of the ultrasound beam to allow us to detect an error in the emission settings. This error was confirmed by looking at the logs of the Exablate neuro that automatically record all the parameters that have been used at each sonication.

The thermochromic phantom is inexpensive and straightforward to establish and set up in a lab. The cost of the materials are summarized in Table 2.

The 3mm sheet of AptFlex F28 was cut with scissors into smaller pieces for use in the thermochromic phantoms. It is reasonable to assume that a piece with length and width of 2.5 cm would suffice (for transducers with diameter or focal spot diameter less than 2.5 cm), so the 300x300x3 mm sheet of F28 could be cut into greater than 100 pieces of 30x30x3 mm absorbers. Since the sheets of Hallcrest TLC are slightly larger in length and width than the AptFlex F28, the same cutting idea was applied. The cost per phantom is estimated to be seven dollars. Furthermore, the TLC material used here changes color reversibly and can be re-used. In this paper we used two phantoms for the single element measurements: one for each element. Alternatively, F28 could be replaced by a cheaper rubber. This would bring the cost of the phantom down drastically. However, the acoustic and thermal properties of the rubber may need to be measured in order to use the phantom with confidence.

For larger apertures, a 3D printed or other custom holder is recommended in order to limit the amount of error a user can produce from assembling the setup. A holder ensures the phantom is at a fixed distance from the
transducer so that the pressure at the absorber face closest to the transducer can assumed to be constant for each trial.

The thermochromic phantom calibration method is straightforward and relatively rapid. Once the user has the phantom, ultrasound gel, and holding mechanism (either a 3D printed holder or some equivalent) calibration can be done in minutes. If calibration with one sonication at high power is sufficient for an experiment, then the calibration can be done in about a minute – the user places the transducer in the holder, puts ultrasound gel at interfaces, turns on the transducer, then waits to see a color change. The color-change time for a high power sonication would range from 2-10 seconds. If more data points are necessary, the user has multiple options. If the user has more than one phantom, they can switch out the gel pad and phantom and sonicate again without waiting between trials. This would ensure that the initial state of the system is the same between sonications. If the user only has one absorber, they can either run the phantom under cold water or wait 5-10 minutes between sonications to roughly ensure that the state of the system is the same between sonications. The temperature in the absorber could be estimated by an infrared thermometer or by a thermocouple if necessary. Regardless, the calibration time frame is in the order of minutes and not hours.

Overall, this work is a preliminary proof of concept and further developments are needed to assess the sensitivity of the approach. Different potential sources of errors could affect the approach.

Air bubbles are a possible source of error. They can form inside the gel pad, in between interfaces coupled by ultrasound gel, and in between the TLC sheet and the absorber. Air bubbles in the gel pad near the transducer surface can potentially damage the transducer. Air bubbles in the gel pad or near an interface may interfere with the ultrasound propagation and alter the lesion time recorded by the user. Air bubbles in-between the TLC sheet and absorber may alter heat transfer at the air interface and alter the lesion time recorded by the user. Fortunately, air bubbles in the gel pad are easy for the user to see and seldom form. Air bubbles in-between the TLC and absorber are harder to see.

## Ambient temperature

The ambient temperature of the room may change over time. Since the TLC shows absolute temperature change, a change in the ambient temperature would change the lesion time recorded by the user.

## Heterogeneity in Materials.
It is possible that the properties of the TLC sheet or the acoustic absorber are heterogeneous. Since each phantom is created by cutting away one section from each material, it is possible that each phantom could have different thermal or acoustic properties due to heterogeneities in the materials.

*Bonding Difference*.

It is possible that the bond between the TLC sheet and the acoustic absorber is different for different phantoms. If so, this would mean that heat would conduct from the absorber to the TLC sheet at different rates for different phantoms. This implies that the color-change time would be different depending on which phantom is used.

Prior work on thermochromic phantoms was in the form of a traditional, volumetric ultrasound phantom (Negussie et al. 2016). While this phantom proved useful for determining the volume of tissue mimic that experienced a peak temperature of 60 °C, it was a single-use phantom because of the permanent nature of the change in color of the thermochromic ink used. While the phantom we present is not volumetric, the ability to reuse the phantom dozens of times contributes to the consistency / longevity of the measurement accuracy and the low relative cost of the phantom. Such an approach could benefit to recent low cost devices developed for central nervous system applications (Biase et al. 2019; Chen et al. 2020b; Chen et al. 2021; Ferri et al. 2019; Maimbourg et al. 2018a; Maimbourg et al. 2018b; Maimbourg et al. 2018c; Pouget et al. 2020; Rabut et al. 2020; Schafer et al. 2021; Wu et al. 2018; Zou et al. 2020), low back pain treatment (Aginsky et al. 2021; Chen et al. 2020a; Perez et al. 2021) or varicose veins (Barnat et al. 2020a; Barnat et al. 2020b; Obermayer et al. 2020).

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309 **CONCLUSION**

310 We demonstrated on two transducers and two DQA phantom geometries that the thermochromic phantom
311 approach is an effective way to assess the stability of focused ultrasound transducers. Next steps involve refining
312 the simulation methods used to better validate the DQA design and deploying custom-mounted DQA phantoms
313 to end users. For example, FUS mobile (Alpharetta, GA, USA) started using such a thermochromic phantom for
314 targeting confirmation and for validation of the full functionalities of the Neurolyser platform.

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317 **ACKNOWLEDGEMENTS**

318 This work was supported by the Focused Ultrasound Foundation.

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321 **CONFLICT OF INTEREST**

322 The authors have no conflict of interest to declare.

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<figure><img src="image_2.png" /></figure>

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**Figure 1:** Top: View of the phantom after sonication with a single element flat transducer. Bottom: View of the phantom after sonication with a multi-element transducer: at the appearance of the thermal spot (a), at the maximum extent of the spot 10s after appearance of the spot (b), just before disappearance of the spot 12s after the previous picture (c).

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<figure><img src="image_3.png" /></figure>

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**Figure 2.** CAD drawing of the geometry of the phantom. X-, Y-, and Z-axes are red, green, and blue, respectively. Left: single element transducer (Model XDR058, Sonic Concepts). Right: multielement array (Exablate Neuro, Insightec)

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<figure><img src="image_4.png" /><figcaption>**Figure 3:** Transducer casing temperature, demonstrating minimum required cool-down period between sonications.</figcaption></figure>

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<figure><img src="image_5.png" /><figcaption>**Figure 4:** (a) Pressure field generated by the XDR058 single element transducer, measured by a needle hydrophone. (b) 3D pressure field generated by the Insightec Neuro measured by a needle hydrophone. For the single element, the corresponding measured peak pressure was 0.5 MPa for 2.5 W, for the multi-element transducer, the corresponding measured peak pressure was 0.9 MPa for 1.5 W.</figcaption></figure>

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<figure><img src="image_6.png" /><figcaption>**Figure 5:** Time to see a color change with the single element transducer. Error bars indicate standard deviation across measurements with the same transducer and prescribed power.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 6:** Time to see a color change with the Exablate Neuro system operated at 670 kHz.</figcaption></figure>
<table><thead><tr><th></th><th>Apt-Flex</th><th>Ceramic</th><th>WATER</th><th>AIR</th></tr></thead><tbody><tr><td>Density (kg/m³)</td><td>1010<sup>(1)</sup></td><td>7500<sup>(5)</sup></td><td>997<sup>(8)</sup></td><td>1.21<sup>(9)</sup></td></tr><tr><td>Speed of sound (m/s)</td><td>1500<sup>(1)</sup></td><td>4350<sup>(5)</sup></td><td>1523<sup>(8)</sup></td><td>345<sup>(4)</sup></td></tr><tr><td>Attenuation (Np/m/MHz)</td><td>330<sup>(1)</sup></td><td>2.5x10<sup>-3</sup></td><td>0<sup>(7)</sup></td><td>18.9<sup>(3)</sup></td></tr><tr><td>Specific heat (J/kg*K)</td><td>1800<sup>(2)</sup></td><td>420<sup>(6)</sup></td><td>4310<sup>(7)</sup></td><td>1006<sup>(9)</sup></td></tr><tr><td>Thermal conductivity (W/m*K)</td><td>0.165<sup>(2)</sup></td><td>1.45<sup>(6)</sup></td><td>0.603<sup>(7)</sup></td><td>0.025<sup>(9)</sup></td></tr></tbody></table>

(1) Provided by manufacturer in the datasheet: Precision Acoustics Ltd (Dorchester, Dorset, UK)

(2) Provided by manufacturer (personal communication): Precision Acoustics Ltd (Dorchester, Dorset, UK)

(3) Jakevičius, Leonas, and Andriejus Demčenko. "Ultrasound attenuation dependence on air temperature in closed chambers." *Ultragarsas* (Ultrasound) 63.1 (2008): 18-22

(4) Cramer, Owen. "The variation of the specific heat ratio and the speed of sound in air with temperature, pressure, humidity, and CO2 concentration." *The Journal of the Acoustical Society of America* 93.5 (1993): 2510-2516.

(5) ONDA longitudinal ceramics, PZT5H

(6) CTS PZT5H Tech data sheet, 3203HD

(7) Meaney, P. M., Clarke, R. L., ter Haar, G. R., & Rivens, I. H. (1998). A 3-D finite-element model for computation of temperature profiles and regions of thermal damage during focused ultrasound surgery exposures. Ultrasound in medicine & biology, 24(9), 1489-1499

(8) Ten Eikelder, H. M. M., et al. "Modelling the temperature evolution of bone under high intensity focused ultrasound." *Physics in Medicine & Biology* 61.4 (2016): 1810.

(9) https://www.engineeringtoolbox.com/

<table><caption>TABLE 1: Simulation Material Properties.</caption><thead><tr><th>Material</th><th>COST</th></tr></thead><tbody><tr><td>3D printed holder</td><td>$0-10</td></tr><tr><td>12x12in sheets of Hallcrest TLC</td><td>$150</td></tr><tr><td>300x300x3mm sheet of AptFlex F28</td><td>$630</td></tr></tbody></table>

<caption>TABLE 2: Cost of the thermochromic pahntom materials.</caption>