X-ray acoustic imaging for external beam radiation therapy dosimetry ...

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X-ray acoustic imaging for external beam radiation therapy dosimetry using a commercial ultrasound scanner. Diego R. T. Sampaio, João H. Uliana, Antonio ...
10.1109/ULTSYM.2015.0400

X-ray acoustic imaging for external beam radiation therapy dosimetry using a commercial ultrasound scanner Diego R. T. Sampaio, João H. Uliana, Antonio A. O. Carneiro, Juliana F. Pavoni and Theo Z. Pavan

Leandro F. Borges

Department of Physics, FFCLRP University of São Paulo Ribeirão Preto, SP, Brazil

Service of Radiotherapy at University Hospital, FMRP University of São Paulo Ribeirão Preto, SP, Brazil

Abstract— Photoacoustic (PA) imaging has been used for numerous applications in clinical medicine and preclinical studies. Usually, nanosecond laser pulses are used to generate PA signals. In laser-based PA imaging, the contrast is based on optical absorption. The generated PA signals can be detected at greater depths, given that the limits imposed by photon scattering of purely optical based techniques are overcome. However, less effort has been directed towards the use of x-ray photons, which have more penetration depth than photons in the visible NIR region. Modern linear accelerators can provide polychromatic xray with sufficient power density to produce microseconds x-ray pulses capable of inducing ultrasonic waves in the material. Based on this concept, the x-ray acoustic computer tomography (XACT) was proposed to generate images by combining x-ray excitation and ultrasonic detection. In XACT an ultrasonic single element transducer with central frequency of 500 kHz was moved 360º around the sample. The present study proposes the use of xray photons to generate x-ray acoustic (XA) imaging using a commercial ultrasound system, where a linear ultrasound probe was used to acquire XA signals during external beam radiation therapy (RT). To validate our system, lead samples were positioned inside a water tank and then were irradiated to generate XA signals for 6 MV and 15 MV energies. The XA signals were captured by the ultrasound transducer operating in a frequency range between 5 MHz and 14 MHz, using the delay and sum beamforming to generate the images. We obtained XA images of lead samples consistent to XACT and the signals analysis showed XA signals with amplitude increased for higher dose-rates. These results demonstrate the feasibility of generating XA images, which provided dosimetric information during RT using a linear accelerator and a commercial ultrasound system. Keywords — ultrasound, photoacoustic, x-ray acoustic, dosimetry

I. INTRODUCTION In 1880, Alexander Graham Bell discovered the photoacoustic (PA) effect [1], which occurs when light is delivered to absorptive media, generating pressure waves due to a local thermal expansion [2]. Soft tissues PA imaging is based on this effect, wherein ultrasonic pressure waves are produced, using nanosecond pulses from a laser-based infrared radiation source [3]. Since PA imaging combines optical absorption with ultrasound detection, greater depths can be

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imaged, given that the limits imposed by photon scattering of purely optical-based techniques are overcome [4]. Recently, many groups have focused on the development of different PA applications, such as anatomic tomography, molecular-level imaging and spectroscopy [5]–[7], which can be applied in clinical and preclinical studies [8]. Nevertheless, less effort has been directed towards the use of x-ray photons. When these high-energy photons have sufficient power, they can serve as a thermoelastic source, producing detectable PA effect due to xray absorption, namely x-ray acoustic (XA) effect. Furthermore, x-ray has the advantage of presenting lower scattering than in visible light, penetrating deeper into the sample. The generated signal is, therefore, dependent on the = 82), which allows us material atomic number (e.g. Lead to analyze more deeply the intrinsic characteristic properties of matter [9]. The external beam radiotherapy (RT) procedure uses short duration pulses (around 4 μs) of megavoltage (MV) x-rays, produced by a linear accelerator (LINAC), to treat cancer patients [10]. Initial studies [11] found that XA waves arising from the interaction of x-rays generated by a LINAC equipment, could be detected at the ultrasonic frequency range, to create 2-D XA images. A technique called x-ray acoustic computed tomography (XACT) combines x-ray excitation and ultrasonic detection to produce two dimensional tomographic images, using back-projection algorithm [5]. Since the XACT technique is based on x-ray absorption, the amplitude of the detected signal is closely related to dose deposition, permitting, therefore, dose measurement and analysis [11]. However, the instrumentation to detect XA signals still needs to be improved, and a more practical approach or protocol, which is necessary to provide a useful XA dose measurement, has to be developed. In the present study, we used a commercial ultrasound scanner coupled to a broadband ultrasonic linear probe (4-15 MHz) to obtain XA images generated using MV x-rays (6 or 15 MV) RT beams. We also show an optimized setup and a dedicated software for XA dosimetry in RT using a commercial ultrasound system.

2015 IEEE International Ultrasonics Symposium Proceedings

II. X-RAY ACOUSTIC IMAGING X-ray energy generates XA waves when a pulse of radiation is absorbed by the sample, due to interactions of the photons with the inner shells electrons, producing photoelectrons and Auger electrons. The excited electrons transfer part of their kinetic energy to the surround medium, raising the local temperature and producing lattice vibration [11]. The inverse source problem approaching of pressure waves [5] supposes that the XA acoustic pressure , , at position and time , for a homogeneous medium is: (1)

Fig. 1. Schematic setup of the XA imaging. The x-ray beam irradiated the lead plate, producing XA signals, which were captured by an ultrasound transducer to create XA images.

, is the heat source, is where is the speed of sound, the volumetric expansion coefficient, and is the specific heat. The heat source is the x-ray illumination times the xray spatial absorption at a given position : , = .

The XA imaging studies were performed irradiating a rectangular plate (94.5 mm x 20.3 mm) of lead (ZPb=82, ρ=11.34g/cm³) with thickness of 1.6 mm. The plate was positioned inside an acrylic (1.18g/cm³) tank (300 mm x 300 mm x 400 mm) with wall thickness of 5 mm filled with water (1.0 g/cm³) at room temperature.

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III. MATERIALS AND METHODS Fig. 1 shows the schematic setup used for XA imaging. The experimental setup consists of a LINAC (Oncor, Siemens), which was used to provide the x-ray beams for either 6 MV or 15 MV energies. These beams were delivered using a pulse repetition period (PRP) of approximately 4.5 ms. The Oncor LINAC delivered 4 μs pulses for both 6 MV and 15 MV energies, at a monitor unit (MU) rate or dose-rate of 300 MU/min and 500 MU/min, respectively. For clinical purpose, the LINAC was calibrated to deliver 1cGy/MU at the depth of maximum dose in a fixed source surface distance (SSD) of 100 cm and using a field size of 10 cm x 10 cm. The XA signals were captured using a commercial ultrasound equipment (Sonix RP, Ultrasonix) coupled with a 128 elements linear probe (L14-5/38, Ultrasonix), with central frequency of 7.2 MHz and fractional bandwidth (-6 dB) of 70%, and a field-of-view of 38 mm. The ultrasound equipment was running a dedicated research platform written using C++, Qt4 and the development kit Texo™ 5.7.4 (Ultrasonix) to provide access to low-level hardware and to acquire raw data. The platform was configured to use a passive delay-andsum (DAS) beamforming. Briefly, this method works without emitting ultrasonic pulses, and recording each A-line as a delay-weighted average of the adjacent A-lines. This beamforming was implemented in the Sonix RP using 32 physical channels at a sampling rate of 40 MHz. This procedure was sequentially repeated for all elements of the transducer through its entire field-of-view. In addition, a synchronization trigger signal obtained from the Oncor console was connected to the Sonix RP to synchronize the emitted xray pulse with an A-line acquisition. A personal computer (PC) was used to access the ultrasound machine through a local area network cable to collect the dataset without entering the RT room.

The tank’s surface was positioned at a SSD of 55 cm to increase the x-ray dose-rate compared to that delivered at a SSD of 100 cm. The dose-rate at the tank’s surface, estimated applying inverse squared distance law [13], was approximately 990 MU/min for 6 MV (or 1650 MU/min for 15MV). Fig. 2(a) shows a depiction of the acrylic sample holder used for fixing the sample and the probe, which was attached to a positioning rail to allow movement along the central axis. The probe was positioned perpendicular and parallel to the long dimension of the sample. We configured the x-ray field size to cover the entire sample (field size of 20 cm x 9 cm), avoiding the probe. For a fixed position from the tank’s surface (water depth = 5 cm), we irradiated the sample for approximately 1 minute, for each energy (i.e., 6 MV and 15 MV) with PRP of 4.5 ms. We repeated that procedure for a water depth of 10 cm.

Fig. 2. (a) Depiction of the sample holder used for positioning the ultrasonic transducer perpendicular and parallel to the long dimension of the sample and (b) the procedure to estimate dose at different depths.

IV. RESULTS AND DISCUSSION Fig. 3 shows a comparison between pulse-echo B-mode ultrasound images, Figs. 3(a) and 3(c), and 6MV-XA images, Figs. 3(b) and 3(d), when the transducer was positioned perpendicular, Figs. 3(a) and 3(b), and parallel, Figs. 3(c) and 3(d), to the long dimension of the sample. The XA images clearly indentified a peak pressure amplitude at the lead/water interface, which is consistent with the results shown in [11]. The spatial XA pressure distribution is in agreement with that obtained for the B-mode ultrasound images. Therefore, these results show that a commercial ultrasound scanner

programmed with DAS beamforming can be applied to generate XA images. It is interesting to notice that two pressure peaks can be observed in the XA images. Fig 4(a), which is a zoom of the image shown in Fig 3(b), clearly shows the two pressure peaks. We believe that this observation could be related to the temporal length of the x-ray beam pulse, which is around 4 μs. Sun et al [12] described a theory of photoacoustic generation for different laser pulse durations. In this study, the authors show that for laser pulses shorter than the absorber’s characteristic oscillation time, the stress confinement condition is achieved. Under this limit, the generated PA pressure amplitude and bandwidth are improved. For example, in the case of nanoseconds laser pulses, tissues do not have sufficient time to expand and relax during the pulse (stress confinement condition). However, we believe that the lead plate characteristic oscillation time was shorter than the microsecond pulses, giving rise to a pressure wave in the expansion and another during the relaxation. From Fig. 4(b) we estimated a time delay of around 3.3μs between the two pressure peaks. The phase inversion between both signals, further strengthen this hypotheses. Xiang et al [11] obtained XACT images by rotating a ultrasound single-element 360º around a lead block sample, and reconstructed the image using a back-projection algorithm. They showed that the pressure waves originated from the boundaries of sample presented higher amplitude than those originated inside the sample. In our study, images were obtained using a commercial ultrasound machine and the same boundary enhancement was observed.

Fig. 3. (a,c) - Pulse-echo B-mode images and (b,d) - 6MV-XA images obtained from the lead plate when the transducer was positioned: (a-b) perpendicular and (c-d) parallel to the long dimension of the sample.

Fig. 4. (a) XA image cropped to observe the region nearby the maximum amplitude peak. (b) A-line used to measure the time-delay between the two peaks.

Fig. 5(a) shows XA-6MV signals obtained when the lead block was at a depth of 5 cm and 10 cm. The signals were normalized by the maximum value obtained at 5 cm, and clearly decreased when the lead plate was positioned at a depth of 10 cm. Generally, increasing the depth decreases the amplitude of XA signals, because of the x-ray beam attenuation [13]. It shows that less dose was deposited for the 10 cm case. The dose-deposition was studied by calculating the relative energy, which was estimated using trapezoidal integration of a squared modulus of XA signal or squared magnitude spectrum (Parseval’s theorem)[14]. The measurements taken at 5 cm had a signal-to-noise ratio (SNR) of 14 dB. Using the data acquired at 5 cm as a reference, the cumulative energy was computed over 1 minute in steps of 3 seconds. Fig. 5(b) shows the cumulative energy for different amounts of absorbed dose, suggesting that XA imaging could work as a linear dosimeter, which can be useful for radiotherapy treatment planning. The graph shows a higher cumulative energy for 6 MV compared to that obtained for 15 MV, for the same range of dose deposition. One possible explanation for this difference is that for MV beams, photoelectrons have a higher production probability at lower energies (i.e., 6 MV). The photoelectron production is the primarily interaction of 6-15 MV photons with Pb. Xing et al. [11] explained that main source of heat production to induce XA waves arises from the photoelectron produced by the interaction of the x-ray with the sample.

Fig. 5. (a) Relative energy calculated for measurements taken at depths of 5 cm, and 10 cm in water. (b) A cumulative sum of the energy over 3 seconds.

CONCLUSION In this study, we acquired XA imaging using a commercial ultrasound system. The XA waves were produced from the interaction of x-rays produced by a LINAC commonly used in radiotherapy treatments. The spatial pressure distribution of the XA images were in agreement with that obtained for the Bmode. Our results suggest that XA imaging setup can be extended to become a dosimetry system capable of dose-

deposition measurement and real-time assessment of doseprofile, providing a new tool for radiotherapy dosimetrists. ACKNOWLEDGMENT This study was supported by CAPES, CNPq grant 476671/2013-2, FAPESP grant 2013/18854-6, and the USP NAP-FisMed grant. In addition, Mr. Carlos Renato da Silva, Mr. Agnelo Bastos, Mr. Andre Delfini and Gphantom company for technical support. REFERENCES [1] [2] [3] [4] [5]

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