Reconstruction of thermoacoustic emission sources induced by proton irradiation using numerical time reversal.

Objective.Mapping of dose delivery in proton beam therapy can potentially be performed by analyzing thermoacoustic emissions measured by ultrasound arrays. Here, a method is derived and demonstrated for spatial mapping of thermoacoustic sources using numerical time reversal, simulating re-transmission of measured emissions into the medium.Approach.Spatial distributions of thermoacoustic emission sources are shown to be approximated by the analytic-signal form of the time-reversed acoustic field, evaluated at the time of the initial proton pulse. Given calibration of the array sensitivity and knowledge of tissue properties, this approach approximately reconstructs the acoustic source amplitude, equal to the product of the time derivative of the radiation dose rate, mass density, and Grüneisen parameter. This approach was implemented using two models for acoustic fields of the array elements, one modeling elements as line sources and the other as rectangular radiators. Thermoacoustic source reconstructions employed previously reported measurements of emissions from proton energy deposition in tissue-mimicking phantoms. For a phantom incorporating a bone layer, reconstructions accounted for the higher sound speed in bone. Dependence of reconstruction quality on array aperture size and signal-to-noise ratio was consistent with previous acoustic simulation studies.Main results.Thermoacoustic source distributions were successfully reconstructed from acoustic emissions measured by a linear ultrasound array. Spatial resolution of reconstructions was significantly improved in the azimuthal (array) direction by incorporation of array element diffraction. Source localization agreed well with Monte Carlo simulations of energy deposition, and was improved by incorporating effects of inhomogeneous sound speed.Significance.The presented numerical time reversal approach reconstructs thermoacoustic sources from proton beam radiation, based on straightforward processing of acoustic emissions measured by ultrasound arrays. This approach may be useful for ranging and dosimetry of clinical proton beams, if acoustic emissions of sufficient amplitude and bandwidth can be generated by therapeutic proton sources.

Thermoacoustic range verification during pencil beam delivery of a clinical plan to an abdominal imaging phantom.

PURPOSE: The purpose of this phantom study is to demonstrate that thermoacoustic range verification could be performed clinically. Thermoacoustic emissions generated in an anatomical multimodality imaging phantom during delivery of a clinical plan are compared to simulated emissions to estimate range shifts compared to the treatment plan. METHODS: A single-field 12-layerproton pencil beam scanning (PBS)treatment plancreated in Pinnacle prescribing6 Gy/fractionwas delivered by a superconducting synchrocyclotron to a triple modality (CT, MRI, and US) abdominal imaging phantom.Data was acquired by four acoustic receivers rigidly affixed to a linear ultrasound array. Receivers 1-2 were located distal to the treatment volume, whereas 3-4 were lateral. Receivers' room coordinates were computed relative to the ultrasound image plane after co-registration to the planning CT volume. For each prescribed beamlet, a set of thermoacoustic emissions corresponding to varied beam energies were computed. Simulated emissions were compared to measured emissions to estimate shifts of the Bragg peak. RESULTS: Shifts were small for high-dose beamlets that stopped in soft tissue. Signals acquired by channels 1-2 yielded shifts of -0.2±0.7mm relative to Monte Carlo simulations for high dose spots (~40 cGy) in the second layer. Additionally, for beam energy ≥125 MeV, thermoacoustic emissions qualitatively tracked lateral motion of pristine beams in a layered gelatin phantom, and time shifts induced by changing phantom layers were self-consistent within nanoseconds. CONCLUSIONS: Acoustic receivers tuned to spectra of thermoacoustic emissions may enable range verification during proton therapy.

Thermoacoustic range verification in the presence of acoustic heterogeneity and soundspeed errors - Robustness relative to ultrasound image of underlying anatomy.

PURPOSE: To demonstrate robustness of thermooacoustic range verification to acoustic heterogeneity and discrepancies between assumed and true propagation speed, i.e., soundspeed errors. METHODS: A beam sweeper was used to deliver 250 ns pulses that deposited 0.26 Gy of 16 MeV protons and 2.3 Gy of 60 MeV helium ions into water and oil targets, respectively. Thermoacoustic signals were detected by a 96-channel ultrasound array with a 1-4 MHz sensitivity band (-6 dB), bandpass filtered and backprojected to create thermoacoustic images in the plane of the ultrasound array. The same soundspeed and transducer array were used to estimate range and generate the ultrasound images onto which Bragg peak locations were overlaid. An air-gap phantom that displaced the Bragg peak by 6.5 mm demonstrated accuracy. Robustness to soundspeed errors was demonstrated in a waterbath as the assumed propagation speed scanner setting was altered by ± 5 % . Tissue-mimicking gelatin and a bone sample were introduced to demonstrate robustness to acoustic heterogeneity relative to ultrasound images of the underlying morphology. RESULTS: Single ion pulse measurements sufficed during the helium run, but signal averaging was required for protons. Range and entry point into the target were estimated from data collected by transducers placed at least 6 cm distal to the Bragg peak. When ultrasound images depicted the air-target interface where the beam enters, estimates of the entry point agreed with ultrasound images and range estimates agreed with Monte Carlo simulations to within 300 µm, even when thermoacoustic emissions traveled through a strongly scattering bone sample. Estimated Bragg peak locations were translated 6.5 mm by the air-gap phantom and correctly identified scenarios when the beam stopped inside the bone. CONCLUSIONS: Soundspeed errors dilate and acoustic heterogeneities deform ultrasound images. When thermoacoustic receivers are co-located with the ultrasound imaging array, the same transformations shift thermoacoustic range estimates. Therefore, thermoacoustic range verification is robust relative to ultrasound images of underlying anatomy. When the treatment target is visible in ultrasound, e.g., prostate, online thermoacoustic range estimates could verify that the treatment spot is inside the target.

Two-stage ionoacoustic range verification leveraging Monte Carlo and acoustic simulations to stably account for tissue inhomogeneity and accelerator-specific time structure - A simulation study.

PURPOSE: Range errors constrain treatment planning by limiting choice of ion beam angles and requiring large margins. Ionoacoustic range verification requires recovering the location of an acoustic source from low frequency signals. A priori information is applied to stably overcome resolution limits of inverse acoustic source imaging in this simulation study. In particular, the accuracy and robustness of ionoacoustic range verification for lateral and oblique delivery of high-energy protons to the prostate is examined. METHODS: Dose maps were computed using GEANT4 Monte Carlo simulations via the TOPAS user interface. Thermoacoustic pulses were propagated using k-Wave software, with initial pressures corresponding to instantaneous dose deposition and piecewise constant maps of tissue properties derived from the planning CT. A database of dose maps with corresponding thermoacoustic emissions and Bragg peak locations, referred to as "control points," were precomputed. Corresponding thermoacoustic emissions were also precomputed. Pulses were recorded at four coplanar locations corresponding to the outer surface of a virtual transrectal array. To model experimental beam delivery, k-Wave results were convolved in time with a Gaussian envelope to account for noninstantaneous proton delivery by a synchrocyclotron. Thermoacoustic pulses were bandlimited below 150 kHz, and amplitudes were directly proportional to charge delivered. To test robustness of our method, white noise was added. Range was estimated in a two-step process. The first step obtained a preliminary range estimate by one-way beamforming. The second step was taken using data corresponding to the "control point" nearest to the preliminary range estimate. For each receiver, the time of flight difference, ∆t, between the measured and control thermoacoustic signals were accurately estimated by applying the Fourier shift theorem. Receiver-Bragg peak distance was then estimated by adding vs ∆t to the known distance of the control point, where vs is soundspeed. A linear system of equations based upon all receiver locations and distances was solved to recover the Bragg peak location. All simulations were performed relative to the planning CT. Because ultrasound (US) images were not available, results were overlaid onto the planning CT. RESULTS: Beamformed estimates from noise-free data tracked all beam locations within 1 cm. Final estimates for oblique and lateral beams were accurate to within 1.0 and 1.6 mm respectively. Average errors of final range estimates for oblique beams from data with SNR = 0 dB were no greater than 2.0 mm. CONCLUSIONS: Ionoacoustic range verification may improve current practice. Ionoacoustic range estimates can be inherently co-registered to ultrasound images of underlying anatomy. To ensure estimates are robust in clinical practice, dose maps based upon the planning CT should be overlaid onto ultrasound volumes acquired at time of treatment and acoustic simulations re-computed to provide a database of control points and corresponding thermoacoustic emissions. Computation times for beamformed estimates are already fast enough for online range verification, but are not accurate enough for a measurement aperture limited to the surface of a transrectal ultrasound probe. Accelerated acoustic simulations will be required to enable online two-stage correction, but offline calculation is already suitable for adaptive planning.

Toward Quantitative Whole Organ Thermoacoustics With a Clinical Array Plus One Very Low-Frequency Channel Applied to Prostate Cancer Imaging.

Thermoacoustics has the potential to provide quantitative images of intrinsic tissue properties, most notably electrical conductivity in Siemens/meter, much as shear wave elastography provides tissue stiffness in kilopascal. Although thermoacoustic imaging with optical excitation has been commercialized for small animals, it has not yet made the transition to clinic for whole organ imaging in humans. The purpose of this work was to develop and validate specifications for a clinical ultrasound array for quantitative whole organ thermoacoustic imaging. Imaging a large organ requires exciting thermoacoustic pulses throughout the volume and broadband detection of those pulses because tomographic image reconstruction preserves frequency content. Applying the half-wavelength limit to a [Formula: see text] inclusion inside a 7.5-cm diameter organ requires measurement sensitivity to frequencies ranging from 4 MHz to 10 kHz, respectively. A dual-transducer system utilizing a P4-1 array connected to a Verasonics V1 system as well as a focused single-element transducer sensitive to lower frequencies was developed. Very high-frequency (VHF) irradiation generated thermoacoustic pulses throughout a [Formula: see text] volume. In the VHF regime, electrical conductivity drives thermoacoustic signal production. Simultaneous acquisition of thermoacoustic pulses by both transducers enabled comparison of transducer performance. Data from the clinical array generated a stack of 96 images with a separation of 0.3 mm, whereas the single-element transducer imaged only in a single plane. In-plane resolution and quantitative accuracy were quantified at isocenter. The array provided volumetric imaging capability with superior resolution whereas the single-element transducer provided superior quantitative accuracy in axial images. Combining axial images from both transducers preserved resolution of the P4-1 array and improved image contrast. Neither transducer was sensitive to frequencies below 50 kHz, resulting in a dc offset and low-frequency shading over fields of view exceeding 15 mm. Fresh human prostates were imaged ex vivo and volumetric reconstructions reveal structures rarely seen in diagnostic images. In conclusion, quantitative whole-organ thermoacoustic tomography will be feasible by sparsely interspersing transducer elements sensitive to the low end of the ultrasonic range.

Measurement of broadband temperature-dependent ultrasonic attenuation and dispersion using photoacoustics.

The broadband ultrasonic characterization of biological fluids and tissues is important for the continued development and application of high-resolution ultrasound imaging modalities. Here, a photoacoustic technique for the transmission measurement of temperature-dependent ultrasonic attenuation and dispersion is described. The system uses a photoacoustic plane wave source constructed from a polymethylmethacrylate substrate with a thin optically absorbent layer. Broadband ultrasonic waves are generated by illuminating the absorbent layer with nanosecond pulses of laser light. The transmitted ultrasound waves are detected by a planar 7-microm high-finesse Fabry-Perot interferometer. Temperature-induced thickness changes in the Fabry-Perot interferometer are tracked to monitor the sample temperature and maintain the sensor sensitivity. The measured -6 dB bandwidth for the combined source and sensor is 1 to 35 MHz, with an attenuation corrected signal level at 100 MHz of -10 dB. The system is demonstrated through temperature-dependent ultrasound measurements in castor oil and olive oil. Power law attenuation parameters are extracted by fitting the experimental attenuation data to a frequency power law while simultaneously fitting the dispersion data to the corresponding Kramers-Krönig relation. The extracted parameters are compared with other calibration measurements previously reported in the literature.

Computation of unmeasured third-generation VCT views from measured views.

We compute unmeasured cone-beam projections from projections measured by a third-generation helical volumetric computed tomography system by solving a characteristic problem for an ultrahyperbolic differential equation [John (1938)]. By working in the Fourier domain, we convert the second-order PDE into a family of first-order ordinary differential equations. A simple first-order integration is used to solve the ODEs.