Ajay Anand

Three-dimensional spatial and temporal temperature imaging in gel phantoms using backscattered ultrasound

Thermal therapies such as radio frequency, heated saline, and high-intensity focused ultrasound ablations are often performed suboptimally due to the inability to monitor the spatial and temporal distribution of delivered heat and the extent of tissue necrosis. Ultrasound-based temperature imaging recently was proposed as a means to measure noninvasively the deposition of heat by tracking the echo arrival time shifts in the ultrasound backscatter caused by changes in speed of sound and tissue thermal expansion. However, the clinical applicability of these techniques has been hampered by the two-dimensional (2-D) nature of traditional ultrasound imaging, and the complexity of the temperature dependence of sound speed for biological tissues. In this paper, we present methodology, results, and validation of a 3-D spatial and temporal ultrasound temperature estimation technique in an alginate-based gel phantom to track the evolution of heat deposition over a treatment volume. The technique was experimentally validated for temperature rises up to ~10degC by comparison with measurements from thermocouples that were embedded in the gel. Good agreement (rms difference=0.12degC, maximum difference=0.24degC) was observed between the noninvasive ultrasound temperature estimates and thermocouple measurements. Based on the results obtained for the temperature range studied in this paper, the technique demonstrates potential for applicability in image guidance of thermal therapy for determining the location of the therapeutic focal spot and assessing the extent of the heated region at subablative intensities.

Two‐Dimensional Real‐time Ultrasound Technique to Control Lesion Size during HIFU Therapy

The lack of accurate and low‐cost techniques for real‐time monitoring and feedback during ablative therapies such as HIFU has limited its widespread clinical adoption. A good ultrasound solution would enable a much more widespread usage of HIFU therapy. An acoustic radiation force (ARF) based technique for controlling the lesion size and its placement during HIFU therapy has been developed. A series of experiments were performed in excised bovine liver tissue to evaluate the proposed technique. The change in the ARF induced displacements during therapy at each spatial location was quantified by a unitless parameter, normalized displacement difference (NDD), defined as the difference between normalized displacement at the therapy endpoint and a reference determined from the data. The 2‐D displacement map (normalized to the value before therapy commenced) illustrated that displacements initially increased due to the temperature rise, reached a maximum, and then decreased with continuing therapy. Strong corr...

Ultrasonic Determination of Three-Dimensional Spatial and Temporal Thermal Distribution for Therapy Monitoring

Thermal therapies such as radio frequency, heated saline, and high-intensity focused ultrasound ablations are often performed sub-optimally due to the inability to monitor the spatial and temporal distribution of delivered heat and the extent of the necrotic tissue. Ultrasound imaging can be used to measure the deposition of heat through local thermally related changes in speed of sound; however the clinical applicability of these techniques have been hampered by the two-dimensional nature of traditional ultrasound imaging. In this paper, we present methodology, results, and validation of an imaging technique that uses a three-dimensional ultrasound imaging system to track the spatial and temporal evolution of heat deposition

Integrated ultrasound thermometry and multiphysics modeling for liver RF ablation monitoring: Ex vivo studies

Monitoring of radiofrequency ablation (RFA) is essential to ensure accurate treatment coverage of large liver tumors. Ultrasound is an excellent option for monitoring of ablations. Since ultrasound B-mode imaging lacks the ability to precisely estimate the extent of the lesion, there has been considerable effort and interest in other ultrasound based monitoring techniques. Ultrasound thermometry is one such technique that has the potential to monitor thermal ablations. However, drawback of the approach is its applicability only in sub-ablative temperatures. We attempt to overcome this limitation by integrating a predictive thermal model with ultrasound thermometry and extend its applicability to ablative temperatures. In this paper, we validate this integrated approach through ex vivo studies on bovine liver tissue. Specifically, sub-ablative temperature rise is measured using ultrasound thermometry in a plane distal to the metallic RFA tine. Independently, temperature distribution in the tissue is obtained from a COMSOL™-based multiphysics model (based on combining the Laplace and bio-heat transfer (BHTE) equation) customized for the ablation device geometry and boundary conditions. Subsequently, the experimentally determined ultrasound temperature estimates and the model temperature estimates are compared to estimate the local in situ unknown tissue properties. Finally, the adapted model is utilized during the ablation to estimate the ablation size. The thermal and electrical conductivity estimated using our approach for N=5 samples are 0.59±0.07 W/m/°C and 0.20±0.02 S/m respectively. Further, the lateral and axial lesion widths are 14.6 ±2.3 mm and 30.6±1.1 mm respectively and compare well with gross pathology estimates of 12.6±1.8 mm and 25.6±1.3 mm. In ex-vivo conditions devoid of blood perfusion effects, we have demonstrated the successful integration of the multiphysics model with the experimentally measured ultrasound backscatter data and the feasibility of the approach to monitor ablations.

Validating Ultrasound‐based HIFU Lesion‐size Monitoring Technique with MR Thermometry and Histology

In order to control and monitor HIFU lesions accurately and cost‐effectively in real‐time, we have developed an ultrasound‐based therapy monitoring technique using acoustic radiation force to track the change in tissue mechanical properties. We validate our method with concurrent MR thermometry and histology. Comparison studies have been completed on in‐vitro bovine liver samples. A single‐element 1.1 MHz focused transducer was used to deliver HIFU and produce acoustic radiation force (ARF). A 5 MHz single‐element transducer was placed co‐axially with the HIFU transducer to acquire the RF data, and track the tissue displacement induced by ARF. During therapy, the monitoring procedure was interleaved with HIFU. MR thermometry (Philips Panorama 1T system) and ultrasound monitoring were performed simultaneously. The tissue temperature and thermal dose (CEM43 = 240 mins) were computed from the MR thermometry data. The tissue displacement induced by the acoustic radiation force was calculated from the ultrasou...

P1B-3 Noninvasive Bleeding Detection and Localization Using Three Dimensional Doppler Ultrasound

Blood loss from extremity wounds is the number one cause of preventable battlefield death today. In civilian casualties, exsanguinations due to internal bleeding are the most significant cause of death in trauma victims. The goal of DARPAs deep bleeder acoustic coagulation (DBAC) program is to stop bleeding quickly enough to prevent the transition from nonprogressive shock to progressive shock, which occurs when the soldier loses 25% of the blood volume. Coagulative therapies such as HIFU and electrocautery can be used to quickly stop internal bleeding to prevent onset of progressive and irreversible hemorrhagic shock, which ultimately leads to death. However, the onset of bleeding must be detected and the site spatially localized in order to treat these trauma wounds effectively. Towards meeting the final goal of the DBAC program, we have performed preliminary studies on in vitro tissue mimicking phantoms to identify unique Doppler based signatures that are indicative of bleeding. In this study, we present the results and validation of a 3D Doppler ultrasound technique to detect and localize the bleeding site by tracking the change in resistance index (RI) at the bleed origin. Significant RI change was obtained at the intersection of the primary vessel (feeding the bleeder) and the bleeding site (jet). Pulsatile flow with near zero diastolic flow was present in the primary vessel and the non-bleeding branches. Within the bleeding jet, continued or elevated forward flow was present during diastole resulting in reduction of RI. The estimated bleed location showed excellent agreement with independent ground truth estimates. The results illustrate potential for the applicability of the technique in battlefield trauma and civilian emergency care applications.

Transrectal Array Configurations Optimized For Prostate HIFU Ablation

The objectives of this study were to evaluate and compare steering and ablation rates from several types of transrectal arrays operated at different frequencies for whole prostate ablation. Three‐dimensional acoustic and thermal modeling (Rayleigh‐Sommerfield and Penne’s BHTE) were performed. Treatment volumes up to 70cc and anterior‐posterior distances up to 6 cm were considered. The maximum transducer dimensions were constrained to 5 cm (along rectum) and 2.5 cm (elevation), and the channel count was limited to 256. Planar array configurations for truncated‐annular, 1/1.5D, and 2D random arrays were evaluated at 1, 2, and 4 MHz for capability to treat the entire prostate. The acoustic intensity at the surface was fixed at 10 W/cm2. The maximum temperature was restricted to 80° C. The volumetric ablation rate was computed to compare the treatment times amongst different configurations. The 1.5D Planar array at 1 MHz ablated the whole prostate in the shortest amount of time while maintaining adequate stee...

P2H-3 Ultrasonic Spatial and Temporal Determination of Heat Deposition in Three Dimensions

Thermal therapies such as radio frequency, heated saline, and high-intensity focused ultrasound ablations are often performed sub-optimally due to the inability to monitor the spatial and temporal distribution of delivered heat and the extent of tissue necrosis. Ultrasound-based temperature imaging has been recently proposed as a means to measure non-invasively the deposition of heat by tracking the echo arrival time shifts in the ultrasound backscatter caused by changes in speed of sound and tissue thermal expansion. However, the clinical applicability of these techniques has been hampered by the two-dimensional nature of traditional ultrasound imaging. In this study, we present methodology, results, and validation of a three-dimensional spatial and temporal ultrasound temperature estimation technique to track the evolution of heat deposition over a treatment volume. Ultrasonic backscattered radio-frequency data were captured using a commercial clinical imaging system to monitor heat generated by a complex, three-dimensional geometrical construct of resistive wires in an ultrasonic phantom. Thermocouples were embedded at various locations in the phantom to independently measure the temperature evolution during 90 seconds of heating (peak temperature increase of 8.5 degC) and the subsequent cool-down. X-ray computed tomography was used to ensure accurate placement of thermocouples. Good agreement was observed between the ultrasound derived temperature estimates and the invasive thermocouple measurements throughout the heating and cool-down phase. The mean difference between the ultrasound and thermocouple data was calculated to be 0.06 degC, RMS difference: 0.12 degC, and maximum difference: 0.24 degC. Time varying, three-dimensional ultrasonic measurements of temperature yielded parametric maps of thermal deposition within the phantom, and illustrated the complex three-dimensional geometry of the heat source. The results demonstrate potential for the applicability of the technique in monitoring and guidance of thermal therapy. To the best of the knowledge of the authors, this work represents the first demonstration of three-dimensional spatial and temporal ultrasound thermometry