Thermal dose optimization via temporal switching in ultrasound surgery

Large-Volume Focused-Ultrasound Mild Hyperthermia for Improving Blood-Brain Tumor Barrier Permeability Application

Mild hyperthermia can locally enhance permeability of the blood-tumor barrier in brain tumors, improving delivery of antitumor nanodrugs. However, a clinical transcranial focused ultrasound (FUS) system does not provide this modality yet. The study aimed at the development of the transcranial FUS technique dedicated for large-volume mild hyperthermia in the brain. Acoustic pressure, multiple-foci, temperature and thermal dose induced by FUS were simulated in the brain through the skull. A 1-MHz, 114-element, spherical helmet transducer was fabricated to verify large-volume hyperthermia in the phantom. The simulated results showed that two foci were simultaneously formed at (2, 0, 0) and (−2, 0, 0) and at (0, 2, 0) and (0, −2, 0), using the phases of focusing pattern 1 and the phases of focusing pattern 2, respectively. Switching two focusing patterns at 5 Hz produced a hyperthermic zone with an ellipsoid of 7 mm × 6 mm × 11 mm in the brain and the temperature was 41–45 °C in the ellipsoid as the maximum intensity was 150 W/cm² and sonication time was 3 min. The phased array driven by switching two mode phases generated a 41 °C-contour region of 10 ± 1 mm × 8 ± 2 mm × 13 ± 2 mm in the phantom after 3-min sonication. Therefore, we have demonstrated our developed FUS technique for large-volume mild hyperthermia.

A new thermal dose model based on Vogel-Tammann-Fulcher behaviour in thermal damage processes

Thermal dose models are metrics that quantify the thermal effect on tissues based on the temperature and the time of exposure. These models are used to predict and control the outcome of hyperthermia (up to 45^°C) treatments, and of thermal coagulation treatments at higher temperatures (>45^°C). The validity and accuracy of the commonly used models (CEM43) are questionable when heating above the hyperthermia temperature range occurs, leading to an over-estimation of the accumulation of thermal damage. A new CEM43 dose model based on an Arrhenius-type, Vogel-Tammann-Fulcher, equation using published data, is introduced in this work. The new dose values for the same damage threshold that was produced at different in-vivo skin experiments were in the same order of magnitude, while the current dose values varied by two orders of magnitude. In addition, the dose values obtained using the new model for the same damage threshold in 6 lesions in ex-vivo liver experiments were more consistent than the current model dose values. The contribution of this work is to provide new modeling approaches to inform more robust thermal dosimetry for improved thermal therapy modeling, monitoring, and control.

Comparison of computer simulations and clinical treatment results of magnetic resonance-guided focused ultrasound surgery (MRgFUS) of uterine fibroids

PURPOSE

Magnetic resonance-guided focused ultrasound surgery (MRgFUS) can be used to noninvasively treat symptomatic uterine fibroids by heating with focused ultrasound sonications while monitoring the temperature with magnetic resonance (MR) thermometry. While prior studies have compared focused ultrasound simulations to clinical results, studies involving uterine fibroids remain scarce. In our study, we perform such a comparison to assess the suitability of simulations for treatment planning.

METHODS

Sonications (N = 67) were simulated retrospectively using acoustic and thermal models based on the Rayleigh integral and Pennes bioheat equation followed by MR-thermometry simulation in seven patients who underwent MRgFUS treatment for uterine fibroids. The spatial accuracy of simulated focus location was assessed by evaluating displacements of the centers of mass of the thermal dose distributions between simulated and treatment MR thermometry slices. Temperature-time curves and sizes of 240 equivalent minutes at 43°C (240EM43 ) volumes between treatment and simulation were compared.

RESULTS

The simulated focus location showed errors of 2.7 ± 4.1, -0.7 ± 2.0, and 1.3 ± 1.2 mm (mean ± SD) in the anterior-posterior, foot-head, and right-left directions for a fibroid absorption coefficient of 4.9 Np m-1  MHz-1 and perfusion parameter of 1.89 kg m-3 s-1 . Linear regression of 240EM43 volumes of 67 sonications of patient treatments and simulations utilizing these parameters yielded a slope of 1.04 and a correlation coefficient of 0.54. The temperature rise ratio of simulation to treatment near the end of sonication was 0.47 ± 0.22, 1.28 ± 0.60, and 1.49 ± 0.71 for 66 sonications simulated utilizing fibroid absorption coefficient of 1.2, 4.9, and 8.6 Np m-1  MHz-1 , respectively, and the aforementioned perfusion value. The impact of perfusion on peak temperature rise is minimal between 1.89 and 10 kg m-3 s-1 , but became more substantial when utilizing a value of 100 kg m-3 s-1 .

CONCLUSIONS

The results of this study suggest that perfusion, while in some cases having a substantial impact on thermal dose volumes, has less impact than ultrasound absorption for predicting peak temperature elevation at least when using perfusion parameter values up to 10 kg m-3 s-1 for this particular array geometry, frequencies, and tissue target which is good for clinicians to be aware of. The results suggest that simulations show promise in treatment planning, particularly in terms of spatial accuracy. However, in order to use simulations to predict temperature rise due to a sonication, knowledge of the patient-specific tissue parameters, in particular the absorption coefficient is important. Currently, spatially varying patient-specific tissue parameter values are not available during treatment, so simulations can only be used for planning purposes to estimate sonication performance on average.

Feasibility study of MR-guided pancreas ablation using high-intensity focused ultrasound in a healthy swine model

Purpose: Pancreatic cancer is typically diagnosed in a late stage with limited therapeutic options. For those patients, ultrasound-guided high-intensity focused ultrasound (US-HIFU) can improve local control and alleviate pain. However, MRI-guided HIFU (MR-HIFU) has not yet been studied extensively in this context. To facilitate related research and accelerate clinical translation, we report a workflow for the in vivo HIFU ablation of the porcine pancreas under MRI guidance.Materials and methods: The pancreases of five healthy German landrace pigs (35-58 kg) were sonicated using a clinical MR-HIFU system. Acoustic access to the pancreas was supported by a specialized diet and a hydrogel compression device for bowel displacement. Organ motion was suspended using periods of apnea. The size of the resulting thermal lesions was assessed using the thermal threshold- and dose profiles, non-perfused volume, and gross examination. The effect of the compression device on beam path length was assessed using MRI imaging.Results: Eight of ten treatments resulted in clearly visible damage in the target tissue upon gross examination. Five treatments resulted in coagulative necrosis. Good agreement between the four metrics for lesion size and a clear correlation between the delivered energy dose and the resulting lesion size were found. The compression device notably shortened the intra-abdominal beam path.Conclusions: We demonstrated a workflow for HIFU treatment of the porcine pancreas in-vivo under MRI-guidance. This development bears significance for the development of MR-guided HIFU interventions on the pancreas as the pig is the preferred animal model for the translation of pre-clinical research into clinical application.

Image-guided ultrasound phased arrays are a disruptive technology for non-invasive therapy

Focused ultrasound offers a non-invasive way of depositing acoustic energy deep into the body, which can be harnessed for a broad spectrum of therapeutic purposes, including tissue ablation, the targeting of therapeutic agents, and stem cell delivery. Phased array transducers enable electronic control over the beam geometry and direction, and can be tailored to provide optimal energy deposition patterns for a given therapeutic application. Their use in combination with modern medical imaging for therapy guidance allows precise targeting, online monitoring, and post-treatment evaluation of the ultrasound-mediated bioeffects. In the past there have been some technical obstacles hindering the construction of large aperture, high-power, densely-populated phased arrays and, as a result, they have not been fully exploited for therapy delivery to date. However, recent research has made the construction of such arrays feasible, and it is expected that their continued development will both greatly improve the safety and efficacy of existing ultrasound therapies as well as enable treatments that are not currently possible with existing technology. This review will summarize the basic principles, current statures, and future potential of image-guided ultrasound phased arrays for therapy.

High-Intensity Focused Ultrasound: Current Status for Image-Guided Therapy

Image-guided high-intensity focused ultrasound (HIFU) is an innovative therapeutic technology, permitting extracorporeal or endocavitary delivery of targeted thermal ablation while minimizing injury to the surrounding structures. While ultrasound-guided HIFU was the original image-guided system, MR-guided HIFU has many inherent advantages, including superior depiction of anatomic detail and superb real-time thermometry during thermoablation sessions, and it has recently demonstrated promising results in the treatment of both benign and malignant tumors. HIFU has been employed in the management of prostate cancer, hepatocellular carcinoma, uterine leiomyomas, and breast tumors, and has been associated with success in limited studies for palliative pain management in pancreatic cancer and bone tumors. Nonthermal HIFU bioeffects, including immune system modulation and targeted drug/gene therapy, are currently being explored in the preclinical realm, with an emphasis on leveraging these therapeutic effects in the care of the oncology patient. Although still in its early stages, the wide spectrum of therapeutic capabilities of HIFU offers great potential in the field of image-guided oncologic therapy.

MRI-controlled ultrasound thermal therapy

Advances in medical imaging have enabled the development of new minimally and completely noninvasive therapies that produce a desired biological effect in a target, such as a tumor, with minimal damage to the surrounding tissue. One means of noninvasively achieving bioeffects in tissue is the use of ultrasound to generate heat. Specialized ultrasound transducers can be used to generate focal regions of heating non invasively, without inserting anything into the body or affecting the tissue outside the target region. Ultrasound thermal therapy can be used with magnetic resonance (MR) imaging (MRI) guidance and MRI temperature feedback to automatically control temperature distributions during heating, producing accurate thermal lesions, or maintaining optimal conditions to enhance drug delivery.

MR-guided focused ultrasound surgery, present and future

MR-guided focused ultrasound surgery (MRgFUS) is a quickly developing technology with potential applications across a spectrum of indications traditionally within the domain of radiation oncology. Especially for applications where focal treatment is the preferred technique (for example, radiosurgery), MRgFUS has the potential to be a disruptive technology that could shift traditional patterns of care. While currently cleared in the United States for the noninvasive treatment of uterine fibroids and bone metastases, a wide range of clinical trials are currently underway, and the number of publications describing advances in MRgFUS is increasing. However, for MRgFUS to make the transition from a research curiosity to a clinical standard of care, a variety of challenges, technical, financial, clinical, and practical, must be overcome. This installment of the Vision 20∕20 series examines the current status of MRgFUS, focusing on the hurdles the technology faces before it can cross over from a research technique to a standard fixture in the clinic. It then reviews current and near-term technical developments which may overcome these hurdles and allow MRgFUS to break through into clinical practice.

Temperature feedback based heating strategy for ultrasound thermal surgery

Phased array transducer has the ability to generate multiple-focus pattern by adjusting the driving signal of individual element of the array. For the treatment of large target volume, several multiple-focus pattern could be used by using temporal switching technique among these power patterns. Typically, to obtain a uniform thermal dose, properly adjust the power level within the target volume is important. In this study, we proposed a temperature feedback based heating strategy without the need of power level adjustment. Several parameters, such as the setting temperature, sonication time, power level and blood perfusion, may affect the final thermal dose. Simulation results show that setting temperature is the key parameter to determine the final thermal dose while the effects of blood perfusion could be neglected and the sonication time should be as short as possible. Small power level is not suggested because the resulting thermal dose would extended owing to the thermal conduction.

Reduction of peak acoustic pressure and shaping of heated region by use of multifoci sonications in MR-guided high-intensity focused ultrasound mediated mild hyperthermia

PURPOSE

Ablative hyperthermia (>55 °C) has been used as a definitive treatment for accessible solid tumors not amenable to surgery, whereas mild hyperthermia (40-45 °C) has been shown effective as an adjuvant for both radiotherapy and chemotherapy. An optimal mild hyperthermia treatment is spatially accurate, with precise and homogeneous heating limited to the target region while also limiting the likelihood of unwanted thermal or mechanical bioeffects (tissue damage, vascular shutoff). Magnetic resonance imaging-guided high-intensity focused ultrasound (MR-HIFU) can noninvasively heat solid tumors under image-guidance. In a mild hyperthermia setting, a sonication approach utilizing multiple concurrent foci may provide the benefit of reducing acoustic pressure in the focal region (leading to reduced or no mechanical effects), while providing better control over the heating. The objective of this study was to design, implement, and characterize a multifoci sonication approach in combination with a mild hyperthermia heating algorithm, and compare it to the more conventional method of electronically sweeping a single focus.

METHODS

Simulations (acoustic and thermal) and measurements (acoustic, with needle hydrophone) were performed. In addition, heating performance of multifoci and single focus sonications was compared using a clinical MR-HIFU platform in a phantom (target = 4-16 mm), in normal rabbit thigh muscle (target = 8 mm), and in a Vx2 tumor (target = 8 mm). A binary control algorithm was used for real-time mild hyperthermia feedback control (target range = 40.5-41 °C). Data were analyzed for peak acoustic pressure and intensity, heating energy efficiency, temperature accuracy (mean), homogeneity of heating (standard deviation [SD], T10 and T90), diameter and length of the heated region, and thermal dose (CEM(43)).

RESULTS

Compared to the single focus approach, multifoci sonications showed significantly lower (67% reduction) peak acoustic pressures in simulations and hydrophone measurements. In a rabbit Vx2 tumor, both single focus and multifoci heating approaches were accurate (mean = 40.82±0.12 °C [single] and 40.70±0.09 °C [multi]) and precise (standard deviation = 0.65±0.05 °C [single] and 0.64±0.04 °C [multi]), producing homogeneous heating (T(10-90) = 1.62 °C [single] and 1.41 °C [multi]). Heated regions were significantly shorter in the beam path direction (35% reduction, p < 0.05, Tukey) for multifoci sonications, i.e., resulting in an aspect ratio closer to one. Energy efficiency was lower for the multifoci approach. Similar results were achieved in phantom and rabbit muscle heating experiments.

CONCLUSIONS

A multifoci sonication approach was combined with a mild hyperthermia heating algorithm, and implemented on a clinical MR-HIFU platform. This approach resulted in accurate and precise heating within the targeted region with significantly lower acoustic pressures and spatially more confined heating in the beam path direction compared to the single focus sonication method.The reduction in acoustic pressure and improvement in spatial control suggest that multifoci heating is a useful tool in mild hyperthermia applications for clinical oncology.

MRI-guided monitoring of thermal dose and targeted drug delivery for cancer therapy

Application of localized hyperthermia treatment for solid tumor therapy is under active clinical investigation. The success of this treatment methodology, whether for tumor ablation or drug delivery, requires accurate target localization and real-time temperature mapping of the targeted region. Magnetic Resonance Imaging (MRI) can monitor temperature elevations in tissues in real-time during tumor therapy. MRI can also be applied in concert with methods such as High Intensity Focused Ultrasound (HIFU) to enable image-guided drug delivery (IGDD) from temperature sensitive nanocarriers, by exploiting not only its anatomic resolution, but its ability to detect and measure drug release using markers co-loaded with drugs within the nanocarriers. We review this rapidly emerging technology, providing an overview of MRI-guided tissue thermal dose monitoring for HIFU and Laser therapy, its role in targeted drug delivery and its future potential for clinical translation.

The role of acoustic nonlinearity in tissue heating behind a rib cage using a high-intensity focused ultrasound phased array

The goal of this study was to investigate theoretically the effects of nonlinear propagation in a high-intensity focused ultrasound (HIFU) field produced by a therapeutic phased array and the resultant heating of tissue behind a rib cage. Three configurations of focusing were simulated: in water, in water with ribs in the beam path and in water with ribs backed by a layer of soft tissue. The Westervelt equation was used to model the nonlinear HIFU field, and a 1 MHz phased array consisting of 254 circular elements was used as a boundary condition to the model. The temperature rise in tissue was modelled using the bioheat equation, and thermally necrosed volumes were calculated using the thermal dose formulation. The shapes of lesions predicted by the modelling were compared with those previously obtained in in vitro experiments at low-power sonications. Intensity levels at the face of the array elements that corresponded to the formation of high-amplitude shock fronts in the focal region were determined as 10 W cm(-2) in the free field in water and 40 W cm(-2) in the presence of ribs. It was shown that exposures with shocks provided a substantial increase in tissue heating, and its better spatial localization in the main focal region only. The relative effects of overheating ribs and splitting of the focus due to the periodic structure of the ribs were therefore reduced. These results suggest that utilizing nonlinear propagation and shock formation effects can be beneficial for inducing confined HIFU lesions when irradiating through obstructions such as ribs. Design of compact therapeutic arrays to provide maximum power outputs with lower intensity levels at the elements is necessary to achieve shock wave regimes for clinically relevant sonication depths in tissue.

MRIgHIFU: a tool for image-guided therapeutics

High-intensity focused ultrasound (HIFU) provides focal delivery of mechanical energy deep into the body. This energy can be used to elevate the tissue temperature to such a degree that ablation is achieved. The elevated temperature can also be used to release drugs from temperature-sensitive carriers or activate therapeutic molecules using mechanical or thermal energy. Lower dose exposures modify the vasculature to allow large molecules to diffuse from blood in the surrounding tissue for local drug delivery. The energy delivery can be targeted and monitored using magnetic resonance imaging (MRI). The online image guidance and monitoring provides treatment delivery that is customized to each patient such that optimal, effective treatment can be achieved. This ability to localize and customize treatment delivery may further enhance the future potential of targeted drugs that are personalized for each patient. This review examines the rapid development of MRI-guided HIFU (MRIgHIFU) methods over the past few years and discuss their future potential.

Mild hyperthermia with magnetic resonance-guided high-intensity focused ultrasound for applications in drug delivery

PURPOSE

Mild hyperthermia (40-45 °C) is a proven adjuvant for radiotherapy and chemotherapy. Magnetic resonance guided high intensity focused ultrasound (MR-HIFU) can non-invasively heat solid tumours under image guidance. Low temperature-sensitive liposomes (LTSLs) release their drug cargo in response to heat (>40 °C) and may improve drug delivery to solid tumours when combined with mild hyperthermia. The objective of this study was to develop and implement a clinically relevant MR-HIFU mild hyperthermia heating algorithm for combination with LTSLs.

MATERIALS AND METHODS

Sonications were performed with a clinical MR-HIFU platform in a phantom and rabbits bearing VX2 tumours (target = 4-16 mm). A binary control algorithm was used for real-time mild hyperthermia feedback control (target = 40-41 °C). Drug delivery with LTSLs was measured with HPLC. Data were compared to simulation results and analysed for spatial targeting accuracy (offset), temperature accuracy (mean), homogeneity of heating (standard deviation (SD), T10 and T90), and thermal dose (CEM43).

RESULTS

Sonications in a phantom resulted in better temperature control than in vivo. Sonications in VX2 tumours resulted in mean temperatures between 40.4 °C and 41.3 °C with a SD of 1.0-1.5 °C (T10 = 41.7-43.7 °C, T90 = 39.0-39.6 °C), in agreement with simulations. 3D spatial offset was 0.1-3.2 mm in vitro and 0.6-4.8 mm in vivo. Combination of MR-HIFU hyperthermia and LTSLs demonstrated heterogeneous delivery to a partially heated VX2 tumour, as expected.

CONCLUSIONS

An MR-HIFU mild hyperthermia heating algorithm was developed, resulting in accurate and homogeneous heating within the targeted region in vitro and in vivo, which is suitable for applications in drug delivery.

Heat localization for targeted tumor treatment with nanoscale near-infrared radiation absorbers

Focusing heat delivery while minimizing collateral damage to normal tissues is essential for successful nanoparticle-mediated laser-induced thermal cancer therapy. We present thermal maps obtained via magnetic resonance imaging characterizing laser heating of a phantom tissue containing a multiwalled carbon nanotube inclusion. The data demonstrate that heating continuously over tens of seconds leads to poor localization (∼ 0.5 cm) of the elevated temperature region. By contrast, for the same energy input, heat localization can be reduced to the millimeter rather than centimeter range by increasing the laser power and shortening the pulse duration. The experimental data can be well understood within a simple diffusive heat conduction model. Analysis of the model indicates that to achieve 1 mm or better resolution, heating pulses of ∼2 s or less need to be used with appropriately higher heating power. Modeling these data using a diffusive heat conduction analysis predicts parameters for optimal targeted delivery of heat for ablative therapy.

Magnetic resonance imaging-guided volumetric ablation of symptomatic leiomyomata: correlation of imaging with histology

PURPOSE

To describe the preliminary safety and accuracy of a magnetic resonance (MR) imaging-guided high-intensity-focused ultrasound (HIFU) system employing new technical developments, including ablation control via volumetric thermal feedback, for the treatment of uterine leiomyomata with histopathologic correlation.

MATERIALS AND METHODS

In this phase I clinical trial, 11 women underwent MR-guided HIFU ablation (Sonalleve 1.5T; Philips Medical Systems, Vantaa, Finland), followed by hysterectomy within 30 days. Adverse events, imaging findings, and pathologic confirmation of ablation were assessed. The relationship between MR imaging findings, thermal dose estimates, and pathology and HIFU spatial accuracy were assessed using Bland-Altman analyses and intraclass correlations.

RESULTS

There were 12 leiomyomata treated. No serious adverse events were observed. Two subjects decided against having hysterectomy and withdrew from the study before surgery. Of 11 women, 9 underwent hysterectomy; all leiomyomata demonstrated treatment in the expected location. A mean ablation volume of 6.92 cm(3) ± 10.7 was observed at histopathologic examination. No significant differences between MR imaging nonperfused volumes, thermal dose estimates, and histopathology ablation volumes were observed (P > .05). Mean misregistration values perpendicular to the ultrasound beam axis were 0.8 mm ± 1.2 in feet-head direction and 0.1 mm ± 1.0 in and left-right direction and -0.7 mm ± 3.1 along the axis.

CONCLUSIONS

Safe, accurate ablation of uterine leiomyomata was achieved with an MR-guided HIFU system with novel treatment monitoring capabilities, including ablation control via volumetric thermal feedback.

A study of heating duration and scanning path in focused ultrasound surgery

A conventional method of avoiding normal tissue overheating during focused ultrasound surgery (FUS) is to apply equal heating duration for each sonication in between cooling intervals. However, this method is time-consuming and expensive. A novel method with unequal heating duration in different scanning paths without cooling intervals was investigated in this paper. This method was compared with the conventional method through the ablation of a 10 × 10 cm(2) target. The simulation results indicated that the new method was able to reduce treatment time by more than 50%, with higher than 95% coverage index, and more uniform thermal dose distribution. The method was further verified through the ablation of a circular lesion. Ex vivo experiments were also performed to confirm the simulation results. Both simulation and experimental results have proven that the new method can increase heating efficiency, and are a promising new approach in FUS.

The effect of electronically steering a phased array ultrasound transducer on near-field tissue heating

PURPOSE

This study presents the results obtained from both simulation and experimental techniques that show the effect of mechanically or electronically steering a phased array transducer on proximal tissue heating.

METHODS

The thermal response of a nine-position raster and a 16-mm diameter circle scanning trajectory executed through both electronic and mechanical scanning was evaluated in computer simulations and experimentally in a homogeneous tissue-mimicking phantom. Simulations were performed using power deposition maps obtained from the hybrid angular spectrum (HAS) method and applying a finite-difference approximation of the Pennes' bioheat transfer equation for the experimentally used transducer and also for a fully sampled transducer to demonstrate the effect of acoustic window, ultrasound beam overlap and grating lobe clutter on near-field heating.

RESULTS

Both simulation and experimental results show that electronically steering the ultrasound beam for the two trajectories using the 256-element phased array significantly increases the thermal dose deposited in the near-field tissues when compared with the same treatment executed through mechanical steering only. In addition, the individual contributions of both beam overlap and grating lobe clutter to the near-field thermal effects were determined through comparing the simulated ultrasound beam patterns and resulting temperature fields from mechanically and electronically steered trajectories using the 256-randomized element phased array transducer to an electronically steered trajectory using a fully sampled transducer with 40 401 phase-adjusted sample points.

CONCLUSIONS

Three distinctly different three distinctly different transducers were simulated to analyze the tradeoffs of selected transducer design parameters on near-field heating. Careful consideration of design tradeoffs and accurate patient treatment planning combined with thorough monitoring of the near-field tissue temperature will help to ensure patient safety during an MRgHIFU treatment.

Assisted hepatic resection using a toroidal HIFU device: an in vivo comparative study in pig

PURPOSE

Bleeding is the main cause of postoperative complications during hepatic surgery. Blood loss and transfusions increase tumor recurrence in liver metastases from colorectal cancer. A high intensity focused ultrasound (HIFU) device with an integrated ultrasound imaging probe was developed for the treatment of colorectal liver metastasis.

METHODS

The HIFU toroidal-shaped transducer contains 256 elements (working frequency: 3 MHz) and can create a single conical lesion of 7 cm3 in 40 s. Then, the volume of treatment can be significantly increased by juxtaposing single lesions. Presented here is the use of this device in an animal model as a complementary tool to improve surgical resection in the liver. Before transecting the liver, a wall of coagulative necrosis was performed using this device in order to minimize blood loss and dissection time during hepatectomy. Resection assisted by HIFU was compared to classical dissections with clamping [intermittent Pringle maneuver (IPM) group] and without clamping (control group). For each technique, 14 partial liver resections were performed in seven pigs. Blood loss per dissection surface area and resection time were the main outcome parameters.

RESULTS

Conserving liver blood inflow during hepatic resection assisted by HIFU did not increase total blood loss (7.4 +/- 3.3 ml cm(-2)) compared to hepatic resection performed during IPM and controlled blood inflow (11.2 +/- 2.2 ml cm(-2)). Lower blood loss was measured on average when using HIFU, even though difference with clamping (IPM) was not statistically significant (p = 0.09). Resection assisted by HIFU reduced blood loss by 50% compared to control group (14.0 +/- 3.4 ml cm(-2), p = 0.03). The duration of transection when using HIFU (13 +/- 3 min) was significantly lower compared to clamping (23 +/- 4 min, p < 0.01) and control (18 +/- 3 min, p = 0.02). Precoagulation also resulted in sealing blood vessels with a diameter of less than 5 mm, and therefore the number of clips needed in the HIFU group was significantly lower (0.8 +/- 0.2 cm(-2)) when compared to clamping (1.6 +/- 0.2 cm(-2), p < 0.01) and control (1.8 +/- 0.4 cm(-2), p < 0.01).

CONCLUSIONS

This method holds promise for future clinical applications in resection of liver metastases.

Focusing of high-intensity ultrasound through the rib cage using a therapeutic random phased array

A method for focusing high-intensity ultrasound (HIFU) through a rib cage that aims to minimize heating of the ribs while maintaining high intensities at the focus (or foci) was proposed and tested theoretically and experimentally. Two approaches, one based on geometric acoustics and the other accounting for diffraction effects associated with propagation through the rib cage, were investigated theoretically for idealized source conditions. It is shown that for an idealized radiator, the diffraction approach provides a 23% gain in peak intensity and results in significantly less power losses on the ribs (1% vs. 7.5% of the irradiated power) compared with the geometric one. A 2-D 1-MHz phased array with 254 randomly distributed elements, tissue-mimicking phantoms and samples of porcine rib cages are used in experiments; the geometric approach is used to configure how the array is driven. Intensity distributions are measured in the plane of the ribs and in the focal plane using an infrared camera. Theoretical and experimental results show that it is possible to provide adequate focusing through the ribs without overheating them for a single focus and several foci, including steering at +/- 10-15 mm off and +/- 20 mm along the array axis. Focus splitting caused by the periodic spatial structure of ribs is demonstrated both in simulations and experiments; the parameters of splitting are quantified. The ability to produce thermal lesions with a split focal pattern in ex vivo porcine tissue placed beyond the rib phantom is also demonstrated. The results suggest that the method is potentially useful for clinical applications of HIFU, for which the rib cage lies between the transducer(s) and the targeted tissue.

MR-guided transcranial brain HIFU in small animal models

Recent studies have demonstrated the feasibility of transcranial high-intensity focused ultrasound (HIFU) therapy in the brain using adaptive focusing techniques. However, the complexity of the procedures imposes provision of accurate targeting, monitoring and control of this emerging therapeutic modality in order to ensure the safety of the treatment and avoid potential damaging effects of ultrasound on healthy tissues. For these purposes, a complete workflow and setup for HIFU treatment under magnetic resonance (MR) guidance is proposed and implemented in rats. For the first time, tissue displacements induced by the acoustic radiation force are detected in vivo in brain tissues and measured quantitatively using motion-sensitive MR sequences. Such a valuable target control prior to treatment assesses the quality of the focusing pattern in situ and enables us to estimate the acoustic intensity at focus. This MR-acoustic radiation force imaging is then correlated with conventional MR-thermometry sequences which are used to follow the temperature changes during the HIFU therapeutic session. Last, pre- and post-treatment magnetic resonance elastography (MRE) datasets are acquired and evaluated as a new potential way to non-invasively control the stiffness changes due to the presence of thermal necrosis. As a proof of concept, MR-guided HIFU is performed in vitro in turkey breast samples and in vivo in transcranial rat brain experiments. The experiments are conducted using a dedicated MR-compatible HIFU setup in a high-field MRI scanner (7 T). Results obtained on rats confirmed that both the MR localization of the US focal point and the pre- and post-HIFU measurement of the tissue stiffness, together with temperature control during HIFU are feasible and valuable techniques for efficient monitoring of HIFU in the brain. Brain elasticity appears to be more sensitive to the presence of oedema than to tissue necrosis.

Improved volumetric MR-HIFU ablation by robust binary feedback control

Volumetric high-intensity focused ultrasound (HIFU) guided by multiplane magnetic resonance (MR) thermometry has been shown to be a safe and efficient method to thermally ablate large tissue volumes. However, the induced temperature rise and thermal lesions show significant variability, depending on exposure parameters, such as power and timing, as well as unknown tissue parameters. In this study, a simple and robust feedback-control method that relies on rapid MR thermometry to control the HIFU exposure during heating is introduced. The binary feedback algorithm adjusts the durations of the concentric ablation circles within the target volume to reach an optimal temperature. The efficacy of the binary feedback control was evaluated by performing 90 ablations in vivo and comparing the results with simulations. Feedback control of the sonications improved the reproducibility of the induced lesion size. The standard deviation of the diameter was reduced by factors of 1.9, 7.2, 5.0, and 3.4 for 4-, 8-, 12-, and 16-mm lesions, respectively. Energy efficiency was also improved, as the binary feedback method required less energy to create the desired lesion. These results show that binary feedback improves the quality of volumetric ablation by consistently producing thermal lesions of expected size while reducing the required energy as well.

Volumetric HIFU ablation under 3D guidance of rapid MRI thermometry

A volumetric sonication method is proposed that produces volume ablations by steering the focal point along a predetermined trajectory consisting of multiple concentric outward-moving circles. This method was tested in vivo on pig thigh muscle (32 ablations in nine animals). Trajectory diameters were 4, 12, and 16 mm with sonication duration depending on the trajectory size and ranging from 20 to 73 s. Despite the larger trajectories requiring more energy to reach necrosis within the desired volume, the ablated volume per unit applied energy increased with trajectory size, indicating improved treatment efficiency for larger trajectories. The higher amounts of energy required for the larger trajectories also increased the risk of off-focus heating, especially along the beam axis in the near field. To avoid related adverse effects, rapid volumetric multiplane MR thermometry was introduced for simultaneous monitoring of the temperature and thermal dose evolution along the beam axis and in the near field, as well as in the target region with a total coverage of six slices acquired every 3 s. An excellent correlation was observed between the thermal dose and both the nonperfused (R=0.929 for the diameter and R=0.964 for the length) and oedematous (R=0.913 for the diameter and R=0.939 for the length) volumes as seen in contrast-enhanced T1-weighted difference images and T2-weighted postsonication images, respectively. Histology confirmed the presence of a homogeneous necrosis inside the heated volumes. These results show that volumetric high-intensity focused ultrasound (HIFU) sonication allows for efficiently creating large thermal lesions while reducing treatment duration and also that the rapid multiplane MR thermometry improves the safety of the therapeutic procedure by monitoring temperature evolution both inside as well as outside the targeted volume.

A waveform diversity method for optimizing 3-d power depositions generated by ultrasound phased arrays

A waveform-diversity-based approach for 3-D tumor heating is compared to spot scanning for hyperthermia applications. The waveform diversity method determines the excitation signals applied to the phased array elements and produces a beam pattern that closely matches the desired power distribution. The optimization algorithm solves the covariance matrix of the excitation signals through semidefinite programming subject to a series of quadratic cost functions and constraints on the control points. A numerical example simulates a 1444-element spherical-section phased array that delivers heat to a 3-cm-diameter spherical tumor located 12 cm from the array aperture, and the results show that waveform diversity combined with mode scanning increases the heated volume within the tumor while simultaneously decreasing normal tissue heating. Whereas standard single focus and multiple focus methods are often associated with unwanted intervening tissue heating, the waveform diversity method combined with mode scanning shifts energy away from intervening tissues where hotspots otherwise accumulate to improve temperature localization in deep-seated tumors.

Feasibility of using lateral mode coupling method for a large scale ultrasound phased array for noninvasive transcranial therapy

A hemispherical-focused, ultrasound phased array was designed and fabricated using 1372 cylindrical piezoelectric transducers that utilize lateral coupling for noninvasive transcranial therapy. The cylindrical transducers allowed the electrical impedance to be reduced by at least an order of magnitude, such that effective operation could be achieved without electronic matching circuits. In addition, the transducer elements generated the maximum acoustic average surface intensity of 27 W/cm(2). The array, driven at the low (306-kHz) or high frequency (840-kHz), achieved excellent focusing through an ex vivo human skull and an adequate beam steering range for clinical brain treatments. It could electronically steer the ultrasound beam over cylindrical volumes of 100-mm in diameter and 60-mm in height at 306 kHz, and 30-mm in diameter and 30-mm in height at 840 kHz. A scanning laser vibrometer was used to investigate the radial and length mode vibrations of the element. The maximum pressure amplitudes through the skull at the geometric focus were predicted to be 5.5 MPa at 306 kHz and 3.7 MPa at 840 kHz for RF power of 1 W on each element. This is the first study demonstrating the feasibility of using cylindrical transducer elements and lateral coupling in construction of ultrasound phased arrays.

Two-dimensional ultrasound phased array design for tissue ablation for treatment of benign prostatic hyperplasia

This paper describes the design, construction and evaluation of a two-dimensional ultrasound phased array to be used in the treatment of benign prostatic hyperplasia. With two-dimensional phased arrays, the focal point position can be controlled by changing the electrical power and phase to the individual elements for focusing and electronically steering in a three-dimensional volume. The array was designed with a steering angle of +/-14 degrees in both transverse and longitudinal directions. A piezoelectric ceramic (PZT-8) was used as the material of the transducer, since it can handle the high power needed for tissue ablation and a matching layer was used for maximum acoustic power transmission to tissue. Analysis of the transducer ceramic and cable impedance has been designed for high power transfer with minimal capacitance and diameter. For this initial prototype, the final construction used magnet compatible housing and cabling for future application in a clinical magnetic resonance imaging system for temperature mapping of the focused ultrasound. To verify the capability of the transducer for focusing and steering, exposimetry was performed and the results correlated well with the calculated field. Ex vivo experiments were performed and indicated the capability of the transducer to ablate tissue using short sonications. For sonications with exposure time of 10, 15 and 20 s, the lesion size was roughly 1.8, 3.0 and 4.3 mm in diameter, respectively, which indicates the feasibility of this device.

Optimal power deposition patterns for ideal high temperature therapy/hyperthermia treatments

If it were possible to achieve, an ideal high temperature therapy or hyperthermia treatment would involve a single heating session and yield a desired thermal dose distribution in the tumour that would be attained in the shortest possible treatment time without heating critical normal tissues excessively. Simultaneously achieving all of these goals is impossible in practice, thus requiring trade-offs that allow clinicians to approach more closely some of these ideal goals at the expense of others. To study the basic nature of a subset of these trade-offs, the present simulation study looked at a simple, ideal case in which the tumour is heated by a single, optimized (with respect to space) power pulse, with no power deposition in the normal tissue. Results were obtained for two different clinical strategies (i.e. trade-off approaches), including: (1) an 'aggressive' approach, wherein the desired, uniform thermal dose is completely delivered to the tumour during the power-on period. This approach gives the clinician the satisfaction of knowing that the tumour was treated completely while power was being delivered, and yields the shortest attainable tumour dose delivery time. However, that benefit is attained at the cost of both 'overdosing' the tumour during the subsequent cool down period and, paradoxically, requiring a longer, overall treatment time. Here, the treatment time is considered as that time interval from the initiation of the heating pulse to the time at which the entire tumour has decayed to a specified 'safe' temperature--below 43 degrees C for our calculations. And, (2) a 'conservative' approach is considered, wherein the desired uniform dose is attained at the post-heating time at which the complete tumour cools back down to 'basal' conditions, taken as 4 h in this study. This conservative approach requires less applied power and energy and avoids the 'overdosing' problem, but at the cost of having a tumour dose delivery time that can be significantly longer than the heating pulse duration. This approach can require that clinicians wait a significant time after the power has been turned off before being able to confirm that the desired tumour thermal dose was reached. The present findings show that: (1) for both clinical strategies, an optimal power deposition shape (with respect to position in the tumour) can always be found that provides the desired uniform thermal dose in the tumour, regardless of the heating pulse duration chosen or the tumour perfusion pattern; and (2) shorter heating pulses are preferable to longer ones in that they require less total energy, take less total time to treat the patients, and have optimal power deposition patterns less influenced by perfusion. On the other hand, shorter pulses always require higher temperatures, and for the 'aggressive' clinical approach, they give significantly larger excess thermal doses in the tumour. The aggressive approach always requires longer treatment times than comparable conservative treatments. The optimal power patterns for both strategies involve a high-power density at the tumour boundary, which frequently creates a 'thermal wave' that contributes significantly to the final thermal dose distribution attained.

MR guided focused ultrasound: technical acceptance measures for a clinical system

Magnetic resonance (MR) guided focused ultrasound (MRgFUS) is a hybrid technique which offers efficient and safe focused ultrasound (FUS) treatments of uterine fibroids under MR guidance and monitoring. As a therapy device, MRgFUS requires systematic testing over a wide range of operational parameters prior to use in the clinical environment. We present technical acceptance tests and data for the first clinical MRgFUS system, ExAblate 2000 (InSightec Inc., Haifa, Israel), that has been FDA approved for treating uterine fibroids. These tests characterize MRgFUS by employing MR temperature measurements in tissue mimicking phantoms. The coronal scan plane is empirically demonstrated to be most reliable for measuring temperature elevations resulting from high intensity ultrasound (US) pulses ('sonications') and shows high sensitivity to changes in sonication parameters. Temperatures measured in the coronal plane were used as a measure of US energy deposited within the focal spot for a range of sonication parameters used in clinical treatments: spot type, spot length, output power, sonication duration, US frequency, and depth of sonication. In addition, MR images acquired during sonications were used to measure effective diameters and lengths of available sonication spot types and lengths. At a constant 60 W output power, the effective spot type diameters were measured to vary between 4.7 +/- 0.3 mm and 6.6 +/- 0.4 mm; treatment temperatures were found to decrease with increasing spot diameter. Prescribing different spot lengths was found to have no effect on the measured length or on measured temperatures. Tests of MRgFUS positioning accuracy determined errors in the direction parallel to the propagation of the US beam to be significantly greater than those in the perpendicular direction; most sonication spots were erroneously positioned towards the FUS transducer. The tests reported here have been demonstrated to be sufficiently sensitive to detect water leakage inside the FUS transducer. The data presented could be used for comparison by those conducting acceptance tests on other clinical MRgFUS systems.

Real-time monitoring of radiofrequency ablation of rabbit liver by respiratory-gated quantitative temperature MRI

PURPOSE

To evaluate the feasibility and precision of magnetic resonance imaging (MRI) thermometry for monitoring radiofrequency (RF) liver ablation in vivo and predicting the size of the ablation zone.

MATERIALS AND METHODS

At 1.5T, respiratory-triggered real-time MR temperature mapping (the proton resonance frequency (PRF) method) was used to monitor RF ablation in rabbit liver (N = 6) under free breathing. The size of the ablation zones, as assessed by histological analyses, was compared with that predicted from MR thermal dose (TD) maps or derived from conventional T1-weighted (T1w), T2-weighted (T2w), and T1w gadolinium (Gd)-enhanced (T1w-Gd) images acquired immediately after the ablation, and on days 4 and 8 postprocedure.

RESULTS

MR temperature uncertainty remained under 1-2 degrees C even during RF deposition. The TD maps were shown to be more predictive and precise than the other MR images, with an average predictive precision for the final ablation zone size of about 1 mm as compared to the histologically proven lesion on day 8.

CONCLUSION

Quantitative temperature MRI during RF ablation is feasible and offered a precise indication of the ablation zone size in this preclinical study based on the lethal dose threshold.

Focused beam control for ultrasound surgery with spherical-section phased array: sound field calculation and genetic optimization algorithm

This study aims at a sound field calculation for the spherical-section phased array and an optimization algorithm for the focus patterns of phased array ultrasound surgery. An efficient field calculation formula represented as an explicit expression is derived by the strategies of projection and binomial expansion. An optimization algorithm based on genetic algorithm is constructed by the suitable fitness function and the selection strategies. The simulation results of 256-element spherical-section phased array show the capability of controlling focus accurately and effectively with the combined method made up of the explicit expression method and the genetic optimization algorithm. The simulation results of single focus, multiple foci, on-axial focus, and off-axial focus further convince the feasibility of three-dimensional (3-D) focus steering with excellent acoustic performances. A single focus with the focus dimension of 1.25 mm x 1.25 mm x 7 mm and with the intensity of 6080 W/cm2 is formed. The multiple-focus pattern can enlarge the treatment volume 22 times larger than that of single focus with a sonication. In addition, a comparison between the explicit expression approach and the point source approach testifies to the applicability of the explicit expression approach. The experiment and simulation results of 16-element array actually confirm the feasibility of the combined method.

Scanning path optimization for ultrasound surgery

One of the problems in ultrasound surgery is the long treatment times when large tumour volumes are sonicated. Large tumours are usually treated by scanning the tumour volume using a sequence of individual focus points. During the scanning, it is possible that surrounding healthy tissue suffers from undesired temperature rise. The selection of the scanning path so that the tumour volume is treated as fast as possible while temperature rise in healthy tissue is minimized would increase the efficiency of ultrasound surgery. The main purpose of this paper is to develop a computationally efficient method which optimizes the scanning path. The optimization algorithm is based on the minimum time formulation of the optimal control theory. The developed algorithm uses quadratic cost criteria to obtain the desired thermal dose in the tumour region. The derived method is evaluated with numerical simulations in 3D which are applied to ultrasound surgery of the breast in simplified geometry. Results from the simulations show that the treatment time as well as the total applied energy can be decreased from 16% to 43% as compared to standard sonication. The robustness of the optimized scanning path is studied by varying the perfusion and absorption in the tumour region.

A 63 element 1.75 dimensional ultrasound phased array for the treatment of benign prostatic hyperplasia

Background

Prostate cancer and benign prostatic hyperplasia are very common diseases in older American men, thus having a reliable treatment modality for both diseases is of great importance. The currently used treating options, mainly surgical ones, have numerous complications, which include the many side effects that accompany such procedures, besides the invasive nature of such techniques. Focused ultrasound is a relatively new treating modality that is showing promising results in treating prostate cancer and benign prostatic hyperplasia. Thus this technique is gaining more attention in the past decade as a non-invasive method to treat both diseases.

Methods

In this paper, the design, construction and evaluation of a 1.75 dimensional ultrasound phased array to be used for treating prostate cancer and benign prostatic hyperplasia is presented. With this array, the position of the focus can be controlled by changing the electrical power and phase to the individual elements for electronically focusing and steering in a three dimensional volume. The array was designed with a maximum steering angle of ± 13.5° in the transverse direction and a maximum depth of penetration of 11 cm, which allows the treatment of large prostates. The transducer piezoelectric ceramic, matching layers and cable impedance have been designed for maximum power transfer to tissue.

Results

To verify the capability of the transducer for focusing and steering, exposimetry was performed and the results correlated well with the calculated field. Ex vivo experiments using bovine tissue were performed with various lesion sizes and indicated the capability of the transducer to ablate tissue using short sonications.

Conclusion

A 1.75 dimensional array, that overcame the drawbacks associated with one-dimensional arrays, has been designed, built and successfully tested. Design issues, such as cable and ceramic capacitances, were taken into account when designing this array. The final prototype overcame also the problem of generating grating lobes at unwanted locations by tapering the array elements.

Focal beam distortion and treatment planning in abdominal focused ultrasound surgery

Recent clinical trials show promising results in using MRI and MRI-based thermometry to guide focused ultrasound surgery to treat uterine fibroids. During treatment, large variation in the focal temperature distribution has been observed. It is possible that some of this variation is due to abdominal tissue inhomogeneity, which might be causing focal beam distortion, and might largely decrease the focusing ability in deep-seated tissues. The purpose of this study was to numerically demonstrate this effect and also show the feasibility of restoring the focal beam patterns by employing the phase correction procedure for phased arrays. Abdominal MR data from four uterine fibroid patients were obtained to reconstruct the three-dimensional meshes of interfaces used in simulations, and one patient was selected to perform the analysis of key parameters in focused ultrasound surgery. Results show that, without phase correction, the focused beam can be severely distorted while using a frequency above 1 MHz or delivering ring-shape focal patterns. Different focal positions at the same depth may require a different power to induce the same ultrasonic intensity level (up to 179% among the different focal patterns). After adding a phase correction procedure, the distorted focal beams can be restored, and the peak intensity can be largely recovered (up to 85% among the different focal patterns). This study may offer important implications and information for treatment planning toward optimizing focused ultrasound surgery in uterine fibroid or other abdominal tumor treatments.

MRI guided and monitored focused ultrasound thermal ablation methods: a review of progress

This paper reviews the current status in using magnetic resonance imaging (MRI) to guide and monitor thermal coagulation of tumours using focused ultrasound. The patient treatment procedure with a second generation phased array system will be described. Several clinical trials have found that patient treatments are feasible and that MRI thermometry allows noninvasive monitoring of clinical treatments. Overall, this emerging modality holds significant potential for non-invasive tumour treatment of both benign and malignant tumours.

Simulation study for thermal dose optimization in ultrasound surgery of the breast

In this paper the previously published optimization algorithm for thermal dose optimization is tested with numerical simulations. The simulations concern the thermal dose optimization in ultrasound surgery of the breast. The optimization algorithm is extended by setting the inequality constraint approximations to temperature in healthy tissue as well as in tumor region. In addition, the simulations are accomplished in realistic 3D geometry with varying thermal parameters. Another topic of the paper is to show the potential of the hemispherical phased array applicator for ultrasound surgery of the breast. With such an applicator larger tissue volumes can be treated with shorter time as compared to single element transducers. In simulations the geometrical focus of the applicator was placed mechanically in the middle of the treatable region. The whole tumor region was then scanned electrically by changing the phase of the emitted wave from individual elements. The simulations indicate that a feasible treatment plan can be achieved. In simulated cases the desired thermal dose was achieved for tumors with diameter from 1.5 to 2.4 cm, depending on the position of tumor. The maximum temperature limitations of 45 degrees C in healthy region and 80 degrees C in tumor region can be maintained in all simulations.

Pilot point temperature regulation for thermal lesion control during ultrasound thermal therapy

The fundamental goal of ultrasound thermal therapy is to provide proper thermal lesion formations for effective tumour treatment. The quality of the therapy depends mostly on its positional precision. To date, most ultrasound thermal therapy treatments have focused on the formation of power or temperature patterns. The non-linear and time-delay effects of thermal dose formation prohibit direct control of the thermal dose distribution. In the paper, the control of thermal lesions by regulation of the temperature of a pilot point is proposed. This scheme utilises the high correlation between temperature elevation and thermal dose at the forward boundary of thermal lesions. To verify the feasibility, a 2D ultrasound phased array system was used to generate thermal lesions of various sizes, and the temperature elevation required to generate a thermal dose threshold was investigated. Results showed that the required temperature elevation was found to be a reasonably constant value of 52.5 degrees C under differing conditions when the focal area was small. When the focal area under consideration was large, the required temperature elevation became a monotonic function of blood perfusion rate, ranging from 49.2 to 52.5 degrees C. When the reference temperature of the pilot point was set at a conservative value (52.5 degrees C), the thermal lesions were controlled precisely under a wide range of blood perfusion and power pattern changes, tested by using a more realistic model that takes into account thermal-induced attenuation and blood perfusion changes. This changed the complex thermal dose control problem into a simple temperature regulation problem, which makes implementation of thermal lesion control easier, giving the scheme a high potential for application to current ultrasound thermal therapy systems.

Thermal dose optimization method for ultrasound surgery

In this paper, a model-based optimization method is derived to control the thermal dose in biological tissues for ultrasound surgery. The optimization method uses the bioheat equation as a system model and quadratic cost criteria for the desired thermal dose. Time-harmonic quasi-stationary ultrasound fields are used as the heat source. In this method the optimal phase and the amplitude trajectories are found directly by minimizing the associated cost function. The approach also allows for maximum input amplitude constraints. The method is based on the Hamiltonian form of the system and results in a large dimensional nonlinear optimization problem which is solved with a gradient-type iterative scheme. The performance of the optimization method is tested with 2D simulations and it is shown that the approach is able to yield a feasible nominal solution. This nominal evolution would then eventually be sought to be maintained with the help of a feedback controller during the actual sonication.

Ultrasound cylindrical phased array for transoesophageal thermal therapy: initial studies

This work was undertaken to investigate the feasibility of constructing a cylindrical phased array composed of 64 elements spread around the periphery (OD 10.6 mm) for transoesophageal ultrasound thermotherapy. The underlying operating principle of this applicator is to rotate a plane ultrasound beam electronically. For this purpose, eight adjacent transducers were successively excited with appropriate delay times so as to generate a plane wave. The exposure direction was changed by exciting a different set of eight elements. For these feasibility studies, we used a cylindrical prototype (OD 10.6 mm) composed of 16 elementary transducers distributed over a quarter of the cylinder, all operating at 4.55 MHz. The active part was mechanically reinforced by a rigid damper structure behind the transducers. It was shown that an ultrasound field similar to that emitted by a plane transducer could be generated. Ex vivo experiments on pig's liver demonstrated that the ultrasound beam could be accurately rotated to generate sector-based lesions to a suitable depth (up to 19 mm). Throughout these experiments, exposures lasting 20 s were delivered at an acoustic intensity of 17 W cm(-2). By varying the power from exposure to exposure, the depth of the lesion at different angles could be controlled.

The use of quantitative temperature images to predict the optimal power for focused ultrasound surgery: in vivo verification in rabbit muscle and brain

In this study, we investigated the use of MRI-derived thermal imaging for determining the exposure parameters for focused ultrasound (FUS) surgery. Since the temperature rise induced by a FUS beam scales linearly with power, the temperature maps acquired during subthreshold sonications can be used to determine the power necessary to produce thermal tissue damage with a desired size. Thermal images acquired during multiple sonications delivered at different locations in rabbit thigh muscle and brain tissue in vivo were analyzed to test this hypothesis. First, the linearity of the induced temperature rise with the acoustic power was tested. Next, the temperature maps acquired during preliminary low power sonications were scaled up until the estimated size of the tissue damage was equal to the tissue damage size of subsequent high power sonications. A threshold thermal dose was used to estimate the onset of thermal damage. The predicted power (based on amount of scaling required to reach the target size) was then compared to the true high power value. Overall, the temperature rise varied linearly with power (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 0.97, 0.93 for pairs of sonications at each location in brain, muscle). The predicted power matched the true high power in the brain sonications (slope = 1.04). The predicted power underestimated the true high power in the muscle sonications (slope = 0.87). This under-prediction was due to a deviation from linearity in those cases where tissue damage was detected in subsequent MR images (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 1.02, 0.84 for no tissue damage, tissue damage). The source of this deviation was not clear from these experiments. Even with this underestimation of the power, this method will be useful because it will allow an estimate of the proper power to use during FUS surgery without exact knowledge of the tissue parameters.

Optimization of power deposition and a heating strategy for external ultrasound thermal therapy

The purpose of this paper is to examine the thermal dose distribution, to configure the optimal absorbed power deposition, and to design an appropriate heating strategy for ultrasound thermal therapy. This work employs simulation programs, which are based on the transient bio-heat transfer equation and an ideal absorbed power deposition or an ideal temperature elevation within a cube of tissue, to study the optimal absorbed power deposition. Meanwhile, a simplified model of a scanned ultrasound transducer power deposition (a cone with convergent/divergent shape) is used to investigate the heating strategy for a large tumor with a sequence of heating pulses. The distribution of thermal dose equivalence defined by Sapareto and Dewey is used to evaluate the heating result for a set of given parameters. The parameters considered are the absorbed power density, heating duration, temperature elevation, blood perfusion, and the size of heating cube. The results demonstrate that the peak temperature is the key factor determining the thermal dose for this short-duration heating. Heat conduction has a very strong influence on the responses of temperature and thermal dose for a small heating cube and the boundary portion of a large heating cube. Hence, for obtaining the same therapeutic result, a higher power density is required for these two conditions to compensate the great temperature difference between the heating cube and the surrounding tissue. The influence of blood perfusion on the thermal dose is negligible on the boundary portion of the heating cube, while in the central portion it may become a crucial factor as a lower power density is used in this portion to save the delivered energy. When using external ultrasound heating method to treat a large tumor, the size of heating unit, the sequence of heating pulses, and the cooling-time interval between the consecutive heating pulses are the important factors to be determined to have an appropriate treatment within a reasonable overall treatment time.

A theoretical assessment of the relative performance of spherical phased arrays for ultrasound surgery

Computer modeling of spherical-section phased arrays for ultrasound surgery (tissue ablation) is described. The influence on performance of the number of circular elements (68 to 1024), their diameter (2.5 to 10 mm), frequency (1 to 2 MHz), and degree of sparseness in the array is investigated for elements distributed randomly or in square, annular, and hexagonal patterns on a spherical shell (radius of curvature, 120 mm). Criteria for evaluating the quality of the intensity distributions obtained when focusing the arrays both on and away from their center of curvature, and in both single focus and simultaneous multiple foci modes, are proposed. Of the arrays studied, the most favorable performance, for both modes, is predicted for 256 5-mm diameter, randomly distributed elements. For the single focus mode, this performed better than regular arrays of 255 to 1024 elements and, for the case of nine simultaneous foci produced on a coplanar 3x3 grid with 4-mm spacing, better than square, hexagonal, or annular distributed arrays with a comparable number of elements. Randomization improved performance by suppressing grating lobes significantly. For single focus mode, a several-fold decrease in the number of elements could be made without degrading the quality of the intensity distribution.

In vivo demonstration of noninvasive thermal surgery of the liver and kidney using an ultrasonic phased array

A 256-element, continuous-wave ultrasonic phased array has been used to thermally coagulate deep-seated liver and kidney tissue. The array elements were formed on a 1-3 piezocomposite bowl with a 10-cm radius of curvature and 12-cm diameter. The 0.65 x 0.65 cm2 projection elements were driven at 1.1 MHz by a custom-built amplifier system. A series of in vivo porcine experiments demonstrated the ability to coagulate liver and kidney tissue using the large-scale phased array. The temperature response of the treatment was guided and monitored using magnetic resonance (MR) images. Focal lesion volumes greater than 0.5 cm3 in kidney and 2 cm3 in liver were formed from a single 20-s sonication.

A comparison of ultrasonic beams for thermal treatment of ocular tumors

OBJECTIVE

This study examined the relative merits of different ultrasonic beams and exposure modalities for treating ocular melanomas.

METHODS

Simulations were conducted to evaluate temperature patterns and lesion shapes induced by intense-ultrasound treatment of ocular tumors. In-vitro insonification experiments were conducted in bovine lenses.

RESULTS

Simulated hyperthermia exposures did not effectively treat tumor margins because of thermal conduction into nearby fluid-like media. Standard high-intensity focused beams produced narrow lesions during 2-s exposures. A high-intensity, multi-lobed beam, produced by a transducer with strip electrodes, generated asymmetric lesions with a single large dimension; this lesion shape could expedite the production of lesion matrices within large tumors. In-vitro cataract shapes were consistent with simulation results for focused high-intensity beams.

CONCLUSIONS

Thermal conduction and perfusion can cause underheating of tumor margins during hyperthermia unless special beam designs are used. The strip-electrode transducer configuration promises to expedite treatment of extended tumor volumes.

Thermal dose optimization for ultrasound tissue ablation

A formal and general thermal dose optimisation method is developed and tested. Prior methods require brute force searches wherein the temperature and dose distributions must be computed at each iteration by solving the bioheat transfer equation (BHTE) numerically. This is extremely time-consuming and can only be used to compare a few prespecified strategies instead of obtaining a more general optimal result. With the method developed here, dose distribution can be calculated without solving the BHTE numerically. This can be done in a matter of a few minutes compared with many hours. Moreover, general thermal dose optimization can now be performed to find the optimal strength and location of each focus so that an optimal dose distribution is obtained while the specified constraint is satisfied. The algorithm developed here consists of a closed-form solution to the BHTE, a Gaussian model for parameterizing a temperature distribution created by a power deposition pattern, and a two-step optimization technique for obtaining the model parameters that optimize the thermal dose distribution. Several examples are given to demonstrate the effectiveness of the algorithm and its robustness under different initial conditions and under assumptions of different sizes of the target region and different numbers of foci. The algorithm developed here provides an efficient and effective tool for treatment planning in ultrasound tissue ablation.