Feasibility study of ultrasonic computed tomography-guided high-intensity focused ultrasound

Real-time assessment of high-intensity focused ultrasound heating and cavitation with hybrid optoacoustic ultrasound imaging

High-intensity focused ultrasound (HIFU) enables localized ablation of biological tissues by capitalizing on the synergistic effects of heating and cavitation. Monitoring of those effects is essential for improving the efficacy and safety of HIFU interventions. Herein, we suggest a hybrid optoacoustic-ultrasound (OPUS) approach for real-time assessment of heating and cavitation processes while providing an essential anatomical reference for accurate localization of the HIFU-induced lesion. Both effects could clearly be observed by exploiting the temperature dependence of optoacoustic (OA) signals and the strong contrast of gas bubbles in pulse-echo ultrasound (US) images. The differences in temperature increase and its rate, as recorded with a thermal camera for different HIFU pressures, evinced the onset of cavitation at the expected pressure threshold. The estimated temperatures based on OA signal variations were also within 10-20 % agreement with the camera readings for temperatures below the coagulation threshold (∼50 °C). Experiments performed in excised tissues as well as in a post-mortem mouse demonstrate that both heating and cavitation effects can be effectively visualized and tracked using the OPUS approach. The good sensitivity of the suggested method for HIFU monitoring purposes was manifested by a significant increase in contrast-to-noise ratio within the ablated region by > 10 dB and > 5 dB for the OA and US images, respectively. The hybrid OPUS-based monitoring approach offers the ease of handheld operation thus can readily be implemented in a bedside setting to benefit several types of HIFU treatments used in the clinics.

Feasibility of ultrasound tomography-guided localized mild hyperthermia using a ring transducer: Ex vivo and in silico studies

BACKGROUND

As of 2022, breast cancer continues to be the most diagnosed cancer worldwide. This problem persists within the United States as well, as the American Cancer Society has reported that ∼12.5% of women will be diagnosed with invasive breast cancer over the course of their lifetime. Therefore, a clinical need continues to exist to address this disease from a treatment and therapeutic perspective. Current treatments for breast cancer and cancers more broadly include surgery, radiation, and chemotherapy. Adjuncts to these methods have been developed to improve the clinical outcomes for patients. One such adjunctive treatment is mild hyperthermia therapy (MHTh), which has been shown to be successful in the treatment of cancers by increasing effectiveness and reduced dosage requirements for radiation and chemotherapies. MHTh-assisted treatments can be performed with invasive thermal devices, noninvasive microwave induction, heating and recirculation of extracted patient blood, or whole-body hyperthermia with hot blankets.

PURPOSE

One common method for inducing MHTh is by using microwave for heat induction and magnetic resonance imaging for temperature monitoring. However, this leads to a complex, expensive, and inaccessible therapy platform. Therefore, in this work we aim to show the feasibility of a novel all-acoustic MHTh system that uses focused ultrasound (US) to induce heating while also using US tomography (UST) to provide temperature estimates. Changes in sound speed (SS) have been shown to be strongly correlated with temperature changes and can therefore be used to indirectly monitor heating throughout the therapy. Additionally, these SS estimates allow for heterogeneous SS-corrected phase delays when heating complex and heterogeneous tissue structures.

METHODS

Feasibility to induce localized heat in tissue was investigated in silico with a simulated breast model, including an embedded tumor using continuous wave US. Here, both heterogenous acoustic and thermal properties were modeled in addition to blood perfusion. We further demonstrate, with ex vivo tissue phantoms, the feasibility of using ring-based UST to monitor temperature by tracking changes in SS. Two phantoms (lamb tissue and human abdominal fat) with latex tubes containing varied temperature flowing water were imaged. The measured SS of the water at each temperature were compared against values that are reported in literature.

RESULTS

Results from ex vivo tissue studies indicate successful tracking of temperature under various phantom configurations and ranges of water temperature. The results of in silico studies show that the proposed system can heat an acoustically and thermally heterogenous breast model to the clinically relevant temperature of 42°C while accounting for a reasonable time needed to image the current cross section (200 ms). Further, we have performed an initial in silico study demonstrating the feasibility of adjusting the transmit waveform frequency to modify the effective heating height at the focused region. Lastly, we have shown in a simpler 2D breast model that MHTh level temperatures can be maintained by adjusting the transmit pressure intensity of the US ring.

CONCLUSIONS

This work has demonstrated the feasibility of using a 256-element ring array transducer for temperature monitoring; however, future work will investigate minimizing the difference between measured SS and the values shown in literature. A hypothesis attributes this bias to potential volumetric average artifacts from the ray-based SS inversion algorithm that was used, and that moving to a waveform-based SS inversion algorithm will greatly improve the SS estimates. Additionally, we have shown that an all-acoustic MHTh system is feasible via in silico studies. These studies have indicated that the proposed system can heat a tumor within a heterogenous breast model to 42°C within a narrow time frame. This holds great promise for increasing the accessibility and reducing the complexity of a future all-acoustic MHTh system.

Ultrasonic Thermal Monitoring of the Brain Using Golay-Coded Excitations-Feasibility Study

Thermal monitoring during focused ultrasound (FUS) transcranial procedures is mandatory and commonly performed by MRI. Transcranial ultrasonic thermal monitoring is an attractive alternative. Furthermore, using the therapeutic FUS transducer itself for this task is highly desirable. Nonetheless, such application is challenged by massive skull-induced signal attenuation and aberrations. This study examined the feasibility of implementing the Golay-coded excitations (CoE) for temperature monitoring in bovine brain samples in the range of 35 °C-43 °C (hyperthermia). Feasibility was assessed using computer simulations, water-based phantoms, and ex vivo bovine brain white-matter samples. The samples were gradually heated to about 45 °C and sonicated during cool down with a 1-MHz therapeutic FUS implementing Golay CoE. Initially, a calibration curve correlating the normalized time-of-flight (TOF) changes and the temperature was generated. Next, a bovine bone was positioned between the FUS and the brain samples, and the scanning process was repeated for different fresh samples. The calibration curve was then used as a mean for estimating the temperature, which was compared to thermocouple measurements. The simulations demonstrated a substantial improvement in signal-to-noise ratio (SNR) and suggested that the implementation of 4-bit sequences is advantageous. The experimental measurements with bone demonstrated good temperature estimation with an average absolute error for the water phantoms and brains of 1.46 °C ± 1.22 °C and 1.23 °C ± 0.99 °C, respectively. In conclusion, a novel noninvasive method utilizing the Golay CoE for ultrasonic thermal monitoring using a therapeutic FUS transducer is introduced. This method can lead to the development of an acoustic tool for brain thermal monitoring.

Speed of Sound and Attenuation Temperature Dependence of Bovine Brain: Ex Vivo Study

OBJECTIVES

Brain treatments using focused ultrasound (FUS) offer a new range of noninvasive transcranial therapies. The acoustic energy deposition during these procedures may induce a temperature elevation in the tissue; therefore, noninvasive thermal monitoring is essential. Magnetic resonance imaging is the current adopted monitoring modality, but its high operational costs and limited availability may hinder the accessibility to FUS treatments. Aiming at the development of a thermometric ultrasound (US) method for the brain, the specific objective of this investigation was to study the acoustic thermal response of the speed of sound (SOS) and attenuation coefficient (AC) of different brain tissues: namely white matter (WM) and cortical matter.

METHODS

Sixteen ex vivo bovine brain samples were investigated. These included 7 WM and 9 cortical matter samples. The samples were gradually heated to about 45°C and then repeatedly scanned while cooling using a computerized US system in the through-transmission mode. The temperature was simultaneously registered with thermocouples. From the scans, the normalized SOS and AC for both tissues were calculated.

RESULTS

The results demonstrated a characteristic cooldown temporal behavior for the normalized AC and SOS curves, which were related to the temperature. The SOS curves enabled clear differentiation between the tissue types but depicted more scattered trajectories for the WM tissue. As for the AC curves, the WM depicted a linear behavior in relation to the temperature. However, both tissue types had rather similar temperature patterns.

CONCLUSIONS

These findings may contribute to the development of a US temperature-monitoring method during FUS procedures.

Experimental Study of Tumor Therapy Mediated by Multimodal Imaging Based on a Biological Targeting Synergistic Agent

Purpose

The high-intensity focused ultrasound (HIFU) ablation of tumors is inseparable from synergistic agents and image monitoring, but the existing synergistic agents have the defects of poor targeting and a single imaging mode, which limits the therapeutic effects of HIFU. The construction of a multifunctional biological targeting synergistic agent with high biosafety, multimodal imaging and targeting therapeutic performance has great significance for combating cancer.

Methods

Multifunctional biological targeting synergistic agent consisting of Bifidobacterium longum (B. longum), ICG and PFH coloaded cationic lipid nanoparticles (CL-ICG-PFH-NPs) were constructed for targeting multimode imaging, synergistic effects with HIFU and imaging-guided ablation of tumors, which was evaluated both in vitro and in vivo.

Results

Both in vitro and in vivo systematical studies validated that the biological targeting synergistic agent can simultaneously achieve tumor-biotargeted multimodal imaging, HIFU synergism and multimodal image monitoring in HIFU therapy. Importantly, the electrostatic adsorption method and the targeting of B. longum to tumor tissues allow the CL-ICG-PFH-NPs to be retained in the tumor tissue, achieve the targeting ability of synergistic agent. Multimodal imaging chose the best treatment time according to the distribution of nanoparticles in the body to guide the efficient and effective treatment of HIFU. CL-ICG-PFH-NPs could serve as a phase change agent and form microbubbles that can facilitate HIFU ablation by mechanical effects, acoustic streaming and shear stress. This lays a foundation for the imaging and treatment of tumors.

Conclusion

In this work, a biological targeting synergistic agent was successfully constructed with good stability and physicochemical properties. This biological targeting synergistic agent can not only provide information for early diagnosis of tumors but also realize multimodal imaging monitoring during HIFU ablation simultaneously with HIFU treatment, which improves the shortcomings of HIFU treatment and has broad application prospects.

Preliminary analysis of ultrasound elastography imaging-based thermometry on non-perfused ex vivo swine liver

AIMS

Real-time monitoring of tissue temperature during percutaneous tumor ablation improves treatment efficacy, leading clinicians in adjustment of treatment settings. This study aims at assessing feasibility of ultrasound thermometry during laser ablation of biological tissue using a specific ultrasound imaging techniques based on elastography acoustic radiation force impulse (ARFI).

METHODS

ARFI uses high-intensity focused ultrasound pulses to generate 'radiation force' in tissue; this provokes tissue displacements trackable using correlation-based ultrasound methods: the sensitivity of shear waves velocity is able to detect temperature changes. Experiments were carried out using a Nd:YAG laser (power: 5 W) in three non-perfused ex vivo pig livers. In each organ, a thermocouple was placed close to the applicator tip (distance range 1.5-2.5 cm) used to record a reference temperature. Positioning of laser applicator and thermocouple was eco-guided. The organ was scanned by an echography system equipped with ARFI; propagation velocity was measured in a region of interest of 1 × 0.5 cm located close to thermocouple, to investigate influence of tissue temperature on shear waves velocity.

RESULTS

Shear wave velocity has a very low sensitivity to temperature up to 55-60 °C, and in all cases, velocity is < 5 m s^-1; for temperature > 55-60 °C, velocity shows a steep increment. The system measures a value "over limit", meaning a velocity > 5 m s^-1.

CONCLUSIONS

Ultrasound thermometry during laser ablation of biological tissue based on elastography shows an abrupt output change at temperatures > 55-60 °C. This issue can have a relevant clinical impact, considering tumor necrosis when temperature crosses 55 °C to define the boundary of damaged volume.

Assessment of Coded Excitation Implementation for Estimating Heat-Induced Speed of Sound Changes

Speed of sound (SoS) is an acoustic property that is highly sensitive to changes in tissues. SoS can be mapped non-invasively using ultrasonic through transmission wave tomography. This however, practically limits its clinical use to the breast. A pulse-echo-based method that has broader clinical use and that can reliably measure treatment-induced changes in SoS even under poor signal-to-noise ratio (SNR) is highly desirable. The aim of this study was to evaluate the implementation of coded excitations (CoEs) to improve pulse-echo monitoring of heat-induced changes in the SoS. In this study, a binary phase modulated Barker sequence and a linear frequency-modulated chirp were compared with a common Gaussian pulse transmission. The comparison was conducted using computer simulations, as well as transmissions in both agar-gelatin phantoms and ex vivo bovine liver. SoS changes were experimentally induced by heating the specimens with a therapeutic ultrasound system. The performance of each transmission signal was evaluated by correlating the relative echo shifts to the normalized SoS measured by through transmission. The computer simulations indicated that CoEs are beneficial at very low SNR. The Barker code performed better than both the chirp and Gaussian pulses, particularly at SNRs <10 dB (R² = 0.81 ± 0.06, 0.68 ± 0.07 and 0.55 ± 0.08, respectively, at 0 dB). At high SNRs, the CoEs performed statistically on par with the Gaussian pulse. The experimental findings indicated that both Barker and chirp codes performed better than the Gaussian pulse on ex vivo liver (R² = 0.80 ± 0.15, 0.79 ± 0.15 and 0.54 ± 0.17, respectively) and comparably on agar-gelatin phantoms. In conclusion, CoEs can be beneficial for assessing temperature-induced changes in the SoS using the pulse-echo method under poor SNR.

Advantages of cooled fiber for monitoring laser tissue ablation through temporal and spectral analysis of RF ultrasound signal: A case study

The promising minimally invasive laser thermal therapy technique may be improved if thermal lesions induced into the tissue can be carefully monitored in extension and morphology during the treatment. According to results obtained in several recent experimentations, solutions that avoid tissue carbonization during the treatment have been proposed, in order to allow deeper and longer lasting light penetration in treated tissue and to reduce failures of the applicator tip and fiber optic, dangerous for patients. In the work the advantages in using a cooled fiber are shown, in order not only to induce efficient lesions but also in performing an accurate monitoring by ultrasound. Indeed, one important limit of the ultrasound control is caused by the gas bubbles generation, which represent an acoustic barrier that invalidate the ultrasonic image representation of the treated tissue. Ultrasonic radiofrequency signals were acquired from the same bovine liver ex vivo sample by using both bare and cooled fiber and processed to produce B-mode and spectral parametric images by implementing TUV (Thermotherapy Ultrasonic View) algorithm. Radiofrequency signals, B-mode and TUV images were analysed and compared in order to evaluate the different tissue heating processes during ablation and the different lesion extensions induced into the tissue after the treatment. Cooled fiber avoided carbonization and strongly reduced gas bubbles generation inducing a larger lesion and allowing a more effective ultrasound monitoring. Moreover by correlating optical images of the lesions and the corresponding Integral TUV images, by using Dice and Jaccard coefficients, it was proven that TUV algorithm is able to characterize the tissue portions differently modified by ablation exhibiting better performances in the case of cooled fiber and revealing to be a potential tool capable to improve the laser delivery settings control.

Fiber Optic Sensors for Temperature Monitoring during Thermal Treatments: An Overview

During recent decades, minimally invasive thermal treatments (i.e., Radiofrequency ablation, Laser ablation, Microwave ablation, High Intensity Focused Ultrasound ablation, and Cryo-ablation) have gained widespread recognition in the field of tumor removal. These techniques induce a localized temperature increase or decrease to remove the tumor while the surrounding healthy tissue remains intact. An accurate measurement of tissue temperature may be particularly beneficial to improve treatment outcomes, because it can be used as a clear end-point to achieve complete tumor ablation and minimize recurrence. Among the several thermometric techniques used in this field, fiber optic sensors (FOSs) have several attractive features: high flexibility and small size of both sensor and cabling, allowing insertion of FOSs within deep-seated tissue; metrological characteristics, such as accuracy (better than 1 °C), sensitivity (e.g., 10 pm·°C⁻¹ for Fiber Bragg Gratings), and frequency response (hundreds of kHz), are adequate for this application; immunity to electromagnetic interference allows the use of FOSs during Magnetic Resonance- or Computed Tomography-guided thermal procedures. In this review the current status of the most used FOSs for temperature monitoring during thermal procedure (e.g., fiber Bragg Grating sensors; fluoroptic sensors) is presented, with emphasis placed on their working principles and metrological characteristics. The essential physics of the common ablation techniques are included to explain the advantages of using FOSs during these procedures.

Non-invasive Measurement of Thermal Diffusivity Using High-Intensity Focused Ultrasound and Through-Transmission Ultrasonic Imaging

Thermal diffusivity at the site ablated by high-intensity focused ultrasound (HIFU) plays an important role in the final therapeutic outcome, as it influences the temperature's spatial and temporal distribution. Moreover, as tissue thermal diffusivity is different in tumors as compared with normal tissue, it could also potentially be used as a new source of imaging contrast. The aim of this study was to examine the feasibility of combining through-transmission ultrasonic imaging and HIFU to estimate thermal diffusivity non-invasively. The concept was initially evaluated using a computer simulation. Then it was experimentally tested on phantoms made of agar and ex vivo porcine fat. A computerized imaging system combined with a HIFU system was used to heat the phantoms to temperatures below 42°C to avoid irreversible damage. Through-transmission scanning provided the time-of-flight values in a region of interest during its cooling process. The time-of-flight values were consequently converted into mean values of speed of sound. Using the speed-of-sound profiles along with the developed model, we estimated the changes in temperature profiles over time. These changes in temperature profiles were then used to calculate the corresponding thermal diffusivity of the studied specimen. Thermal diffusivity for porcine fat was found to be lower by one order of magnitude than that obtained for agar (0.313×10(-7)m(2)/s vs. 4.83×10(-7)m(2)/s, respectively, p < 0.041). The fact that there is a substantial difference between agar and fat implies that non-invasive all-ultrasound thermal diffusivity mapping is feasible. The suggested method may particularly be suitable for breast scanning.

The benefits of compression methods in acoustic coherence tomography

Pulse compression methods improve the quality of ultrasonic medical images. In comparison with standard broadband pulse techniques, these methods enhance the contrast-to-noise ratio (CNR) and increase the probing depth without any perceptible loss of spatial resolution. The Golay compression technique is analyzed here in the context of ultrasonic computed tomography, first on a one-dimensional target and second on a very low-contrast phantom probed using a half-ring array tomograph. The imaging performances were assessed based on the image CNR. The improvement obtained (up to 40%) depends, however, on the number of coherently associated diffraction projections. Beyond a certain number, few advantages were observed. Advances in ultrasound computed tomography suggest that pulse compression methods should provide a useful means of optimizing the trade-off between the image quality and the probing sampling density.

Advances of high intensity focused ultrasound (HIFU) for pancreatic cancer

High intensity focused ultrasound (HIFU) is a novel therapeutic modality. Several preclinical and clinical studies have investigated the safety and efficacy of HIFU for treating solid tumours, including pancreatic cancer. Preliminary studies suggest that HIFU may be useful for the palliative therapy of cancer-related pain in patients with unresectable pancreatic cancer. This review provides a brief overview of HIFU, describes current clinical applications of HIFU for pancreatic cancer, and discusses future applications and challenges.

Design, fabrication, and characterization of a single-aperture 1.5-MHz/3-MHz dual-frequency HIFU transducer

High-intensity focused ultrasound (HIFU) treatment efficiency is critical in maximizing the hyperthermia and reducing the surgery time. In this paper, a single-aperture, 1.5 MHz/3 MHz dual-frequency HIFU transducer was designed, fabricated, and characterized for tissue ablation enhancement. Double PZT-2 layers were configured in serial and dual-frequency ultrasound waves can be concurrently generated by exciting one of the PZT-2 layers. Impulse responses from the prototype showed that the wave amplitudes at 1.5 and 3 MHz were about the same, and both are more than 12 dB larger than those of higher orders of harmonics. Tissue ablation tests demonstrated that higher temperature rise can be achieved with dual-frequency ultrasound than with single-frequency ablation at the same acoustic power.