Real-time monitoring of HIFU treatment using pulse inversion

Suppressing HIFU interference in ultrasound images using 1D U-Net-based neural networks

Objective.One big challenge with high-intensity focused ultrasound (HIFU) is that the intense acoustic interference generated by HIFU irradiation overwhelms the B-mode monitoring images, compromising monitoring effectiveness. This study aims to overcome this problem using a one-dimensional (1D) deep convolutional neural network.Approach. U-Net-based networks have been proven to be effective in image reconstruction and denoising, and the two-dimensional (2D) U-Net has already been investigated for suppressing HIFU interference in ultrasound monitoring images. In this study, we propose that the one-dimensional (1D) convolution in U-Net-based networks is more suitable for removing HIFU artifacts and can better recover the contaminated B-mode images compared to 2D convolution.Ex vivoandinvivoHIFU experiments were performed on a clinically equivalent ultrasound-guided HIFU platform to collect image data, and the 1D convolution in U-Net, Attention U-Net, U-Net++, and FUS-Net was applied to verify our proposal.Main results.All 1D U-Net-based networks were more effective in suppressing HIFU interference than their 2D counterparts, with over 30% improvement in terms of structural similarity (SSIM) to the uncontaminated B-mode images. Additionally, 1D U-Nets trained usingex vivodatasets demonstrated better generalization performance ininvivoexperiments.Significance.These findings indicate that the utilization of 1D convolution in U-Net-based networks offers great potential in addressing the challenges of monitoring in ultrasound-guided HIFU systems.

Ultrasound Monitoring of Simultaneous high-intensity focused ultrasound (HIFU) therapy using minimum-peak-sidelobe coded excitation

Bipolar sequences can be readily transmitted by ultrasound (US) pulser hardware with the full driving voltage to boost the echo magnitude in B-mode monitoring of HIFU treatment. In this study, a novel single-transmit bipolar sequence with minimum-peak-sidelobe (MPS) level is developed not only to restore the image quality of US monitoring but also remove acoustic interference from simultaneous HIFU transmission. The proposed MPS code is designed with an equal number of positive and negative bits and the bit duration should be an integer multiple of the period of the HIFU waveform. In addition, different permutations of code sequence are searched in order to obtain the optimal encoding. The received imaging echo is firstly decoded by matched filtering to cancel HIFU interference and to enhance the echo magnitude of US monitoring. Then, Wiener filtering is applied as the second-stage pulse compression to improve the final image quality. Simulations and phantom experiments are performed to compare the single-transmit MPS decoding with conventional two-transmit methods such as pulse-inversion subtraction (PIS) and Golay decoding for their performance in simultaneous US monitoring of HIFU treatment. Results show that the MPS decoding effectively removes HIFU interference even in the presence of tissue motion. The image quality of PIS and Golay decoding, on the other hand, is compromised by the uncancelled HIFU components due to tissue motion. Simultaneous US monitoring of tissue ablation using the proposed MPS decoding has also demonstrated to be feasible in ex-vivo experiments. Compared to the notch filtering that also allows single-transmit HIFU elimination, the MPS decoding is preferrable because it does not suffer from the tradeoff between residual HIFU and speckle deterioration in US monitoring images.

Improved source localization in passive acoustic mapping using delay-multiply-and-sum beamforming with virtually augmented aperture

High-intensity focused ultrasound (HIFU) is a promising non-invasive treatment method whose applications include tissue ablation, hemostasis, thrombolysis and blood-brain barrier opening etc. Its therapeutic effects come from the thermal necrosis and the mechanical destruction associated with acoustic cavitation. Passive acoustic mapping (PAM) is capable of simultaneous monitoring of HIFU-induced cavitation events using only receive beamforming. Nonetheless, conventional time exposure acoustics (TEA) algorithm has poor spatial resolution and suffers from the X-shaped artifacts. These factors lead to difficulties in precise localization of cavitation source. In this study, we proposed a novel adaptive PAM method which combines Delay-Multiply-and-Sum (DMAS) beamforming with virtual augmented aperture (VA) to overcome the problem. In DMAS-VA beamforming, the magnitude of each channel waveform is scaled by p-th root while the phase is multiplied by L. The p and L correspond respectively to the degree of signal coherence in DMAS beamforming and the augmentation factor of aperture size. After channel sum, p-th power is applied to restore the dimensionality of source strength and then the PAM image is reconstructed by accumulating the signal power over the observation time. Based on simulation and experimental results, the proposed DMAS-VA has better image resolution and image contrast compared with the conventional TEA. Moreover, since the VA method may introduce grating lobes into PAM because of the virtually augmented pitch size, DMAS coherent factor (DCF) is further developed to alleviate these image artifacts. Results indicate that, with DCF weighting, the PAM image of DMAS-VA beamforming could be constructed without detectable image artifacts from grating lobes and false main lobes.

Immunomodulation and targeted drug delivery with high intensity focused ultrasound (HIFU): Principles and mechanisms

High intensity focused ultrasound (HIFU) is a non-invasive and non-ionizing sonic energy-based therapeutic technology for inducing thermal and non-thermal effects in tissues. Depending on the parameters, HIFU can ablate tissues by heating them to >55 °C to induce denaturation and coagulative necrosis, improve radio- and chemo-sensitizations and local drug delivery from nanoparticles at moderate hyperthermia (~41-43 °C), and mechanically fragment cells using acoustic cavitation (also known as histotripsy). HIFU has already emerged as an attractive modality for treating human prostate cancer, veterinary cancers, and neuromodulation. Herein, we comprehensively review the role of HIFU in enhancing drug delivery and immunotherapy in soft and calcified tissues. Specifically, the ability of HIFU to improve adjuvant treatments from various classes of drugs is described. These crucial insights highlight the opportunities and challenges of HIFU technology and its potential to support new clinical trials and translation to patients.

Golay-Encoded Ultrasound Monitoring of Simultaneous High-Intensity Focused Ultrasound Treatment: A Phantom Study

Ultrasound (US) imaging has high potential in monitoring high-intensity focused US (HIFU) treatment due to its superior temporal resolution. However, US monitoring is often hindered by strong HIFU interference, which overwhelms the echoes received by the imaging array. In this study, a method of Golay-encoded US monitoring is proposed to visualize the imaged object for simultaneous HIFU treatment. It effectively removes HIFU interference patterns in real-time B-mode imaging and improves the metrics of image quality, such as peak signal-to-noise ratio (PSNR), structural similarity (SSIM), and contrast ratio (CR). Compared to the pulse-inversion sequence, the N -bit Golay sequence can boost the echo magnitude of US monitoring by another N times and, thus, exhibits higher robustness. Simulations show that a sinusoidal HIFU waveform can be fully eliminated using Golay decoding when the bit duration of the N -bit Golay sequence ( N is the power of 4) coincides with either odd (Case I) or even (Case II) integer multiples of the HIFU quarter period. Experimental results also show that the Golay decoding with Case II can increase the PSNR of US monitoring images by more than 30 dB for both pulse- and continuous-wave HIFU transmissions. The SSIM index also effectively improves to about unity, indicating that the B-mode image with HIFU transmission is visually indistinguishable from that acquired without HIFU transmission. Though Case I is inferior to Case II in the elimination of even-order HIFU harmonic, they together enable a more flexible selection of imaging frequencies to meet the required image resolution and penetration for Golay-encoded US monitoring.

FUS-Net: U-Net-Based FUS Interference Filtering

Imaging applications tailored towards ultrasound-based treatment, such as high intensity focused ultrasound (FUS), where higher power ultrasound generates a radiation force for ultrasound elasticity imaging or therapeutics/theranostics, are affected by interference from FUS. The artifact becomes more pronounced with intensity and power. To overcome this limitation, we propose FUS-net, a method that incorporates a CNN-based U-net autoencoder trained end-to-end on 'clean' and 'corrupted' RF data in Tensorflow 2.3 for FUS artifact removal. The network learns the representation of RF data and FUS artifacts in latent space so that the output of corrupted RF input is clean RF data. We find that FUS-net perform 15% better than stacked autoencoders (SAE) on evaluated test datasets. B-mode images beamformed from FUS-net RF shows superior speckle quality and better contrast-to-noise (CNR) than both notch-filtered and adaptive least means squares filtered RF data. Furthermore, FUS-net filtered images had lower errors and higher similarity to clean images collected from unseen scans at all pressure levels. Lastly, FUS-net RF can be used with existing cross-correlation speckle-tracking algorithms to generate displacement maps. FUS-net currently outperforms conventional filtering and SAEs for removing high pressure FUS interference from RF data, and hence may be applicable to all FUS-based imaging and therapeutic methods.

Real-Time Visualization of a Focused Ultrasound Beam Using Ultrasonic Backscatter

Focused ultrasound (FUS) therapies induce therapeutic effects in localized tissues using either temperature elevations or mechanical stresses caused by an ultrasound wave. During an FUS therapy, it is crucial to continuously monitor the position of the FUS beam in order to correct for tissue motion and keep the focus within the target region. Toward the goal of achieving real-time monitoring for FUS therapies, we have developed a method for the real-time visualization of an FUS beam using ultrasonic backscatter. The intensity field of an FUS beam was reconstructed using backscatter from an FUS pulse received by an imaging array and then overlaid onto a B-mode image captured using the same imaging array. The FUS beam visualization allows one to monitor the position and extent of the FUS beam in the context of the surrounding medium. Variations in the scattering properties of the medium were corrected in the FUS beam reconstruction by normalizing based on the echogenicity of the coaligned B-mode image. On average, normalizing by echogenicity reduced the mean square error between FUS beam reconstructions in nonhomogeneous regions of a phantom and baseline homogeneous regions by 21.61. FUS beam visualizations were achieved, using a single diagnostic imaging array as both an FUS source and an imaging probe, in a tissue-mimicking phantom and a rat tumor in vivo with a frame rate of 25-30 frames/s.

Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging

Real-time monitoring of high-intensity focused ultrasound (HIFU) surgery is essential for safe and accurate treatment. However, ultrasound imaging is difficult to use for treatment monitoring during HIFU surgery because of the high intensity of the HIFU echoes that are received by an imaging transducer. Here, we propose a real-time HIFU treatment monitoring method based on pulse inversion of imaging ultrasound; an imaging transducer fires ultrasound twice in 0° and 180° phases for one scanline while HIFUs of the same phase are transmitted in synchronization with the ultrasound transmission for imaging. By doing so, HIFU interferences can be eliminated after subtracting the two sets of the signals received by the imaging transducer. This function was implemented in a commercial research ultrasound scanner, and its performance was evaluated using the excised bovine liver. The experimental results demonstrated that the proposed method allowed ultrasound images to clearly show the echogenicity change induced by HIFU in the excised bovine liver. Additionally, it was confirmed that the moving velocity of the organs in the abdomen due to respiration does not affect the performance of the proposed method. Based on the experimental results, we believe that the proposed method can be used for real-time HIFU surgery monitoring that is a pivotal function for maximized treatment efficacy.

Ultrasound-assisted photothermal therapy and real-time treatment monitoring

Photothermal therapy (PTT) has the capability for selective treatment, in which light delivered to the target is converted into heat and subsequently causes coagulative necrosis. However, optical scattering in biological media limits light penetration, thus reducing therapeutic efficacy. Here, we demonstrate that the temperatures generated by light and ultrasound energies can be added constructively in resected melanoma cancers, which causes an increase in treatment depth. This method is called dual thermal therapy (DTT). It is also shown that combined ultrasound and photoacoustic images acquired using the pulse sequence proposed in this paper can be used for real-time monitoring of DTT.

Pulse inversion enhances the passive mapping of microbubble-based ultrasound therapy

Therapeutic ultrasound combined with preformed circulating microbubbles has enabled non-invasive and targeted drug delivery into the brain, tumors, and blood clots. Monitoring the microbubble activity is essential for the success of such therapies; however, skull and tissues limit our ability to detect low acoustic signals. Here, we show that by emitting consecutive therapeutic pulses of inverse polarity, the sensitivity in the detection of weak bubble acoustic signals during blood-brain barrier opening is enhanced compared to therapeutic pulses of the same polarity. Synchronous passive mapping of the cavitation activity was conducted using delay-and-sum beamforming with absolute time delays, which offers superior spatial resolution compared to the existing asynchronous passive imaging techniques. Sonication with pulse inversion allowed filter-free suppression of the tissue signals by up to 8 dB in a tissue-mimicking phantom and by 7 dB in vivo, compared to exposure without pulse inversion, enabling enhanced passive mapping of microbubble activity. Both therapeutic schemes resulted in similar free-field microbubble activation in vitro and efficient blood-brain barrier opening in vivo.

Increased light penetration due to ultrasound-induced air bubbles in optical scattering media

Light is an attractive tool for high spatial- and contrast-resolution imaging, highly sensitive molecular imaging, and target-selective therapy, and it does not exhibit the risks associated with ionizing radiation. The main limitation of using light in clinical applications is its superficial imaging and therapeutic depth caused by high optical scattering in biological media. Here, we demonstrate that the scattering and thus defocusing of the incident light can be alleviated when simultaneously delivered ultrasound generates air bubbles in the pathway of the incident light, thus increasing the light penetration. The bubbles are temporally induced by ultrasound with an intensity that is sufficiently low to avoid tissue damage and act as a Mie scattering medium in which light is scattered predominantly in the forward direction. The change in the optical scattering property caused by the ultrasound is undone after cessation of the insonification. From the results, it is expected that this proposed method will open a new route for overcoming the limitations of current optical imaging and therapeutic techniques.

Microbubbles used for contrast enhanced ultrasound and theragnosis: a review of principles to applications

Ultrasound was developed several decades ago as a useful imaging modality, and it became the second most popular diagnostic tool due to its non-invasiveness, real-time capabilities, and safety. Additionally, ultrasound has been used as a therapeutic tool with several therapeutic agents and in nanomedicine. Ultrasound imaging is often used to diagnose many types of cancers, including breast, stomach, and thyroid cancers. In addition, ultrasound-mediated therapy is used in cases of joint inflammation, rheumatoid arthritis, and osteoarthritis. Microbubbles, when used as ultrasound contrast agents, can act as echo-enhancers and therapeutic agents, and they can play an essential role in ultrasound imaging and ultrasound-mediated therapy. Recently, various types of ultrasound contrast agents made of lipid, polymer, and protein shells have been used. Air, nitrogen, and perfluorocarbon are usually included in the core of the microbubbles to enhance ultrasound imaging, and therapeutic drugs are conjugated and loaded onto the surface or into the core of the microbubbles, depending on the purpose and properties of the substance. Many research groups have utilized ultrasound contrast agents to enhance the imaging signal in blood vessels or tissues and to overcome the blood-brain barrier or blood-retina barrier. These agents are also used to help treat diseases in various regions or systems of the body, such as the cardiovascular system, or as a cancer treatment. In addition, with the introduction of targeted moiety and multiple functional groups, ultrasound contrast agents are expected to have a potential future in ultrasound imaging and therapy. In this paper, we briefly review the principles of ultrasound and introduce the underlying theory, applications, limitations, and future perspectives of ultrasound contrast agents.

Design and Fabrication of Double-Focused Ultrasound Transducers to Achieve Tight Focusing

Beauty treatment for skin requires a high-intensity focused ultrasound (HIFU) transducer to generate coagulative necrosis in a small focal volume (e.g., 1 mm³) placed at a shallow depth (3–4.5 mm from the skin surface). For this, it is desirable to make the F-number as small as possible under the largest possible aperture in order to generate ultrasound energy high enough to induce tissue coagulation in such a small focal volume. However, satisfying both conditions at the same time is demanding. To meet the requirements, this paper, therefore, proposes a double-focusing technique, in which the aperture of an ultrasound transducer is spherically shaped for initial focusing and an acoustic lens is used to finally focus ultrasound on a target depth of treatment; it is possible to achieve the F-number of unity or less while keeping the aperture of a transducer as large as possible. In accordance with the proposed method, we designed and fabricated a 7-MHz double-focused ultrasound transducer. The experimental results demonstrated that the fabricated double-focused transducer had a focal length of 10.2 mm reduced from an initial focal length of 15.2 mm and, thus, the F-number changed from 1.52 to 1.02. Based on the results, we concluded that the proposed double-focusing method is suitable to decrease F-number while maintaining a large aperture size.

Using passive cavitation images to classify high-intensity focused ultrasound lesions

Passive cavitation imaging provides spatially resolved monitoring of cavitation emissions. However, the diffraction limit of a linear imaging array results in relatively poor range resolution. Poor range resolution has limited prior analyses of the spatial specificity and sensitivity of passive cavitation imaging in predicting thermal lesion formation. In this study, this limitation is overcome by orienting a linear array orthogonal to the high-intensity focused ultrasound propagation direction and performing passive imaging. Fourteen lesions were formed in ex vivo bovine liver samples as a result of 1.1-MHz continuous-wave ultrasound exposure. The lesions were classified as focal, "tadpole" or pre-focal based on their shape and location. Passive cavitation images were beamformed from emissions at the fundamental, harmonic, ultraharmonic and inharmonic frequencies with an established algorithm. Using the area under a receiver operating characteristic curve (AUROC), fundamental, harmonic and ultraharmonic emissions were found to be significant predictors of lesion formation for all lesion types. For both harmonic and ultraharmonic emissions, pre-focal lesions were classified most successfully (AUROC values of 0.87 and 0.88, respectively), followed by tadpole lesions (AUROC values of 0.77 and 0.64, respectively) and focal lesions (AUROC values of 0.65 and 0.60, respectively).

Thermometry and ablation monitoring with ultrasound

In this review we present the current status of ultrasound thermometry and ablation monitoring, with emphasis on the diverse approaches published in the literature and with an eye on which methods are closest to clinical reality. It is hoped that this review will serve as a guide to the expansion of sonographic methods for treatment monitoring and thermometry since the last brief review in 2007.

An effective pulse sequence for simultaneous HIFU insonation and monitoring

The HIFU interference cancellation method using pulse inversion is useful for real-time treatment monitoring; however, this method suffers from residual interference when a high duty cycle is employed. In this paper, a pulse sequence is proposed to overcome the problem. It was experimentally verified that all interference could be removed using the pulse sequence. This implies that the HIFU interference cancellation method with the pulse sequence can be utilized for simultaneous HIFU insonation and monitoring under any duty cycle condition.

Thermal therapeutic method for selective treatment of deep-lying tissue by combining laser and high-intensity focused ultrasound energy

Photothermal therapy is performed by delivering laser radiation into the target lesion containing tissue chromophores so as to induce localized heating. For high treatment efficacy, the laser wavelength should be selected to maximize the absorption of incident laser radiation in the tissue chromophores. However, even with the optimal laser wavelength, both the absorption and the scattering of laser energy in tissue openly hamper treatment efficacy in deep-lying lesions. To overcome the limitation, we propose a dual thermal therapeutic method in which both laser and acoustic energies are transmitted to increase therapeutic depth while maintaining high target selectivity of photothermal therapy. Through skin-mimicking phantom experiments, it was verified that the two different energies are complementary in elevation of tissue temperature, and the treatment depth using laser radiation is increased along with acoustic energy.