Pilot Study on Boiling Histotripsy Treatment of Human Leiomyoma Ex Vivo.

OBJECTIVE: As an alternative to surgical excision and magnetic resonance-guided thermal high-intensity focused ultrasound ablation of uterine leiomyoma, this work was aimed at pilot feasibility demonstration of use of ultrasound-guided boiling histotripsy for non-invasive non-thermal fractionation of human uterine leiomyoma ex vivo. METHODS: A custom-made sector ultrasound transducer of 1.5-MHz operating frequency and nominal f-number F# = 0.75 was used to produce a volumetric lesion (two layers of 5 × 5 foci with a 1 mm step) in surgically resected human leiomyoma ex vivo. A sequence of 10 ms pulses (P+/P-/As = 157/-25/170 MPa in situ) with 1% duty cycle was delivered N = 30 times per focus under B-mode guidance. The treatment outcome was evaluated via B-mode imaging and histologically with hematoxylin and eosin and Masson's trichrome staining. RESULTS: The treatment was successfully performed in less than 30 min and resulted in formation of a rectangular lesion visualized on B-mode images during the sonication as an echogenic region, which sustained for about 10 min post-treatment. Histology revealed loss of cellular structure, necrotic debris and globules of degenerated collagen in the target volume surrounded by injured smooth muscle cells. CONCLUSION: The pilot experiment described here indicates that boiling histotripsy is feasible for non-invasive mechanical disintegration of human uterine leiomyoma ex vivo under B-mode guidance, encouraging further investigation and optimization of this potential clinical application of boiling histotripsy.

Elastic Properties of Aging Human Hematoma Model In Vitro and Its Susceptibility to Histotripsy Liquefaction.

OBJECTIVE: Tissue susceptibility to histotripsy disintegration has been reported to depend on its elastic properties. This work was aimed at investigation of histotripsy efficiency for liquefaction of human hematomas, depending on their stiffness and degree of retraction over time (0-10 d). METHODS: As an in vitro hematoma model, anticoagulated human blood samples (200 mL) were recalcified at different temperatures. In one set of samples, the shear modulus was measured by shear wave elastography during blood clotting at 10℃, 22℃ and 37℃, and then daily during further aging. The ultrastructure of the samples was analyzed daily with scanning electron microscopy (SEM). Another set of blood samples (50-200 mL) were recalcified at 37℃ for density and retraction measurements over aging and exposed to histotripsy at varying time points. Boiling histotripsy (2.5 ms pulses) and hybrid histotripsy (0.2 ms pulses) exposures (2 MHz, 1% dc, P+/P-/As = 182/-27/207 MPa in situ) were used to produce either individual cigar-shaped or volumetric (0.8-3 mL) lesions in samples incubated for 3 h, 5 d and 10 d. The obtained lesions were sized, then the lysate aspirated under B-mode guidance was analyzed ultrastructurally and diluted in distilled water for sizing of residual fragments. RESULTS: It was found that clotting time decreased from 113 to 25 min with the increase in blood temperature from 10℃ to 37℃. The shear modulus increased to 0.53 ± 0.17 kPa during clotting and remained constant within 8 d of incubation at 2℃. Sample volumes decreased by 57% because of retraction within 10 d. SEM revealed significant echinocytosis but unchanged ultrastructure of the fibrin meshwork. Liquefaction rate and lesion dimensions produced with the same histotripsy protocols correlated with the increase in the degree of retraction and were lower in retracted samples versus freshly clotted samples. More than 80% of residual fibrin fragments after histotripsy treatment were shorter than 150 µm; the maximum length was 208 µm, allowing for unobstructed aspiration of the lysate with most clinically used needles. CONCLUSION: The results indicate that hematoma susceptibility to histotripsy liquefaction is not entirely determined by its stiffness, and correlates with the retraction degree.

Initial Assessment of Boiling Histotripsy for Mechanical Ablation of Ex Vivo Human Prostate Tissue.

Boiling histotripsy (BH) is a focused ultrasound technology that uses millisecond-long pulses with shock fronts to induce mechanical tissue ablation. The pulsing scheme and mechanisms of BH differ from those of cavitation cloud histotripsy, which was previously developed for benign prostatic hyperplasia. The goal of the work described here was to evaluate the feasibility of using BH to ablate fresh ex vivo human prostate tissue as a proof of principle for developing BH for prostate applications. Fresh human prostate samples (N = 24) were obtained via rapid autopsy (<24 h after death, institutional review board exempt). Samples were analyzed using shear wave elastography to ensure that mechanical properties of autopsy tissue were clinically representative. Samples were exposed to BH using 10- or 1-ms pulses with 1% duty cycle under real-time B-mode and Doppler imaging. Volumetric lesions were created by sonicating 1-4 rectangular planes spaced 1 mm apart, containing a grid of foci spaced 1-2 mm apart. Tissue then was evaluated grossly and histologically, and the lesion content was analyzed using transmission electron microscopy and scanning electron microscopy. Observed shear wave elastography characterization of ex vivo prostate tissue (37.9 ± 22.2 kPa) was within the typical range observed clinically. During BH, hyperechoic regions were visualized at the focus on B-mode, and BH-induced bubbles were also detected using power Doppler. As treatment progressed, hypoechoic regions of tissue appeared, suggesting successful tissue fractionation. BH treatment was twofold faster using shorter pulses (1 ms vs. 10 ms). Histological analysis revealed lesions containing completely homogenized cell debris, consistent with histotripsy-induced mechanical ablation. It was therefore determined that BH is feasible in fresh ex vivo human prostate tissue producing desired mechanical ablation. The study supports further work aimed at translating BH technology as a clinical option for prostate ablation.

Ultrastructural Analysis of Volumetric Histotripsy Bio-effects in Large Human Hematomas.

Large-volume soft tissue hematomas are a serious clinical problem, which, if untreated, can have severe consequences. Current treatments are associated with significant pain and discomfort. It has been reported that in an in vitro bovine hematoma model, pulsed high-intensity focused ultrasound (HIFU) ablation, termed histotripsy, can be used to rapidly and non-invasively liquefy the hematoma through localized bubble activity, enabling fine-needle aspiration. The goals of this study were to evaluate the efficiency and speed of volumetric histotripsy liquefaction using a large in vitro human hematoma model. Large human hematoma phantoms (85 cc) were formed by recalcifying blood anticoagulated with citrate phosphate dextrose/saline-adenine-glucose-mannitol solution. Typical boiling histotripsy pulses (10 or 2 ms) or hybrid histotripsy pulses using higher-amplitude and shorter pulses (0.4 ms) were delivered at 1% duty cycle while continuously translating the HIFU focus location. Histotripsy exposures were performed under ultrasound guidance with a 1.5-MHz transducer (8-cm aperture, F# = 0.75). The volume of liquefied lesions was determined by ultrasound imaging and gross inspection. Untreated hematoma samples and samples of the liquefied lesions aspirated using a fine needle were analyzed cytologically and ultrastructurally with scanning electron microscopy. All exposures resulted in uniform liquid-filled voids with sharp edges; liquefaction speed was higher for exposures with shorter pulses and higher shock amplitudes at the focus (up to 0.32, 0.68 and 2.62 mL/min for 10-, 2- and 0.4-ms pulses, respectively). Cytological and ultrastructural observations revealed completely homogenized blood cells and fibrin fragments in the lysate. Most of the fibrin fragments were less than 20 µm in length, but a number of fragments were up to 150 µm. The lysate with residual debris of that size would potentially be amenable to fine-needle aspiration without risk for needle clogging in clinical implementation.

Holographic extraction of plane waves from an ultrasound beam for acoustic characterization of an absorbing layer of finite dimensions.

For the acoustic characterization of materials, a method is proposed for interpreting experiments with finite-sized transducers and test samples in terms of the idealized situation in which plane waves are transmitted through an infinite plane-parallel layer. The method uses acoustic holography, which experimentally provides complete knowledge of the wave field by recording pressure waveforms at points on a surface intersected by the acoustic beam. The measured hologram makes it possible to calculate the angular spectrum of the beam to decompose the field into a superposition of plane waves propagating in different directions. Because these waves cancel one another outside the beam, the idealized geometry of an infinite layer can be represented by a sample of finite size if its lateral dimensions exceed the width of the acoustic beam. The proposed method relies on holograms that represent the acoustic beam with and without the test sample in the transmission path. The method is described theoretically, and its capabilities are demonstrated experimentally for silicone rubber samples by measuring their frequency-dependent phase velocities and absorption coefficients in the megahertz frequency range.

Biofabrication of a Functional Tubular Construct from Tissue Spheroids Using Magnetoacoustic Levitational Directed Assembly.

In traditional tissue engineering, synthetic or natural scaffolds are usually used as removable temporal support, which involves some biotechnology limitations. The concept of "scaffield" approach utilizing the physical fields instead of biomaterial scaffold has been proposed recently. In particular, a combination of intense magnetic and acoustic fields can enable rapid levitational bioassembly of complex-shaped 3D tissue constructs from tissue spheroids at low concentration of paramagnetic agent (gadolinium salt) in the medium. In the current study, the tissue spheroids from human bladder smooth muscle cells (myospheres) are used as building blocks for assembling the tubular 3D constructs. Levitational assembly is accomplished at low concentrations of gadolinium salts in the high magnetic field at 9.5 T. The biofabricated smooth muscle constructs demonstrate contraction after the addition of vasoconstrictive agent endothelin-1. Thus, hybrid magnetoacoustic levitational bioassembly is considered as a new technology platform in the emerging field of formative biofabrication. This novel technology of scaffold-free, nozzle-free, and label-free bioassembly opens a unique opportunity for rapid biofabrication of 3D tissue and organ constructs with complex geometry.

Acoustic holography as a metrological tool for characterizing medical ultrasound sources and fields.

Acoustic holography is a powerful technique for characterizing ultrasound sources and the fields they radiate, with the ability to quantify source vibrations and reduce the number of required measurements. These capabilities are increasingly appealing for meeting measurement standards in medical ultrasound; however, associated uncertainties have not been investigated systematically. Here errors associated with holographic representations of a linear, continuous-wave ultrasound field are studied. To facilitate the analysis, error metrics are defined explicitly, and a detailed description of a holography formulation based on the Rayleigh integral is provided. Errors are evaluated both for simulations of a typical therapeutic ultrasound source and for physical experiments with three different ultrasound sources. Simulated experiments explore sampling errors introduced by the use of a finite number of measurements, geometric uncertainties in the actual positions of acquired measurements, and uncertainties in the properties of the propagation medium. Results demonstrate the theoretical feasibility of keeping errors less than about 1%. Typical errors in physical experiments were somewhat larger, on the order of a few percent; comparison with simulations provides specific guidelines for improving the experimental implementation to reduce these errors. Overall, results suggest that holography can be implemented successfully as a metrological tool with small, quantifiable errors.

Experimental Study of Acoustic Radiation Force of an Ultrasound Beam on Absorbing and Scattering Objects.

Acoustic radiation force is a nonlinear acoustic effect caused by the transfer of wave momentum to absorbing or scattering objects. This phenomenon is exploited in modern ultrasound metrology for measurement of the acoustic power radiated by a source and is used for both therapeutic and diagnostic sources in medical applications. To calculate radiation force an acoustic hologram can be used in conjunction with analytical expressions based on the angular spectrum of the measured field. The results of an experimental investigation of radiation forces in two different cases are presented in this paper. In one case, the radiation force of an obliquely incident ultrasound beam on a large absorber (which completely absorbs the beam) is considered. The second case concerns measurement of the radiation force on a spherical target that is small compared to the beam diameter.

A multi-element interstitial ultrasound applicator for the thermal therapy of brain tumors.

Interstitial thermal therapy is a minimally invasive treatment modality that has been used clinically for ablating both primary and secondary brain tumors. Here a multi-element interstitial ultrasound applicator is described that allows for increased spatial control during thermal ablation of tumors as compared to existing clinical devices. The device consists of an array of 56 ultrasound elements operating at 6 MHz, oriented on the seven faces of a 3.2 mm flexible catheter. The device was first characterized using the acoustic holography method to examine the functioning of the array. Then experiments were performed to measure heating in tissue-mimicking gel phantoms and ex vivo tissue samples using magnetic resonance imaging-based thermometry. Experimental measurements were compared with results obtained using numerical simulations. Last, simulations were performed to study the feasibility of using the device for thermal ablation in the brain. Experimental results show that the device can be used to induce a temperature rise of greater than 20 °C in ex vivo tissue samples and numerical simulations further demonstrate that tumors with diameters of greater than 30-mm could potentially be treated.