Histotripsy methods in mechanical disintegration of tissue: towards clinical applications

Abstract

In high intensity focused ultrasound (HIFU) therapy, an ultrasound beam is focused within the body to locally affect the targeted site without damaging intervening tissues. The most common HIFU regime is thermal ablation. Recently there has been increasing interest in generating purely mechanical lesions in tissue (histotripsy). This paper provides an overview of several studies on the development of histotripsy methods toward clinical applications. Two histotripsy approaches and examples of their applications are presented. In one approach, sequences of high-amplitude, short (microsecond-long), focused ultrasound pulses periodically produce dense, energetic bubble clouds that mechanically disintegrate tissue. In an alternative approach, longer (millisecond-long) pulses with shock fronts generate boiling bubbles and the interaction of shock fronts with the resulting vapour cavity causes tissue disintegration. Recent preclinical studies on histotripsy are reviewed for treating benign prostatic hyperplasia (BPH), liver and kidney tumours, kidney stone fragmentation, enhancing anti-tumour immune response, and tissue decellularisation for regenerative medicine applications. Potential clinical advantages of the histotripsy methods are discussed. Histotripsy methods can be used to mechanically ablate a wide variety of tissues, whilst selectivity sparing structures such as large vessels. Both ultrasound and MR imaging can be used for targeting and monitoring the treatment in real time. Although the two approaches utilise different mechanisms for tissue disintegration, both have many of the same advantages and offer a promising alternative method of non-invasive surgery.

Keywords: High intensity focused ultrasound; physics; ultrasound.

Figure 1

(A) Highly asymmetric nonlinear pressure waveform with shock fronts at the focus typical for histotripsy. Different pulsing schemes used in (B) cavitation cloud histotripsy and (C) boiling histotripsy approaches; (D) Cavitation cloud histotripsy lesion in ex vivo porcine myocardial muscle. (E) H&E section at boundary of the lesion showing the bisection of myocytes and the complete homogenization of tissue. (After Parson et al. [12]) (F) Boiling histotripsy lesion in ex vivo bovine liver tissue. (G) H&E section through the lesion boundary showing complete disintegration of cells in the lesion.

Figure 2

(A) Histotripsy was applied to the prostate transcutaneously in anesthetized canine subjects. A transrectal ultrasound imaging probe provided real-time visualization of the prostate and cavitation bubble cloud during treatment. (B) H&E stained cross-section of the prostate four weeks after treatment. The targeted volume (devoid of debris) is seen within the glandular prostate communicating with the urethra.

Figure 3

(A) H&E slide showing intact vessels (indicated by arrows) remained in the completely fractionated liver. (B) There is no statistical significance in the number of vessels above 300µm diameter in the treated and control regions. (This figure is adapted from [52])

Figure 4

Ultrasound images of the femoral vein captured by a linear array imaging probe between treatments of a thrombus. (A) and (B) show the original appearance of the vessel on 2D imaging and with color Doppler. (C) and (D) show the thrombus after 240 seconds of treatment (one scan). (E) and (F) show the final condition of the clot after 720 seconds (three scans). Note the decreased echogenicity in the lumen on (C) and (E) compared with (A). Also, a flow channel is clearly visible on (D) and (F), while none was present before treatment in (B). (This figure is adapted from [68])

Figure 5

(A) Fragment size distributions from abbreviated lithotripter and histotripsy treatments. (B) Histotripsy erosion rates for various pulse rates and pressures showing saturation effect. (C) Measured comminution rates for SWL, histotripsy, and combined therapy.

Figure 6

(A) A boiling histotripsy transducer built to treat through large overlying tissue paths such as liver. (B) Three mechanical lesions generated at shallow depth in ex-vivo bovine liver tissue, showing the tadpole shape. (C) Lesions generated through different liver thicknesses showing consistent dimensions. Ultrasound was generated from the top of the frames. (D) In-situ pressure waveform determined using nonlinear derating method from characterization measurements in water [18, 88].

Figure 7

(A) Experimental set-up for generating boiling histotripsy lesions in ex-vivo porcine kidney. (B) Mechanical lesion in a cortex of the kidney treated with boiling histotripsy. (C) Histologic appearance of a section of boiling histotripsy lesion demonstrating homogenization of the targeted region.

Figure 8

(A) Diagram of the experimental set-up for boiling histotripsy of subcutaneous B16 melanoma tumors in mice. (B) Frames of B-mode ultrasound recorded before and during boiling-histotripsy treatment of the tumor. (C) The dynamics of the tumor growth before and after boiling histotripsy or sham treatment (n = 4 per group). Although the treatment delayed the tumor growth, it did not affect the subsequent tumor growth rate.

Figure 9

(A) Photos of boiling histotripsy lesions in ex-vivo bovine liver in cross-section after rinsing (left). Rinsed lesions reveal remaining vasculature and connective tissue that were preserved after sonication. Histomicrographs of representative sections stained with NADH-d (center) and Picro sirius red (right). Unstained regions in the NADH-d stained sections indicate thermal damage. Orange-red birefringence in the Picro sirius red stain sections indicate fibrillar collagen (yellow arrows). Large caliber vessels can be observed to be intact. Dashed yellow line indicates the border of the lesions. Scale bar represents 5 mm. B) Masson’s trichome stained sections showing small caliber patent vessels (yellow arrows). Scale bar represents 100 μm. C) Cross section of volumetric lesion with lysed cell debris washed out and with the remaining connective tissue manipulated. Scale bar represents 5mm.

Figure 10

Microtripsy resulting from a single principal negative half cycle exceeding the intrinsic threshold for cloud cavitation histotripsy. (A) A representative acoustic waveform for a 500 kHz histotripsy pulse measured in water with a fiber optic probe hydrophone (FOPH). (B) Cavitational bubble cloud and (C) corresponding lesion generated in a red blood cell (RBC) phantom using 500 kHz histotripsy pulses with an estimated peak negative pressure of 26.2 MPa (estimated using linear summation). (D) Demonstration of fine spatial resolution as evidenced in the phrase reading “M” (vertical scale bar 1 mm). (Produced using a 3 MHz transducer, and this figure is adapted from Lin et al. [14])