A Multimodal Phantom for Visualization and Assessment of Histotripsy Treatments on Ultrasound and X-Ray Imaging

Evaluation of targeting accuracy of cone beam CT guided histotripsy in an in vivo porcine model

PURPOSE

The application of histotripsy, an emerging noninvasive, non-ionizing, and non-thermal tumor treatment, is currently limited by the inherent limitations of diagnostic ultrasound as the sole targeting modality. This study evaluates the feasibility and accuracy of cone beam computed tomography (CBCT) guidance for histotripsy treatments in an in vivo porcine model.

MATERIALS AND METHODS

Histotripsy treatments were performed in the liver of seven healthy swine under the guidance of a C-arm CBCT system that was calibrated to the robotic arm of the histotripsy system. For each treatment, pseudotumors (small histotripsy treatments of 15 mm) were created using conventional US guidance to serve as targets for subsequent CBCT guided treatments. A pretreatment CBCT with intravenous contrast was acquired for each swine and the center of the pseudotumor was selected as the target. The robotic arm automatically aligned the transducer to the selected target location. Ultrasound based aberration offset correction was performed when possible, and a 25 mm diameter treatment was performed. A post-treatment CBCT with intravenous contrast was then acquired to evaluate coverage, treatment size, and distance between the pseudotumor target and actual treatment zone center.

RESULTS

Treatments were technically successful and pseudotumors were completely covered in all seven treatments (7/7). The average treatment diameter was 39.3 ± 4.2 mm. The center-to-center distance between pseudotumor and actual treatments was 3.8 ± 1.3 mm.

CONCLUSION

CBCT provides accurate targeting for histotripsy treatment in vivo. While future work is required to assess safety and efficacy in the presence of obstructions, the proposed approach could supplement ultrasound imaging for targeting.

Calibration correction to improve registration during cone-beam CT guided histotripsy

BACKGROUND

Histotripsy is a non-invasive, non-ionizing, non-thermal focused ultrasound technique. High amplitude short acoustic pulses converge to create high negative pressures that cavitate endogenous gas into a bubble cloud leading to mechanical tissue destruction. In the United States, histotripsy is approved to treat liver tumors under diagnostic ultrasound guidance but in initial clinical cases, some areas of the liver have not been treated due to bone or gas obstructing the acoustic window for targeting. To address this limitation in visualization, cone-beam computed tomography (CBCT) guided histotripsy was developed to expand the number of tumors and patients that can be treated with histotripsy.

PURPOSE

The purpose of this work is to improve the accuracy of CBCT guided histotripsy by calibrating the therapeutic bubble cloud location relative to the histotripsy robot arm.

METHODS

The calibration correction involves creating a bubble cloud sized treatment (a few mm) in an agar-based phantom consisting of 11 layers with alternating high and low x-ray attenuation. The layers were spaced ∼3 mm apart to allow visualization of mixing after mechanical disintegration from the histotripsy treatment. Bubble cloud treatments were localized using an automated algorithm that minimized a cost function based on the intensity difference within the treatment region on the pre- and post-treatment CBCT. The actual treatment location can be compared to the theoretical bubble cloud location (focal point based on the CAD model of the transducer assembly) to calculate a 3D offset (X, Y, Z), which is used as the calibration correction between the therapeutic bubble cloud location and the histotripsy robot arm. The phantom and algorithm were analyzed to determine parameters that maximized bubble cloud treatment detection (treatment duration, localization accuracy of the phantom, number of bubble clouds) and were tested on four different histotripsy transducers.

RESULTS

Bubble cloud locations were accurately identified with the automated algorithm from post-treatment CBCT images of the multilayer agar phantom. Treating the phantom for 20 seconds was associated with the greatest change in CBCT intensity. The phantom and algorithm were able to localize changes in bubble cloud location with mean residual errors (MRE) between the measured and planned translations of 0.3 ± 0.3 mm in X, -0.2 ± 0.6 mm in Y, and 0.1 ± 1.0 mm in Z. A multi-bubble cloud calibration approach with four adjacent bubble clouds provided a statistically significant lower mean absolute deviation (MAD) in measured 3D offset (0.1, 0.0 and 0.2 mm in X, Y, and Z, respectively) compared to using a single bubble cloud (MAD of 0.2, 1.1 and 1.2 mm in X, Y, and Z, respectively). The calibration correction method measured statistically significantly different 3D transducer offsets between the four histotripsy transducers.

CONCLUSIONS

Creating and analyzing four adjacent bubble clouds together produced more accurate and reproducible 3D offset measurements than analyzing individual bubble clouds. The presented histotripsy bubble cloud calibration correction method is automated, accurate, and can be easily integrated in the current histotripsy workflow to improve accuracy of CBCT guided histotripsy.

A target containing phantom for accuracy assessment of cone-beam CT-guided histotripsy

PURPOSE

Histotripsy is a nonionizing, noninvasive, and nonthermal focal tumor therapy. Cone-beam computed tomography (CBCT) guidance was developed for targeting tumors not visible on ultrasound. This approach assumes cavitation is formed at the geometrical focal point of the therapy transducer. In practice, the exact location might vary slightly between transducers. In this study, we present a phantom with an embedded target to evaluate CBCT-guided histotripsy accuracy and assess the completeness of treatments.

METHODS

Spherical (2.8 cm) targets with alternating layers of agar and radiopaque barium were embedded in larger phantoms with similar layers. The layer geometry was designed so that targets were visible on pre-treatment CBCT scans. The actual histotripsy treatment zone was visualized via the mixing of adjacent barium and agar layers in post-treatment CBCT images. CBCT-guided histotripsy treatments of the targets were performed in six phantoms. Offsets between planned and actual treatment zones were measured and used for calibration refinement. To measure targeting accuracy after calibration refinement, six additional phantoms were treated. In a separate investigation, two groups (N = 3) of phantoms were treated to assess visualization of incomplete treatments ("undertreatment" group: 2 cm treatment within 2.8 cm tumor, "mistarget" group: 2.8 cm treatment intentionally shifted laterally). Treatment zones were segmented (3D Slicer 5.0.3), and the centroid distance between the prescribed target and actual treatment zones was quantified.

RESULTS

In the calibration refinement group, a 2 mm offset in the direction of ultrasound propagation (Z) was measured. After calibration refinement, the centroid-to-centroid distance between prescribed and actual treatment volumes was 0.5 ± 0.2 mm. Average difference between the prescribed and measured treatment sizes in the incomplete treatment groups was 0.5 ± 0.7 mm. In the mistarget group, the distance between prescribed and measured shifts was 0.2 ± 0.1 mm.

CONCLUSION

The proposed prototype phantom allowed for accurate measurement of treatment size and location, and the CBCT visible target provided a simple way to detect misalignments for preliminary quality assurance of CBCT-guided histotripsy.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.