Comparison of computer simulations and clinical treatment results of magnetic resonance guided focused ultrasound surgery (MRgFUS) of uterine fibroids

Development and validation of a radiomics model based on T2-weighted imaging for predicting the efficacy of high intensity focused ultrasound ablation in uterine fibroids

Background

The heterogeneity of uterine fibroids in magnetic resonance imaging (MRI) is complex for a subjective visual evaluation, therefore it is difficult for an accurate prediction of the efficacy of high intensity focused ultrasound (HIFU) ablation in fibroids before the treatment. The purpose of this study was to set up a radiomics model based on MRI T2-weighted imaging (T2WI) for predicting the efficacy of HIFU ablation in uterine fibroids, and it would be used in preoperative screening of the fibroids for achieving high non-perfused volume ratio (NPVR).

Methods

A total of 178 patients with uterine fibroids were consecutively enrolled and treated with ultrasound-guided HIFU under conscious sedation between February 2017 and December 2021. Among them, 96 patients with 108 uterine fibroids with high ablation efficacy (NPVR ≥80%, h_NPVR) and 82 patients with 92 fibroids with lower ablation efficacy (NPVR <80%, l_NPVR) were retrospectively analyzed. The transverse T2WI images of fibroids were selected, and the fibroids were delineated slice by slice using ITK-SNAP software. The radiomics analysis was performed to find the imaging biomarker for the construction of a predicting model for the evaluation of the ablation efficacy, including the feature extraction, feature selection and model construction. The prediction model was built by logistic regression and assessed by receiver operating characteristic (ROC) curve, and the prediction efficiency of the two models was compared by Delong test. The ratio of the training set to the testing set was 8:2.

Results

The logistic regression model showed that the mean area under the curve (AUC) of the training set was 0.817 [95% confidence interval (CI): 0.755-0.882], and the testing set was 0.805 (95% CI: 0.670-0.941), respectively, which indicated a strong classification ability. The Delong test showed that there was no significant difference in the area under the ROC curve between the training set and testing set (P>0.05).

Conclusions

The radiomics model based on T2WI is feasible and effective for predicting the efficacy of HIFU ablation in treatment of uterine fibroids.

Evaluation of acoustic-thermal simulations of in vivo magnetic resonance guided focused ultrasound ablative therapy

PURPOSE

To evaluate numerical simulations of focused ultrasound (FUS) with a rabbit model, comparing simulated heating characteristics with magnetic resonance temperature imaging (MRTI) data collected during in vivo treatment.

METHODS

A rabbit model was treated with FUS sonications in the biceps femoris with 3D MRTI collected. Acoustic and thermal properties of the rabbit muscle were determined experimentally. Numerical models of the rabbits were created, and tissue-type-specific properties were assigned. FUS simulations were performed using both the hybrid angular spectrum (HAS) method and k-Wave. Simulated power deposition patterns were converted to temperature maps using a Pennes' bioheat equation-based thermal solver. Agreement of pressure between the simulation techniques and temperature between the simulation and experimental heating was evaluated. Contributions of scattering and absorption attenuation were considered.

RESULTS

Simulated peak pressures derived using the HAS method exceeded the simulated peak pressures from k-Wave by 1.6 ± 2.7%. The location and FWHM of the peak pressure calculated from HAS and k-Wave showed good agreement. When muscle acoustic absorption value in the simulations was adjusted to approximately 54% of the measured attenuation, the average root-mean-squared error between simulated and experimental spatial-average temperature profiles was 0.046 ± 0.019 °C/W. Mean distance between simulated and experimental COTMs was 3.25 ± 1.37 mm. Transverse FWHMs of simulated sonications were smaller than in in vivo sonications. Longitudinal FWHMs were similar.

CONCLUSIONS

Presented results demonstrate agreement between HAS and k-Wave simulations and that FUS simulations can accurately predict focal position and heating for in vivo applications in soft tissue.

Factors influencing magnetic resonance imaging changes associated with pelvic bone injury after high-intensity focused ultrasound ablation of uterine fibroids: a retrospective case-control study

Background

The application of high-intensity focused ultrasound (HIFU) in the treatment of uterine fibroids is becoming increasingly widespread, and postoperative collateral thermal damage to adjacent tissue has become a prominent subject of discussion. However, there is limited research related to bone injury. Therefore, the aim of this study was to investigate the potential factors influencing unintentional pelvic bone injury after HIFU ablation of uterine fibroids with magnetic resonance imaging (MRI).

Methods

A total of 635 patients with fibroids treated with HIFU in the First Affiliated Hospital of Chongqing Medical University were enrolled. All patients underwent contrast-enhanced MRI (CE-MRI) pre- and post-HIFU. Based on the post-treatment MRI, the patients were divided into two groups: pelvic bone injury group and non-injury group, while the specific site of pelvic bone injury of each patient was recorded. The univariate and multivariate analyses were used to assess the correlations between the factors of fibroid features and treatment parameters and pelvic bone injury, and to further analyze the factors influencing the site of injury.

Results

Signal changes in the pelvis were observed on CE-MRI in 51% (324/635) of patients after HIFU. Among them, 269 (42.4%) patients developed sacral injuries and 135 (21.3%) had pubic bone injuries. Multivariate analyses showed that patients with higher age [P=0.003; odds ratio (OR), 1.692; 95% confidence interval (CI): 1.191-2.404], large anterior side-to-skin distance of fibroid (P<0.001; OR, 2.297; 95% CI: 1.567-3.365), posterior wall fibroid (P=0.006; OR, 1.897; 95% CI: 1.204-2.989), hyperintensity on T2-weighted imaging (T2WI, P=0.003; OR, 2.125; 95% CI: 1.283-3.518), and large therapeutic dose (TD, P<0.001; OR, 3.007; 95% CI: 2.093-4.319) were at higher risk of postoperative pelvic bone injury. Further analysis of the factors influencing the site of the pelvic bone injury showed that some of the fibroid features and treatment parameters were associated with it. Moreover, some postoperative pain-related adverse events were associated with the pelvic bone injury.

Conclusions

Post-HIFU treatment, patients may experience pelvic injuries to the sacrum, pubis, or a combination of both, and some of them experienced adverse events. Some fibroid features and treatment parameters are associated with the injury. Taking its influencing factors into full consideration preoperatively, slowing down treatment, and prolonging intraoperative cooling phase can help optimize treatment decisions for HIFU.

Aberration correction in abdominal histotripsy

In transabdominal histotripsy, ultrasound pulses are focused on the body to noninvasively destroy soft tissues via cavitation. However, the ability to focus is limited by phase aberration, or decorrelation of the ultrasound pulses due to spatial variation in the speed of sound throughout heterogeneous tissue. Phase aberration shifts, broadens, and weakens the focus, thereby reducing the safety and efficacy of histotripsy therapy. This paper reviews and discusses aberration effects in histotripsy and in related therapeutic ultrasound techniques (e.g., high intensity focused ultrasound), with an emphasis on aberration by soft tissues. Methods for aberration correction are reviewed and can be classified into two groups: model-based methods, which use segmented images of the tissue as input to an acoustic propagation model to predict and compensate phase differences, and signal-based methods, which use a receive-capable therapy array to detect phase differences by sensing acoustic signals backpropagating from the focus. The relative advantages and disadvantages of both groups of methods are discussed. Importantly, model-based methods can correct focal shift, while signal-based methods can restore substantial focal pressure, suggesting that both methods should be combined in a 2-step approach. Aberration correction will be critical to improving histotripsy treatments and expanding the histotripsy treatment envelope to enable non-invasive, non-thermal histotripsy therapy for more patients.

In Situ Measurement of Acoustic Attenuation for Focused Ultrasound Ablation Surgery Using a Boiling Bubble at the Focus

OBJECTIVE

Acoustic attenuation in the propagation path of focused ultrasound ablation surgery determines the energy loss toward the focal region and is critical to the consequent treatment outcomes. In situ non-invasive, reliable, and accurate measurement is challenging for multi-layered heterogeneous tissues within the focusing angle.

METHODS

A novel measurement approach is proposed and its performance is evaluated using ex vivo porcine tenderloin and bovine heart. A big boiling bubble (i.e., larger than a few millimeters in size) was produced at the focus as a strong reflector inside the tissue, and the echo amplitudes were used to determine the acoustic attenuation. Two models, acoustic ray and energy loss, were developed to derive the equivalent acoustic attenuation coefficient for a focused beam.

RESULTS

The measured acoustic attenuation coefficients of ex vivo porcine tenderloin and bovine heart at 0.97 MHz and a thickness of 3 cm are 0.159 ± 0.002 and 0.250 ± 0.005 Np/cm, respectively, which are all within the scope of measured values in the literature. In addition, the echo amplitude is sensitive to the conditions of the propagation path, and the inverse acoustic attenuation coefficient of the silicone gel pad placed in front of the tissue sample was 0.807 ± 0.002 Np/cm, which is comparable to the measurement using the insertion substitution method, 0.766 ± 0.003 Np/cm.

CONCLUSION

Our proposed approach could determine the tissue acoustic attenuation for focused ultrasound ablation surgery reliably and accurately in situ. The easy operating protocol may allow clinical translation and adoption for improved safety and efficacy.

An endoluminal cylindrical sectored-ring ultrasound phased-array applicator for minimally-invasive therapeutic ultrasound

BACKGROUND

The size of catheter-based ultrasound devices for delivering ultrasound energy to deep-seated tumors is constrained by the access pathway which limits their therapeutic capabilities.

PURPOSE

To devise and investigate a deployable applicator suitable for minimally-invasive delivery of therapeutic ultrasound, consisting of a 2D cylindrical sectored-ring ultrasound phased array, integrated within an expandable paraboloid-shaped balloon-based reflector. The balloon can be collapsed for compact delivery and expanded close to the target position to mimic a larger-diameter concentric-ring sector-vortex array for enhanced dynamic control of focal depth and volume.

METHODS

Acoustic and biothermal simulations were employed in 3D generalized homogeneous and patient-specific heterogeneous models, for three-phased array transducers with 32, 64, and 128 elements, composed of sectored 4, 8, and 16 tubular ring transducers, respectively. The applicator performance was characterized as a function of array configuration, focal depth, phasing modes, and balloon reflector geometry. A 16-element proof-of-concept phased array applicator assembly, consisting of four tubular transducers each divided into four sectors, was fabricated, and characterized with hydrophone measurements along and across the axis, and ablations in ex vivo tissue.

RESULTS

Simulation results indicated that transducer arrays (1.5 MHz, 9 mm OD × 20 mm long), balloon sizes (41-50 mm expanded diameter, 20-60 mm focal depth), phasing mode (0-4) and sonication duration (30 s) can produce spatially localized acoustic intensity focal patterns (focal length: 3-22 mm, focal width: 0.7-8.7 mm) and ablative thermal lesions (width: 2.7-16 mm, length: 6-46 mm) in pancreatic tissue across a 10-90 mm focal depth range. Patient-specific studies indicated that 0.1, 0.46, and 1.2 cm³ volume of tumor can be ablated in the body of the pancreas for 120 s sonications using a single axial focus (Mode 0), or four, and eight simultaneous foci in a toroidal pattern (Mode 2 and 4, respectively). Hydrophone measurements demonstrated good agreement with simulation. Experiments in which chicken meat was thermally ablated indicated that volumetric ablation can be produced using single or multiple foci.

CONCLUSIONS

The results of this study demonstrated the feasibility of a novel compact ultrasound applicator design capable of focusing, deep penetration, electronic steering, and volumetric thermal ablation. The proposed applicator can be used for compact endoluminal or laparoscopic delivery of localized ultrasound energy to deep-seated targets.