Characterization of Lesion Formation and Bubble Activities during High Intensity Focused Ultrasound Ablation using Temperature-Derived Parameters

Proactive esophageal cooling during laser cardiac ablation: A computer modeling study

BACKGROUND AND OBJECTIVES

Laser ablation is increasingly used to treat atrial fibrillation (AF). However, atrioesophageal injury remains a potentially serious complication. While proactive esophageal cooling (PEC) reduces esophageal injury during radiofrequency ablation, the effects of PEC during laser ablation have not previously been determined. We aimed to evaluate the protective effects of PEC during laser ablation of AF by means of a theoretical study based on computer modeling.

METHODS

Three-dimensional mathematical models were built for 20 different cases including a fragment of atrial wall (myocardium), epicardial fat (adipose tissue), connective tissue, and esophageal wall. The esophagus was considered with and without PEC. Laser-tissue interaction was modeled using Beer-Lambert's law, Pennes' Bioheat equation was used to compute the resultant heating, and the Arrhenius equation was used to estimate the fraction of tissue damage (FOD), assuming a threshold of 63% to assess induced necrosis. We modeled laser irradiation power of 8.5 W over 20 s. Thermal simulations extended up to 250 s to account for thermal latency.

RESULTS

PEC significantly altered the temperature distribution around the cooling device, resulting in lower temperatures (around 22°C less in the esophagus and 9°C in the atrial wall) compared to the case without PEC. This thermal reduction translated into the absence of transmural lesions in the esophagus. The esophagus was thermally damaged only in the cases without PEC and with a distance equal to or shorter than 3.5 mm between the esophagus and endocardium (inner boundary of the atrial wall). Furthermore, PEC demonstrated minimal impact on the lesion created across the atrial wall, either in terms of maximum temperature or FOD.

CONCLUSIONS

PEC reduces the potential for esophageal injury without degrading the intended cardiac lesions for a variety of different tissue thicknesses. Thermal latency may influence lesion formation during laser ablation and may play a part in any collateral damage.

Advances in Biological Application of and Research on Low-Frequency Ultrasound

In recent years, the in-depth study of low-frequency sonophoresis (LFS) has greatly elucidated its biological effects in various therapeutic applications, including drug delivery, enhanced healing, thrombolytic technology, anti-inflammatory effects and tumor treatment. Specifically, numerous studies have reported its use in drug delivery and synergistic antitumor activity, indicating a new treatment direction for cancer. However, there are significant gaps in the understanding of LFS in terms of frequency and sound intensity safety; these issues are becoming increasingly important in understanding the biological effects of LFS ultrasound. This article reviews the treatment mechanism and current applications of LFS technology and discusses and summarizes its safety and application prospects.

HIFU-induced changes in optical scattering and absorption of tissue over nine orders of thermal dose

The optical properties of tissue change during thermal ablation. Multi-modal methods such as acousto-optic (AO) and photo-acoustic (PA) imaging may provide a real-time, direct measure of lesion formation. Baseline changes in optical properties have been previously measured over limited ranges of thermal dose for tissues exposed to a temperature-controlled water bath, however, there is scant data for optical properties of lesions created by HIFU. In this work, the optical scattering and absorption coefficients from 400-1300 nm of excised chicken breast exposed to HIFU were measured using an integrating sphere spectrophotometric technique. HIFU-induced spatiotemporal temperature elevations were measured using an infrared camera and used to calculate the thermal dose delivered to a localized region of tissue. Results obtained over a range of thermal dose spanning 9 orders of magnitude show that the reduced scattering coefficient increases for HIFU exposures exceeding a threshold thermal dose of CEM₄₃  =  600  ±  81 cumulative equivalent minutes. HIFU-induced thermal damage results in changes in scattering over all optical wavelengths, with a 2.5-fold increase for thermal lesions exceeding 70 °C. The tissue absorption coefficient was also found to increase for thermally lesioned tissue, however, the magnitude was strongly dependent on the optical wavelength and there was substantial sample-to-sample variability, such that the existence of a threshold thermal dose could not be determined. Therapeutic windows, where the optical penetration depth is expected to be greatest, were identified in the near infrared regime centered near 900 nm and 1100 nm. These data motivate further research to improve the real-time AO and PA sensing of lesion formation during HIFU therapy as an alternative to thermometry.

Temperature dependence of speed of sound and attenuation of porcine left ventricular myocardium

The temperature dependence of soft tissue acoustic properties is relevant for monitoring tissue hyperthermia. In the current work, the propagation speed and attenuation of healthy porcine left ventricular myocardium (N=5) was investigated in a frequency range relevant for clinical diagnostic imaging, i.e. 2.5-13.0MHz. Each tissue sample was held in a water bath at a temperature T=25°C, heated to 45°C, and allowed to cool back down to 25°C. Due to initial tissue swelling, the data for decreasing temperatures was considered more reliable. In this case, the slope of the phase velocity versus temperature relation was measured to be 1.10±0.04m/s/°C, and the slope of the attenuation was -0.11±0.04dB/cm/°C at 10MHz, or -0.0041±0.0015dB/cm/MHz^1.4336/°C as a function of frequency.

Microscale characterization of the viscoelastic properties of hydrogel biomaterials using dual-mode ultrasound elastography

Characterization of the microscale mechanical properties of biomaterials is a key challenge in the field of mechanobiology. Dual-mode ultrasound elastography (DUE) uses high frequency focused ultrasound to induce compression in a sample, combined with interleaved ultrasound imaging to measure the resulting deformation. This technique can be used to non-invasively perform creep testing on hydrogel biomaterials to characterize their viscoelastic properties. DUE was applied to a range of hydrogel constructs consisting of either hydroxyapatite (HA)-doped agarose, HA-collagen, HA-fibrin, or preosteoblast-seeded collagen constructs. DUE provided spatial and temporal mapping of local and bulk displacements and strains at high resolution. Hydrogel materials exhibited characteristic creep behavior, and the maximum strain and residual strain were both material- and concentration-dependent. Burger's viscoelastic model was used to extract characteristic parameters describing material behavior. Increased protein concentration resulted in greater stiffness and viscosity, but did not affect the viscoelastic time constant of acellular constructs. Collagen constructs exhibited significantly higher modulus and viscosity than fibrin constructs. Cell-seeded collagen constructs became stiffer with altered mechanical behavior as they developed over time. Importantly, DUE also provides insight into the spatial variation of viscoelastic properties at sub-millimeter resolution, allowing interrogation of the interior of constructs. DUE presents a novel technique for non-invasively characterizing hydrogel materials at the microscale, and therefore may have unique utility in the study of mechanobiology and the characterization of hydrogel biomaterials.

Calibration and Evaluation of Ultrasound Thermography Using Infrared Imaging

Real-time monitoring of the spatiotemporal evolution of tissue temperature is important to ensure safe and effective treatment in thermal therapies including hyperthermia and thermal ablation. Ultrasound thermography has been proposed as a non-invasive technique for temperature measurement, and accurate calibration of the temperature-dependent ultrasound signal changes against temperature is required. Here we report a method that uses infrared thermography for calibration and validation of ultrasound thermography. Using phantoms and cardiac tissue specimens subjected to high-intensity focused ultrasound heating, we simultaneously acquired ultrasound and infrared imaging data from the same surface plane of a sample. The commonly used echo time shift-based method was chosen to compute ultrasound thermometry. We first correlated the ultrasound echo time shifts with infrared-measured temperatures for material-dependent calibration and found that the calibration coefficient was positive for fat-mimicking phantom (1.49 ± 0.27) but negative for tissue-mimicking phantom (-0.59 ± 0.08) and cardiac tissue (-0.69 ± 0.18°C-mm/ns). We then obtained the estimation error of the ultrasound thermometry by comparing against the infrared-measured temperature and revealed that the error increased with decreased size of the heated region. Consistent with previous findings, the echo time shifts were no longer linearly dependent on temperature beyond 45°C-50°C in cardiac tissues. Unlike previous studies in which thermocouples or water bath techniques were used to evaluate the performance of ultrasound thermography, our results indicate that high-resolution infrared thermography is a useful tool that can be applied to evaluate and understand the limitations of ultrasound thermography methods.

Electrophysiological changes correlated with temperature increases induced by high-intensity focused ultrasound ablation

To gain better understanding of the detailed mechanisms of high-intensity focused ultrasound (HIFU) ablation for cardiac arrhythmias, we investigated how the cellular electrophysiological (EP) changes were correlated with temperature increases and thermal dose (cumulative equivalent minutes [CEM43]) during HIFU application using Langendorff-perfused rabbit hearts. Employing voltage-sensitive dye di-4-ANEPPS, we measured the EP and temperature during HIFU using simultaneous optical mapping and infrared imaging. Both action potential amplitude (APA) and action potential duration at 50% repolarization (APD50) decreased with temperature increases, and APD50 was more thermally sensitive than APA. EP and tissue changes were irreversible when HIFU-induced temperature increased above 52.3 ± 1.4°C and log10(CEM43) above 2.16 ± 0.51 (n = 5), but were reversible when temperature was below 50.1 ± 0.8°C and log10(CEM43) below -0.9 ± 0.3 (n = 9). EP and temperature/thermal dose changes were spatially correlated with HIFU-induced tissue necrosis surrounded by a transition zone.