Real-time spectroscopic assessment of thermal damage: implications for radiofrequency ablation

Portable Nd:YAG Laser-Enhanced Photoacoustic Spectral Sensing for Breast Tissues: Toward Oncological Theranostics

A solid-state laser is preferred for generating light in a photoacoustic (PA) system because of its high energy and coherence. However, conventional Nd:YAG lasers are bulky, complex, and expensive. This article introduces a portable alternative: a custom-built Nd:YAG laser with an in-house power supply that delivers 0-30 A current pulses with a 1500 μs pulse width, providing efficient thermal management. A pockels cell driver generates 10 ns pulses with 3.84 mJ/cm 2 laser energy density. Implemented for noninvasive breast cancer diagnosis, The peak frequency obtained from three different samples was 0.23 ± 0.1, 0.26 ± 0.13, and 1.80 ± 0.14 MHz, respectively, for Normal, Fibrotic, and Tumor tissues. In addition to the dominant frequency peaks the spectral energy of the PASR spectra has also been investigated to characterize the breast tissue samples. The developed laser successfully differentiates between carcinoma, fibrocystic disease, and normal breast tissue based on quantitative PA spectral parameters.

Interstitial Optical Monitoring of Focal Laser Ablation

Focal laser ablation is a minimally invasive method of treating cancerous lesions in organs such as prostate, liver and brain. Oncologic control is achieved by inducing hyperthermia throughout the target while minimizing damage to surrounding tissue. Consequently, successful clinical outcomes are contingent upon achieving desired ablation volumes. Magnetic resonance thermometry is frequently used to monitor the formation of the induced thermal damage zone and inform the decision to terminate energy delivery. However, due to the associated cost and complexity there is growing interest in the development of alternative approaches. Here we investigate the utility of real-time interstitial interrogation of laser-tissue interaction as an inexpensive alternative monitoring modality that provides direct assessment of tissue coagulation without the need for organ specific calibration. The optical contrast mechanism was determined using a Monte Carlo model. Subsequently, four interstitial probe designs were manufactured and assessed in a tissue mimicking phantom under simultaneous magnetic resonance imaging. Finally, the optimal probe design was evaluated in ex vivo bovine muscle. It was found to be capable of providing sufficient feedback to achieve pre-defined ablation radii in the range 4-7mm with a mean absolute error of 0.3mm. This approach provides an inexpensive monitoring modality that may facilitate widespread adoption of focal laser ablation.

Methods of monitoring thermal ablation of soft tissue tumors - A comprehensive review

Thermal ablation is a form of hyperthermia in which oncologic control can be achieved by briefly inducing elevated temperatures, typically in the range 50-80°C, within a target tissue. Ablation modalities include high intensity focused ultrasound, radiofrequency ablation, microwave ablation, and laser interstitial thermal therapy which are all capable of generating confined zones of tissue destruction, resulting in fewer complications than conventional cancer therapies. Oncologic control is contingent upon achieving predefined coagulation zones; therefore, intraoperative assessment of treatment progress is highly desirable. Consequently, there is a growing interest in the development of ablation monitoring modalities. The first section of this review presents the mechanism of action and common applications of the primary ablation modalities. The following section outlines the state-of-the-art in thermal dosimetry which includes interstitial thermal probes and radiologic imaging. Both the physical mechanism of measurement and clinical or pre-clinical performance are discussed for each ablation modality. Thermal dosimetry must be coupled with a thermal damage model as outlined in Section 4. These models estimate cell death based on temperature-time history and are inherently tissue specific. In the absence of a reliable thermal model, the utility of thermal monitoring is greatly reduced. The final section of this review paper covers technologies that have been developed to directly assess tissue conditions. These approaches include visualization of non-perfused tissue with contrast-enhanced imaging, assessment of tissue mechanical properties using ultrasound and magnetic resonance elastography, and finally interrogation of tissue optical properties with interstitial probes. In summary, monitoring thermal ablation is critical for consistent clinical success and many promising technologies are under development but an optimal solution has yet to achieve widespread adoption.

Optical signatures of radiofrequency ablation in biological tissues

Accurate monitoring of treatment is crucial in minimally-invasive radiofrequency ablation in oncology and cardiovascular disease. We investigated alterations in optical properties of ex-vivo bovine tissues of the liver, heart, muscle, and brain, undergoing the treatment. Time-domain diffuse optical spectroscopy was used, which enabled us to disentangle and quantify absorption and reduced scattering spectra. In addition to the well-known global (1) decrease in absorption, and (2) increase in reduced scattering, we uncovered new features based on sensitive detection of spectral changes. These absorption spectrum features are: (3) emergence of a peak around 840 nm, (4) redshift of the 760 nm deoxyhemoglobin peak, and (5) blueshift of the 970 nm water peak. Treatment temperatures above 100 °C led to (6) increased absorption at shorter wavelengths, and (7) further decrease in reduced scattering. This optical behavior provides new insights into tissue response to thermal treatment and sets the stage for optical monitoring of radiofrequency ablation.

Photoacoustic Spectral Sensing Technique for Diagnosis of Biological Tissue Coagulation: In-Vitro Study

Thermal coagulation of abnormal tissues has evolved as a therapeutic technique for different diseases including cancer. Tissue heating beyond 55 °C causes coagulation that leads to cell death. Noninvasive diagnosis of thermally coagulated tissues is pragmatic for performing efficient therapy as well as reducing damage of surrounding healthy tissues. We propose a noninvasive, elasticity-based photoacoustic spectral sensing technique for differentiating normal and coagulated tissues. Photoacoustic diagnosis is performed for quantitative differentiation of normal and coagulated excised chicken liver and muscle tissues in vitro by characterizing a dominant frequency of photoacoustic frequency spectrum. Pronounced distinction in the spectral parameter (i.e., dominant frequency) was observed due to change in tissue elastic property. We confirmed nearly two-fold increase in dominant frequencies for the coagulated muscle and liver tissues as compared to the normal ones. A density increase caused by tissue coagulation is clearly reflected in the dominant frequency composition. Experimental results were consistent over five different sample sets, delineating the potential of proposed technique to diagnose biological tissue coagulation and thus monitor thermal coagulation therapy in clinical applications.

Monitoring of tissue optical properties during thermal coagulation of ex vivo tissues

BACKGROUND AND OBJECTIVE

Real-time monitoring of tissue status during thermal ablation of tumors is critical to ensure complete destruction of tumor mass, while avoiding tissue charring and excessive damage to normal tissues. Currently, magnetic resonance thermometry (MRT), along with magnetic resonance imaging (MRI), is the most commonly used technique for monitoring and assessing thermal ablation process in soft tissues. MRT/MRI is very expensive, bulky, and often subject to motion artifacts. On the other hand, light propagation within tissue is sensitive to changes in tissue microstructure and physiology which could be used to directly quantify the extent of tissue damage. Furthermore, optical monitoring can be a portable, and cost-effective alternative for monitoring a thermal ablation process. The main objective of this study, is to establish a correlation between changes in tissue optical properties and the status of tissue coagulation/damage during heating of ex vivo tissues.

MATERIALS AND METHODS

A portable diffuse reflectance spectroscopy system and a side-firing fiber-optic probe were developed to study the absorption (μa (λ)), and reduced scattering coefficients (μ's (λ)) of native and coagulated ex vivo porcine, and chicken breast tissues. In the first experiment, both porcine and chicken breast tissues were heated at discrete temperature points between 24 and 140°C for 2 minutes. Diffuse reflectance spectra (430-630 nm) of native and coagulated tissues were recorded prior to, and post heating. In a second experiment, porcine tissue samples were heated at 70°C and diffuse reflectance spectra were recorded continuously during heating. The μa (λ) and μ's (λ) of the tissues were extracted from the measured diffuse reflectance spectra using an inverse Monte-Carlo model of diffuse reflectance. Tissue heating was stopped when the wavelength-averaged scattering plateaued.

RESULTS

The wavelength-averaged optical properties, <μ's (λ)> and <μa (λ)>, for native porcine tissues (n = 66) at room temperature, were 5.4 ± 0.3 cm(-1) and 0.780 ± 0.008 cm(-1) (SD), respectively. The <μ's (λ)> and <μa (λ)> for native chicken breast tissues (n = 66) at room temperature, were 2.69 ± 0.08 cm(-1) and 0.29 ± 0.01 cm(-1) (SD), respectively. In the first experiment, the <μ's (λ)> of coagulated porcine and chicken breast tissue rose to 56.4 ± 3.6 cm(-1) at 68.7 ± 1.7°C (SD), and 52.8 ± 1 cm(-1) at 57.1 ± 1.5°C (SD), respectively. Correspondingly, the <μa (λ)> of coagulated porcine (140.6°C), and chicken breast tissues (130°C) were 0.75 ± 0.05 cm(-1) and 0.263 ± 0.004 cm(-1) (SD). For both tissues, charring was observed at temperatures above 80°C. During continuous monitoring of porcine tissue (with connective tissues) heating, the <μ's (λ)> started to rise rapidly from 13.7 ± 1.5 minutes and plateaued at 19 ± 2.5 (SD) minutes. The <μ's (λ)> plateaued at 11.7 ± 3 (SD) minutes for porcine tissue devoid of connective tissue between probe and tissue surface. No charring was observed during continuous monitoring of thermal ablation process.

CONCLUSION

The changes in optical absorption and scattering properties can be continuously quantified, which could be used as a diagnostic biomarker for assessing tissue coagulation/damage during thermal ablation. Lasers Surg. Med. 48:686-694, 2016. © 2016 Wiley Periodicals, Inc.

Monitoring of tumor radio frequency ablation using derivative spectroscopy

Despite the widespread use of radio frequency (RF) ablation, an effective way to assess thermal tissue damage during and after the procedure is still lacking. We present a method for monitoring RF ablation efficacy based on thermally induced methemoglobin as a marker for full tissue ablation. Diffuse reflectance (DR) spectra were measured from human blood samples during gradual heating of the samples from 37 to 60, 70, and 85°C. Additionally, reflectance spectra were recorded real-time during RF ablation of human liver tissue ex vivo and in vivo. Specific spectral characteristics of methemoglobin were extracted from the spectral slopes using a custom optical ablation ratio. Thermal coagulation of blood caused significant changes in the spectral slopes, which is thought to be caused by the formation of methemoglobin. The time course of these changes was clearly dependent on the heating temperature. RF ablation of liver tissue essentially led to similar spectral alterations. In vivo DR measurements confirmed that the method could be used to assess the degree of thermal damage during RF ablation and long after the tissue cooled.

Optical spectroscopy: current advances and future applications in cancer diagnostics and therapy

Optical spectroscopy (OS) is a tissue-sensing technique that could enhance cancer diagnosis and treatment in the near future. With OS, tissue is illuminated with a selected light spectrum. Different tissue types can be distinguished from each other based on specific changes in the reflected light spectrum that are a result of differences on a molecular level between compared tissues. Therefore, OS has the potential to become an important optical tool for cancer diagnosis and treatment monitoring. In recent years, significant progress has been made in the discriminating abilities of OS techniques between normal and cancer tissues of multiple human tissue types. This article provides an overview of the advances made with diffuse reflectance, fluorescence and Raman spectroscopy techniques in the field of clinical oncology, and focuses on the different clinical applications that OS could enhance.

Use of endogenous NADH fluorescence for real-time in situ visualization of epicardial radiofrequency ablation lesions and gaps

Radiofrequency ablation (RFA) aims to produce lesions that interrupt reentrant circuits or block the spread of electrical activation from sites of abnormal activity. Today, there are limited means for real-time visualization of cardiac muscle tissue injury during RFA procedures. We hypothesized that the fluorescence of endogenous NADH could be used as a marker of cardiac muscle injury during epicardial RFA procedures. Studies were conducted in blood-free and blood-perfused hearts from healthy adult Sprague-Dawley rats and New Zealand rabbits. Radiofrequency was applied to the epicardial surface of the heart using a 4-mm standard blazer ablation catheter. A dual camera optical mapping system was used to monitor NADH fluorescence upon ultraviolet illumination of the epicardial surface and to record optical action potentials using the voltage-sensitive probe RH237. Epicardial lesions were seen as areas of low NADH fluorescence. The lesions appeared immediately after ablation and remained stable for several hours. Real-time monitoring of NADH fluorescence allowed visualization of viable tissue between the RFA lesions. Dual recordings of NADH and epicardial electrical activity linked the gaps between lesions to postablation reentries. We found that the fluorescence of endogenous NADH aids the visualization of injured epicardial tissue caused by RFA. This was true for both blood-free and blood-perfused preparations. Gaps between NADH-negative regions revealed unablated tissue, which may promote postablation reentry or provide pathways for the conduction of abnormal electrical activity.

Use of a radiofrequency probe for tenoscopic-guided annular ligament desmotomy

REASONS FOR PERFORMING STUDY

Annular ligament desmotomy is commonly performed in horses with chronic tenosynovitis. Previously reported tenoscopic techniques have limitations related to haemorrhage and awkward instrumentation. Radiofrequency (RF) energy affords precision and excellent haemostasis and may be a good alternative to sharp transection of the annular ligament in horses.

OBJECTIVE

To describe a technique for using a RF probe for tenoscopic-guided annular ligament desmotomy and to report the clinical outcome of horses in which it was performed.

METHODS

Cadaver specimens (n = 14) and live horses undergoing unrelated terminal procedures (n = 2) were used to optimise the tenoscopic-guided RF annular ligament desmotomy technique. Records were examined for all horses undergoing annular ligament desmotomy with an RF probe from 2003 to 2008 for which follow-up of >1 year post operatively was available.

RESULTS

The annular ligament was successfully transected in the cadaver and live horse model limbs using 2 different commercially available RF probes. Complete transection was achieved with practice and confirmed on gross dissection. Histopathology did not reveal any collateral damage to surrounding tissue. Follow-up of >1 year was available for 6 of 7 clinical cases. Four of 6 horses returned to work. Owners were satisfied with the outcome in all cases.

CONCLUSIONS

Desmotomy using a RF probe allows precise tissue transection under tenoscopic guidance without damage to surrounding structures or haemorrhage. With experience, it is an easily performed technique. In clinical patients, an acceptable outcome may be expected.

POTENTIAL RELEVANCE

Tenoscopic-guided RF annular ligament desmotomy offers advantages, including reliable haemostasis and precise tissue transection, over previously reported techniques and is a viable surgical alternative for treating horses with annular ligament desmitis and other complex pathology within the tendon sheath.

Perturbative diffusion theory formalism for interpreting temporal light intensity changes during laser interstitial thermal therapy

In an effort to understand dynamic optical changes during laser interstitial thermal therapy (LITT), we utilize the perturbative solution of the diffusion equation in heterogeneous media to formulate scattering weight functions for cylindrical line sources. The analysis explicitly shows how changes in detected interstitial light intensity are associated with the extent and location of the volume of thermal coagulation during treatment. Explanations for previously reported increases in optical intensity observed early during laser heating are clarified using the model and demonstrated with experimental measurements in ex vivo bovine liver tissue. This work provides an improved understanding of interstitial optical signal changes during LITT and indicates the sensitivity and potential of interstitial optical monitoring of thermal damage.

In Vivo Assessment of Radio Frequency Induced Thermal Damage of Kidney Using Optical Spectroscopy

PURPOSE

Radio frequency ablation is a promising modality for treating small renal tumors. Several studies have been published showing its efficacy. A major drawback in the current management of renal tumors with radio frequency ablation is the lack of an effective modality to accurately monitor the progress of the ablation zone in real-time fashion. Previous studies have demonstrated the feasibility of using optical spectroscopy to assess tissue thermal damage, especially in hepatic lesions. We examined the feasibility of this technology in the setting of renal radio frequency ablation.

MATERIALS AND METHODS

A portable spectroscopic system was used to acquire in vivo fluorescence and diffuse reflectance spectra from porcine renal tissue undergoing radio frequency ablation in real-time fashion with simultaneous temperature recordings. Fluorescence and diffuse reflectance spectral data were then correlated with various degrees of thermal damage and temperature recordings.

RESULTS

The most noticeable change in fluorescence characteristics of renal tissue resulting from thermal coagulation was a strong decrease in fluorescence intensity between 400 and 550 nm. When fully coagulated, a significant increase in diffuse reflectance intensity was observed between 500 and 800 nm.

CONCLUSIONS

Optical spectroscopy, specifically fluorescence and diffuse reflectance spectroscopy, differs significantly in porcine renal tissues with varying degrees of thermal damage from radio frequency ablation in an in vivo setting. Future clinical studies with sufficient sample size are required to validate the potential of these findings. Optical diagnostics may prove to be a rapid, noninvasive, low cost option for monitoring the tumor response to radio frequency based ablative techniques. It may be integrated into future radio frequency ablation probes.

Relationship between damaged fraction and reflected spectra of denaturing tissues

BACKGROUND AND OBJECTIVES

During thermal therapy of tissue, such as induced by microwave heating, the initiation of denaturation should be monitored for proper thermal dosage. Additionally, denaturation should be confined to the pathologic volume, while preserving surrounding healthy tissue. The relationship between the damaged fraction and reflected spectra of denaturing tissues was investigated for a variation of the temperature of the tissues.

STUDY DESIGN/MATERIALS AND METHODS

Denaturation of muscle, liver, and milk was studied in vitro by measuring the temperature-varying reflectance spectrum as heating occurs. A high-resolution fiber optic spectrometer was used to measure the reflectance changes. Temperature was monitored using a thermocouple embedded within the tissue along the side of the optical fiber probe.

RESULTS

The values of average free energy to initiate denaturation in muscle and liver at about 60 degrees C were 94.8 and 96.3 kJ/mole, respectively. The reflectance spectra increased in amplitude for muscle and liver, and the peak shifted from approximately 700 to 720 nm in accordance with the damage fraction of tissue. The reflectance spectrum for milk was essentially unchanged.

CONCLUSIONS

Spectral changes from heated muscle and liver reflect denaturation of proteins contained therein. The spectral information at 800 nm can be used to determine the average free energy for the initiation of denaturation.

Fluorescence spectroscopy accurately detects irreversible cell damage during hepatic radiofrequency ablation

BACKGROUND

A current limitation of hepatic radiofrequency ablation (RFA) is an inability to detect ablation margins in real time. Thermal injury from RFA alters the biochemical properties governing tissue fluorescence. We hypothesized that the changes in hepatic fluorescence measured during hepatic RFA could be used to detect irreversible hepatocyte damage accurately and to determine ablation margins in real time.

METHODS

RFA was performed on healthy pig livers and monitored in vivo simultaneously for fluorescence and temperature by a fiberoptic micro-interrogation probe connected to a spectroscopy system. Ablations were stopped based on previously established real-time fluorescence spectral data, not based on temperature or time. To determine where in the ablated tissue cell death occurred, biopsies for transmission electron microscopy were taken from 4 areas of 3 specimens: (1) nonablated liver, (2) hemorrhagic zone/normal liver interface, (3) hemorrhagic zone/coagulated zone interface, and (4) coagulated zone. In vitro fluorescence emission intensity was determined at each biopsy site.

RESULTS

Peak hepatic fluorescence intensity occurred at 470 nm and decreased as RFA progressed. Transmission electron microscopy evidence of irreversible hepatocyte damage occurred at the interface of the coagulation zone and the hemorrhagic zone and correlated with a 87.5% +/- 9% decrease in fluorescence emission intensity. Tissue fluorescent changes from thermal injury were unaffected by tissue cooling.

CONCLUSION

Fluorescence spectroscopy accurately detected hepatocellular thermal injury from RFA in real time and can detect irreversible cell damage during tissue thermal therapy.