Experimental assessment of CT-based thermometry during laser ablation of porcine pancreas

Reproducible spectral CT thermometry with liver-mimicking phantoms for image-guided thermal ablation

Objectives. Evaluate the reproducibility, temperature tolerance, and radiation dose requirements of spectral CT thermometry in tissue-mimicking phantoms to establish its utility for non-invasive temperature monitoring of thermal ablations.Methods. Three liver mimicking phantoms embedded with temperature sensors were individually scanned with a dual-layer spectral CT at different radiation dose levels during heating (35 °C-80 °C). Physical density maps were reconstructed from spectral results using varying reconstruction parameters. Thermal volumetric expansion was then measured at each temperature sensor every 5 °C in order to establish a correlation between physical density and temperature. Linear regressions were applied based on thermal volumetric expansion for each phantom, and coefficient of variation for fit parameters was calculated to characterize reproducibility of spectral CT thermometry. Additionally, temperature tolerance was determined to evaluate effects of acquisition and reconstruction parameters. The resulting minimum radiation dose to meet the clinical temperature accuracy requirement was determined for each slice thickness with and without additional denoising.Results. Thermal volumetric expansion was robustly replicated in all three phantoms, with a correlation coefficient variation of only 0.43%. Similarly, the coefficient of variation for the slope and intercept were 9.6% and 0.08%, respectively, indicating reproducibility of the spectral CT thermometry. Temperature tolerance ranged from 2 °C to 23 °C, decreasing with increased radiation dose, slice thickness, and iterative reconstruction level. To meet the clinical requirement for temperature tolerance, the minimum required radiation dose ranged from 20, 30, and 57 mGy for slice thickness of 2, 3, and 5 mm, respectively, but was reduced to 2 mGy with additional denoising.Conclusions. Spectral CT thermometry demonstrated reproducibility across three liver-mimicking phantoms and illustrated the clinical requirement for temperature tolerance can be met for different slice thicknesses. The reproducibility and temperature accuracy of spectral CT thermometry enable its clinical application for non-invasive temperature monitoring of thermal ablation.

A review on radiofrequency, laser, and microwave ablations and their thermal monitoring through fiber Bragg gratings

Thermal ablation of tumors aims to apply extreme temperatures inside the target tissue to achieve substantial tumor destruction in a minimally invasive manner. Several techniques are comprised, classified according to the type of energy source. However, the lack of treatment selectivity still needs to be addressed, potentially causing two risks: i) incomplete tumor destruction and recurrence, or conversely, ii) damage of the surrounding healthy tissue. Therefore, the research herein reviewed seeks to develop sensing systems based on fiber Bragg gratings (FBGs) for thermal monitoring inside the lesion during radiofrequency, laser, and microwave ablation. This review shows that, mainly thanks to multiplexing and minimal invasiveness, FBGs provide an optimal sensing solution. Their temperature measurements are the feedback to control the ablation process and allow to investigate different treatments, compare their outcomes, and quantify the impact of factors such as proximity to thermal probe and blood vessels, perfusion, and tissue type.

Reproducible spectral CT thermometry with liver-mimicking phantoms for image-guided thermal ablation

Objectives

Evaluate the reproducibility, temperature sensitivity, and radiation dose requirements of spectral CT thermometry in tissue-mimicking phantoms to establish its utility for non-invasive temperature monitoring of thermal ablations.

Materials and Methods

Three liver mimicking phantoms embedded with temperature sensors were individually scanned with a dual-layer spectral CT at different radiation dose levels during heating and cooling (35 to 80 °C). Physical density maps were reconstructed from spectral results using a range of reconstruction parameters. Thermal volumetric expansion was then measured at each temperature sensor every 5°C in order to establish a correlation between physical density and temperature. Linear regressions were applied based on thermal volumetric expansion for each phantom, and coefficient of variation for fit parameters was calculated to characterize reproducibility of spectral CT thermometry. Additionally, temperature sensitivity was determined to evaluate the effect of acquisition parameters, reconstruction parameters, and image denoising. The resulting minimum radiation dose to meet the clinical temperature sensitivity requirement was determined for each slice thickness, both with and without additional denoising.

Results

Thermal volumetric expansion was robustly replicated in all three phantoms, with a correlation coefficient variation of only 0.43%. Similarly, the coefficient of variation for the slope and intercept were 9.6% and 0.08%, respectively, indicating reproducibility of the spectral CT thermometry. Temperature sensitivity ranged from 2 to 23 °C, decreasing with increased radiation dose, slice thickness, and iterative reconstruction level. To meet the clinical requirement for temperature sensitivity, the minimum required radiation dose ranged from 20, 30, and 57 mGy for slice thickness of 2, 3, and 5 mm, respectively, but was reduced to 2 mGy with additional denoising.

Conclusions

Spectral CT thermometry demonstrated reproducibility across three liver-mimicking phantoms and illustrated the clinical requirement for temperature sensitivity can be met for different slice thicknesses. Moreover, additional denoising enables the use of more clinically relevant radiation doses, facilitating the clinical translation of spectral CT thermometry. The reproducibility and temperature accuracy of spectral CT thermometry enable its clinical application for non-invasive temperature monitoring of thermal ablation.

Techniques for Temperature Monitoring of Myocardial Tissue Undergoing Radiofrequency Ablation Treatments: An Overview

Cardiac radiofrequency ablation (RFA) has received substantial attention for the treatment of multiple arrhythmias. In this scenario, there is an ever-growing demand for monitoring the temperature trend inside the tissue as it may allow an accurate control of the treatment effects, with a consequent improvement of the clinical outcomes. There are many methods for monitoring temperature in tissues undergoing RFA, which can be divided into invasive and non-invasive. This paper aims to provide an overview of the currently available techniques for temperature detection in this clinical scenario. Firstly, we describe the heat generation during RFA, then we report the principle of work of the most popular thermometric techniques and their features. Finally, we introduce their main applications in the field of cardiac RFA to explore the applicability in clinical settings of each method.

Feature-based automated segmentation of ablation zones by fuzzy c-mean clustering during low-dose computed tomography

PURPOSE

Intra-procedural monitoring and post-procedural follow-up is necessary for a successful ablation treatment. An imaging technique which can assess the ablation geometry accurately is beneficial to monitor and evaluate treatment. In this study, we developed an automated ablation segmentation technique for serial low-dose, noisy ablation computed tomography (CT) or contrast-enhanced CT (CECT).

METHODS

Low-dose, noisy temporal CT and CECT volumes were acquired during microwave ablation on normal porcine liver (four with non-contrast CT and eight with CECT). Highly constrained backprojection (HYPR) processing was used to recover ablation zone information compromised by low-dose noise. First-order statistic features and normalized fractional Brownian features (NBF) were used to segment ablation zones by fuzzy c-mean clustering. After clustering, the segmented ablation zone was refined by cyclic morphological processing. Automatic and manual segmentations were compared to gross pathology with Dice's coefficient (morphological similarity), while cross-sectional dimensions were compared by percent difference.

RESULTS

Automatic and manual segmentations of the ablation zone were very similar to gross pathology (Dice Coefficients: Auto.-Path. = 0.84 ± 0.02; Manu.-Path. = 0.76 ± 0.03, P = 0.11). The differences in ablation area, major diameter and minor diameter were 17.9 ± 3.2%, 11.1 ± 3.2% and 16.2 ± 3.4%, respectively, when comparing automatic segmentation to gross pathology, which were lower than the differences of 32.9 ± 16.8%, 13.0 ± 9.8% and 21.8 ± 5.8% when comparing manual segmentation to gross pathology. Manual segmentations tended to overestimate gross pathology when ablation area was less than 15 cm2 , but the automated segmentation tended to underestimate gross pathology when ablation zone is larger than 20 cm2 .

CONCLUSION

Fuzzy c-means clustering may be used to aid automatic segmentation of ablation zones without prior information or user input, making serial CT/CECT has more potential to assess treatments intra-procedurally.

Evaluation of the thermal sensitivity of porcine liver in CT-guided cryoablation: an initial study

PURPOSE

To evaluate computed tomography (CT)-based thermometry in cryoablation, the thermal sensitivity of an ex-vivo porcine liver was determined in an initial study design.

METHODS

The CT-guided cryoablation was performed in three porcine liver samples over a period of 10 min. Fiber optic temperature probes were positioned parallel to the shaft of the cryoprobe in an axial slice orientation. During ablation, temperature measurements were performed simultaneously with CT imaging at 5 s intervals. On the CT images, the average CT number was calculated for a region of interest of 3 × 3 pixels just below the tip of each temperature probe. A linear regression analysis was performed using eleven data sets to determine the dependence of the CT number on the temperature.

RESULTS

With decreasing temperature, an increasing hypodense area around the tip of the cryoprobe was observed on the CT images and decreasing values of the CT number were determined. Starting at a temperature of - 40°C a linear relation between the CT number and the temperature was determined and a thermal sensitivity of 0.95 HU/°C (R²  = 0.73) was obtained. The thermal sensitivity was used to calculate color-coded temperature maps. The calculated temperature distribution corresponds quantitatively to the increasing hypodense area.

CONCLUSIONS

A noninvasive CT-based temperature determination during cryoablation in a normal ex vivo porcine liver is feasible. A thermal sensitivity of 0.95 HU/°C was determined by linear regression analysis. A color-coded map of the temperature distribution was presented.

In vivo noninvasive three-dimensional (3D) assessment of microwave thermal ablation zone using non-contrast-enhanced x-ray CT

PURPOSE

To develop an image processing methodology for noninvasive three-dimensional (3D) quantification of microwave thermal ablation zones in vivo using x-ray computed tomography (CT) imaging without injection of a contrast enhancing material.

METHODS

Six microwave (MW) thermal ablation procedures were performed in three pigs. The ablations were performed with a constant heating duration of 8 min and power level of 30 W. During the procedure images from sixty 1 mm thick slices were acquired every 30 s. At the end of all ablation procedures for each pig, a contrast-enhanced scan was acquired for reference. Special algorithms for addressing challenges stemming from the 3D in vivo setup and processing the acquired images were prepared. The algorithms first rearranged the data to account for the oblique needle orientation and for breathing motion. Then, the gray level variance changes were analyzed, and optical flow analysis was applied to the treated volume in order to obtain the ablation contours and reconstruct the ablation zone in 3D. The analysis also included a special correction algorithm for eliminating artifacts caused by proximal major blood vessels and blood flow. Finally, 3D reference reconstructions from the contrast-enhanced scan were obtained for quantitative comparison.

RESULTS

For four ablations located >3 mm from a large blood vessel, the mean dice similarity coefficient (DSC) and the mean absolute radial discrepancy between the contours obtained from the reference contrast-enhanced images and the contours produced by the algorithm were 0.82 ± 0.03 and 1.92 ± 1.47 mm, respectively. In two cases of ablation adjacent to large blood vessels, the average DSC and discrepancy were: 0.67 ± 0.6 and 2.96 ± 2.15 mm, respectively. The addition of the special correction algorithm utilizing blood vessels mapping improved the mean DSC and the mean absolute discrepancy to 0.85 ± 0.02 and 1.19 ± 1.00 mm, respectively.

CONCLUSIONS

The developed algorithms provide highly accurate detailed contours in vivo (average error < 2.5 mm) and cope well with the challenges listed above. Clinical implementation of the developed methodology could potentially provide real time noninvasive 3D accurate monitoring of MW thermal ablation in-vivo, provided that the radiation dose can be reduced.

Imaging-based internal body temperature measurements: The journal Temperature toolbox

Noninvasive imaging methods of internal body temperature are in high demand in both clinical medicine and physiological research. Thermography and thermometry can be used to assess tissue temperature during thermal therapies: ablative and hyperthermia treatments to ensure adequate temperature rise in target tissues but also to avoid collateral damage by heating healthy tissues. In research use, measurement of internal body temperature enables us the production of thermal maps on muscles, internal organs, and other tissues of interest. The most used methods for noninvasive imaging of internal body temperature are based on different parameters acquired with magnetic resonance imaging, ultrasound, computed tomography, microwave radiometry, photoacoustic imaging, and near-infrared spectroscopy. In the current review, we examine the aforementioned imaging methods, their use in estimating internal body temperature in vivo with their advantages and disadvantages, and the physical phenomena the thermography or thermometry modalities are based on.

Association of Liver Tissue Optical Properties and Thermal Damage

BACKGROUND AND OBJECTIVES

Complete thermocoagulation of tumors is vital to minimize the risk of local tumor recurrence after a thermal ablation. Histological assessments are not real-time and require experienced pathologists to grade the thermal damage (histopathology) [Correction added on 21 January, 2020 after first online publication: After thermal damage in the preceding sentence, (histopathology) was added]. Real-time assessment of thermal tissue damage during an ablation is necessary to achieve optimal tumor ablation. In our previous studies, we found that continuous monitoring of the wavelength-averaged (435-630 nm) tissue absorption coefficient (µ_a ) and the reduced scattering coefficient ( μ s ' ) during heating of a porcine liver at 100°C follows a sigmoidal growth curve. Therefore, we concluded that increases in the tissue µ_a and μ s ' during thermocoagulation were correlated with true thermal damage. The goal of this study was to determine if increases in the tissue µ_a and μ s ' during thermocoagulation are correlated with true thermal damage.

STUDY DESIGN/MATERIALS AND METHODS

In this paper, continuously measured values of µ_a and μ s ' during heating of the porcine liver tissue were compared with the histology-assessed thermal damage scores at four different temperature points (37°C, 55°C, 65°C, and 75°C).

RESULTS

The damage scores for the tissues in Group 3 (65°C) and Group 4 (75°C) were significantly different from each other and from the other groups. The damage scores were not significantly different between Group 1 (37°C) and Group 2 (55°C).

CONCLUSION

The results indicate that relative changes in µ_a and μ s ' can be used to classify thermal damage (histopathology) scores with an overall accuracy of 72.5% up to 75°C. [Correction added on 21 January, 2020 after first online publication: After thermal damage in the preceding sentence, (histopathology) was added]. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Preliminary analysis of ultrasound elastography imaging-based thermometry on non-perfused ex vivo swine liver

AIMS

Real-time monitoring of tissue temperature during percutaneous tumor ablation improves treatment efficacy, leading clinicians in adjustment of treatment settings. This study aims at assessing feasibility of ultrasound thermometry during laser ablation of biological tissue using a specific ultrasound imaging techniques based on elastography acoustic radiation force impulse (ARFI).

METHODS

ARFI uses high-intensity focused ultrasound pulses to generate 'radiation force' in tissue; this provokes tissue displacements trackable using correlation-based ultrasound methods: the sensitivity of shear waves velocity is able to detect temperature changes. Experiments were carried out using a Nd:YAG laser (power: 5 W) in three non-perfused ex vivo pig livers. In each organ, a thermocouple was placed close to the applicator tip (distance range 1.5-2.5 cm) used to record a reference temperature. Positioning of laser applicator and thermocouple was eco-guided. The organ was scanned by an echography system equipped with ARFI; propagation velocity was measured in a region of interest of 1 × 0.5 cm located close to thermocouple, to investigate influence of tissue temperature on shear waves velocity.

RESULTS

Shear wave velocity has a very low sensitivity to temperature up to 55-60 °C, and in all cases, velocity is < 5 m s^-1; for temperature > 55-60 °C, velocity shows a steep increment. The system measures a value "over limit", meaning a velocity > 5 m s^-1.

CONCLUSIONS

Ultrasound thermometry during laser ablation of biological tissue based on elastography shows an abrupt output change at temperatures > 55-60 °C. This issue can have a relevant clinical impact, considering tumor necrosis when temperature crosses 55 °C to define the boundary of damaged volume.

Hyperspectral imaging for thermal effect monitoring in in vivo liver during laser ablation

Thermal ablation is a minimally invasive technique used to induce a controlled necrosis of malignant cells by increasing the temperature in localized areas. This procedure needs an accurate and real-time monitoring of thermal effects to evaluate and control treatment outcome. In this work, a hyperspectral imaging (HSI) technique is proposed as a new and non-invasive method to monitor ablative therapy. HSI provides images of the target object in several spectral bands, hence the reflectance/absorbance spectrum for each pixel. This paper presents a preliminary and original HSI-based analysis of the thermal state in the in vivo porcine liver undergoing laser ablation. In order to compare the spectral response between treated and untreated areas of the organ, proper Regions of Interest (ROIs) were chosen on the hyperspectral images; for each ROI, the absorbance variation for the selected wavelengths (i.e., 630, 760, and 960nm, for deoxyhemoglobin, methemoglobin, and water respectively) was assessed. Results obtained during and after laser ablation show that the absorbance of the methemoglobin peaks increases up to 40% in the burned region with respect to the non-ablated one. Conversely, the relative change of deoxyhemoglobin and water peaks is less marked. Based on these results, absorbance threshold values were retrieved and used to visualize the ablation zone on the images. This preliminary analysis suggests that a combination of the absorbance information is essential to achieve a more accurate identification of the ablation region. The results encourage further studies on the correlation between thermal effects and the spectral response of biological tissues undergoing thermal ablation, for final clinical use.

Thermal ablation of pancreatic cancer: A systematic literature review of clinical practice and pre-clinical studies

PURPOSE

Pancreatic cancer is a challenging malignancy with low treatment option and poor life expectancy. Thermal ablation techniques were proposed as alternative treatment options, especially in advanced stages and for unfit-for-surgery patients. This systematic review describes the thermal ablative techniques -i.e., Laser (LA), Radiofrequency (RFA), Microwave (MWA) Ablation, High-Intensity Focused Ultrasound (HIFU) and cryoablation- available for pancreatic cancer treatment. Additionally, an analysis of the efficacy, complication rate and overall survival for each technique is conducted.

MATERIAL AND METHODS

This review collects the ex vivo, preclinical and clinical studies presenting the use of thermal techniques in the pancreatic cancer treatment, searched up to March 2018 in PubMed and Medline. Abstracts, letters-to-the-editor, expert opinions, reviews and non-English language manuscripts were excluded.

RESULTS

Sixty-five papers were included. For the ex vivo and preclinical studies, there are: 12 records for LA, 8 for RFA, 0 for MWA, 6 for HIFU, 1 for cryoablation and 3 for hybrid techniques. For clinical studies, 1 paper for LA, 14 for RFA, 1 for MWA, 17 for HIFU, 1 for cryoablation and 1 for hybrid techniques.

CONCLUSIONS

Important technological advances are presented in ex vivo and preclinical studies, as the real-time thermometry, nanotechnology and hybrid techniques to enhance the thermal outcome. Conversely, a lack of standardization in the clinical employment of the procedures emerged, leading to contrasting results on the safety and feasibility of some analyzed techniques. Uniform conclusions on the safety and feasibility of these techniques for pancreatic cancer will require further structured investigation.

Optical flow and image segmentation analysis for noninvasive precise mapping of microwave thermal ablation in X-ray CT scans - ex vivo study

PURPOSE

To develop image processing algorithms for noninvasive mapping of microwave thermal ablation using X-ray CT.

METHODS

Ten specimens of bovine liver were subjected to microwave ablation (20-80 W, 8 min) while scanned by X-ray CT at 5 s intervals. Specimens were cut and manually traced by two observers. Two algorithms were developed and implemented to map the ablation zone. The first algorithm utilises images segmentation of Hounsfield units changes (ISHU). The second algorithm utilises radial optical flow (ROF). Algorithm sensitivity to spatiotemporal under-sampling was assessed by decreasing the acquisition rate and reducing the number of acquired projections used for image reconstruction in order to evaluate the feasibility of implementing radiation reduction techniques.

RESULTS

The average radial discrepancy between the ISHU and ROF contours and the manual tracing were 1.04±0.74 and 1.16±0.79mm, respectively. When diluting the input data, the ISHU algorithm retained its accuracy, ranging from 1.04 to 1.79mm. By contrast, the ROF algorithm performance became inconsistent at low acquisition rates. Both algorithms were not sensitive to projections reduction, (ISHU: 1.24±0.83mm, ROF: 1.53±1.15mm, for reduction by eight fold). Ablations near large blood vessels affected the ROF algorithm performance (1.83±1.30mm; p < 0.01), whereas ISHU performance remained the same.

CONCLUSION

The two suggested noninvasive ablation mapping algorithms can provide highly accurate contouring of the ablation zone at low scan rates. The ISHU algorithm may be more suitable for clinical practice as it appears more robust when radiation dose reduction strategies are employed and when the ablation zone is near large blood vessels.

Linearly chirped fiber Bragg grating response to thermal gradient: from bench tests to the real-time assessment during in vivo laser ablations of biological tissue

The response of a fiber optic sensor [linearly chirped fiber Bragg grating (LCFBG)] to a linear thermal gradient applied on its sensing length (i.e., 1.5 cm) has been investigated. After these bench tests, we assessed their feasibility for temperature monitoring during thermal tumor treatment. In particular, we performed experiments during ex vivo laser ablation (LA) in pig liver and in vivo thermal ablation in animal models (pigs). We investigated the following: (i) the relationship between the full width at half maximum of the LCFBG spectrum and the temperature difference among the extremities of the LCFBG and (ii) the relationship between the mean spectrum wavelength and the mean temperature acting on the LCFBG sensing area. These relationships showed a linear trend during both bench tests and LA in animal models. Thermal sensitivity was significant although different values were found with regards to bench tests and animal experiments. The linear trend and significant sensitivity allow hypothesizing a future use of this kind of sensor to monitor both temperature gradient and mean temperature within a tissue undergoing thermal treatment.

Laser Ablation for Cancer: Past, Present and Future

Laser ablation (LA) is gaining acceptance for the treatment of tumors as an alternative to surgical resection. This paper reviews the use of lasers for ablative and surgical applications. Also reviewed are solutions aimed at improving LA outcomes: hyperthermal treatment planning tools and thermometric techniques during LA, used to guide the surgeon in the choice and adjustment of the optimal laser settings, and the potential use of nanoparticles to allow biologic selectivity of ablative treatments. Promising technical solutions and a better knowledge of laser-tissue interaction should allow LA to be used in a safe and effective manner as a cancer treatment.

US-guided percutaneous laser ablation of refractory metastatic retroperitoneal lesions: A care-compliant case report

RATIONALE

Retroperitoneal metastatic lymph node is rare but severe, which has important structures like the gastrointestinal tract and large blood vessels around and may challenge excision, inducing serious complications like hemorrhage, intestinal adhesion, and even death after injury.

PATIENT CONCERNS

We described the case of a 60-year-old man with a history of right liver resection in 2010, pulmonary wedge resection in 2012, and transarterial chemoembolization twice in 2014, in which the postoperative pathology suggested the mixed liver cancer, and poorly differentiated lung cancer from liver metastasis.

DIAGNOSES

Preoperative magnetic resonance (MR) imaging scan showed a refractory retroperitoneal metastatic lymph node.

INTERVENTIONS

Then this patient repeatedly received 4 ablations with US-guided laser ablation within a month.

OUTCOMES

After 4 ablations due to residual tumor, MR, and CT images of 5-month follow-up showed the partial response. No obvious side effects were discovered in this case during these procedures.

LESSONS

This suggested US-guided laser ablation appears to be a useful technique for retroperitoneal metastatic lymph node with poor general condition or those refusing surgical therapy.

Noninvasive microwave ablation zone radii estimation using x-ray CT image analysis

PURPOSE

The aims of this study were to noninvasively and automatically estimate both the radius of the ablated liver tissue and the radius encircling the treated zone, which also defines where the tissue is definitely untreated during a microwave (MW) thermal ablation procedure.

METHODS

Fourteen ex vivo bovine fresh liver specimens were ablated at 40 W using a 14 G microwave antenna, for durations of 3, 6, 8, and 10 min. The tissues were scanned every 5 s during the ablation using an x-ray CT scanner. In order to estimate the radius of the ablation zone, the acquired images were transformed into a polar presentation by displaying the Hounsfield units (HU) as a function of angle and radius. From this polar presentation, the average HU radial profile was analyzed at each time point and the ablation zone radius was estimated. In addition, textural analysis was applied to the original CT images. The proposed algorithm identified high entropy regions and estimated the treated zone radius per time. The estimated ablated zone radii as a function of treatment durations were compared, by means of correlation coefficient and root mean square error (RMSE) to gross pathology measurements taken immediately post-treatment from similarly ablated tissue.

RESULTS

Both the estimated ablation radii and the treated zone radii demonstrated strong correlation with the measured gross pathology values (R(2) ≥ 0.89 and R(2) ≥ 0.86, respectively). The automated ablation radii estimation had an average discrepancy of less than 1 mm (RMSE = 0.65 mm) from the gross pathology measured values, while the treated zone radii showed a slight overestimation of approximately 1.5 mm (RMSE = 1.6 mm).

CONCLUSIONS

Noninvasive monitoring of MW ablation using x-ray CT and image analysis is feasible. Automatic estimations of the ablation zone radius and the radius encompassing the treated zone that highly correlate with actual ablation measured values can be obtained. This technique can therefore potentially be used to obtain real time monitoring and improve the clinical outcome.

Multipoint temperature monitoring in liver undergoing computed tomography-guided radiofrequency ablation with fiber Bragg grating probes

In this work, we investigated the temperature increment experienced by biological tissue during radiofrequency ablation (RFA). The measurements were performed by using two custom-made thermal probes based on fiber optic sensors (fiber Bragg gratings, FBGs). The two probes embed a total of 9 FBGs. Experiments were performed during RFA of an ex vivo healthy porcine liver. The RFA heating module was equipped with 5 thermocouples. Results show that the temperature increment close to the applicator (i.e., 0.6 cm-0.7 cm) reaches the temperature which is set as a target on the RFA module (i.e., approximately 100 °C). The distance from the applicator also has an impact on the dynamics of the heating phenomenon: at short distances the tissue temperature reaches a steady state condition after a few minutes, on the other hand the sensors placed at a distance ≥2cm did not reach the steady-state conditions during the 14-minute procedure. The multipoint temperature monitoring, which uses sensors at several distances from the applicator, can provide useful information regarding the boundary of damaged volume. This approach can be combined with the monitoring temperature system embedded in the heating equipment, to better control the damaged volume, and to improve the treatment outcomes.

Temperature monitoring during radiofrequency ablation of liver: in vivo trials

Radiofrequency ablation (RFA) is a minimally invasive procedure used to treat tumors by means of hyperthermia, mostly through percutaneous approach. The tissue temperature plays a pivotal role in the achievement of the target volume heating, while sparing the surrounding healthy tissue from thermal damage. Several techniques for thermometry during RFA are investigated, most of them based on the use of single-point measurement system (e.g., thermocouples). The measurement of temperature map is crucial for the real-time control and fine adjustment of the treatment settings, to optimize the shape and size of the ablated volume. The recent interest about fiber optic sensors and, among them, fiber Bragg gratings (FBGs) for the monitoring of thermal effects motivated further investigation. In particular, the feature of FBGs to form an array of several elements, thus to be inscribed within the same fiber, allows the use of a single probe for the multi-points monitoring of the tissue temperature during RFA. Hence, the aim of this study is the development and characterization of a needle-like probe embedding an array of three FBGs, which was tested on pig liver during in vivo trials. The needle allows a safe and easy insertion of the fiber optic within the liver. It was inserted by ultrasound guidance into the liver, and monitored the change of tissue temperature during RFA controlled by the roll-off technique. Also the measurement error induced by breathing movements of the liver was assessed (less than 3 °C). Results encourage the use of the probe in clinical settings, as well as the improvement of some features, e.g., a higher number of FBGs for performing quasi-distributed measurement.

Analysis of iodinated contrast delivered during thermal ablation: is material trapped in the ablation zone?

Intra-procedural contrast-enhanced CT (CECT) has been proposed to evaluate treatment efficacy of thermal ablation. We hypothesized that contrast material delivered concurrently with thermal ablation may become trapped in the ablation zone, and set out to determine whether such an effect would impact ablation visualization. CECT images were acquired during microwave ablation in normal porcine liver with: (A) normal blood perfusion and no iodinated contrast, (B) normal perfusion and iodinated contrast infusion or (C) no blood perfusion and residual iodinated contrast. Changes in CT attenuation were analyzed from before, during and after ablation to evaluate whether contrast was trapped inside of the ablation zone. Visualization was compared between groups using post-ablation contrast-to-noise ratio (CNR). Attenuation gradients were calculated at the ablation boundary and background to quantitate ablation conspicuity. In Group A, attenuation decreased during ablation due to thermal expansion of tissue water and water vaporization. The ablation zone was difficult to visualize (CNR  =  1.57  ±  0.73, boundary gradient  =  0.7  ±  0.4 HU mm(-1)), leading to ablation diameter underestimation compared to gross pathology. Group B ablations saw attenuation increase, suggesting that iodine was trapped inside the ablation zone. However, because the normally perfused liver increased even more, Group B ablations were more visible than Group A (CNR  =  2.04  ±  0.84, boundary gradient  =  6.3  ±  1.1 HU mm(-1)) and allowed accurate estimation of the ablation zone dimensions compared to gross pathology. Substantial water vaporization led to substantial attenuation changes in Group C, though the ablation zone boundary was not highly visible (boundary gradient  =  3.9  ±  1.1 HU mm(-1)). Our results demonstrate that despite iodinated contrast being trapped in the ablation zone, ablation visibility was highest when contrast is delivered intra-procedurally. Therefore, CECT may be feasible for real-time thermal ablation monitoring.

Fiber Optic Sensors for Temperature Monitoring during Thermal Treatments: An Overview

During recent decades, minimally invasive thermal treatments (i.e., Radiofrequency ablation, Laser ablation, Microwave ablation, High Intensity Focused Ultrasound ablation, and Cryo-ablation) have gained widespread recognition in the field of tumor removal. These techniques induce a localized temperature increase or decrease to remove the tumor while the surrounding healthy tissue remains intact. An accurate measurement of tissue temperature may be particularly beneficial to improve treatment outcomes, because it can be used as a clear end-point to achieve complete tumor ablation and minimize recurrence. Among the several thermometric techniques used in this field, fiber optic sensors (FOSs) have several attractive features: high flexibility and small size of both sensor and cabling, allowing insertion of FOSs within deep-seated tissue; metrological characteristics, such as accuracy (better than 1 °C), sensitivity (e.g., 10 pm·°C⁻¹ for Fiber Bragg Gratings), and frequency response (hundreds of kHz), are adequate for this application; immunity to electromagnetic interference allows the use of FOSs during Magnetic Resonance- or Computed Tomography-guided thermal procedures. In this review the current status of the most used FOSs for temperature monitoring during thermal procedure (e.g., fiber Bragg Grating sensors; fluoroptic sensors) is presented, with emphasis placed on their working principles and metrological characteristics. The essential physics of the common ablation techniques are included to explain the advantages of using FOSs during these procedures.

An MR-compatible force sensor based on FBG technology for biomedical application

Fiber Bragg Grating (FBG) technology is very attractive to develop sensors for the measurement of thermal and mechanical parameters in biological applications, particularly in presence of electromagnetic interferences. This work presents the design, working principle and experimental characterization of a force sensor based on two FBGs, with the feature of being compatible with Magnetic Resonance. Two prototypes based on different designs are considered and characterized: 1) the fiber with the FBGs is encapsulated in a polydimethylsiloxane (PDMS) sheet; 2) the fiber with the FBGs is free without the employment of any polymeric layer. Results show that the prototype which adopts the polymeric sheet has a wider range of measurement (4200 mN vs 250 mN) and good linearity; although it has lower sensitivity (≈0.1 nm-N(1) vs 7 nm-N(1)). The sensor without polymeric layer is also characterized by employing a differential configuration which allows neglecting the influence of temperature. This solution improves the linearity of the sensor, on the other hand the sensitivity decreases. The resulting good metrological properties of the prototypes here tested make them attractive for the intended application and in general for force measurement during biomedical applications in presence of electromagnetic interferences.

Temperature monitoring during microwave ablation in ex vivo porcine livers

OBJECTIVE

The aim of the present study was to assess the temperature map and its reproducibility while applying two different MWA systems (915 MHz vs 2.45 GHz) in ex vivo porcine livers.

MATERIALS AND METHODS

Fifteen fresh pig livers were treated using the two antennae at three different settings: treatment time of 10 min and power of 45 W for both systems; 4 min and 100 W for the 2.45 GHz system. Trends of temperature were recorded during all procedures by means of fiber optic-based probes located at five fixed distances from the antenna, ranging between 10 mm and 30 mm. Each trial was repeated twice to assess the reproducibility of temperature distribution.

RESULTS

Temperature as function of distance from the antenna can be modeled by a decreasing exponential trend. At the same settings, temperature obtained with the 2.45 GHz system was higher than that obtained with the 915 MHz thus resulting into a wider area of ablation (diameter 17 mm vs 15 mm). Both systems showed good reproducibility in terms of temperature distribution (root mean squared difference for both systems ranged between 2.8 °C and 3.4 °C).

CONCLUSIONS

When both MWA systems are applied, a decreasing exponential model can predict the temperature map. The 2.45 GHz antenna causes higher temperatures as compared to the 915 MHz thus, resulting into larger areas of ablation. Both systems showed good reproducibility although better results were achieved with the 2.45 GHz antenna.

A Needlelike Probe for Temperature Monitoring During Laser Ablation Based on Fiber Bragg Grating: Manufacturing and Characterization

Temperature distribution monitoring in tissue undergoing laser ablation (LA) could be beneficial for improving treatment outcomes. Among several thermometric techniques employed in LA, fiber Bragg grating (FBG) sensors show valuable characteristics, although their sensitivity to strain entails measurement error for patient respiratory movements. Our work describes a solution to overcome this issue by housing an FBG in a surgical needle. The metrological properties of the probes were assessed in terms of thermal sensitivity (0.027 nm °C−1 versus 0.010 nm °C−1 for epoxy liquid encapsulated probe and thermal paste one, respectively) and response time (about 100 ms) and compared with properties of nonencapsulated FBG (sensitivity of 0.010 nm °C−1, response time of 43 ms). The error due to the strain caused by liver movements, simulating a typical respiratory pattern, was assessed: the strain induces a probes output error less than 0.5 °C, which is negligible when compared to the response of nonencapsulated FBG (2.5 °C). The metallic needle entails a measurement error, called artifact, due to direct absorption of the laser radiation. The analysis of the artifact was performed by employing the probes for temperature monitoring on liver undergoing LA. Experiments were performed at two laser powers (i.e., 2 W and 4 W) and at nine distances between the probes and the laser applicator. The artifact decreases with the distance and increases with the power: it exceeds 10 °C at 4 W, when the encapsulated probes are placed at 3.6 mm and 0 deg from the applicator, and it is lower than 1 °C for distance higher than 5 mm and angle higher than 30 deg.

Emerging clinical applications of computed tomography

X-ray computed tomography (CT) has recently been experiencing remarkable growth as a result of technological advances and new clinical applications. This paper reviews the essential physics of X-ray CT and its major components. Also reviewed are recent promising applications of CT, ie, CT-guided procedures, CT-based thermometry, photon-counting technology, hybrid PET-CT, use of ultrafast-high pitch scanners, and potential use of dual-energy CT for material differentiations. These promising solutions and a better knowledge of their potentialities should allow CT to be used in a safe and effective manner in several clinical applications.

Non-invasive temperature monitoring and hyperthermic injury onset detection using X-ray CT during HIFU thermal treatment in ex vivo fatty tissue

PURPOSE

This paper examines X-ray CT, to serve as an image-guiding thermal monitoring modality for high intensity focused ultrasound (HIFU) treatment of fatty tissues.

MATERIALS AND METHODS

Six ex vivo porcine fat tissue specimens were scanned by X-ray CT simultaneously with the application of HIFU. Images were acquired during both heating and post-ablation stages. The temperature at the focal zone was measured simultaneously using a thermocouple. The mean values of the Hounsfield units (HU) at the focal zone were registered and plotted as a function of temperature.

RESULTS

In all specimens studied, the HU versus temperature curves measured during the heating stage depicted a characteristic non-linear parabolic trajectory (R(2) > 0.87). The HU-temperature trajectory initially decreased to a minimum value at about 44.5 °C and then increased substantially as the heating progressed. The occurrence of this nadir point during the heating stage was clearly detectable. During post-ablation cooling, on the other hand, the HU increased monotonically with the decreasing temperature and depicted a clearly linear trajectory (R(2) ≥ 0.9).

CONCLUSIONS

Our results demonstrate that the HU-temperature curve during HIFU treatment has a characteristic parabolic trajectory for fat tissue that might potentially be utilised for thermal monitoring during HIFU ablation treatments. The clear detection of 44.5 °C, presumably marking the onset of hyperthermic injury, can be detected non-invasively as an occurrence of a minimum on the HU-time curve without any need to relate the HU directly to temperature. Such features may be helpful in monitoring and optimising HIFU thermal treatment for clinically applicable indications such as in the breast by providing a non-invasive monitoring of tissue damage.

Estimation of anisotropy coefficient and total attenuation of swine liver at 850 nm based on a goniometric technique: influence of sample thickness

Estimation of optical properties of biologic tissue is crucial for theoretical modeling of laser treatments in medicine. Tissue highly absorbs and scatters the light between 650 nm and 1300 nm, where the laser provides therapeutic effects. Among other properties, the characteristic of biological tissues to scatter the light traveling trough, is described by the anisotropy coefficient (g). The relationship between g and the distribution of the scattered light at different angles is described by Henyey-Greenstein phase function. The measurement of angular distribution of scattered light is performed by the goniometric technique. This paper describes the estimation of g and attenuation coefficient, μt, of swine liver at 850 nm, performed by an ad hoc designed goniometric-based system, where a spectrometer measures intensities of scattered light at fixed angles (0°, 30°, 45°, 60, 120°, 135° and 150°). Both one-term and two-term Henyey-Greenstein phase function have been employed to estimate anisotropy coefficient for forward (gfs) and backward scattering (gbs). Measurements are performed on samples of two thicknesses (60 um and 30 urn) to investigate the influence of this factor on g, and repeated 6 times for each thickness. The estimated values of gfs were 0.947 and 0.951 for thickness of 60 μm and 30 μm, respectively; the estimations of gfs were -0.498 and -0.270 for thickness of 60 μm and 30 μm, respectively. Moreover, μt of liver has been estimated (i.e., 90±20 cm(1)), through Lambert-Beer equation. The comparison of our results with data reported in literature encourages the use of the ad hoc designed tool for performing experiments on other tissue, and at other wavelengths.

Estimation of liver iron concentration by dual energy CT images: influence of X-ray energy on sensitivity

In hemochromatosis an abnormal accumulation of iron is present in parenchymal organs and especially in liver. Among the several techniques employed to diagnose the iron overload, magnetic resonance imaging (MRI) and Computed Tomography (CT) are the most promising non-invasive ones. MRI is largely used but shows limitation including an overestimation of iron and inability to quantify iron at very high concentrations. Therefore, some research groups are focusing on the estimation of iron concentration by CT images. Single X-ray CTs are not able to accurately perform this task in case of the presence of confounding factors (e.g., fat). A potential solution to overcome this concern is the employment of Dual-Energy CT (DECT). The aim of this work is to investigate influence of the kVp and mAs on CT number sensitivity to iron concentration. A phantom with test tubes filled with homogenized porcine liver at different iron concentrations, has been scanned with DECT at different mAs. The images have been analyzed using an ad-hoc developed algorithm which allows minimizing the influence of air bubbles present in the homogenized. Data show that the sensitivity is strongly influenced by kVp (its value almost halves from 80 kVp to 140 kVp; e.g. 0.41 g·μmol(-1) and 0.19 g·μmol(-1) at 80 kVp/120 mAs and 140 kVp/60 mAs respectively), on the other hand the influence of mAs value is negligible.

Feasibility assessment of CT-based thermometry for temperature monitoring during thermal procedure: Influence of ROI size and scan setting on metrological properties

Computed tomography (CT) thermometry belongs to the wide class of non-invasive temperature monitoring techniques, which includes ultrasound and Magnetic Resonance thermometry. Non-invasive techniques are particularly attractive to be used in hyperthermal procedures for their ability to produce a three-dimensional temperature map and because they overcome the risks related to the insertion of sensing elements.

Perspectives of breast cancer thermotherapies

In this article, the use of different types of thermotherapies to treat breast cancer is reviewed. While hyperthermia is most commonly used as an adjuvant in combination with radiotherapy, chemotherapy, targeted therapy or cryotherapy to enhance the therapeutic effect of these therapies, thermoablation is usually carried out alone to eradicate small breast tumors. A recently developed thermotherapy, called magnetic hyperthermia, which involves localized heating of nanoparticles under the application of an alternating magnetic field, is also presented. The advantages and drawbacks of these different thermotherapies are highlighted.

Optical fiber-based MR-compatible sensors for medical applications: an overview

During last decades, Magnetic Resonance (MR)--compatible sensors based on different techniques have been developed due to growing demand for application in medicine. There are several technological solutions to design MR-compatible sensors, among them, the one based on optical fibers presents several attractive features. The high elasticity and small size allow designing miniaturized fiber optic sensors (FOS) with metrological characteristics (e.g., accuracy, sensitivity, zero drift, and frequency response) adequate for most common medical applications; the immunity from electromagnetic interference and the absence of electrical connection to the patient make FOS suitable to be used in high electromagnetic field and intrinsically safer than conventional technologies. These two features further heightened the potential role of FOS in medicine making them especially attractive for application in MRI. This paper provides an overview of MR-compatible FOS, focusing on the sensors employed for measuring physical parameters in medicine (i.e., temperature, force, torque, strain, and position). The working principles of the most promising FOS are reviewed in terms of their relevant advantages and disadvantages, together with their applications in medicine.