Disturbing Effect of Intra-Tissue Temperature Sensors in Pre-Clinical Experimental Studies of Radiofrequency Cardiac Ablation: A Computer-Based Modeling Study

Round 1

Reviewer 1 Report

There are fewer English language  errors. e.g in line 181 the word used should be "farther" and not further..

There is lack of qualitative and quantitative analysis in the manuscript. The authors can show some initial simulations.

 

In section 3.2 , the authors have started with a simultaneous application of , thermocouples , and optical fiber sensors. This make the analysis subtle . Can we make it in a superposition fashion. Each one independently. and discuss the results first independently and then combines. The discussion part is also packing 

 

The model is oversimplified . Can we add other tissue layers , for instance fat layer and its effect towards and heating and temperature evaluation.

More references can be added. 

 

 

Author Response

Many thanks for the useful comments. We conducted a major revision, adding a new case of arrangement of temperature sensors, and including the raw data as Supplementary material. Below are the responses to each comment as well as a description of how the manuscript has been modified accordingly.

COMMENT #1: “There are fewer English language errors. e.g in line 181 the word used should be "farther" and not further.”

Change in manuscript: The entire text has been revised.

 

COMMENT #2: “There is lack of qualitative and quantitative analysis in the manuscript. The authors can show some initial simulations.”

Response: We understand that the manuscript reports not only qualitative information (in form of temperature distributions for the different cases, e.g. Fig. 6 and 9), but also quantitative information (in the form of differences between temperature evolutions with and without sensors, e.g. Fig. 7, 8 and 10). The main values are given in the text.

 

COMMENT #3: “In section 3.2, the authors have started with a simultaneous application of , thermocouples , and optical fiber sensors. This make the analysis subtle. Can we make it in a superposition fashion. Each one independently and discuss the results first independently and then combines. The discussion part is also packing.”

Response: This was done because while in the case of sensors in parallel with the catheter axis there are different subcases at different depths of the sensor tip (see Fig. 2). This was not the case with the sensors perpendicular to the catheter axis, in which there is only one set of distances (see Fig. 3). There are much fewer results in the second case, allowing a more direct comparison between thermocouples and fiber optics, as shown in Figures 9 and 10.

 

COMMENT #4: “The model is oversimplified. Can we add other tissue layers, for instance fat layer and its effect towards and heating and temperature evaluation.”

Response: While previous RFCA modeling studies included the different layers of tissue around the ablation electrode with the aim of imitating a real clinical scenario (such as myocardium, fat, and connective tissue [1]), the aim of our study was to assess the thermal impact of the presence of sensors within the tissue, which is normally performed during pre-clinical studies using homogeneous material [2,3] such as agar or ex vivo cardiac tissue. We think that considering heterogeneous tissue would introduce "noise" in the analysis since the results would be strongly influenced by the relative position of the sensor within the different layers of tissue. Simplifying by assuming a homogeneous tissue therefore seems to be relevant considering the objective of the study.

Change in manuscript: This comment has been added in the Discussion section.

 

COMMENT #5: “More references can be added.”

Change in manuscript: Only the strictly necessary references have been added based on the reviewers' comments.

 

References

1. Pérez, J.J.; Gonzalez‐Suárez, A.; Maher, T.; Nakagawa, H.; D'Avila, A.; Berjano, E. Relationship between luminal esophageal temperature and volume of esophageal injury during RF ablation: In silico study comparing low power-moderate duration vs. high power-short duration. Cardiovasc. Electrophysiol. 2022, 33, 220–230. https://doi.org/10.1111/jce.15311.
2. Rossmann C, Motamarry A, Panescu D, Haemmerich D. Computer simulations of an irrigated radiofrequency cardiac ablation catheter and experimental validation by infrared imaging. Int J Hyperthermia. 2021;38(1):1149-1163. doi: 10.1080/02656736.2021.1961027.
3. Demazumder D, Mirotznik MS, Schwartzman D. Comparison of irrigated electrode designs for radiofrequency ablation of myocardium. J Interv Card Electrophysiol. 2001 Dec;5(4):391-400. doi: 10.1023/a:1013241927388.

 

Reviewer 2 Report

 

This work assesses through numerical modelling the impact of temperature sensors inside the target region during radiofrequency cardiac ablation (RFCA). Two kinds of sensors are considered and compared, i.e., thermocouples and optical fibers, arranged in two different spatial configurations.

The measurement artifacts introduced by thermocouples during RFCA and the causes of such effects, attributable to the conductive metallic components which constitute thermocouples, are well known and already reported in literature (e.g., D. Chakraborty and I. Brezovich, Error sources affecting thermocouple thermometry in RF electromagnetic fields, J. Microw. Power 1982, 17, 17–28; D. P. Chakraborty and I. Brezovich, A source of thermocouple error in radiofrequency electric fields. Electron. Lett. 1980, 16, 853–854). Moreover, the paper lacks comparison with experimental analyses, the simulations do not consider blood flow and parametric studies with treatment power and time are not reported. However, the comparison with optical fiber sensors can be interesting.

I suggest the paper can be reconsidered after the following major revisions:

- Section 2.2: Why the thickness of the polyimide coating of the fibers is 0.15 mm if its outer diameter is 0.3 mm? And since polyimide is sensitive also to humidity why it has been chosen as fiber coating? Please clarify.

- Correct some typos like “°C” and the question mark in Eq. 3.

- Section 3.1: A figure like Fig. 6 but for the fiber optic sensors should be reported to show the difference of the thermal maps between thermocouples and optical fiber.

- Figure 9 is missing A, B, and C in the image.

 

 

Author Response

Many thanks for the useful comments. We conducted a major revision, adding a new case of arrangement of temperature sensors, and including the raw data as Supplementary material. Below are the responses to each comment as well as a description of how the manuscript has been modified accordingly.

 

COMMENT #1: “The measurement artifacts introduced by thermocouples during RFCA and the causes of such effects, attributable to the conductive metallic components which constitute thermocouples, are well known and already reported in literature (e.g., D. Chakraborty and I. Brezovich, Error sources affecting thermocouple thermometry in RF electromagnetic fields, J. Microw. Power 1982, 17, 17–28; D. P. Chakraborty and I. Brezovich, A source of thermocouple error in radiofrequency electric fields. Electron. Lett. 1980, 16, 853–854). Moreover, the paper lacks comparison with experimental analyses, the simulations do not consider blood flow and parametric studies with treatment power and time are not reported. However, the comparison with optical fiber sensors can be interesting.”

 Response: The articles by Chakraborty and Brezovich cited by the reviewer are really well-known classic in the field of hyperthermia thermometry [1,2] and though they are highly relevant and valuable references, they focus on the electromagnetic interference coupled by RF fields at 13.56 MHz, both capacitively and inductively, which causes two types of error: “pick-up” errors and genuine errors. The first are due to stray capacitances between the electrodes and the thermocouples, which result in electrical currents flowing into the measurement system (delicate electronic circuit), leading to erroneous temperature readings. In contrast, the second are due to true temperature changes in the thermocouple junction induced by the RF currents. In fact, these studies propose minimization techniques based on shielding and filtering. Note that, as we emphasized in the limitations, the capacitive or inductive coupling was not taken into account in our simulations, since we solved a static electrical problem, and the sensing zone of the thermocouples was assumed to be completely covered by insulation material (see Fig. 1B). In this regard, our model would represent an ideal case in which filtering and shielding techniques are 100% effective (to completely remove the “pick-up” errors) and there is no RF-induced self-heating in the metal thermocouple parts. Our objective was to assess the thermal impact due to the mere presence of the sensor, due to the fact that it is composed of materials with very different properties to biological tissue. In fact, the few studies dealing with the interference between thermocouples and RF fields [3-5] suggest that the RF power interruption when measuring temperatures, which would not be useful in the context of our study in which we found that the disturbance is caused by thermal conduction through the sensor itself, especially in thermocouples due to the high thermal conductivity of the metal parts.

Furthermore, what all these studies conclude is that the errors due to the invasive measurement of temperature in the vicinity of RF fields is highly dependent on the specific conditions of each case: size and type of sensor, relative position, thermal characteristics of the tissue, etc. This is precisely one of our conclusions, in which the relative position of the different temperature sensors with respect to the ablation electrode, as well as the presence or absence of metallic parts inside it, can affect the temperature measurement, suggesting that computational modeling would complement bench studies with invasive point temperature probes in order to predict any possible biases and correct the experimental values accordingly.

Finally, the comparison between thermocouples and optical fibers in terms of the absence of metallic parts in the latter is undoubtedly one of the novelties of the study.

Change in manuscript: This information, along with the new references, has been added and discussed in the revised version.

 

COMMENT #2: “Section 2.2: Why the thickness of the polyimide coating of the fibers is 0.15 mm if its outer diameter is 0.3 mm? And since polyimide is sensitive also to humidity why it has been chosen as fiber coating? Please clarify.”

 Response: We found that this material and these dimensions are employed in the case of the optical fiber modeled in our study, particularly the Luna model ODiSI 6000 sensor (Roanoke, VA, USA). In its technical specs it claims that “Luna’s high-definition strain sensors are constructed using polyimide-coated low bend loss fiber with a diameter of 155 μm” (https://lunainc.com/sites/default/files/assets/files/data-sheet/LUNA%20ODiSI%206000%20Data%20Sheet.pdf)

Change in manuscript: These decisions have been clarified in the revised version.

 

COMMENT #3: “Correct some typos like “°C” and the question mark in Eq. 3.”

 Change in manuscript: Done.

 

COMMENT #4: “Section 3.1: A figure like Fig. 6 but for the fiber optic sensors should be reported to show the difference of the thermal maps between thermocouples and optical fiber.”

 Change in manuscript: Done.

 

COMMENT #5: “Figure 9 is missing A, B, and C in the image.”

 Change in manuscript: Done (it is the current Fig. 10).

 

References

1. Chakraborty DP, Brezovich I. Error sources affecting thermocouple thermometry in RF electromagnetic fields. J. Microw. Power. 1982, 17:17-28. doi: 1080/16070658.1982.11689261.
2. Chakraborty DP, Brezovich I. A source of thermocouple error in radiofrequency electric fields. Electron. Lett. 1980, 16:853-854. doi: 1049/EL:19800606.
3. Dunscombe PB, Constable RT, McLellan J. Minimizing the self-heating artefacts due to the microwave irradiation of thermocouples. Int J Hyperthermia. 1988 Jul-Aug;4(4):437-45. doi: 10.3109/02656738809016496.
4. Kaatee RS, Crezee H, Visser AG. Temperature measurement errors with thermocouples inside 27 MHz current source interstitial hyperthermia applicators. Phys Med Biol. 1999 Jun;44(6):1499-511. doi: 10.1088/0031-9155/44/6/305.
5. Song CW, Rhee JG, Lee CK, Levitt SH. Capacitive heating of phantom and human tumors with an 8 MHz radiofrequency applicator (Thermotron RF-8). Int J Radiat Oncol Biol Phys. 1986 Mar;12(3):365-72. doi: 10.1016/0360-3016(86)90352-4.

Round 2

Reviewer 1 Report

The authors have addressed majority of the comments. The manuscript in present form can go for publishing

Reviewer 2 Report

The authors implemented the required revisions. I suggest this work can be accepted.