Three-Line Microstrip Array for Whole-Body MRI System at 7 T

Round 1

Reviewer 1 Report

The paper might be interesting since it raises the important issue of implementing new MRI research methods. However I'm not convinced. The work is purely theoretical.

 

The author writes that there are currently no transmitting coils that allow the generation of a B1 field for 7T MRI magnets. What about bird-cage coils? So how does the GE 7T MR950 work? The SAR parameter is not something we can directly influence except decreasing an experimental time or reducing hot-spots, which by the way, author' solution generate quite many!

SAR it is directly related to the B1 field amplitude, frequency and tissue type, that is all. Another problem with 7T scanners and bigger is problems with gradient coils. The so-called "bannana modes" gradient coils are difficult to remove, and the coils can influence the flow of liquid helium inside the cryostats, magnets become unstable.

I do not like the fact that the author focused solely on theoretical research. When writing about the prototype, this seems plain misconception.

 

Problems: I am not entirely convinced that the system will transmit the B1 field to the whole organism as written in the abstract. Fig. 2 It is clear that the penetration depth is remarkably small, on the order of 20-30% of the width of the strip. Besides, the heterogeneity of the field also leaves much to be desired. Fig. 4. From what I understand, the author wanted to reduce the SAR parameter, and the figure shows that the authors only counted the B1 field, which incidentally is extremely heterogeneous (Fig.4c). I would say the comparison favors the bird-cage coil. The B1 field can be scaled with the transmitter power, and this is not a technical problem. Fig. 5 Similar analysis, sagittal section uniformity for the bird-cage coil is the best. Imagine what T1 weighted images would look like with such heterogeneity? Fig. 6 The SAR parameter is again related to the heterogeneity of the transmitting coil and here again it can be seen that nothing better than the bird-cage coil has been invented. I would be convinced if author would apply this solution in reality.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

This is a brief study about the possible use of three-line microstrip arrays for whole body MRI system at 7T instead of other more proven technologies like birdcage coils. This work is exclusively computational using commercial software and a virtual phantom model. Certainly it is difficult to find other works in the scientific literature with a similar idea, so that this novel approach merit attention. However, many details about the calculations are missing, like the phantom distribution of human tissues and its accuracy as a model of the real human body response to RF. Other cuestions like cost vs performance compared to the birdcage approach are not addressed at all. I think that this study would benefit from a more detailed description of the calculations, concrete numerical algorithms used by the simulation software, expected errors and limitations ot the model, etc.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The author made a number of corrections to the article. Although I do not fully agree with some of the conclusions. I have this feeling that my previous remarks were not fully addressed. Furthermore, I still believe that the authors did not fully prove that their solution is better than those currently used. What is more, a computer model is not a prototype (used twice in the manuscript).

Author Response

We have modified the design of the microstrip, in order to improve the field uniformity and also the SAR distribution. We understand the reviewer comments in terms of the performance of the proposed design. For this reason we opted to keep using the triple line microstrips but at a smaller scale such that it can be used in an array configuration capable to make a circular polarized field, that can improve the field uniformity and at the same time reduce the SAR.

We have also added more computational information about the design of the triple microstrip, such as the dependence on the height of the dielectric, and the distance of the adjacent coupled lines.

We changed the simulations with the Birdcage coil. We performed tuning of the BC coil to 300MHz, the values of the capacitors has been added to the manuscript.

We have updated the figures and the text with the new design and data. We also added a table showing the SAR mean and max value for some of the tissues of more interest along the body and compared between the three RF transmitters.

We removed the word prototype.

The article was reviewed by English editors.

Reviewer 2 Report

Although many of the review questions have been addressed, I think that the author could design a relatively simple scaled experiment using simulants or pig tissues, for example. I understand that this can be slower, but it is much more rigorous. The experiment can be done without much expense, because a full body scale is not necessary. My main concern is the low resolution of the numerical model and the lack of experimental references, so that field inhomogeneites are not conveniently described which could produce hot spots or image defects. If the author cannot perform real experiments, at least I suggest a very detailed series of computational models showing field distributions in tissue interfaces, different tissues, different body parts, etc.

Author Response

We thank the reviewer for the constructed comments, and suggestions. We are not able to make the MRI experiments at this moment; but we have included more studies on the field uniformity and SAR.

We have modified the design of the microstrip, in order to improve the field uniformity and also the SAR distribution. We understand the reviewer comments in terms of the performance of the proposed design. For this reason we opted to keep using the triple line microstrips but at a smaller scale such that it can be used in an array configuration capable to make a circular polarized field, that can improve the field uniformity and at the same time reduce the SAR.

We have also added more computational information about the design of the triple microstrip, such as the dependence on the height of the dielectric, and the distance of the adjacent coupled lines.

We changed the simulations with the Birdcage coil. We performed tuning of the BC coil to 300MHz, the values of the capacitors has been added to the manuscript.

We have updated the figures and the text with the new design and data.

We also added a table showing the SAR mean and max value for some of the tissues of more interest along the body and compared between the three RF transmitters.

-The article was reviewed by English editors.

Round 3

Reviewer 2 Report

The author have done a great effort to improve the paper. I understand that real experiments are out of reach for this work, but simulations have been improved and explained to the point that the paper can be accepted. Surely, this work could be better, but the idea deserves been published, and I hope that the author will be able to provide further experimental confirming studies in the future.