MRI of Implantation Sites Using Parallel Transmission of an Optimized Radiofrequency Excitation Vector

Round 1

Reviewer 1 Report

This is an interesting pre-clinical study about the optimization of MRI imaging of orthopedic implants. The study seems well-developed and deals with an interesting topic. The methods represent the most interesting part of the paper as the Authors elaborated brilliant solutions to conduct their research.
I have a few questions/comments for the Authors:
- why did you choose to develop this technique based on 7.0 T magnetic field? Usually, a 1.5 T magnetic field is sufficient to evaluate orthopedic implants and it is less prone to artifacts or heating.
- why did you choose copper for the wire?
- why did you use 3D gradient echo sequences?
- I would add a brief paragraph on the possible employments of this technology in clinical practice.

Author Response

"Please see the attachment." 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript “MRI of implantation sites using parallel transmission of an optimized 2 radiofrequency excitation vector” contains a complete theoretical and experimental work and presents a new approach to B1 field optimization of interest for the MRI community.

Before publication it is my opinion that some issues could be resolved to improve its readability.

Title/Abstract/Text
The authors generically use the word “implant” which includes a quite wide class of devices. In the manuscript the authors focus on a specific case “A cylindrical element (L=70 mm, R=1 mm, ?=153 5.8×108 S?⁄)” with a one-dimensional shape. I invite the authors to describe the type of clinical devices they are considering with their simulations/experiments and specifically refer to them in the title/abstract/text. In the Discussion the authors state that
“Our results demonstrate that the GA-based approach addresses both these challenges, and thus represents a promising option en-route to safe clinical MRI of orthopedic implants,…”
and
“Hence, the GA method is not only capable of eliminating unwanted RF induced effects of orthopedic implants in MRI but may also be used for shaping the transmission field around other passively conducting (interventional) devices. These include for example standard titanium implants, catheters, intracoronary stents, guide wires or metallic needles.”
Please provide the arguments to support these statements considering that the authors only numerically/experimentally tested metallic implants with a one-dimensional configuration.

Section 2.1

-       Different types of implants can be hosted in different body districts which require different coils. For different body districts the coil size could change, and this could result in important changes in the RF response of the coil (some of the A-H array designs could not be feasible for a large body coil). The authors should clearly state the body district they are focusing on when presenting the eight RF transceiver configurations examined and, in the discussion section, discuss the possible extension of their results to coils of different size.

-       In section 2.1 it is stated that
“Neighboring loop elements were decoupled with transformers [40].”
but nothing is reported about non-adjacent loops coupling which, for increasing loop size, could become an issue. Is isolation below -15 dB even for them?

Section 2.2

-       The authors define three parameters used for the definition of the objectives (lines 172-175), but I couldn’t understand their interplay in the complex excitation vector K_{GA} determination. The optimization tolerance parameter is defined, the optimization parameter no. Are the above mentioned parameters used to defined, with some weights, an energy which is minimized? Please explain this point which is at the very heart of the proposed method.

-       At line 186 there is a reference in superscript form.

Section 2.3

-       At line 227 there is a “22” as superscript

Section 2.5

-       At line 272 the calibration coefficients are defined but their later use is not detailed. Please provide information about their use in the procedure.

-       If the calibration coefficients are needed for a correct application of the proposed method, how the authors imagine a clinical workflow should be?

Section 2.6

-       Please insert here all the relevant info concerning the RF coil used for the experiments, including the decoupling level among adjacent and next-to-adjacent coils to provide a crosscheck of correspondence with the simulated coil.

Section 3.1

-       I do not understand the meaning of
“Yet our simulations showed that in some scenarios OP excitation vectors produced a superficial SAR10g,max value that is outside of the implantation site and thus its maximum SAR10g,max is still in the range of the results derived from the CP mode.”
neither what is intended for superficial SAR.

Section 3.2

-       Please move the hardware information in Section 2.6.

Section 4

-       I found misleading the sentence
“Increasing the number of RF channels may introduce more resistance (i.e., copper, lumped elements etc.) to the total resistance seen from the RF transceiver ports, which constrains the signal-to-noise ratio of MRI.”
since a typical UHF-MRI detection channel is dominated by the load noise. Increasing the RF channels may change this situation if the sensible area of each channel is reduced, the load noise is reduced (as well as radiation losses), and the copper and lumped elements noise could come into play.

Please note that in many sentences the references are inserted after the full stop.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf