Abstract

In this article, we describe the acquisition of depth profiles, in particular of paint layers, in the static gradient of a high field magnet, providing a superior sensitivity. The main objective are reference profiles that help to understand scans made with noninvasive unilateral nuclear magnetic resonance (NMR), which often suffers from poor signal-to-noise ratio when working with real samples. Various technical aspects like the coil geometry and the limit of resolution are investigated. A major advancement is the use of frequency-modulated pulses that are very broadband and at the same time very short (25 mu s). The latter is necessary to allow the acquisition of a CPMG echo train of old, rigid paint material. Despite being far from adiabatic, they provide uniform excitation and refocusing over 1 MHz, which corresponds to about 400 mu m with the used gradient. We show that the uniformity is even sufficient to obtain biexponential relaxation profiles. With these tools, a paint sample from a restoration campaign is analyzed with different contrast criteria: The original and two layers from former restoration attempts can be visualized, and furthermore, the relaxation profiles allow to study the migration of plasticizing molecules.


Authors

Taugeron, Pierre;  Bricaud, Sullivan;  Kehlet, Cindie;  Dittmer, Jens

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  • pre-publication peer review (FINAL ROUND)
    Decision Letter
    2020/05/03

    03-May-2020

    Dear Dr. Dittmer

    MANUSCRIPT ID: MRC-20-0002.R1
    MANUSCRIPT TITLE: Profiles of paint layer samples obtained in the fringe field of a high field magnet by means of very short broadband frequency modulated pulses
    MANUSCRIPT TYPE: Special Issue Research Article

    It is a pleasure to accept the above manuscript in its current form for publication in Magnetic Resonance in Chemistry. If applicable, any referees' comments will be included at the foot of this email.

    If your paper contains Supporting Information:

    Materials submitted as Supporting Information are authorized for publication alongside the online version of the accepted paper. No further Supporting Information can be submitted after acceptance. It is the responsibility of the authors to supply any necessary permissions to the editorial office.

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    Thank you for submitting your manuscript to Magnetic Resonance in Chemistry. I hope that you will again consider Magnetic Resonance in Chemistry to publish your research.

    Yours sincerely

    Dr. Noemi Proietti
    Review Editor, Magnetic Resonance in Chemistry
    noemi.proietti@cnr.it

    Magnetic Resonance in Chemistry is published by Wiley

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    Referees comments:

    Reviewer: 1

    Comments to the Author
    I am happy to recommend this manuscript for publication.

    Reviewer: 2

    Comments to the Author
    Thank you for addressing the comments and improving the syntax. The text is now more readily accessible for the non-NMR expert.

    Decision letter by
    Cite this decision letter
    Reviewer report
    2020/05/02

    Thank you for addressing the comments and improving the syntax. The text is now more readily accessible for the non-NMR expert.

    Reviewed by
    Cite this review
    Reviewer report
    2020/04/19

    I am happy to recommend this manuscript for publication.

    Reviewed by
    Cite this review
    Author Response
    2020/03/24

    Dear Dr. Proietti

    Please find attached the revised version of our manuscript entitled “Profiles of paint layer samples obtained in the fringe field of a high field magnet by means of very short broadband frequency modulated pulses”, as well as the replies to the referees.

    We appreciate the work and time the referees have invested in the revision of the manuscript and their constructive criticism. We have corrected the manuscript following most of their suggestions. Please find the replies to their specific points below.

    Editor, Magnetic Resonance in Chemistry
    noemi.proietti@cnr.it

    Referees' Comments:

    Reviewer: 1

    Comments to the Author
    The manuscript deals with one dimensional magnetic resonance images of a paint chip which has been flaked off a wooden sculpture. These depth profiles show that the paint consists of three layers: the original and two restoration layers. In order to achieve the required sensitivity and micrometer resolution, the measurements were performed in the stray field of a standard NMR magnet with a modified probe head and a special imaging procedure including frequency modulated pulses. The equipment and the imaging routine was thoroughly tested before.

    The authors took great experimental effort resulting in very nice images. The manuscript is well written and all important experimental details are given as well as a benchmark of the hardware and routines.

    There is one thing in the interpretation which should be discussed. From Fig. 5a the authors conclude that probably plasticizer accumulates in the interface between the two restoration layers. From the data, I would conclude that there is a concentration gradient of plasticizer inside the second restoration layer, which has a maximum at the bottom of this layer. This interpretation fits to a slow evaporation of the plasticizer on the surface.

    R: The reason why we set up the hypothesis of an accumulation of the plasticizer or plasticizing molecules is the shift in the maximum of I1 compared to I0, which is experimentally significant. If the region of the transition between the two layers in the profile of I0 is due to rugosity and imperfect orientation with respect to the field gradient, it is difficult to imagine a scenario that explains that the maximum of I1 is so far below the maximum region of I0. We however agree with the referee that the plasticizing molecules are probably from restoration layer II because the flank is less steep to that side. We have expanded the discussion and added the evaporation argument of the referee. Furthermore we have added lines to the graphs in Fig. 5 to facilitate the comparison of the I0 and I1 profiles.

    For further analysis, a higher spatial resolution would be preferable. Since the resolution is limited to the inhomogeneity of the paint thickness as well as to the transversal (xy-) inhomogeneity of the magnetic field. Both could be decreased by using a smaller piece of paint. I am aware that the signal will also decrease. This leads me to another question: Is the magnetic field profile known? Why was the position of 220 MHz chosen: highest gradient, low xy homogeneity or a compromise?

    R: The field profile has been measured; the gradient is close to the maximal gradient, but not at the absolute maximum (there would not have been significant further gain). The resolution limits given by the above magnetic field parameters are tested (Fig. 2) and considered to be largely sufficient for the paint sample. No systematic optimization of the xy gradient (as it has been shown in the literature) has been done yet. The reviewer is right, higher resolution is always desirable, and the sensitivity of the system leaves some more potential for increasing it by reducing the size of the sample (concomitantly reducing the coil) in the future. (A parameter we have not tested either is a potential perturbation of the magnetic susceptibility by the coil.)

    Minor points:
    Fig 1: a and b are reversed

    R: Corrected.

    Fig 4: red curve is missing

    R: Is now in Figs. 5 and S3 (S4 in the new version); caption corrected.

    In this context, I would suggest to include S3 and S4 to the main manuscript, since they are heavily discussed in the text.

    R: We discuss them but we think that more profiles in the main article could take the focus away from the principle results.

    Reviewer: 2

    Comments to the Author
    The subject matter will be of clear interest to the readers of MRC and the broader field of conservation science, but it is poorly written and edited. This must be addressed before publication. Examples include the caption for Figure S1 containing a reminder to the author to check parameters, but more generally the syntax needs addressing.
    In the first instance it is not apparent what the aim of the work was until much later in the text. It wasn’t until reaching the ‘Phase’ section that it became clear that the aim was to assess the potential for a single measurement and the impact that differences in field strength might have on the profile signal. The introduction did not lay out clearly what was being investigated and what the application was. A very clear statement of intent needs to be made much earlier.

    R: We have outlined the structure of the manuscript more precisely at the end of the introduction.

    Additionally, it is not clear who the target audience is. If it is intended for the expert user then I think the level of technical explanation is appropriate. However, if this paper is intended to inform the fields of conservation and conservation science there are too many assumptions made of the reader. For instance, the section on Frequency Modulated Pulses would benefit from a couple of sentences outlining the relationship between frequency and resolution. The same goes for the results section and the discussions around Figure 5 and proton density.

    R: We are in general working on technical improvements which are often inspired or demanded by real application projects, as in the present case. Therefore the target audience is always both, the conservation scientist and the NMR expert. The conservation scientist can see what is possible (but will perhaps not understand all technical details) and the NMR expert can implement the experiment or is inspired to develop further advances. As this is a manuscript for a NMR journal, we think that a presentation of technical progress (rather a pure application study) would be appropriate.

    Specifics

    Pg. 2, line 4: You state ‘measurements have taken up to a week per scan’. This is a meaningless statement without context.
    Pg. 2, line 8 ‘Often reference materials are tested, or there are samples of the
    artwork. In case of paint we often obtain chips that are flaked off’
    by this stage you haven’t said what you are planning to investigate so again it is not clear what the focus is or why you are making this statement.

    R: The two points are connected. We’ve extended the first phrase, introducing paint as example, and, for the first point, have added “of old, thick paint chips with high resolution”.

    Pg. 2, line 15 You begin the sentence ‘Another application’, but you haven’t yet defined the previous application

    R: We have rewritten this phrase.

    Pg. 2, line 23 the use of the term ‘image’ is puzzling in this context. It is used throughout, but images are not being constructed but line graphs or profiles.

    R: Replaced by “profile”, except for p. 2, line 19, where the term “profile” is defined.

    Pg. 2, line 37 what is meant by ‘cleaner selectivity’?

    R: Clarified.

    Pg. 3, line 4 ‘frequency encoding of the paint’ sounds like jargon. Rephrase.

    R: “of the protons” added.

    Pg. 3, line 9 This study does not determine the type of paint, it exploits the differences in the relaxation properties of paints in order to identify the presence of paint strata. It does not characterise the type of paint. Rephrase.

    R: This phrase refers to a previous study, where the types of paint were (among others) characterized by solid state NMR. We’ve slightly changed it to make this more clear.

    Pg. 3, line 23 Depending on the intended audience a schematic of the magnet/coil setup would be helpful here.

    R: We’ve added a figure with sketches of the geometries to the SI (new Figure S1).

    Pg. 4, line 6 ‘the circuitry band width can give a limitation’. Can you expand on these limitations?

    R: The circuitry band width is well known, can be measured and calculated, could be enlarged, but all this is not subject of this manuscript and therefore not further discussed.

    Pg. 4, line 25 ‘One can achieve a reasonable degree of excitation and refocusing over a broad range with relatively short pulses’. What are the drawbacks/trade-offs?

    R: The main drawback is the limitation of the echo lengths (in the shown example not shorter than 72 s). This is discussed later in the paragraph “Echo times” of the “Discussion” part.

    Pg. 4, line 35 I believe that Figures 1a and 1b are swapped around. Figure 1a shows the 2nd and 3rd echoes.

    R: Indeed. (Corrected.)

    Pg. 6, line 3 Can you define what you mean by ‘homogenous sample’. Do you mean laterally homogenous?

    R: Laterally homogeneous and vertically homogeneous, thus just homogeneous.

    Pg. 6, line 59 ‘According to the shape’. Shape of what?

    R: “of the profile” added.

    Pg. 8, line 44 Can you illustrate the drift?

    R: We – unfortunately – see this temperature drift, which reflects the temperature variation in our laboratory. However, the reader is probably not so much interested in the poor performance of the air conditioner in our lab, which sometimes causes temperature changes of some degrees; therefore we do not show an extra figure.

    We have marked all changes in the text in red in an additional version for review. Changes in the figures concern
    Fig. 1 (a and b exchanged)
    Fig. 5 (lines added)
    Fig. S1 (new, on the request of referee 2)

    We hope you will find the revised version acceptable for Magnetic Resonance in Chemistry.

    With best regards

    Jens Dittmer
    on behalf of all authors



    Cite this author response
  • pre-publication peer review (ROUND 1)
    Decision Letter
    2020/02/24

    24-Feb-2020

    Dear Dr. Dittmer

    I am writing to let you know that the review process on the following manuscript is now complete:

    Manuscript ID MRC-20-0002
    Profiles of paint layer samples obtained in the fringe field of a high field magnet by means of very short broadband frequency modulated pulses

    The reports of the referees are included at the foot of this EMAIL or as an attachment. You can also view these reports via your Author Center in ScholarOne Manuscripts.

    The referees have recommended publication, but also suggest some minor revisions to your manuscript. I invite you to respond to the referees' comments and to revise your manuscript.

    Your revised manuscript is due on: 10-Mar-2020

    You can upload your revised manuscript and submit it through your Author Center on MRC-ScholarOne Manuscripts.

    You can use this direct link:
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    Alternatively, log into https://mc.manuscriptcentral.com/mrc and access your Author Center, where you will find your manuscript listed under "Manuscripts with Decisions". Please do not submit the revised manuscript as a new manuscript.

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    Once again, thank you for submitting your manuscript to Magnetic Resonance in Chemistry. I look forward to receiving your revised manuscript.

    Yours sincerely

    Dr. Noemi Proietti
    Editor, Magnetic Resonance in Chemistry
    noemi.proietti@cnr.it

    Referees' Comments:

    Reviewer: 1

    Comments to the Author
    The manuscript deals with one dimensional magnetic resonance images of a paint chip which has been flaked off a wooden sculpture. These depth profiles show that the paint consists of three layers: the original and two restoration layers. In order to achieve the required sensitivity and micrometer resolution, the measurements were performed in the stray field of a standard NMR magnet with a modified probe head and a special imaging procedure including frequency modulated pulses. The equipment and the imaging routine was thoroughly tested before.

    The authors took great experimental effort resulting in very nice images. The manuscript is well written and all important experimental details are given as well as a benchmark of the hardware and routines.

    There is one thing in the interpretation which should be discussed. From Fig. 5a the authors conclude that probably plasticizer accumulates in the interface between the two restoration layers. From the data, I would conclude that there is a concentration gradient of plasticizer inside the second restoration layer, which has a maximum at the bottom of this layer. This interpretation fits to a slow evaporation of the plasticizer on the surface.

    For further analysis, a higher spatial resolution would be preferable. Since the resolution is limited to the inhomogeneity of the paint thickness as well as to the transversal (xy-) inhomogeneity of the magnetic field. Both could be decreased by using a smaller piece of paint. I am aware that the signal will also decrease. This leads me to another question: Is the magnetic field profile known? Why was the position of 220 MHz chosen: highest gradient, low xy homogeneity or a compromise?

    Minor points:
    Fig 1: a and b are reversed
    Fig 4: red curve is missing
    In this context, I would suggest to include S3 and S4 to the main manuscript, since they are heavily discussed in the text.

    Reviewer: 2

    Comments to the Author
    The subject matter will be of clear interest to the readers of MRC and the broader field of conservation science, but it is poorly written and edited. This must be addressed before publication. Examples include the caption for Figure S1 containing a reminder to the author to check parameters, but more generally the syntax needs addressing.
    In the first instance it is not apparent what the aim of the work was until much later in the text. It wasn’t until reaching the ‘Phase’ section that it became clear that the aim was to assess the potential for a single measurement and the impact that differences in field strength might have on the profile signal. The introduction did not lay out clearly what was being investigated and what the application was. A very clear statement of intent needs to be made much earlier.
    Additionally, it is not clear who the target audience is. If it is intended for the expert user then I think the level of technical explanation is appropriate. However, if this paper is intended to inform the fields of conservation and conservation science there are too many assumptions made of the reader. For instance, the section on Frequency Modulated Pulses would benefit from a couple of sentences outlining the relationship between frequency and resolution. The same goes for the results section and the discussions around Figure 5 and proton density.

    Specifics

    Pg. 2, line 4: You state ‘measurements have taken up to a week per scan’. This is a meaningless statement without context.
    Pg. 2, line 8 ‘Often reference materials are tested, or there are samples of the
    artwork. In case of paint we often obtain chips that are flaked off’
    by this stage you haven’t said what you are planning to investigate so again it is not clear what the focus is or why you are making this statement.
    Pg. 2, line 15 You begin the sentence ‘Another application’, but you haven’t yet defined the previous application
    Pg. 2, line 23 the use of the term ‘image’ is puzzling in this context. It is used throughout, but images are not being constructed but line graphs or profiles.
    Pg. 2, line 37 what is meant by ‘cleaner selectivity’?
    Pg. 3, line 4 ‘frequency encoding of the paint’ sounds like jargon. Rephrase.
    Pg. 3, line 9 This study does not determine the type of paint, it exploits the differences in the relaxation properties of paints in order to identify the presence of paint strata. It does not characterise the type of paint. Rephrase.
    Pg. 3, line 23 Depending on the intended audience a schematic of the magnet/coil setup would be helpful here.
    Pg. 4, line 6 ‘the circuitry band width can give a limitation’. Can you expand on these limitations?
    Pg. 4, line 25 ‘One can achieve a reasonable degree of excitation and refocusing over a broad range with relatively short pulses’. What are the drawbacks/trade-offs?
    Pg. 4, line 35 I believe that Figures 1a and 1b are swapped around. Figure 1a shows the 2nd and 3rd echoes.
    Pg. 6, line 3 Can you define what you mean by ‘homogenous sample’. Do you mean laterally homogenous?
    Pg. 6, line 59 ‘According to the shape’. Shape of what?
    Pg. 8, line 44 Can you illustrate the drift?

    Decision letter by
    Cite this decision letter
    Reviewer report
    2020/02/23

    The subject matter will be of clear interest to the readers of MRC and the broader field of conservation science, but it is poorly written and edited. This must be addressed before publication. Examples include the caption for Figure S1 containing a reminder to the author to check parameters, but more generally the syntax needs addressing.
    In the first instance it is not apparent what the aim of the work was until much later in the text. It wasn’t until reaching the ‘Phase’ section that it became clear that the aim was to assess the potential for a single measurement and the impact that differences in field strength might have on the profile signal. The introduction did not lay out clearly what was being investigated and what the application was. A very clear statement of intent needs to be made much earlier.
    Additionally, it is not clear who the target audience is. If it is intended for the expert user then I think the level of technical explanation is appropriate. However, if this paper is intended to inform the fields of conservation and conservation science there are too many assumptions made of the reader. For instance, the section on Frequency Modulated Pulses would benefit from a couple of sentences outlining the relationship between frequency and resolution. The same goes for the results section and the discussions around Figure 5 and proton density.

    Specifics

    Pg. 2, line 4: You state ‘measurements have taken up to a week per scan’. This is a meaningless statement without context.
    Pg. 2, line 8 ‘Often reference materials are tested, or there are samples of the
    artwork. In case of paint we often obtain chips that are flaked off’
    by this stage you haven’t said what you are planning to investigate so again it is not clear what the focus is or why you are making this statement.
    Pg. 2, line 15 You begin the sentence ‘Another application’, but you haven’t yet defined the previous application
    Pg. 2, line 23 the use of the term ‘image’ is puzzling in this context. It is used throughout, but images are not being constructed but line graphs or profiles.
    Pg. 2, line 37 what is meant by ‘cleaner selectivity’?
    Pg. 3, line 4 ‘frequency encoding of the paint’ sounds like jargon. Rephrase.
    Pg. 3, line 9 This study does not determine the type of paint, it exploits the differences in the relaxation properties of paints in order to identify the presence of paint strata. It does not characterise the type of paint. Rephrase.
    Pg. 3, line 23 Depending on the intended audience a schematic of the magnet/coil setup would be helpful here.
    Pg. 4, line 6 ‘the circuitry band width can give a limitation’. Can you expand on these limitations?
    Pg. 4, line 25 ‘One can achieve a reasonable degree of excitation and refocusing over a broad range with relatively short pulses’. What are the drawbacks/trade-offs?
    Pg. 4, line 35 I believe that Figures 1a and 1b are swapped around. Figure 1a shows the 2nd and 3rd echoes.
    Pg. 6, line 3 Can you define what you mean by ‘homogenous sample’. Do you mean laterally homogenous?
    Pg. 6, line 59 ‘According to the shape’. Shape of what?
    Pg. 8, line 44 Can you illustrate the drift?

    Reviewed by
    Cite this review
    Reviewer report
    2020/01/29

    The manuscript deals with one dimensional magnetic resonance images of a paint chip which has been flaked off a wooden sculpture. These depth profiles show that the paint consists of three layers: the original and two restoration layers. In order to achieve the required sensitivity and micrometer resolution, the measurements were performed in the stray field of a standard NMR magnet with a modified probe head and a special imaging procedure including frequency modulated pulses. The equipment and the imaging routine was thoroughly tested before.

    The authors took great experimental effort resulting in very nice images. The manuscript is well written and all important experimental details are given as well as a benchmark of the hardware and routines.

    There is one thing in the interpretation which should be discussed. From Fig. 5a the authors conclude that probably plasticizer accumulates in the interface between the two restoration layers. From the data, I would conclude that there is a concentration gradient of plasticizer inside the second restoration layer, which has a maximum at the bottom of this layer. This interpretation fits to a slow evaporation of the plasticizer on the surface.

    For further analysis, a higher spatial resolution would be preferable. Since the resolution is limited to the inhomogeneity of the paint thickness as well as to the transversal (xy-) inhomogeneity of the magnetic field. Both could be decreased by using a smaller piece of paint. I am aware that the signal will also decrease. This leads me to another question: Is the magnetic field profile known? Why was the position of 220 MHz chosen: highest gradient, low xy homogeneity or a compromise?

    Minor points:
    Fig 1: a and b are reversed
    Fig 4: red curve is missing
    In this context, I would suggest to include S3 and S4 to the main manuscript, since they are heavily discussed in the text.

    Reviewed by
    Cite this review
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