7-Tesla Functional Cardiovascular MR Using Vectorcardiographic Triggering—Overcoming the Magnetohydrodynamic Effect

Round 1

Reviewer 1 Report

This is an experimental small study, focused on the magnetohydrodynamic effects generating  ECG signal distorsion, especially using B0 at ultra-high field.  

General comment:

Cardiac gating is usually performed using electrocardiogram (ECG) triggering. In general, vectorcardiography (VCG)- based QRS detection algorithms are used, which aim to detect the R-wave in their peak by recognizing the R-wave’s rising edge. However, ECG signal distortions from several effects have been challenging at ultra-high field. The interaction of the conductive fluid blood with the static magnetic field B0, for instance, induces a voltage perpendicular to B0 and the direction of flow that superimposes on the ECG signal. This so-called MHD effect is particularly large during the early systolic phase, when the blood is ejected from the left ventricle into the aortic arch. Hence, it mainly affects the Twave of the ECG signal. The probability that a rising edge of an MHD artifact is similar to the rising edge of the R-wave is generally nonzero. Consequently, deteriorated cardiac synchronization is likely in the presence of strong MHD artifacts that are similar to the R-wave’s rising edge. Problems have been observed at clinical field strengths such as 3T, and because the MHD effect scales with B0, distortions have been reported to be worse at ultra-high field. In addition, the time-varying magnetic gradient fields, which induce voltage perturbations in the ECG leads, also distort the ECG signal. To avoid motion artifacts, the lengthening of scan times and scan repetitions that result from poor ECG triggering, the establishment of a stable cardiac synchronization technique is essential to advance ultra-high field CMR.

As possible solutions, pulse triggering is often used in cases were the conventional ECG approach fails. However, being derived from softly shaped peaks in the pulse wave signal, the trigger events are subject to immanent enhanced jittering, which commonly introduces trigger-related image artifacts. Because the trigger events are also delayed with respect to the R-wave, pulse triggering is unsuitable for modalities like myocardial tagging that require an accurate detection of the R-wave. Doppler ultrasound  and acoustic trigger devices  as well as self-navigation  and pilot tone navigation  have recently been explored as alternative tools to conventional ECG triggering.

In this study, advanced ECG detection algorithm has been proposed, showing promising results. From a clinical point of view, only a limited number of healthy subjects have been examined. Thus, it is important to note that the overall number of RR-intervals analyzed outside of the magnet bore is rather low. Moreover, it is well-known that the success of ECG triggering can be highly subject-dependent, and certainly, a larger number of subjects need to be examined in future work to fully reveal the performance of the trigger technology at hand. Apart from that, ECG triggering can be particularly challenging in patient cohorts with cardiac arrhythmia, where the MHD effect can be more severe and variable. Finally,  beyond the LV an RV volumes, and  the phase contrast, in clinical practice the advantage of B0 at ultra-high field using this algorithm should be testes for evaluation of T2, for edema, DE, for viability and scar and for T1 and T2 mapping.

To improve the manuscript the authors should:

Increase the sample size of subject analyzed

Include patients with arrhythmia (atrial fibrillation)

Added a paragraph  reporting alternative potential solutions of MHD artifacts

Added a paragraph to discuss  which patients/subjects as ideal candidate to  7T MRI.

Author Response

1. Increase the sample size of subject analyzed.

 

Response: We agree that more subjects may have been useful to increase the number of R-waves analyszed. However, this study aimed to assess the clinical feasibility of the advanced ECG detection algorithm and learning phase on acquisition of SSFP and Phase Contrast flow imaging at 7.0T. The number of healthy subjects was sufficient to test the hypothesis that VCG gating with a learning phase would allow for CMR acquisitions which can be analyzed for left and right ventricular volumes, ejection fraction and vascular flow. The number of subjects was limited by the funding from UQ Academic Title Holders Research Grant and thus cannot be further extended for the purposes of this paper.

 

2. Include patients with arrhythmia (atrial fibrillation)

 

Response: We agree that it would be interesting to perform a study on subjects with arrythmias. However, the aim of the present study was to assess the CMR acquisition quality at 7T, and thus patients with arrythmias were excluded from enrolment. The MHD effect on the R and Twaves would be the same in sinus rhythm or in AF, thus we expect that the learning-phase VCG detection algorithm should perform equally well. Clinically, however, when we acquire CMR for patients in AF we use advanced prospective gating or real-time imaging , which are sequences not currently available on this Siemens investigative 7T platform.

We have added a sentence in the Limitations:

“Subjects with arrythmias such as atrial fibrillation were excluded, thus the performance of the VCG-gating algorithm in patients with arrhythmias is not known.”

 

3. Added a paragraph  reporting alternative potential solutions of MHD artifacts

Response: Thank you. This is discussed in the manuscript.

 

4. Added a paragraph to discuss  which patients/subjects as ideal candidate to  7T MRI.

 

Response: Thank you. We have added a sentence:

“Ideal candidates for imaging at 7.0T are those in sinus rhythm, able to breath-hold, and with no relative contraindications to MRI.”

Author Response File: Author Response.docx

Reviewer 2 Report

This paper describes an application of 7-Tesla Functional Cardiovascular MR to overcome the magnetohydrodynamic effect. This is a nice and novel application and could be interesting for readers of the journal. Some minor problems are listed in the followings:

1. In the section “abstract,” the acronyms ECG (line 15), MR (line 16), MRI (line 18), VCG (line 25), LVEF (line 28), and RCEF (line 28) should write the full words. Some acronyms such as EF, LCEF, RVEF, MHD and VCG, should be removed, because they only appear once.
2. Line 32, the sentence “…this method of ECG triggering..” has two dots.
3. In the section “introduction,” the characteristics of the proposed approaches should be enhanced and detailed.
4. In the section “results,” can the authors select a baseline system for comparison?

Author Response

This paper describes an application of 7-Tesla Functional Cardiovascular MR to overcome the magnetohydrodynamic effect. This is a nice and novel application and could be interesting for readers of the journal.

Response: We thank the Reviewer for the positive comments.

Some minor problems are listed in the followings:

1. In the section “abstract,” the acronyms ECG (line 15), MR (line 16), MRI (line 18), VCG (line 25), LVEF (line 28), and RCEF (line 28) should write the full words. Some acronyms such as EF, LCEF, RVEF, MHD and VCG, should be removed, because they only appear once.

 

Response: Thank you. The abstract has been corrected to remove acronyms and use full words if appearing only once.

 

1. Line 32, the sentence “…this method of ECG triggering..” has two dots.

 

Response: Thank you, this has been corrected.

 

1. In the section “introduction,” the characteristics of the proposed approaches should be enhanced and detailed.

 

Response: We have previously published a detailed manuscript on the VCG algorithm (Tomography. 2016, 2:167-174. 10.18383/j.tom.2016.00193) and do not wish to duplicate previous works.

 

 

1. In the section “results,” can the authors select a baseline system for comparison?

 

Response: The authors is not clear what “baseline system” could be used for comparison; this was not a comparative efficacy study, but a feasibility study of SSFP and Flow at 7.0T examining the ability to generate clinically-acceptable imaging for volumetric and quantitative analysis. Subjects were not scanned on another system for the purposes of this study.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Some  questions remain to be solved, especially regarding the application  of this tool in clinical practice in patients  with  atrial  fibrillation.