# Quantitative Confounder Analysis of Electrocardiogram Signals in Cardiac Magnetic Resonance at 1.5, 3 and 7 T—Assessing Standardized Electrode Positions and Sequence Types—Towards Quality Assurance

## TL;DR

This study examines how magnetic field strength, electrode placement, and imaging sequences affect ECG signals in cardiac MRI, finding that higher field strengths and alternative electrode positions cause more signal distortion.

## Contribution

The study quantifies ECG signal distortion at 1.5T, 3T, and 7T using standardized electrode positions and sequence types, offering practical guidance for quality assurance in cardiac MRI.

## Key findings

- Higher magnetic field strengths (7T) caused significantly more ECG signal distortion compared to 1.5T and 3T.
- Vendor-recommended electrode positions (pos1/pos2) resulted in less signal distortion than alternative positions (pos3/pos4).
- Imaging sequences showed similar levels of ECG distortion, with no major differences between them.

## Abstract

The electrocardiogram (ECG) used for gating in cardiac MRI may be compromised by multiple confounders inside the scanner bore.

To quantify the influence of magnetic field strengths (1.5 T/3 T/7 T), standardized electrode positions, and imaging sequences on ECG signals used for gating.

Prospective.

Sixteen healthy volunteers (eight male; mean age 26.25 ± 7.67 years).

Balanced steady‐state free precession cine (1.5 T/3 T), fast low‐angle shot cine (7 T), and 4D flow (1.5 T/3 T/7 T) sequences.

ECG‐signals were recorded during breath‐hold and non‐breath‐hold short axis cine (sax‐bh and sax‐nbh, respectively) and 4D flow scans at 1.5 T/3 T/7 T. All scans were repeated with 4 standardized electrode positionings (pos1–4) at each field strength. Pos1/2 were vendor‐recommended positionings for 1.5 T/3 T/7 T scans, respectively, whereas pos3/4 were alternative positionings recommended in previous studies. Similarity between confounded ECG‐signals and unconfounded baseline ECG‐signals was assessed by QRS‐feature correlation. Cine image quality (IQ) was assessed by 3 readers (with 6, 10, and 22 years experience) on a four‐point Likert scale.

Linear mixed models with type III tests of fixed effects (overall) and t tests with adjusted degrees of freedom (pairwise subgroup‐comparisons) at significance level p < 0.05.

Increasing field strength resulted in significantly decreasing similarity to baseline measurements, with r values (provided with 95% confidence interval) of 1.5 T: 97% (92.6–101.3); 3 T: 91.4% (87.1–95.8); 7 T: 50.4% (46–54.9) and lower IQ: 1.5 T: 2.33 (2.12–2.55); 3 T: 1.96 (1.75–2.17); 7 T: 0.91 (0.7–1.12). Vendor‐specified electrode positions pos1: 91.8% (87.2–96.5), pos2: 88.3% (83.7–92.9) showed significantly higher correlation with baseline measurements than alternative positions pos3: 67.5% (62.9–72.1) and pos4: 70.8% (66.2–75.4). The evaluated standardized sequences showed similar amounts of electrocardiogram distortion, with r values of: sax‐bh: 77.3% (73–81.7); 4D: 79.3% (75–83.7), p = 0.54; sax‐nbh: 82.1% (77.8–86.5), p = 0.31, but the difference between sax‐bh and sax‐nbh: 4.8% (2.88–6.72) was significant.

Increasing field strength leads to significant ECG signal distortions. Vendor‐specified positions 1/2 resulted in less distorted ECG signals than alternative positions 3/4 recommended in previous publications.

2.

Stage 5.

The electrical signal of the heart plays an important role in synchronizing cardiac MR imaging with the heart's motion. This study measured how much this signal is distorted by magnetic field strengths (1.5, 3, and 7 T), electrode positioning, and imaging sequences. The results showed higher amounts of distortion with increasing field strengths but less distortion with standard electrode positions. The tested imaging sequences showed little difference. These results highlight challenges for ultra‐high‐field MRI in clinical practice and provide practical guidance to optimize electrode placement. They may pave the way for more reliable heart imaging at increasing magnetic field strengths.