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.

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

Purpose: 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.

Study type: Prospective.

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

Field strength/sequence: 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.

Assessment: 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.

Statistical tests: 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.

Results: 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.

Data conclusion: 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.

Level of evidence: 2:

Technical efficacy: Stage 5.