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Clinical & Translation

Clinical & Translation 11: CMR in Athletes and Sport Cardiology

Exercise CMR Comparison Between 3T and 0.55T: Image Quality and Exercise Performance

Saturday, February 1, 2025
8:50 AM – 9:00 AM East Coast USA Time
Location: Regency Room
Background: In-bore Exercise Cardiac Magnetic Resonance (EXCMR) offers the potential to combine a cardiopulmonary stress examination with CMR to improve diagnosis for a range of cardiac diseases [1,2]. Exercise is the preferred modality of stress as exercise performance provides important independent prognosis [3]. In-bore exercise can be challenging due to the space limitations of standard 60cm – 70cm diameter patient bores. A 0.55T scanner with 80 cm wide bore allows for greater range of motion and can accommodate larger patients; however, lower field strength and reduced gradient performance present challenges for real-time cine image quality. We sought to evaluate the feasibility of in-bore exercise and EXCMR image quality at 0.55T compared to 3T.

Methods: Healthy adults underwent resting CMR and EXCMR on both 0.55T (80cm*167cm bore) and 3T (70cm*185cm bore) MRI scanners (MAGNETOM Free.Max and Vida, Siemens Healthcare, Germany). In-bore exercise consisted of a ramped protocol (+20watts/60sec) that was held constant during peak exercise, defined as 85% age-predicted max HR or subject expression of volitional fatigue (rating of perceived exertion > 17) [5]. Real-time (RT) short axis (SAX) cine images were acquired at rest and during peak exercise using highly accelerated RT cine balanced steady state free precession sequences (Table 1) and reconstructed using inline compressed sensing [6]. SAX cines at rest and peak exercise were randomized and scored for overall image quality by an expert reader using a 4 point Likert scale (1=poor, 2=acceptable, 3=good, 4=excellent). Cardiac function was analyzed using SuiteHeart (Neosoft LLC, Pewaukee, WI) at rest and peak exercise. Cardiac function and image quality was compared between magnets at rest and peak exercise using paired t-tests.

Results: 9 subjects participated in this pilot study. 2 subjects were unable to exercise in the 3T magnet due to their knees hitting the bore but were able to exercise in the 0.55T; all data is presented with 7 subjects (4male/3female; age: 32±8yrs; body mass index: 25± 3kg/m2). There was no difference in resting HR (p=0.28) or cardiac output (CO) (p=0.27) between the 3T (CO: 5.1±1.5 L/min; HR: 68±8bpm) and 0.55T (CO: 5.6±1.5 L/min; HR:71±9bpm). Greater peak HR was achieved on the 0.55T magnet (170±8bpm) compared to the 3T (164±10bpm; p=0.058) (Fig 1). Resting RT cine quality was scored similarly between 0.55T (3.4±0.5) and 3T (3.9±0.4) (p=0.60). While still scored as “acceptable”, image quality was worse at peak exercise on the 0.55T (1.9±0.4) compared to 3T (3.1±0.6; p=0.001) (Fig 2). Image quality at 0.55T was notably reduced on basal SAX images during exercise due to flow artifacts that worsened at high heart rates, likely caused by longer TE and TR necessitated by slower gradients.

Conclusion: EXCMR is feasible on a low-field ultrawide bore magnet with acceptable image quality, while better accommodating a wider range of patient body habitus. Increased exercise range of motion resulted in greater peak HR.

Figure 1. Within subject heart rates at peak exercise on the ultrawide low field magnet and standard sized higher field magnet. Solid lines represent individual subjects. The dotted line represents group averages on each magnet.

Figure 2. Real time cine images acquired during peak exercise on standard sized 70cm 3T (182 bpm) and ultrawide 80cm 0.55T (178bpm) systems at end-diastole and end-systole.