Comparison of Cardiac Output Values Calculated from Breath Held and Free Breathing 2D Phase Contrast Flow Quantification Obtained at 0.55 Tesla MRI with Two Gradient Performance Settings

Background: Mid-field MRI has been shown to be capable of producing high-quality cardiac imaging with good clinical measurement concordance. Commercial mid-field systems, however, offer lower gradient specifications than is typically used for cardiac MRI. To assess the importance of the gradient specifications for obtaining 2D phase contrast flow measurements, we compared cardiac output values calculated from breath held and free breathing 2D phase contrast flow sequences obtained at 0.55T with two distinct modes of gradient performance.

Methods: Twenty-two healthy volunteers underwent cardiac MRI on a 0.55T MRI system (investigational Free.Max, Siemens Healthineers Forchheim, Germany), with prototype high-performance gradient hardware. Software limits were used to operate gradients in two modes with High Performance Mode: Gmax = 33mT/m, Smax = 160T/m/s, and Product Mode: Gmax = 25mT/m, Smax = 40T/m/s. 2D phase contrast imaging of the ascending aorta (Qs) and pulmonary artery (Qp) (VENC 150 cm/s) was obtained using a free-breathing and a single breath held sequence at both gradient modes, resulting in 4 separate Qs and Qp assessments for each exam. Sequence parameters are provided in Table 1. Of note, whereas the acceleration factor of the free breathing protocol was the same between gradient modes, the breath-held protocol required a higher acceleration rate in Product Mode to achieve reasonable durations. Flow was subsequently quantified using commercial software (CMR Segment, Medviso AB). As there was substantial interpersonal variation in Qs measurements, we considered a linear mixed effects model with a random intercept and fixed effects: Free Breathing vs Breath Hold, and High-Performance Mode vs Product Mode. Bland-Altman analyses were also performed, and the Qp/Qs ratio was examined.

Results: Healthy volunteers were 50% female and 45% White, 36% Black and 19% Asian. The average age was 32 years (range 19-55). Qualitatively, the breath-held acquisition had lower SNR compared to free-breathing in both gradient modes whereas the free-breathing acquisitions appeared similar between gradient settings (Figure 1). There were no significant differences in the mean Qs of between the groups when accounting for the different gradients (p=0.32) or the different breathing conditions (p=0.29). Similarly, there were no significant differences in Qp/Qs ratios between the groups when accounting for the different gradients (p=0.43) or the different breathing conditions (p=0.10). Bland-Altman plots and boxplots of pairwise comparisons of Qs are shown in Figure 2.

Conclusion: Our study shows that high fidelity cardiac output and Qp/Qs measurements can be made with lower gradient specification, even with a breath-held acquisition where quality appears visually degraded. Further work is needed to assess how flows obtained at 0.55T compare with flows obtained at 1.5T, and it remains to be seen if similarly equivalent findings would be observed in patients with abnormal flows.

Sequence Parameters

	Breath Held Phase Contrast MRI		Free Breathing Phase Contrast MRI
	High-Performance Mode	Product Mode	High-Performance Mode	Product Mode
TE/TR (ms)	2.6/4.8	3.6/6.2	5.0/7.9	5.6/9.1
Flip Angle	15	15	25	25
Pixel Resolution (mm)	2.0 x 2.2	1.5 x 2.0	1.7 x 1.7	1.7 x 1.7
Slice Thickness (mm)	6.0	8.0	8.0	8.0
FOV (mm)	380	380	380	380
Base Resolution	192	256	224	224
Acceleration	2	3	2	2
Bandwidth (Hz/px)	449	561	298	298
Segments	5	5	2	2
Temporal Resolution (ms)	47.6	61.8	31.6	36.3
Averages	1	1	3	3
Scan Time (@60 bpm)	15s	11s	2m 42s	2m 42s

Comparison of Breath-Held and Free-Breathing magnitude and phase images obtained with Product and High Performance gradient modes in the same healthy volunteer.
A) Bland-Altman plots, and B) boxplots demonstrating pairwise comparisons of Qs between groups. HP = high-performance, Prod = product, FB = free-breathing, BH = breath-held.