Real-time flow assessment: From model to patients

Background In stratification of heart valve diseases blood flow assessment often plays a key role. When echocardiography struggles, phase contrast magnetic resonance imaging (PC-MRI) may be considered as an alternative (Srichai et al. AJR 2009). Arrhythmias are a major limitation of conventional segmented PC-MRI (SEG). Real-time sequences (RT) could overcome it. The purpose of this study is to evaluate RT in a flow model as well as in volunteers and patients. We hypothesize to measure equal velocities and flow compared to SEG as reference in sinus rhythm and aimed to show feasibility in atrial fibrillation (Afib).


Background
In stratification of heart valve diseases blood flow assessment often plays a key role. When echocardiography struggles, phase contrast magnetic resonance imaging (PC-MRI) may be considered as an alternative (Srichai et al. AJR 2009). Arrhythmias are a major limitation of conventional segmented PC-MRI (SEG). Real-time sequences (RT) could overcome it. The purpose of this study is to evaluate RT in a flow model as well as in volunteers and patients. We hypothesize to measure equal velocities and flow compared to SEG as reference in sinus rhythm and aimed to show feasibility in atrial fibrillation (Afib).

Methods
In a flow model (I), volunteers and prospectively enrolled patients (II) we compared a highly accelerated RT (temp. res. 40 ms, TE 5.6 ms, ETL 7, T-PAT 3×, matrix 128×104px) using shared velocity encoding (Lin et al. MRM 2009) with SEG (temp. res. 48 ms, TE 2.3 ms, ETL 5, I-PAT 2×, matrix 192×156px) on a 1.5 T scanner (Avanto, Siemens Healthcare, Germany) with a 12 channel cardiac coil. I The model generated adjustable constant flow. 81 PC images were acquired 25 mm from an interchangeable aortic stenosis-like narrowing with different areas (0.6 cm2, 1.3 cm2, 2.0 cm2) perpendicular to tube running ( Figure 1). II In vivo studies were measured at sinotubular junction perpendicular to the aorta. We quantified (cvi42, Circle CVI, Canada) with equalized voxel size: in I mean velocity and flow in II mean peak velocity, stroke volume and regurgitation fraction In patients with Afib only RT was applied.

Conclusions
The evaluated real-time PC sequence can access flow reliably and in good correlation to a conventional segmented version in model experiments, volunteers and patients. Hence it might become an useful alternative to doppler echocardiography in arrhythmic patients.

Funding
Funded by the general research budget of the working group.

Figure 2
In vivo results: RT shows good correlation to SEG regarding peak velocity (a), stroke volume (b) and regurgitation fraction (c). Stray bullets had at least moderate aortic valve stenosis. On Afib RT shows frequency-dependent variability of stroke volumes (d).