Feasibility of Real-Time Radial Tagging at 0.55T Using bSSFP and Spiral Readouts

Background: Low-field MRI offers reduced cost, improved accessibility, and fewer susceptibility artifacts, making it an appealing platform for cardiac imaging. However, myocardial tagging at low field is challenged by short T₁ relaxation and lower signal strength, which diminish tagline visibility and persistence. Radial tagging—well suited to the geometry of the left ventricle (LV)—has not previously been demonstrated at low field. This study evaluates the feasibility of performing real-time radial myocardial tagging at 0.55T using balanced steady-state free precession (bSSFP) and spiral readouts.

Methods: A triggered radial tagging sequence was implemented on a 0.55T MRI scanner (prototype MAGNETOM Aera, Siemens Healthineers) equipped with high-performance shielded gradients (45mT/m amplitude, 200T/m/s slew rate) using the RTHawk platform (Vista.ai). Radial taglines were generated by spatial modulation of magnetization [1] right after each ECG trigger and followed by a bSSFP readout with 28 interleaved spiral trajectories (TR/TE/Tread = 5.43/1.6/2.55 ms; ~150 ms per frame). Imaging was performed in phantoms and 5 healthy volunteers using a 6-channel body coil (superior) and 6 channels from a 12-channel spine array (anterior). Reconstruction used conventional gridding with coil sensitivity-based sum-of-squares combination. Tagline persistence and visibility were assessed qualitatively across time frames. Contrast-to-noise ratio (CNR) was estimated at early systole and compared with published 3T tagging data.

Results: Taglines appeared sharp and continuous with minimal motion artifact and were visible for ~400 ms—sufficient to capture systolic deformation and early recoil. The spiral acquisition minimized motion blur and off-resonance artifacts. Figure 1 shows a representative short-axis time series with radial taglines originating from the LV center, demonstrating myocardial motion and tagline fading. Initial CNR values were consistently greater than 10, which, while lower than prior measurements on 3T (~17) [2], remains notable given the significantly reduced signal strength at 0.55T. Qualitative comparisons with 3T images showed similar tagline geometry and visibility.

Conclusion: This study demonstrates the feasibility of real-time radial myocardial tagging at 0.55T using SSFP and spiral readouts. Taglines persisted long enough to capture key systolic deformation, and early CNR was comparable to prior 3T studies. These findings highlight the potential of low-field MRI as a cost-effective, accessible platform for functional cardiac imaging.

(a) Sequential short-axis images acquired at 0.55T in a healthy volunteer using radial myocardial tagging. Radial taglines originate from the center of the left ventricle (LV) and remain clearly visible across early systole. The series demonstrates myocardial deformation and progressive tagline fading over time. (b) Representative radial tagging image from a prior 3T study, demonstrating similar tagline visibility and pattern. Used here as a qualitative benchmark.