Evaluation of a Portable Doppler Ultrasound Gating Device for Fetal Cardiac MR Imaging: Initial Results at 1.5T and 3T

Abstract

Purpose: Fetal cardiac MRI has the potential to play an important role in the assessment of fetal cardiac pathologies, but it is up to now not feasible due to a missing gating method. The purpose of this work was the evaluation of Doppler ultrasound (DUS) for external fetal cardiac gating with regard to compatibility, functionality, and reliability. Preliminary results were assessed performing fetal cardiac MRI.

Methods: An MRI conditional DUS device was developed to obtain a gating signal from the fetal heart. The MRI compatibility was evaluated at 1.5T and 3T using B₁ field maps and gradient echo images. The quality and sensitivity of the DUS device to detect the fetal heart motion for cardiac gating were evaluated outside the MRI room in 15 fetuses. A dynamic fetal cardiac phantom was employed to evaluate distortions of the DUS device and gating signal due to electromagnetic interferences at 1.5T and 3T. In the first in vivo experience, dynamic fetal cardiac images were acquired in four-chamber view at 1.5T and 3T in two fetuses.

Results: The maximum change in the B₁ field and signal intensity with and without the DUS device was <6.5% for 1.5T and 3T. The sensitivity of the DUS device to detect the fetal heartbeat was 99.1%. Validation of the DUS device using the fetal cardiac phantom revealed no electromagnetic interferences at 1.5T or 3T and a high correlation to the simulated heart frequencies. Fetal cardiac cine images were successfully applied and showed good image quality.

Conclusion: An MR conditional DUS gating device was developed and evaluated revealing safety, compatibility, and reliability for different field strengths. In a preliminary experience, the DUS device was successfully applied for in vivo fetal cardiac imaging at 1.5T and 3T.

Keywords: Doppler ultrasound; cardiovascular system; fetal heart; magnetic resonance imaging.

Fig. 1

(A) Used Doppler ultrasound (DUS) system with the ultrasound transducer (1) connected to the transmission line containing the cable traps, which are covered by a surrounding isolating sleeve (2). The ultrasound transducer is finally connected to the DUS device (3). The application of the DUS device to derive a fetal cardiac gating signal is shown schematically in (B). The transducer is placed above the fetal heart and is connected with the DUS electronic via a cable including cable traps to attenuate radiofrequency (RF) interferences. After signal acquisition, the signal processing algorithm is outlined schematically. The gating output is finally connected to the physiologic unit of the MRI.

Fig. 2

The setup and dimensions of the used biventricular heart phantom are shown in (a–c). An example of the measured Doppler ultrasound signal from the moving heart phantom mimicking fetal heart movement is shown in (d) with corresponding detected gating signals (BPM = 118).

Fig. 3

Shown are coronal B₁ flip angle maps and axial gradient echo images at 1.5T (upper row) and at 3T (bottom row) for cases with transducer inside the scanner (dashed line) on top of the phantom in (b) and (f) and without transducer inside the magnetic resonance scanner in (a) and (e). No effects on B₁ field homogeneity due to the cable or transducer are visible for 1.5T or for 3T as shown in the image difference in (c) and (g). No geometric distortions or signal loss due to common-mode currents were visible with the transducer inside the MRI using Gradient echo images for 1.5T (d) and for 3T (h).

Fig. 4

Shown are exemplary steady-state free precession (SSFP) images of the fetal cardiac phantom at 1.5T (upper row) and at 3T (bottom row). Images during mid-diastole are shown in (a) and in (c) with their corresponding projection over one whole cardiac cycle in (b) and (d).

Fig. 5

The results of the Doppler ultrasound (DUS) gating validation using the heart phantom during MRI acquisition for heart rates ranging from 75 to 200 BPM is shown in (a) and (b), for the in vivo validation outside of MRI in (c) and (d). The dashed lines of the Bland Altman plot in (b) and (d) represent the confidence interval of ±1.96 of the standard deviation.

Fig. 6

Exemplary the Doppler ultrasound (DUS) signal during MRI acquisition for one fetus at 1.5T. The dashed lines represent the detected gating time points of DUS device which are sent to the physiologic unit of the MRI scanner for gating.

Fig. 7

Preliminary fetal cardiac steady-state free precession (SSFP) cine images in four-chamber view for 1.5T (upper row) and 3T (bottom row). Gated images are shown in diastole in (a) and (d), for systole in (b) and (e). The corresponding projection over one cardiac cycle is shown in (c) and (f).