Respiratory navigation with the pencil-beam excitation technique^{1, 2} is widely used in abdominal MRI. Oscillating gradient pulses are applied during the pencil-beam excitation for two-dimensionally selective excitation, causing large acoustic noise. The purpose of this work was to develop and demonstrate the feasibility of a silent navigator technique with whole volume excitation for respiratory motion detection and correction.

Navigator Pulse Sequence and Signal Processing: A non-selective hard RF pulse was used for excitation without any gradient pulses for not generating high levels of acoustic noise (Fig. 1a), resulting in whole volume excitation. A k-space center (DC) navigator signal acquisition followed the excitation without a read-out gradient pulse, which has been demonstrated in 2D self-navigated imaging previously. ³ For respiratory motion detection with DC signals from the whole volume, DC signals only from the superior half of the coil elements were combined (Fig. 1b). Superior coil elements were chosen because their sensitivities cover the region where the liver volume changed greatly by respiratory motion. Superiority of the coil elements were determined by a low resolution one-dimensional profile acquisition scan performed prior to the actual respiratory navigator signal acquisition. Magnitude of the combined DC signal was used as the final navigator signal for respiratory motion detection. We refer to this navigator with the whole volume excitation RF pulse as vNav technique.

Data Acquisition: We performed all experiments on GE 3 T MR imaging systems. The A-weighted continuous equivalent sound pressure level (LAeq) was measured for background, the conventional pencil-beam navigator (pNav), and vNav scans using a Bruel & Kjaer hand-held analyzer type 2270. A microphone mounted in the isocenter of the magnet was connected to the analyzer. Each noise measurement continued for 30 seconds without a receiver RF coil and a phantom. In the pNav and vNav scan, the navigator sequence was repeated every 100 ms. Free-breathing volunteer scans were performed with a 32-channel body coil. For comparison of the navigator signals to the bellows signals, the vNav sequence was repeated every 100 ms without imaging for 30 seconds. Imaging with motion correction was performed using navigator-gated coronal 3D-SPGR scan^4,5 with the pNav and vNav techniques. Non-gated 3D-SPGR scan was also conducted. Imaging parameters in the 3D-SPGR volunteer scan included: parallel imaging using ARC with an acceleration factor of 2 × 1, TR/TE = 4.2 ms/1.9 ms, slice thickness = 4.0 mm, 34 slices, FOV = 44 × 44 cm, matrix = 320 × 224, NEX = 0.71, receiver bandwidth = ±166.7 kHz and flip angle = 12°.

The measured LAeq values of background, pNav and vNav were 68.59 dB(A), 101.97 dB(A) and 68.99 dB(A), respectively, which shows that vNav reduced acoustic noise to almost the same level as background. Figure 2 shows the waveforms of the vNav signals processed with different coil combination methods. All the waveforms were well correlated with the bellows signal, but the signals combined from the upper channels (Fig. 2b) had larger signal variations responding sensitively to the respiratory motion. Navigator gated 3D-SPGR images showed less motion-related artifacts for both pNav (Fig. 3b) and vNav (Fig. 3c) than the non-gate 3D SPGR image (Fig. 3a).We have demonstrated that the vNav technique enabled silent respiratory navigation and reduced motion artifacts in free-breathing 3D-SPGR imaging. The vNav integration into the silent imaging sequence such as soft-gradient scan⁶ should be examined in the next step.