Cardiorespiratory (CR) synchronization is routinely effected in short TR sequences using retrospective gating techniques, in which multiple repeats of each phase (or projection) encoded line are acquired over the cardiac and respiratory cycles.[1-3] Data are ordered using reference signals, sometimes obtained for the MRI data, prior to reconstruction[2]. This scan mode infers that fewer than one unique phase encoded line can be acquired per heartbeat, and it commonplace to include data acquired during the respiratory motion[3]. We demonstrate, in the context of balanced Steady State Free Precession (b-SSFP) imaging that ordered k-space segmentation in conjunction with prospective cardio-respiratory synchronization and maintenance of the steady state magnetization allows a significant reduction in imaging time, compared to existing techniques, whilst maintaining image fidelity.MRI was performed at 4.7 T (Agilent VNMRS), using a 25 mm quadrature birdcage coil (Rapid Biomedical). CR-synchronised b-SSFP scans were performed volumetrically, using hard pulse excitation, at an isotropic resolution of 250 microns in times of ca. 240 s. Acquisitions were repeated with and without RF pulse phase alternation to allow production of MIP images with reduced banding due to poor shim quality. The scan timing and gating schematic is shown in Fig. 1. Following receipt of a CR control signal one segment of an 8-step, centre-out phase encoded bSSFP scan was acquired (TE=1.25, TR=2.5, FA=20, bandwidth=178 kHz, 192 read points, 96x96 phase encode steps), The scan then entered a steady state maintenance loop in which RF pulses and evaluations of the state of the CR control signal were repeated at the same TR as for the imaging segment but without application of the imaging gradients (as these were not necessary). This loop was terminated by receipt of the next CR control signal after which the next k-space segment was acquired, and this was repeated until 12 segments had been successfully acquired. Heartbeats occurring during the respiratory motion were not used for imaging and the segments from the 2 heartbeats preceding any breath were reacquired immediately following the same breath in order to minimize respiratory motion artifact. The digital CR-gating control signals were generating using custom-made timer switches interfaced to a gating digitizer (Biopac DTU-200), and acquisition of each centre-out k-space segment commenced within one TR of detection of the r-wave. ECG was detected using subcutaneously implanted electrode needles placed in the left forelimb and right flank, and respiration was monitored pneumatically. 4 black-6 mice were anaesthetized using isoflurane in 30% oxygen/70% air giving a respiration rate of 40-60 breath/minute and a heart rate of ca. 450 beats/minute and temperature was maintained at 35-37 deg C for the duration of the scan sessions before recovery.Coronal views through the bodies of 4 living mice are presented in Fig. 2, and showed bright blood in the vena cava and liver blood vessels (due to inflow) and ‘T₂’-weighted intensities for the other organs, including the heart. The gall bladder showed bright due to its fluid-based long T₂. The myocardium showed with a low degree of motion-related blurring or ghosting, as expected from the CR-gated and k-space ordered 20 ms long acquisition frame (equivalent to that cited as acceptable for retrospective gating)[2]. The CR-gated, 8-line k-space segmentation used allowed the scan to operate over 8 times faster than possible with retrospective gating for equivalent spatial encoding, as the need to acquire the same k-space line over more than one heartbeat was removed. Gradients were not required during the steady-state maintenance loop due to the zero-integral of the b-SSFP gradient play for this volumetric acquisition mode. R-wave detection was therefore very straightforward as there was no corruption of the ECG waveform. That said this technique has also been applied equally successfully when gradients are applied during the steady state maintenance module for FLASH, FISP and PSIF scan modes. Increasing the number of k-space line acquired per heartbeat reduces scan time but increases motion-related intensity corruption in these areas.

Steady state maintenance can be achieved in conjunction with CR synchronisation in order to allow motion-desensitised imaging of the moving mouse heart and lung. The scan described allowed a significant reduction in imaging time compared to that achievable with retrospective techniques as multiple k-space lines can be acquired per heartbeat, and this was demonstrated for b-SSFP imaging of the mouse thorax.