Unlike the adult mouse heart, the neonatal heart has been reported to regenerate following myocardial injury during the few days after birth¹. With ischaemic heart disease being a leading cause of death in the developed world, research into the mechanisms of cardiac regeneration has been actively pursued with the aim to translation to the clinic². In the mouse, this regenerative ability is lost by one week of age¹. In order to dissect the molecular and genetic basis of cardiac regeneration, the ability to quantify lesion size and rate of resolution is critical. MRI has long been established as the method of choice for non-invasive functional imaging of the heart in adult mice, but for neonates only limited work has attempted to assess cardiac function in three-day old mice^3,4. This study aimed to establish the framework for reproducible and reliable functional imaging of hearts in newborn (i.e. one-day old, P1) mice, with successful recovery. Such a method would permit longitudinal imaging of myocardial injury models allowing monitoring of the process of myocardial regeneration.Neonatal wild type mice (CD1, N=5) were imaged at P1. General anaesthesia was induced using 4% isoflurane in oxygen and the animal placed prone in a custom designed, 3D printed, 12.5 mm animal cradle (Figure 1). Animal physiology was maintained using ~2% isoflurane in oxygen and a heated air blanket placed over the back. Respiration was monitored using a pressure transducer placed beneath the abdomen. All imaging was conducted using a 9.4 T horizontal bore scanner with Direct Drive2 console and 60 mm i.d., 1 T/m, shielded gradient set (Agilent Technologies). A 13 mm i.d. quadrature driven birdcage resonator (Rapid Biomedical) was used for signal transmission/reception. Following scouting and shimming, short axis, retrospectively gated, 2D gradient echo multi-slice cine images were acquired covering the entire left ventricle (2x compressed sensing acceleration)^5,6. Imaging parameters were: matrix size 192x192, 10˚ flip angle, TR/TE=4.6/1.9 ms, 50 frames, 6 repetitions (28800 signals). Field of view was 15x15 mm with slice thickness of 0.5 mm. Total acquisition time including scouting and shimming was ~30 minutes. The mice were recovered and the entire litter returned to the mother following imaging. Twenty frames per slice were reconstructed off-line and subjected to image analysis.The neonatal mice recovered rapidly following imaging with all animals surviving the imaging procedure. Cine images could be successfully reconstructed for all mice with full ventricular coverage (~6-7 slices). Image contrast and resolution were sufficient for manual segmentation, and derivation of functional parameters (Figure 2; Table 1).To the best of our knowledge, this is the first report of successful cardiac functional imaging of P1 mice with recovery. Cardiac MR imaging of neonatal mice with recovery has been reported for P3, although ECG electrodes were placed in the forelimbs in order to enable cardiac gating and the mice were not recovered^3,4. As the ability to regenerate the myocardium following injury is lost between 4 and 7 days after birth, imaging and recovering the mouse in the first 24 to 48 hours following birth is desirable in order to study the process and rate of myocardial regeneration. By employing retrospectively gated, compressed sensing accelerated imaging, the scan time has been reduced sufficiently to allow easy recovery of the animals following general anaesthesia without compromising spatial resolution. The use of retrospectively gated imaging means that ECG electrodes are not required, thereby preventing the potential injury to the forelimbs of the mouse and aiding recovery. The diameter of the RF coil was chosen to be as small as was practical to fit the neonatal mouse, anaesthetic nose cone, respiratory monitor and heating in order to maximize the signal to noise ratio. Extension of this work to longitudinal imaging of neonatal mice following myocardial infarction will allow the process of cardiac regeneration to be monitored and quantified and ultimately to shed new light onto myocardial regeneration.These proof of concept data demonstrate that cardiac functional MR imaging with recovery in newborn mice is practical. The methods used in this study offer the possibility to investigate the process of myocardial regeneration, and loss of regenerative ability, during the first days of life.