The longitudinal relaxation time T₁ is an important parameter to characterize myocardial morphology¹ and to assess functional parameters² with cardiovascular magnetic resonance (CMR). Since studies in murine models are of great interest, the development of robust T₁ measurements adjusted to small rodents is important. However, cardiac inversion recovery (IR) measurements are challenging due to the very rapid heart and respiratory rate and require a synchronization of the measurement with the cardiac and respiratory motion. The most common cardiac synchronization technique uses the signal of an electrocardiogram (ECG). At ultrahigh fields, however, the ECG signal can be hampered by the rapidly switching readout gradients³. As alternative approach a self-navigated cardiac T₁ mapping technique using a radial inversion recovery fast low angle shot (IRSF) sequence was proposed⁴. The introduced technique allows for the extraction of self-navigation signals even from highly dynamic background signals. However, as radial imaging technique the cardiac synchronization signal and the reconstructed T₁ maps can be prone to trajectory errors caused by B₀ inhomogeneities and gradient delays and also be hampered by flow artifacts. The use of shorter echo times would reduce this susceptibility and hence would result into more robust self-navigation⁵. In this work a self-navigated ultra short echo time (UTE) IR sequence is introduced for self-navigated cardiac T₁ mapping. Using this UTE sequence the echo time could be reduced to 0.57 ms, which results into a more stable self-navigation signal and into a better image quality. All measurements are carried out on a 7 T small animal system with a 470 mT/m gradient system and a 35 mm birdcage coil. A 2D-UTE IR sequence was realized by synchronizing the start of the acquisition with the ramping of the readout gradients. An echo time of 0.57 ms (middle of excitation pulse to start of acquisition) was achieved with a gaussian-shaped excitation pulse (duration: 0.2 ms). The repetition time was TR=2.5 ms and the sampling bandwidth was 75.6 kHz. A set of 20 global inversions with 3200 golden ratio readouts, respectively, was acquired. The total scan time was 5.8 minutes. For reconstruction the trajectory was measured in a separate experiment⁶. For comparison T₁ was also measured with a retrospectively triggered radial IRSF sequence using the following parameters: TR/TE = 3.0/1.1 ms, 3200 readouts per inversion, echo position: 25% and a sinc pulse (duration: 0.4 ms) for excitation. After the measurement self-navigation signals were extracted from the center k-space signal using the algorithm described previously⁴. Projections were selected according to their cardiac phase and the inversion time and were re-gridded to an isotropic spatial resolution of 234x234 µm2 (FOV 30x30 mm2, slice thickness: 1.5 mm, zero-filling to 117x117 µm2). Steady state cines and T₁ maps were obtained from both data sets. Radial IRSF and UTE IR data were obtained in a healthy mouse (cardiac period: 110±3 ms). In order to test the capability of the UTE sequence no emphasize was put on a good B₀ homogeneity. Figure 1 displays the temporal deviations of the trigger time stamps determined with the self-navigation signal relative to an additionally monitored ECG reference signal. In the IRSF measurement the self-navigation signal was hampered by trajectory deviations caused by susceptibility differences and gradient imperfections, which lead to a broadened distribution of trigger points (Fig. 1a). In the UTE IR measurement (Fig. 1b) the deviations are 37% smaller and more in range with the expected deviations due to natural heart rate variations (± 5ms). Figure 2 shows end-diastolic cine frames (top) and the corresponding cardiac T₁ maps (bottom) for both measurements. Note that in the left ventricle (green arrow) and in a vessel inside the lung (red arrow) signal cancellations caused by flow are noticeable, which are almost eliminated in the UTE measurement on the right-hand side. Moreover, more blurring is present in the IRSF measurement due to the less accurate self-navigation. Flow artifacts (black arrow) and blurring are also present in the T₁ map determined with the radial measurement while in the UTE measurement both ventricles can be clearly distinguished from the myocardium.Self-navigated radial acquisitions provide a potential alternative to ECG-synchronized measurements and enable "wireless" cardiac T₁ mapping without the need of additional navigator echoes, which would prolong the echo time. However, the extracted cardiac synchronization signal and the reconstructed images can be susceptible to B₀ inhomogeneities and flow. Using an UTE IR sequence this influence could be significantly reduced and the extraction of more accurate self-gating signals achieved. This information was used for the reconstruction of more robust cardiac T₁ maps.