Mapping longitudinal relaxation times in 3D is a promising quantitative and non-invasive imaging tool to assess cardiac remodeling. Few methods are proposed in the literature allowing us to perform 3D T1 mapping. These methods often require long scan times and use a low number of 3D images to calculate T1 . In this project, a fast 3D T1 mapping method using a stack-of-spirals sampling scheme and regular RF pulse excitation at 7 T is presented. This sequence, combined with a newly developed fitting procedure, allowed us to quantify T1 of the whole mouse heart with a high spatial resolution of 208 × 208 × 315 µm(3) in 10-12 min acquisition time. The sensitivity of this method for measuring T1 variations was demonstrated on mouse hearts after several injections of manganese chloride (doses from 25 to 150 µmol kg(-1) ). T1 values were measured in vivo in both pre- and post-contrast experiments. This protocol was also validated on ischemic mice to demonstrate its efficiency to visualize tissue damage induced by a myocardial infarction. This study showed that combining spiral gradient shape and steady RF excitation enabled fast and robust 3D T1 mapping of the entire heart with a high spatial resolution.

The purpose of this study was to demonstrate the feasibility of steady-state True fast imaging with steady precession (True-FISP) four-dimensional imaging of mouse heart at high resolution and its efficiency for cardiac volumetry. Three-dimensional cine-imaging of control and hypoxic mice was carried out at 4.7 T without magnetization preparation or ECG-triggering. The k-space lines were acquired with the TrueFISP sequence (pulse repetition time/echo time ‫؍‬ 4/2 ms) in a repeated sequential manner. Retrospective reordering of raw data allowed the reconstruction of 10 three-dimensional images per cardiac cycle. The acquisition scheme used an alternating radiofrequency phase and sum-of-square reconstruction method. Black-blood three-dimensional images at around 200 m resolution were produced without banding artifact throughout the cardiac cycle. High contrast to noise made it possible to estimate cavity volumes during diastole and systole. Right and left ventricular stroke volume was significantly higher in hypoxic mice vs controls (20.2 ؎ 2 vs 15.1 ؎ 2; P < 0.05, 24.9 ؎ 2 vs 20.4 ؎ 2; P < 0.05, respectively). Owing to the many possibilities offered by transgenic and surgical approaches, numerous models of cardiovascular diseases have been developed in mouse. This provides not only a better understanding of physiology but is also of great use in preclinical trials, e.g., gene therapy or pharmacological treatment.MRI has become a reference noninvasive tool to characterize variations in structure and function of mouse heart. Until now, most of the MR studies on mouse models were performed with two-dimensional white-blood cine-imaging combined with cardiac triggering for MR acquisition synchronization (1-4). Parameters like end-systolic and end-diastolic volumes can be measured in order to evaluate both stroke volume and ejection fraction. Nevertheless, routine MRI techniques suffer from many drawbacks. First of all, two-dimensional imaging results in a limited spatial resolution in the slice dimension. In fact, typical out-ofplane resolutions are in the range of 1 mm, whereas mouse heart does not exceed 10 mm in the long axis. This could result in measurement errors due to a partial volume effect.To overcome this issue, some authors have shown interest in performing four-dimensional (4D) (three-dimensional [3D]-cine) imaging (5-7). Feintuch et al. (6) demonstrated the feasibility of 4D mouse imaging with an isotropic spatial resolution of 200 m. Nevertheless, scan times were very long (between 80 and 120 min) for bright blood imaging. More recently, Bucholz et al. (7) presented impressive images with exceptional resolution (80 m), but injection of a nonconventional liposomal gadolinium contrast agent was required. The other drawback of such experiments is that, in many cases, images are acquired with a bright blood contrast. It is now widely accepted that MR sequences with suppression of the blood signal (black or dark blood sequence) are more appropriate to reduce interobserver variability in the drawing of epicar...

BackgroundTo show that 3D sequences with ultra-short echo times (UTEs) can generate a positive contrast whatever the magnetic field (4.7, 7 or 9.4 T) and whatever Ultra Small Particles of Iron Oxide (USPIO) concentration injected and to use it for 3D time-resolved imaging of the murine cardiovascular system with high spatial and temporal resolutions.MethodsThree different concentrations (50, 200 and 500 μmol Fe/kg) of USPIO were injected in mice and static images of the middle part of the animals were acquired at 4.7, 7 and 9.4 T pre and post-contrast with UTE (TE/TR = 0.05/4.5 ms) sequences. Signal-to-Noise Ratio (SNR) and Contrast-to-Noise Ratio (CNR) of blood and static tissus were evaluated before and after contrast agent injection. 3D-cine images (TE/TR = 0.05/3.5 ms, scan time < 12 min) at 156 μm isotropic resolution of the mouse cardiopulmonary system were acquired prospectively with the UTE sequence for the three magnetic fields and with an USPIO dose of 200 μmol Fe/kg. SNR, CNR and signal homogeneity of blood were measured. High spatial (104 μm) or temporal (3.5 ms) resolution 3D-cine imaging (scan time < 35 min) isotropic resolution were also performed at 7 T with a new sequence encoding scheme.ResultsUTE imaging generated positive contrast and higher SNR and CNR whatever the magnetic field and the USPIO concentration used compared to pre-contrast images. Time-resolved 3D acquisition enables high blood SNR (66.6 ± 4.5 at 7 T) and CNR (33.2 ± 4.2 at 7 T) without flow or motion artefact. Coronary arteries and aortic valve were visible on images acquired at 104 μm resolution.ConclusionsWe have demonstrated that by combining the injection of iron nanoparticles with 3D-cine UTE sequences, it was possible to generate a strong positive contrast between blood and surrounding tissues. These properties were exploited to produce images of the cardiovascular system in small animals at high magnetic fields with a high spatial and temporal resolution. This approach might be useful to measure the functional cardiac parameters or to assess anatomical modifications to the blood vessels in cardio-vascular disease models.Electronic supplementary materialThe online version of this article (doi:10.1186/s12968-015-0167-4) contains supplementary material, which is available to authorized users.