Comparison between prospective and retrospective triggering for mouse cardiac MRI Edwin Heijman, 1 * Wolter de Graaf, 1 Petra Niessen, 2 Arno Nauerth, 3 Guillaume van Eys, 2 Larry de Graaf, 1 Klaas Nicolay 1 and Gustav J. Strijkers 1 1 Biomedical NMR, Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands 2 Department of Molecular Genetics, Cardiovascular Research Institute Maastricht (CARIM), U ¨ niversity of Maastricht, Maastricht, The Netherlands 3 Bruker BioSpin MRI GmbH, Ettlingen, Germany Received 1 August 2006; Revised 13 September 2006; Accepted 13 September 2006 ABSTRACT: High-resolution magnetic resonance imaging (MRI) has evolved into one of the major non-invasive tools to study the healthy and diseased mouse heart. This study presents a Cartesian CINE MRI protocol based on a fast low-angle shot sequence with a navigator echo to generate cardiac triggering and respiratory gating signals retrospectively, making the use of ECG leads and respiratory motion sensors obsolete. MRI of the in vivo mouse heart using this sequence resulted in CINE images with no detectable cardiac and respiratory motion artefacts. The retrospective method allows for steady-state imaging of the mouse heart, which is essential for quantitative contrast-enhanced MRI studies. A comparison was made between prospective and retrospective methods in terms of the signal-to-noise ratio and the contrast-to-noise ratio between blood and myocardial wall, as well as global cardiac functional indices: end-diastolic volume, end-systolic volume, stroke volume and ejection fraction. The retrospective method resulted in almost constant left-ventricle wall signal intensity throughout the cardiac cycle, at the expense of a decrease in the signal-to-noise ratio and the contrast-to-noise ratio between blood and myocardial wall as compared with the prospective method. Prospective and retrospective sequences yielded comparable global cardiac functional indices. The largest mean relative difference found was 8% for the end-systolic volume. Copyright # 2006 John Wiley & Sons, Ltd. KEYWORDS: MRI; heart; mouse; triggering; navigator; retrospective; self-gated INTRODUCTION Cardiac MR images without motion artefacts can only be accomplished by synchronizing the image sampling of the MRI scanner with the cardiac cycle (cardiac triggering) (1) and excluding or stopping the image sampling during the respiratory period (respiratory gating) (2). Several solutions already exist for obtaining the cardiac triggering and respiratory gating signals for cardiac MRI of small laboratory animals, such as rats and mice. The conventional way to measure the triggering and gating signals is using respiratory and electrocardiogram (ECG) sensors (3–9). The respiratory sensor is positioned on the chest of the animal to determine the respiratory motion, and typically two ECG pads are connected to the paws or the tail to measure the ECG signal of the heart. Triggering pulses are derived from the ECG signal by a computer or a dedicated ECG monitoring system. Within these devices the respiratory gating pulses, which are based on the measured respiratory motion, can also be determined and used to suppress the cardiac trigger pulses during respiratory activity (10). For human cardiac MRI, breath holding is often used to prevent displacement of the heart during scanning (11). This method, however, is not easily applicable to small rodents such as mice, because the anaesthetized animals are usually allowed to breath freely during the experiment. The measured ECG and respiratory signals can be used in two ways. Either the MRI scanner is directly triggered by the cardiac triggering pulses, the prospective way (12), or ECG and respiratory information are stored together with the MRI data, which are then used to sort and reconstruct the cardiac images after the experiment, the so-called list-mode or retro- spective triggering method (13). In addition, there is a convenient way to circumvent these ECG and respiratory motion measurements with sensors. Since any echo or free induction decay signal in an MRI sequence is sensitive to the heart and respiratory motion, which lies obviously at the basis of why synchronization between heart and respiratory motion and the MRI measurement is needed in the first place, time-dependent changes in these signals as a consequence NMR IN BIOMEDICINE NMR Biomed. 2007; 20: 439–447 Published online 22 November 2006 in Wiley InterScience (www.interscience.wiley.com) DOI:10.1002/nbm.1110 *Correspondence to: E. Heijman, Biomedical NMR, Department of Biomedical Engineering, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: e.heijman@tue.nl Contract/grant sponsor: EC FP6 project DiMI; contract/grant number: LSHB-CT-2005-512146. Contract/grant sponsor: BSIK programme; contract/grant number: BSIK03033. Copyright # 2006 John Wiley & Sons, Ltd. NMR Biomed. 2007; 20: 439–447