## 1 Introduction
#### 2.2.3 Image-Based Navigation  
In image-based navigation single 2D [57,58], orthogonal 2D [59], or 3D [60] real-time images are acquired in every heart-beat prior to CMRA acquisition. The main advantage of this approach is that the moving heart can be spatially isolated from surrounding static tissues, which allows for improved respiratory motion estimation. Furthermore, image-based motion compensation has intrinsically more degrees of freedom for motion correction, which is not limited just to translational motion in several directions but also includes rotation and non-rigid motion correction. A potential risk of using image-based navigation is that the navigator acquisition may saturate signal or cause artifacts in the subsequent and spatially overlapping CMRA acquisition. One solution to this problem is to use spectrally selective RF pulses for the navigator to image and estimate motion from the epicardial fat, while the CMRA acquisition relies on water signal from blood and tissues [61]. An alternative is to use a so called trailing navigator, where the navigator image acquisition is performed after the CMRA acquisition in the cardiac cycle. However, this approach is incompatible with prospective respiratory gating and correction. The feasibility of prospective image-based motion correction has recently been demonstrated [57,58], although compared to 1D navigation such as the diaphragmatic navigator or self-navigation the increased computational complexity of multi-dimensional image reconstruction, registration and correction makes real-time implementations more challenging. A solution to this problem could be to implement the navigator image post-processing on graphics processing units [62,63] as most of the steps can be parallelized. However, even more crucial for accurate motion estimation using image-based navigation is the close temporal proximity of the navigator with the CRMA sequence to ensure good correlation between the estimated motion at the time of the navigator acquisition and the actual motion during the CMRA acquisition. [PERSON] _et al._ found that a short time delay between the navigator acquisition and the imaging, _i.e._, temporal proximity, was critical to minimize motion artifacts in high resolution MRI, whereas the navigator spatial resolution was of minor importance [64]. The main drawback of image-based navigation compared to self-navigation is that the navigator may not be as readily extracted from the image acquisition which means that additional scan planning is necessary to define the navigator image location. To combine the advantages of self-navigation with those of image-based navigation, an approach has been proposed whereby the navigator signal is acquired using the startup echoes [65] of a balanced steady-state free precession CMRA sequence [66]. In this approach, the navigator has nearly identical imaging parameters including location and orientation (self-navigation) while still generating a 2D image, by spatially encoding the startup echoes by adding phase encoding gradients (image-based navigation), as shown in Figure 4. Another image-based self-navigation approach divides the respiratory cycles into several states (so called bins), whereby undersampled radial CMRA data from several cardiac cycles but the same respiratory bin can be reconstructed and used to estimate respiratory motion between bins [67,68]. Similar approaches, using respiratory binning, have been implemented for Cartesian sampling as well. However, in these approaches the respiratory motion estimation is based on data acquisition performed in addition to the CMRA data [11,69]. All binning approaches require an initial motion estimation to define the respiratory bin for each cardiac cycle, typically with the d1D NAV, and thereafter the respiratory motion is estimated from the heart motion between respiratory bins. **Figure 4.** Sequence diagram of two-dimensional image-based self-navigation (2D NAV) using the startup echoes of a 3D balanced Steady-State Free Precession (bSSFP) sequence (**a**).