Mixed echo train acquisition displacement encoding with stimulated echoes : An optimized DENSE method for in vivo functional imaging of the human heart

Cardiac functional imaging based on displacement encoding with stimulated echoes (DENSE) has been proposed as a high-resolution method for evaluating cardiac contractility (1–5). This was first combined with a segmented echo-planar imaging readout (fast-DENSE), and its application has been demonstrated with normal volunteers (6). However, two issues arose with respect to utilizing this methodology for routine patient cardiac functional evaluation. First, the signal-to-noise ratio (SNR) of the images acquired was insufficient for encoding the entire systolic interval. Specifically, only the last 100–150 ms of systole were captured for functional evaluation. Second, the breath-holds required for such exams were on the order of 26 heartbeats. This certainly posed a problem for evaluating function in patients with myocardial infarction. The solution to both issues is presented in this work. It involves a new way of sampling the stimulated echo (STE) signal and improved resolution by suppressing the FID signal. This method, mixed echo train acquisition DENSE (meta-DENSE), is applied successfully in both normal volunteers and patients to quantitatively evaluate cardiac function at high resolution in situ. In our previous work with fast-DENSE, the duration of the encoding interval and data acquisition speed were both constrained by the SNR, as already mentioned (Fig. 1). Since DENSE is based on STEs, there is an inherent 50% signal loss. In addition, myocardial strain alters the shape of the voxels during the encoding period. As a result, intravoxel dephasing can further lower the SNR, especially where strain is high along the longest dimension of the voxel, i.e., along the slice axis (6,7). Intravoxel dephasing signal loss is accentuated if strong TE gradient pulses are applied along the slice axis (Fig. 1). Such pulses are routinely used in stimulated echo acquisition mode (STEAM) experiments to eliminate unwanted signal contributions that can cause artifacts. The previous implementation (fast-DENSE) used a low RF flip angle segmented-EPI readout. The position-encoded magnetization was recalled in small portions onto the XY-plane and then discarded. Despite its ability to acquire data fast, the EPI-based readout is inefficient for acquiring DENSE data. Unlike other EPI applications, with DENSE the available magnetization that contains useful phase-encoded displacement information is limited. A fast readout scheme that recycles the available magnetization is more suitable for sampling the STE in DENSE experiments. One approach to this goal has been to utilize a fast-spin-echo (FSE) readout (8,9). That method yielded 3D static displacement strain images. However, there were significant artifacts with the normal FSE readout. The source of these artifacts can be traced to the mixing of the original STEAM signal with additional STEs that are created by the FSE pulse train (10). FIG. 1 Fast-DENSE image acquired with a segmented EPI readout. Note the signal loss due to intravoxel dephasing in the myocardium (arrow). This magnitude image was acquired with an encoding interval of 100 ms covering the end portion of systole. Alternative FSE readouts applied to diffusion weighting can be utilized to sample the STEAM signal. In one such approach, diffusion weighting was accomplished via the Stejskal Tanner sequence (11) and read out was performed in a single-shot via phase-insensitive rapid acquisition with relaxation enhancement (RARE) preparation (12). This preparation, via a dephasing gradient, divides the signal evenly between a Meiboom Gill (MG) (13) and an orthogonal component. The latter is then nutated onto the z-axis, where it remains during image acquisition, allowing the MG component to be imaged free of artifacts. It is important to note that if a signal has already lost its non-MG component, such a phase-insensitive RARE preparation is no longer required. One such signal is that generated by the STEAM preparation in DENSE, as will be shown. Therefore, the STEAM signal can be sampled by a train of 180° pulses with no artifacts. Another approach to sampling the STEAM signal is split acquisition of fast spin-echo signals for diffusion imaging (14), which has also utilized an FSE readout. However, SPLICE reduces artifacts by reconstructing two separate magnitude images from two echo trains (14) created by a readout gradient imbalance rather than exploiting the inherent MG properties of the STEAM signal. Indeed, the two echo trains of SPLICE can be forced to coincide with the STE and the stimulated anti-echo (STAE) (15) of STEAM and result in artifact-free images, as will be described later. In this work, the underlying cause of artifacts generated in STEAM displacement imaging with a conventional FSE readout is explained. The use of the STAE along with the conventional notion of the STE will facilitate the description process. It will be shown that the DENSE preparation is inherently compatible with a specialized readout train of 180° refocusing pulses, and as such artifact-free phase images can be obtained. The STEAM signal, which is comprised of two components, is RF-refocused so that not only is phase-contrast consistency maintained but also any additional STEs, created by imperfect 180°s during the readout, combine coherently with its two components. Additionally, an inversion-recovery nulling method will be used to eliminate unencoded water and fat signal contributions, which can cause artifacts when low encoding strengths are used in conjunction with high resolution. Finally, data acquired with this new method from normal volunteers and a patient in 14 heartbeats will be shown.