Latest pulse sequence for displacement-encoded MR imaging incorporates essential technical improvements for multiphase measurement of intramyocardial strain.

Abstract

The article by Kim et (1) presents a significant step forward in the evolution of magnetic resonance (MR) imaging techniques to measure two-dimensional intramyocardial displacement and strain. The pulse sequence described in reference 1 is the latest in a set of sequences that have built on an innovative technique called displacement encoding with stimulated echoes (DENSE) (2). The DENSE pulse sequence directly encodes the shift in position (ie, displacement) of tissue in the phase of the transverse magnetization. The first sequence, proposed in reference 2, required a 4-minute acquisition for one end-systolic displacementencoded MR image at one section location, while this latest publication reports a breath-hold sequence using short echo train echo-planar MR imaging (fast gradient-recalled-echo echo train) (3) with a versatile artifact-suppression technique (4) that acquires a time series of 12 displacement-encoded MR images across roughly 3⁄4 of the cardiac cycle at one section location in 13 heartbeats. At this stage, a clinical evaluation of this sequence in comparison with standard tagging methods using the latest tag identification postprocessing algorithms is warranted. Displacement encoding by phase contrast is a significant innovation, on the same level as velocity encoding by phase contrast. The mechanism whereby the magnetization phase angle becomes a function of the shift in position (ie, displacement, with units of length) of the tissue is not immediately obvious. The publication on DENSE MR imaging by Aletras et al (2) described the process of displacement encoding. Transverse magnetization is initially position encoded by an encoding gradient pulse, stored temporarily as longitudinal magnetization during the “mixing time” and then refocused just prior to data acquisition by an “unencoding” gradient pulse. The unencoding gradient pulse causes the transverse magnetization to refocus at a phase angle that is linearly proportional to the displacement that occurred during the mixing time. In later publications (1,5,6), it was made more clear to the reader that the unencoding gradient pulse refocuses only half of the transverse magnetization that had been stored and that the other half of the magnetization is not refocused and destroys the displacement encoding if it is allowed to contribute signal to the data prior to Fourier image reconstruction. The encoding and unencoding gradient pulses are each given a large zeroth moment (gradient strength duration), so that the spatial frequencies of this “unrefocused” magnetization are shifted to outside the spatial frequency range of the acquired data. The unwanted signal from this magnetization is present as an oscillating current in the receiver coil, but most if not all of this signal is eliminated by band-pass analog or digital filtering of the signal during data acquisition. The use of the unencoding gradient pulse to shift the spatial frequencies, to effectively eliminate the unrefocused magnetization, and to preserve the refocused magnetization whose phase is linearly proportional to the displacement is a key innovation distinguishing DENSE MR imaging from amplitude-modulation methods, such as spatial modulation of magnetization (7). Before the development of DENSE MR imaging, spatial tagging and velocity-encoded phase imaging were used to measure myocardial motion and deformation (7–10) and allowed clinicians to establish a significant diagnostic potential for these types of measurements. Scanning options to improve image quality and new postprocessing algorithms to improve automation, speed, and reliability of the motion and deformation measurements continue to be developed. Nevertheless, DENSE MR imaging provides several fundamental technical advantages