Evaluation of Cardiac Function

Technical Foundations: How is it done?

Orlando P. Simonetti, Ph.D.

The Ohio State University

The basic technique for ECG-triggered cine imaging of the heart was first described over 20 years ago[1]. The original prospectively-triggered spoiled gradient echo (GRE) technique, which required several minutes to acquire a single slice and was prone to respiratory motion artifact, has evolved significantly over the years. Segmented k-space cine acquisition, first described by Atkinson and Edelman in 1991[2], provided a means to trade temporal resolution for scan time by collecting several k-space lines for each cine frame in each cardiac cycle; this enabled acquisition of a complete cine series within a reasonable breath-hold. Modifications such as echo-sharing and retrospective gating [3]are commonly used to recover some of the lost temporal resolution caused by k-space segmentation. Retrospective gating has the added advantage of generating image frames across the entire cardiac cycle; this can be critical in the evaluation of diastolic function and also provides a smooth, complete cine loop that facilitates qualitative evaluation of regional myocardial wall motion.

The next major advance in cine MRI came with the replacement of GRE with the steady-state free precession (SSFP) pulse sequence [4]. Whereas GRE is primarily T1-weighted and relies on in-flow enhancement for its “bright-blood” signal,SSFP is inherently "bright-blood" as its signal is dependent (approximately) on T2/T1. SSFP requires extremely short repetition times (TR) to limit its sensitivity to field inhomogeneity and the resulting characteristic “dark-band” artifacts; therefore, while the SSFP technique was originally described in 1986[5], it did not become practical until the advent of fast gradient hardware in the late 1990’s. The very short TR of SSFP also results in higher scan efficiency, and thus shorter scan times and/or improved temporal resolution, than spoiled gradient echo. The most commonly used method for MR cine imaging remains a segmented k-space acquisition with SSFP readout, although GRE still enjoys some popularity at high field (3T and above) due to its low specific absorption rate (SAR) and relative insensitivity to field inhomogeneity.

The advantages of “real-time” imaging (no ECG synchronization or breath-holding) were recognized early on, and an echo planar (EPI) sequence for real-time cine imaging of the heart was described as far back as 1987 [6,7]. Single-shot EPI of the heart has never proven to provide reliable image quality, but the combination of fast SSFP readout with parallel imaging methods (i.e., SENSE or GRAPPA) has made real-time imaging practical on most modern MRI systems. While still requiring some sacrifice of spatial and temporal resolution relative to segmented k-space sequences, real-time imaging is feasible in practically any patient. New methods applying alternative k-space trajectories and reconstruction strategies are constantly improving the spatial and temporal resolution achievable with real-time imaging; undoubtedly, real-time will ultimately replace segmented k-space as the standard cine acquisition technique.

While conventional cine images are often utilized to quantitatively measure global cardiac function parameters (ejection fraction, stroke volume, cardiac output, etc.), a number of variations on cine imaging have been developed and employed to facilitate quantitative evaluation of regional cardiac function. Myocardial tagging [8], HARP [9], DENSE [10], and phase contrast MRI are all used to quantify myocardial motion and to derive myocardial strain and strain rate. While these techniques have not been widely adopted for clinical imaging, they serve as important clinical and pre-clinical research tools.

This presentation will cover the basic principles of MRI cine imaging of the heart, as well as some of the advanced techniques that are more commonly used in clinical and research applications. The basic approaches to quantitative evaluation of global and regional cardiac function will also be described.

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