Motion Reduction Techniques for Coronary MRA

Abstract

Significant progress has been made in coronary magnetic resonance angiography (MRA) (1–16). Motion reduction techniques have contributed significantly to this progress (1,2,9–16). Motion of the heart consists of cardiac contraction and respiration. For effective reduction of the effects of cardiac contraction, coronary data acquisition is enabled only during a brief diastolic period ( 100 msec) of the cardiac cycle when the cardiac contraction is minimum and coronary arterial flow is maximum. Many cardiac cycles are accordingly required to complete acquisition of the coronary tree, and respiration occurred during data acquisition is the major factor limiting image quality. The challenge to reduce respiration effects in magnetic resonance imaging (MRI) is that respiration cannot be modeled or predicted accurately. Breathholding has been a popular method used to reduce respiration effects. Multiple breathholds are typically required to acquire the whole coronary tree with sufficient spatial resolution and signal-to-noise ratio. Misregistration between different breathholds may limit diagnostic accuracy (7,10). A systematic and effective approach to reduce respiration effects in coronary MRA as well as in other areas of MRI affected by motion is the navigator-based motion reduction method: Motion is monitored using navigator echoes and data acquisition is modified accordingly. This concept was introduced in the early 1950s for correcting the distorting effects of the Earth’s atmosphere in ground-based astronomical imaging, where the angular resolution is limited by the atmospheric turbulence deforming the image on a millisecond time scale, not by the size of the primary mirrors of telescopes (17). Adaptive optical systems consisting of natural or artificial guide stars, wavefront sensors, and real-time phase-delay corrections have significantly improved image resolution (18). Spatial resolution of coronary MRA is similarly limited by physiological motion, not by the capability of the MRI system to sample high spatial frequency information. Navigator-based motion gating and correction techniques were separately introduced in MRI in the late 1980s to reduce motion effects (19,20). Navigator-based motion correction techniques are currently used widely to correct undesired motion effects in diffusion imaging (21–24), neuro-fMRI (26–28), body MRA (29–32), and spectroscopy (33). The initial results from our group and others vindicate the effectiveness of these navigatorbased methods for reducing motion effects in coronary MRA (9–16). These navigator methods will very likely play a significant role in future development of coronary MRA. This chapter will attempt to review systematically navigator-based motion reduction techniques for coronary MRA: (1) kinematics and imaging effects of respiration; (2) navigator echo techniques for monitoring motion; and (3) motion reduction techniques for coronary MRA.