The future of CMR: All-in-one vs. real-time CMR (Part 2)

Abstract

Cardiac Magnetic Resonance (CMR) protocols can be lengthy and complex, which has driven the research community to develop new technologies to make these protocols more efficient and patient-friendly. Two different approaches to improving CMR have been proposed, specifically "all-in-one" CMR, where several contrasts and/or motion states are acquired simultaneously, and "real-time" CMR, in which the examination is accelerated to avoid the need for breathholding and/or cardiac gating. The goal of this two-part manuscript is to describe these two different types of emerging rapid CMR protocols. To this end, the vision of all-in-one and real-time imaging are described, along with techniques which have been devised and tested along the pathway of clinical implementation. The pros and cons of the different methods are presented, and the remaining open needs of each are detailed. Part 1 tackles the "All-in-One" approaches, and Part 2 focuses on the "Real-Time" approaches along with an overall summary of these emerging methods.

Keywords: Cardiac MRI; Compressed sensing; Magnetic Resonance Fingerprinting; Multitasking; Parallel imaging; Quantitative imaging; Rapid imaging; Real-time imaging.

Fig. 1

For real-time CMR, often trajectories are applied which allow image reconstruction from an arbitrary bin size,b. Prominent examples are radial and spiral trajectories, often combined with tiny or regular golden angle angular increment. For improving the “apparent” temporal resolution, the increment, t, between subsequent reconstructions is normally chosen smaller than b, yielding a classical sliding window reconstruction with almost free choice of a compromise between spatial and temporal resolution, and aliasing artifact level. CMR: cardiovascular magnetic resonance

Fig. 2

Cine in a patient (49 y.o. man with atrial fibrillation) experiencing arrhythmia during the MRI exam shows acceptable quality during systole (A-B), but strong artifacts by mid-diastole (C), using conventional retrospective ECG-gated cine, which can be avoided using prospective ECG-gating or real-time imaging. Another subject (36 y.o. man with bigeminy), where conventional cine (D) was obtained by scanning over two heart-beats with 1.6x 1.6 x 6 mm³ resolution, and (E) real-time cine (3x3x8mm³ resolution) was also acquired, providing similar information.

Fig. 3

Left ventricular function of a subject with an arrhythmia as imaged using a real-time sequence. A) Two short-axis slice locations are shown with temporal projections (B) as shown by the red line. The irregular heart rhythm led to impaired segmented cine acquisition and the irregular pattern can be visualized with real-time imaging. Images taken from Contijoch et al. .

Fig. 4

Example of a real-time monitoring method which can be used to evaluate changes in cardiac performance during a 3 min exercise protocol. The top row shows magnitude (left) and phase (middle) images with a flow curve on the right. Beat-to-beat changes in heart rate (left), stroke volume (middle), and cardiac output (right) are shown in the bottom.

Fig. 5

A small region of enhancement on LGE in a patient (66 year old woman with suspected myocarditis), which can be well observed on both conventional breath-hold 2D LGE (1.6 ×2.2x8mm³), free-breathing MoCo (1.7 ×2.2 ×8 mm³), and high resolution navigator-gated 3D LGE (1.5 ×1.5 ×3.6 mm³). However, a midwall line of enhancement on the free wall of the myocardium can only be observed with high resolution 3D LGE, highlighting the value of a high-resolution methods, still not available with real-time acquisitions.