The ECG tracing can reveal the origin of the electrical activity, rhythm, rate, conduction abnormalities and even give clues about cardiac ischemia or prior myocardial infarctions. For the purpose of cardiac imaging, we are most concerned about the heart rate and rhythm pattern. Heart rate (HR) can be expressed in beats per minute (bpm) as determined by the number of QRS complexes per minute. Cardiac activity is also described by the R–R interval, the cardiac cycle duration between R waves in milliseconds (ms). Heart rate and R-R intervals are related by Eq. 9.1. Table 9.1 lists typical heart rates with the corresponding R-R intervals.

Heart rhythm is defined by the origin of each electrical impulse and the regularity of the stimuli. Rhythm regularity is most important during CMR and it is easily determined by observing the R-R interval variability. The normal sinus rhythm is regular and usually between 60 and 100 bpm corresponding to R-R intervals of 1000–600 ms. However, individuals referred for CMR may often have slow (bradycardia; <60 bpm), fast (tachycardia; >100 bpm) or irregular heart rhythms (arrhythmias). The most common irregular rhythms seen in typical patients with cardiac disease referred for CMR examination are atrial fibrillation, premature atrial or ventricular contractions, and non-sustained ventricular tachycardia. Prompt recognition of these abnormalities allows the imager to adjust the acquisition technique to achieve optimal image quality and make sure that the exam is completed safely or terminated early if needed to facilitate rhythm management. Later in the chapter we will discuss how to tailor the exam in the setting of cardiac arrhythmias.

The goal of ECG-gating is to synchronize data acquisition to the cardiac cycle, which reduces cardiac motion-induced artifacts and optimizes image quality. The R-wave is the most prominent and easily detected feature on the ECG and serves as a temporal reference point within the cardiac cycle. Thus, it is the preferred marker to gate and trigger data acquisition. Although the terms often used interchangeably, ECG-triggering and ECG-gating are not synonymous. ECG–triggering specifically refers to the initiation of the MRI pulse sequence after detection of an R-wave, whereas ECG–gating refers to the process of matching the acquired data to the corresponding temporal phase within the RR-interval.

Two primary ECG gating modes are commonly used in CMR: prospective and retrospective. In prospective gating, detection of an R wave triggers the MR pulse sequence to acquire a fixed number of data lines at a given timepoint within the R-R interval. Thus, data is prospectively collected at a particular time in the cardiac cycle with respect to the R-wave detection. In retrospective mode, data is collected continuously over several heart beats and retrospectively binned into a fixed number of reconstructed cardiac phases using the detected R-waves as reference points.

These ECG gating modes are combined with two data collection strategies: single–shot or segmented acquisitions. Single-shot pulse sequences are typically prospectively gated and all the data needed to make an image is acquired during a single heart beat. Alternatively, during segmented acquisitions, a fraction of the total number of data lines is acquired with each heartbeat, and the data needed to reconstruct a single image is obtained over multiple R-R intervals. In the next sections we will describe in detail the various ECG-gating and acquisition strategy combinations that are used in a cardiac exam. Table 9.2 summarizes typical ECG-gating and acquisition mode combinations, their respective applications, and their advantages and limitations.

Single-heartbeat (also termed ‘single-shot’) pulse sequences are typically prospectively gated and all the data needed to make an image is acquired during a single cardiac cycle (Fig. 9.3). During a prospectively gated acquisition, data is acquired after a programmed delay, or trigger delay (TD), from the R-wave. Data is then collected for a period called the acquisition window (AW), which typically ends several milliseconds prior to the next heartbeat. This gap, or trigger window (TW), allows the scanner to detect the next R-wave. As a result, data is not sampled during this end-diastolic period of the cardiac cycle. For still frames or anatomic images, the TD and AW are adjusted to allow data collection during diastasis which coincides with the T-P segment (Fig. 9.1). One or multiple slice locations can be acquired during each breath hold.

Prospectively gated single-shot sequences are used for anatomic or static imaging. Examples include dark-blood half-Fourier turbo spin echo (HASTE) and bright-blood steady-state free precession (SSFP) images which are used as scouts during the initial survey of cardiac exams or to image cardiac anatomy (Fig. 9.4). Single-shot sequences may be used instead of segmented acquisitions for applications such as late gadolinium enhanced imaging (LGE) in patients with arrhythmias or in subjects who cannot perform adequate breath holding to acquire diagnostic segmented images.