Most cardiac magnetic resonance images are acquired over several heart beats. If the heart rate is regular, the resulting images have enhanced spatial and temporal resolution. However, if the heart rate is irregular, the resulting images are degraded, and functional interpretations may be misleading. In patients with atrial fibrillation or frequent premature contractions, global systolic function may appear worse than reality.
Arrhythmias comprise a substantial fraction of the patient population. So it is essential to have a strategy for dealing with them. Fortunately, one can obtain an accurate assessment of LV volumes and global systolic function, if image acquisition is changed such that each slice is acquired from a single heart beat (i.e., “real time” acquisition mode).
The figure below shows a “real time” acquisition of a series of short axis slices from a patient with an irregular cardiac rhythm.
In this case, end diastole is the last cardiac phase for each slice. However, the end systolic cardiac phase varies with the slice location. This is because each slice represents a unique heart beat of different duration. End systole appears in earlier cardiac phases for those heart beats of longer duration, and in later cardiac phase for those heart beats of shorter duration. To accurately determine LV volumes, the user must determine end-diastole and end systole on a slice-by-slice basis. This can be appreciated in the traces below and the accompanying grid which shows on a slice by slice basis the cardiac phase that was chosen for end diastole (red) and end systole (blue):
In patients with irregular cardiac rhythms, the left ventricular end diastolic volume, end systolic volume, stroke volume, and ejection fraction vary on a beat-to-beat basis. In particular, cardiac function is influenced by the degree of LV filling before the onset of systole. For example, premature beats start with an abnormally small left ventricular end diastolic volume because the left ventricle hasn’t had time to completely fill. As a result, premature beats have an abnormally low stroke volume and ejection fraction. The volumetric values obtained from analyzing a series of short axis “real time” images are a weighted average of all of the individual heart beats.