Assessment of Left Atrial Size and Function by Cardiac Magnetic Resonance

Left atrial (LA) size and function are powerful prognostic markers in a variety of clinical conditions. Cardiac magnetic resonance (CMR) is considered the gold-standard to assess the volumes of heart chambers by contouring cavity borders at end-diastole and end-systole from a short-axis cine stack. This method is routinely used to measure ventricular volumes and ejection fraction, but has never been implemented for LA volume measurements, due to the need of a significant lengthening of both CMR acquisition and post-processing times. Current CMR guidelines do not provide any specific recommendation on LA imaging in daily routine, and LA volume is usually evaluated by using the biplane area-length algorithm in standard cine long-axis images. Yet, the latter methodology implies geometric assumptions about the LA shape that are often not fulfilled in routine patients. Moreover, the standard long-axis views are oriented according to the major axis of the left ventricle (LV), which lies in a plane with a different spatial orientation compared with the LA. As a result, standard long-axis views optimized according to the maximal LV length commonly foreshorten the LA and therefore underestimate its size.

Accordingly, a previous echocardiographic study showed that long-axis views focused on the LA were more accurate to derive LA maximal volume vs. standard long-axis views.

Differently from echocardiography, CMR allows to orientate images during acquisition process with no dependency from acoustic windows. Capturing the true LA long-axis by focused imaging is therefore more feasible with CMR vs. two-dimensional echocardiography. Compared to current CMR practice, implementation of focused LA long-axis views might result in improved accuracy of LA volumes estimation with no significant delay on acquisition times (Figure).

LA dysfunction, as assessed by deformation analysis, is emerging as an early prognostic marker in several cardiac diseases. At present, standard long-axis cine images are used for CMR feature tracking (CMR-FT), which allows the assessment of atrial phasic functions. Of core importance, CMR-FT derived strain parameters [i.e., LA total strain (εs), conduit strain (εe), and booster strain (εa)] are sensible to plan orientation and might be affected by foreshortening.

Current CMR protocols use long-axis cine images oriented on the LV long axis and may systematically underestimate LA size and function. We hypothesize that LA-focused long-axis cine images at CMR are more accurate in estimating LA volumes and function than current practice.