Previously, EWI required image reconstruction over multiple cardiac cycles and could only be used to observe periodic cardiac rhythms. Now, researchers at Columbia University (New York, NY) have developed a high-frame-rate technique that images the entire heart within a single heartbeat. This allows observation, and potentially diagnosis and treatment monitoring, of non-periodic arrhythmias such as atrial fibrillation, the most common abnormal heart rhythm (Phys. Med. Biol. 56 L1).

EWI uses standard ultrasound equipment to map the electromechanical wave (EW) – the transient deformations that follow electrical activation of the heart. Displacements between consecutive image frames are used to determine the inter-frame strains that depict the EW. Visualizing the EW at high resolution and over a large field-of-view, however, poses a major technical challenge.

The Columbia team, headed up by Elisa Konofagou, overcame this challenge by developing ultrasound imaging sequences based on flash and wide transmit beams. These larger ultrasound beams enable imaging of the entire heart at 2000 frames/s (a temporal resolution of 0.5 ms).

In vivo validation

EWI was performed using a Verasonics ultrasound system with a 64-element phased-array probe. To generate the flash transmit beam, every transducer element fires with the same amplitude but delayed such that the wavefront propagates as if emitted from a single point. A wide beam, meanwhile, is generated by reducing the number of transmit elements and modulating their amplitudes.

Konofagou and colleagues first validated the technique by performing wide-transmit-beam EWI on an open-chest canine during pacing from the apical region of the lateral wall. A 64-electrode basket catheter was used simultaneously to generate 3D maps of electrical activation.

EWI showed activation originating from the apical region of the lateral wall, followed by activation of the right-ventricular wall and finally the septum. Electrical activation mapped using the basket catheter was highly correlated with the EWI-measured EW.

The team then examined a closed-chest setting, which is more prone to artefacts such as reflections from the rib cage. They performed EWI on four conscious canines using the flash sequence, during normal heart beating and a non-periodic rhythm. An electrocardiogram (ECG) was acquired during the scan to correlate the EW with the various components of a heartbeat.

The total acquisition time was 3.5 s, compared with 15–20 s using previous methods. This enables free breathing acquisition and limits potential motion artefacts. The 3.5 s comprised a 2-s EWI measurement, plus a 1.5-s B-mode ultrasound scan, acquired as a base image onto which the strain maps were overlaid.

During normal heart rhythm, the sinus node in the right atrium acts as a natural pacemaker. Signals generated at this node travel through the atrium during the P-wave (the first deflection observed in an ECG) to the atrio-ventricular node, and finally to the ventricular myocardium (during the QRS complex, the next three deflections). The imaged EW was seen to follow the above pattern, originating from the right atrium, propagating towards the left atrium and finally in the ventricles.

Each animal was then imaged after implantation of a pacemaker in the right ventricle and ablation of the atrio-ventricular node. In this situation, activation of the sinus node does not necessarily lead to activation of the ventricles – as was confirmed by the ECG trace.

EWI during the P-wave showed that the EW was initiated in the right atrium and propagated towards the left atrium, as expected. Activation during the QRS complex (triggered by the pacemaker) also followed the expected pattern, starting in the right-ventricular wall and then propagating towards the septum and lateral wall.

According to Konofagou, the use of flash and wide-beam imaging sequences in a clinical setting could expand the applications of EWI to include: "visualizing and localizing arrhythmia origins in both atria and ventricles ", as well as studying atrial flutter and heart attacks.

"Clinical studies are already underway and we are in the process of submitting another manuscript on those," Konofagou told medicalphysicsweb. "We have performed EWI in patients during cardiac resynchronization therapy (CRT), atrial fibrillation and atrial flutter."