Velocity quantification by electrocardiography-gated phase contrast magnetic resonance imaging in patients with cardiac arrhythmia: a simulation study based on real time transesophageal echocardiography data in atrial fibrillation

Abstract

Objective: To systematically investigate the impact of beat-to-beat variations on electrocardiography (ECG)-gated multibeat flow imaging with phase contrast (PC) magnetic resonance imaging (MRI) based on real time in vivo transesophageal echocardiography (TEE) data in patients with known arrhythmia.

Methods: Real-time 2-dimensional Doppler TEE was performed in five patients with atrial fibrillation (4 men, age = 64 ± 8.7 years). The TEE data provided real-time left atrial (LA) and left ventricular (LV) flow velocities in consecutive cardiac cycles with different RR interval durations. The PC MRI acquisitions were simulated from the TEE velocity measures by constructing time-resolved k-space data for segmented sampling schemes typically used for ECG-gated 2-dimensional PC MRI. Each simulation was repeated 100 times to minimize effects from data that may be weighted to a particular beat in the center of k-space. The resulting LA and LV velocities were compared to the average TEE velocities and data from individual cardiac cycles.

Results: Despite beat-to-beat variations of velocities in TEE data, ECG-gated flow imaging with MRI could reproduce persistent average LA and LV mean velocities within 7.0% to 7.4% compared to TEE.

Conclusions: The PC MRI velocity measurements in patients with varying RR interval durations are not significantly different from time-averaged real-time velocity data for a typical segmented k-space data acquisition schemes. Though beat-to-beat variations in atrial velocities that were observed with TEE cannot be detected with ECG-gated multibeat PC MRI, it can reliably assess average flow patterns across multiple beats.

Figure 1

Flowchart of the MR simulation methodology. Original TEE data (A) are separated into RR-intervals and converted to grayscale (B, left), and velocity measures are extracted from the color Doppler images (B, right). Each complex image (magnitude: grayscale, phase: velocity) undergoes a Fourier transform to simulate MRI k-space (C). K-space lines are randomly selected using one of three acquisition schemes (sample selection from three heartbeats shown in (C) in (D, left)) to create 2D PC MRI k-space data (D., right), which undergoes an inverse Fourier transform to calculate a simulated MR reconstruction of 2D PC MRI magnitude and velocity images (E). LA: left atrium, LV: left ventricle.

Figure 2

K-space acquisition sampling patterns used in 2D PC-MRI simulations. In MRI 1, k-space was divided into 4 segments, with one line filled in each segment per heartbeat. In MRI 2, four consecutive lines of k-space are filled for each heartbeat. In MRI 3, four lines of k-space are placed randomly into unfilled locations per heartbeat.

Figure 3

Representative TEE data for a single patient with atrial fibrillation at systole (top row), and diastole (bottom row) for three consecutive heartbeats (R-R intervals of 750ms, 660ms, and 580ms). LA: left atrium, LV: left ventricle

Figure 4

Average TEE data over all acquired heartbeats for one representative atrial fibrillation patients at systole (top row) and diastole (bottom row). The reconstructed velocity images from the MR simulations are also shown. MRI 1–3 denote the 3 different k-space sampling schemes used for the simulation of prospectively gated 2D PC-MRI data. LA: left atrium, LV: left ventricle.