A pulsatile 3D flow relevant to thoracic hemodynamics: CFD - 4D MRI comparison

Application Challenge AC7-04 © copyright ERCOFTAC 2021

Test Data

Overview of Tests

The experimental method presented in this section is described in detail by Puiseux et al. (2019) [2]. However, the MRI machine and sequence parameters differ. The PC-MRI scans were performed on a 1.5T scanner (Siemens MAGNETOM Sola, Siemens Medical Systems, Erlangen, Germany).

Description of Experiment

A scheme presenting the experimental setup is shown on Fig. 1. The pulsatile flow is delivered by a computer-controlled pump (CardioFlow 5000MR, Shelley Medical Imaging Technologies, London, Ontario, Canada), which allows the user to define the desired waveforms. To control the flow rate it delivers, an ultrasonic flowmeter (UF25B100 Cynergy3 components Ltd, Wimborne, Dorset, UK) is used. Futhermore to reduce the swirling motion of the fluid entering the phantom, a flow straightener is positioned downstream of the flowmeter and upstream of the phantom. Inside the scanner room, the pump is positioned outside the 5 Gauss Line for MR safety considerations.

Concerning the PC-MRI acquisitions, one acquisition was made for 4D Flow (duration of 42.6 min) and several for 2D cine PC-MRI (duration of 3.6 min each). For each acquisition, the full k-space was recorded (ie there was no parallel imaging acceleration), where the k-space refers to the complex-values Fourier transform of the MR images measured. Indeed the signal collected during the MR experiment corresponds to the Fourier transform of the transverse magnetization signal and is thus expressed in terms of spatial frequencies. The MR images are reconstructed through an inverse Fourier transform applied on this k-space. All scans were retrospectively cardiac gated, which allows to reconstruct a certain number of phases of the whole cardiac cycle. 20 phases were acquired for the 4D Flow scan and 30 for the 2D cine PC-MRI scan. Concerning the spatial resolution, isotropic voxels of 2 mm were acquired for the 4D Flow scan. For the 2D cine PC-MRI, the isotropic pixel size was 0.8 mm, with a slice thickness of 6 mm to enhance the signal-to-noise ratio.

These settings and additional MR information are given in the following table:

	4D Flow	2D cine PC-MRI
Acquisition Duration	42.6 min	3.6 min
Cardiac Phases Number	20	30
Acquisition Plane	Coronal	Transverse
Voxel size	2 x 2 x 2 mm	0.78125 x 0.78125 x 6 mm
Flip Angle	7°	20°
VENC	0.7 m/s (in-plane velocity)	0.7 m/s (through-plane velocity)
VENC	0.2 m/s (through-plane velocity)	0.2 m/s (in-plane velocity)
TR	51.84 ms	30.32 ms
Effective TE	4.15 ms	4.84 ms

Table 1: MR sequences parameters

Boundary Data

The inlet and outlet of the phantom are both connected to the whole hydraulic circuit and care is taken to avoid any air running into it. Before the measurements, the fluid was freely running through the circuit. Besides the flowmeter measurements, the high-resolution 2D cine PC-MRI acquisitions at the inlet enable to check whether the prescribed pulsatile flow is achieved. The averaged flowrate obtained at the inlet for 2D cine PC-MRI acquisitions is displayed in Fig. 4, together with the flow rate obtained for 4D Flow at the same location.