Heart applications of 4D flow

Abstract

Four-dimensional (4D) flow sequences are an innovative type of MR sequences based upon phase contrast (PC) sequences which are a type of application of Angio-MRI together with the Time of Flight (TOF) sequences and Contrast-Enhanced Magnetic Resonance Acquisition (CE-MRA). They share the basic principles of PC, but unlike PC sequences, 4D flow has velocity encoding along all three flow directions and three-dimensional (3D) anatomic coverage. They guarantee the analysis of flow with multiplanarity on a post-processing level, which is a unique feature among MR sequences. Furthermore, this technique provides a completely new level to the in vivo flow analysis as it allows measurements in never studied districts such as intracranial applications or some parts of the heart never studied with echo-color-doppler, which is its sonographic equivalent. Furthermore, this technique provides a completely new level to the in vivo flow analysis as it allows accurate measurement of the flows in different districts (e.g., intracranial, cardiac) that are usually studied with echo-color-doppler, which is its sonographic equivalent. Of note, the technique has proved to be affected by less inter and intra-observer variability in several application. 4D-flow basic principles, advantages, limitations, common pitfalls and artefacts are described. This review will outline the basis of the formation of PC image, the construction of a 4D-flow and the huge impact the technique is having on the cardiovascular non-invasive examination. It will be then studied how this technique has had a huge impact on cardiovascular examinations especially on a central heart level.

Keywords: 4D flow; Quantitative MRI; angio-MRI; cardiac MRI.

Figure 1

Magnitude and phase image of a aorta with a sagittal oriented acquisition. Phase image is based on phase differences of moving objects.

Figure 2

4D flow applied to the carotid flow. (A) A left carotid streamline; (B) a left carotid vector fields representation. Images obtained with a 1.5 T system following parameters: flip angle, 8°; TE, 2.2 ms; TR, 4.1 ms; velocity sensitivity (VENC), 200 cm/s; spatial resolution, 2.4 × 2.39 × 2.50 mm³; slab thickness, 50.4 mm; number of sections 55; and temporal resolution, 64.9 ms. The software reconstruction provides different visual representation (streamlines or vector fields in these examples).

Figure 3

4D flow applied to the carotid flow. (A) Pressure difference visualization in a right carotid stenosis. Pressure is an important parameter of the pathophysiology of a stenosis genesis; (B) flow visualization and quantification in a right carotid stenosis. The selection of plans to quantitatively analyze is made in post processing and this is one of the main advantages of 4D flow acquisition. Software usability is fundamental to speed up this process.

Figure 4

Retrospective flow quantification in a right carotid artery at 6 different levels.