Clinical application of flow imaging has been accepted since clinical MRI was introduced. Various MR acquisition techniques are applied for imaging vessels of body organs, including time-of-flight, inflow-dependent inversion recovery (IR), velocity selective and phase-contrast (PC) MR angiography as well as contrast-enhanced MR angiography.¹ Flow-encoding PC techniques are also used for flow measurements. Depending on clinical purposes, blood flow should be visualized as static images for vascular morphology and/or measured for hemodynamic evaluation.

Respiratory-gated Inflow-dependent IR MR angiography is widely used for imaging renal arteries. In this technique, the pulse sequence consists of a slab-selective inversion pulse that suppresses stationary background and venous signals, a subsequent waiting time (blood-traveling inversion time), and a 3D balanced steady-state free precession (bSSFP) readout. Arranging the slab-selective inversion pulse-field provides visualization of other target vessels such as hepatic arteries and portal veins. Although static angiographic images are obtained, variable waiting time settings allow visualization of flow dynamics. Other readout techniques such as 3D single-shot fast spin echo and 3D ultrashort echo time alternative to 3D bSSFP can be implemented to evaluate pulmonary arteries and visceral artery disease after endovascular therapy.² Although it is noninvasive, a potential drawback of inflow-dependent IR MR angiography includes a limitation of balancing between superior-inferior coverage and tagging efficiency of upstream arterial blood.

Contrast-enhanced MR angiography relies on signal enhancement by flow-independent T1-shortening effect of blood after injection of gadolinium contrast media. However, consecutive imaging using time-resolved techniques allows qualitative evaluation of hemodynamics. Especially, current time-resolved methods with a temporal resolution of 1-2 seconds can visualize the flow direction of the complex vascular systems in body organs.³ With the advantages of excellent contrasts and anatomical information, they can provide critical information for interventional radiology procedures such as retrograde flow and shunt flow. Imaging can be performed under breath-holding or shallow breathing. The operator-independent technique is widely accepted in clinical settings.

2D cine PC MRI has been widely used to quantitatively evaluate regional hemodynamics since the 1980s.^4-6 Magnetic field gradients encode the spatial movement of spins on PC MRI. Sequence schemes are characterized by velocity-encoded bipolar gradients consisting of two lobes of equal area and opposite gradient polarity.^7,8 The bipolar gradients generate phase shifts, resulting in net-zero shifts for stationary spins. However, moving spins gain a nonzero phase along with changes of the encoding magnetic field strength with the location of the spins. The velocity information can be obtained as a phase-difference image, created by subtracting two phase images. Ideally, the velocity encoding is set as low as possible yet not below the maximum velocity. Background phase errors caused by the eddy current, concomitant gradient field, and gradient non-linearity should be assessed for accurate quantification.⁹ The 2D PC cross-section is generally set perpendicular to the flow direction to obtain through-plane velocity. Breath-holding is preferred during acquisition. A series of flow-velocity and anatomical images over cardiac cycles in the target vessel are generated. 2D PC MRI is a reliable method to measure flow velocity and volume. Clinical applications for body organs include evaluation of renal blood flow, visceral artery stenosis, portal hypertension, portal venous flow under stress challenges, surgical bypass function, etc.^10-14 Drawbacks of 2D PC MRI are lack of anatomical information, limited imaging volumes, and technical difficulties in setting imaging planes for curved visceral vessels. Therefore, morphological imaging such as MR angiography should be combined during the examination.

Recently, the development of imaging techniques led to the acquisition of time-resolved PC MRI with velocity encoding along x-, y- and z-axes directions and 3D volumetric coverage, also termed 4D flow MRI.¹⁵ In the body field, respiratory gating is implemented depending on the target vessels. Non-respiratory gating with increased number of excitations is possible for greater vessels. Nonetheless, 4D flow MRI in the body inheres a trade-off between spatial resolution and limitation of scan time. A large amount of image data was generated after completing the 4D flow acquisition, including magnitude images and three sets of phase-difference images over the cardiac cycle. Therefore, post-processing software that integrates data sets is essential for evaluation. The post-processing of 4D flow MRI allows for 2D flow analysis on any cross-sections, 3D dynamic blood flow visualization, and analysis of various hemodynamic parameters, all of which are significant advantages over other imaging modalities. Advanced hemodynamic parameters such as wall shear stress, pressure gradients across lesions, and pulse wave velocity may become imaging biomarkers. Similar to 2D PC MRI, applications of 4D flow MRI in different body organs are introduced.^16-19 However, the wide use of 4D flow MRI in clinical settings is still limited primarily due to its longer scan time. Further technological developments for accelerated scanning and user-friendly post-processing software are expected for routine use.

In this talk, the following will be introduced: 1) basic knowledge of associations between flow and signals, 2) non-contrast-enhanced inflow-dependent MR angiography and contrast-enhanced MR angiography for qualitative flow visualization, and 3) principles of 2D PC and 4D flow MRI for flow visualization and measurement. Future directions of 4D flow MRI to overcome the time-consuming process will also be discussed.

Acknowledgements

No acknowledgement found.

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