4D Flow MRI: Applications in Cardiology

Magnetic resonance imaging has been an invaluable radiological tool in evaluating and determining cardiovascular diseases. This imaging technique can provide healthcare professionals with structural and functional information regarding the heart, aorta and surrounding blood vessels¹.

With the advancement of technology, over the years, the nature of MRIs has also evolved — from standard two-dimensional phase contrast (PC) procedures to volumetric three-dimensional methods, and now, cutting-edge four-dimensional (4D) flow MRIs find their place in clinical practices.

The Fourth Dimension in MRIs

4D flow MRI spins a new angle on the medical imaging system — movement — enabling clinicians to view an improved and zoomed in version of the heart and blood flow to specific areas. Here, all three spatial directions relative to space and time encode flow and velocity. Therefore, a time-resolved 3D PC-MRI (time + 3D) can be termed as 4D flow MRI, also known as 4D flow cardiovascular magnetic resonance (CMR) ^{1, 6}. It is a non-invasive, free-breathing, short process that comprehensively accesses and investigates the cardiovascular region with one scan ⁹.

The results itself are in the form of colorful and visually stunning images and can be read by experts already trained in reading MRIs ².

Former editor of Journal of American College of Cardiology, Dr. Anthony De Maria, discussing this new entrant said, “The ability to do 4D flow really has the potential to quantify valvular lesions with an accuracy we haven't had the ability to do before — in doing that you have the confidence to make a decision about a therapeutic strategy⁹.”

Comparing MRI Methods: 2D vs 3D vs 4D

2D MRI scanning has been in use since the 1970s. The image from this, a single slice, is a product of a powerful magnetic field and radio frequencies. Here, magnetic gradients are applied in only two directions to figure out the location of frequencies in vivo. A few decades later, the third dimension, volume, was added to the mix (as multiple stacked 2D slices) that used three magnetic field gradients so as to provide a higher spatial resolution of the anatomy ⁸.

When 4D flow MRI was considered as an option for medical imaging a few years ago, it was seen to be a vastly improved technique over 2D MRIs. Complete information about temporal and spatial coverage of the area in question and a detailed visualization of the direction of blood flow allowed professionals to see extensively and quantify flow ⁷. Recent studies have shown that a complete analysis of complex blood flow patterns is viable with 4D MRIs that was not entirely feasible with the other methods.

Take, for example, congenital heart disease. Determining the origin of the abnormalities or defects requires advanced diagnostic imaging, such as 3D or 4D, because the blood flow could assume random, and not predictable, patterns in some cases. A flat, two-dimensional image can display flow but of limited velocities².

As Michael Marks, a radiologist at the Stanford University of California, said, “To capture what is going on in these [congenital heart] diseases, you really need to quantify flow in multiple locations and this technique lends itself to do that.” ²

One of the biggest challenges that the fourth dimension overcomes is obtaining scans of patients in their natural state, and them not having to hold their breaths during the process. The vasculature around the heart can, then, be determined by any planes of interest ^{1, 7}. This method is in contrast to the conventional 2D format where since the imaging plane is limited, patients must control their breathing, too.

Applications and Future of 4D Flow MRI in Cardiology

Currently, MRIs are being used to evaluate patients with heart conditions such as aneurysms, atherosclerosis, tumors, cardiac failure, and heart valve diseases ³. But with 4D flow MRI, a more detailed assessment of hemodynamics in vessel walls and the myocardium, and a structured approach towards therapy management are possible ^{6, 7}. Research has compared dynamics in healthy versus diseased individuals using this technology.

The method has allowed delving deeper into mysterious parts of the cardiovascular system, such as movement of blood between heart chambers, retrograde flow pathways, as published in research papers earlier ¹. One of the most common abnormalities in the United States is a bicuspid aortic valve, and cardiologists and radiologists see the new MRI option as a tool to manage patients with this condition for the related complications it could cause ². Similarly, a study was conducted using 4D flow to assess post-stenotic dilation in patients with aortic stenosis and left ventricular remodeling ⁴.

Apart from congenital heart disease, 4D analysis finds application in adjudging associated stroke risk in people with atrial fibrillation, as this method has become a faster, better and clearer (resolution-wise) means to do so ¹. Studying blood flow dynamics can help scientists further understand its role in health, disease, and aspects of cardiovascular physiology ^{2, 6}. It is the accuracy and robustness of 4D flow MRI that is an exciting introduction in the clinical world, especially, in terms of increasing the quality of pharmaceutical and surgical therapies, and treatments.

References:

1. Stankovic, Z. et al. (2014), ‘4D flow imaging with MRI’, Cardiovasc Diagn Ther, 4 (2), Pp 173-192
2. 4D Flow MRI: Another Dimension for Congenital Heart Disease, Cardiovascular Business, http://www.cardiovascularbusiness.com/topics/imaging/4d-flow-mri-another-dimension-congenital-heart-disease, (accessed 13 Mar 2018)
3. Four-Dimensional Magnetic Resonance Imaging (4D MRI), Northwestern Medicine, https://www.nm.org/conditions-and-care-areas/tests/four-dimensional-magnetic-resonance-imaging, (accessed 14 Mar 2018)
4. Knobelsdorff-Brenkenhoff, F. et al. (2016), ‘Evaluation of Aortic Blood Flow and Wall Shear Stress in Aortic Stenosis and Its Association With Left Ventricular Remodeling’, Circ Cardiovasc Imaging
5. 4D Heart Imaging - The Future of Cardiac MRI Assessment, NSW Cardiology, http://www.nswcardiology.com.au/4d-heart-imaging-the-future-of-cardiac-mri-assessment/, (accessed 15 Mar 2018)
6. Assessment of Cardiovascular Blood Flow Using 4D Flow MRI, Linkoping University, https://liu.se/en/article/assessment-of-cardiovascular-blood-flow-using-4d-flow-mri, (accessed 15 Mar 2018)
7. Dyverfeldt, P. et al. (2015), ‘4D flow cardiovascular magnetic resonance consensus statement’, Journal of Cardiovascular Magnetic Resonanc e
8. Questions and Answers in MRI, http://mriquestions.com/2d-vs-3d-mra.html, (accessed 15 Mar 2018)
9. 4D Flow Post-Processing, Arterys, https://arterys.com/4d-flow, (accessed 15 Mar 2018)