Magnetic resonance imaging (MRI) is an imaging technique that is central to the diagnosis and evaluation of cardiovascular disease. Over the years, this radiological tool has evolved from 2D phase-contrast procedures to 3D methods and today’s advanced 4D flow MRIs.
Four-dimensional flow MRIs use movement to provide clinicians with another perspective of the heart and blood flow. The procedure takes into account the three spatial directions of 3D (length, width, and height), as well as time and space, enabling encoding into flow and velocity. This time-resolved form of 3D MRI is therefore referred to as 4D flow MRI.
In this article, we will discuss the technical aspects and benefits of 4D Flow for cardiac MRI, track the transition from 2D to 4D Flow, compare 4D MRI Flow to 2D and 3D, and ask what the applications and future of 4D Flow MRI are in cardiology.
Technical aspects of 4D Flow
As we have outlined above, 4D Flow MRI can also be termed as 3D time-resolved phase-contrast (PC) MRI. This refers to the time-resolved 3D velocity field that is created by the encoding of velocity through the cardiac cycle along all three spatial dimensions, enabling the recording of three-directional velocity measurements with the use of interleaved four-point velocity encoding.
In 4D Flow, as in 2D MRI, data acquisition is synchronized with the cardiac cycle. K-space segmentation techniques are used to distribute data collection over a series of cardiac cycles. In each cardiac cycle, a small fraction of the total 4D flow data is measured, and this successive collection of data leads to the generation of four time-resolved 3D data sets. Velocity encoding, or Venc, is an important parameter that is user-defined. This sets the maximum flow velocity that can be acquired. To avoid velocity aliasing and ensure accurate flow visualization and quantification, the measured blood flow velocity shouldn’t exceed the acquisition setting.
Acceleration techniques such as k-space sampling and sparsity exploitation have been key to reducing scan times and making 4D Flow MRI clinically viable. The scan time has now been brought down to as little as 2 to 8 minutes. Preprocessing is central to avoiding systematic velocity encoding errors that can occur in 4D flow MRI data. This allows the calculation of a 3D PC MR angiography in order to guide segmentation and flow visualization.
Validation of the blood flow velocities measured by 4D Flow is necessary. But this is done without a gold standard measurement technique for the in vivo assessment of time-resolved 3D blood flow velocities. Alternative approaches such as the use of dedicated pulsatile and steady in vitro flow phantoms, and comparisons with Doppler ultrasound and standard 2D PC MRI in healthy volunteers, have been studied. Advanced post-processing options include the derivation of advanced hemodynamic measures including vorticity and helicity, WSS, and pressure gradients.
The benefits of 4D Flow for MRI
Now we’ve discussed the workings of 4D Flow for MRI, let’s identify the main benefits of the technique. Not only can 4D Flow MRI be effective in providing comprehensive visualization of blood flow and advanced 4D flow hemodynamic markers in the heart and large vessels, but the technique can also be used throughout the human circulatory system, in vascular territories such as the carotid arteries, cerebral large arteries, peripheral vessels, and abdominal vessels.
4D Flow MRI has proven effective in accurately measuring velocity in all spatial directions. Compared to 2D cardiovascular MRI, 4D Flow MRI offers superior spatial coverage, which could make it “better at capturing the peak velocity of a stenotic jet”.
From 2D to 4D Flow MRI
2D MRI scanning has been used since the 1970s, and standard 2D PC MRI emerged in the late 1980s and was characterized by the through-plane assessment of blood flow fields and velocities. Over time, in-plane PC sequence development enabled time-resolved cine sequence that made 3D velocity encoding possible. This technique is known as 4D Flow MRI. Despite its introduction in the 1990s, 4D Flow MRI hasn’t been suitable for clinical use until recent years. It was previously used predominantly for research.
4D MRI Flow Vs 2D & 3D
2D MRI scanning uses radio frequencies and a powerful magnetic field to create a single slice image. With 2D MRI, the application of magnetic gradients is in two directions, allowing the location of frequencies to be determined. 3D MRI involved the introduction of a third dimension: Volume. This was created with the stacking of multiple 2D slices, and was able to offer a higher spatial resolution of the anatomy by using three magnetic field gradients.
4D Flow MRI goes further than 3D MRI, being able to provide a more detailed visualization of blood flow direction and comprehensive information on temporal and spatial coverage of a given area. This has allowed clinicians to view and quantify flow more extensively than is possible with 2D and 3D MRI. This ability to completely analyze complex blood flow patterns has made it easier to determine the cause of heart defects and abnormalities.
Applications and Future of 4D Flow MRI in Cardiology
In the future, we can expect to see the applications of 4D Flow MRI in cardiology increase. The procedure has already improved the assessment of hemodynamics and therapy management. Its potential has been highlighted for applications such as the management of patients with bicuspid aortic valves, one of the most common abnormalities. As studies continue into data processing automation, sequence optimization, and new flow metrics, the number of applications for 4D Flow MRI look set to increase.
cvi42 with 4D Flow MRI
cvi42, the multi-modality imaging software solution from Circle CVI, includes a 4D Flow Plug-In that offers semi-automated PC-MRA segmentation, complex blood flow pattern visualizations with advanced options for WSS, pressure, and energy loss, and provides quantitative volumetric assessment.