Aortic valve regurgitation, also known as aortic valve insufficiency or aortic valve incompetence, is a valvopathy that describes leaking of the aortic valve during diastole that causes blood to flow in the reverse direction from the aorta and into the left ventricle. Patients with valvular conditions are referred to MR following an inconclusive Doppler ultrasound exam. While Doppler ultrasound is the current gold standard, it cannot always provide a precise answer on whether the patient should undergo surgery in cases with a poor acoustic window.
A cardiac MR (CMR) exam is comprised of various sequences that can provide a detailed assessment of the aortic valve and left ventricular function. It is a highly accurate method to determine the size of the aortic root, assess regurgitant parameters, determine ejection fraction, measure left ventricular size and detect underlying etiologies. However, acquiring quality cardiac sequences to quantify cardiac function and flow has historically been a complex and time-consuming exam to perform, requiring technologist expertise and physician supervision with little room for error when capturing constantly moving anatomy. Conventional CMR techniques like 2D phase contrast require multiple slice acquisitions that are perpendicular to the flow of the blood. For some pathologies, this would require the patient to hold their breath — in many cases greater than 20 times in an exam. Considering that patients who typically receive a CMR exam often have heart disease, it can be difficult for them to repeatedly hold their breath and, therefore, exams may suffer from sub-optimal or non-diagnostic image quality. Despite the value of CMR, these limitations continue to complicate image acquisition.
New technology could shift this paradigm. Several techniques enabling free-breathing flow acquisitions are under investigation. Real-time CMR is one approach to image acquisition during free breathing that is analogous to echocardiography, or cardiac ultrasound. Using acceleration techniques, the data is rapidly acquired throughout the breathing cycle and then reconstructed to provide an average heartbeat. Alternatively, 3D CMR data can be acquired during free-breathing over several minutes using respiratory motion compensation, with data then reconstructed retrospectively.
Figure 1.
cmr42 post-processing for streamlined visualization.
To improve data acquisition in our institution, we have implemented the ViosWorks 4D Flow sequence in standard CMR exams. With this technique, the technologist simply places the imaging volume over the patient’s chest and data acquisition is completed with no breath-holds. There is little interaction necessary on the front end and immediate reconstruction of the images in order to review instantly, which is helpful to ensure the proper velocity encoding (VENC) of the vessel before the patient gets off the table. The image can be reformatted to an arbitrary plane and blood flow in the entire volume can be quantified retrospectively in offline processing.
The 4D Flow data can be used to measure blood flow velocity and direction in any part of the cardiovascular system, including flow quantification in the ascending aorta and main pulmonary artery, as well as in patients with congenital heart disease. This approach is particularly attractive because these patients frequently require flow measurements to be made in multiple vessels and at various levels within that vessel. Using traditional 2D CMR sequences, flow in each location is measured from a separate acquisition that needs to be set up precisely from separately acquired localizer images, resulting in prolonged scan times.
With ViosWorks 4D Flow, the volume of data is enough to cover the entire chest. Isotropic images are gated and timed to the breathing cycle to provide high spatial resolution with 2 mm3 slices, enabling retrospective reformatting in any image orientation.
Patient history
A 24-year-old male with an unremarkable medical history presented with recurring shortness of breath on exertion. On auscultation, the patient was found to have a heart murmur. He was referred for further assessment with echocardiogram, which revealed severe aortic regurgitation with dilated left ventricle and systolic dysfunction. The patient underwent CMR for an accurate measurement of aortic regurgitation and to investigate the cause of left ventricle dilatation.
Technique
The ViosWorks 4D Flow sequence was performed on a 1.5T SIGNA™ Artist and was completed in as little as 10 minutes with the patient free-breathing. This technique provides quantitative cardiac measurements including flow, regurgitant fraction, stroke volume, ventricular volumes and ejection fraction.
Figure 2.
Using cmr42 it is possible to detect regurgitation flow (arrows).
Figure 3.
(A) 2-chamber FIESTA Cine and (B) 4-chamber FIESTA Cine.
MR findings
Patient diagnosed with bicuspid aortic valve, a congenital disorder, and vortex, or twisted, flow. Aortic flow (obtained by analyzing
4D Flow):
- Total forward volume: 93 ml
- Backward volume: 35 ml
- Forward volume: 58 ml
- Regurgitation fracture: 38%
Left ventricle function and volume:
- Ejection fraction: 60%
- LVEDV/BSA: 141 ml/m2 (dilated)
ViosWorks 4D Flow provided a complete view of anatomy of the heart, including the flow within the four chambers and large vessels. This allowed us to study flow patterns throughout the cardiac cycle and to visualize turbulences and quantify flows such as regurgitations. As a result, we were able to diagnose vortex, or twisted flow, which is a blood flow that has separated from the central streamlines within a vessel and countercurrent to the main flow direction. This condition was only diagnosed by using 4D Flow and was not seen on conventional 2D phase contrast or echocardiography.
Discussion
The 4D Flow technique provides the information needed for basic flow quantification and appears promising for more advanced hemodynamic analysis, including pressure gradients, wall shear stress, pulse wave velocity and kinetic energy. It delivers high-resolution images depicting volumetric, cardiac-motion-resolved heart anatomy and blood flow with improved exam efficiency and minimal or no breath-holding, addressing many of the challenges facing CMR today. With ViosWorks, for the first time all seven dimensions of information — 3D in space, 1D in time and 3D in velocity — can be captured in a 10-minute or less free-breathing cardiac exam.
Figure 4.
(A) 3-chamber LVOT FIESTA Cine for evaluating function; (B) short axis FIESTA Cine; and (C) sagittal oblique FIESTA Cine perpindicular to the aortic valve demonstrating the flow jet from regurgitation.
Figure 5.
(A) 2-chamber, (B) 4-chamber and (C) short axis PS MDE, post-contrast.
ViosWorks 4D Flow has enabled us to accurately measure trans stenotic pressure gradients non-invasively in aortic coarctation. Previously, this could only be measured invasively in the cardiac catheterization lab.1 It may also be possible to identify alterations in hemodynamics that can affect the growth of aneurysms or development of atherosclerotic plaque.
As important, 4D Flow has simplified image acquisition and reduced overall exam time in patients with congenital and valvular heart disease. The use of an acceleration technique has played a significant role in reducing scan time by exploiting data correlations in space. Prescription of the image plane is important to obtain a true double oblique image. Otherwise it is possible for the flow data to be incorrect. Further, a key benefit of 4D Flow is that it acquires comprehensive flow data for the entire data set. We can then go back and process the data retrospectively; if a clinical question is raised after the exam, we can go back and perform additional measurements and process additional flow information from the vessel in the field of view without rescanning the patient.
We are also using the Circle cvi42 cmr42 post-processing software based on deep learning for faster image analysis. What once took hours of computer processing time now can be accomplished in minutes.
Today, ViosWorks 4D Flow is preferred clinically at The Royal Hospital Muscat. 4D Flow is a technology that may change our cardiac imaging practice and we are convinced that this technique will play a more important role in the near future for evaluating patients with cardiac disease.
References
- Fusman B, Faxon D, Feldman T. Hemodynamic rounds: Transvalvular pressure gradient measurement. Catheter Cardiovasc Interv. 2001 Aug;53(4):553-61.
