Both cardiac MRI and echo have been considered the gold standard for measuring ejection fraction (EF); the amount of blood ejected from the left ventricle of the heart each time it contracts. These two imaging modalities are both viable methods of calculating EF, helping to assess how well the heart is pumping blood, and play a role in diagnosing and monitoring heart failure.
In this article, we will ask whether cardiac MRI or echo is best for measuring ejection fraction, weighing up the pros and cons of each.
Cardiac MRI for ejection fraction
In recent years, cardiac MRI has come to be recognized as the gold standard for EF. Cardiac MRI is considered the best modality for calculating EF, due to the range of hemodynamic measurements it provides compared to other non-invasive modalities, and the 3D representation of images that it offers. The modality is favored for its high signal and high contrast resolution, which ensures a well-defined endocardial border that is important for measuring EF.
Cardiac MRI provides the manual or automated calculation of EF. The EF formula divides the amount of blood pumped from the ventricle in each contraction (stroke volume, or SV) by the total amount of blood in the ventricle (the end-diastolic volume, or EDV). EF is given as a percentage by multiplying this figure by 100. To summarize EF = (SV/EDV) x 100.
Aside from its superb spatial resolution, MRI offers benefits such as multiplanar 3-dimensional analysis, an unrestricted field of view, excellent reproducibility, lack of ionizing radiation, and no need to use a contrast agent.
Cardiac MRI can measure both left-ventricle ejection fraction (LVEF) and right-ventricle ejection fraction (RVEF).
Cardiac MRI ejection fraction accuracy
Accurate LVEF measurement has an important input on clinical decisions. Other imaging modalities can produce errors related to manual measurement of the LV cavity, due to different human perceptions of the endocardial border and variability in the techniques used. Cardiac MRI’s high contrast resolution results in a well-defined endocardial border. The modality allows automatic LVEF calculation and according to research has the “least variability” among volumetric imaging techniques.
Left ventricular ejection fraction
Manual, semi-automated, or automated methods can be used to calculate LVEF with cardiac MRI. The test is accurate and reproducible, can also measure left ventricular masses and volumes, and is used to assess indications such as various cardiomyopathies, new onset heart failure, and acute myocardial infarction.
The Simpson disk summation method enables the calculation of LVEF, using short-axis cine steady-state free precession images of the LV. To determine the ventricular cavity area for each image slice, the left ventricle endocardial borders are traced manually on each short-axis image. The traced area for each slice is multiplied by the slice thickness + image gap (slice interval), which gives a volume for each slice. LV volume is determined by adding up these volumes.
Cardiac MRI is the gold standard for measuring LVEF, providing highly accurate and reproducible data, involving no ionizing radiation exposure, and possessing low variability which means serial studies can be compared with confidence.
Right ventricular ejection fraction
Cardiac MRI is also useful in measuring right-ventricle ejection fraction (RVEF). RVEF measures the volume of oxygen-poor blood pumped to the lungs from the right side of the heart. In patients with right-sided heart failure, RVEF measurement with MRI can provide powerful prognostic information in patients with congestive heart failure (CHF), particularly patients with a low LVEF.
Pitfalls and considerations
There are circumstances in which cardiac MRI accuracy for EF may be decreased. This includes cases of patients with ectopic beats or cardiac arrhythmias, which may lead to degraded image quality. Furthermore, patients with implantable devices that can cause metallic susceptibility artifacts - such as cardioverter defibrillators and pacemakers – are not suitable for MRI.
Myocardial performance index, the echocardiographic parameter, has been demonstrated by research to be superior to MRI in the detection of severely reduced RVEF (≤30%).
Echocardiogram for ejection fraction
Echocardiography is the most widely used test for measuring EF. It is a favored modality due to its low cost, portability, and lack of ionizing radiation. Various methods have been used to measure LVEF with echocardiography, with 1D, 2D or 3D measurements being obtained.
The modified Quinones method is commonly used with 2D echocardiography, employing linear measurements including single measurements of the LV cavity in the mid-ventricle in both end-diastole and end-systole.
There is also the modified Simpson method; a 2D echocardiographic technique method that necessitates area tracings of the LV cavity and the LV endocardial border.
Echocardiography ejection fraction accuracy
1D and 2D echocardiography accuracy is compromised in patients with regional variation in systolic function. This is because measurements may be obtained from an area of the LV cavity where the function contradicts the overall ventricular function. Incorrect imaging planes may also cause reduced accuracy.
EF can also be measured using 3D echocardiography, with reconstruction techniques able to acquire 3D data of the heart, allowing calculation of LV volumes. This type of echocardiography is more accurate than 1D or 2D methods due to the detection of the whole LV cavity but relies on the sufficient quality of the acoustic window for complete delineation of the LV cavity endocardial border.
Left ventricular ejection fraction
The modified Quinones method is used with 2D echocardiography for measuring LVEF. This method requires only adequate visualization of the endocardium in the mid-ventricle to perform but does involve assumptions relating to LV chamber geometry being made as the circumferential contraction is measured in a single plane.
The modified Simpson method, also used with 2D echocardiography, is recommended by the American Society of Echocardiography for measuring LVEF. This method requires fewer geometric assumptions of LV shape compared to the modified Quinones method.
LVEF can also be calculated with 3D echocardiography, using reconstruction techniques to acquire 3D data of the heart and calculate LV volumes. These techniques typically require data obtained over several heartbeats with 3D imaging probes.
Right ventricular ejection fraction
There is currently no established method of estimating RVEF with 2D transthoracic echocardiography. However, 3D echocardiography has been used to measure RVEF, providing useful prognostic information in cardiovascular diseases.
Pitfalls and considerations
Despite its low cost, portability, and lack of ionizing radiation, echo involves more geometric assumptions than MRI. In some patients there can be insufficient acoustic windows that lead to poor image quality. And in terms of accuracy, compared to MRI, echo has been shown to consistently underestimate LVEF.
2D echocardiography suffers from the need for significant geometrical assumptions to be made to accommodate linear measurement changes, and in cases of asymmetry of contraction, such as wall motion abnormalities, the technique has considerable limitations.
In 3D echocardiography, as image data is typically obtained over several heartbeats, breathing during the imaging time or an ectopic beat lead to artifacts that may change the endocardial border, with segments of the left ventricle appearing to contract at different times. Inaccurate LVEF measurements can also stem from 3D echocardiography’s requirement to manually assign some points in the left ventricle.
Summary
The low variability of cardiac MRI makes it superior to echo volumetric imaging techniques. While echo offers several significant benefits – including low cost, portability, and lack of ionizing radiation – its requirement of more geometric assumptions and the possibility of insufficient acoustic windows make it inferior to MRI, which is the gold standard for measuring EF.
Sources:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1767357/
https://pubmed.ncbi.nlm.nih.gov/21900300/
http://www.ncbi.nlm.nih.gov/pubmed/16376782
https://cardiovascularultrasound.biomedcentral.com/articles/10.1186/s12947-017-0096-5