Ejection fraction (EF) of the heart refers to the amount of blood ejected (pumped) out of the left ventricle with each contraction. Measuring EF is important in diagnosing and monitoring heart failure, as well as assessing how well the heart is pumping blood.
Cardiac magnetic resonance imaging (MRI) is the most accurate test for calculating EF, being considered the gold standard for measuring and diagnosing EF. In this article, we explain how EF is calculated with cardiac MRI, discussing the modality’s accuracy and comparing it to other techniques such as echocardiography (echo) and computed tomography (CT).
How to calculate ejection fraction
As we’ve touched upon, EF is the amount of blood (given as a percentage) ejected from the left ventricle of the heart each time it contracts. The higher the number, the healthier the heart. A low EF of 45% or less could be evidence of heart disease or heart failure. From 50-70%is considered a healthy heart. Some people can have a normal ejection fraction but also have heart failure – this is referred to as heart failure with preserved ejection fraction (HFpEF).
EF represents the percentage of blood that is ejected from the ventricle from each contraction. The EF formula is worked out by dividing the amount of blood pumped from the ventricle in each contraction (stroke volume, or SV) by the end-diastolic volume (EDV), which is the total amount of blood in the ventricle. To give the number as a percentage, it is multiplied by 100. Thus, EF = (SV/EDV) x 100.
What is the gold standard for ejection fraction?
Cardiac MRI has been considered the gold standard for assessing and measuring EF. While cardiac MRI has its advantages and disadvantages, it is considered the best modality for calculating EF. Cardiac MRI can provide more hemodynamic measurements than other non-invasive modalities and offers a 3D representation of images.
Before the emergence of cardiac MRI, echo was the gold standard for EF. It remains the most common test for measuring EF, offering advantages such as wide availability, portability, and low cost.
Cardiac MRI for ejection fraction
Cardiac MRI is favored for measuring EF due to its high signal and high contrast resolution, which can ensure that the endocardial border is well-defined.
MRI can calculate ejection fraction manually or automatically. The Simpson disk summation – which uses short-axis cine steady-state free precession images of the left ventricle – is the most popular method. The ventricular cavity area for each slice is determined by tracing the left ventricle endocardial borders manually on each short-axis image. This traced area for each image slice is then multiplied by the slice thickness + image gap (slice interval) to obtain a volume for each slice. The volumes of the slices are added up to determine the LV volume.
Aside from MRI’s excellent spatial resolution, the modality offers advantages such as:
- Reproducibility
- An unrestricted field of view
- Multiplanar 3-dimensional analysis
- Lack of ionizing radiation
- No requirement to use a contrast agent
Cardiac MRI can be used to measure not only left-ventricle ejection fraction (LVEF) but right-ventricle ejection fraction (RVEF), too. RVEF measures the amount of oxygen-poor blood that is pumped to the lungs for oxygen from the right side of the heart. Measuring RVEF is important for patients with right-sided heart failure, and can provide powerful prognostic information in patients with congestive heart failure (CHF), especially if they have a low LVEF. Right-sided heart failure is less common than left-sided heart failure.
Cardiac MRI ejection fraction accuracy
LVEF needs to be measured accurately in order to inform clinical decisions on prognosis and therapy. While several modalities are capable of measuring LVEF, there is the potential for measurement errors which may result in inaccurate LVEF calculation.
One potential source of errors is a manual measurement of the LV cavity, with different perceptions of the endocardial border and measurement technique variations resulting in human error. Cardiac MRI allows automatic LVEF calculation and benefits from high contrast resolution which makes the endocardial border well-defined.
A paper which examined techniques and potential pitfalls of measuring LVEF concluded that “the extent of variability of a volumetric technique depends on both its ability to accurately delineate the LV cavity and the amount of operator interaction, especially in defining the LV–left atrial interface. Among the volumetric imaging techniques, MRI is generally considered to have the least variability.”
Decreased accuracy in cardiac MRI for EF may result from degraded image quality in patients with ectopic beats or cardiac arrhythmias. MRI is not suitable for patients with implantable devices such as cardioverter defibrillators and pacemakers, which can cause metallic susceptibility artifacts.
Research suggests that the echo-derived myocardial performance index is superior to MRI in detecting severely reduced RVEF (≤30%).
Cardiac MRI vs echo vs CT
Why is cardiac MRI the gold standard for EF, compared to other modalities; specifically echo and cardiac CT? The key to MRI’s effectiveness for measuring EF is its low variability compared to other volumetric imaging techniques.
While echo offers low cost, portability and lack of ionizing radiation, it involves making more geometric assumptions than MRI as the LV cavity border can’t be traced in its entirety. In some patients – such as those who are obese or have chronic obstructive pulmonary disease – there may be insufficient acoustic windows, leading to poor image quality. Echo has been shown to consistently underestimate LVEF compared to MRI, irrespective of the technique used.
For more information, please refer to our article on cardiac MRI vs echo for ejection fraction.
Like MRI, CT involves a few assumptions of LV shape being made, allowing the tracing of the whole LV cavity. Images can be obtained with a single breath hold, and there is a high contrast and spatial resolution. However, CT uses a contrast agent, exposes patients to ionizing radiation, and presents the challenge of timing contrast injection and scanning.
Conclusion
Cardiac MRI is now considered the gold standard for EF, offering a larger field of view and superior reproducibility to echo.
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Sources:
https://www.mayoclinic.org/tests-procedures/ekg/expert-answers/ejection-fraction/faq-20058286
http://www.ncbi.nlm.nih.gov/pubmed/17478238
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1767357/