Coronary artery disease (CAD) is a condition where the heart’s arteries become narrowed, potentially restricting blood flow to the heart muscle. Diagnosis traditionally relied on imaging techniques that showed the physical structure of these arteries and identified blockages. Computed Tomography Fractional Flow Reserve (CT FFR) is a newer non-invasive advancement that combines detailed imaging with sophisticated computational analysis. This technology determines the actual functional impact of a blockage on blood flow, moving beyond merely identifying a narrowing. CT FFR provides a more comprehensive picture of heart health, helping clinicians make informed decisions about patient care.
Understanding the Role of CT FFR in Heart Health
Standard coronary CT angiography (CTA) is excellent at visualizing the anatomy of the coronary arteries and identifying the presence and severity of plaque buildup, known as stenosis. A limitation of CTA is that a physical narrowing seen on an image does not always translate into a restriction of blood flow that causes symptoms or damage to the heart muscle. This lack of functional information means that CTA alone can sometimes overestimate the severity of a blockage, potentially leading to unnecessary invasive procedures.
CT FFR addresses this functional gap by calculating the Fractional Flow Reserve, a measurement of pressure differences across a narrowing, without needing an invasive catheter. It determines if a blockage visible on the CT scan is restricting the blood supply to the heart muscle. This tool is particularly useful for patients with intermediate blockages, typically between 50% and 70% narrowing, where the clinical significance is often uncertain. By establishing the hemodynamic significance of a lesion, CT FFR helps physicians decide which patients can be managed with medication and which require an invasive procedure like stenting or bypass surgery.
The Technology Behind the Calculation
The CT FFR value is generated through image processing and a technology called Computational Fluid Dynamics (CFD). First, high-resolution images acquired during a standard coronary CT angiography scan are used to create a detailed three-dimensional (3D) model of the patient’s coronary artery tree. This model reconstructs the vessel geometry, including the location and dimensions of any plaque or narrowing.
The raw CT data is then uploaded to computing systems where the CFD simulation takes place. CFD involves solving hundreds of thousands of mathematical equations, derived from the laws of fluid dynamics, to simulate how blood flows through the modeled arteries. These equations, such as the incompressible Navier-Stokes equations, calculate the pressure and flow velocity at every point within the 3D model.
The simulation accounts for factors like the viscosity of blood and assumes conditions of maximum blood flow, which is known as hyperemia. By comparing the calculated blood pressure before a narrowing to the pressure after the narrowing, the software derives the FFR value. The process is non-invasive, using only the images obtained from the initial CT scan to create a digital twin of the patient’s coronary circulation.
What to Expect During the CT FFR Process
The CT FFR process begins with a standard coronary CT angiography (CTA) scan. Patients may be asked to fast for a few hours beforehand and may need to adjust certain medications, such as those for diabetes. To ensure clear images, a contrast dye is injected, and medication may be given to slow the heart rate.
During the scan, the patient lies on a table that moves through the CT scanner while the machine captures images of the heart. The patient may be asked to hold their breath briefly to minimize movement and prevent image blurring. Once the CTA scan is finished, the patient is free to leave, as the FFR calculation occurs after the imaging is complete.
The raw image data is transmitted to a specialized center for the computational analysis. The time required for the CFD modeling can vary, but results are often returned to the ordering physician within 24 to 48 hours. The CT FFR analysis does not require a second appointment, additional scans, or any further physical interaction.
Interpreting the FFR Value and Treatment Planning
The Fractional Flow Reserve (FFR) is expressed as a ratio, representing the maximum blood flow in a stenotic artery compared to the maximum flow that would exist in a normal artery. Mathematically, the calculation is the ratio of the mean blood pressure measured immediately after the blockage to the mean blood pressure measured in the aorta, under hyperemic conditions. A normal, unrestricted artery would have an FFR value of 1.0.
A clinical threshold for determining a significant blockage is an FFR value of 0.80. An FFR value greater than 0.80 suggests that the blockage is not causing a significant restriction of blood flow to the heart muscle, and the patient can be managed with optimal medical therapy. Conversely, a value of 0.75 or less is considered abnormal, indicating a hemodynamically significant blockage that warrants consideration for an invasive procedure.
Values in the borderline range, such as 0.76 to 0.80, require consideration of the patient’s clinical scenario, symptoms, and other imaging findings. CT FFR accurately differentiates between functionally significant and non-significant blockages, helping to avoid unnecessary invasive coronary angiography, which carries risks. This functional information guides cardiologists toward a personalized treatment pathway, ensuring invasive interventions are reserved for blockages that restrict blood flow.