Identifying Coronary Artery Calcification on Non-gated Computed Tomography Scans

Here, we present a protocol to reliably and systematically identify coronary artery calcification (CAC) on non-gated computed tomography (CT) scans of the chest or abdomen. CAC provides an objective measure of coronary artery disease for both research and clinical purposes.

This method can help answer key questions about the clinical interpretation of radiographic images, specifically, how to reliably interpret coronary calcification on non-gated computed tomography, or CT scans. The main advantage of this technique is that it's applicable in a wide range of studies and provides a consistent approach to the identification of a powerful prognostic tool. Begin by opening the image viewer and using an institutional username and password to log in to the program.

Click on the Study List icon, select With Patient ID Equal To in the dropdown menu, and enter the patient's hospital identification number. Under Modalities, click All Modalities to unselect all of the imaging modalities, and select the Computed Tomography modality. Under Body Regions, leave the default set to All Body Regions.

Then, click Find. Click Performed On to organize the list by the date of study, and select the study of interest. Use the tile tool to highlight a single tile, and click to make the screen a single pane.

Hover the mouse over the series icon to identify the series that has an about three millimeter slice thickness, and drag this icon to the center of the viewing screen. Scroll through the images to ensure an adequate visualization of the coronary tree until an image with one of the coronary arteries is visualized. Right-click to select the Window/Level option, and click on Interactive Window/Level.

As a starting point, type 500 in the Window field and 150 in the Level field. Manually adjust the window and level sizes until the fat, including the epicardial fat, appears dark gray to black, the myocardium is a slightly lighter gray, and the calcium and metal are white. Then close the window and level boxes to begin viewing the images.

To identify the coronary artery calcification, scroll through the image, looking at one coronary artery at a time and recording whether coronary artery calcification is present or absent in each of the four major epicardial coronary arteries. To determine whether a questionable area is coronary artery calcification, right-click to open the menu and select Annotate. Click Elliptical Region Of Interest, and draw an outline around the potential calcification, as well as some epicardial fat, without including any other chambers.

After adjusting the outline size and location as necessary, create a region of interest over the sternum and over the ascending aorta. Then compare the maximum Hounsfield unit in the area of potential calcification to the maximum Hounsfield units in the selected ascending aorta and sternum regions. The typical locations for evaluating the coronary vessels are easily identified in most patients.

In these representative gated and non-gated scans, calcification within the right coronary artery can be visualized. In these non-gated and gated CT chest scans of a different patient, cardiac motion obscured the mild coronary calcification in the proximal left circumflex artery. General considerations for identifying the severity of the calcification include the number of vessels involved, the number of plaques in each vessel, and the density of calcification in each plaque.

While attempting this protocol, it is important to remember to be consistent in applying each of these steps. Following this procedure, other methods such as coronary artery calcium quantification can be performed to answer additional questions about the severity of coronary artery disease. This technique has allowed researchers to retrospectively review CT scans to incorporate this important imaging finding in large data sets without the need for expensive prospective studies.

Although the image processing itself is very safe, it is important to take the necessary precautions to ensure patient confidentiality when viewing the images for research purposes.