Computed tomography angiography (CTA) is a procedure gaining usage in the diagnosis of aneurysms located in the aorta, carotid arteries, and in other locations and has also shown promise in the planning of stent placement procedures. Recently, automatic vessel segmentation programs have been developed that can extract the entire aortic vessel tree and provide information to the user regarding the size, length, and tortuosity of the blood vessels. This study was designed to determine if using the full width at half maximum (FWHM) value is an accurate method of determining the diameter of contrast-enhanced blood vessels. A phantom used to simulate vessels of various diameters was filled with a nonionic iodine solution and scanned using a 16-detector CT scanner (Mx8000IDT, Philips Medical Systems, Inc.). The phantom was scanned with varying concentrations of contrast solution to emulate the variation of enhancement that may be seen clinically. The data was analyzed using an application on a workstation (MxView, Philips Medical Systems, Inc.), which allowed for the calculation of FWHM of a user-defined region of interest. The results indicate that the full width at half maximum is an accurate method of calculating the diameter of a blood vessel, regardless of contrast concentration. The full width at half maximum is an easily calculated value, which could potentially be used in an automatic segmentation algorithm to determine the diameters of extracted vessels.