Contrast optimization and scan timing for single and multidetector-row computed tomography.

Various complex pharmacokinetic interrelationships affect the use of contrast media for computed tomography (CT) imaging. These include factors related to each patient's unique body habitus and their degree of health, such as their age, gender, height, weight, and cardiovascular status. Of equal importance are factors related to the contrast material injection. The volume, concentration, and rate of injection all affect the degree of enhancement that is achieved with an injection of contrast material. In addition, the injection technique--whether the contrast is infused at a constant injection rate (uniphasic injection) or whether the rate is altered during the injection (multiphasic injection)--also affects the magnitude and duration of contrast enhancement. In body CT imaging, the liver poses unique challenges in managing the use of intravenous contrast material because of its dual blood supply and the need to complete imaging before equilibrium occurs between the intravascular and extravascular compartments. The magnitude of hepatic enhancement that is ultimately achieved is related primarily to the amount of iodinated contrast material that accumulates in the extravascular space within the target organ, independent of the speed of the CT scanner. The key determinant of the onset of the equilibrium phase is the injection duration. Given that a high injection flow rate (4-5 ml/s) is desirable for arterial phase imaging, the injection duration is maintained by using an appropriate contrast volume. Thus, modifications of the total iodine dose are best performed by altering the contrast concentration. Use of a high contrast concentration (400 mg iodine/ml) may be advantageous in heavy patients, or in patients in whom routine imaging is married with a need for high-detail imaging of the vasculature with high resolution CT angiography. The magnitude of arterial enhancement that is achieved is related to both the concentration of contrast and the rate of administration. The speed of the scanner determines its ability to record image data during the most advantageous time period, the peak of arterial enhancement. Thus, rapid imaging is particularly advantageous for optimal contrast use in CT angiography as well as in multiphasic imaging of the parenchymal organs.