Intravenous Contrast

Excerpt

The concept of contrast is the foundation upon which imaging rests. Contrast is simply the ability to distinguish two objects. In medical imaging, it allows adjacent substances or tissue to be distinguishable and visualized. Contrast allowed Marie Curie to identify her bones from the outline of her hand. The ability to distinguish target tissue from the surrounding structures is how anatomy is defined, pathology identified, and diagnoses are made. While specific anatomical structures have inherent contrast due to their physical properties, others are naturally poorly delineated. Radiographs easily define bones against a background of muscle and fat, but identifying early hepatocellular carcinoma from a background of a nodular cirrhotic liver is limited using routine CT imaging. To increase the contrast of these very similar tissues, an agent can be used to change the appearance of the target tissue, the background tissue, or both.

The optimal use of contrast will depend on the modality and physics of the imaging system. Radiographs and consequently computed tomography (CT) reflect how a target tissue depletes an energy signal as it passes through the tissue. The variables contributing to the signal loss include both the energy beam’s and the target’s physical properties. By changing the beam’s wavelength, amplitude, and frequency, inherent contrast can be maximized. Different techniques are utilized to evaluate for a rib fracture versus pneumonia, despite both studies covering the same anatomical structures. The physical density, atomic structure, and location of lung parenchyma are different from the bones. While technique can be manipulated to maximize some of the tissue’s characteristics, the pathology or tissue of concern may be beyond the contrast resolution of the system unless there is introduction of outside variables. A material added to delineate or better discern these otherwise subtle findings is called contrast media.

Various types of contrast media are useful in medical imaging. The two largest groups include CT and magnetic resonance imaging (MRI) agents. The contrast groups are not interchangeable. Additional, less commonly utilized contrast agents, including fluoroscopy-based air or CO2, and molecular imaging (nuclear imaging), will not be discussed in depth here. A contrast medium can be utilized internally or externally to a patient, and administration is frequently enterally or intravascularly. Intravascular phases can be arterial, venous, or lymphatic. This article will be limited to administration intravenously (IV).