Contrast Agents Commonly Applied In Medical Imaging

Contrast agents emphasize or magnify the physiological features or functions that would otherwise, be invisible, weak or obscured in images. Contrast agents are commonly applied in vasculature and also employed in applications where the uptake of an agent by an organ is used to indicate the degree of functional or metabolic activity.

Each agent is designed to function with particular physics of the intended imaging modality as well as the region of indication of the body. These agents include:

• Ultrasound Agents
• Computed Tomography Agents
• Magnetic Resonance Imaging Agents
• Positron Emission Tomography Agents

Ultrasound Agents

Ultrasound contrast agents are small microspheres that are used to highlight the passage of blood in regions otherwise difficult to image, such as blood, which is nearly invisible to ultrasound without special processing. These agents are usually microspheres filled with air or gases such as perfluorocarbon, with thin, flexible shells typically made of human serum albumin or surfactants, and are about 4 μm in diameter, similar to the size of the red blood cells.

The ultrasound contrast agents are injected into the venous system to act as blood cell tracers. While in the blood, ultrasound agents significantly improve the ability and sensitivity of ultrasound to follow the flow of blood especially in small vessels, and the perfusion of blood in muscle, especially those of the heart (opacification). After travelling along in blood, the spheres diffuse and are released as they pass through the lungs.

Ultrasound contrast agents are designed for different applications & effects and are matched to operate at certain frequencies. They act as miniature nonlinear resonators that greatly enhance their reflectivity for ultrasound. Under certain conditions, the unusual properties of these agents are changes in size, cavitation, fragmentation, or directed movement.

Related: Ultrasound Scanning Techniques

Computed Tomography Agents

Intravenously administered CT contrast agents containing iodine or gadolinium are used in the visualization of blood vessels. For instance, for a CT pulmonary angiogram, emboli and obstructions can be seen through the attenuation of the x-rays or opacification, which brightens the appearance of flow in a vessel. Through the enhancement of blood vessels, other applications include visualization of the internal structures of the brain, spine, and liver among other organs. Subsequently, the kidney and liver normally aid in passing the agents out of the system. In some cases, these agents may involve, some risk and possible allergic reactions such as those of the kidney.

For the diagnosis of abdominal ailments and injuries, CT contrast agents are administered either orally or rectally. A dilute suspension of barium sulfate is used in most cases. Applications include the diagnosis and characterization of cancer and the investigation of sources or severe abdominal pain and trauma.

Related: What is CAT scan?

Magnetic Resonance Imaging Agents

For MRI, the contrast agents distort or alter the magnetic fields locally, where they are concentrated so there is a relative change in a region. The effects achieved can either intensify or weaken the signal close by, depending on the image weighting selected and the context of the agent. The agents tend to be either paramagnetic or super magnetic materials that are modified to be ingested or infused into the body safely and passed out. The main type of agent used is chelated gadolinium in various forms. The key applications are intravascular and gastrointestinal.

Related: Magnetic Resonance Imaging (MRI)

Positron Emission Tomography Agents

In Positron Emission Tomography, gamma rays are detected that come from a positron-emitting radionuclide material. The locations and concentration of these tracer materials in the body are reconstructed from multiple detectors in 3D image. It is the concentrations of this substance that are taken up by tissues and indicate the local metabolic activity. Only concentrations of this contrast agent are displayed, hanging in space, so it is difficult to determine locations; therefore PET images are often combined with another imaging modality such as CT.

Related: Positron Emission Tomography (PET)