Contrast agents, also known as contrast media, are often used during medical imaging examinations to highlight specific parts of the body and enhance their contrast. Contrast agents can help detect diseases earlier and more precisely. Moreover, they can provide relevant diagnostic information that physicians are otherwise frequently unable to obtain without using invasive methods. They thus play a crucial part in guiding, monitoring and following up treatment.
Contrast agents can be used with many types of imaging examinations, including X-ray and magnetic resonance imaging (MRI). They use different mechanisms depending on the underlying imaging technique.
Fig. 1: Dual Source CT: contrast agents make it possible to visualize the inflamed area around the phalangeal joints of a gout patient. (Courtesy of Christoph Becker, LMU Munich)
X-ray contrast media
Whereas bony structures of the body can easily be imaged using classic X-ray techniques or computed tomography, distinctions between soft tissues are often much less clear. Contrast agents are necessary to create an artificial contrast between the organ or tissue to be diagnosed on the one hand, and vascular or surrounding anatomic structures on the other. Most X-ray contrast media contain chemical elements of high atomic number such as iodine. They attenuate X-rays, thereby enhancing contrast with organ structures. The company formerly known as Schering, which is now part of Bayer HealthCare Pharmaceuticals, was one of the pioneering developers of these substances, launching the first iodine-containing injectable X-ray contrast medium as early as 1931. Today, 60-80% of all computed-tomography (CT) procedures require contrast enhancement.
Fig. 2: Brain MRI, left image: without contrast no lesion is visible; right: contrast-enhanced MRI leads to the detection of a brain metastasis (white spot) (Courtesy of M. Taupitz, Germany)
MRI contrast media
The secret behind the mode of action of MRI contrast media lies in their ability to interfere with the electromagnetic behavior of tissues, leading to improved contrast on an MR image.
In MRI the patient is exposed to a powerful magnetic field. Certain atomic nuclei in the body – mainly the hydrogen protons in the water molecules – are then electromagnetically stimulated by radio frequency impulses. While they are “recovering” – or relaxing again – they transmit signals that are recorded and compiled into an image.
Most MRI contrast agents are based on the rare earth element gadolinium, adding paramagnetic properties to the compound. Gadolinium causes the atomic nuclei to relax more quickly and thus to transmit stronger signals, leading to an improved contrast.
Bayer HealthCare Pharmaceuticals launched the first contrast medium for magnetic resonance imaging in 1988, a milestone in the development of MR imaging. Today, a broad range of MR contrast agents is available, helping in the diagnosis of breast, liver and brain tumors, and in the detection of occluded blood vessels, to name just a few applications.