Detection and Description of Tissue Disease: Advances in the Use of Nanomedicine for Medical Imaging - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 243

8.2 Contrast Media for Computed Tomography

Computed tomography (CT) is an imaging modality in which x-rays
are projected through the subject at diff erent angles and the resulting
data are then reconstructed into three-dimensional images. X-rays
are able to pass through the diff erent types of tissue to varying
degrees, with bone being relatively radiodense, air being essentially
radiotransparent, and water lying between these two extremes.
The attenuation of the x-rays is measured in Hounsfield Units (HU),
with air and water arbitrarily set to 0 and 1000, respectively. As
soft tissues tend to have very similar radiodensities, CT imaging
can be significantly improved by injecting radiopaque compounds
intravenously, improving the definition of vascular structures and
providing some degree of soft tissue targeting and diff erentiation.
There is both a need and an opportunity for new CT contrast
agents that target specific tissues or provide prolonged imaging
times. Magnetic resonance imaging (MRI) can also provide high-
resolution anatomic images as well as quantitative information on
blood flow, but CT off ers both technical and pragmatic advantages
compared with MRI: There is a direct proportionality between x-ray
absorption and contrast agent concentration, simplifying quantitative
studies, and CT is less expensive and more widely available than MRI.
The primary disadvantage of CT compared with MRI is that CT scans
involve exposure to ionizing radiation, but the radiation dose can
be minimized by only imaging the region of interest and by careful
selection of imaging parameters (i.e., power (kvp) and x-ray current
(mA)).
CT contrast media must be very water soluble because of the
large quantities that are injected. The solubility requirement results
in rapid clearance from the blood, which in turn limits the available
imaging time. Prolonging the clearance time, by means such as
increasing the molecular weight of the agent, has, therefore, been an
important research focus. A second research focus is the development
of CT contrast agents that target specific tissues. This objective
has proved difficult to achieve because a significant mass of the
contrast agent must accumulate in the target tissue before the tissue
density increases enough to improve the contrast of the image. This
requirement, in turn, increases the risk of toxicity. While iodine is the
primary element used in CT contrast agents, primarily because of its