Biomedical Engineering Reference
In-Depth Information
16.5.2 Magnetic Resonance Imaging Agents
For MRI, 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;
they work by reducing T
1
and T
2
relaxation times nearby. The agents tend to be either para-
magnetic or supermagnetic materials that are modified to be ingested or infused into the
body safely and passed out. The major type of agent used is chelated gadolinium in various
forms. The main applications are intravascular and gastrointestinal.
16.5.3 Positron Emission Tomography Agents
For positron emission tomography (PET, introduced in Section 15.3.3), gamma rays are
detected that come from a positron-emitting radionuclide material. The location and con-
centration of these tracer materials in the body are reconstructed from multiple detectors
into a 3D image. The tracer most often used is 18F-FDG, a glucose analog. It is concentra-
tions of this substance that are taken up by tissues and indicate local metabolic activity.
Only concentrations of this contrast agent are displayed, hanging in space, so it is difficult
to determine spatial landmarks to determine locations; therefore, PET images are often
combined with another imaging modality such as CT (described in Section 16.7).
16.5.4 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
m in diameter, similar to the size of red blood cells.
They are injected into the venous system to act as blood cell tracers. While in the blood, ultra-
sound 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, and to identify walls of the moving chambers of the heart (opacification). After
traveling along in blood, the spheres diffuse and are released as they pass through the lungs.
Ultrasound contrast agents are designed for different applications and effects and are
matched to operate at certain insonifying frequencies. They act as miniature nonlinear resona-
tors 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.
m
16.6 COMPARISON OF IMAGING MODES
How can imaging modalities be compared? Each major diagnostic imaging method is
examined in the following sections, and the overall results are tallied in Table 16.4. Exam-
ples of three imaging modalities—CT, MRI, and ultrasound—are shown as different images
of the right kidney in Figure 16.55.
