Biomedical Engineering Reference
In-Depth Information
agents is that sufficient mass of radiodense material must
accumulate at the site of interest to cause a corresponding change
in the attenuation of the incident x-ray beam. Sufficient mass is also
necessary for ultrasound contrast agents, except that they must also
possess echogenicity for detection, placing a further developmental
constraint on these nanoparticles. It is also necessary that there
be a change in the attenuation of the incident x-ray beam or that
sufficient radiodense material accumulate at the site of interest.
Neither (x-rays or radiodensity) is applicable to US. We will now
consider how these imaging probes are being developed.
8.3 Ultrasound
Medical ultrasonography uses reflected sound waves with
frequencies above 20 MHz to construct images. This technology
has been used for several decades to visualize a range of anatomic
features in the human body. Ultrasound can be used to evaluate
cardiac abnormalities, such as blood vessel wall inflammation, as an
early indicator of heart disease. It can also be used to detect blood
clots, such as deep-vein thromboses. More intense US beams can also
be used therapeutically. Thus, one of the more interesting features of
ultrasound is that, in some cases, it can be used both to detect and
to treat disease.
Ultrasound has a number of advantages over other imaging
modalities: It is relatively inexpensive and portable and has a low
risk profile, with no exposure to ionizing radiation and no inherent
toxicity associated with the potential contrast agents (i.e., no iodine
or gadolinium is required). The primary disadvantage is that imaging
is confined to a relatively small area of interest. Advantages over MRI
include high resolution (of the order of 10's of microns, enabling
imaging of small blood vessels) and that repeat scans can be made
immediately, which is difficult with MRI if repeated administration
of contrast agents is required.
Until relatively recently, all US was carried out without the use
of contrast agents. The past 10 years, however, have witnessed the
introduction of the first US contrast agents. The agents enhance
the US image by creating diff erences in the sound reflection, or
echogenicity, between diff erent anatomic compartments. For
example, intravenous administration of encapsulated air bubbles
(“microbubbles”) results in an increased diff erence in echogenicity
 
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