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to detect regions of biologic activity that may be associated with disease. Relatively
short lived isotope, such as Technetium 99mTC, Iodine 123I, Gallium 67Ga or 18F
is administered to the patient. Isotopes are often preferentially absorbed by biologi-
cally active tissue in the body, and can be used to identify tumours or fracture points
in bone. Images are acquired after collimated photons are detected by a crystal that
gives off a light signal, which is in turn amplified and converted into an image. It
is used heavily in clinical oncology (medical imaging of tumours and the search for
metastases), and for clinical diagnosis of certain diffuse brain diseases such as those
causing various types of dementias. PET is also an important research tool to map
normal human brain (see Fig. 12.4) and heart function.
Fig. 12.3 MRIimagesliceofahead
Ultrasound: The ultrasound image is the result of reflection, refraction and de-
flection of ultrasound waves in the megahertz range, from different types of tissues
with different acoustic impedance (see Fig. 12.5). Usually, the contrast in ultra-
sound images is very low and boundaries between region of interest and background
are fuzzy. The sound wave is partially reflected from the layers between different
tissues. Specifically, sound is reflected anywhere there are density changes in the
body: e.g. blood cells in blood plasma, small structures in organs, etc. This is com-
monly associated with imaging the fetus in pregnant women. Uses of ultrasound
are much broader, however. Other important uses include imaging the abdominal
organs, heart, breast, muscles, tendons, arteries and veins.
 
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