Diagnosis and Therapeutics
Last modified on June 5, 2010
For a list of rule-outs for rodent infectious diseases, click here.
For a list of fish diseases, click here.
This
section is supposed to incorporate internal medicine and diagnostic imaging. For
clinical pathology (including techniques and interpretation of results in
virology, microbiology, hematology and other -ologies, go to the Clinical
Pathology pages.
Imaging
DEXA
MRI
PET
SPECT
Bioluminescence
MicroCT
MicroCT (left, middle), microPET (bottom), microMRI (right)
images of mice.
Dual Energy X-Ray Absorptiometry (DEXA)
Bone density can be measured using DEXA, which originated in 1963 with
single-photon absorptiometers. Dual photon units were developed to improve
compensation for differences in thickness and shape of soft tissues, resulting
in quicker scans that were more accurate. X-ray absorptiometers were first
introduced in 1987, further reducing scanning time. The "pencil beams"
are narrowly collimated, yielding high resolution with low exposure to X-rays.
Computer software enables measurement of bone, lean tissue, and fat composition
in addition to bone mineral content.{4090}
One
use of DEXA is in the measurement of body composition of animals throughout
growth and development. There are significant differences due to age and gender
in the cat. Starting at 45 days of age, there is no difference in basic
composition. Males grow larger and for a longer period of time, making larger
gains in lean body mass up to 1-2 years of age, whereas females stopped growing
lean tissue at 6-10 months. Bone mineral content (in grams) did not decrease in
males after neutering. There is high individual variation that makes it
necessary to use large group sizes for study.{4090}
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Magnetic Resonance Imaging (MRI)
MRI uses magnetic fields and radio signals. T1 images (spin-lattice
relaxation) are obtained as hydrogen nuclei release energy imparted by the
magnet, which is detected by an antenna. The time between excitation and
recovery is the spin-lattice relaxation. T2 images are obtained by changing the
scanner to a different mode, and is sensitive to different tissues. In T2 mode
the hydrogen molecules are made to precess in phase with each other. When the
radio-frequency pulse is discontinued the signal decays as the molecules fall
out of phase; this is called the spin-spin relaxation time. Small-animal MRI
units can have magnets in the range of 10 Tesla and resolutions of 50x50x50µm.{4186}
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Positron Emission Tomography (PET)
PET images the anatomical distribution of radiolabeled molecules which are
natural body compounds (i.e. carbon-11, nitrogen-13, oxygen-15 and fluorine-18)
which must be produced by a nearby cyclotron. The subject fits into a ring of
detectors, which pick up two 511-keV gamma rays traveling along opposite paths.
Resolution is 1-2mm.{4186}
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Single-Photon Emission Tomography (SPECT)
SPECT is similar to PET but the isotopes used produce single gamma rays which
are picked up through collimators located above and below the subject. The
isotopes are commonly available and less expensive. Compounds like
technicium-99m, indium-111, iodine-131 and iodine-123 can be chemically attached
to molecules of interest, sometimes in the investigator's lab. The resolution is
1-2mm.{4186} Top
Bioluminescence imaging uses visible light, i.e.
that generated by luciferase. The light is detectable by CCD (cooled charged
coupled device) cameras. This technology has been used to map bacterial
growth, tumor growth and gene expression in living mice.{4186}
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Micro-Computed Tomography
X-ray micro computed tomography (micro CT) equipment is less expensive than
MRI, simple to operate, and has resolution comparable to MRI. The scanner is
very sensitive to bone and fat and can also be used to image soft tissue if
contrast agents are given.{4186}
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