Animal Models for Preclinical Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 461

standardized positioning between subjects and a reduced motion artifact. The posi-
tion of internal organs within the body varies with position. For comparable images,
it is important to standardize the position of each subject.
Breathing motion can introduce significant reduction in resolution for imaging,
particularly in small animals. This becomes most important for regions near the
lungs or abdomen. Gating of image acquisition based on the heart rate is necessary
for cardiac imaging and requires fast data acquisition. Breath and cardiac gating of
acquisition can be automated by inputting physiologic signals from the monitoring
systems to trigger data acquisition. An alternative to gated image acquisition is
postprocessing of image data. Automated processing can best be accomplished by
programmatic selection based on physiologic monitoring acquired simultaneously
with the imaging data. otherwise, a manual selection process can be effective, but
time-consuming.

16.3.6 other considerations

For optical, ultrasound, and photoacoustic (optoacoustic) imaging experiments, the
hair covering the region of interest must be removed. Hair significantly attenuates
signal in these modalities. Hair removal must be done carefully to prevent damage to
the skin that may cause artifacts in imaging. Gentle clipping can be followed with
chemical depilatory such as Nair® to remove hair over a large section of the body.
It is not practical to remove all of the hair. Nude mice that are naturally hairless are
often used in optical imaging for this reason.

16.4 choice of AnimAl model

The primary considerations in choosing an animal model should be based on the
biological question at hand. In development of diagnostic agents, the first question is
usually: “can we detect the target?” In this case, the decisions on the choice of animal
model should include the imaging modality for sensitivity and resolution. Selections
of animal models include the following criteria: (i) similarity in biology to human
disease, (ii) quality of model (ease of producing, reproducibility, and short time of
onset), and (iii) translation of results to human medicine.
In many studies, the question is, can we detect a biological response? Imaging is
a very powerful method for noninvasive detection of anatomical and molecular
responses. MRI, CT, and ultrasound have been used for many years to measure the
size of tumors in preclinical chemotherapy development. As with the clinical analog,
response to chemotherapy is determined by the reduction or lack of reduction in
tumor size. In the current age of molecular imaging, responses can now be measured
by glucose utilization, incorporation of radiolabeled nucleotides, or alterations in
other molecular markers of disease progression. Again here, the choice of animal
model depends heavily on the imaging modality for resolution and sensitivity. In
general, well-established models of response that have traditionally been used for
survival studies are good choices for these studies. For example, traditional methods