TITLE:  Micro-PET CT procedures for brain imaging of rats
AUTHORS: christopher.cawthorne, María Sanchiz Calvo, eduard.bentea

[DESCRIPTION]
 Micro-PET CT procedures for brain imaging of rats

[STEPS]
1.  A range of PET radiotracers and radioligands (“radiopharmaceuticals”) can be used to
image biochemical flux and receptor density in rodents. PET
radiopharmaceuticals are administered in very low ‘tracer’ doses to avoid
pharmacological effects, the specific activity of the tracer must be carefully
evaluated to ensure this assumption is met.
2. Typically rats are anaesthetised throughout the scanning process. Commonly this is with
isoflurane (as described here), but other regimes may be necessary depending on
the biological system to be studied. Whether animals are anaesthetised
throughout the entire period of tracer uptake depends on both the relation of
the tracer with the biological system being studied and the quantitation method
used
3. For studies of the rat brain, full length dynamic scanning allows a range of
quantitative approaches to be compared. Typically rats are anesthetized in a
warmed chamber before transfer to a warm mat for tail vein cannulation.
Catheters are best constructed in-house using pharma-grade tubing of low
diameter, to allow optimum length to be determined to allow injection when the
animal is in the imaging cell in the PET scanner. Catheter needles are
typically 23G or larger. Catheters should be checked for integrity immediately
before scanning using sterile saline solution. Heparin may be added, though if
the time between cannulation and injection is minimised this is optional
4. The tail is cleaned with an ethanol wipe and warmed before a lateral vein is
cannulated (this should be visible). In rats, the potency of the line can be
confirmed by withdrawing a small amount of blood before reinjection. Cannulated
animals must be placed in the specialised imaging cell before transfer to the
scanner. Imaging cells differ between manufacturer, however temperature and
respiration are usually monitored and this must be confirmed before scanning is
implemented
5.  Syringe activities before injection should be measured in a dose calibrator, and
residues after injection measured in the same way – importantly the exact time
of measurement should be recorded. All equipment should be set to the same
time. 
6.  Typically the scan will be started before radiotracer is injected. Injection should be
done over 20-30 seconds and the beginning and end of injection recorded, though
if backpressure is felt this indicates the cannula is no longer potent and the
injection should be abandoned. For this reason, syringe pump use requires
additional validation. At the end of injection, the catheter should be removed
if possible to allow measurement of residue. For quantitative scans, the
catheters are not flushed.
7.  Dynamic PET images should be acquired for long enough to allow kinetic modelling. This
depends on the radiotracer but times of  90-120 min are not uncommon. The brain should
be at, or close to, the center of the field of view.
8.   After PET scanning, a CT image should be acquired for attenuation correction. The CT
field of view must be equal or larger to the PET field of view.