Micro-PET CT procedures for brain imaging of rats

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.

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

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

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

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.

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.

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.

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.