The Brain & Neuroimaging: PET & SPECT

Question 1

Describe PET and what does it stand for?

Answer 1

Positron Emission Tomography
* PET neuroimaging assumes that areas of high radioactivity are associated with brain activity
* It measures flow of blood of brain indirectly
* Invasive technique
* Blood flow to different (& active) parts of brain is correlated to oxygen consumption & glucose use using a radioactive tracer oxygen-15
* The biologically active molecule chosen for PET as a tracer is fluorodeoxyglucose (FDG) which is an analogue of glucose & therefore indicates regional metabolic activity corresponding to glucose
* In practice, since brain is normally a rapid user of glucose & since brain pathologies greatly ↓brain metabolism of both glucose & oxygen in tandem, standard FDG-PET of brain, which measures regional glucose use, may also be successfully used to tell you what is wrong

Question 2

What is PET useful and what receptors does it use?

Answer 2

• Several radioligands which act like tracers have been developed for specific neuroreceptor subtypes such as dopamine D2/D3 receptors, serotonin receptors & nicotinic acetylcholine receptors
• These agents permit visualisation of neuroreceptor pools in context of a plurality of neurological conditions
• Its greatest effect is in detecting Alzheimer’s Disease

Question 3

Which Techniques are useful in epilepsy and how are they useful?

Answer 3

PET, SPECT, MRI, MEG & EEG
* Used in conjunction with single photon emission tomography (SPECT) and magnetic resonance imaging (MRI)or EEG, magnetoencephalography (MEG) most effectively to detect epileptic foci prior to neurosurgery
o Get good idea of epileptic focus – if need to take out bits of brain as the epilepsy is intractable

Question 4

Describe SPECT and what does it stand for?

Answer 4

Single Photon Emission (Computerised) Tomopgraphy
* SPECT has practical capacity to image blood flow functional changes that occur during seizures in routine clinical setting
* In single photon emission computed tomography (SPECT) of brain, dual & triple-head Anger gamma cameras are used providing very high-resolution images (~ 7mm full width half maximum (FWHM) extrinsic resolution)
o This is good because it can account for noise as a result of px motion
 For e.g., a 30-min scan can be divided into 2 15-min segments, each obtaining a 360° set of projection images
 If px moves during last 15-min imaging segment, the first 15-minute imaging segment can be used for reconstruction of complete set of tomographic images