Imaging

	SPECT	PET
Spatial resolution	2-3mm	5-10mm
Radiation dose	5mSv	10mSv
Isotope	99Tcm-HMPAO (Ceretec) 99Tcm ECD (Neurolite)	18F-fluoro-deoxyglucose (FDG)
T1/2	ECD 6hours HMPAO 4hours	110 min
Radiation	Gamma rays	Positron – annihilates with electron and emits two photons at 180deg to each other
Method	Isotope is distributed with blood flow and is fixed on first pass Stays fixed for 12-24 hours Gamma camera rotates around patient at 6deg intervals in various planes to map slices of gamma radiation	FDG distributes with glucose utilisation Fixed ring detects two matching photons to work out point of decay
Advantages	Allows ictal imaging Longer half life makes it more accessible and cheaper	Better spatial resolution
Disadvantages	Poor spatial resolution Blood flow not glucose metabolism?	Expensive

SPECT agents:

99Tcm-HMPAO (Ceretec)

• Image within 2- 4hours

• Gets trapped in cell by conversion to hydrophilic form

99Tcm ECD (Neurolite)

• Faster excretion – better background

• Image within 4 hours (25% cleared by 4hour hours)

Thallium 201 – crosses if BBB abnormal – e.g. tumour

Iodine 123 – PIB scanning for PD

Use of PET and SPECT in epilepsy

• Desai et al Epilepsia 2013

• 53 patients

• Focus localised in 45

• PET co-localised in 25 (56% sensitivity)

• SPECT co-localised in 39 (87% sensitivity)

• If Intracranial EEG and PET disagreed – SPECT co-localised with EEG in 16(80%)

• If intracranial EEG and SPECT disagreed – PET co-localised with EEG in 2(33%)

• No significant effect of imaging concordance on seizure outcome

• 64% had excellent surgical outcome

Other statistics:

PET

• Sensitivity

o TLE 70-90%

o Extra TLE 30-60%

o MRI negative TLE 66%

o Cortical dysplasia 81%