Radiotracers: Thallium and Technetium

	Thallium 201	Technetium 99m (Sestamibi/ Cardiolite, Tetrofosmin/Myoview)
	Potassium analog; monovalent cation	Lipid soluble, cationic
Preparation	Cyclotron	Generator (eluted from Mo 99m)
Half life	73.1 hours	6 hours (therefore larger doses can be given resulting in higher counts, less scatter and less tissue attenuation)
Decay	Electron capture to Hg-201/ X rays	Isomeric transition/ Gamma ray emission
Energy release	68-80 keV	140 keV
First pass extraction	High (85%)	Lower (55%)
Pharmacodynamics	Active membrane transport in to the myocyte through Na/K pumps Rapid clearance from intravascular space	Diffuses in to cells- no active uptake Uptake depends on blood flow Enters mitochondria due to transmembrane energy potentials (mitochondria à negatively charged membrane)
Myocardial extraction coefficient	Highest: 85% (peak concentration 10 mins after injection)	65% (S) 54% (T)
Redistribution	Mono-exponential  (depends on initial tracer concentration in the myocyte and myocardial blood flow) washout that starts 10-15 mins after injection	Negligible washout therefore no redistribution
Clearance	Kidneys	Hepatobiliary system (tetrofosmin clears liver earlier than sestamibi)
Dose administered	2.5-4 mCi	10-20 mCi: sestamibi 5-33 mCi: tetrofosmin
Whole body radiation dose	0.68 rad	0.5 rad
Whole body effective dose	6.3mSv/mCi of Tl-201 injected	0.3mSv/mCi of Tc 99m injected
Collimator	GAP/ LEAP	High Resolution (LEHR)
Imaging protocol	Exercise/Pharm Stress àTh-201 injection à 10-15mins àImaging (stress) à 2-4 hours à Imaging (rest/redistribution)	1 day protocol: Patient at rest à  Tc injection à 45 mins à Imaging à -Exercise Stress à Tc injection (2-3 times higher dose) à15-20 mins àImaging -Pharm stress à Tc injection (2-3 times higher dose) à 45mins à Imaging 2 day protocol*: Stress imaging on day 1; rest imaging on day 2 (if needed)
Time to imaging Rest/ resdistribution: Pharmacologic stress: Exercise:	2.5- 4 hours after stress 10-15 mins 10-15 mins	45-60 mins (S)/ 30-45 mins (T) 60 mins (S)/ 45 mins (T) 15-20 (S)/ 10-15 mins (T)
Limitations	Low energy emission, therefore: -More image attenuation (especially obese patients) -Longer imaging times	Short shelf life: must be ordered everyday/ twice a day
Clinical uses	CAD diagnosis Viability assessment	CAD diagnosis: sharper images LV function
Minimum number of counts	100 counts/ pixel	200 counts/pixel
Minimum number of projections	32	60-64

*2-day protocol used in BMI >30; female patients where significant breast attenuation in expected