Impact of Dopamine Transporter Scan in Parkinson’s Disease

1.1 Technique of nigrostriatal FP-CIT SPECT The density of the presynaptic dopamine reuptake transporters (DAT) can be quantified with [123I]FP-CIT SPECT. The tracer Fluorpropyl-Carbomethoxy-Iodophenyl-Tropan (FPCIT), marked by radioactive 123iodine [123I] (therefore the name [123I]FP-CIT), binds reversibly to the DAT on the presynaptic membrane. Before application of FP-CIT, the iodine uptake of the thyroid gland is blocked with sodium perchlorate. Then the radioactive tracer [123I]FP-CIT (110 185 MBq, specific activity: 580 1040 GBq/mg) is injected intravenously. 3 – 6 hours following the intravenous tracer injection, cerebral SPECT images are obtained by a gamma camera. At the moment, FP-CIT is the preferred tracer to visualize DAT due to its pharmacokinetic properties including fast (3 6 hours after intravenous injection) and highly affine binding (Ki = 3.1 nM) at DAT (4, 22). The exposure to radiation at the examination of an individual with a body weight of 70 kilograms amounts to 4.35 milli-Sievert (mSv). For comparison: the average effective exposure to natural background radiation in middle Europe at sea level runs up to 2.4 mSv. Due to the similarity between dopamine reuptake transporters and serotonin reuptake transporters, serotonin reuptake inhibitors, widely used as antidepressants, must not be taken in the last two weeks before FP-CIT SPECT examination (fluoxetine for a period of six weeks). Furthermore therapy with selegiline should be discontinued to avoid interaction of its metabolites with FP-CIT at the dopamine transporter (19). Further antiparkinsonian medication may be continued during SPECT examination, since present data (11, 16, 19, 21) do not show any effect of antiparkinsonian drugs on dopamine transporter binding. Beyond FP-CIT-SPECT there exist further nuclear medicine methods to quantify the nigrostriatal presynaptic DAT density: [123I]-beta-CIT SPECT (2beta-CarbomethoxyIodophenyl-Tropan; late SPECT after application of beta-CIT; [9, 15, 16]) and TRODAT SPECT (6, 23).