Early detection of brain disorders by MR and PET

The main pathology of Parkinson's disease (PD) is a loss of nigrostriatal dopaminergic neurons. Although the substantia nigra par compacta (SNc) is a small structure, SNc can be seen as an isointensity area between low intensity areas of the red nucleus and substantia nigra par reticulata in T2 weighted images of magnetic resonance imaging (MRI). There is a narrowing of isointensity area of the SNc in PD. There is a trend toward narrowing of an isointensity area of the SNc as the disease progress in PD. Positron emission tomography (PET) with (18F)6-fluoro-L-dopa ((18F)dopa) provides a measure of the integrity of nigrostriatal dopaminergic neurons. (18F)dopa passes through blood-brain barrier after intravenous injection and is metabolized to (18F)dopamine by L-aromatic amino acid decarboxylase. Then (18F)dopamine is stored in the synaptic vesicles in the terminals of nigrostriatal neurons. A linear correlation has been demonstrated between striatal (18F)dopa uptake and nigral cell counts in a human and a monkey studies. Patients develop Parkinsonian symptoms when striatal (18F)dopa uptake reduces by 30% compared with normal controls, indicating patients can be diagnosed as having PD in the presymptomatic period by (18F)dopa-PET. (18F)dopa-PET also provides a measure of viability of fetal tissue transplanted in the striatum. There are several appropriate ligands labelled with positron emitters for measuring dopamine D1 and D2 receptors. It is useful for the differential diagnosis of PD to image dopamine receptors with PET. Recent development of single photon emission tomography(SPECT) ligands has made it possible to measure dopamine transporter and dopamine receptors with SPECT. Early or presymptomatic detection of PD may be possible with SPECT. Recently, appropriate PET and SPECT ligands for cholinergic systems have been developed. Theses techniques may be useful for the diagnosis and therapeutic monitoring of Alzheimer disease (AD). (J.P.N.)