Delivery of imaging agents into brain.

Delivery of diagnostic agents to the central nervous system (CNS) poses several challenges as a result of the special features of CNS blood vessels and tissue fluids. Diffusion barriers exist between blood and neural tissue, in the endothelium of parenchymal vessels (blood-brain barrier, BBB), and in the epithelia of the choroid plexuses and arachnoid membrane (blood-CSF barriers), which severely restrict penetration of several diagnostic imaging agents. The anatomy of large vessels can be imaged using bolus injection of X-ray contrast agents to identify sites of malformation or occlusion, and blood flow measured using MRI and CT, while new techniques permit analysis of capillary perfusion and blood volume. Absolute quantities can be derived, although relative measures in different CNS regions may be as useful in diagnosis. Local blood flow, blood volume, and their ratio (mean transit time) can be measured with high speed tomographic imaging using MRI and CT. Intravascular contrast agents for MRI are based on high magnetic susceptibility agents such as gadolinium, dysprosium and iron. Steady-state imaging using agents that cross the BBB including (123)I- and (99m)Tc-labelled lipophilic agents with SPECT, gives a 'snapshot' of perfusion at the time of injection. Cerebral perfusion can also be measured with PET, using H(2)(15)O, (11)C- or (15)O-butanol, and (18)F-fluoromethane, and cerebral blood volume measured with C(15)O. Recent advances in MRI permit the non-invasive 'labelling' of endogenous water protons in flowing blood, with subsequent detection as a measure of blood flow. Imaging the BBB most commonly involves detecting disruptions of the barrier, allowing contrast agents to leak out of the vascular system. Gd-DTPA is useful in imaging leaky vessels as in some cerebral tumors, while the shortening of T(1) by MR contrast agents can be used to detect more subtle changes in BBB permeability to water as in cerebral ischemia. Techniques for imaging the dynamic activity of the brain parenchyma mainly involve PET, using a variety of radiopharmaceuticals to image glucose transport and metabolism, neurotransmitter binding and uptake, protein synthesis and DNA dynamics. PET methods permit detailed analysis of regional function by comparing resting and task-related images, important in improving understanding of both normal and pathological brain function.