Quantification of convection-enhanced delivery to the ischemic brain

Abstract

Convection-enhanced delivery (CED) could have clinical application in the delivery of neuroprotective agents following ischemic stroke. However, ischemic brain tissue changes such as cytotoxic edema, in which cellular swelling decreases the fractional volume of the extracellular space, would be expected to significantly alter the distribution of neuroprotective agents delivered by CED. We sought to predict and characterize these effects using the magnetic resonance contrast agent gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) as a model therapeutic agent. CED was observed using MRI in a normal rat brain and in a middle cerebral artery (MCA) occlusion rat model of brain ischemia. Gd-DTPA was infused to the caudate putamen in the normal rat (n = 6) and MCA occlusion model (n = 6). In each rat, baseline apparent diffusion coefficient images were acquired prior to infusion, and T1 maps were then acquired 13 times throughout the duration of the experiment. These T1 maps were used to compute Gd-DTPA concentrations throughout each brain. In the MCA occlusion group, CED delivered Gd-DTPA to a comparatively larger volume with lower average tissue concentrations. Following the infusion, the total content of Gd-DTPA decreased more slowly in the MCA occlusion group than in the normal group. This quantitative characterization confirms that edematous ischemic tissue changes alter the distribution of agents by CED. These findings may have important implications for CED in the treatment of brain injury, and will assist in future efforts to model the distribution of therapeutic agents.