This work describes quantitative MRI assessment of primate brain maturation. Nine young baboons were followed from the age of one to 30 months. Assessment of myelination was based on the gray/white matter contrast on MR images and the evolution of T2 relaxation time respectively. The brain maturation began in the posterior fossa and progressed to the olfactory bulbs corresponding to decreasing white matter T2 values. Relaxation parameters provide new opportunities to trace the myelination process in vivo.

Purpose: Using magnetic resonance (MR) and isotopic imaging to investigate the cerebral alterations after high-dose single-fraction irradiation on a pig model. We assessed the nuclear magnetic resonance (NMR) relaxation times as early markers of radiation injury to the healthy brain.Methods and Materials: A total of 17 animals was studied; 15 irradiated and 2 unirradiated controls. Pigs were irradiated with a 12 MeV electron beam at a rate of 2 Gy/min. Ten animals received 40 Gy at the 90% isodose, five animals received 60 Gy, and two animals were unirradiated. The follow-up intervals ranged from 2 days to 6 months. T-1-weighted scans, T-2-weighted scans, and scintigrams were performed on all animals to study neurological abnormalities, cerebral blood flow, and blood-brain barrier (BBB) integrity. T-1 and T-2 relaxation times were measured in selected regions of interest (ROIs) within the irradiated and contralateral hemispheres. A ratio T-1 after irradiation/T-1 before irradiation, and a ratio T-2 after irradiation/T-2 before irradiation, were calculated, pooled for each dose group, and followed as a function of time after irradiation.Results: Scintigraphy visualized the brain perfusion defect and BBB disruption in all irradiated brains. The ratio T-2 after irradiation/T-2 before irradiation was proportional to the effective dose received. The T-2 ratio kinetics could be analyzed in three phases: an immediate and transient phase, two long-lasting phases, which preceded compression of the irradiated lateral ventricle, and edema and necrosis at later stages of radiation injury, respectively. The magnetic resonance imaging (MRI) observations correlated well with histological analysis.Conclusion: The results show that quantitative imaging is a sensitive in vivo method for early detection of cerebral radiation injury. The reliability and dose dependence of T-2 relaxation time may offer new opportunities to detect and understand brain pathophysiology after high-dose single-fraction irradiation.

PURPOSE: To use the pig brain as a large-animal model to examine the effects of high-dose single-fraction irradiation on MR images, T2 relaxation time, and histologic integrity. METHODS: A total of 24 Meishan pigs were studied: 20 irradiated animals and 4 unirradiated controls. A high dose was delivered to the right hemisphere of the animals, using a 12-MeV electron beam. Ten animals received 40 Gy at the 90% isodose, and 10 animals received 60 Gy. Quantitative measurement of T2 relaxation time was compared with qualitative analysis of T2-weighted images and histologic studies. RESULTS: Quantitative analysis revealed a reproducible increase of the T2 parameter within the irradiated areas. The T2 kinetic could be analyzed in two phases, which appeared before the visualization of ventricle compression, necrosis, and edema. The first is characterized by vascular inflammation and the latter by radiation necrosis and edema. Both are dose dependent. CONCLUSION: These results underline the ability of quantitative MR for early diagnosis of brain radiation lesions in vivo.