MR Imaging of Brain Development
The human brain undergoes tremendous changes in size, shape and complexity over the first few years of life. On physical exam, we observe a change in the head shape and the head circumference, as well as the attainment of developmental milestones. Magnetic Resonance Imaging (MRI) is perfectly suited for characterization of the underlying structural changes in the brain that accompany the outward changes that parents and pediatrician observe. MRI does not use ionizing radiation so there is no risk of cancer, which is important in the pediatric population since they are more sensitive to ionizing radiation than adults.
MRI of the brain uses radiofrequency (RF) energy in the presence of magnetic fields to excite the protons in water and then capture the signal the is emitted when the excess energy dissipates. The process of energy dissipation is called relaxation. Two fundamental tissue relaxation constants govern this process: T1 and T2. Almost all MRI protocols produce images with either T1-weighting or T2-weighting. T1-weighted images are hyperintense (bright) when T1 relaxation occurs quickly. The key driver for T1 relaxation in the brain is lipid (fat). Brain lipid content is greatest in the myelin sheath that covers the axons. At birth, there is very little mature myelin and thus very little lipid. As the brain matures, the white matter myelinates, which changes contrast from dark to bright on T1-weighted images. T2-weighted image contrast, on the other hand, is governed predominately by the tissue water content. More water in the brain leads to brighter signal on T2-weighted images. Over the first few years of life, the brain dries out in general and the water content in the white matter drops substantially. The result is that white matter changes contrast from bright at birth to dark by age 2 on T2-weighted images. At first glance, one would say that brain image contrast reverses on T1- and T2-weighted images between birth and age 2.
By observing the signal changes on T1- and T2-weighted images, we can track the change in brain size, the decrease in brain water content and the increased complexity of the folding pattern that result from the maturational process. What is more, we can see regional changes in white matter myelination that follow a predictable pattern and time course over the first few years of life. Specifically, brain myelination proceeds from bottom to top, back to front, and inside out. The onset of the signal changes due to myelination is delayed for T2-weighted images relative to T1-weighted scans, which adds to the precision of our temporal estimates.
After attending this presentation, you should be able to describe how water and myelin content changes during brain development influence the MR image appearance. Armed with this knowledge, you will understand that the time course and regional variation of brain MRI signal is dictated by maturational changes. The final step in the process will be to use a systematic approach to determine the child's brain age using MRI.