Magnetic resonance imaging (MRI) is an intriguing medical imaging technology that probes the state of atomic nuclei in the body and their responses to magnetic fields. For dozens of years clinical MRI procedures have been used to acquire spatially localized information that allow structures in the body to be distinguished from each other, and the field continues to evolve as it turns more and more to acquiring precise quantitative information that can be attributed to the exact properties of materials in the body.
A great deal of research effort, starting with basic science investigations and on into clinical trials, goes into searching for and characterizing these properties. After years of development, several parameters that measure these properties are beginning to be used to image humans and the information used for clinical assessment. Magnetization transfer (MT) is a phenomenon that occurs when protons in different states, tightly bound or more free, are probed with a magnetic field and exchange a property called magnetization with each other. The measurement can determine the amount of protons in each condition and the rates of exchange of magnetization between them. The year 2013 has seen the publication of several studies of MT imaging of the body. While they are by no means the very first studies of MT on humans, they demonstrate that the measurement is finding its way into more routine use in supporting medicine.
Recently, researchers at Brown University and King’s College London conducted a study in which they measured the amount of myelin in children during their first few years of life (Deoni et al. 2013). The goal was to determine how myelin development potentially differs with breastmilk or formula feeding. Myelin, which surrounds the axons of neurons, is an example of a material in the body with bound protons that are sensitive to MT, and MT imaging studies can identify the amounts and locations of the myelin. In the study, children of varying ages who had been fed breastmilk, formula, or a mixture of the two since birth were imaged using MT procedures, and maps were made of the fraction of the protons in the bound state. The researchers found that breastfeeding was associated with greater development of white matter in several regions of the brain, such as the frontal and temporal white matter and others, areas that have been associated with executive functioning, planning, social-emotional function, and language.
On the opposite end of the body, MT has also been used to visualize peripheral nerves in the foot. A group of Swiss scientists sought to use the strengths of MT imaging to help distinguish soft-tissue structures (Mekle et al. 2013). They used a measurement called the magnetization transfer ratio (MTR) which measures MR signal with and without a preparation magnetic field. Because the nerves in the foot exhibited different MTR values from surrounding tissue, the researchers were able to use image processing methods to render the nerves three-dimensionally, allowing for their visualization.
MRI, and medical imaging in general, is more than just pretty pictures. Each pixel can represent measurements that make use of subtle features of tissue that reflect differences of more than just shape. As the field develops, each year brings more interesting applications to the human body, allowing physicians and patients to see more and do more with that information.
Deoni, S.C.L. et al., 2013. Breastfeeding and early white matter development: A cross-sectional study. NeuroImage, 82, pp.77-86. Available at: http://www.ncbi.nlm.nih.gov/pubmed/23721722 [Accessed November 7, 2013].
Mekle, R. et al., 2013. Magnetization transfer-based 3D visualization of foot peripheral nerves. Journal of Magnetic Resonance Imaging, 37(5), pp.1234-7. Available at: http://www.ncbi.nlm.nih.gov/pubmed/23023888 [Accessed November 7, 2013].
Heather Whitney is assistant professor of physics at Wheaton College.
Copyright © 2013 Books & Culture. Click for reprint information.
