Fundamentals of structural MR imaging

What is MRI, and how does MR work? Magnetic resonance imaging (MRI) provides exquisitely detailed images of internal body structures and is especially useful for brain imaging. This chapter will provide an overview of the fundamental elements of MRI. Chapter 4 will introduce MRI techniques which permit assessment of brain physiology, function, and metabolism. State-of-the-art MRI machines employ large, tube-shaped superconducting magnets to produce a strong and homogeneous static magnetic field. Magnetic field strengths are measured in units of tesla (T), with typical human MRI scanners ranging from 0.3 to 3.0 T. One tesla is equal to 10 000 gauss. The Earth’s magnetic field is about 0.5 gauss. Using the magnetic field, along with additional, but much weaker timevarying magnetic fields (radio frequency (RF) fields and pulsed gradient magnetic fields), computers reconstruct images from signals emitted by proton nuclei. Nuclear magnetic resonance (NMR) is the foundation on which MRI is built, although commercial interest in human applications of NMR led to the discarding of the term “nuclear” because of its negative perception. NMR is a physical phenomenon that occurs when certain elements (nuclei with an odd number of protons and/or neutrons) interact with a magnetic field. NMR is the process by which the signal detected in MRI is generated. In human tissue, which is composed largely of hydrogen-containing water (H₂O), hydrogen is the most abundant of all the NMR-capable nuclei. For this reason, human MRI is focused almost exclusively on hydrogen. Since the hydrogen nucleus contains only a single proton and nothing more, hydrogen nuclei are also referred to merely as protons. The protons within the nucleus of any atom contain electric charge and generate a magnetic field, termed a dipole, and we use a single vector to describe the magnetic field of the dipole. The orientation of this vector indicates the orientation of the dipole and the length of the vector indicates its strength. In the absence of any magnetic field external to the nucleus, orientation of the dipoles of a group of nuclei will be random. In the presence of an externally applied magnetic field, however, the dipoles will align with the externally applied magnetic field.