Introductory magnetic resonance imaging physics

This chapter introduces the key conceptual underpinnings of magnetic resonance imaging (MRI) with an intuitive description of the underlying physics. The behavior of proton magnetisation within an externally applied magnetic field is described, as is the use of radiofrequency magnetic field pulses to manipulate magnetisation and create detectable signal. Determinants of image contrast are explored by describing the growth and disappearance of longitudinal and transverse magnetisation due to the effects of the inherent tissue T1 and T2 relaxation rates during the repetition time and echo time. Spatial localisation by magnetic field gradients is explained, including a conceptual description of k-space and Fourier transformation. These elements of signal creation, image contrast creation and spatial localisation are brought together as building blocks of the basic MRI pulse sequence. Other commonly used techniques are introduced, including diffusion-weighted and susceptibility-weighted imaging and motion compensation methods. Finally, selected emerging areas are explored, including rapid, continuous, comprehensive imaging and artificial intelligence.