The article provides an introduction to Magnetic Resonance Imaging (MRI) and the underlying principles of Nuclear Magnetic Resonance (NMR). It begins by explaining the Bloch equation, which models the interactions between applied magnetic fields and nuclear spins. The Bloch equation describes the evolution of the magnetization vector under the influence of these fields, including relaxation processes. The article then discusses the basic imaging experiment, where a sample is polarized by a static magnetic field and excited by a time-dependent radio frequency (RF) pulse, followed by the measurement of the resulting signal. The signal is proportional to the integral of the spin density function weighted by the exponential decay of the transverse magnetization. The article also covers selective excitation techniques, where only a specific region of the sample is excited, and spin-warp imaging, a practical technique for measuring the 2D Fourier transform of a "slice" of the sample. The signal-to-noise ratio (SNR) and contrast between different materials in MRI are discussed, highlighting the large contrast achievable in MRI compared to X-ray imaging. Finally, the article explains how the contrast and resolution in MRI images are influenced by various imaging parameters and the effects of finite sampling and relaxation processes.The article provides an introduction to Magnetic Resonance Imaging (MRI) and the underlying principles of Nuclear Magnetic Resonance (NMR). It begins by explaining the Bloch equation, which models the interactions between applied magnetic fields and nuclear spins. The Bloch equation describes the evolution of the magnetization vector under the influence of these fields, including relaxation processes. The article then discusses the basic imaging experiment, where a sample is polarized by a static magnetic field and excited by a time-dependent radio frequency (RF) pulse, followed by the measurement of the resulting signal. The signal is proportional to the integral of the spin density function weighted by the exponential decay of the transverse magnetization. The article also covers selective excitation techniques, where only a specific region of the sample is excited, and spin-warp imaging, a practical technique for measuring the 2D Fourier transform of a "slice" of the sample. The signal-to-noise ratio (SNR) and contrast between different materials in MRI are discussed, highlighting the large contrast achievable in MRI compared to X-ray imaging. Finally, the article explains how the contrast and resolution in MRI images are influenced by various imaging parameters and the effects of finite sampling and relaxation processes.