Unit
- Nuclear magnetic resonance: Hydrogen protons in the body align with the main magnetic field (B0) when placed inside an MRI scanner. The net magnetization vector precesses at the Larmor frequency, which is proportional to the field strength (42.57 MHz/T for hydrogen).
- Excitation and relaxation: A radiofrequency (RF) pulse at the Larmor frequency tips the magnetization into the transverse plane. After the pulse ends, two independent relaxation processes occur: T1 (longitudinal recovery) and T2 (transverse decay). These tissue-specific constants produce the contrast that distinguishes anatomy and pathology.
- T1 relaxation: Also called spin-lattice relaxation, it reflects the time required for longitudinal magnetization to recover to 63% of its equilibrium value. T1 times differ between tissues; for example, fat has a short T1 (bright on T1-weighted images) while water has a long T1 (dark).
- T2 relaxation: Also called spin-spin relaxation, it describes the exponential decay of transverse magnetization. T2 values are always shorter than or equal to T1. Fluids have long T2 (bright on T2-weighted images) and solid tissues have shorter T2.
- Proton density: The concentration of hydrogen protons in a tissue also influences signal intensity. Tissues with more mobile protons (e.g., cerebrospinal fluid) produce higher signal than those with fewer (e.g., cortical bone). Proton density weighting minimizes T1 and T2 effects.
- Magnetic field gradients: Gradient coils produce linear variations in the magnetic field along three axes (x, y, z). These gradients encode spatial information by causing protons at different locations to precess at slightly different frequencies, enabling image reconstruction via Fourier transformation.
- Signal reception: The transverse magnetization induces a voltage in the receiver coil according to Faraday's law of induction. This signal, called the free induction decay (FID), is digitized and processed to create the final image. Coil design (phased-array surface coils) significantly affects signal-to-noise ratio.
- Main magnetic field strength: Clinical MRI systems operate at field strengths from 0.2 T to 3 T and higher. Higher field increases signal-to-noise ratio but also increases susceptibility artifacts, specific absorption rate (SAR), and cost. The choice of field strength depends on clinical indications and operational priorities.
- K-space encoding: The digitized MRI signal fills a spatial frequency domain called k-space. Each data point in k-space corresponds to a specific spatial frequency component of the image. The center of k-space determines overall contrast and brightness, while the periphery encodes fine details and edges. Efficient k-space sampling strategies, such as partial Fourier or parallel imaging, reduce scan time.
- Pulse sequences: A pulse sequence is a precisely timed series of RF pulses and gradient pulses that generates a specific type of image weighting. Common sequences include spin echo, which refocuses T2 decay, and gradient echo, which uses gradient reversals instead of an 180 degree pulse. Sequence parameters like echo time and repetition time control whether the image is T1 weighted, T2 weighted, or proton density weighted.
- Slice selection: To acquire an image from a specific plane, a slice selective gradient is applied along one axis while transmitting a narrow bandwidth RF pulse. The gradient causes only protons within a thin slab to have a Larmor frequency matching the RF frequency. The slice thickness is determined by the gradient strength and the RF pulse bandwidth, allowing precise anatomical localization.
- Safety and artifact considerations: MRI safety involves controlling specific absorption rate (SAR) to avoid tissue heating, managing gradient induced peripheral nerve stimulation, and preventing projectile effects from ferromagnetic objects. Common artifacts include chemical shift from fat water frequency differences, susceptibility artifacts near air tissue interfaces, and motion artifacts from patient movement. Understanding these factors is critical for safe operation and diagnostic image quality.
Terminology
- Larmor frequency
- The precessional frequency of hydrogen protons in a magnetic field, given by omega = gamma * B0, where gamma is the gyromagnetic ratio.
- Precession
- The wobbling motion of the magnetization vector around the direction of the main magnetic field.
Quick Revision
- MRI signal originates from hydrogen protons aligned with B0 and tipped by RF pulses.
- T1 and T2 relaxation times determine image contrast between tissues.
- Gradients encode spatial location, and receiver coils detect the resulting signal.
- Higher field strength improves SNR but introduces trade-offs in safety and artifact.