Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool that provides detailed images of the human body without using ionizing radiation. The technology relies on advanced physics principles, particularly nuclear magnetic resonance (NMR), to generate high-resolution images. This article explores the fundamental physics behind MRI, how it works, and its significance in modern medicine.
The Physics Behind MRI: Basic Principles
1. Magnetic Fields and Proton Alignment
At the core of MRI physics is the interaction between hydrogen protons and a strong external magnetic field. Since the human body is composed largely of water (H₂O), hydrogen atoms are abundant. Each hydrogen nucleus (a single proton) behaves like a tiny magnet due to its intrinsic spin.
When a patient is placed inside an MRI scanner, the powerful magnetic field (measured in Tesla, typically 1.5T or 3T) forces these hydrogen protons to align with the field. Some align parallel, while others align antiparallel, creating a net magnetization.
2. Radiofrequency (RF) Pulses and Resonance
Once the protons are aligned, the MRI system applies a brief but strong radiofrequency (RF) pulse at a specific resonance frequency (Larmor frequency). This pulse disturbs the alignment, causing protons to absorb energy and move into a higher energy state.
When the RF pulse is turned off, the protons begin to relax, returning to their original alignment. As they do so, they release energy, which is detected by MRI coils and used to generate an image.
3. Relaxation Times: T1 and T2
The emitted signals vary depending on the tissue type, which helps differentiate between organs and abnormalities.
- T1 Relaxation (Longitudinal Relaxation Time): The time it takes for protons to realign with the magnetic field. T1-weighted images provide detailed anatomical structures.
- T2 Relaxation (Transverse Relaxation Time): The time it takes for protons to lose coherence among neighboring spins. T2-weighted images highlight fluid-filled structures and pathology like edema or tumors.
How MRI Scans Are Created: Image Formation
MRI images are generated through a process called Fourier Transformation, which converts raw signal data into a detailed visual representation. Several key factors influence MRI image quality:
- Magnetic Field Strength: Higher Tesla scanners (3T, 7T) produce more detailed images.
- Pulse Sequences: Different sequences like T1-weighted, T2-weighted, and FLAIR (Fluid Attenuated Inversion Recovery) are used to enhance specific tissues.
- Gradient Coils: These create spatial variations in the magnetic field, allowing precise localization of signals.
- Contrast Agents: Gadolinium-based agents improve tissue contrast for better diagnosis.