Table of Contents
MRI (Magnetic Resonance Imaging) is a medical imaging technique that uses strong magnetic fields and radio waves to generate detailed images of the body’s internal structures. Understanding the physics behind MRI involves grasping how signals are formed and how contrast is achieved between different tissues.
Basic Principles of MRI Signal Formation
The core of MRI signal formation relies on the behavior of hydrogen nuclei in the body’s tissues. When placed in a magnetic field, these nuclei align with the field. Radiofrequency pulses disturb this alignment, and as nuclei return to equilibrium, they emit signals that are detected and used to create images.
Calculations of Signal Intensity
The signal intensity in MRI depends on several factors, including the density of hydrogen nuclei, relaxation times (T1 and T2), and the strength of the magnetic field. The basic signal equation can be expressed as:
S = k * ρ * (1 – e^(-TR/T1)) * e^(-TE/T2)
where S is the signal, ρ is the proton density, TR is the repetition time, TE is the echo time, and k is a proportionality constant.
Contrast Mechanisms in MRI
Contrast in MRI images is primarily achieved by manipulating relaxation times and sequence parameters. T1 contrast emphasizes differences in longitudinal relaxation, while T2 contrast highlights differences in transverse relaxation. Adjusting TR and TE values allows for selective tissue imaging.
- Proton density
- Relaxation times (T1 and T2)
- Magnetic field strength
- Sequence parameters (TR and TE)