An MRI scan uses strong magnetic fields, magnetic field gradients, and radio waves to generate detailed images of the organs and tissues in your body. The process involves several key steps:
1. Strong Magnetic Field Alignment
The patient is placed inside a powerful MRI scanner which generates a strong, uniform magnetic field, typically 1.5 to 3 Tesla (T), although research scanners can reach much higher field strengths. This field aligns the magnetic moments of the protons (hydrogen nuclei) in the body, primarily in water molecules, either parallel or anti-parallel to the external magnetic field. A slight excess of protons aligns parallel, creating a net magnetization in that direction.
2. Radiofrequency (RF) Pulse Excitation
A radiofrequency (RF) pulse, tuned to the specific resonant frequency of the protons (determined by the Larmor equation), is emitted by the scanner. This RF pulse temporarily knocks the aligned protons out of alignment, causing them to tip into a transverse plane (perpendicular to the main magnetic field).
3. Relaxation and Signal Emission
Once the RF pulse is turned off, the excited protons begin to "relax" back to their original aligned state, releasing energy in the form of radiofrequency signals. There are two main relaxation processes:
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T1 Relaxation (Longitudinal Relaxation): Protons realign with the main magnetic field (longitudinal axis). The time constant for this process is T1, and it represents the time it takes for 63% of the longitudinal magnetization to recover.
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T2 Relaxation (Transverse Relaxation): Protons lose phase coherence in the transverse plane, leading to a decay of the transverse magnetization. The time constant for this process is T2, and it represents the time it takes for 63% of the transverse magnetization to decay. T2 relaxation is generally much faster than T1 relaxation.
These relaxation processes are different for different tissues, providing contrast in the final image.
4. Gradient Coils for Spatial Encoding
To determine where the signals are coming from, MRI scanners use gradient coils. These coils generate small, spatially varying magnetic fields that are superimposed on the main magnetic field. By changing the gradient fields, the resonant frequency of the protons varies depending on their location. This allows the scanner to encode the signal's origin in three dimensions. Three types of gradients are used:
- Slice Selection Gradient: Selects a specific slice or volume for imaging.
- Frequency Encoding Gradient: Encodes the position along one axis within the selected slice based on frequency.
- Phase Encoding Gradient: Encodes the position along another axis within the selected slice based on phase.
5. Signal Detection and Image Reconstruction
The radiofrequency signals emitted by the relaxing protons are detected by receiver coils. These signals are then processed using a Fourier transform to reconstruct an image. The intensity of each pixel in the image corresponds to the strength of the signal from that particular location.
6. Image Formation and Interpretation
By manipulating the timing and strength of the RF pulses and gradients, different contrasts can be generated, highlighting specific tissues or pathologies. Radiologists then interpret these images to diagnose various medical conditions.
In summary, MRI leverages the magnetic properties of atomic nuclei, radiofrequency waves, and magnetic field gradients to create detailed cross-sectional images of the body. The differential relaxation properties of various tissues allow for high soft tissue contrast, making MRI a powerful diagnostic tool.
