Magnetic Resonance Imaging (MRI) Overview

Magnetic Resonance Imaging (MRI)

• MRI is a medical imaging technique that uses nuclear magnetic resonance (NMR) to visualize detailed internal structures of the body.
• NMR utilizes the magnetic properties of atomic nuclei, particularly protons, which are the nuclei of hydrogen atoms.
• MRI displays the signal intensities emitted by magnetized tissues during the imaging process.

Advantages of MRI

• Non-ionizing radiation: MRI does not use ionizing radiation, making it safe for repeated scans without long-term health effects.
• High contrast resolution: MRI provides exceptional contrast between different tissues, aiding in accurate diagnosis.
• Variable slice thickness and plane selection: MRI allows for imaging in various planes (axial, coronal, sagittal) and slice thicknesses.
• Various sequences: Multiple MRI sequences are available, enabling detailed tissue characterization.
• Detailed visualization without contrast agents: Many anatomical structures can be visualized without intravenous contrast injection.

Disadvantages of MRI

• High cost: MRI scans are expensive due to the complex technology involved.
• Contraindications for patients with metallic implants: Patients with certain metallic implants (pacemakers, aneurysm clips) cannot undergo MRI.
• Claustrophobia: MRI scans can be uncomfortable for individuals with claustrophobia due to the enclosed scanner.
• Movement artifacts: Patient movement during the scan can result in blurry images.
• RF-related burns: Improper handling of RF transmitters can lead to severe burns.
• Limited availability: MRI scanners are not as widely available as some other imaging modalities.

Components of the MRI System

1. Superconducting Magnet

• The heart of the MRI system: It generates a powerful magnetic field that aligns the protons in the body.
• Strongest field for clinical use: MRI scanners typically use magnets with a field strength of up to 3 Tesla (T).
• Superconducting and cryogenically cooled: The magnet is superconducting, meaning it requires constant cooling with liquid helium (LHe) to maintain its magnetic field.
• Horizontal bore design: The magnet is designed with a horizontal bore to accommodate the patient during the scan.

2. Gradient Coils

• Produce localized magnetic fields: Gradient coils alter the main magnetic field to define specific areas for imaging.
• Three sets of coils: One set for each spatial direction (x, y, z).
• Slice selection, phase, and frequency encoding: Gradient coils are essential for selecting specific slices of the body and encoding spatial information for image reconstruction.

3. RF Coils

• Radio frequency (RF) antennas: RF coils transmit and receive radio waves used in MRI.
• Circular or rectangular loop designs: RF coils can be circular or rectangular, depending on the area being imaged.
• Excitation and relaxation: RF pulses excite the protons, causing them to align temporarily with the magnetic field. When the pulse stops, the protons relax back to their original state, emitting a signal that is detected by the RF coil.

4. Other Components:

• Power supplies: Provide electrical power to the magnet, gradient coils, and RF coils.
• Computer system: Processes the data from the RF coils and reconstructs images.
• Documentation system: Records patient information, images, and scan parameters.
• Cooling system: Maintains the low temperature needed for the superconducting magnet.
• Monitoring camera: Allows the technologist to observe the patient during the scan.