Magnetic Resonance Imaging (MRI) Ch 13 PP

MRI Definition: Medical imaging technique that does not use ionizing radiation.
First Developed: Emerged in the early 1980s.

Detection Mechanism: MRI detects and shows the concentration of free-floating hydrogen molecules in tissue.
Contrast Enhancement: MRI offers superior subject contrast in comparison to CT scans and X-rays.
Evaluation Superiority: MRI is particularly effective for imaging:
- Brain
- Spinal cord
- Soft tissue structures such as ligaments and cartilage.

Risks: The strong magnetic fields used in MRI machines can be very dangerous.

Atom Behavior: Atoms possess a spin and wobbling motion known as precession.
Magnetic Polarity: All atoms and molecules demonstrate magnetic polarity, allowing them to align and become magnetized in a magnetic field.
Procedure Steps:
1. Technician applies a radio frequency that elevates atoms' energy state.
2. Hydrogen atoms in the patient align their magnetism at a 90° angle from their original alignment in the magnetic field.
3. These atoms wobble (precess) in unison.
4. Upon removing the radio frequency, molecules return to their normal state, emitting energy in the form of radio waves.
5. The emitted radio waves are detected by the radio antenna, allowing the MRI scanner to produce an image based on the received signals.

Earth’s Magnetic Field: Approximately 1 gauss (G).
Tesla Measurement: Field strength of medical magnets described in Tesla (T), where 10,000 ext{ G} = 1.0 ext{ T}.
Types of Medical MRI Magnets:
- Permanent Magnets: Consists of two slabs of magnetic material with field strengths of less than 0.3 T.
- Electromagnets (Resistive Magnets): Created via electrical charge through copper wire, with field strengths of less than 0.6 T.
- Superconducting Magnets: Most common type, remain superconductive at low temperatures. Uses liquid helium, the coldest substance known (around -459°F), to maintain the magnetic field.

Gradient Coils: Located within the magnetic bore, enabling image creation in various planes as chosen by the technician.
Radio Antenna: Strong antenna used to send and receive radio waves to/from the patient; its capabilities are compared to radio station technology.
Surface Coils: Coils designed to receive (sometimes send) radio waves, specifically molded for different body parts such as the brain, spine, and joints (e.g., hock). The smaller the coil, the better the resulting image. Each coil is custom-made for specific scanners and is not interchangeable.
Computing System: Required to convert radio signals into clear imaging output.

Signal Intensity vs. Density: The contrast seen in MRI scans is based on signal intensities, not densities.
Shades of Gray:
- High Signal Intensity: Appears brighter or closer to white.
- Low Signal Intensity: Appears darker shades of gray.
T1 vs. T2 Weighting:
- T1 Images: Cerebrospinal Fluid (CSF) appears black, fat and skin appear bright.
- T2 Images: CSF appears bright, skin appears black.

Gadolinium: A paramagnetic substance that enhances the T1 properties of tissues.
Kidney Function Requirement: Gadolinium is excreted by the kidneys and is administered only to patients with normal kidney function.

Projectile Effect: Due to high magnetic field strength, metal objects can be propelled towards the magnet, resulting in potential injuries or fatalities.
Radio Frequency Safety: Patients with implanted devices (e.g., pacemakers, certain aneurysm clips) must avoid MRI machines due to risks.
Noise Levels: MRI scans often produce noise exceeding 60-90 decibels, necessitating ear protection for patients and staff in the MRI room during procedures.
Special Note: MRI studies on animals, such as dogs, can significantly advance understanding of animal brains, as explored in studies like "Studying The Dog Brain - Inside the Animal Mind" by BBC.