Magnetic Resonance and Medical Imaging

(l) Principles of Magnetic Resonance

Precession of Nuclei: When placed in a magnetic field, nuclei with an odd number of protons (e.g., hydrogen) precess around the magnetic field direction.

Resonance: When an external radiofrequency (RF) pulse matches the Larmor frequency of the nuclei, they absorb energy and move to a higher energy state.

Relaxation Time: The time taken for nuclei to return to their equilibrium state after the RF pulse is switched off. It consists of:

  • T1 relaxation (Spin-lattice relaxation): Energy transfer to surrounding tissues.

  • T2 relaxation (Spin-spin relaxation): Dephasing due to interactions between spins.

Larmor Frequency: Given by the equation: where is frequency (Hz), is the magnetic field strength (T).

(m) MRI in Diagnostics

Utilizes strong magnetic fields and RF pulses to produce detailed images of soft tissues.

Particularly effective for imaging the brain, spinal cord, joints, and organs.

Provides high contrast between different types of soft tissue.

No ionizing radiation exposure, making it safer for repeated use.

(n) Advantages and Disadvantages of Imaging Techniques

Ultrasound:

  • Advantages:

    • Non-ionizing,

    • real-time imaging,

    • safe for pregnancy.

  • Disadvantages:

    • Limited penetration depth,

    • poor imaging of bones

    • air-filled structures.

X-ray Imaging:

  • Advantages:

    • Good for bone imaging,

    • fast

    • widely available.

  • Disadvantages:

    • Ionizing radiation exposure,

    • poor soft tissue contrast.

MRI:

  • Advantages:

    • Excellent soft tissue contrast,

    • no ionizing radiation.

  • Disadvantages:

    • Expensive,

    • time-consuming,

    • unsuitable for patients with metallic implants.

(o) Effects of Radiation on Living Matter

α-radiation: Highly ionizing but low penetration; dangerous if ingested or inhaled.

β-radiation: Moderate penetration and ionization; can cause skin burns.

γ-radiation: Highly penetrating but weakly ionizing; can damage internal organs and DNA.

Absorbed and Equivalent Dose

  • Gray (Gy): Unit of absorbed dose, defined as energy absorbed per kilogram (J/kg).

  • Sievert (Sv): Unit of equivalent dose, accounts for radiation type and biological effect.

  • Absorbed dose (D): Energy absorbed per kilogram.

Equivalent dose: H = DWg (absorbed dose × radiation weighting factor).

Effective dose: E = HWT (equivalent dose × tissue weighting factor).

(q) Radionuclides as Tracers

Technetium-99m (Tc-99m):

  • Short half-life (~6 hours), reducing radiation exposure.

  • Used in imaging bones, heart, and kidneys.

  • Emits -rays, detected by gamma cameras.

(r) Gamma Camera Principles

Collimator: Allows only photons traveling in a specific direction to enter.

Scintillation Counter: Converts -rays into light flashes.

Photomultiplier Tubes: Amplify light signals and convert them into electrical signals.

CCD (Charge-Coupled Device): Converts electrical signals into digital images.

(s) Positron Emission Tomography (PET)

Uses positron-emitting radionuclides (e.g., fluorine-18).

Positrons annihilate with electrons, producing gamma rays.

Detected gamma rays create high-resolution images.

Used in cancer detection and monitoring metabolic activity in the brain and heart.