Questions
2–4 questions in professional university exams
Difficulty
Medium
Importance
High yield for BMRIT and Medical Radiography boards
Overview
Magnetic Resonance Imaging (MRI) is a non-ionizing medical imaging modality that utilizes powerful magnetic fields and radiofrequency pulses to generate detailed anatomical images. Understanding its physical principles, specifically nuclear magnetic resonance, is crucial for both theory exams and clinical viva voce. Aspirants must master the transition between signal acquisition and spatial encoding to succeed.
Principles of MRI
The fundamental principle of MRI relies on the behavior of hydrogen protons in the body when placed in a strong external magnetic field (B0). Protons align with the field, and upon the application of a 90-degree radiofrequency (RF) pulse, they absorb energy and precess in phase, creating a detectable signal. As the RF pulse is removed, the protons undergo relaxation, returning to equilibrium and emitting an RF signal.
- Larmor Frequency: ω = γB0
- Hydrogen nuclei (single proton) are the primary source of signal
- Precession occurs at the Larmor frequency specific to the tissue type
- RF pulses shift the net magnetization vector
- Faraday's Law of Induction used for signal detection via receiver coils
T1 vs T2 Weighted Imaging
T1 and T2 weighted images represent different relaxation processes that occur after the RF pulse is turned off. T1-weighted images highlight longitudinal relaxation (recovery of magnetization along the z-axis), while T2-weighted images highlight transverse relaxation (decay of magnetization in the xy-plane). Recognizing the contrast differences is essential for interpreting common diagnostic scans.
- T1 recovery: Longitudinal relaxation time (Spin-lattice relaxation)
- T2 decay: Transverse relaxation time (Spin-spin relaxation)
- T1 weighting: Fat appears bright, water/CSF appears dark
- T2 weighting: Water/CSF appears bright, fat appears darker
- TE (Echo Time) and TR (Repetition Time) determine the weighting
Contraindications and Safety
Due to the intense static magnetic field and high-energy radio waves, strict safety protocols are required. Metal objects can become projectiles, and electronic implants can malfunction or heat up during the scan. Screening protocols must be rigorously followed to prevent patient injury or equipment damage.
- Ferromagnetic implants are absolute contraindications
- Cardiac pacemakers and cochlear implants are high-risk
- Monitoring for RF heating and acoustic noise levels
- Screening questionnaires are mandatory before entry to the MRI suite
- Claustrophobia management protocols
Formula Sheet
Larmor Frequency equation: ω = γB0
T1 recovery: Mz(t) = M0(1 - e^(-t/T1))
T2 decay: Mxy(t) = Mxy(0) * e^(-t/T2)
Exam Tip
Always relate T1 recovery to 'fat-bright' and T2 decay to 'fluid-bright' to solve image interpretation questions instantly.
Common Mistakes
- Confusing T1 and T2 appearances for CSF, leading to misinterpretation of neurological scans
- Failing to mention the Larmor equation when asked about the underlying physics of MRI
- Ignoring the difference between absolute and relative contraindications for non-ferromagnetic metals
More Revision Notes
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