The Basic principle of MRI (Magnetic Resonance Imaging) is based on the behavior of hydrogen nuclei (protons) in a strong magnetic field.
Hydrogen nuclei absorb radiofrequency energy in a strong magnetic field and emit signals while returning to equilibrium. These signals are converted into images.
Magnetic Resonance Imaging is an imaging technique that produces detailed images of internal body structures .
Strong magnetic field
The patient is placed inside a powerful magnet.
Hydrogen protons in the body (mainly in water and fat) align with the magnetic field.
Radiofrequency (RF) pulse
A radio wave pulse is applied at a specific frequency.
This pulse disturbs the aligned protons and tips them away from their normal direction.
Relaxation
When the RF pulse is turned off, the protons return to their original alignment.
During this process, they release energy as radio signals.
Signal detection
Receiver coils detect these emitted signals.
Image formation
A computer processes the signals to create detailed images of body tissues.
Important concepts
T1 relaxation → longitudinal recovery
T2 relaxation → transverse
Different tissues relax at different rates, producing image contrast.
Why MRI is useful ?
Excellent soft tissue contrast
No ionizing radiation
Useful for brain, spine, joints, muscles, heart, and tumors.
Why Hydrogen is Used
Hydrogen is ideal because:
The human body contains a large amount of water and fat
Each hydrogen nucleus contains one proton
Protons behave like tiny magnets and spin continuously
Normally-
Hydrogen protons point in random directions
After entering the magnetic field:
Some protons align parallel
Some align anti-parallel
A small excess aligns parallel
This creates a net magnetization vector.
Precession of Protons
The aligned protons rotate around the magnetic field axis.
This motion is called:
Precession
The precession frequency is called:
Larmor Frequency
It depends on magnetic field strength.
Where:
ω₀ = Larmor frequency
γ = gyromagnetic ratio
B₀ = magnetic field strength
Application of Radiofrequency (RF) Pulse
An RF pulse at the Larmor frequency is transmitted.
The protons absorb energy and:
Move away from alignment
Tip into the transverse plane
This condition is called:
Resonance
Different tissues have different T1 and T2 values, producing contrast in MRI images.
Signal Detection
The emitted RF signals are detected by receiver coils.
These signals contain information about:
Tissue type
Position
Proton density
Spatial Encoding
Gradient magnets are used to identify signal location in:
X-axis
Y-axis
Z-axis
Three gradients are used:
Slice selection gradient
Phase encoding gradient
Frequency encoding gradient
Image Formation
A computer processes the signals using:
Fourier Transformation
The final image is reconstructed into cross-sectional body images.
Components of MRI System
Main parts include:
Main magnet
Gradient coils
RF transmitter coil
RF receiver coil
Computer system
Patient table
Types of Magnets Used
Permanent magnet
Resistive magnet
Superconducting magnet (most common)
MRI Contrast Depends On
Image contrast depends on:
T1 relaxation time
T2 relaxation time
Proton density
Flow of blood
Pulse sequence
Advantages of MRI
No ionizing radiation
Excellent soft tissue contrast
Multiplanar imaging
Detects early disease changes
Disadvantages of MRI
Expensive
Long scan time
Claustrophobia
Cannot be used with some metallic implants
Common Clinical Uses
MRI is commonly used for:
Brain imaging
Spine imaging
Joint imaging
Cardiac imaging
Tumor detection
Ligament injuries
Simple Flow Chart
Strong Magnetic Field
Protons Excited
One-Line Definition
MRI is a medical imaging technique based on nuclear magnetic resonance of hydrogen protons in a strong magnetic field.
What is MRI?
Magnetic Resonance Imaging is a non-invasive imaging technique that uses:
Strong magnetic field
Radiofrequency (RF) energy
Electromagnetic principles
Computer processing
to produce detailed images of organs and soft tissues.
MRI is based on the principle of:
Pulse Sequences
MRI images are created using different pulse sequences.
Common sequences:
Spin Echo (SE)
Gradient Echo (GRE)
FLAIR
STIR
Diffusion Weighted Imaging (DWI)
Spin Echo Principle
Uses:
90° RF pulse
180° RF pulse
Purpose:
Corrects dephasing
Produces strong signal
Important MRI Parameters
TR (Repetition Time)
Time between successive RF pulses.
TE (Echo Time)
Time between RF pulse and signal measurement.
T1-Weighted Image
Features:
Fat bright
Fluid dark
Good anatomy
T2-Weighted Image
Features:
Fluid bright
Edema visible
Pathology detection
Proton Density Imaging
Contrast mainly depends on hydrogen concentration.
Superconductivity in MRI
Most MRI systems use superconducting magnets cooled by:
Liquid Helium
Temperature:
Around −269°C
This reduces electrical resistance.
MRI Safety
MRI uses no ionizing radiation.
But caution is required with:Not allowed
Pacemakers
Metallic implants
Aneurysm clips
Ferromagnetic objects
Contrast Agents
Common MRI contrast:
Gadolinium
It shortens T1 relaxation time and enhances tissues.
Functional MRI (fMRI)
Measures brain activity using:
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