MRI

PRELIM

MRI

Medical imaging technique using magnetic fields and RF waves to produce high-contrast soft tissue images

Nuclear Magnetic Resonance (NMR)

Physical phenomenon where nuclei absorb and emit RF energy in a magnetic field

Reason “nuclear” was dropped

Psychological fear of radiation despite MRI using non-ionizing energy

Felix Bloch

Co-discoverer of magnetic resonance in 1946

Edward Purcell

Independent co-discoverer of magnetic resonance in 1946

Raymond Damadian

Demonstrated different relaxation times between normal and tumor tissue

Paul Lauterbur

Introduced spatial encoding using gradients and produced first MR image

Peter Mansfield

Developed Echo-Planar Imaging and fast MRI techniques

Richard Ernst

Introduced Fourier Transform and frequency/phase encoding to MRI

CT significance in MRI history

Proved hospitals would invest in expensive imaging technology

Main advantage of MRI

Superior soft tissue contrast without ionizing radiation

Main magnetic field (B₀)

Strong uniform static magnetic field aligning nuclear spins

Permanent magnet

Ferromagnetic magnet producing low field strength up to 0.4T

Electromagnet

Magnet created using electric current through coils

Resistive magnet

Electromagnet using normal conductive coils with high power consumption

Superconducting magnet

Electromagnet with zero resistance at cryogenic temperatures

Clinical standard MRI field strength

1.5 Tesla

Liquid helium

Coolant used to maintain superconductivity at 4K

Field homogeneity

Uniformity of magnetic field measured in parts per million

Shimming

Process of correcting magnetic field inhomogeneity

Passive shimming

Mechanical correction using metal pieces

Active shimming

Electrical correction using shim coils

RF coil

Device that transmits RF pulses and receives MR signals

Volume coil

RF coil providing homogeneous excitation over large regions

Surface coil

RF coil placed close to anatomy providing high SNR but limited depth

Quadrature coil

Coil design using two channels 90° apart to improve SNR

Phased array coil

Multiple surface coils combined for high SNR and large coverage

Faraday cage

RF shielding room preventing external interference

Atomic nucleus

Central part of atom containing protons and neutrons

Hydrogen nucleus

Single proton nucleus used for MRI signal generation

Gyromagnetic ratio

Constant relating magnetic field strength to precessional frequency

Gyromagnetic ratio of hydrogen

42.57 MHz per Tesla

Magnetization

Net magnetic moment produced by aligned spins

Parallel alignment

Low-energy proton orientation with B₀

Antiparallel alignment

High-energy proton orientation opposite B₀

Net magnetization vector (M₀)

Resultant magnetization aligned along Z-axis

Longitudinal magnetization

Magnetization component parallel to B₀

Larmor frequency

Precessional frequency of spins in a magnetic field

Larmor equation

ω₀ = γB₀

Precession

Wobbling motion of spins around B₀

Phase coherence

Spins precessing together producing strong signal

Dephasing

Loss of phase coherence resulting in signal decay

RF pulse

Oscillating magnetic field applied perpendicular to B₀

Resonance

Condition when RF frequency equals Larmor frequency

Flip angle

Angle by which net magnetization is tipped from Z-axis

90° RF pulse

Rotates magnetization into transverse plane maximizing signal

Excitation

Absorption of RF energy by protons

Relaxation

Return of spins to equilibrium after excitation

T1 relaxation

Recovery of longitudinal magnetization via energy transfer to lattice

Spin-lattice relaxation

Another term for T1 relaxation

T1 definition

Time for longitudinal magnetization to recover to 63%

Fat T1 characteristic

Short T1 due to efficient energy exchange

Water T1 characteristic

Long T1 due to inefficient energy exchange

T2 relaxation

Decay of transverse magnetization due to spin-spin interactions

Spin-spin relaxation

Another term for T2 relaxation

T2 definition

Time for transverse magnetization to decay to 37%

Fat T2 characteristic

Short T2 due to rapid dephasing

Water T2 characteristic

Long T2 due to slow dephasing

T2*

Apparent transverse decay including field inhomogeneities

Relationship of relaxation times

T1 > T2 > T2*

Spin echo

Pulse sequence using 180° RF pulse to refocus dephasing

Gradient echo

Pulse sequence using gradients instead of 180° pulse

FID

Signal immediately following RF excitation

Fourier Transform

Mathematical conversion from time domain to frequency domain

Gradient coil

Coil producing small spatially varying magnetic fields

Slice selection gradient

Gradient determining slice location

Phase encoding gradient

Gradient determining spatial position in one direction

Frequency encoding gradient

Gradient determining spatial position during readout

K-space

Raw data matrix containing frequency and phase information

Center of k-space

Determines image contrast

Edges of k-space

Determine spatial resolution

Pulse sequence

Timing diagram of RF and gradient events

Gradient strength

Maximum amplitude of gradient field

Slew rate

Speed at which gradient reaches maximum strength

Signal-to-noise ratio (SNR)

Ratio of useful signal to background noise

Voxel

Three-dimensional volume element of tissue

Pixel

Two-dimensional image element

Matrix

Grid defining number of pixels in image

Inter-slice gap

Spacing between slices to reduce cross-talk

TR (Repetition Time)

Time between successive RF pulses controlling T1 contrast

TE (Echo Time)

Time between RF pulse and signal readout controlling T2 contrast

NEX

Number of signal averages used to improve SNR

Receiver bandwidth

Frequency range sampled during readout

MRI contrast agent

Substance that alters relaxation times of tissues

Gadolinium contrast

T1-shortening agent producing bright signal

DWI

Imaging technique measuring Brownian motion of water

ADC

Quantitative map confirming true diffusion restriction