Understanding MRI Physics: Principles,
Components, and Artifacts
Magnetic Resonance Imaging (MRI) is a complex imaging technique used
to visualize detailed internal structures. This piece consolidates multiple
documents to provide a comprehensive explanation of core MRI principles,
components, and artifacts.
Basic MRI Principles
Nuclear Magnetic Resonance (NMR)
NMR: Phenomenon occurring when nuclei with non-zero spin are placed
in a magnetic field and absorb energy from RF pulses matching their
precessional frequency (Larmor Frequency), causing resonance.
Spin: Quantum property of protons, e.g., hydrogen atoms have a non-
zero spin value of 1/2.
Magnetic Moment: Describes the magnetic field around a proton with
non-zero spin.
Alignment: Protons align with the external magnetic field (B₀).
Precession: Rotation of proton's magnetic moment around B₀, occurring
at Larmor Frequency.
Larmor Frequency and Resonance
Larmor Frequency: Depends on the gyromagnetic ratio of the element
and the magnetic field strength.
Resonance: Occurs when RF pulse matches Larmor Frequency, causing
energy absorption and phase resonance.
MRI System Components
Main Magnet and Coils
, Main Magnet/Coil: Generates the strong, static main magnetic field (B₀).
Gradient Coils: Create small magnetic field variations for spatial
encoding.
Slice Selection Gradient: Applied during RF pulse to select tissue
slice along the z-axis.
Frequency Encoding Gradient: Applied during signal acquisition
along the x-axis.
Phase Encoding Gradient: Briefly applied between excitation pulse
and readout along the y-axis.
Radio Frequency (RF) Coils
RF Coils: Transmit RF pulses and receive signals. Types vary based on
body part imaged.
Shims
Passive Shims: Magnetic sheets or ferromagnetic metals placed within
MRI bore to manipulate the magnetic field.
Active Shims: Coils with current supply dynamically adjusted to
improve B₀ homogeneity.
Superconductivity and Quenching
Superconductivity: Materials exhibit zero electrical resistance below
critical temperature, enabling strong magnetic fields.
Quenching: Sudden loss of superconductivity, usually causing rapid
cryogen boil-off and magnetic field loss.
MRI Signal Formation and Acquisition
Signal Components and Relaxation
Components, and Artifacts
Magnetic Resonance Imaging (MRI) is a complex imaging technique used
to visualize detailed internal structures. This piece consolidates multiple
documents to provide a comprehensive explanation of core MRI principles,
components, and artifacts.
Basic MRI Principles
Nuclear Magnetic Resonance (NMR)
NMR: Phenomenon occurring when nuclei with non-zero spin are placed
in a magnetic field and absorb energy from RF pulses matching their
precessional frequency (Larmor Frequency), causing resonance.
Spin: Quantum property of protons, e.g., hydrogen atoms have a non-
zero spin value of 1/2.
Magnetic Moment: Describes the magnetic field around a proton with
non-zero spin.
Alignment: Protons align with the external magnetic field (B₀).
Precession: Rotation of proton's magnetic moment around B₀, occurring
at Larmor Frequency.
Larmor Frequency and Resonance
Larmor Frequency: Depends on the gyromagnetic ratio of the element
and the magnetic field strength.
Resonance: Occurs when RF pulse matches Larmor Frequency, causing
energy absorption and phase resonance.
MRI System Components
Main Magnet and Coils
, Main Magnet/Coil: Generates the strong, static main magnetic field (B₀).
Gradient Coils: Create small magnetic field variations for spatial
encoding.
Slice Selection Gradient: Applied during RF pulse to select tissue
slice along the z-axis.
Frequency Encoding Gradient: Applied during signal acquisition
along the x-axis.
Phase Encoding Gradient: Briefly applied between excitation pulse
and readout along the y-axis.
Radio Frequency (RF) Coils
RF Coils: Transmit RF pulses and receive signals. Types vary based on
body part imaged.
Shims
Passive Shims: Magnetic sheets or ferromagnetic metals placed within
MRI bore to manipulate the magnetic field.
Active Shims: Coils with current supply dynamically adjusted to
improve B₀ homogeneity.
Superconductivity and Quenching
Superconductivity: Materials exhibit zero electrical resistance below
critical temperature, enabling strong magnetic fields.
Quenching: Sudden loss of superconductivity, usually causing rapid
cryogen boil-off and magnetic field loss.
MRI Signal Formation and Acquisition
Signal Components and Relaxation
