Nuclear Magnetic Resonance (NMR) and Spin

Nuclear Magnetic Resonance and Spin

• Nuclear magnetic resonance (NMR) depends on the reaction of certain nuclei to external magnetic fields in MRI.
• NMR understanding requires understanding spin.
• Spin can be illustrated using classical and quantum mechanical models.
• The classical model uses a charged particle rotating around its axis, resulting in angular momentum.
• Rotating charged particles create a magnetic field.
• Magnetic Moment (a vector) represents the strength and direction of the magnetic field.
• The relative strength of magnetic fields is indicated using arrows.
• The classical model is analogous, not physically accurate within particles.
• The classical model has some limitations.
• Neutrons lack charge but possess a Magnetic Moment, which contradicts the classical model.
• Quantum physics accounts for the Magnetic Moment in a neutron.
• Quantum physics expresses properties in discrete, measurable values
• Quantum properties like charge, mass, color, and spin are distinct and measurable
• Spin measures how a subatomic particle interacts with an external magnetic field.
• Spin angular momentum is measurable in discrete values.
• Protons are formed of up and down quarks with spin values, which are bound by gluons.
• Protons possess a one-half net spin value, indicating spin angular momentum.

Spin Values and Magnetic Fields

• Electrons possess a spin of one-half.
• Within the same nucleus, neutrons exhibit spin-up and spin-down states.
• Spin up and spin down states have opposing magnetic moments that cancel.
• Nuclei having even numbers of protons and neutrons have a net spin value of zero.
• External magnetic fields do not affect zero-spin atoms.
• Oxygen-16 and carbon-12 possess canceled spin values because they have even numbers of protons and neutrons.
• Hydrogen is vulnerable to external magnetic fields because it has a proton with one-half spin.
• Pali Exclusion Principle, Heisenberg Uncertainty Principle, Schrodinger equation, and entanglement relate to these ideas.
• When assessing a group of protons, a net magnetization vector is applicable.
• The sum of all magnetic moments gives the net magnetization vector.

Why Hydrogen Is Used In MRI

• The most prevalent isotope in the body is hydrogen.
• Of all isotopes, hydrogen's Magnetic Moment is the biggest.
• Tissue can be compared because hydrogen is abundant in many tissues.
• The terms free hydrogens, protons, and spins can be interchanged.
• Hydrogen may be in spin-up and spin-down states at the same time.
• Hydrogen's state is measurable.
• The net Magnetic Moment of a group of hydrogens is important.
• Magnetic Moment describes how hydrogen protons react to an external magnetic field.
• The gyromagnetic ratio connects Magnetic Moment magnitude and proton spin.
• The hydrogen atom has a 42.5 MHz per Tesla gyromagnetic ratio.

Larmor Frequency

• In a magnetic field, an atom having a non-zero spin aligns and precesses at a defined frequency.
• Precessional frequency relates to magnetic field strength and atom type.
• Gyromagnetic ratio and magnetic field strength allow calculation of the Larmor frequency.
• Larmor Frequency = gyromagnetic ratio * magnetic field strength
• Knowing the magnetic field strength and gyromagnetic ratio enables calculation of hydrogen proton precessional frequency.
• Use a radio frequency (RF) pulse that matches the precessional frequency.
• At 1.5 Tesla main magnetic field strength, frequency rises 50%.
• Larmor frequency computes the precessional frequency of the relevant atom.
• The type of atom in the magnetic field determines the frequency.

Net Magnetization Vector

• Protons align with the magnetic field and create an energy differential between spin up and spin down.
• The net magnetization vector runs parallel to the external magnetic field.
• Protons precess out of phase, thus canceling transverse magnetization values.
• The net magnetization is oriented along the longitudinal or Z axis.
• The net magnetization vector within the MRI scanner should not be precessing.
• Although hydrogen atoms precess at a defined rate, the magnetization vector remains constant.
• The transverse magnetization values offset one another.

Radio Frequency Pulse

• A radio frequency pulse at a defined frequency creates transverse magnetization.
• The radio frequency pulse (B1) runs perpendicular to the main magnetic field.
• Applying a radio frequency magnetic force at the same frequency introduces energy.
• The net magnetization vector is displaced off the longitudinal or Z plane.
• Transverse magnetization rises.
• Resonance arises when the radio frequency pulse matches hydrogen atoms precessional frequency.
• The net magnetization vector fans out due to resonance.
• Hydrogen spins begin to spin in sync.
• In the X Y plane, signals are measurable when transverse magnetization exists.
• Transverse magnetization occurs when hydrogen nuclei are in phase and resonance happens.

Flip Angle and Signal Measurement

• Time-controlled radio frequency pulse application flips the net magnetization vector by a flip angle.
• A signal can be read at a 60-degree flip angle.
• Signal strength is proportional to transverse magnetization of hydrogen atoms processing in phase.
• A longer radio frequency pulse raises the flip angle to 90 degrees.
• At 90 degrees to the main magnetic field, hydrogen nuclei achieve maximum signal.
• Resonance causes transverse magnetization, which enables signal measuring.
• Larmor equation calculations for specific locations are enabled by gradients in the longitudinal direction.
• The magnetic field strength changes as the gyromagnetic ratio stays constant.
• Frequency of hydrogen protons varies as magnetic field strength changes.
• A certain slice is chosen to match the frequency of a radio frequency pulse.
• Radio frequency pulse and precessional frequency must match.
• Resonance enables signal measuring and selection of specific hydrogen atoms using Larmor frequency.

Flip Angle Timing

• Transverse magnetization gain occurs during radio frequency pulse application.
• A 45-degree magnetization vector has a defined signal; a 90-degree vector produces the highest signal.
• Using the 45-degree technique saves half the time while producing 70% of the 90-degree signal.
• Short flip angles are used in sequences requiring rapid signal measurements.
• Quantum properties enable a 180-degree flip of the net magnetization vector.
• Increasing the radio frequency pulse application time by twofold achieves a full 180-degree flip.
• All transverse magnetization is lost.
• A 180-degree vector is in the anti-parallel higher energy state relative to the main magnetic field.
• Protons can be in both spin-up and spin-down states.
• Pulse sequences need a 180-degree vector flip.
• The net magnetization vector returns to its resting state by waiting.
• Helpful when attempting a true T2 signal generation.
• Spin angular momentum causes magnetic moment within a proton.
• Resonance is inducible using forces perpendicular to precessional frequency.
• Nuclear magnetic resonance describes the overall process.