MAP.10 MRI1 Principle of nuclear magnetic imaging

Learning Outcomes

Recall the principles of electrical induction and electromagnets.
Describe the components and standard working procedure of standard NMR.
Describe precession of proton spins in external magnetic fields and calculate their Larmor frequency.
Differentiate between spin flips by resonant absorption of radio frequency (RF) photons or by spontaneous decay.
Explain the different origins of the longitudinal and transverse magnetic fields.
Explain spin-lattice relaxation (T1) and spin-spin relaxation (T2).

Electricity and magnetism are interconnected through electromagnetic waves.
Understanding how electricity generates magnetism is essential for exploring Magnetic Resonance Imaging (MRI).

Definition: An electric current flowing through a wire induces a magnetic field around it.
Ampere's Law relates magnetic field (B) to the electric current (I) via the permeability of free space (μ0):
- Equation: B ~ μ0 I

Coil + Current → Magnetic Field: Sending current through a long coil produces a uniform magnetic field comparable to that of a bar magnet (e.g., electromagnetic recycling crane).

MRI = NMR + Imaging: MRI is based on NMR principles and used for imaging.
NMR Instrumentation: Strong magnets are required with RF coils acting as transmitters and detectors of signals.

Place the sample in a strong magnetic field.
Send a short RF pulse; some energy is absorbed.
Detect decaying electromagnetic signals from the sample.
- Classic NMR focuses on quantifying signal strength at various field strengths for chemical groups.
- MRI measures T1 and T2 signal decay rates for tissue properties.

Each proton has a positive charge and spin, creating a small magnetic field (Bp), forming a proton magnet.
Bp's direction is important and can be aligned (spin up) or anti-aligned (spin down) relative to external magnetic field Bz.

Precession: Spins move in a circular motion around the external magnetic field direction due to angular momentum.
Definition of Precession: Slow movement of a rotating axis around another due to torque influences.
Example: Similar to a spinning top; in MRI, it occurs in the xy-plane due to an external z-axis field.

The frequency at which Bp precesses around Bz, given by the Larmor equation:
- FL = γ'Bz, where γ' is the gyromagnetic ratio (42.58 MHz/T for H-1).
Stronger Bz results in higher FL.

The overall magnetization of nuclei depends on both protons and neutrons. Only H-1 nuclei are relevant for MRI because they contain a single proton.
When subjected to Bz, H-1 protons have a higher likelihood of aligning parallel to Bz, contributing to a net longitudinal magnetic field (BL) in the z-direction.

Longitudinal and transverse components contribute differently due to the alignment and random phase of spins.
Net Longitudinal Magnetic Field (BL): Small non-zero value aligned with Bz, created by a higher number of spin-up protons.
Net Transverse Magnetic Field (BT): Induced by a pules of RF, which gets excited in phase, allowing for measurement.

Protons absorb RF energy when the frequency matches their Larmor frequency, enabling transition from low energy (spin up) to high energy (spin down) states.
Upon cessation of RF, protons transition back to lower energy states, emitting RF photons during decay.

Spin-Lattice Relaxation (T1): After the RF pulse, BL recovers to ~63% of its original after a time T1.
Spin-Spin Relaxation (T2): The decay of BT happens simultaneously post-RF and is characterized by the time constant T2 (decay to 37% of its maximum).
T1 levels vary based on tissue’s thermal energy exchange while T2 reflects atomic homogeneity around the nucleus.

Inhomogeneities in the magnetic fields (both extrinsic and intrinsic) can speed up BT decay, leading to variations in how T2 is measured.

Understanding NMR principles is crucial for interpreting MRI results, with T1 and T2 providing insight into tissue characteristics.