MRI Notes

Magnetism in MRI

Magneton

The apparatus used in MRI.

Magnets and Magnetic Fields

  • A magnet attracts metallic objects.
  • A magnetic field influences its environment by orienting it.

Magnetic Materials

  • Magnets attract iron; this is called ferromagnetism.
  • Ferromagnetism exists in iron, cobalt, manganese, gadolinium, and dysprosium, as well as certain alloys.

Magnetic Poles

  • Magnets have a North (N) and South (S) pole.
  • Like poles repel, opposite poles attract.

Magnetic Fields Defined

  • A magnet influences its surroundings, represented by a vector field called the magnetic field.

Magnetic Spectra Visualization

  • Iron filings are used to visualize magnetic spectra.
  • The filings align with the magnetic field, forming lines of force.

Magnetic Field and Electric Current

  • Electric current in a wire creates a magnetic field (Oersted's experiment, 1820).
  • A moving magnetic field generates electric current (Faraday's experiment, 1831).

Magnetic Field of a Straight Conductor

  • No N or S poles.
  • The magnetic field direction depends on the current direction.
  • Field intensity increases with current and decreases with distance from the conductor.
  • Magnetic field lines are concentric circles around the conductor.

Earth's Magnetic Field

  • The Earth has a magnetic field used for navigation.
  • The magnetic south pole is near the geographic north pole, and vice versa.

Magnetic Field of a U-shaped Magnet

  • The magnetic field (B) is uniform between the branches of the magnet.

Magnetic Field of a Straight Magnet

  • Field lines exit the N pole and enter the S pole.

Magnetic Field Lines

  • Magnetic field lines indicate the direction and magnitude of the magnetic vector B.
  • Denser lines indicate a stronger field.

Broken Magnet Experiment

  • Breaking a magnet results in two magnets, each with N and S poles.
  • Magnetic poles cannot be isolated.
  • Ferromagnetic materials are made of microscopic magnets that are randomly organized when not magnetized.
  • In a magnetic field, these elementary magnets align.

Nuclear Magnetic Resonance

Spin

  • Atoms consist of a nucleus and electrons; the nucleus contains nucleons (protons and neutrons).
  • Protons are positively charged, and nuclei spin, possessing a spin or angular momentum S.

Gyromagnetic ratio

  • µ = γS where γ is the gyromagnetic ratio.

Nucleons

  • Protons and neutrons pair up, canceling magnetic moments.
  • Atoms with an odd number of nucleons have a net magnetic moment.
  • Key nuclei for MRI: hydrogen ^{1}H, carbon ^{13}C, fluorine ^{19}F, phosphorus ^{31}P, sodium ^{23}Na.

Hydrogen in MRI

  • Hydrogen is abundant in soft tissues (70-90% water).
  • Its nucleus (a single proton) has a magnetic spin moment.
  • Most MRI uses hydrogen nuclei.
  • Gyromagnetic ratios (MHz/tesla): H=42.58, F=40, Na=11.2, C13=10.7, N=3

Magnetic Moments

  • Normally, hydrogen nuclei's magnetic moments are randomly oriented, so net magnetization is zero (\Sigmaµ = 0).

Absence of External Magnetic Field

  • Protons orient randomly; the microscopic magnetization vectors sum to zero (\Sigma m = 0).
  • No macroscopic magnetization vector M exists.

Presence of External Field

  • Protons align with the external field (Oz), resulting in a macroscopic magnetization vector Mzo.

Larmor Frequency

  • In an external magnetic field, protons precess at an angular frequency \omega0 = γ B0 (rad/s).
  • B_0 = magnetic field (tesla).
  • γ = gyromagnetic ratio.
  • Larmor frequency: ν0 = γ B0 / 2π (Hertz).

Energy Levels

  • Protons align either parallel or anti-parallel to B_0.
  • Parallel alignment is slightly more common and has lower energy (E1); anti-parallel is higher energy (E2).

Macroscopic Magnetization

  • The few extra parallel protons create a net macroscopic magnetic moment along B_0, called Mz0.
  • This overall magnetic moment Mz0 is weak and requires dynamic physical methods (RF transfer) to detect.

RF Pulses

  • RF pulses cause Mz0 to tilt towards the xOy plane, decreasing longitudinal magnetization (Mz) and creating transverse magnetization (Mxy).

Resonance

  • Applying a radiofrequency wave perpendicular to B0 modifies the precession angle: \omega = γB0.
  • Equilibrium State: M aligns with Oz, M = Mzo, Mxy = 0
  • 90° RF Pulse: Mz decreases, Mxy increases, achieving Mz = 0 and Mxy = maximum.

Magnet Components

  • The MRI machine includes shim coils, cryostat, superconducting coil, head antenna, magnet, gradient coils, and body antenna.

Magnet Characteristics

  • Magnetic field intensity:
    • Low fields (<0.25 T)
    • Medium fields (<1 T)
    • High fields (>1 T, 1.5, 3T)
  • Earth's magnetic field = 0.5 x 10^{-4} Tesla
  • Homogeneity

Magnet Types

  • Permanent magnets: no electricity or cooling; heavy, inhomogeneous fields.
  • Resistive magnets: electromagnets with high heat dissipation and electricity consumption, require cooling.
  • Superconducting magnets

Superconducting Magnets

  • Electromagnets using superconducting metal
  • Generate intense, uniform B_0 fields.
  • Lower electricity consumption.
  • Cooled by Helium (cryostat).

Constant Magnetic Field

  • The magnetic field B_0 is constant and always present.
  • MRI rooms include a Faraday cage.

Gradient Coils

  • Electromagnetic coils introduce gradients in B_0.
  • Characterized by amplitude (mT/m), rise time (µsec), and slew rate.

RF Antennas

  • Transmit and receive RF signals.
  • Types: cylindrical (deep exploration), surface (superficial exploration).
  • Can be used alone or in phased arrays.
  • Phased arrays offer small fields with good signal-to-noise ratio (S/B) and larger coverage.

Antenna Use

  • The magnet can serve as a "body antenna."
  • Dedicated antennas improve the signal-to-noise ratio and spatial resolution.
  • Choice depends on the region to be explored.

Antenna Positioning

  • Center the region of interest in the antenna and at the magnet's center.

MRI Other Components

  • Examination table, computer system, acquisition and image processing consoles, archiving, and Faraday cage.

Faraday Cage

  • Surrounds the room.
  • Reduces magnetic fields and isolates from external RF interference.

RF Pulses and Magnetization

  • 180° RF Pulse: Mz decreases and inverts, resulting in M = -Mzo.

RF Pulse Values

  • 90° Pulse: tips M into the OXY plane, maximizing Mxy while Mz is zero (saturated system).
  • 180° Pulse: inverts M to the opposite of B_0, inverting Mz while Mxy is zero (inverted system).

Summary of MRI Principles

  • Larmor frequency: \omega0 = γ B0.
  • Electromagnetic wave: rotating magnetic field B_1.
  • Equilibrium: excess protons create a magnetization vector M, aligned with B_0.
  • B_1 perturbs equilibrium, tilting M into the xOy plane (90° pulse) => M = Mxy.
  • Two mechanisms: equalization of protons across two energy levels and phase alignment of protons.
  • 180° RF pulse inverts protons across energy levels > M.

Relaxation

  • Upon stopping the RF wave, a signal is collected as an RF wave.
  • This free induction decay (FID) signal is characterized by its initial intensity (RHO) and decay, depending mainly on T2* and T2.

Relaxation - Return to Equilibrium

  • After excitation, the system returns to equilibrium.
  • Longitudinal magnetization recovers; transverse magnetization disappears.
  • Transverse and longitudinal relaxations have different mechanisms.

Tissue Magnetization

  • Two types of tissue magnetization: longitudinal (Mz, parallel to B0, related to T1) and transverse (Mxy, perpendicular to B0, related to T2).

Longitudinal Relaxation (T1)

  • Spins return to the parallel state.
  • Longitudinal magnetization regrows.
  • T1 is the time to recover approximately 63% of longitudinal magnetization.

Transverse Relaxation (T2)

  • Spins dephase.
  • Transverse component disappears.
  • T2 is the time for approximately 63% of transverse magnetization to disappear.

Relaxation Times

  • Depend on the tissue structure and are the primary factor for contrast.
  • T1 = longitudinal relaxation time.
  • T2 = transverse relaxation time.

T1 and T2 Contrasts

  • Tissues have different T1 values, even with the same proton density, resulting in varying signal intensities.
  • T1 short = fast regrowth (e.g., fat).
  • T1 long = slow regrowth (e.g., liquids).
  • T1 relaxation time increases with increasing B_0.

T2 Characteristics

  • T2 = time for MT to lose 63% of its value.
  • Depends on proton dephasing.
  • T2 short = rapid decay (liquids).
  • T2 long = slow decay (fat).
  • T2 relaxation time is linked to molecular heterogeneities.

Relaxation Summary

  • Longitudinal relaxation (T1) = regrowth of Mz.
  • Transverse relaxation (T2) = loss of phase coherence of protons.
  • T1 and T2 are distinct with different durations.
  • T2 is much faster than T1.

Signal Measurement

  • Antennas convert tissue magnetization into an electrical signal.
  • Free induction decay (FID) reflects tissue properties.

Free Induction Decay (FID)

  • Depends on the nucleus's resonance frequency.
  • Amplitude is determined by the RF wave intensity, B_0 intensity, number of protons, and the physicochemical environment.

Tissue contrast dependence

  • Proton density.
  • T1 relaxation time.
  • T2 relaxation time.
  • Flow.

Proton Density Contrast

  • Tissues without protons (calcification, air) produce no signal.

Programming

  • MRI machines are programmed to emphasize contrast elements.
  • Sequences are programmed to favor a particular contrast (T1, T2, proton density, or flow).

Image Acquisition

  • Images are derived from multiple sequences.
  • The time between sequence repetitions is the repetition time (TR), a key parameter to set.

Spin Echo Sequence

  • Employs a 90° pulse followed by a 180° pulse to create an echo.
  • TR = time between two 90° pulses.
  • TE = echo time = time between the 90° pulse and signal reception.

TR Influence

  • TR influences T1 contrast.
  • If TR is long compared to tissue T1, longitudinal magnetization fully recovers.
  • If TR is short, longitudinal magnetization is less than maximal.

TE Influence

  • TE influences T2 contrast.
  • If TE is short, T2 decay differences are minimized.
  • If TE is long, differences in T2 are highlighted.

Long Sequences

  • Long TE (40-100ms) and TR (1000-3000 ms)
  • Ponderation in T2
    • TR long (>2000msec)
    • TE long

Short Sequences

  • Short TE (<30 ms) and TR (<600msec)
  • Ponderation in T1
    • TR short.
    • TE short.

Mixed Sequences

  • Less common.
  • TE long, TR short.
  • TE short, TR long: Ponderation in proton density.

Inversion Recovery Sequence

  • Improves T1 contrast.

Spatial Encoding

Spatial Localization of Signal

  • Without spatial discrimination, signals from the sample are irresolvable.

Magnetic Gradient Creation

  • By = B0 + g_y * y

Selective Excitation

  • Selective excitation of a plane can be done if the radiofrequency is the resonance frequency of protons in that plan.

Gradient Determinations

  • Gradients determine the orientation of the slice.
  • The central frequency of the RF pulse sets the slice level.
  • The RF pulse bandwidth controls slice thickness.

Slice Selection

  • A gradient (Gz) defines the slice.

Fourier Transform

  • Mathematical calculation to classify frequencies and transform them into RMN signals.
  • Allows transformation into a gray scale image.

Image Acquisition Time

  • One cycle provides information for one line.
  • The phase gradient is shifted to acquire subsequent lines.
  • TA = TR x N x n; N = number of lines, n = number of accumulations.

Reducing Acquisition Time

  • Use asymmetric matrices.
  • Reduce TR.
  • Reduce flip angle.

Asymmetric Matrix

  • Only the number of lines affects acquisition time.

Gradient Echo Sequence

  • Signal echo achieved by applying a reading gradient Gx.
  • Reduces acquisition time.

Contrast with Gradient Echo

  • If angle > 60°, equivalent to SE; TR and TE determine T1 or T2 contrast.
  • If angle < 30°:
    • TE > 30 ms: T2* images.
    • TE short and TR > 100 ms: DP.

Image Quality Trade-offs

  • Signal-to-noise ratio (S/B), acquisition time, spatial resolution, number of slices, and weighting.

Contrast Modification

  • Fat suppression sequences (IR, STIR) and fat saturation.
  • Contrast agents.

Gadolinium Chelates

  • Decrease T1 in tissues.
  • Used with T1-weighted sequences after IV administration.

Artifacts

  • Motion, susceptibility, aliasing, chemical shift, truncation, partial volume, system defects.

Motion Artifacts - non-periodic

  • Caused by patient movements, propageted along the phase and/or the frequency
  • Correction: Information, Contention, Sedation

Motion Artifacts - periodic

  • Caused by respiratory or cardiovascular movements
  • Propageted along the phase, generating ghost images
  • Correction: fast imagery, saturation bands, synchronisation

Susceptibility Artifacts

  • Metallic objects distort the image due to local B changes.

Aliasing Artifact

  • Occurs when the object is larger than the field of view (FOV).
  • Correction: increase the FOV.

Chemical Shift Artifact

  • Linked to localization errors along the encoding gradient, specifically at fat-water interfaces.
  • correction: fat supression.

Truncation Artifact

  • Occurs at abrupt signal changes.
  • Correction: increase matrix and higher spatial resolution.

Partial Volume Artifact

  • Similar to CT artifact = reduce slice thickness.

Magnet Safety

  • All materials must be non-magnetic.
  • Patients must be screened for contraindications.

Absolute Contraindications

  • Certain devices (pacemakers, implanted pumps, neurostimulators) and ferromagnetic foreign bodies.

Relative Contraindications

  • Orthopedic prostheses, claustrophobia, involuntary movements, pregnancy (first trimester).

Patient Preparation

  • Confirm identity, check for contraindications, explain the exam.

Room Preparation

  • Verify emergency equipment.
  • Ensuring hygiene

Antenna Preparation

  • Select the appropriate antenna and proper connection.

Patient Installation

  • Ensure patient comfort and safety.

Console Settings

  • Select the appropriate sequence, plane, weighting, matrix, FOV, echo time, slice thickness, and fat suppression.

Image Interpretation

  • T1, proton density, and T2 contrasts each provide different information about tissue properties.
  • For T1-weighted images, fat appears hyperintense.
  • Calcium and air typically appear hypointense on all sequences.
  • Water/CSF will appear hypointense on T1, hyperintense on T2, and very hyperintense on FLAIR.