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 $\omega<em>0 = γ B</em>0$ (rad/s).
• $B_0$ = magnetic field (tesla).
• $γ$ = gyromagnetic ratio.
• Larmor frequency: $ν<em>0 = γ B</em>0 / 2π$ (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 $B<em>0$ modifies the precession angle: $\omega = γB</em>0$.
• 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: $\omega<em>0 = γ B</em>0$.
• 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 $B<em>0$, related to T1) and transverse (Mxy, perpendicular to $B</em>0$, 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

• $B<em>y = B</em>0 + g_y * y$

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.