Chapter 9: MRI Magnetism, Shimming, Gradients & RF Systems

Main Static Magnetic Field and Magnetism Basics

• Atomic Structure:
  • Atoms consist of a central nucleus made up of positive protons and neutral neutrons, surrounded by orbiting negative electrons.
  • The atomic number and specific element are determined by the number of protons.
  • An atom becomes an ion if the number of electrons does not equal the number of protons.
• Hydrogen Protons in MRI:
  • Hydrogen is the critical element manipulated to construct MRI images.
  • Hydrogen is selected due to its abundance in body tissues, specifically water and fat.
• Magnetic Behavior of Electrons:
  • The degree of magnetism depends directly on the number of unpaired electrons: a higher count of unpaired electrons yields a stronger magnetic effect.
  • Memory Rule for Electron Pairs:
  • Paired electrons result in repulsion: PAIR = PUSH.
  • Unpaired electrons result in attraction: UNPAIRED = PULL.

Categories of Magnetism

• Diamagnetism:
  • Electron Configuration: Contains paired electrons.
  • Magnetic Effect: Slightly repelled by magnetic fields.
  • Safety and Compatibility: Generally MRI Safe or Compatible.
  • Material Examples: Water, copper, lead, sodium chloride, sulfur.
• Paramagnetism:
  • Electron Configuration: Contains unpaired electrons.
  • Magnetic Effect: Slightly attracted to magnetic fields.
  • Material Examples: Tungsten, cesium, aluminum, lithium, magnesium, sodium, gadolinium.
• Superparamagnetism:
  • Magnetic Strength: Possesses a magnetic moment greater than paramagnetic materials, but less than ferromagnetic materials.
  • Material Examples: Iron oxide particles, specific MRI contrast media.
• Ferromagnetism:
  • Magnetic Behavior: Exhibits strong magnetic attraction and alignment.
  • Hazards and Safety: Usually MRI Unsafe; presents extreme torque and projectile hazards, and can permanently retain magnetism after exposure.
  • Material Examples: Iron, steel, nickel, cobalt.
• High-Yield Rule for Mixed Properties:
  • If a material possesses both paramagnetic and diamagnetic characteristics, it will behave as a paramagnetic material.

MRI Magnet Systems and Configurations

• Permanent Magnet Systems:
  • Physical Design: Typically an open magnet design; extremely heavy.
  • Field Strength: Low static field strength ranging from $0.2\,\text{T}$ to $0.3\,\text{T}$.
  • Operational Characteristics: The field is always ON; requires low power and minimal maintenance; produces a very low fringe field.
  • Quick Memory Hook: Always ON + low field.
• Resistive Magnet Systems:
  • Construction: Uses copper-wound solenoid electromagnets.
  • Thermal Dynamics: Electrical resistance creates significant heat.
  • Operational Characteristics: Can be turned OFF; requires high power input; reaches field strengths up to $0.3\,\text{T}$.
  • Quick Memory Hook: RESISTANCE = HEAT.
• Superconducting Magnet Systems:
  • Prevalence: Most common system used in clinical and research imaging.
  • Field Strength: Standard clinical field strengths range from $0.5\,\text{T}$ to $3\,\text{T}$.
  • Cooling Mechanism: Utilizes cryogens, primarily liquid helium, to reduce conductor resistance to zero, reaching superconductivity.
  • Field Characteristics: Highly stable with high magnetic field homogeneity; generates a large fringe field.
  • Quick Memory Hook: SUPER = COLD + STRONG.
  • Quench Event: The sudden emergency or purposeful release/loss of cryogens that rapidly destroys the main magnetic field.
• Scanner Configurations:
  • Closed-bore: The patient is completely enclosed in a cylindrical bore with the target region of interest centered at the magnet isocenter.
  • Open: The patient is positioned between magnetic poles, offering maximum accessibility but operating at weaker field strengths and lower spatial resolution.
  • Extremity: Dedicated to imaging limbs (arms or legs), typically utilizing permanent magnets operating below $1\,\text{T}$.
• Magnetic Field Measurements and Safety Boundaries:
  • Field Strength Measurement Units: Expressed in gauss ($\text{G}$) or tesla ($\text{T}$).
  • Earth's Magnetic Field: Approximately $0.5\,\text{G}$ to $0.6\,\text{G}$.
  • Fringe Field: The peripheral magnetic field surrounding the scanner bore and extending into adjacent space.
  • 5-Gauss Line: The strict perimeter boundary used to divide MRI Safe zones from MRI Unsafe areas for equipment and personnel.
  • Field Unit Conversion: $0.5\,\text{mT} = 5\,\text{G}$.

Magnet Homogeneity and Shimming Systems

• Homogeneity Definitions and Thresholds:
  • Homogeneity describes the overall uniformity of the main static magnetic field ($B_0$).
  • Field Inhomogeneity causes image artifacts and compromised image quality.
  • Measurement: Expressed in units of parts per million ($\text{ppm}$).
  • Factory Homogeneity: Raw manufacturing standard is approximately $100\,\text{ppm}$.
  • Clinical Imaging Homogeneity: Requires approximately $4\,\text{ppm}$.
  • Clinical Spectroscopy Homogeneity: Requires precision better than $1\,\text{ppm}$.
  • Coil Loading Effect: Positioning a patient inside the bore decreases magnetic field homogeneity.
• Shimming Concepts:
  • Shimming is the physical or electromagnetic correction of magnetic field imperfections to optimize homogeneity.
  • Passive Shimming:
  • Timing: Performed exclusively during magnet installation.
  • Hardware: Steel discs or metal plates embedded inside the main magnet casing.
  • Procedure: Service engineers adjust plate placement using phantoms to smooth field inconsistencies.
  • Application: Corrects environmental and inherent structural magnet inhomogeneities, reducing variance from approximately $100\,\text{ppm}$ down to $1\,\text{ppm}$ to $4\,\text{ppm}$.
  • Active Shimming:
  • Timing: Executed continuously during individual patient scan routines.
  • Hardware: Electromagnetic solenoid coils placed inside the main magnet housing.
  • Application: Corrects dynamic, patient-induced field inhomogeneities regardless of patient mass, size, or tissue composition.
  • Operational Impact: Directly responsible for the brief prescan delay prior to sequence acquisition.
• Shimming vs. Shielding Distinction:
  • Shimming adjusts $B_0$ to correct homogeneity.
  • Shielding limits and confines the outer fringe field.
  • Passive Shielding: Heavy steel plates installed directly into the scan room walls during construction.
  • Active Shielding: Dedicated superconducting coils integrated inside the scanner housing that actively collapse and reduce stray fringe fields.

Gradient System Principles and Characteristics

• Gradient Coils and Spatial Encoding:
  • Gradient definition: A deliberate, linear slope or variation in magnetic field strength across distance.
  • Hardware: Electromagnets integrated along the inner bore walls of the main magnet.
  • Acoustic Noise: Rapid electrical current pulses switching between positive and negative polarities produce the characteristic loud knocking MRI sound.
  • Spatial Coordinate Coils:
  • X Gradient: Positioned on the lateral sides of the cylinder; controls Left to Right spatial encoding; defines the Sagittal image plane.
  • Y Gradient: Positioned at the top and bottom of the cylinder; controls Anterior to Posterior spatial encoding; defines the Coronal image plane.
  • Z Gradient: Wrapped completely around each end of the cylindrical bore; controls Head to Foot spatial encoding; defines the Axial image plane.
  • Gradient Power Amplifiers: Dedicated power supply units assigned to each gradient coil set; high-speed gradient switching can demand power spikes up to $1000\,\text{kW}$.
  • Core Functions of Gradients:
  1. Slice selection.
  2. Spatial encoding.
  3. Phase encoding.
  4. Frequency encoding.
• Magnetic Field Alterations via Gradients:
  • Physical variables controlling field changes: Number of wire loops, electrical current flowing through loops, loop diameter, and loop spacing.
  • Spacing vs. Uniformity: Loop count, current, and diameter generate uniform changes; variable loop spacing produces non-uniform magnetic field alterations required for gradient imaging.
  • Frequency Modulation Dynamics:
  • Gradient flowing opposite $B_0$: Net main magnetic field strength decreases, causing the proton precessional frequency to slow down.
  • Gradient flowing in direction of $B_0$: Net main magnetic field strength increases, causing the proton precessional frequency to speed up.
  • Units of Gradient Strength: Expressed in Gauss per centimeter ($\text{G/cm}$) or millitesla per meter ($\text{mT/m}$).
  • Unit Conversion: $1\,\text{G/cm} = 10\,\text{mT/m}$.
• Key Gradient Performance Characteristics:
  • Rise Time: The time required for a gradient to ramp up from zero amplitude to its maximum peak amplitude.
  • Maximum Amplitude: The peak magnetic field strength generated by the gradient ($\text{mT/m}$). High amplitude enables higher spatial resolution, thinner slices, smaller fields of view (FOV), and shorter echo and overall scan times.
  • Slew Rate: The combined measure of rise time and maximum amplitude, defining how quickly the gradient can switch polarities between positive and negative. Directly dictates overall data acquisition speed.
  • Duty Cycle: The percentage of total repetition time ($\text{TR}$) that the gradient coil is permitted to operate at its maximum peak amplitude. Higher duty cycle settings restrict the maximum number of slices that can be acquired per $\text{TR}$.
• Gradient Safety and Biological Effects:
  • Rapid gradient switching induces electrical currents in conductive patient tissue, leading to Peripheral Nerve Stimulation (PNS).
  • Symptoms include mild cutaneous sensations, involuntary muscle twitching or contractions, and visual light flashes (retinal phosphenes).
  • Food and Drug Administration (FDA) regulations establish strict upper limits on gradient switching rates to remain within patient safety limits.

Radiofrequency (RF) System and Safety

• Operational Responsibilities:
  1. Transmit RF energy precisely at the Larmor/resonant frequency of hydrogen to disturb or excite protons in specific targeted anatomical regions.
  2. Receive extremely small RF signals emitted by relaxing hydrogen protons to reconstruct diagnostic images.
• Physical Orientation:
  • Transmitted RF fields must be positioned at an angle of $90^\circ$ relative to the main static magnetic field ($B_0$).
• Functional Classification of RF Coils:
  • Transmit Coils, Receive Coils, or Transceivers (capable of both transmitting and receiving).
• Specific RF Coil Configurations:
  • Volume Coils:
  • Design: Completely surrounds the target anatomical area.
  • SNR Characteristics: Inherent low signal-to-noise ratio (SNR); quadrature (circularly polarized) hardware configurations are utilized to improve SNR performance.
  • Applications: Head, extremity, whole-body imaging.
  • Surface / Local Coils:
  • Design: Positioned directly against or adjacent to the patient's body surface.
  • SNR Characteristics: High local SNR due to extreme proximity, but restricted to a shallow depth and small field of view.
  • Applications: Endorectal, endovascular, endovaginal, urethral, esophageal, temporomandibular joint (TMJ).
  • Phased Array Coils:
  • Design: Formed by combining multiple small surface coil elements.
  • SNR Characteristics: Retains the elevated SNR performance of small surface coils while combining individual signal channels to cover extensive anatomical regions.
  • Applications: Spine, pelvic, breast, cardiac, TMJ.
  • Parallel Imaging Coils:
  • Design: Multi-channel array system containing up to $32$ independent receive channels.
  • Performance: Significantly enhances SNR and substantially reduces overall image acquisition times.
• Safety and Handling Protocols for RF Coils:
  • Plug-in Verification: Ensure all connectors are securely plugged into scanner ports.
  • Cable Routing Hazards: Never loop RF coil cables. Cable loops inside strong magnetic fields induce high electrical currents.
  • Prevention of Patient Thermal Burns: RF coils and cables must never directly contact the patient's skin. Insulating padding must separate coils/cables from tissue to avoid high-temperature thermal burns.
  • Care and Inspection: Coils contain delicate micro-electronics. Never drop, throw, or toss coils. Maintain cleanliness and perform routine inspections for cable fraying, tears, or bent/missing connection pins.
• RF Shielding and the Faraday Cage:
  • Purpose: RF pulses share the electromagnetic spectrum with commercial radio broadcasts and ambient electronic emissions. Shielding prevents exterior radio wave interference from degrading image quality.
  • Implementation: A continuous Faraday Cage composed of solid copper sheets line the floor, walls, and ceiling of the scanner room, with conductive copper mesh integrated into the observation window.

Active Recall Questions and Answers

• Question 1: What is $B_0$?
  • Answer: The main static magnetic field.
• Question 2: What is a fringe field?
  • Answer: The peripheral magnetic field surrounding the scanner bore and extending outside the magnet structure.
• Question 3: What does the 5-gauss line represent?
  • Answer: The standard safety perimeter boundary separating MRI Safe areas from unsafe regions ($0.5\,\text{mT} = 5\,\text{G}$).
• Question 4: What is the difference between diamagnetic and paramagnetic?
  • Answer: Diamagnetic materials contain paired electrons and are slightly repelled by magnetic fields. Paramagnetic materials contain unpaired electrons and are slightly attracted.
• Question 5: Which type of magnetism has strong attraction, torque, and projectile risk?
  • Answer: Ferromagnetism.
• Question 6: Name the three main MRI magnet systems.
  • Answer: Permanent magnets, resistive electromagnets, and superconducting electromagnets.
• Question 7: Why do resistive magnets produce heat?
  • Answer: Electrical resistance in the copper wire coils generates heat.
• Question 8: Why are superconducting magnets surrounded by cryogens?
  • Answer: Cryogens (liquid helium) eliminate electrical resistance in the conductor, allowing it to maintain high superconducting current states.
• Question 9: What is a quench?
  • Answer: The rapid boiling off or release of cryogens that abruptly collapses the main static magnetic field, used during extreme emergencies or magnet decommission/relocation.
• Question 10: What is shimming?
  • Answer: The process of adjusting and correcting magnetic field imperfections to maximize field homogeneity.
• Question 11: What is the difference between passive and active shimming?
  • Answer: Passive shimming is performed at installation using steel plates/discs to compensate for environmental and manufacturing imperfections. Active shimming is performed dynamically during patient prescan routines using electromagnetic solenoids to correct patient-induced field distortions.
• Question 12: Name the X, Y, and Z gradient directions and image planes.
  • Answer: X gradient operates Left to Right (Sagittal plane); Y gradient operates Anterior to Posterior (Coronal plane); Z gradient operates Head to Foot (Axial plane).
• Question 13: What are the four main uses of gradients?
  • Answer: Slice selection, spatial encoding, phase encoding, and frequency encoding.
• Question 14: What happens when a gradient adds to $B_0$? What happens when it opposes $B_0$?
  • Answer: Adding to $B_0$ increases net field strength and speeds up proton precessional frequency. Opposing $B_0$ decreases net field strength and slows down precessional frequency.
• Question 15: What units are used for gradient strength?
  • Answer: Gauss per centimeter ($\text{G/cm}$) or millitesla per meter ($\text{mT/m}$), where $1\,\text{G/cm} = 10\,\text{mT/m}$.
• Question 16: Define rise time, maximum amplitude, slew rate, and duty cycle.
  • Answer: Rise time is the time required to ramp up from zero to peak amplitude. Maximum amplitude is the peak gradient strength. Slew rate measures how rapidly the gradient changes polarity (combination of rise time and maximum amplitude). Duty cycle is the percentage of total $\text{TR}$ the gradient operates at peak amplitude.
• Question 17: What safety problems can rapid gradient switching cause?
  • Answer: Peripheral nerve stimulation (PNS), cutaneous skin tingling, involuntary muscle twitching, and visual light flashes (retinal phosphenes).
• Question 18: What are the two responsibilities of the RF system?
  • Answer: Transmitting RF energy at the resonant frequency of hydrogen to disturb/excite protons, and receiving tiny RF signals emitted by relaxing hydrogen protons to form the image.
• Question 19: What is the difference between volume, surface, and phased-array coils?
  • Answer: Volume coils encompass whole body parts for uniform coverage but lower SNR. Surface coils sit close to local anatomy for high SNR over small areas. Phased-array coils link multiple small surface coils together for high SNR across large areas.
• Question 20: Why should RF coil wires not be looped?
  • Answer: Looped cables in a strong magnetic field can induce dangerous electric currents.
• Question 21: Why should RF coils not touch the patient's skin?
  • Answer: Direct contact during RF transmission can cause thermal skin burns.
• Question 22: What is the purpose of RF shielding/Faraday's Cage?
  • Answer: To block ambient external radio wave emissions from entering the scanner room and creating image artifacts.

Summary Memory Maps and Study Protocol

• Core Memory Associations:
  • Electron Pairing: PAIR = PUSH; UNPAIRED = PULL.
  • Shimming Hardware: PASSIVE = PLATES; ACTIVE = COILS.
  • Superconducting Systems: SUPER = COLD + STRONG.
  • Gradient Spatial Encoding Planes: X = Sagittal; Y = Coronal; Z = Axial.
  • System Functions: SHIM = Homogeneity; SHIELD = Environmental Interference / Stray Field Protection; GRADIENT = Spatial Location; RF = Transmission + Reception.
• Four-Round Study Protocol:
  • Round 1: Review memory sheets and high-yield operational associations.
  • Round 2: Cover answer fields in concept tables and perform verbal active recall.
  • Round 3: Answer all 22 active recall questions independently without referencing source material.
  • Round 4: Re-evaluate missed topics later the same day and repeat testing on the following day.