Physics week 3 study guide

Main magnet (B₀) role in MRI

Creates the static magnetic field used to align protons.

Shim system purpose

Improves the homogeneity (uniformity) of the main magnetic field B₀.

Gradient system main function

Provides spatial localization by varying the magnetic field linearly over space.

RF transmit system role

Excites protons using RF (radiofrequency) energy.

RF receive system role

Detects the MR signal emitted from the patient.

Computer systems in MRI roles

Control hardware, reconstruct images, and display/store data.

Nature of B₀ (static vs time-varying)

B₀ is static: constant over time.

Meaning of B₀ homogeneity

B₀ is uniform over space within the imaging volume.

Unit for B₀ homogeneity

Parts per million (ppm).

Effect of higher ppm on homogeneity

Higher ppm = worse homogeneity.

Overall purpose of shimming in MRI

To improve the homogeneity of the main magnetic field B₀.

Active shimming definition

Shimming using electromagnetic shim coils to adjust the field (b0).

Passive shimming definition

Shimming using strategically placed metal pieces to adjust the field.

When shimming is performed

Typically during installation and maintenance; technologists can apply limited shims via software.

Fringe field definition

The portion of the B₀ field that extends beyond the magnet bore.

Usefulness of fringe field for imaging

Not useful for imaging and can pose safety risks.

Purpose of magnetic shielding

To concentrate and control the fringe field, improving safety and siting.

Passive magnetic shielding

Shielding using iron or steel structures around the magnet.

Active magnetic shielding

Shielding using bucking coils that produce fields opposing B₀ to confine the fringe field.

Advantage of active shielding

Allows smaller, lighter magnet rooms and reduced siting requirements.

Advantages of permanent magnets

Low operating costs; no power required to maintain the field.

Disadvantages of permanent magnets

Low field strength (≤ 0.5T); very heavy/difficult to site; B₀ cannot be turned off.

Can permanent magnet B₀ be turned off?

No, the magnetic field of a permanent magnet cannot be turned off.

Advantage of resistive magnets

Lower initial cost; B₀ can be turned off.

Disadvantages of resistive magnets

High power consumption; limited field strength.

Can resistive magnet B₀ be turned off?

Yes, by switching off the current.

Properties of superconducting magnets

High field strength (1.5T+); require cryogens; better image quality and faster scans; higher capital cost.

Why cryogens are needed in MRI

Superconductivity requires very low temperatures, near 4 K.

Main cryogen for MRI superconducting coils

Liquid helium.

Function of cold heads

Help recapture and recondense evaporated helium, reducing cryogen loss.

Danger of cryogen loss in a quench

Rapid gas release can displace oxygen (asphyxiation risk) and cause over-pressurization.

Primary role of gradient magnetic fields

Provide spatial localization of the MR signal.

How gradient fields vary over distance

They change linearly over space.

Three gradient directions in MRI

X, Y, and Z.

How gradient fields are generated

By applying currents in gradient coils that create linear variations in the magnetic field.

Gradient amplitude definition and unit

Strength of the gradient, usually in millitesla per meter (mT/m).

Gradient rise time definition

Time required for a gradient to reach its set amplitude.

Gradient slew rate definition

How quickly the gradient reaches its amplitude (amplitude + rise time), usually in mT/m/s.

Type of power used by gradients

Gradients use DC (direct current) power that is rapidly switched on and off.

Two main safety concerns of rapid gradient switching

Peripheral nerve stimulation and loud acoustic noise (hearing damage).

Type of current used by RF transmission system

RF transmission uses AC (alternating current) current.

What RF transmit system produces

Oscillating electromagnetic fields (RF) used to excite protons.

How RF energy is delivered

In short RF pulses.

What RF receive coils detect

The MR signal emitted by the patient (induced voltage from precessing magnetization).

RF coils act as what type of device?

Antennas for transmitting and/or receiving RF signals.

Effect of coil size on SNR and coverage

Smaller coils: better SNR, less coverage. Larger coils: more coverage, lower efficiency/SNR.

Typical role of body coil

Built-in transceiver coil that can transmit and often receive RF signals.

Surface coils usage

Receive-only coils used to improve SNR in localized regions.

Volume coils definition

Coils that surround a body part and can transmit and/or receive over a larger volume.

Phased-array coil definition

A coil with multiple independent elements.

Main advantage of phased-array coils

Higher SNR and better parallel imaging performance.

Orientation of RF coils relative to B₀

Physical coils are parallel to B₀ so that the RF field is perpendicular to B₀.

Purpose of RF shielding (Faraday cage)

To prevent outside RF interference from entering and degrading image quality.

Common material for RF shielding

Copper in the walls, doors, and windows.

Definition of “safety hazard” in MRI

A situation with potential for permanent, irreversible injury.

Definition of “biologic response” in MRI safety

A temporary and reversible effect on the patient.

Primary risk of static magnetic field

Projectile accidents and ferromagnetic attraction (displacement/torque on metal objects).

Two risks associated with gradient fields

Peripheral nerve stimulation and hearing damage from acoustic noise.

Main safety concern with RF fields

Tissue heating and burns, controlled via SAR.

SAR full name

Specific Absorption Rate.

What SAR measures

RF energy absorbed by tissue over time.

Who sets clinical SAR limits

Regulatory bodies such as the FDA (and IEC).

Why SAR is limited

In order to prevent excessive tissue heating and burns.

MRI Zone I definition

Public access area, uncontrolled (e.g., general hospital corridors).

MRI Zone II definition

Screening zone where patients and visitors are interviewed and supervised.

MRI Zone III definition

Restricted access area controlled by MRI staff due to proximity to the magnet.

MRI Zone IV definition

MRI scanner room where the magnet is located.

Four main implant risks in MRI

Heating, displacement, torque, and malfunction.

Only definitive source of implant MRI safety status

Implant manufacturer’s labeling and documentation.

What to do if implant safety cannot be verified

Do not scan the patient.

Examples of high-risk implants

Ferromagnetic aneurysm clips; many pacemakers (without MRI-conditional labeling); recently implanted devices.

Examples of generally lower-risk implants

Orthopedic hardware; coronary stents after an appropriate incorporation period.

When to use Emergency Stop (E-stop)

In true emergencies such as electrical fire or flooding affecting the system.

Should E-stop be used for image-quality or routine issues?

No, it should not be used for routine workflow or image-quality problems.