MRI Phys Midterm

What is B₀ in MRI?

The static magnetic field produced by the main magnet and the foundation of MRI.

How do hydrogen nuclei align in B₀?

Parallel (low energy spin up) and antiparallel (high energy).

What creates the net magnetization vector (NMV)?

A slight excess of protons in the low-energy parallel state.

At thermal equilibrium where is the NMV?

Aligned longitudinally parallel to B₀.

Why is there no transverse signal at equilibrium?

Protons are out of phase despite same precessional frequency.

What unit measures magnetic field strength?

Tesla (T).

Why are superconducting magnets used?

They eliminate resistance using liquid helium to maintain a strong stable field.

What is B₁?

The oscillating magnetic field produced by the RF transmitter coil.

What is resonance in MRI?

When RF energy matches the Larmor frequency allowing energy absorption.

What happens during excitation?

The NMV is tipped away from the longitudinal axis.

What does a 90° RF pulse do?

Places magnetization in the transverse plane and synchronizes protons.

What is FID?

The signal immediately after excitation (Free Induction Decay).

What is T1 recovery?

Recovery of longitudinal magnetization as energy is released to the lattice.

What is T1 time?

Time for 63 percent recovery of longitudinal magnetization.

What is T2 decay?

Loss of phase coherence in the transverse plane.

What is T2 time?

Time for 63 percent decay of transverse magnetization.

What is T2*?

T2 decay plus dephasing from field inhomogeneities.

What controls image contrast in spin echo imaging?

TR (repetition time) and TE (echo time).

What produces T1-weighted images?

Short TR and short TE.

What produces T2-weighted images?

Long TR and long TE.

What produces proton density weighting?

Long TR and short TE.

What does TR control?

T1 recovery effects.

What does TE control?

T2 decay effects.

What is the function of gradients?

Spatial localization of MRI signal.

What do gradient coils produce?

Small rapidly changing magnetic fields.

What is gradient amplitude?

Strength of the gradient measured in mT/m or G/cm.

What is slew rate?

Speed at which gradient strength changes.

What is rise time?

Time required to reach maximum gradient strength.

What is duty cycle?

Percentage of time gradients are active.

Why is MRI loud?

Rapid gradient switching causes coil vibration and acoustic noise.

What safety precaution is required due to noise?

Hearing protection.

What biological effect can gradients cause?

Peripheral nerve stimulation.

What is shimming?

Improving magnetic field homogeneity.

Does the shim system create B₀?

No it only corrects imperfections.

What is Level 1 MRI personnel?

Individuals with basic MRI safety knowledge.

What is Level 2 MRI personnel?

Individuals with advanced MRI safety training.

What are the implant safety categories?

MR Safe MR Conditional MR Unsafe.

Which implant term is outdated?

MR compatible.

What is required if orbital metal is suspected?

CT scan before MRI.

Why must metallic foreign bodies be ruled out?

To prevent movement and injury in the magnetic field.

How can RF burns occur?

From conductive loops or cable contact with skin.

How do you prevent RF burns?

Avoid loops and keep cables off the skin.

What is SAR?

Specific absorption rate measuring RF energy absorbed by the body.

What increases SAR?

Higher field strength larger flip angles shorter TR more RF pulses.

Which sequence has higher SAR?

Fast spin echo.

What is a Faraday cage?

Copper-lined room preventing RF interference.

Why is a Faraday cage important?

Prevents external signals from degrading image quality.

What is magnetic shielding?

Containing the fringe field within the scan room.

What is Faraday’s law of induction?

A changing magnetic field induces voltage in a conductor.

Why is Faraday’s law important in MRI?

Explains signal detection and induced currents in metals.

What determines Larmor frequency?

Magnetic field strength and gyromagnetic ratio.

What happens to frequency as field strength increases?

It increases proportionally.

What is the role of the main magnet?

Aligns protons.

What is the role of the RF system?

Excites protons and produces signal.

What is the role of gradients?

Provide spatial encoding.

What is the role of the shim system?

Improves magnetic field uniformity.

What are key ARRT focus areas?

TR TE relationships T1 vs T2 definitions SAR safety screening system roles.