MRI Principles, Gradients, and Safety

Comprehensive review flashcards covering MRI gradient coils, slice selection, pulse sequences, relaxation times, image weighting, and magnetic field safety.

What are the three gradient magnetic fields inside an MRI magnet called?

The X, Y, and Z gradients.

What is the primary function of gradient magnetic fields in an MRI scanner?

To locate signals and isolate specific anatomical slices.

Why are different radiofrequencies utilized for consecutive slices during an MRI scan?

To isolate each individual slice by mapping specific spatial locations to distinct frequencies.

How are the magnetic moments of protons oriented in the human body prior to entering the MRI magnet?

They are oriented randomly in all directions

When placed into the main magnetic field of an MRI scanner, in what two directions do protons align?

Parallel and antiparallel.

What type of magnetization must be created by tipping protons to generate a detectable MRI signal?

Transverse magnetization.

What is the standard radiofrequency flip angle commonly used to knock protons into the transverse plane?

$$90^\circ$$

At what frequency does an excited slice emit its signal back relative to the transmitted pulse frequency?

At the exact same frequency that was transmitted into the slice.

What is the term for the frequency emitted when protons relax and realign with the main magnetic field?

Resonant (or resonate) frequency.

In MRI slice orientation, what does an 'orthogonal' slice refer to?

Any slice orientation that deviates from a true axial (transverse), sagittal, or coronal plane.

How does MRI differ from CT regarding the generation of sagittal and coronal planes?

MRI acquires axial, sagittal, and coronal planes directly without requiring post-processing CAD reformatting.

Which gradient coil is assigned for slice selection when performing a coronal scan?

The Y gradient.

Which gradient coil is assigned for slice selection when performing an axial scan?

The Z gradient.

Which gradient coil is assigned for slice selection when performing a sagittal scan?

The X gradient.

When the Y gradient is designated for slice selection in coronal imaging, how are the X and Z gradients utilized?

They are utilized for frequency encoding and phase encoding.

Approximately how long did early axial T2-weighted pulse sequences take to acquire on older MRI scanners?

23 minutes for a single pulse sequence.

What designation is given to the main static magnetic field, which aligns with the Z gradient axis?

$$B_0$$

How is T1 relaxation time defined in the lecture?

The time it takes for longitudinal magnetization to recover back to its original origin.

How is T2 relaxation time defined in the lecture?

The time it takes for magnetization to recover back from its original value in transverse magnetization.

What are the two most common types of MRI magnets used in clinical practice today?

Superconducting magnets and permanent magnets.

What generic terms are used to contrast superconducting and permanent magnets regarding patient enclosure?

Closed magnets and open magnets.

What four imaging performance advantages do high-field MRI systems offer over low-field systems?

Better SNR (signal-to-noise ratio), better CNR (contrast-to-noise ratio), better spatial resolution, and better temporal resolution.

What field strengths were compared when discussing modern high-field magnets versus modern low-field open magnets?

$1.5\,\text{Tesla}$ for high-field and $1.2\,\text{Tesla}$ for low-field (a difference of $0.3\,\text{Tesla}$).

What is the magnetic field orientation of a conventional closed high-field magnet versus an open low-field magnet?

A high-field magnet has a longitudinal field, whereas an open low-field magnet has a vertical field.

How does the lecture characterize the fundamental clinical value of spatial resolution?

The ability to pinpoint exact physical locations and determine how small of structures can be resolved.

Why is high spatial resolution critical when detecting breast cancer in the terminal ductal lobular unit?

Detecting cancer early in the terminal ductal lobular unit provides a cure rate of almost 100% before it spreads to glandular tissue.

What does the abbreviation TR stand for in an MRI pulse sequence?

Time to repetition (repetition time).

What does the abbreviation TE stand for in an MRI pulse sequence?

Time to echo (echo time).

Which image weighting is TE primarily responsible for controlling in an MRI sequence?

T2 weighting.

What is the primary clinical purpose of T1-weighted images according to the lecture?

Anatomical evaluation to assess structural details and determine if anatomy is normal or abnormal.

Between T1-weighted and T2-weighted images, which sequence better visualizes a hemorrhage or muscle bruise?

T2-weighted images.

When viewing an axial cross-sectional MR image of a patient's legs, on which side of the display is the patient's right leg?

On the viewer's left side.

Which two radiofrequency pulses are specific to a conventional spin echo pulse sequence?

A $90^\circ$ excitation pulse and a $180^\circ$ refocusing pulse.

What three gradient encoding operations are energized during an MRI pulse sequence?

Slice selection gradient, phase encoding gradient, and frequency encoding gradient.

What hazardous mechanical effect can the static magnetic field cause on ferromagnetic implants such as aneurysm clips?

It can cause them to move or torque.

What critical danger arises if a ferromagnetic aneurysm clip torques inside the MRI scanner?

It can tear the blood vessel to which it is attached, potentially resulting in fatal hemorrhage.

What functional change can exposure to the static magnetic field induce in an implanted cardiac pacemaker?

It can make the pacemaker dysfunctional or change it from a demand pacemaker to a fixed pacemaker.

Why must patients avoid moving or bobbing their heads during a brain MRI scan?

Motion produces artifacts and disrupts slice data acquisition because slices rely on precise multi-gradient timing over the sequence duration.