***Chapter 13 - Magnetic Resonance Imaging

Question 1

MR ARTIFACTS are classified into three broad areas

Answer 1

1. Machine
2. Patient
3. Signal processing

Question 2

Ratio of the induced internal magnetization in a tissue to the external magnetic field

Answer 2

Magnetic susceptibility

Question 3

The most common susceptibility changes occur at ____ , which a cause a signal loss due to more rapid dephasing (T2*) at the tissue-air interface

Answer 3

Tissue-air interfaces (e.g. Lungs and sinuses)

Question 4

Defines the volume (slice)

Answer 4

Slice select gradient

Question 5

Provide the spatial information in the other two dimensions

Answer 5

Phase and frequency encoding gradients

Question 6

Proximal to the surface coil , receive signals are intense , and with distance , signal intensity is attenuated, resulting in grayscale shading and loss of brightness in the image

Answer 6

RF coil artifacts

Question 7

Creates noise patterns perpendicular to the frequency encoding direction

Answer 7

Narrow-band noise

Question 8

A narrow band pattern of black/white alternating noise produces ______

Answer 8

“Zipper” artifact

Question 9

RF energy received by adjacent slices during a multiplicity acquisition excite and saturate protons in adjacent slices, chiefly due to RF pulses without sharp off/on/off transitions

Answer 9

Cross-excitation

Question 10

A technique to mitigate cross excitation by reordering slices into two groups with gaps

Answer 10

Slice interleaving

Question 11

Affect all areas of the reconstructed image , and cause the artifactual superimposition of wave patterns across the FOV

Answer 11

K-space errors

Question 12

Faint copies of the image displaced along the phase encode direction, are the visual result of patient motion

Answer 12

Ghost images

Question 13

Resonance frequency variations resulting from intrinsic ,magnetic shielding of anatomic structures

Answer 13

Chemical shift

Question 14

Occur with GE images, resulting from the rephasing and dephasing of the echo in the same direction relative to the main magnetic field

Answer 14

Chemical shift artifacts of the second kind

Question 15

Occurs near sharp boundaries and high-contrast transitions in the image, and appears as multiple, regularly spaced parallel bands of alternating bright and dark signal that slowly fades with distance

Answer 15

Ringing artifacts (aka Gibbs phenomenon)

*commonly occurs at skull/brain interfaces, where there is a large transition in signal amplitude

Question 16

A result of the mismapping of anatomy that lies outside of the FOV but within the slice volume

-anatomy is usually displaced at the opposite side of the image

Answer 16

Wraparound artifacts

Question 17

Arise from the finite size of the voxel over which the signal is averaged

-results in a loss of detail and spatial resolution

Answer 17

Partial volume artifacts