Exam 2

In what direction does phase encoding occur?

Perpendicular to frequency encoding direction; L-R on axial, R-L on coronal, Top to bottom on sagittal

In what direction does frequency encoding occur in?

Perpendicular to phase encoding; R-L on axial; Z axis on sagittal coronal

What methods can be used to compensate for phase encoding artifacts?

Increase FOV; add SAT bands; swap frequency and phase direction

What are three methods to reduce/eliminate cross talk?

add slice gap; adjust slice excitation order; add slice concatenation

Which cross talk elimination method does not reduce scan time?

Increasing slice gap

What causes alliancing artifacts in frequency direction?

Frequencies in echo aren’t sampled at the proper rate; frequency outside of FOV; and outside signal is encoded and included into FOV

What is the software option that compensates for frequency aliasing artifacts?

Frequency filter

What is the suggested temperature for the MRI scan room?

18-22 C

What is the optimal humidity range in MRI?

30-50% RH

What activity references to proactive and reactive?

Proactive - improve CONTINUOUSLY over time

Reactive - improve ONLY after a event

What is a quality control check that is performed on a phantom that is performed for something other than image quality?

Central frequency and transmitter gain

How often does a physicist check the coils?

Once a year

If there are air bubbles in a phantom (other than level bubbles), is it okay to proceed with the quality check?

No

What is the weekly allowed central frequency drift from the ACR menu?

1.5 ppm per week

What is “Table ok” checking for?

Accuracy of laser localizer

Are all shimming effects performed during installation only?

No

What material is used for RF shielding and magnetic shielding?

Copper, aluminum and steel

What value must the fringe field be restricted to inside the room?

5 gauss

What artifact can be made by a leak in RF shielding? How do you eliminate it?

Zipper artifact, can be eliminated by checking any RF leak such as door opened

What type of magnetic shielding is located inside the scanner?

Active shielding

What are two differences between MRI and CT contrast? (other than contrast agent?

CT contrast is in higher volume of contrast fluid and at a higher rate of injection

Screening MRI devices should locate and identify what type of objects?

Ferromagnetic objects

What are 3 types of hardware operation electronic devices for magnetic detectors?

Beat-frequency oscillator (BFO); pulse induction (PI); VLF

Describe a situation where a screening device can be helpful?

Can be helpful for patients with limited mobility

What are two cons of having screening devices?

Alarm fatigue and false safety

What are two type of lines that MRI techs are allowed to remove?

IV line and midline

What is the speed and volume for studies using power injector? (dynamic studies)

22 G can inject up to 3 ml/sec; 24 G at 1.5-2 ml/sec

What are some types of artifacts?

Motion artifact, physical artifacts, sampling artifacts, susceptibility artifacts, technical error, sampling artifacts, and hardware artifacts

Phase aliasing artifact

Shows: Anatomy overlaid onto anatomy inside FOV in direction of phase encoding (L→R)

Cause: Anatomy outside of FOV; outside signal is encoded and included into FOV; Extra signal is potentially at the same phase value as anatomy being scanned

Fix: Increase FOV (but lowers resolution); add sat bands (but increases SAR); add no phase wrap; swap phase and frequency direction

Frequency aliasing artifact

Shows: Anatomy overlaid onto anatomy inside FOV in direction of frequency encoding (top to bottom)

Cause: Frequencies in echo aren’t sampled at the proper rate; frequency outside of FOV; and outside signal is encoded and included into FOV

Fix: use frequency filter

Cross talk

Shows: Slices next to each other will have different tissue contrast

Cause: RF profile has protons in adjacent slices are already excited which = low/none signal

Fix: add slice gap; adjust slice excitation order; add slice concatenation

Slice overlap/cross slice

Shows: dark area with no signal

Cause: multi slice regions that are overlapping and saturated protons can’t produce signal

Fix: try position FOV or separation into two sequences (AX T1 Upper; AX T1 Lower)

Truncation artifact (Gibbs artifact)

Shows Low bright/dark lines though high intensity area

Cause: Under sampling of data so not enough k-space filled

Fix: Larger matrix size

Zipper artifact

Shows: Mini zipping lines across picture

Cause: Outside RF enters the room; defect in room RF shielding;

Fix: Make sure scan room door is shut; must find leak so call service engineer if not found

Spike artifact (k-space artifact)

Shows: Parallel bands

Cause: wrong electrical discharge gets encoded into k-space

Fix: Repeat scan; can’t be corrected

Propeller artifact (k-space artifact)

Cause: Using a propeller to fill k-space but anatomy is too large for FOV

Fix: Repeat scan and increase FOV

Moire artifact

Show: Shown only on gradient echo pulse sequences. Black and white banding near edge of FOV

Cause: wrap on one part of the body to the other; caused by field inhomogeneities

Fix: Use spin echo sequences; make sure body is not touching bore

Shading artifact

Show: Shading around anatomy

Cause: Uneven uniform excitation; coil coupling; field inhomogeneities; coil selection

Fix: Select right size coil for anatomy; prevent anatomy from touching coil; make sure pre-scan calculation is performed by system

Quadrature coil artifact

Show: 180 degree ghosting

Cause: Signals from quadrature channels are summed incorrectly

Fix: Have coil serviced or repaired

Geometric distortion artifact

Show: Image distortion and squishing

Cause: Improper gradient/gradient amplifier function which creates poor linear variation

Fix: Call service

Would more encoding be done to compensate a phase or frequency aliasing artifact? What direction?

Yes for phase encoding aliasing artifact so phase direction

ACR phantom; what value in mm is it measured in?

Sagittal is 148 mm and A/P and R/L is 190 mm with a ± 2mm allowed

ACR middle is shown to

measure whether laser/isocenter match

The arrows show in ACR

Outer edges show to measure diameter from A/P and L/R (in grid/slice 5/axial); arrows in sagittal localizer show SI length