Part 2 powers MRI

Extrinsic parameters are what type of parameters?

Geometric

FOV stands for what?

Field of view

FOV comes from which gradient?

Frequency encoding (readout)

FOV defines size in which two directions?

Phase + frequency

FOV affects image contrast heavily?

No

FOV heavily affects which 2 things?

SNR + resolution

Pixel size formula

FOV ÷ matrix

Frequency Encoding Gradient (FEG) is also called what?

Readout gradient

FEG amplitude controls what?

FOV

Shallow FEG produces what FOV size?

Large FOV

Steep FEG produces what FOV size?

Small FOV

Shallow FEG may allow what timing change?

Shorter min TE

If you double FOV, SNR changes by what factor?

4x increase

Changing FOV changes what?

Pixel size

Increasing FOV (same matrix) makes pixels:

Bigger

Increasing FOV (same matrix) resolution:

Decreases

Increasing FOV (same matrix) SNR:

Increases

Increasing FOV increases what effect?

Partial volume

Increasing FOV increases anatomical:

Coverage

Increasing FOV changes wrap/aliasing risk:

Decreases

Partial volume is best described as:

Averaging within a voxel

If black + white tissues share a voxel, the pixel appears:

Gray

Slice thickness is mainly controlled by which gradient?

SSG (slice select)

SSG amplitude vs slice thickness relationship

Inverse

Steeper SSG gives what slice?

Thinner

Shallow SSG gives what slice?

Thicker

Another way to change slice thickness besides SSG

Transmitter bandwidth

To get thicker slice at constant SSG, transmitter bandwidth should:

Increase

If transmitter bandwidth increases, RF pulse duration:

Longer

RF pulse must cross what?

Zero point

Longer RF pulse may cause TR and/or min TE to:

Increase

Thick slice tradeoff: SNR

Increases

Thick slice tradeoff: resolution

Decreases

Thick slice tradeoff: partial volume

Increases

Thin slice tradeoff: SNR

Decreases

Thin slice tradeoff: resolution

Increases

Thin slice tradeoff: partial volume

Decreases

Voxel dimension formula

FOV ÷ matrix

Isotropic voxel means:

Equal dimensions

Anisotropic voxel means:

Unequal dimensions

Best voxel type for reformatting (MIPs)

Isotropic

Scan time formula

TR × phase × NEX

Increasing phase encodes affects scan time how?

Increases

Increasing phase encodes affects resolution how?

Increases

Increasing phase encodes affects SNR how?

Decreases

More phase encodes requires more:

TRs (fill k-space)

Frequency matrix tells receiver what?

Samples of the echo

Higher frequency matrix tends to do what to min TE?

Increases (likely)

Receiver bandwidth (RBW) is what?

Sampling rate

Sampling high matrix slowly tends to:

Increase min TE

Sampling high matrix slowly tends to SNR:

Increase

Sampling high matrix slowly affects chemical shift:

Decreases

Wide bandwidth = sample rate is:

Fast

Wide/fast bandwidth SNR:

Decreases

Wide/fast bandwidth minimum TE:

Shorter

Wide/fast bandwidth chemical shift artifact:

Smaller

Wide bandwidth summary:

Wide = everything decreases

Narrow bandwidth = sample rate is:

Slow

Narrow/slow bandwidth SNR:

Increases

Narrow/slow bandwidth minimum TE:

Longer

Narrow/slow bandwidth chemical shift artifact:

Bigger

Narrow bandwidth summary:

Narrow = everything increases

NEX (GE) stands for:

Number of excitations

Acq (Siemens) stands for:

Acquisitions

NSA (Philips/Hitachi/Toshiba) stands for:

Number of signal averages

What does NEX/Acq/NSA mean in k-space?

how many times the scaler will fill each line of k-space on successive TRs

Increasing acquisitions does what to scan time?

Doubles (if doubled)

SNR relationship to acquisitions

SNR = √Acq

√2 is approximately:

1.4

√3 is approximately:

1.7

√4 equals:

2

Successive TRs help with what 2 things?

Motion + TR×matrix

Concatenation is used to maintain what?

T1 contrast

Concatenation also helps maintain:

Fat sat

Voxel volume formula

Pixel dimension × thickness

SNR in 2D/3D sequential is the square of?

Partions/slices

Increasing partitions does what to SNR?

Increases

Larmor frequency shorthand

W0 = B0 × 42.57 MHz

Gauss to Tesla conversion

1 G = 0.0001 T

Gauss to milliTesla conversion

1 G = 0.1 mT