MRI in Practice ch.4

Matrix size in phase direction is determined by?

# of phase encoding performed

Which of these is a consequence of decreasing the TR from 2500 ms to 500 ms? a)Scan time decreases b)T1 weighting decreases c)Spatial resolution decreases d)Slice number available increases

(a). (b), (c) and (d) are all consequences of increasing the TR. TR is a scan time parameter as it dictates the time interval between filling a line of K space for a particular slice.

When the number of signal averages is doubled, by how much does the scan time change? a)× 2 b)1/2 c)22 d)√2

(a). Number of signal averages increases the scan time proportionally

Which of these is a consequence of decreasing the phase matrix? a)Truncation artefact improves b)SNR increases c)Spatial resolution increases d)Scan time increases

(b). (a), (c) and (d) are consequences of increasing the phase matrix, not decreasing it. SNR increases because each voxel gets bigger as the phase matrix is decreased.

Which of the following does NOT occur when the receive bandwidth is reduced? a)SNR increases b)Chemical shift artefact improves c)Number of slices available decreases d)TE increases

(b). SNR increases as reducing the RBW reduces the number of noise frequencies that are sampled. Chemical shift artefact gets worse as the RBW is reduced as fewer frequencies are mapped across each pixel. TE increases as reducing the RBW reduces the digital sampling rate and therefore the sampling time and the TE increase. Slice number available will decrease as the TE increases; fewer slices may be acquired for a given TR.

Halving a square FOV reduces the SNR by: a)1/2 b)1/4 c)3/4 d)1/8

(c). Halving the FOV halves its dimension along both phase and frequency encoding axes. Therefore the voxel volume is reduced to 1/4 or by 3/4.

Which of these is NOT a consequence of decreasing the TR from 2500 ms to 500 ms? a)Scan time decreases b)T1 weighting increases c)Slice number available decreases d)SNR increases

(d). (a), (b) and (c) are all consequences of decreasing the TR between the given values. As the TR decreases, SNR decreases.

Which of these is NOT a consequence of decreasing the phase matrix? a)SNR increases b)Spatial resolution decreases c)Truncation artefact gets worse d)Scan time increases

(d). (a), (b) and (c) are all consequences of decreasing the phase matrix. Scan time decreases as the phase matrix decreases, as fewer K space lines are filled

Doubling the number of signal averages changes the SNR by how much? a)× 2 b)1/2 c)22 d)√2

(d). As noise is random the SNR is only increased by a square root of 2 or 1.4 (40%) when the number of signal averages is doubled.

Increasing the field strength increases SNR because: a)the flip angle increases b)the NMV increases in size because the high energy population of spins increases c)the precessional frequency increases d)the NMV increases in size because the low energy population of spins increases

(d). Increasing the field strength increases the energy jump between high and low energy spins. Hence fewer nuclei have enough energy to align their magnetic moments in opposition to the stronger field. The low energy population therefore increases in size relative to the high energy population so increasing the NM

The factors that affect SNR are?

*magnetic field strength of the system, *PD of the area under examination, *voxel volume, *TR, TE & flip angle, *NEX, *receive bandwidth, *coil type

Of the following parameters, which would give the best spatial resolution? a)256 × 256, 3 mm slice thickness, 12 cm FOV, 1 NEX b)256 × 128, 8 mm slice thickness, 40 cm FOV, 4 NEX c)512 × 256, 6 mm slice thickness, 14 cm FOV, 2 NEX

A

Spatial resolution is determined by?

FOV, matrix size, slice thickness

List three ways of improving the CNR between pathology and normal tissue

MTC, T2 weighted images, contrast enhancement or chemical suppression

The 4 main considerations determining image quality are?

SNR, CNR, spatial resolution & scan time

Which could you change without affecting image contrast or scan time?

Slice thickness and FOV

Flip angle controls the amount of weighting for?

T1 & PD

TR determines the amount of weighting for?

T1 & PD

TE controls the amount of weighting for?

T2

Scan time is proportional to?

TR, # of phase encodings, NEX

Scan time is doubled if you double?

TR, phase matrix, NEX

How is K space filling altered in a 50% rectangular FOV?

The incremental step between each phase encode is doubled, which halves the FOV in the phase direction relative to frequency and halves the number of phase encodes performed. The phase resolution is however maintained because the most outer lines of K space are still filled but the scan time is halved because only the phase encoding steps are completed.

How would you achieve equal resolution in all reformatting planes in a volume acquisition?

Voxel must be isotropic, i.e. equal dimensions in all planes. Keep matrix square. Calculate pixel dimension by dividing FOV by number of pixels. Select slice thickness that equals this dimension.

To increase CNR you can use:

a T2 image, contrast agents, chemical pre-sat techniques, magnetization transfer contrast (MTC)

Of the following parameters, which would give the highest SNR? a)256 × 256, 3 mm slice thickness, 12 cm FOV, 1 NEX b)256 × 128, 8 mm slice thickness, 40 cm FOV, 4 NEX c)512 × 256, 6 mm slice thickness, 14 cm FOV, 2 NEX

b

Volume imaging _______ the SNR as a whole volume of excited tissue

increases

Increasing the slice # _____ the SNR and it _____ the scan time

increases, increases

_______ voxels give equal resolution in every plane

isotropic

A ___ TR increases SNR and a ______ TR reduces SNR?

long; short

A _____ TE reduces SNR and a _____ TE increases SNR

long; short

The ____ the flip angle, the lower the ____

lower; SNR

Volume imaging allows ______ in any plane

reformatting

For higher SNR you use:

se pulse sequences, not using a very short TR and a very long TE, correct coil & pt placement, coarse matrix, large FOV, thick slices, as many NEX as possible

Scan time depends on?

slice # & the TR, phase encoding #, NEX

Voxel size is affected by?

slice thickness, FOV, # of pixels or matrix

A change in voxel size affects SNR in 3 ways by changing?

slice thickness, image matrix & FOV

Size of the frequency FOV is determined by?

slope of the frequency encoding gradient

Slice thickness is determined by?

slope of the slice select gradient

To achieve thin slices,a small FOV, and a fine matrix than the slope of each different gradient used is?

steep

Spatial resolution can be maintained by selecting?

thin slices, fine matrix, small FOV, rectangular FOV when possible

To achieve the shortest scan time possible:

use a short TR, a coarse matrix, minimum NEX

Spatial resolution is controlled by?

voxel size

List the factors that affect SNR

• Field strength • Proton density • Coil type • TR • TE • Flip angle • Slice thickness • Matrix • FOV • Receive bandwidth • NEX

List four consequences of decreasing the phase matrix

• Shorter scan time. • Decreased spatial resolution. • Increased SNR. • Increased truncation artefact

List four consequences of decreasing the TR

• Shorter scan time. • Increased T1 weighting. • Decreased SNR. • Decreased slice number