Parameters & Trade Offs CH. 4

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

MTC, T2 weighted images, contrast enhancement or chemical suppression.

Which of the following occurs when the receive bandwidth is reduced? a)SNR increases b)Chemical shift artefact improves c)Number of slices available decreases d)TE increases

SNR increases Number of slices available decreases TE increases 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.

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

Slice thickness and FOV.

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.

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)256 × 256, 3 mm slice thickness, 12 cm FOV, 1 NEX

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)Scan time decreases (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)× 2 Number of signal averages increases the scan time proportionally

When the field of view is increased, signal to noise ratio ___________. a. Increases b. Decreases c. Stays the same

a. Increases

The size of the area being displayed on an MR image is known as __________. a. field of view b. voxel size c. pixel size d. all of the above

a. field of view

Increasing the TE: a. increases the contrast based on T2 relaxation times of the tissues b. reduces the contrast based on T2 relaxation times of the tissues c. reduces the contrast based on T1 relaxation times of the tissues d. a and c

a. increases the contrast based on T2 relaxation times of the tissues

If a given conventional spin echo pulse sequence takes 12 minutes to acquire, a fast spin echo sequence using an ETL of six, with all other factors that affect scan time remaining the same, will take: a. 2 minutes b. 1 minute c. 4 minutes d. 6 minutes

a. 2 minutes (time divided by ETL)

Reducing the TR: a. decreases the signal to noise ratio of the image b. reduces the contrast based on T2 relaxation times of the tissues c. increases the scan time d. all of the above

a. decreases the signal to noise ratio of the image

The ability to distinguish one structure from another on an image is known as: a. spatial resolution b. contrast to noise c. signal to noise d. all of the above

a. spatial resolution

When TE is long: a. there is time for significant T2 decay b. the image is T1 weighted c. tissues with long T1 appear bright d. tissues with long T2 appear dark

a. there is time for significant T2 decay

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)256 × 128, 8 mm slice thickness, 40 cm FOV, 4 NEX

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)SNR increases (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.

The difference in brightness between two regions of an image is known as: a. Signal to noise b. Image contrast c. Spatial resolution d. All of the above

b. Image contrast

What is the term used to describe a picture element? a. Image element b. Pixel c. Voxel d. Volume element

b. Pixel

The proportion of signal actually used to construct an image relative to the amount of background noise is known as __________. a. Spatial resolution b. Signal to noise ratio c. Image contrast d. All of the above

b. Signal to noise ratio

The proportion of signal actually used to construct an image relative to the amount of background noise is known as __________. a. Spatial resolution b. Signal to noise ratio c. Image contrast d. All of the above

b. Signal to noise ratio

Doubling the number of signal averages (NSA) will: a. decreases the SNR b. increases the SNR c. doubles the SNR d. not affect the SNR

b. increases the SNR

An image with a grainy appearance usually represents an image with ____________. a. high resolution b. low signal to noise c. high signal to noise d. low resolution

b. low signal to noise

When echo time (TE) is increased, signal to noise ratio _________. a. Increases b. Decreases c. Stays the same

b. Decreases

An image with a grainy appearance usually represents an image with ____________. a. high resolution b. low signal to noise c. high signal to noise d. low resolution

b. low signal to noise

The number of picture elements used to make up an image is known as: a. contrast b. matrix c. signal to noise d. all of the above

b. matrix

Three characteristics commonly used to define the quality of an MRI image are ___________________. a. contrast, resolution, matrix b. resolution, contrast, signal to noise c. signal to noise, excitations, contrast d. resolution, matrix, signal to noise

b. resolution, contrast, signal to noise

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

c)3/4 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.

The clarity with which different areas of an image are distinguished is called? a. Image contrast b. Signal to noise c. Spatial resolution d. All of the above

c. Spatial resolution

The term used to describe a volume element is known as ____________. a. pixel b. pixie c. voxel d. picture element

c. voxel

Reducing the FOV: a. increases the TR b. decreases resolution c. increases resolution d. has no effect on resolution

c. increases resolution

Decreasing the receiver bandwidth: a. decreases the SNR b. inverts the SNR c. increases the SNR d. has no effect on SNR

c. increases the SNR

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)SNR increases (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)Scan time increases (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.

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)the NMV increases in size because the low energy population of spins increases 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 NMV.

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

d)√2 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.

Which of these is NOT an element that describes image quality? a. Image contrast b. Signal to noise ratio c. Spatial resolution d. Time of flight

d. Time of flight

SNR is affected by all EXCEPT: a. voxel size b. slice thickness c. signal d. dB/dt

d. dB/dt

Which parameter(s) effect total scan time? a. Repetition time b. Number of phase encoding steps c. Number of excitations d. All of the above

d. All of the above

The scan time for a 3D volume acquisition is given by? a. TR x NEX x bandwidth x slice thickness b. TR x NEX x number of phase encodings x number of slabs c. TR x NEX x number of phase encodings x echo train length d. TR x NEX x number of phase encodings x number of slices

d. TR x NEX x number of phase encodings x number of slices

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.