Scan Principles Image Basics - Calculations*

Question 1

Assuming all other parameters are the same, an echo time of ___________ ms provides the best Signal-to-Noise Ratio

Answer 1

15

Question 2

Fat has a _______ T1 relaxation time and a ________ T2 relaxation time.

Answer 2

short; short

Question 3

Water has a __________ T1 relaxation time and a ________ T2 relaxation time.

Answer 3

long; long

Question 4

If the repetition time is doubled, the scan time will:

Answer 4

Double

TR is directly proportional to scan time (TR x Phase Matrix x NEX)

Question 5

Which of the following is NOT affected by changing the slice thickness?

Answer 5

The T1 contrast

Repetition Time (TR) is the controlling factor of T1 weighting/contrast in an image; slice thickness does nothing to alter the T1 information.

Question 6

Which of the following would NOT compensate for the change in signal-to-noise as a result of decreasing the FOV?

Answer 6

Reducing the number of excitations

A reduction in excitations (NEX/NSA) would reduce SNR, which has the same effect as decreasing the field of view

Question 7

Reducing the phase matrix from 256 to 192 will

Answer 7

All of the above

Reducing the phase encoding matrix yields higher SNR, shorter scan times, but decreased spatial resolution due to the increased pixel size.

Question 8

SNR increases with all the following adjustments, EXCEPT:

Answer 8

Decreasing pixel size

Smaller pixel size (higher phase matrix) leads to improved resolution, but decreased SNR.

Question 9

Another name for a picture element is a:

Answer 9

Pixel

Question 10

What formula is used to calculate the size of the pixel?

Answer 10

FOV ÷ matrix

Question 11

Which parameter will provide the better spatial resolution?

Answer 11

4mm slice thickness; 256 x 256 matrix

Thinner slices yield improved spatial resolution, in addition to smaller pixel size (higher matrix)

Question 12

If a sequence with a matrix of 256 x 512 is adjusted to 512 x 512,

Answer 12

The SNR will decrease and the scan time will increase

Question 13

In a fast spin echo sequence with a 28 cm FOV, 256 x 512 matrix, TR 2200, 90 TE, 5mm slice thickness, 8 ETL and 3 NEX, the scan time would be:

Answer 13

3 minutes 31 seconds

Scan time formula: TR x Phase Matrix x NEX ÷ ETL = total in milliseconds, then ÷ 1000 to achieve scan time in seconds. Be careful if you divide by 60 to achieve the scan time in decimal format (This example computes to 211 seconds, if you divide by 60 you get 3.52 seconds, which is 3 minutes 31 seconds, not 3 minutes 52 seconds)

Question 14

Decreasing the TR will:

Answer 14

Both A and B

Question 15

With regards to k-space, the data containing high resolution are located along:

Answer 15

The outer lines

Referring to k-space, the data containing high resolution are located along the outer lines

Question 16

With regards to k-space, the data containing signal and contrast information are located along:

Answer 16

The central lines

Referring to k-space, the data containing signal and contrast information are located along the central lines.

Question 17

As the number of excitations (NEX) is increased from 1 to 3, which of the following does NOT occur?

Answer 17

The pixel size is increased

Changes in NEX have no effect on the resolution of an image.

Question 18

Reducing the FOV (field of view) only will result in:

Answer 18

Decreased signal-to-noise

A reduction in FOV will yield a decrease in SNR, but an increase in spatial resolution.

Question 19

The scan time of a fast spin echo sequence utilizing a TR of 3500ms, a TE of 90ms, a 256x256 matrix, 1NSA, a 220mm FOV and an echo train length of 5 is ______ minutes, approximately.

Answer 19

3

TR (3500) x Phase Matrix (256) x NSA (1) ÷ ETL (5) = 179200 ms, then divide by 1000 to get 179.2 seconds, or nearly 3 minutes.

Question 20

Assuming all other parameters are the same, a ___________ ms TR provides the best signal-to-noise ratio.

Answer 20

800

The longer the TR, the better the SNR

Question 21

The smallest object that can be resolved in an image acquired with a 20cm FOV and a 224 x 224 matrix is _________ mm.

Answer 21

0.89

200 mm ÷ 224 = 0.892mm

Question 22

With a recFOV that is twice as tall as it is wide, and the phase direction oriented along the smaller “width” dimension of the FOV, which of the following relationships between the frequency and phase steps yields square pixels?

Answer 22

Phase steps must be half the number of frequency steps

With a 50% Rectangular FOV, the phase matrix (rectFOV direction) must be exactly half the frequency matrix in order to yield square pixels

Question 23

Choose the correct slice thickness to create an isotropic voxel for the following: TR 2000, TE 90, Matrix 256 x 256, FOV 32cm.

Answer 23

1.25 mm

An isotropic voxel has equal measurements in all three dimensions (phase, frequency and slice direction) FOV/Matrix = 1.25mm

Question 24

The pixel size of the images which result from a 3D acquisition using a 240mm FOV, a 272 x 272 matrix, 32 slices and a volume of 64 mm is:

Answer 24

0.88 x 0.88 mm

FOV ÷ Matrix = pixel size….. 240 ÷ 272 = 0.88mm

Question 25

An increase in TR will produce all of the following EXCEPT:

Answer 25

Decrease the number of available slices

An increase in repetition time will produce more available slices, per TR period

Question 26

A set of specifically timed instructions to the magnet is known as:

Answer 26

A pulse sequence

A pulse sequence is a set of specifically timed instructions to the magnet telling it how images should look with regards to the tissue being sampled

Question 27

All of the following affect acquisition time EXCEPT:

Answer 27

TE

Question 28

All of the following yield a higher signal-to-noise ratio EXCEPT:

Answer 28

Increase TE

An increase in TE will yield a decrease in SNR

Question 29

All of the following would aid in reducing scan time EXCEPT:

Answer 29

Reducing TE
TE (echo time) has no affect on scan time

Question 30

Each line of k-space is defined by the:

Answer 30

Phase encoding gradient

Question 31

High amplitude signals are stored in the:

Answer 31

Center portion of k-space

High amplitude signals are stored in the center portion of k-space.

Question 32

Acquiring a bit more than half the phase k-space samples, then interpolating the data with zeroes for the remaining half is a technique known as:

Answer 32

A and/or B

Acquiring a bit more than half the phase k-space samples, then interpolating the data with zeroes for the remaining half is a technique known as Half Fourier, or Zero Fill

Question 33

Increasing NSA will:

Answer 33

Improve SNR

Question 34

A reduction in FOV will result in:

Answer 34

Decrease in SNR

A reduction in FOV will result in a decrease in SNR, but will increase the spatial resolution in the image by lowering the pixel size.

Question 35

Reducing the slice thickness will result in:

Answer 35

A decrease in SNR

Reducing the slice thickness will result in a decrease in SNR, but will decrease partial volume averaging

Question 36

The central lines of k-space are associated with _____________ in a fast spin echo sequence.

Answer 36

Contrast information

The central lines of k-space are associated with contrast information in a fast spin echo sequence

Question 37

The timing of the RF pulses in an MRI pulse sequence controls:

Answer 37

Image contrast

The timing of the RF pulses in an MRI pulse sequence controls image contrast

Question 38

A method of reducing MRI scan time by not filling as many lines of k-space in the phase direction, while maintaining spatial resolution, is known as:

Answer 38

Rectangular FOV

A method of reducing MRI scan time by not filling as many lines of k-space in the phase direction, while maintaining spatial resolution, is known as Rectangular FOV

Question 39

The overall signal-to-noise ratio will increase in 3D volume imaging when there is an increase in:

Answer 39

A and/or D

The overall signal-to-noise ratio (SNR) will increase in 3D volume imaging when there is an increase in either the Field of View (FOV) and/or an increase in the number of slices or partitions (chunks).

Question 40

Which of the following pulse sequence parameter combinations would yield an image with the most blurring?

Answer 40

6000TR, 80TE, 256 x 256, 5mm slice thickness, 16 ETL, 4NSA

The longer the ETL (echo train length), the increased chance of blurring in the image.

Question 41

Reducing the number of lines filled in k-space will produce an image

Answer 41

With a decrease in spatial resolution

Question 42

If the NEX is increased from 2 to 6, the signal-to-noise ratio increase would be:

Answer 42

1.7

When increasing NEX/NSA, SNR (signal-to-noise ratio) increases by the square root (√) of the % increase. For example, a sequence with 2 NEX, increased to 6 NEX (3times the total NEX) would have an increase in SNR of √3, or 1.73 times more SNR than the original sequence

Question 43

Calculate the voxel volume for the following pulse sequence parameters: TR 450, TE 20, FOV 24cm, 320 x 320 matrix, 3mm slice thickness.

Answer 43

1.69mm3

To calculate voxel volume, first determine pixel area. FOV/matrix x FOV/matrix = pixel area. Pixel area x slice thickness = voxel volume. 240/320 x 240/320 = 0.5625 mm2. 0.5625 x 3 = 1.69mm3

Question 44

Calculate the voxel volume for the following pulse sequence: TR 2000, TE 85, FOV 28cm, Matrix 272 x 320, 5mm slice thickness, 3 NSA.

Answer 44

4.55 mm3

To calculate voxel volume, first determine pixel area. FOV/matrix x FOV/matrix = pixel area. Pixel area x slice thickness = voxel volume. 280/272 x 280/320 = 0.91 mm2. 0.91 x 5 = 4.55mm3

Question 45

Calculate the pixel area for the following sequence: TR 450, TE 12, FOV 18cm, 224 x 256 matrix, 4 NEX.

Answer 45

0.85 mm2

FOV/matrix x FOV/matrix = pixel area. 280/304 x 280/304 = 0.85 mm2

Question 46

K-space is typically filled:

Answer 46

One line at a time

K-space is typically filled one line at a time

Question 47

_______ is when only half the views of k-space are filled in the frequency axis.

Answer 47

Partial or fractional echo

Partial or fractional echo is when only half the views of k-space are filled in the frequency axis, an acceleration technique used to shorten TR and TE

Question 48

As the number of phase encodings is increased from 256 to 512, SNR (signal to noise ratio):

Answer 48

Decreases

As the number of phase encodings is increased from 256 to 512, SNR (signal to noise ratio) decreases (less protons per voxel)

Question 49

What would the scan time be in a 3D volume acquisition as follows: TR 32 ms, TE 4 ms, Flip Angle 15°, FOV 18cm, 2mm slice thickness, 96 slices, 192 x 256 matrix, 1 NEX?

Answer 49

9 min 50 sec

3D scan time formula: TR x Phase Matrix x NEX x # slices