image basics Flashcards
Assuming all other parameters are the same, an echo time of ___________ ms provides the best Signal-to-Noise Ratio.
15
Fat has a _______ T1 relaxation time and a ________ T2 relaxation time.
short; short
Water has a __________ T1 relaxation time and a ________ T2 relaxation time
long; long
If the repetition time is doubled, the scan time will:
Double
Which of the following is NOT affected by changing the slice thickness?
The T1 contrast
Which of the following would NOT compensate for the change in signal-to-noise as a result of decreasing the FOV?
Reducing the number of excitations
Reducing the phase matrix from 256 to 192 will:
A. Increase the signal-to-noise ratio
B. Decrease imaging time
C. Decrease spatial resolution
D. All of the above
All of the above
SNR increases with all the following adjustments, EXCEPT
Decreasing pixel size
: Another name for a picture element is a:
Pixel
What formula is used to calculate the size of the pixel?
FOV ÷ matrix
Which parameter will provide the better spatial resolution?
A. 4mm slice thickness; 256 x 192 matrix
B. 4mm slice thickness; 256 x 256 matrix
C. 6mm slice thickness; 256 x 192 matrix
D. 6mm slice thickness; 256 x 256 matrix
4mm slice thickness; 256 x 256 matrix
If a sequence with a matrix of 256 x 512 is adjusted to 512 x 512,
The SNR will decrease and the scan time will increase
: 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
3 minutes 31 seconds
Decreasing the TR will:
A. Lower SNR
B. Reduce scan time
C. Increase SNR
D. Both A and B
Both A and B
With regards to k-space, the data containing high resolution are located along:
The outer lines
With regards to k-space, the data containing signal and contrast information are located along:
The central lines
As the number of excitations (NEX) is increased from 1 to 3, which of the following does NOT occur?
The pixel size is increased
Reducing the FOV (field of view) only will result in
Decreased signal-to-noise
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.
3
Assuming all other parameters are the same, a ___________ ms TR provides the best signal-to-noise ratio.
800
The smallest object that can be resolved in an image acquired with a 20cm FOV and a 224 x 224 matrix is _________ mm.
0.89
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?
Phase steps must be half the number of frequency steps
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?
9 min 50 sec
3D scan time formula: TR x Phase Matrix x NEX x # slices
Choose the correct slice thickness to create an isotropic voxel for the following: TR 2000, TE 90, Matrix 256 x 256, FOV 32cm
1.25 mm
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:
: 0.88 x 0.88 mm
FOV ÷ Matrix = pixel size….. 240 ÷ 272 = 0.88mm
An increase in TR will produce all of the following EXCEPT
Decrease the number of available slices
A set of specifically timed instructions to the magnet is known as
A pulse sequence
All of the following affect acquisition time EXCEPT:
TE
All of the following yield a higher signal-to-noise ratio EXCEPT
Increase TE
All of the following would aid in reducing scan time EXCEPT:
Reducing TE
Each line of k-space is defined by the
Phase encoding gradient
High amplitude signals are stored in the
Center portion of k-space
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:
A. Half Fourier
B. Zero fill
C. Fast spin echo
D. A and/or B
A and/or B
Increasing NSA will
Improve SNR
A reduction in FOV will result in:
A. Decrease in spatial resolution
B. Increase in SNR
C. Decrease in SNR
D. None of the above
E. A and C only
Decrease in SNR
Reducing the slice thickness will result in
A. An increase in partial volume averaging
B. A decrease in SNR
C. An increase in SNR
D. A & B only
E. None of the above
A decrease in SNR
The central lines of k-space are associated with _____________ in a fast spin echo sequence.
Contrast information
The timing of the RF pulses in an MRI pulse sequence controls:
Image contrast
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
The overall signal-to-noise ratio will increase in 3D volume imaging when there is an increase in:
A. Number of slices or partitions
B. TE
C. Phase Matrix
D. Field of View
E. A and/or D
A and/or D
Which of the following pulse sequence parameter combinations would yield an image with the most blurring?
6000TR, 80TE, 256 x 256, 5mm slice thickness, 16 ETL, 4NSA
Reducing the number of lines filled in k-space will produce an image:
A. With a decrease in spatial resolution
B. With an increase in spatial resolution
C. With an decrease in overall SNR
D. A and C
With a decrease in spatial resolution
If the NEX is increased from 2 to 6, the signal-to-noise ratio increase would be
1.7
Calculate the voxel volume for the following pulse sequence parameters: TR 450, TE 20, FOV 24cm, 320 x 320 matrix, 3mm slice thickness
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
Calculate the voxel volume for the following pulse sequence: TR 2000, TE 85, FOV 28cm, Matrix 272 x 320, 5mm slice thickness, 3 NSA
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
Calculate the pixel area for the following sequence: TR 450, TE 12, FOV 18cm, 224 x 256 matrix, 4 NEX.
0.56 mm2
FOV/matrix x FOV/matrix = pixel area. 180/224 x 180/256 = 0.56 mm2
Calculate the pixel area for the following sequence: TR 4000, TE 120, FOV 28cm, 304 x 304 matrix, 2 NEX, ETL 12.
0.85 mm2
FOV/matrix x FOV/matrix = pixel area. 280/304 x 280/304 = 0.85 mm2
K-space is typically filled:
: One line at a time
_______ is when only half the views of k-space are filled in the frequency axis
Partial or fractional echo
As the number of phase encodings is increased from 256 to 512, SNR (signal to noise ratio):
Decreases
