Scan Principles Image Basics - Calculations* 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
TR is directly proportional to scan time (TR x Phase Matrix x NEX)
Which of the following is NOT affected by changing the slice thickness?
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.
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
A reduction in excitations (NEX/NSA) would reduce SNR, which has the same effect as decreasing the field of view
Reducing the phase matrix from 256 to 192 will
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.
SNR increases with all the following adjustments, EXCEPT:
Decreasing pixel size
Smaller pixel size (higher phase matrix) leads to improved resolution, but decreased SNR.
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?
4mm slice thickness; 256 x 256 matrix
Thinner slices yield improved spatial resolution, in addition to smaller pixel size (higher 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
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)
Decreasing the TR will:
Both A and B
With regards to k-space, the data containing high resolution are located along:
The outer lines
Referring 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
Referring 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
Changes in NEX have no effect on the resolution of an image.
Reducing the FOV (field of view) only will result in:
Decreased signal-to-noise
A reduction in FOV will yield a decrease in SNR, but an increase in spatial resolution.
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
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.
Assuming all other parameters are the same, a ___________ ms TR provides the best signal-to-noise ratio.
800
The longer the TR, the better the SNR
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
200 mm ÷ 224 = 0.892mm
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
With a 50% Rectangular FOV, the phase matrix (rectFOV direction) must be exactly half the frequency matrix in order to yield square pixels
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
An isotropic voxel has equal measurements in all three dimensions (phase, frequency and slice direction) FOV/Matrix = 1.25mm
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
An increase in repetition time will produce more available slices, per TR period
A set of specifically timed instructions to the magnet is known as:
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
All of the following affect acquisition time EXCEPT:
TE
All of the following yield a higher signal-to-noise ratio EXCEPT:
Increase TE
An increase in TE will yield a decrease in SNR
All of the following would aid in reducing scan time EXCEPT:
Reducing TE
TE (echo time) has no affect on scan time
Each line of k-space is defined by the:
Phase encoding gradient
High amplitude signals are stored in the:
Center portion of k-space
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 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
Increasing NSA will:
Improve SNR
A reduction in FOV will result in:
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.
Reducing the slice thickness will result in:
A decrease in SNR
Reducing the slice thickness will result in a decrease in SNR, but will decrease partial volume averaging
The central lines of k-space are associated with _____________ in a fast spin echo sequence.
Contrast information
The central lines of k-space are associated with contrast information in a fast spin echo sequence
The timing of the RF pulses in an MRI pulse sequence controls:
Image contrast
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
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 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).
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
The longer the ETL (echo train length), the increased chance of blurring in the image.
Reducing the number of lines filled in k-space will produce an image
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
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
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.
0.85 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
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
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
As the number of phase encodings is increased from 256 to 512, SNR (signal to noise ratio):
Decreases
As the number of phase encodings is increased from 256 to 512, SNR (signal to noise ratio) decreases (less protons per voxel)
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?
9 min 50 sec
3D scan time formula: TR x Phase Matrix x NEX x # slices
