MRI Physics Quiz 5
TR SCAN TIME !!!! PD Receive Bandwidth Flip angle Voxel volume TE
All of the following affect SNR except:
TR SCAN TIME !!!! Voxel volume PD TE Flip angle
All of the following affect SNR except:
Decreasing resolution
All of the following factors is the result of decreasing the NEX except which one?
True
Changing the slice thickness from 4mm to 3mm in a thinner slice which has a lower SNR.
NEX TR FOV !!!! Phase matrix
Factors that affect scan time are all of the following except:
True
If FOV is rectangular, the pixels will be square if the matrix selected produces a pixel with the same dimensions along phase as well as frequency directions.
False
If the FOV is square and an uneven matrix is selected (256x128), the pixels are rectangular, resulting in an increased spatial resolution.
40%
If we change the NEX from 1 to 2, the SNR is increasing how much:
True
If we have a voxel of .78mm x .78mm x 1mm, we would have an anisotropic voxel?
INCREASE NUMBER OF SLICES !!!!! Decrease number of slices Decrease SNR DECREASE T1 WEIGHTING !!!!!! INCREASE SNR !!!!
If we increase TR, we: (select as many choices as desired)
False
If we were to select a frequency value of 256 and a phase value of 192, we would have an isotropic pixel?
2x
If you change the NEX from 2 to 4, what is the resulting scan time change?
4x
If you change your FOV from 12cm to 24cm, how much is the SNR increasing?
.78 mm x 1.04 mm
If you have a frequency = 256, phase = 192, and FOV = 200mm, what would be the pixel size? (round off 2 decimals places)
SLICE ENCODINGS !!!!!!! TE FOV Receive bandwidth PHASE ENCODING !!!!!!! TR !!!!!!! NEX !!!!!!!
In volume imaging, which of the following parameters affect scan time (select all that apply)
The NVM increases in size because the low energy population of spin increases
Increasing the field strength increases SNR because:
False
Large voxels contain more spins or nuclei than small voxels, therefore have lower signal and lower SNR.
True
Long TE decreases SNR and a short TE increases SNR
False
Long TR decreases SNR and a short TR increases SNR
FOV divided by matrix size
Pixel area is equal to:
True
Reducing the receive bandwidth does not increase the signal, but rather only reduces the noise that is sampled.
True
Reducing the receive bandwidth increases the SNR, and reduced the noise that is sampled.
True
SNR is reduced with a lower flip angle
FOV !!!! Image matrix NEX TR Flip Angle TE Slice Thickness
Say you are doing an MRI exam and you need to change 1 of the factors to improve SNR. Which of the following has the greatest factor of improving the SNR?
TR
Spatial resolution is determined by all of the following except:
190.5
The following changes were made to a sequence. What is the new SNR value following this change? NEX: 1 > 3 FOV: 20 RFOV: 1.5 Matrix: 224x224 Slice thickness: 3.5mm Original measured SNR valued: 110
140
The following changes were made to a sequence. What is the new SNR value following this change? NEX: 2 > 4 FOV: 20 Matrix: 224x224 Slice thickness: 3.5mm Original measured SNR value: 100
4 times
To double the SNR, we need to increase the NEX and scan time by how much?
Selecting as thin a slice as possible
To improve image quality, spatial resolution must be optimized. The spatial resolution can be maintained by:
Bandwidth
Voxel size is affected by all of the following except:
Slice thickness x pixel area
Voxel volume is equal to:
NEX
Voxel volume signal changes is achieved by all of the following except:
.34 mm^2
What is the pixel area for the following scan parameters (round to nearest hundredth): FOV: 14cm RFOV: .75 Matrix: 256x224 TR: 3000 TE: 120 Flip Angle: 90 Slice Thickness: 4mm
5.14 mm^3
What is the voxel volume for the following parameters (round to the nearest hundredth): FOV: 25.6cm RFOV: 1.5 Matrix: 256x224 TE: 120 TR: 3000 Flip Angle: 90 Slice Thickness: 4.5mm
Decreasing the slice thickness 1/2 mm
When doing a study on a PT's knee you notice that the image is fuzzy (too much partial volume averaging). Which of the following changes would improve your image the most when repeating the sequence?
Increasing FOV
When doing a study on a PT's knee you notice that the image is grainy (low SNR). Which of the following changed would improve your image the most when repeating the sequence?
Increasing the slice thickness 1/2 mm
When doing a study on a PT's knee you notice that the image is grainy (low SNR). Which of the following changed would improve your image the most when repeating the sequence?
Increasing the matrix
When doing a study on a PT's knee you notice that the image is pixelated (poor resolution). Which of the following changes would improve your image the most when repeating the sequence?
True
When imaging on low field systems, SNR may be compromised and steps may have to be taken to boost the SNR, usually resulting in longer scan times.
2x
When the NEX is doubled, how much does the resulting scan time change?
Scan time
Which of the following aspects is also changed when we add slices to our sequence?
SNR increases
Which of the following occurs when the receive bandwidth is reduced?
Matrix: 256x128, slice thickness 6mm, FOV: 36cm, NEX: 4
Which of the following would offer the best SNR?
Matrix: 256x256, slice thickness 5mm, FOV: 12cm, NEX: 4
Which of the following would offer the best spatial resolution?
SNR increases
Which of these is NOT a consequence of decreasing the TR from 2500 ms to 500 ms?
Spatial resolution decreases SCAN TIME INCREASES !!!!!!!! SNR increases Voxel volume increases
Which of these is NOT a consequence of decreasing the phase matrix?
Scan time decreases
Which of these is a consequence of decreasing the TR from 2500 ms to 500 ms?
SNR increases
Which of these is a consequence of decreasing the phase matrix
Increase the bandwidth
You're doing a scan on a knee and notice that the images have a strange outline appearance around each differing tissue type (chemical shift artifact). In order to correct this you make what change to the repeat sequence?
