image formation Flashcards
Which of the following statements is FALSE regarding the use of a narrow receiver bandwidth?
A. It generates lower signal-to-noise ratio images
B. It should be applied in anatomical regions that contain high fat and water interfaces
C. It is utilized with a high readout gradient amplitude
D. All of the above
All of the above
: A ______________ is defined as a picture element and has _________ characteristics.
Pixel, two-dimensional
The maximum intensity projection algorithm
Projects the high intensity pixels which represent blood vessels/ducts into one image
Two gradients applied at the same time during slice selection are utilized for
Encoding oblique slice planes
The readout gradient is usually turned on during the sampling or readout of the peak echo and also during which other process?
Frequency encoding
If the maximum slices in a set TR is 10, and the necessary number of slices for a given sequence’s anatomical coverage is 26, how many acquisitions (packages) will be required
3
Slice ___________ is determined by the slice selection process.
A. Position
B. Orientation
C. Thickness
D. All of the above
All of the above
What effect would decreasing the FOV have on the spatial resolution of an image
Increase
What effect would using a steep slice select slope and/or narrow bandwidth have on slice thickness?
Slices will be thin
Which set of scan parameters below would yield the best spatial resolution?
A. 270 mm FOV, 192 x 512 matrix, 5mm slice thickness, 8 NEX
B. 270 mm FOV, 256 x 512 matrix, 5mm slice thickness, 4 NEX
C. 200 mm FOV, 512 x 512 matrix, 3mm slice thickness, 4 NEX
D. 200 mm FOV, 512 x 512 matrix, 5mm slice thickness, 4 NEX
200 mm FOV, 512 x 512 matrix, 3mm slice thickness, 4 NEX
An isotropic voxel refers to a __________ shaped voxel.
Cubic
: If the radiologist requires 3mm slices axially acquired through the IAC, with a slice gap of 0.5mm, and requires 4.2cm total coverage, how many slices must be selected?
12
Anatomic slice coverage: Slice thickness + interslice gap x # of slices
If 24 4mm slices are planned with a gap of 1mm, the total anatomic coverage is _______ cm.
12
24 slices x 5mm (4 thickness + 1 gap) = 120 mm; ÷ 10 to calculate the coverage in cm.
Which of the following is NOT a result of reducing the FOV?
Anatomical structures are displayed smaller in the image
Creating additional images in various planes from a 3D data set is accomplished by a technique known as:
Multi-Planar reconstruction (MPR
Which of the following can be considered an advantage of selecting a 3D acquisition as opposed to a 2D acquisition?
A. The slices have a greater SNR
B. No crosstalk
C. Longer repetition times
D. All of the above
E. A & B only
A & B only
Multiplanar reconstructions are typically utilized in 3D pulse sequences because
A. 2D slices are too thick to reformat
B. 2D slice groups are too long
C. There are gaps between 2D slices
D. A & C only
E. B & C only
A & C only
The process that digitizes the MR signals is known as
Fourier transform
The signal produced immediately following an RF pulse is:
FID
__________ is defined as a small change in the magnetic field along a particular axis
A gradient
_________ is a file on the computer where the collected echoes are stored prior to being processed into an image by the Fourier Transform
Raw data
In order to generate an MR image, the slice select gradient must be turned on during
RF energy application
Decreasing the slice selection gradient strength will ________ of the resulting slice
Change the slice thickness
The range of transmitted frequencies at each slice position must _____ to maintain slice thickness.
Remain constant
All of the following are true mechanisms of saturation EXCEPT
Partial saturation yields T1 weighting; occurs when NMV is 180°
Applying two gradients simultaneously during slice selection would
Produce an oblique slice
A narrow receiver bandwidth:
Increases susceptibility artifact
Which factors affect the flip angle?
Strength and duration of the RF field
: To increase the voxel volume, which parameters would be adjusted
FOV, slice thickness and matrix
Signal to Noise Ratio (SNR) would increase in a 3D volume acquisition with an increase in:
A. TE
B. Slices
C. Field of view
D. All of the above
E. B and/or C only
B and/or C only
A technique that reduces scan time by not filling as many lines of k-space in the phase direction, while maintaining spatial resolution, is known as:
Rectangular FOV
To obtain a thin slice thickness
A steep gradient is applied with the transmit bandwidth at the Larmor frequency of H
The Ernst angle can be defined as the optimal ___________ that yields the ____________ signal for a particular spin in the least amount of time.
Flip angle; maximum
As the flip angle is increased, SNR(signal to noise ratio):
Increases to a point called the Ernst angle
When gradient moment nulling is used as an imaging option in MR sequences
The minimum TE is increased
Gradient moment nulling is used to compensate for:
I. First order motion
II. Second order motion
III. Slow flowing vessels
IV. Fast flowing vessels
I and III
As the TR is increased,
I. SNR is increased
II. Available number of slices is increased
III. T2 information is maximized
I and II
Increasing the field of view (FOV) would have what effect?
Decrease spatial resolution
2D acquisitions usually require
An interslice gap
In a gradient echo sequence, reducing the flip angle while maintaining the TR reduces
Saturation
Reducing the TE
Decreases the contrast based on T2 tissue relaxation times
Reducing the TR:
A. Increases the SNR of the image
B. Decreases the SNR of the image
C. Decreases the contrast based on the T2 relaxation time
D. Increases saturation effects
E. A and D only
F. B and D only
G. B, C and D only
B and D only
Increasing the TE:
A. Increases the contrast based on T1 tissue relaxation times
B. Decreases the SNR of the image
C. Decreases the contrast based on T2 tissue relaxation times
D. Increases the contrast based on T2 relaxation times
E. A and B only
F. B and C only
G. B and D only
B and D only
Complete saturation is a condition where
: Longitudinal magnetization is not allowed to recover between excitations
Gradient echo sequences use flip angles
To control saturation effects
The presaturation pulses typically are found
A. Between the 90º and the 180º pulses
B. Prior to the TE
C. Following the 180º pulse
D. Prior to the excitation pulse
E. A and D only
Prior to the excitation pulse
Presaturation pulses are often used to:
Reduce flow artifacts
Decreasing rBW (receiver bandwidth)by a factor of two:
Increases SNR by √2
Decreasing the receiver bandwidth (utilizing a narrow BW):
A. Increases the number of slices available
B. Decreases the number of slices available
C. Does not change the number of slices available
D. Decreases SNR
E. A and D only
F. B and D only
Decreases the number of slices available
Decreasing the receiver bandwidth (utilizing a narrow BW):
A. Increases susceptibility artifact
B. Decreases susceptibility artifact
C. Does not affect susceptibility artifact
D. Decreases SNR
E. A and D only
F. B and D only
Increases susceptibility artifact
Decreasing the receiver bandwidth (utilizing a narrow BW):
A. Reduces readout time
B. Increases readout time
C. Does not affect the readout time
D. Increases SNR
E. A and D only
F. B and D only
B and D only
Decreasing the receiver bandwidth (narrow rBW):
A. Increases the contrast based on T1 tissue relaxation times
B. Decreases the SNR of the image
C. Increases the SNR of the image
D. Inverts the SNR of the image
E. Does not affect the SNR of the image
Increases the SNR of the image
The time during which the frequency encoding gradient is on
Increases with a reduction in receiver bandwidth
: If the receiver bandwidth (rBW) is decreased, the sampling rate
Decreases
Increasing the receiver bandwidth (rBW):
Decreases chemical shift artifacts
