Scan Principles - Image Formation Flashcards
Which of the following statements is FALSE regarding the use of a narrow receiver bandwidth?
All of the above
Lower bandwidth = higher signal-to-noise = more chemical shift
The maximum intensity projection algorithm:
Projects the high intensity pixels which represent blood vessels/ducts into one image
The high intensity pixels are reconstructed into composite images using the MIP (maximum intensity projection) reconstruction method.
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
If the set TR allots 10 slices, 3 acquisitions will be required to obtain the entire 26 slice group. Raising the TR to allow for 14 slices would permit the slices to be acquired in two acquisitions, for example
Slice ___________ is determined by the slice selection process.
All of the above
What effect would decreasing the FOV have on the spatial resolution of an image?
Increase
Decreasing the FOV would increase the spatial resolution/detail in an image by reducing the pixel size.
What effect would using a steep slice select slope and/or narrow bandwidth have on slice thickness?
Slices will be thin
Steep slice select gradient slope yields thin slice thickness
Which set of scan parameters below would yield the best spatial resolution?
200 mm FOV, 512 x 512 matrix, 3mm slice thickness, 4 NEX
An isotropic voxel refers to a __________ shaped voxel.
Cubic
An isotropic voxel has equal measurements in all directions, thus making it cubed in shape.
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?
Increased Spatial resolution
Anatomical structures are displayed smaller in the image
Anatomical structures are displayed larger with a reduction in FOV
Creating additional images in various planes from a 3D data set is accomplished by a technique known as:
Multi-Planar reconstruction (MPR)
The MPR function enables the user to create additional images from an acquired 3D data set
Which of the following can be considered an advantage of selecting a 3D acquisition as opposed to a 2D acquisition?
A & B only
Long TR’s are not advantageous in a 3D acquisition, but a 3D sequence does provide greater SNR and elimination of crosstalk artifacts.
Multiplanar reconstructions are typically utilized in 3D pulse sequences because
A & C only
The option to reconstruct multiple planes from a single data set is advantageous in a 3D pulse sequence due to the thin slices used in 3D sequences and the lack of interslice gap
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
A gradient is defined as a small change in the magnetic field along a particular axis.
_________ 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
Raw data is a file on the computer where the collected echos are stored prior to being processed into an image by the Fourier Transform.
In order to generate an MR image, the slice select gradient must be turned on during:
RF energy application
In order to generate an MR image, the slice select gradient must be turned on during the RF energy application.
Decreasing the slice selection gradient strength will ________ of the resulting slice.
Change the slice thickness
Decreasing the slice selection gradient strength will change the slice thickness of the resulting slice.
The range of transmitted frequencies at each slice position must _____ to maintain slice thickness.
Partial saturation yields T1 weighting; occurs when NMV is 180°
Applying two gradients simultaneously during slice selection would:
Produce an oblique slice
Applying two gradients simultaneously during slice selection would produce an oblique slice.
A narrow receiver bandwidth:
Increases susceptibility artifact
A narrow receiver bandwidth increases susceptibility artifact, and should never be used in the presence of metal implants.
Which factors affect the flip angle?
Strength and duration of the RF field
The strength and duration of the RF field affect the flip angle.
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:
B and/or C only
Signal to Noise Ratio (SNR) would increase in a 3D volume acquisition with an increase in the Field of View or the number of slices.
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
A steep gradient is applied with the transmit bandwidth at the Larmor frequency of H in order to achieve a thin slice thickness in MRI imaging
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
The Ernst angle can be defined as the optimal flip angle that yields the maximum signal for a particular spin in the least amount of time.
As the flip angle is increased, SNR(signal to noise ratio):
Increases to a point called the Ernst angle
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
By the application of an imaging option known as gradient moment nulling (flow compensation), it is required that a longer minimum TE is used due to the use of additional gradients
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
Gradient moment nulling (flow compensation)is used to compensate for first order motion (protons moving with constant velocity) and slow flowing vessels
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
2D acquisitions usually require an interslice gap
In a gradient echo sequence, reducing the flip angle while maintaining the TR reduces:
Saturation
In a gradient echo sequence, reducing the flip angle while holding the TR constant reduces saturation (Gradient echo sequences use flip angles to control saturation effects)
Reducing the TE:
Decreases the contrast based on T2 tissue relaxation times
Reducing the TE reduces contrast based on T2 relaxation time
Reducing the TR:
B and D only
Increasing the TE increases the contrast based on the tissue’s relaxation times, and also decreases the SNR of the image.
Complete saturation is a condition where:
Longitudinal magnetization is not allowed to recover between excitations
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:
Prior to the excitation pulse
The presaturation pulses typically are found 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 rBW (receiver bandwidth)by a factor of two increases SNR by √2
Decreasing the receiver bandwidth (utilizing a narrow BW):
Decreases the number of slices available
Decreasing the receiver bandwidth (utilizing a narrow BW) decreases the number of slices available (Decreasing the bandwidth increases the sampling time and lengthens the sequence timing, creating longer minimum TR and TE values
Decreasing the receiver bandwidth (utilizing a narrow BW):
Increases susceptibility artifact
Decreasing the receiver bandwidth (utilizing a narrow BW):
B and D only
Decreasing the receiver bandwidth (utilizing a narrow BW) increases SNR and readout time – increases the sampling time and lengthens the sequence timing, creating longer minimum TR and TE values
Decreasing the receiver bandwidth (narrow rBW):
Increases the SNR of the image
Decreasing the receiver bandwidth (narrow rBW) 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
Increasing the receiver bandwidth (rBW), or utilizing a wider BW, shortens the sampling time, which decreases chemical shift artifacts
