Study questions Flashcards
Which material is commonly used for passive magnetic shielding within the walls of the MRI room?
Iron
For adults, what is the current FDA guideline limit for exposure to a magnetic field?
8 Tesla
For infants less than one month, what is the current FDA guideline limit for exposure to a magnetic field?
4 Tesla
What material is used for RF shielding?
Copper
The FDA guideline limits SAR for the head to
3.2 W/k
The FDA guideline limits SAR averaged over the whole body to:
4.0 W/kg
The normal operating mode with limit to the whole body SAR average to no greater than:
2.0 W/kg
Increasing the flip angle from 45 degrees to 90 degrees will increase SAR by a factor of:
4
Which sequence is of most concern with regard to SAR?
Fast Spin Echo
The magnetic field of the hydrogen proton is known as:
The magnetic moment
Hydrogen protons in the low energy spin state are said to have their magnetic moment:
Aligned with the direction of the external magnetic field.
Within a few seconds of being placed in a magnetic field, the hydrogen protons reach a condition known as:
Thermal equilibrium
Properties of T2 relaxation
Spin-Spin
Exchange of energy among spins
63% decay transverse magnetization
Properties of T1 relaxation
Spin-Lattice
Spins give up/lose energy
63% of longitudinal magnetization has recovered
T2 relaxation times are mostly expressed in:
Milliseconds
Increasing the flip angle from 90 to 180 degrees requires how much of an increase in RF power?
4 times
T1 parameters for TR and TE
TR: 400-700 ms
TE: 25ms or less
T2 parameters for TR and TE
TR: 3000ms or higher
TE: 80-120 ms
Proton Density parameters for TR and TE
TR: 3000ms or higher
TE: 25ms or less
To obtain a TE which is “out of phase”, which type of pulse sequence would be selected?
Gradient echo
Ina gradient echo sequence, T1 weighting can be controlled by:
Both flip angle and TR
To produce a T1 weighted image using an extremely short TR, which gradient echo sequences would be selected?
Spoiled gradient echo
Decreasing flip angle does what to saturation
Decreased FA=Decreased saturation
The Maine function of Fourier transform in MR image reconstruction is to:
Convert time domain to frequency domain
Slice thickness is affected by the ____ of the slice selecting gradient.
Amplitude
What also affects slice thickness besides amplitude?
Transmit Bandwidth
Increasing which of the following will result in a direct increase in scan time?
A.Frequency matrix
B. Slice thickness
C.Phase matrix
D. Transmit bandwidth
Phase matrix
Increasing the NSA results in:
Reduction in noise
With regard to raw data, the data points which provide the greatest signal contribution are:
Low frequencies
Where in k-space is high frequency data?
Outer
With regard to raw data, edge detail is provided by
high frequencies
In a 3D FFT acquisition, SNR is proportional to
The square root of the number of slices.
Reformatting image data into another plane is done through a process known as
MPR
Which scan time reduction technique may require a reference or calibration scan prior to the actual acquisition?
Parallel Imaging
Spin Echo EPI is most often used to acquire which type of MR studies?
Diffusion weighted images
Decreasing matrix size does what to distortion artifacts?
Decreasing matrix=Decreased distortion artifacts
When each gradient is applied:
Slice selection
Phase Encoding
Frequency encoding
Slice-During RF pulse
Phase-During FID
Frequency-During sampling of echo
Low frequencies=
? Signal
? Contrast
Low Frequency =
High Signal
High Contrast
High frequencies=
? Signal
? Edge Detail
? Noise
High Frequency=
Low Signal
High Edge detail
High Noise
Sense vs. Smash Technique
Sense-Utilizes calibration technique
Smash-Utilizes data from raw scan to reconstruct image (no calibration scan)
Half Fourier or Partial Fourier does what to spatial resolution and noise
Spatial Resolution not affected
Noise is increased
Reducing TR = ? Saturation
Reducing TR = Increase in saturation ( Less signal-Not enough time to recover)
Following the injection of gad, which technique is not recommended?
STIR
Which technique of fat suppression can be successfully utilized at all field strengths following the injection of gad?
Dixon Method
Reducing the slice thickness by a factor of 2, will reduce the voxel volume by a factor of
2
Reducing the FOV by a factor of 2 will reduce the voxel volume by a factor of
4
Truncation artifact can be reduced by
Increasing the number of high frequency samples
Utilizing a gradient echo sequence, which parameter is adjusted to produce an out of phase image?
TE
Chemical shift is seen along which direction?
Frequency encoding direction
To reduce magnetic susceptibility, do what to receiver bandwidth?
Increase receiver bandwidth to decrease magnetic susceptibility
Increase FOV 2 =
?Signal
?Noise
Increase FOV 2=
Increase Signal 4
-Noise
Increase Slice thickness 2=
?Signal
?Noise
Increase Slice thickness 2=
Increase Signal 2
-Noise
Increase NSA 2=
?Signal
?Noise
Increase NSA 2=
-Signal
Decrease Noise sq root 2
Increase Receiver Bandwidth 2=
?Signal
?Noise
Increase Receiver Bandwidth 2=
-Signal
Increase Noise sq root 2
In a spin echo, normal flow will appear as:
A Signal Void
In a gradient echo, normal flow will appear as:
High signal
The contrast mechanism for time of flight is
Flow related enhancement
2D TOF TR and Flip angle
Short TR and high flip angle
Optimum signal due to flow is achieved when the blood flow is
Perpendicular to the slice plane
Name the disadvantage of 3D TOF
Best for imaging slow flow
Advantage of 2D TOF
Larger Voxel Volumes lead to large coverage
The contrast mechanism for phase contrast MRA is:
Velocity induced phase shifts
Which pulse sequence is commonly utilized for CE-MRA?
Spoiled Gradient Echo
Which pulse sequence is most commonly used for Diffusion?
Spin Echo EPI
Which sequence is most commonly used to acquire a perfusion study utilizing a bolus of gad?
gradient echo EPI
