MRI Physics Final Exam with Verified
Solutions
Factors affecting the magnitude of entry slice phenomenon Correct Ans-long TR
high flip angle
thin slice thickness
increase in velocity of flow
counter current flow
What are the two different pulse sequences used in TOF? Correct Ans-spin echo pulse
sequence
gradient echo pulse sequence
Spin echo produces Correct Ans-dark blood
Gradient echo produces Correct Ans-bright blood
- GMN reduces intra-voxel dephasing
- Gradients alter magnetic field strength precessional frequency and phase of nuclei
- If flowing nucleus is adjacent to a stationary nucleus in a voxel, there is a phase difference
between the two nuclei, therefore nuclei within the same voxel are out of phase with each other.
This results in a reduction of total signal amplitude from the voxel. Correct Ans-Intra-Voxel
dephasing
,-acquire sound echoes at a multiple of the first TE
-nuclei have to be given the same amount of time to rephrase as they were given to dephase
Correct Ans-Even-echo raphasing
-compensates for altered phase values
-uses gradients to correct altered phase back to original values
-slice select and/or readout gradient
-gradients alters polarity from positive to double negative and back to positive again
-most effective in Laminar flow Correct Ans-Gradient moment nulling
-nullify signal from flowing nuclei so that the effects of entry slice and TOF are minimized
-90 RF pulse is delivered outside the FOV Correct Ans-Spatial pre-saturation
o Signals are received from a series of image slices
o Multiple thin slices are obtained in a plane perpendicular to the course of the imaging vessels
o Values are kept short <30 ms to suppress background tissue and flow
o Optimal in areas of slow velocity flow
o Short scan but susceptible to motion artifact Correct Ans-2D-TOF
o Signals are received from the entire volume of interest
o Offer high spatial resolution in three spatial directions
, o Offers high SNR and thin slices for high resolution
o Works well for high flow arterial systems (Circle of Willis)
o NOT suitable for slow flow Correct Ans-3D-TOF
o 3D sequential multi slab sequence uses a sequence of thin overlapping 3D slabs
o Hybrid between 2D and 3D TOF
o Allows high resolution (3D) and provides large area of coverage (2D) Correct Ans-MOTSA
o Large anatomical coverage
o More sensitive to slow flow (carotid, venous, extremities) Correct Ans-advantages of 2D-
TOF MRA
o Low SNR
o Low resolution
o Insensitive to in-plane flow
o Susceptible to "Venetian Blind" artifacts due to pulsatile flow Correct Ans-disadvantages of
2D-TOF MRA
o High SNR
o High resolution
o Very thin slices with no gap between slices can be reconstructed from within the imaging
volume, with any obliquity, retrospectively
o Optimal for fast flow (intracranial arterial vasculature)
Solutions
Factors affecting the magnitude of entry slice phenomenon Correct Ans-long TR
high flip angle
thin slice thickness
increase in velocity of flow
counter current flow
What are the two different pulse sequences used in TOF? Correct Ans-spin echo pulse
sequence
gradient echo pulse sequence
Spin echo produces Correct Ans-dark blood
Gradient echo produces Correct Ans-bright blood
- GMN reduces intra-voxel dephasing
- Gradients alter magnetic field strength precessional frequency and phase of nuclei
- If flowing nucleus is adjacent to a stationary nucleus in a voxel, there is a phase difference
between the two nuclei, therefore nuclei within the same voxel are out of phase with each other.
This results in a reduction of total signal amplitude from the voxel. Correct Ans-Intra-Voxel
dephasing
,-acquire sound echoes at a multiple of the first TE
-nuclei have to be given the same amount of time to rephrase as they were given to dephase
Correct Ans-Even-echo raphasing
-compensates for altered phase values
-uses gradients to correct altered phase back to original values
-slice select and/or readout gradient
-gradients alters polarity from positive to double negative and back to positive again
-most effective in Laminar flow Correct Ans-Gradient moment nulling
-nullify signal from flowing nuclei so that the effects of entry slice and TOF are minimized
-90 RF pulse is delivered outside the FOV Correct Ans-Spatial pre-saturation
o Signals are received from a series of image slices
o Multiple thin slices are obtained in a plane perpendicular to the course of the imaging vessels
o Values are kept short <30 ms to suppress background tissue and flow
o Optimal in areas of slow velocity flow
o Short scan but susceptible to motion artifact Correct Ans-2D-TOF
o Signals are received from the entire volume of interest
o Offer high spatial resolution in three spatial directions
, o Offers high SNR and thin slices for high resolution
o Works well for high flow arterial systems (Circle of Willis)
o NOT suitable for slow flow Correct Ans-3D-TOF
o 3D sequential multi slab sequence uses a sequence of thin overlapping 3D slabs
o Hybrid between 2D and 3D TOF
o Allows high resolution (3D) and provides large area of coverage (2D) Correct Ans-MOTSA
o Large anatomical coverage
o More sensitive to slow flow (carotid, venous, extremities) Correct Ans-advantages of 2D-
TOF MRA
o Low SNR
o Low resolution
o Insensitive to in-plane flow
o Susceptible to "Venetian Blind" artifacts due to pulsatile flow Correct Ans-disadvantages of
2D-TOF MRA
o High SNR
o High resolution
o Very thin slices with no gap between slices can be reconstructed from within the imaging
volume, with any obliquity, retrospectively
o Optimal for fast flow (intracranial arterial vasculature)
