Advanced MRI Physics Review Questions

MRI - Flow-Related Effects and Phase-Contrast MRA
MRI - Parallel Imaging

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MRI - Flow-Related Effects and Phase-Contrast MRA

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Flow voids in spin echo and fast spin echo sequences are primarily related to:
1. Intravoxel dephasing related to changing velocities over the course of the scan
2. Time elapsed between the 90-degree and 180-degree pulses
3. T2 decay
4. T1 relaxation
5. Accumulation of phase shift by the 90- and 180-degree pulses
Flow voids in spin echo sequences are caused by moving blood not experiencing both the 90- and 180-degree pulses as it passes through the slice.
Flow-related enhancement, which refers to brighter signal seen in flowing blood (even without intravascular contrast) on certain sequences, is primarily related to:
1. Intravoxel dephasing related to turbulence
2. Accumulation of positive phase shift over the course of the scan
3. Fat-saturation pulses as used in time-of-flight imaging
4. Inflow of unsaturated blood into the slice after the initial 90-degree pulse
Flow-related enhancement is caused by 'naive' blood entering the slice. This blood hasn't been prepared by an initial 90-degree pulse to reduce its longitudinal magnetization, so it comes in with more signal. These effects are negated by flow-related dephasing and are only seen well in short TE sequences.
Flow-related dephasing is primarily related to:
1. Accelerated T1 relaxation of protons in moving blood
2. Accumulation of a phase shift related to the 90- and 180-degree pulses
3. Accumulation of a phase shift related to slice-encoding, phase-encoding, and frequency-encoding gradients
4. Inflow of unsaturated blood into the slice after the initial 90-degree pulse
5. Inflow of unsaturated blood into the slice between 90- and 180-degree pulses
Flow-related dephasing is caused when moving protons pick up phase shifts as they pass along spatially-varying gradients. As their phase changes more rapidly than stationary tissue, they lose signal. This can be compensated for by using flow-compensation gradients.
Flow-related dephasing can be minimized by: (advanced)
1. Monophasic readout gradient
2. Biphasic readout gradient
3. Triphasic readout gradient
4. Increased TE
5. Increased TR
A triphasic gradient is needed to fully compensate for protons moving at constant velocity. Higher-order gradients (e.g. 5-lobe) are needed for protons that are accelerating.
Time-of-flight MR angiography primarily utilizes which of the following effects:
1. Flow-related enhancement
2. Flow-related dephasing
3. Flow compensation
4. Flow voids
The TOF technique suppresses signal from non-moving tissue in the slice (with a saturation pulse). Any protons entering the slice will have full signal.
'Bright-blood' steady-state free precession (SSFP) imaging employs which of the following for bright signal in vessels:
1. Flow-related enhancement
2. Flow-related dephasing
3. Flow compensation
4. Flow voids
5. Gadolinium-related T1 shortening
Bright-blood techniques use the inherent T1 and T2 characteristics of blood but need flow compensation gradients to minimize flow-related dephasing.
Dark signal is seen in turbulent flow on bright-blood SSFP sequences because of:
1. Flow void phenomenon from 90- and 180-degree pulses
2. Lack of triphasic readout gradient to compensate
3. Turbulent flow involves rapidly changing velocities
4. T1 effects
Turbulent flow induces such rapid and unpredictable changes in velocity that it cannot be compensated for even with high-order compensation gradients.
Phase-contrast MR angiography relies the same phenomenon as which effect:
1. Flow voids on spin echo sequences
2. Flow-related enhancement
3. Flow-related dephasing
4. Time-of-flight MR angiography
5. Contrast-enhanced MR angiography
Phase-contrast MRA actually measures the phase change induced as protons move through a spatially varying gradient.
Phase-contrast MR angiography employs: (advanced)
1. Monophasic gradients
2. Biphasic gradients
3. Triphasic gradients
4. Spin echoes instead of gradients
In order to compensate for stationary tissue, a biphasic gradient is used. A triphasic gradient would defeat the whole purpose - moving protons would not accumulate any phase shift!
Choosing the correct gradient strength is necessary to avoid:
1. Velocity aliasing
2. Flow-related dephasing
3. Poor signal-to-noise
4. Phase shifts in stationary tissue
Phases can only be measured up to 180 degrees - 181 degrees looks just like 179 degrees. This is the phenomenon of aliasing. Using too high of a Venc gives poor velocity resolution, however.

MRI - Parallel Imaging

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The primary goal of parallel imaging is to:
1. Decrease scan time
2. Increase signal-to-noise
3. Remove motion artifacts
4. Perform 3D acquisitions
Parallel imaging improves scan time, although this can be 'exchanged' for gaining spatial resolution or performing more excitations to improve SNR. Overall, SNR decreases with parallel imaging (assuming other parameters are not changed).
Parallel imaging requires the use of:
1. A volume receive coil
2. A single surface coil
3. Multiple surface coils in an array
4. An endorectal or endovaginal coil
Parallel imaging requires an array of multiple surface coils, each with different spatial sensitivities.
Parallel imaging employs which reconstruction technique:
1. Reconstructing a full image from aliased reduced FOV images
2. Reconstructing a full image from low-resolution images
3. Combining images from selective excitation of tissue near each coil
4. Averaging full-resolution images from each coil
Parallel imaging reconstructs a full FOV image using aliased reduced FOV images (which are faster to acquire). The reconstruction works because each coil is closer to certain parts of the body. Spatial sensitivities are measured from low-resolution full FOV images.