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60 Cards in this Set
- Front
- Back
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What are the gradient-related artifacts?
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Eddy currents
Non-linearity Geometric distortion |
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What happens if signal is undersampled?
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Signal gets spatially mismapped
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Why does an aliasing artifact occur in the frequency (readout) direction?
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Too high frequencies (outside BW) are mapped into the lower frequencies
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How does NFW eliminate frequency wrap?
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frequency oversampling
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How is NFW accomplished?
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Doubling the FOV and Nx in the read direction while reducing the sampling interval delta Ts. keeps sample time fixed. Extra FOV higher frequencies are filtered out. No SNR or resolution penalty.
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How does NPW work?
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Double the FOV and Ny while reducing NEX. Excess FOV is discarded. Maintains SNR spatial resolution and sacan time.
Increased motion artifacts. |
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What is a saturation pulse?
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A band placed outside the FOV which nulls the signal from those areas preventing aliasing.
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What are the remedies for aliasing?
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NFW
NPW Sat pulses Discard end slices from 3D volume |
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What does chemical shift come from?
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H protons are situated in different chemical substances.
Electrons create different mag fields about the protons. Protons precess at different frequencies |
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What is the difference in fat and water precession frequencies in ppm and Hz?
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3.5ppm
220Hz @ 1.5 T |
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What happens to chemical shift as static field increases?
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decreases
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Where can chemical shift occur?
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Orbits
Vertebral endplates Abdomen Anywhere fat and water are close together |
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What is chemical shift of the first kind?
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Shift along the frequency encoding axis?
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What is the SI of chemical shift at lower frequencies
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Bright
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What T2W sequence reduces chemical shift?
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SE
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In spine studies, which direction is water shifted? Fat?
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Water-up
Fat-down |
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What will the SI of the lower vertebral endplate be compared to the upper?
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Low appears dark
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What factors increase chemical shift?
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Stronger static field
Decreasing receiver BW Smaller pixels |
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What can be done to decrease chemical shift?
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Use FATSAT or STIR
Increase pixel size Lower static field Increase BW Switch phase and frequency Use longer TE |
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Why does chemical shift of the 2nd kind occur?
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Fat and water transverse magnetization will be periodically in and out of phase with each other.
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Why does chemical shift of the 2nd kind occur?
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Fat and water transverse magnetization will be periodically in and out of phase with each other.
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In what direction does chemical shift occur?
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Phase and frequency
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What is the boundary effect?
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When the TE is selected to be out of phase then a dark boundary will be seen around organs surrounded by fat.
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Where does the boundary effect occur?
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Kidneys
Muscles |
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What are remedies for chemical shift of the 2nd kind?
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Use SE or FSE
Select in phase TE Use FATSAT |
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At what type of contrast location will a truncation artifact occur?
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High
Spinal cord/CSF Skull/brain Knee meniscus/fluid |
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In what direction does the truncation appear?
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Phase because the phase matrix is usually smaller than the frequency matrix but it can occur in either direction
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What causes truncation?
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Undersampling of data
FT has little ripples |
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How is truncation remedied?
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Increase phase matrix Ny
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What is partial voluming?
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If the slice thickness delta z is too large, then more htan one tissue will exist in a voxel.
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How is the partial voluming artifact remedied?
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Decrease slice thickness z
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What direction will motiona artifacts show up?
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Phase
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Why does motion show up in the phase direction?
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A different amount of accumulated precession phase than expected.
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Why does ghosting occur?
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Phase mismapping
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Why does phase mismapping occur?
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Structures are moving while in a magnetic field gradient. Structures experience different field values - mismapped
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How is ghosting remedied?
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Sat bands
Increase NEX Swap phase and frequency Cardiac gating Flow compensation (GMN) |
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How does respiratory compensation minimize motion artifacts?
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by ordering phase encoding steps according to the respiratory cycle.
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How are CSF flow artificats remedied?
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Flow compensation
Cardiac gating |
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What are examples of common CSF artifacts?
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pseudo anuerysm of basilar artery
pseudo MS plaques in brain stem pseudo disc herniation |
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On which weightings will the magic angle artifact occur?
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T1W and PD
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How can cross-talk be remedied?
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Gaps - 30% - Missing anatomy
Interleave - Increased scan time 3D acquistion |
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What is a RF noise zipper artifact?
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sequence of stripes of alternating dark and bright spots appears along the phase direction at various frequency values
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What is a RF feed-through artifact?
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When the RF excitation pulse continues too long
Occurs in the phase direction but at zero frequency |
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What causes a RF FID zipper artifact?
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Overlap of the FID and the 180 refocusing pulse.
Occurs along the frequency direction at zero phase |
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How is the RF zipper artifact remedied?
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Increase TE
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What causes a shading artifact?
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RF excitation pulse does not excite H protons evenly.
B1 varies so flip angles are not uniform |
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What causes a shading artifact?
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Abnormally loading an RF coil.
Non-uniform sensitivity of a surface coil |
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What causes external magnetic field artifacts?
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Improper shimming
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What sequences reduce magnetic field inhomogeneities?
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SE or FSE with 180 refocusing pulses
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What are the consequences of using FATSAT with magnetic field inhomogeneities
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wrong tissues receiving the spectral sat pulses
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What are the consequences of using GRE with magnetic field inhomogeneities
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Moire
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Where do magnetic susceptibility artifacts occur?
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Interfaces of differing magnetic susceptibility
Paranasal sinuses Skull base sella Distortions in magnetic field lead to dephasing of spins - signal loss and mismapping |
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What is diametnetism?
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Substances with paired electron spins - most tissues - esentially non-magnetic
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What is paramagnetism?
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x>0 unpaired electron spins. Gd, Dy
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What is the order of pulse sequences and magnetic susceptibility
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GRE - most sensitive - good for hemorrhage
CSE - Has a 180 refocusing FSE - Least sensitive -several 180's |
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How is magnetic susceptibility sensitivity minimized?
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Remove all metal
Use FSE or CSE (not sensitive to hemorrhage) |
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What are eddy currents?
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Small electric currents generated by rapid gradient switching on/off
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What is the result of eddy currents?
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Distorted gradient profile leading to distortion
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What does errors in k-space lead to?
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criss-cross striations
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How are errors in data k-space remedied?
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Delete the error and average data about error location
Repeat the sequence |
