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95 Cards in this Set
- Front
- Back
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What is the Larmor frequency for protons at 1 T?
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42 MHz
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What is the number of excess protons in spin-up alignment at 1T?
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4 protons
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What is the Larmor frequency of 19-F and 23-Na at 1 T?
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40 MHz and 11 MHz
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What is the T1 of fat?
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250 ms
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What is the T1 of liver?
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500 ms
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What is the T1 of white matter?
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800 ms
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What is the T1 of grey matter?
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900 ms
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What is the T1 of CSF?
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2400 ms
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What is the T2 of fat?
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80 ms
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What is the T2 of liver?
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45 ms
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What is the T2 of white matter?
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90 ms
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What is the T2 of grey matter?
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100 ms
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What is the T2 of CSF?
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280 ms
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What do contrast agents do to T1?
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Shorten it
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What does magnetic field strength do to T1?
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Increases it
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How much does doubling magnetic field strength affect T1?
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Increases by 2^0.5
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How much of the matrix is acquired with one repetition time TR?
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One line
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How does a long TR affect T1 weighting?
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T1 recovers, so little T1 weighting
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Do tissues with a long or short T1 generate more signal with a short TR?
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short T1 recovers, so more signal
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What are typical TR values for T1 weighted signals at 1.5 T and 3 T?
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300 ms and 450 ms
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Liquids and solids have _____ T1 times while medium-viscosity materials have ____ T1 times.
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long; short
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Short TE values will have ______ T2 weighting.
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minimal (due to no difference in contrast)
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Long TE values will have _____ T2 weighting.
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high
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What is the relationoship between T2, T2*, and spin dephasing due to inhomogeneities (T2_inhomogeneitiy)
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1/T2* = 1/T2 + 1/(T2_inhomogenieity)
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What do paramagnetic and ferromagnetic contrast agents do?
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Shorten T2*
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How can T2* be overcome?
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By generating spin echoes
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Is loss of transverse magnetization due to T2 relaxation reversible?
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No
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What is the earth's magnetic field?
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0.5 G
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What fields are generated by permanent magnets?
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0.35 T
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What fields are generated by resistive magnets?
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0.5 T
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T1 relaxation time, SNR, and RF energy deposition _____ with RF energy deposition.
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increase
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What is the gradient strength on a 1.5 T scanner?
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30 mT/m
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How is a quadrature coil different from a linear coil?
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Recieves signal in both the x and y direction
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What type of coils are used in parallel imaging?
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Phased array
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What materials are used for magnetic shielding?
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Iron plates or steel sheet metal
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What materal is used to provide RF shielding?
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Copper
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When are frequency encode gradients applied?
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During signal detection
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What is the acquisition time to obtain M pixels in teh phase encode direction?
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M * TR
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What is TR and TE in T1 weighting?
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short TR; short TE
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What is TR and TE in T2 weighting?
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long TR; long TE
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What is TR and TE in proton density weighting?
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long TR; short TE
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What do fast spin echo techniques change with each echo?
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phase encoding gradients
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What happens to magnetic field gradients during gradient recalled echo?
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They are reversed
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What is a typical TR in GRE?
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5 ms
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What is the weighting in GRE?
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T2*
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What does FLASH stand for?
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fast low-angle shot
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What does FISP stand for?
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Fast imaging with steady-state precession
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What does GRASS stand for?
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gradient recalled acquision in the steady state
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What is the initial pulse in inversion recovery?
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180 degree pulse
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What happens at the inversion time in inversion recovery sequences?
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90 degree pulse is applied
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What else is the 90-degree pulse known as in inversion recovery?
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readout pulse
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What occurs after the 90 degree pulse in inversion recovery?
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A 180 degree pulse produces an echo at time TE
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What kind of gradients are used in three dimensional imaging?
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Two orthogonal phase encoding gradients and one frequency-encoding gradient
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What kind of RF pulse is used in 3D imaging?
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Nonselective pulse
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What is the imaging time for 3D imaging?
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N1 x N2 x TR
N1 - phase encoding in 1 dimension N2 - phase encoding in the orthogonal plane |
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Short T1 values appear ____ on T1-weighted images.
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bright
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Long T1 values appear _____ on T2-weighted images.
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bright
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What is the tissue difference in proton density weighted images?
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10%
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What is the limiting spatial resolution of clinical MR?
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0.3 lp / mm
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How much does quadrature detection improve MR SNR?
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Sqrt 2
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How does the use of smaller surface coils affect SNR?
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Improves it
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How does decreasing matrix size affect SNR?
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Improves it
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What artifact is caused by slight differences in resonance frequency between water and fat?
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Chemical shift
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What is another name for truncation artifact?
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Gibbs ringing
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What do truncation artifacts simulate in the spinal cord?
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Syrinx
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In what direction do ghost images appear?
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Phase encode direction
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What are two techniques to reduce motion artifacts in body imaging?
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Cardiac respiratory gating
Phase reordering |
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What artifact is caused by undersampling?
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Wraparound
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How can wraparound be avoided?
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By increasing the read samplign rate in the frequency encoding direction
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What do zipper, cetnral point, and RF frield inhomogeneities refer to?
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Artifacts
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What occurs when unsaturated protons enter a slice and generate a greater signal than stationary, partially saturated tissues?
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Flow enhancement
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What is the FDA gradient limit to prevent peripheral nerve stimulation?
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3 T/s
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What produces magneto-phosphenes (light flashes)?
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Time-varying magnetic fields
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What is the measure of dose of RF fields called and what is the unit?
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specific absoprtion rate (SAR); W/kg
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What is the FDA maximum tissue temperature for the head, body, and extremities?
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< 38 C; < 39 C; < 40 C
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What is the maximum rise in core temp?
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1 C
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What is the noise level of MR systems?
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65 to 120 dB
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Is pregnancy a contraindication for scanning the abdomen?
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Yes
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What particles are supramagnetic?
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Fe3O4
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Below what size are Fe3O4 particles superparamagnetic?
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350 A
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What are superparamagnetic crystals used for?
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Imaging liver and reticuloendothelial sytem
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How do positive contrast agents affect T1 and T2?
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Reduce T1 more than T2
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Do positive contrast agents create hyperintensity or hypointensity?
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Hyperintensity
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How do negative contrast agents affect T1 and T2?
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Reduce T2 more than T1
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Do negative contrast agents create hyperintensity or hypointensity?
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Hypointensity
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What contrast is used in MRI mammography?
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Gadolinium
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What are two MRA tehcniques?
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Time of flight
Phase contrast |
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What sequence uses rapidly switching gradients to refocus echos?
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echo planar EPI
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How fast can EPI generate images?
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50 ms
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What thecnique saturates a pool of protons in macromolecules?
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Magnetization transfer
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What are typical voxel sizes for 1H and 31P?
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1 cm3; 8 cm3
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What does brain activity increase locally in functional imaging?
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Venous blood oxygenoation
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What does BOLD stand for?
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blood oxygen level dependent imaging
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What sequences are used in BOLD?
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EPI with T2* weighting
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Which has better temporal and spatial resolution: PET or fMRI?
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fMRI
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