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| Resonant Frequency is determined by... |
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Definition
| isotope being imaged, characteristics of the molecule, strength of the magnetic field (primarily) |
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| Dephasing of protons is caused by... |
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Definition
| non-uniform magnetic field |
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| Averaging the signals from several repeated acquisitions is used to reduce image noise |
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Surface coils generally pick up less of the noise energy from the patients body because they are small compared to the head and body coils.
They pick up a more intense signal because they are closer to the tissue being imaged.
They do not directly affect image detail or slice thickness. |
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Definition
With spin echo imaging, bright blood from flow related enhancement is a characteristics of relatively slow flow.
As the velocity increases to faster flow, the flow-void effect takes over. |
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Definition
T1 weighted = short TE, short TR
T2 weighted = long TE, long TR
PD weighted = short TE, long TR
Short T1 = bright............Long T1 = dark
Tissues with short T1 = bright (fat, intermediate sized molecules)
Tissues with short T2 = dark (solids, large molecules)
Tissues with long T1 = dark (small (water) and large (protein) molecules)
Tissues with long T2 = bright (liquids)
T1 > T2 for most biological tissues |
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Definition
TR = repetition time between pulse sequences
the amount of time that exists between successive pulse sequences applied to the same slice
short TR - T1
long TR - T2 |
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Definition
TE = echo time
represents the time in milliseconds between the application of the 90° pulse and the peak of the echo signal
short TE - T1
long TE - T2 |
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Term
| T1 relaxation ("spin-lattice") |
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Definition
"time for the longitudinal magnetization to return to 63% of its original length"
characterized by the longitudinal return of the net magnetization to its ground state of maximum length in the direction of the main magnetic field
After two T1 times, the magnetization is at 86% of its original length. Three T1 times gives 95%. Spins are considered completely relaxed after 3-5 T1 times.
T1 relaxation is fastest when the motion of the nucleus (rotations and translations or "tumbling rate") matches that of the Larmor frequency. As a result, T1 relaxation is dependent on the main magnetic field strength that specifies the Larmor frequency. Higher magnetic fields are associated with longer T1 times. |
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Term
| T2 relaxation ("spin-spin") |
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Definition
"time required for 63% of the transverse magnetization to decay"
"measure of the time of disappearance of the transverse magnetization"
T2 time is considerably shorter than the T1 time. Ex) CSF, T1=1.9 seconds and T2=0.25 seconds.
Short T2 = BRIGHT
T2* = loss of signal seen with dephasing of individual magnetizations; basically the relaxation of an individual RF echo pulse versus T2 as a whole |
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Definition
pulses that do not refocus T2* decay
As a result, GE sequences demonstrate more susceptibility artifact for ferromagnetic foreign bodies |
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| Signal intensity on MR is determined by? |
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Definition
1. proton density
2. T1 relaxation
3. T2 relaxation
4. flow |
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Definition
time between consecutive 90 degree RF pulses
T1 = short TR
T2 = long TR |
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Definition
time between the initial 90 degree RF pulse and the echo
T1 = short TE
T2 = long TE |
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Definition
Short TE (shorty) - more T1ish
Long TR (longer) - more T2ish |
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Term
| FLAIR (Fluid Attenuated Inversion Recovery) |
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Definition
| Basically T2 w/ CSF suppressed |
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Term
| LARMOR FREQUENCY/RESONANCE |
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Definition
Describes frequency or rate of proton precession
w = yB
w – angular (or precession) frequency (i.e. speed)
y – gyro-magnetic ratio (42.5 for protons)
B – external magnetic field
exposure of an object to RF radiation at the Larmor frequency, causes the net magnetization to spiral away from the Bo field |
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Definition
| over time, anti-parallel protons will "relax" into parallel orientation (parallel to the Bo magnetic field); this causes a cumulative effect leading to a net magnetism; the longitudinal magnetization vector grows exponentially over time and the strength of the vector is determined by the number of protons that align in the parallel orientation which itself is dependent on the tissue type and the strength of the magnetic field |
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Definition
time it takes the longitudinal magnetization vector to grow to 63% of its max value
5 T1s = saturated |
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Definition
amount of RF energy required to equalize parallel and anti-parallel orientation
basically the pulse which equalizes parallel and anti-parallel protons so that the only magnetization is transverse in nature (i.e. protons precess synchronously)
Following RF pulse from copper coil, precession of protons is synchronous and a net transverse magnetization develops |
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Definition
loss of synchronous precession
CSF relaxation time is very long, muscle is not as long |
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time it takes for transverse magnetization vector to diminish to 37% of original length…or put another way the time it takes for transverse vector to diminish 63%; transverse magnetization develops after RF pulse
Observable MR signal from tissue is based on the net transverse vector! |
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| Free Induction Decay (FID) |
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Definition
| diminishing MR signal (i.e. loss of synchronous precession i.e. T2 relaxation) |
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Definition
Signal = Mxy = M0 (1-e-TR/T1) e-TE/T2
Mxy = transverse magnetization vector
M0 = max longitudinal magnetization (contributed by the proton density of the tissue and can't be changed)
TR = repetition time between pulse sequences
T1 = T1 relaxation time; increases with increasing Bo
TE = echo time
T2 = T2 relaxation time; independent of Bo |
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Definition
the size of the smallest detectable detail; in MR this is the size of the VOXEL
Smaller voxels = higher spatial resolution (resolution inversely proportional to voxel)
Voxel = FOV/Matrix size (smaller field of view leads to better resolution) |
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Term
| Small FOVs in MR lead to... |
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Definition
| aliasing (wrap-around) artifact |
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Definition
3D rectangular solid which defines spatial resolution in MR
smaller voxel = higher spatial resolution = sharper image |
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| Voxel size is determined by... |
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Definition
matrix size, FOV, and slice thickness
increasing FOV increases voxel size and decreases resolution
increasing matrix decreases voxel size and increases resolution
increasing slice thickness increases voxel size and decreases resolution |
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Definition
| ST x (FOVphase/MatrixSizePhase) x (FOVRead/MatrixSizeRead) |
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Definition
| determines depth of voxel; directly proportional to voxel volume; increasing slice thickness leads to greater voxel volume which leads to decreased resolution |
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| MR signal is proportional to... |
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Definition
voxel volume
increasing signal leads to increased vox vol which leads to decreased spatial resolution |
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Definition
90o RF pulse >> transverse magnetization >> T2 dephasing (i.e. reduction in transverse mag) >> 180o pulse >> rephrase
After a time TR, sequence is repeated
Only one row of k-space is acquired per each TR period
Less sensitive to magnetic susceptibility and field inhomogeneity; long TR, longer time acq |
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Definition
RF pulse followed by series of refocusing pulses leading to a series of echoes
Each echo fills one row of k-space
T2 signal gradually decreases during echo collection reducing image sharpness (decr SNR)
Useful for when inhomogeneity and mag susceptibility precludes hi-res images by faster sequences |
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Definition
RF pulse less than 90o (creates some transverse mag) >> negative gradient pulse (rapid dephasing of transverse mag) >> positive pulse rephrases transverse mag creating gradient echo @ TE
More sensitive to inhomo field and mag suscept than SE
Shorter TR and fast image time but decreased SNR |
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Definition
| single RF excitation; very short acq times, image contrast T2*; fMRI and DWI |
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Definition
starts with 180o RF pulse which inverts tissue magnetization to prepare image contrast
During inversion time, signal is formed by T1 relaxation; thus TI can be used to control T1 contrast of tissues
After TI, standard spin echo sequence performed
Allows suppression of tissue types by manually reducing magnetization of that tissue (this reduces the signal given off from that tissue)
Least susceptible to mag field inhomo and mag suscept
STIR: short tau inversion recovery – FAT (short T1) suppressed as a result of short TI
FLAIR: fluid attenuated inversion recovery – CSF (long T2) suppressed as a result of long TI |
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Term
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Definition
TR x Np x NEX
TR = repetition time (time it takes to complete one acq cycle)
Np = number of phase encodes (part of matrix size which determines how many times acq cycle must be repeated)
NEX = number of excitations or averages |
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Term
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Definition
Magnetic susceptibility describes the behavior of material when placed in external mag field
Diamagnetic, Para, Ferro are classes of materials by susceptibility
Ferromagnetic substances develop strong internal magnetic field parallel to direction of external field which dramatically increases it
Susceptibility artifacts observed where magnetic susceptibility varies rapidly (such as nasal cavity) and result from the presence of metal objects
Commonly in T2-weighted, GE, and Echo planar (DWI) sequences
Appears as image distortion w/ warped anatomy
Appears as signal voids, surrounded by bright rim |
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Definition
Tissue excited outside FOV is recorded artificially w/in FOV
occurs when FOV is smaller than the surrounding tissue |
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Definition
Duplicated copies of anatomic structures with a reduced intensity
may appears as a blurring or motion
Caused by physiologic and/or patient motion or pulsatile flow (aorta) |
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Term
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Definition
Multiple, alternating bright/dark lines (ringing)
Fourier-based imaging technique leads to image errors near sharp boundaries
Typically involved w/ bright signal boundaries such as spinal cord b/t CSF/tissue, other T2 fluid images, or T1 fat |
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Term
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Definition
shifting of fat in one direction 2/2 misregistration of fat and water
TYPE 1 - fat/water interfaces separate on images - bright band is where fat and water are seen to overlap while dark band is where fat and water are separate
TYPE 2 ("India ink") - boundaries of tissues with large differences in fat/water content are outlined in black - THIS IS OUT OF PHASE IMAGING |
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Definition
caused by detection of extraneous RF signals; usually malfunctioning or incompatible medical equipment (syringe pumps, open door to magnet)
thin stippled/dashed band through the image |
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