Module 1 Flashcards
Basic RF coil
typically single coil loop using singel receiver channel
linear coil
coil set 90 relative to each other
able to receive the MR signal in 2 planes
effect is double the signal and double the noise
noise cancels itself
net result 40% increase in SNR
net 10% increase in SNR from flat coils
quadrature coil
multi coil
series of loops arranged in order to increase coverage or increase SNR over same area
needs RF amplifiers and receivers on system
phased array coils
arranged in coil pairs 90 relative to each other
multi coil
series of loops arranged in order to increase coverage or increase SNR over same area
needs RF amplifiers and receivers on system
Quadrature phased array coils
ability to send RF pulses as well as collect the MR signal
send and receive coils
ability to collect MR signal only
receive only coils
cervical
thoracic
lumbar
full spine
carotid MRA
anterior and soft tissue neck
phased array C/T/L coil
coil for
knee
ankle
foot
phased array extremity coil
amplitude of signal received by coil to the amplitude of the noise
SNR
voltage induced in coil
signal
constant value dependant on the area under exam and electrical background of the system
noise
SE & FSE sequences
long TR & short TE
90 flip angle
increase SNR
well tuned coils
coarse matrix
large FOV
thick slices
increase SNR
narrow bandwidth
high order signal averages
increase SNR
NEX
NSA
increase SNR
difference in the SNR between two adjacent structures
CNR
Contrast Noise Ratio
admin of contrast agents
t2 sequences with fat sat
STIR (tissue suppression)
FLAIR (fluid attenuated Inv Recovery)
sequences that enhance flow (time of flight)
increase CNR
control spatial resolution (voxel size)
thin slices
fine matrices
small FOV
increase CNR
256x128
course matrix
256x256
medium matrix
512x512
fine matrix
1024x1024
very fine matrix
18 cm or less
small FOV
19-29 cm
medium FOV
30-48 cm
large FOV
determined by region of interest (ROI)
FOV determining factor
1-4 mm
thin slice
5-6 mm
medium slice gap
8 mm or more
large slice
industry standards
Reading Rad’s preferences/protocols
slice thickness determining factor
increase SNR
increase slice per acquisition
decreased T1 weighting
increase scan time
TR increased
decreased scan time
increased T1 weighting
decreased SNR
decreased slices per acq
TR decrease
increased T2 weighting
decreased SNR
TE increase
increase SNR
decrease T2 weighting
TE decrease
increased SNR all tissue
reduce motion artifact (signal averaging)
direct proportional increase in scan time
NEX (NSA) increase
direct proportional decrease in scan time
decrease in SNR all tissues
increased motion artifact
NEX (NSA) decrease
increase SNR all tissues
increase coverage of anatomy
decrease spatial resolution and partial voluming
slice thickness increase
increase spatial resolution and reduce partial voluming
slice thickness decrease
increase SNR
increase coverage of anatomy
decrease spatial resolution
FOV increased
decreased SNR
decrease coverage of anatomy
increase likelihood of aliasing
FOV decreased
increase spatial resolution
decrease SNR
increase scan time
matrix increased
increase SNR
decrease scan time
decrease spatial resolution
matrix decreased
decrease minimum TE
decrease in chemical shift
decreased SNR
Rcve bandwidth increased
increase SNR
increase minimum TE
increase chemical shift
decrease bandwidth
90/180 re-phasing
SE
90/multiple 180’s (ETL)
FSE
180/90/180 pulse sequence
IR/FSE-IR
inversion recovery
STIR
FLAIR T1 & T2
use inversion recovery pulses
use variable flip angle followed by gradiaent rephasing to produce GRE
coherent gradiant echo T2
steady state sequence that uses very short TR for rapid acquisition times and large flip angles to increase SNR
balanced gradient echo T2
use variable flip angle and gradient rephasing resulting in a GRE
commonly used in steady state so residual magnetization builds up in the transverse plane
incoherent gradient echo T2
spoiled GRE
steady state sequence uses medium flip angles and short TR to maintain the steady state pulse so residual magnetization builds up in the transverse plane
steady state free precession T2
very fast sequences such as EPI
best for moving structures
real time imaging
rapid acquistion of images either after contrast or to observe movement
dynamic imaging
rap[id technique that acquires images of the brain during activity or stimulus and at rest
functional imaging
demonstrates areas with restricted diffusion of extracellular water such as infarcted tissue
diffusion weighted imaging
DWI
refers to microscopic changes in perfusion when gadolinium first passes through the cappillary bed
perfusion imaging
usually uses an incoherent (spoiled) GRE sequence in conjunction with TR and flip angle combination that saturate background tissue but allow moving spins to enter the slice fresh and return a high signal
TOF
12-20 sec breath holds
used for long vessels
ceMRA
fat
haemangioma
intra osseous ipoma
radiatino change
degen fatty deposit
methaeglobin
cysts w/proteinaceous fluid
paramagnetic contrast agent
slow flowing blood
high T1
CSF
synovial fluid
hemangioma
infection
inflammation
oedema
some tumors
hemorrhage
slow flowing blood
cysts
high T2
cortical bone
AVN
infarction
infection
tumors
sclerosis
cysts
calcification
low T1
cortical bone
bone islands
deoxyhemoglobin
hemosiderin
calcification
T2 paramagnetic agents
low T2
air
fast flowing blood
ligaments
tendons
cortical bone
scar tissue
calcificatoin
low to no T1 and T2
TOF
entry slice
intra voxel dephasing
most common flow phenomena
occurs when nuclei move through the slice may receive only one of the RF pulses applied.
Time of flight phenomena
depends on excitation history of nuclei flowing within a vessel
largely controlled by the direction of flow relative to slice excitation
entry slice phenomena
cuased by presence of gradients that either accelerate or decellerate flowing nuclei as they move from areas of differeing field strenght along the gradient
intra voxel dephasing
spatial presaturation pulses
GMN (FC)
main flow artifact remedies
nullifies signal from nuclei that produce unwanted signal or artefact by applying a 90 RF pulse to selected tissue before the pulse sequence begins
spatial presaturation
produces low signal from flowing nuclei
reduces motion and aliasing if bands are placed over signal producing anatomy
increases the specific absorptin rate (SAR) and may reduce slice number available per TR
mainly reduces TOF and entry slice phenomena
spatial presaturation
utilizes extra gradients to rephase the magnetic momnets of flowing nuclei so that they have a similar phase to their stationary counterparts
GMN
FC
produces high signal from flowing nuclei
increases the minu=imum TE and may reduce slice number available
maine reduces intra-voxel dephasing
gradient moment nulling
GMN
P
Atrial contraction
QRS somplex
contraction of ventricles
T
relaxation of ventricles
represents original pace making pulse imput from the SA node
no more than 3mm high .12 sec in duration
P wave
time between onset of P wave and QRS complex
.12 to .20 sec
PR interval
depolarization (contraction) of ventricles
.08-.11sec in duration
QRS complex
time between the depolarization and beginning of repolarization of ventricles
ST segment
recovery phase after ventricular contraction
flowing blood in body make an artifact in the ECG that obscures normal wave when patient is inside the magnet
T wave
1 R to R interbal
T1 weighting
2-3 R to R intervals
Pd/T2 weighting
15-20% of R to R interval
trigger window
min allowed for maximun # of slices
trigger delay
breath holding
navigators
respiratory
respiraory compensation
forms of respiratory compensation and gating
9 types
gadolinium
strongly paramagnetic
positive contrast agents
historically iron oxide
super para-magnetic
negative contrast agents
