chap 2 Flashcards
low signal on ADC, high signal on extracellular space is small
restricted diffusion
high ADC, extracellular space is large
normal diffusion
T/F: the T1 relaxation times, although inherent to that tissue, are dependent on the field strength of the magnet
true
typical value for a T1 sequence
TR < 800
typical value for a T2 sequence
TR > 2000
how well it can be imaged or determined positive
sensitivity
how well it can distinguish those areas with disease from those without
specificity
effect of substances on relaxation rate
relaxivity
naturally toxic, chelated for excretion, has 7 unpaired electrons
gadolinium
the RF excitation is turned off, H return to the low-energy state. Magnetic moments dephase
relaxation
movement of molecules in extracellular space due to random thermal motion
diffusion
images where contrast is determined by the ADC
DWI
controls how much ADC influences contrast. Function of the amplitude, duration and interval of the gradients in ADC
b-value
uses the magnetic susceptibility b/t tissues to generate image contrast
susceptibility weighting (SWI)
net displacement of molecules diffusing across an area of tissue per second
apparent diffusion coefficient (ADC)
images function/physiology. Relies on BOLD to produce a signal in areas of the brain where there is increased activity
fMRI
effect of a substance on the relaxation rate
relaxivity
inherent to the body’s tissues. Cannot be changed. T1 recovery time, T2 decay time, PD, flow, ADC
intrinsic
Factors set by the technologist, can be changed (TR,TE, flip angle, TI, ETL, b-value)
extrinsic
how would a lesion look on a T1 after gadolinium injection
create bright lesions
how would a lesion look on a T2 after a gadolinium injection
create dark lesions
what series is usually performed after contrast injection
T1
lowering your flip angle also lowers your
SNR
short TR , short TE , large FA
T1
long TR, long TE, small FA
T2
long TR, short TE, small FA
PD
relatively large flip angles (90+) produce
T1 contrast
relatively low flip angle values (45-90) produce
PD & T2 contrast
slow molecular tumbling rate. Closely packed molecules. Short T1 & short T2
fat
fast molecular tumbling rate. Molecules are spaced apart. Long T1 & T2
water
number of mobile hydrogen protons per unit volume of tissue determines contrast. Basic MRI contrast
proton density
controls the amount of T1 weighting
TR
controls the amount of T2 weighting
TE
for a T1 weighted image, the TR must be
short
for a T2 weighted image, the TE must be
long
caused by nuclei giving up energy to surrounding environment or lattice. Recovery of 63% longitudinal magnetization. Aka: spin lattice
T1 recovery
magnetic fields of neighboring nuclei interacting with each other. Results of 63% loss (37% remains) of transverse magnetization. Aka spin spin
T2 decay
the decay of the FID following the RF excitation pulse based on inhomogeneities
T2*
describe T1 recovery
- spin lattice relaxation
- caused by the nuclei giving up their energy to the surrounding environment or lattice
what is the T2 relaxation time of tissue?
the time is takes 63% of the transverse magnetization to decay in the tissue
what factors do T1 & T2 relaxation depend on?
- inherent energy of the tissue
- how closely packed the molecules are
a hyperintense area on your post contrast imaging, where your pre contrast image showed no enhancement, would most likely represent what
pathology
bright fat, dark water
T1
bright water, dark fat
T2
high proton density is bright, low proton density is dark. Basic MRI contrast
PD
gray on the image
intermediate signal
hypointense, black on the image. Small or no transverse component at TE
low signal
hyperintense, white on the image. Large transverse component of coherent magnetization at TE
high signal
images obtain contrast mainly through the mechanisms of
T1 recovery
T2 decay
Proton or spin density
T2 contrast depends predominantly on
T2 times between fat & water
why is T2 weighting long?
give fat and water time to decay
what would happen if the T2 weighting was too short?
no difference in the tissues demonstrated
used for anatomy and pathology post contrast
T1
used for pathology
T2
T/F: the TE would be shorter to take advantage of T2*
true
gadolinium raw element is
ferromagnetic
gadolinium chelate contrast media is
paramagnetic
T1 contrast depends predominantly on
the differences in the T1 times between fat & water
why is the T1 weighting short
neither fat nor water have sufficient time to return to B0
T2 graph
starts high ends low
T1 graph
starts low ends high
what are inhomogeneities
areas that do not match the external field strength
