EXAM #4 — MODULE 4 Flashcards
A. Before RF pulse:
- Net of protons exist in _____ orientation to outside magnetic field (lower state of energy - walking on feet)
a. This creates net longitudinal magnetism along __ axis (upright vector-walking on feet)
A. Before RF pulse:
- Net of protons exist in parallel orientation to outside magnetic field (lower state of energy - walking on feet)
a. This creates net longitudinal magnetism along Z axis (upright vector-walking on feet)
A. Before RF pulse:
- 90° RF pulse introduced - 2 effects (pg. 38):
a. just right strength (pulse frequency) and duration to flip some longitudinal vectors into _____ direction such that # _____ vectors = # _____ vectors
Longitudinal magnetic vector is _____ (goes to __)
b. parallel and antiparallel magnetic vectors gain _____ _____ (precess in _____); just right strength and duration to flip longitudinal vectors into _____ plane
_____ magnetic vector is created (fig 25, pg 39)
A. Before RF pulse:
- 90° RF pulse introduced - 2 effects (pg. 38):
a. just right strength (pulse frequency) and duration to flip some longitudinal vectors into antiparallel direction such that # antiparallel vectors = # parallel vectors
Longitudinal magnetic vector is neutralized (goes to 0)
b. parallel and antiparallel magnetic vectors gain phase coherence (precess in phase); just right strength and duration to flip longitudinal vectors into transverse plane
Transverse magnetic vector is created (fig 25, pg 39)
NET EFFECT: 90° RF PULSE FLIPS NET MAGNETIC VECTOR FROM LONGITUDINAL TO TRANSVERSE AXES
(180° RF pulses also exist)
- 90° RF pulse switched off - 2 effects (pgs. 40 - 41):
a. increasingly more longitudinal vectors return from _____ (on hands) to _____ (on feet) during __ relaxation - causes regeneration of _____ magnetic vector (fig 26)
b. protons precess increasingly out of phase during __ relaxation - Causes degeneration of _____ magnetic vector (fig 26)
NET EFFECT: 90° RF PULSE FLIPS NET MAGNETIC VECTOR FROM LONGITUDINAL TO TRANSVERSE AXES
(180° RF pulses also exist)
- 90° RF pulse switched off - 2 effects (pgs. 40 - 41):
a. increasingly more longitudinal vectors return from antiparallel (on hands) to parallel (on feet) during T1 relaxation - causes regeneration of longitudinal magnetic vector (fig 26)
b. protons precess increasingly out of phase during T2 relaxation - Causes degeneration of transverse magnetic vector (fig 26)
NET EFFECT OF SWITCHING OFF 90° RF PULSE: 90° FLIP OF NET MAGNETIC VECTOR FROM _____ TO _____ AXES
NOTE: LONGITUDINAL REGENERATION AND TRANSVERSE DEGENERATION OCCUR _____ TO, BUT _____ OF ONE ANOTHER.
NET EFFECT OF SWITCHING OFF 90° RF PULSE: 90° FLIP OF NET MAGNETIC VECTOR FROM TRANSVERSE TO LONGITUDINAL AXES
NOTE: LONGITUDINAL REGENERATION AND TRANSVERSE DEGENERATION OCCUR SIMULTANEOUSLY TO, BUT INDEPENDENTLY OF ONE ANOTHER.
- Longitudinal and transverse magnetic vectors add up to a sum magnetic vector
a. this sum vector exists “between” the _____ and _____ magnetic vectors
b. the sum magnetic vector precesses along with the _____ and _____ vector components and spirals upward from the _____ to the _____ axes, forming a “spiral beehive” (fig 27)
c. a changing (moving) magnetic force can induce an _____ _____ in a nearby antenna (figs 28-29) - an antenna is placed at the peripheral edge of the __ plane
- the sum magnetic vector precesses _____ the antenna and _____ from it, creating a _____ in the antenna which peaks and valleys with the frequency of the sum magnetic vector’s _____ _____
- Longitudinal and transverse magnetic vectors add up to a sum magnetic vector
a. this sum vector exists “between” the longitudinal and transverse magnetic vectors
b. the sum magnetic vector precesses along with the longitudinal and transverse vector components and spirals upward from the transverse to the longitudinal axes, forming a “spiral beehive” (fig 27)
c. a changing (moving) magnetic force can induce an electrical signal in a nearby antenna (figs 28-29) - an antenna is placed at the peripheral edge of the x-y plane
- the sum magnetic vector precesses toward the antenna and away from it, creating a signal in the antenna which peaks and valleys with the frequency of the sum magnetic vector’s precessing frequency
- Longitudinal and transverse magnetic vectors add up to a sum magnetic vector
THIS IS THE _____ _____; THE AMOUNT OF NET MAGNETIZATION IN THE _____ PLANE GENERATES THE _____ _____; THE SIGNAL’S FREQUENCY EQUALS THE _____ FREQUENCY
- the peaks and valleys of the signal become less and less intense as the sum magnetic vector spirals _____ away from the _____ plane into the _____ plane (peaks and valleys of the signal’s sine wave become increasingly shallow)
- this decreasing signal is called a _____
- Longitudinal and transverse magnetic vectors add up to a sum magnetic vector
THIS IS THE MRI SIGNAL; THE AMOUNT OF NET MAGNETIZATION IN THE TRANSVERSE PLANE GENERATES THE MR SIGNAL; THE SIGNAL’S FREQUENCY EQUALS THE PRECESSING FREQUENCY
- the peaks and valleys of the signal become less and less intense as the sum magnetic vector spirals upward away from the transverse (x,y) plane into the longitudinal (z) plane (peaks and valleys of the signal’s sine wave become increasingly shallow)
- this decreasing signal is called a “free induction decay (FID) signal”
- TR = _____
a. Time lapse between 1st and 2nd, 2nd and 3rd, and subsequent 90° RF pulses; time from one _____ to _____ administered to same population of protons
b. TR controls how much _____ due to __ relaxation is seen between tissues
c. Short TR: _____ msec; difference in signal intensity due to differences in __ relaxation among various tissue types is more pronounced with short TR (contrast is more __ induced with short TR)
d. Long TR: _____ msec; less difference in signal intensity due to differences in __ relaxation among various tissue types with long TR (contrast is less __ induced with long TR)
- TR = time to repeat
a. Time lapse between 1st and 2nd, 2nd and 3rd, and subsequent 90° RF pulses; time from one 90° pulse to another administered to same population of protons
b. TR controls how much contrast due to T1 relaxation is seen between tissues
c. Short TR: < 500 msec; difference in signal intensity due to differences in T1 relaxation among various tissue types is more pronounced with short TR (contrast is more T1 induced with short TR)
d. Long TR: > 1,500 msec; less difference in signal intensity due to differences in T1 relaxation among various tissue types with long TR (contrast is less T1 induced with long TR)
- TE = _____
a. Time between _____ and the _____
b. Determined by the _____
c. TE controls how much _____ due to __ relaxation is seen between tissues
d. Short TE: _____ msec
e. Long TE: _____ msec
f. Short TE: strong signals from _____; not much _____…; not much _____… due to differing __ relaxation times among varying tissues (contrast is less __ induced with short TE) (fig 37, pg 56)
g. Long TE: transverse magnetization degeneration among various tissues varies _____ over time, creating _____ differences in signal intensity and thus _____ MR image contrast due to differences in __ relaxation among varying tissues with long TE (contrast is more _ induced with long TE)
(Note: very _____ TE causes smaller signal-to-noise ratio - grainy image results since we lose quantity of signal)
- TE = time to echo
a. Time between 90° pulse and the signal
b. Determined by the technologist
c. TE controls how much contrast due to T2 relaxation is seen between tissues
d. Short TE: < 30 msec
e. Long TE: > 80 msec
f. Short TE: strong signals from all tissues; not much transverse magnetic degeneration; not much difference in signal intensity (contrast) due to differing T2 relaxation times among varying tissues (contrast is less T2 induced with short TE) (fig 37, pg 56)
g. Long TE: transverse magnetization degeneration among various tissues varies moreso over time, creating larger differences in signal intensity and thus higher MR image contrast due to differences in T2 relaxation among varying tissues with long TE (contrast is more T2 induced with long TE)
(Note: very long TE causes smaller signal-to-noise ratio - grainy image results since we lose quantity of signal)
- Signal differentiation between differing tissues (figs 30a and 30b)
a. Differing tissues possess different __ and __ relaxation times
Different lengths of time for _____ regeneration and _____ degradation
b. If _____ TR is used, complete longitudinal regeneration and partial (varying) transverse degradation will have occurred for all tissue types
The 2nd RF pulse will flip longitudinal magnetic vectors of equal length into the transverse plane, signals from various tissues will be of _____ intensity (equal vector length causes equal _____ intensity)
c. If _____ TR is used, some tissues will have experienced complete _____ degradation and _____ regeneration (tissue A) while others will have experienced various stages of partial _____ degradation and _____ regeneration (tissue B)
The 2nd RF pulse will flip _____ magnetic vectors of various lengths into the _____ plane, signals from various tissues will be of differing intensities
- Signal differentiation between differing tissues (figs 30a and 30b)
a. Differing tissues possess different T1 and T2 relaxation times
Different lengths of time for longitudinal regeneration and transverse degradation
b. If long TR is used, complete longitudinal regeneration and partial (varying) transverse degradation will have occurred for all tissue types
The 2nd RF pulse will flip longitudinal magnetic vectors of equal length into the transverse plane, signals from various tissues will be of equal intensity (equal vector length causes equal signal intensity)
c. If short TR is used, some tissues will have experienced complete transversal degradation and longitudinal regeneration (tissue A) while others will have experienced various stages of partial transversal degradation and longitudinal regeneration (tissue B)
The 2nd RF pulse will flip longitudinal magnetic vectors of various lengths into the transverse plane, signals from various tissues will be of differing intensities
*WITH SHORT TR: DIFFERING SIGNAL INTENSITIES FROM DIFFERENT TISSUES ARE CAUSED BY THEIR DIFFERENT __ RELAXATION TIMES; THIS RESULTS IN DIFFERING “LENGTHS” OF REGENERATED LONGITUDINAL MAGNETIZATION AMONG THE TISSUES FOLLOWING SHORT TR; 2nd RF PULSE FLIPS THESE INTO TRANSVERSAL MAGNETIZATION VECTORS OF VARIOUS LENGTHS, CREATING DIFFERENT SIGNAL INTENSITIES FROM DIFFERENT TISSUES AND CONTRAST ON THE MR IMAGE
BY CHANGING TR (TIME BETWEEN SUCCESSIVE RF PULSES), TRANSVERSE MAGNETIZATION LENGTHS AND SIGNAL INTENSITIES FROM DIFFERENT TISSUES CAN BE MODIFIED
DIFFERING SIGNAL INTENSITIES FROM DIFFERING TISSUES RESULT IN DIFFERENT RECORDED _____ _____ ON THE MR IMAGE
*WITH SHORT TR: DIFFERING SIGNAL INTENSITIES FROM DIFFERENT TISSUES ARE CAUSED BY THEIR DIFFERENT T1 RELAXATION TIMES; THIS RESULTS IN DIFFERING “LENGTHS” OF REGENERATED LONGITUDINAL MAGNETIZATION AMONG THE TISSUES FOLLOWING SHORT TR; 2nd RF PULSE FLIPS THESE INTO TRANSVERSAL MAGNETIZATION VECTORS OF VARIOUS LENGTHS, CREATING DIFFERENT SIGNAL INTENSITIES FROM DIFFERENT TISSUES AND CONTRAST ON THE MR IMAGE
BY CHANGING TR (TIME BETWEEN SUCCESSIVE RF PULSES), TRANSVERSE MAGNETIZATION LENGTHS AND SIGNAL INTENSITIES FROM DIFFERENT TISSUES CAN BE MODIFIED
DIFFERING SIGNAL INTENSITIES FROM DIFFERING TISSUES RESULT IN DIFFERENT RECORDED TISSUE DENSITIES ON THE MR IMAGE
[Pulse sequence]
90° excitation RF pulse (transmit coil):
Degeneration of __ magnetization
Generation of __ magnetization
90° RF pulse off:
__ magn starts to regenerate
__ magn starts to degenerate
180° rephasing RF pulse
_____ dephasing H+ vectors in transverse plane
_____ generated by 180° rephasing pulse
Signal collected by _____ coil
[Pulse sequence ]
90° excitation RF pulse (transmit coil):
Degeneration of Z magnetization
Generation of XY magnetization
90° RF pulse off:
Z magn starts to regenerate
XY magn starts to degenerate
180° rephasing RF pulse
Rephases dephasing H+ vectors in transverse plane
Echo generated by 180° rephasing pulse
Signal collected by receive coil
TR = _____…
TE = _____…
TR = time to repeat (from the excitation pulse)
TE = time to echo (from the excitation pulse)
[Dual Echo Spin Echo Pulse Sequence ]
180° RF rephasing pulse: counter-acts _____
- Effectively overrides external _____
Does not effectively override _____ _____
The 180° rephasing pulse does not rephase each echo _____
The second echo’s amplitude (signal intensity) is _____ that the previous echo
[Dual Echo Spin Echo Pulse Sequence]
180° RF rephasing pulse: counter-acts dephasers
- Effectively overrides external dephasers
Does not effectively override internal dephasers
The 180° rephasing pulse does not rephase each echo 100%
The second echo’s amplitude (signal intensity) is less that the previous echo
