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Pulse Timing parameters Pulse Timing parameters & Weighting& Weighting
Pulse timing parametersPulse timing parameters
V.G.Wimalasena
Principal
School of Radiography
Summary of the previous lessonSummary of the previous lessonThe NMV (Net Magnetic Vector) is a vector The NMV (Net Magnetic Vector) is a vector quantity.quantity.
It is created by two components at 90It is created by two components at 9000 to each to each other. i.e. magnetization in the longitudinal plane other. i.e. magnetization in the longitudinal plane & magnetization in the transverse plane.& magnetization in the transverse plane.
Before resonance, there is full longitudinal Before resonance, there is full longitudinal magnetization parallel to Bmagnetization parallel to B00
After application of RF pulse, the NMV is flipped After application of RF pulse, the NMV is flipped fully into the transverse plane (assuming fully into the transverse plane (assuming sufficient energy is applied) sufficient energy is applied)
There is now full transverse magnetization There is now full transverse magnetization and zero longitudinal magnetization.and zero longitudinal magnetization.Once the RF pulse is removed, the NMV Once the RF pulse is removed, the NMV recovers.recovers.As this occurs, the longitudinal component As this occurs, the longitudinal component of magnetization grows again, while the of magnetization grows again, while the transverse magnetization decreases.transverse magnetization decreases.As the received signal is related to the As the received signal is related to the magnitude of the transverse component, magnitude of the transverse component, the signal in the coil decays as relaxation the signal in the coil decays as relaxation takes place.takes place.
Pulse timing parametersPulse timing parameters
The magnitude and timing of the RF The magnitude and timing of the RF pulses form the basis of MRIpulses form the basis of MRI
The two Pulse timing parameters areThe two Pulse timing parameters are Pulse Repetition time - TRPulse Repetition time - TR Echo time - TEEcho time - TE
Pulse sequencePulse sequenceA very simplified pulse sequence is a A very simplified pulse sequence is a combination of RF pulses, signals and combination of RF pulses, signals and intervening periods of recovery as shown. intervening periods of recovery as shown.
RF PulseRF Pulse RF Pulse
TR TR
TE TE TE
A pulse sequence consists of several A pulse sequence consists of several componentscomponents..
1. The repetition time (TR):1. The repetition time (TR):This is the time from the application of one This is the time from the application of one RF pulse to the application of the next RF RF pulse to the application of the next RF pulse and is measured in milliseconds pulse and is measured in milliseconds (ms).(ms).The The TRTR determines the amount of determines the amount of relaxationrelaxation that is allowed to occur between that is allowed to occur between the end of one RF pulse and the application the end of one RF pulse and the application of the next. of the next. Therefore Therefore TRTR determines the amount of determines the amount of T1 T1 relaxationrelaxation that has occurred that has occurred
2. The Echo time (TE)2. The Echo time (TE)
(TE) is the time from the application of the (TE) is the time from the application of the RF pulse to the Peak of the signal induced RF pulse to the Peak of the signal induced in the coil and is also measured in ms.in the coil and is also measured in ms.
The TE determines how much The TE determines how much decaydecay of of transverse magnetizationtransverse magnetization is allowed to is allowed to occur before the signal is read.occur before the signal is read.
Therefore TE controls the amount of T2 Therefore TE controls the amount of T2 relaxation that has occurredrelaxation that has occurred
Contrast of MR imagesContrast of MR images
The The application of RF pulsesapplication of RF pulses at certain at certain repetition times (repetition times (TRTR) ) and and The The receiving of signalsreceiving of signals at pre-defined at pre-defined echo times (echo times (TETE) produces contrast in MRI ) produces contrast in MRI images.images.
Image Weighting & Image Weighting & ContrastContrast
Contrast?Contrast?T1 weighted ?T1 weighted ?T2 weighted ?T2 weighted ?
Proton density ?Proton density ?
Contrast ?Contrast ?
o An image has contrast if there are areas of An image has contrast if there are areas of high signalhigh signal ( (whitewhite on the image) as well as on the image) as well as areas of areas of low signallow signal ( (darkdark on the image). on the image).
o Some areas have an intermediate signal Some areas have an intermediate signal (shades of grey in-between white and (shades of grey in-between white and black))black))
The NMV & signal in different The NMV & signal in different tissuestissues
The NMV can be separated into the The NMV can be separated into the individual vectors of the tissues such as individual vectors of the tissues such as CSF, fat and muscle.CSF, fat and muscle.
A tissue has a A tissue has a high signalhigh signal if it has a if it has a large large transverse componenttransverse component of magnetization. of magnetization.
The coil receives a high signal resulting The coil receives a high signal resulting brightbright area on the image area on the image
A tissue has a A tissue has a low signallow signal if it has a if it has a smallsmall transversetransverse component of magnetization component of magnetization
If there is a If there is a small componentsmall component of transverse of transverse magnetization the amplitude of the signal magnetization the amplitude of the signal received by the coil is small resulting in a received by the coil is small resulting in a dark areadark area on the image. on the image.
Generally, the two extremes of contrast in Generally, the two extremes of contrast in MRI are MRI are fatfat and and water.water.
Fat & Water appear differently in MR Fat & Water appear differently in MR imagesimages
Fat & WaterFat & Watero The Larmor frequency of hydrogen in The Larmor frequency of hydrogen in
water is higher than hydrogen in fat .water is higher than hydrogen in fat .
o Hydrogen in fat recovers more rapidly Hydrogen in fat recovers more rapidly along the longitudinal axis than water and along the longitudinal axis than water and loses transverse magnetization faster than loses transverse magnetization faster than In water.In water.
o Subsequently fat and water appear Subsequently fat and water appear differently in MR imagesdifferently in MR images
Fat & WaterFat & WaterB0
Fat vecto
r
Water vector
Transverse components of magnetization
Long
itudi
nal c
ompo
nent
s
Contrast mechanisms Contrast mechanisms Intrinsic factorsIntrinsic factors
Images obtain contrast mainly Images obtain contrast mainly through the mechanisms of :through the mechanisms of : T1 recoveryT1 recovery T2 decayT2 decay Proton or spin densityProton or spin density. (proton . (proton density of a tissue is the number of density of a tissue is the number of protons per unit volume of that protons per unit volume of that tissue)tissue)
T1 & T2 of Fat & WaterT1 & T2 of Fat & Water
Fat Fat WaterWater
T1T1 shortshort longlong
T2T2 Short Short
(= 80ms)(= 80ms)
Long (= Long (= 200ms)200ms)
Recovery time constant -T1Recovery time constant -T1
T1 is the time it T1 is the time it takes 63% of the takes 63% of the longitudinal longitudinal magnetization to magnetization to recover in the recover in the tissuetissue
Time
100%
63%
Sig
nal
inte
nsi
ty
T1
Time constant of decay – T2Time constant of decay – T2
T2 is the time it T2 is the time it takes 63% of the takes 63% of the transverse transverse magnetization to magnetization to be lostbe lost
100%
37%
T2 Time
Sig
nal
inte
nsi
ty
Demonstration of T1 contrastDemonstration of T1 contrast
B0
1st RF pulse
Longitudinal components
Fat
Water
Recovery
Longitudinal components
flipped by next RF pulse
Transverse components
Fat
water
T1 contrast / weighted imageT1 contrast / weighted image
As the T1 time of fat is shorter than water, As the T1 time of fat is shorter than water, the fat vector realigns with Bthe fat vector realigns with B00 faster than faster than
that of water.that of water.The longitudinal component of The longitudinal component of
magnetization of fat is therefore longer magnetization of fat is therefore longer than water.than water.
After a certain TR the next RF pulse is After a certain TR the next RF pulse is applied.applied.
The RF excitation pulse flips the The RF excitation pulse flips the longitudinal components of magnetization of longitudinal components of magnetization of both fat and water into the transverse plane.both fat and water into the transverse plane.
As there is more longitudinal magnetization As there is more longitudinal magnetization in fat before the RF pulse, there is more in fat before the RF pulse, there is more transverse magnetization in fat after RF transverse magnetization in fat after RF pulse. pulse.
FatFat therefore has a therefore has a high signalhigh signal and and appearsappears brightbright on a on a T1 contrastT1 contrast imageimage
As there is less longitudinal magnetization As there is less longitudinal magnetization in water before the RF pulse, there is less in water before the RF pulse, there is less transverse magnetization in water after the transverse magnetization in water after the RF pulse. RF pulse.
WaterWater therefore has a therefore has a low signallow signal and and appears darkappears dark on a on a T1 contrastT1 contrast imageimage..
Such images are called T1 weighted Such images are called T1 weighted images.images.
Demonstration of T1 contrastDemonstration of T1 contrast
B0
1st RF pulse
Longitudinal components
Fat
Water
Recovery
Longitudinal components
flipped by next RF pulse
Transverse components
Fat
water
Weighting?Weighting?
To demonstrate either; To demonstrate either;
T1, proton density or T2 contrast,T1, proton density or T2 contrast,
Specific values of TR and TE are selected Specific values of TR and TE are selected for a given pulse sequence.for a given pulse sequence.
The selection of appropriate TR and TE The selection of appropriate TR and TE weightsweights an imagean image so that so that one contrastone contrast mechanismmechanism predominatespredominates over the other over the other two.two.
T1 weightingT1 weighting
The contrast depends predominantly on the The contrast depends predominantly on the differences in the T1 times between fat and differences in the T1 times between fat and water and therefore all the tissues with water and therefore all the tissues with intermediate signal as wellintermediate signal as well
Because TR controls how far each vector can Because TR controls how far each vector can recover before it is excited by the next RF pulse, recover before it is excited by the next RF pulse, to achieve T1 weighting;to achieve T1 weighting;
TR must be short enough so that neither fat no TR must be short enough so that neither fat no water has sufficient time to fully return to Bwater has sufficient time to fully return to B00
T1 weighting T1 weighting
TR controls the amount of T1weightingTR controls the amount of T1weightingFor T1 weighting TR must be shortFor T1 weighting TR must be short
Signal intensity
Contrast between
fat & water
Short T1 fat
Long T1 water
Short TR Long TRTR (ms)
No contrast between fat & water
T2 contrast / weighted imageT2 contrast / weighted image
The T2 time of fat is shorter than that of The T2 time of fat is shorter than that of waterwater
Therefore the transverse component of Therefore the transverse component of magnetization of fat decays faster.magnetization of fat decays faster.
The magnitude of transverse component The magnitude of transverse component of magnetization of water is large.of magnetization of water is large.
Water has a high signal and appears Water has a high signal and appears bright on a T2 contrast image.bright on a T2 contrast image.
However, the magnitude of transverse However, the magnitude of transverse magnetization in fat is small.magnetization in fat is small.
Fat therefore has a low signal, and Fat therefore has a low signal, and appears dark on a T2 contrast image.appears dark on a T2 contrast image.
Such images are called T2 weighted Such images are called T2 weighted imagesimages
Production of T2 contrastProduction of T2 contrastFat WaterDecay of
transverse magnetization
Large amount of dephasing
small amount of dephasing
Small transverse component
Large transverse component
Short T2
Long T2
Produces low signal Produces high signal
T2 weightingT2 weightingContrast predominately depends on the Contrast predominately depends on the differences in the T2 times between fat differences in the T2 times between fat and water (and therefore all the tissues and water (and therefore all the tissues with intermediate signal as well)with intermediate signal as well)The TE controls the amount of T2 decay The TE controls the amount of T2 decay that is allowed to occur before the signal is that is allowed to occur before the signal is received.received. to achieve T2 weighting, the TE must be to achieve T2 weighting, the TE must be long enough to give both fat and water long enough to give both fat and water time to decaytime to decay
T2 weightingT2 weighting
TE controls the TE controls the amount of T2 amount of T2 weightingweightingFor T2 weighting TE For T2 weighting TE must be longmust be long
Signal intensity Small contrast difference
between fat and water
Large contrast difference between fat and water
Long T2 water
Short T2 fat
Short TE TE (ms) Long TE
Proton density contrastProton density contrast
Proton density contrast refers to the Proton density contrast refers to the differences in signal intensity between differences in signal intensity between tissues which are a consequence of their tissues which are a consequence of their relative number of protons per unit relative number of protons per unit volume.volume.To produce this contrast, the transverse To produce this contrast, the transverse component of magnetization must reflect component of magnetization must reflect these differences.these differences.Tissues with high proton density (e.g. Tissues with high proton density (e.g. brain) have a large transverse component brain) have a large transverse component of magnetization and therefore a high of magnetization and therefore a high signalsignal
They appear brighter on a proton density They appear brighter on a proton density contrast imagecontrast imageTissues with a low proton density (e.g. Tissues with a low proton density (e.g. cortical bone) have a small transverse cortical bone) have a small transverse component of magnetization and therefore component of magnetization and therefore a low signal.a low signal.They appear dark on a proton density They appear dark on a proton density contrast image.contrast image.Proton density contrast is always present Proton density contrast is always present and depends on the patient and the area and depends on the patient and the area being examined.being examined.It is the basic MRI contrast It is the basic MRI contrast
Proton density weightingProton density weighting
The difference in the numbers of protons The difference in the numbers of protons per unit volume in the patient is the main per unit volume in the patient is the main determining factor in forming image determining factor in forming image contrastcontrast
Proton density is always present to some Proton density is always present to some extentextent
To obtain proton density weighting T1 & To obtain proton density weighting T1 & T2 contrast must be diminishedT2 contrast must be diminished
Long TR will diminish T1 and short TE will Long TR will diminish T1 and short TE will diminish T2diminish T2
SummarySummaryFor T1 weightingFor T1 weighting To exaggerate T1 TR is SHORTTo exaggerate T1 TR is SHORT To diminish T2To diminish T2 TE is SHORT TE is SHORT
For T2 weightingFor T2 weighting To exaggerate T2 TE is LONGTo exaggerate T2 TE is LONG To diminish T1 TR is LONGTo diminish T1 TR is LONG
For proton density weightingFor proton density weighting To diminish T2 TE is SHORTTo diminish T2 TE is SHORT To diminish T1 TR is LONGTo diminish T1 TR is LONG
Typical values of TR and TETypical values of TR and TE
Long TR 2000 msLong TR 2000 ms
Short TR 250 – 750 msShort TR 250 – 750 ms
Long TE 60 ms +Long TE 60 ms +
Short TE 20 – 25 msShort TE 20 – 25 ms
Summary….Summary….
Fat has a short T1 and T2 timeFat has a short T1 and T2 timeWater has a long T1 and T2 timeWater has a long T1 and T2 timeTo produce high signal, there must be a To produce high signal, there must be a large component of magnetization in the large component of magnetization in the transverse plane to induce a large signal transverse plane to induce a large signal in the coilin the coilTo produce a low signal, there must be a To produce a low signal, there must be a small component of magnetization in the small component of magnetization in the transverse plane to induce a small signal transverse plane to induce a small signal in the coil.in the coil.
Summary …Summary …
T1 weighted images are characterized by T1 weighted images are characterized by bright fat and dark water.bright fat and dark water.
T2 weighted images are characterized by T2 weighted images are characterized by bright water and dark fat.bright water and dark fat.
Proton density weighted images are Proton density weighted images are characterized by characterized by Areas with high proton density are brightAreas with high proton density are bright Areas with low proton density are darkAreas with low proton density are dark
T1 T1 weighted weighted
ImageImage
T2 T2 weighted weighted
imageimage
Proton Proton Density Density
Weighted Weighted imageimage
EndEnd
