Pulse Timing parameters & Weighting

Presentation on theme: "Pulse Timing parameters & Weighting"— Presentation transcript:

Pulse Timing parameters & Weighting

Summary of the previous lesson
The NMV (Net Magnetic Vector) is a vector quantity. It is created by two components at 900 to each other. i.e. magnetization in the longitudinal plane & magnetization in the transverse plane. Before resonance, there is full longitudinal magnetization parallel to B0 After application of RF pulse, the NMV is flipped fully into the transverse plane (assuming sufficient energy is applied)

There is now full transverse magnetization and zero longitudinal magnetization.
Once the RF pulse is removed, the NMV recovers. As this occurs, the longitudinal component of magnetization grows again, while the transverse magnetization decreases. As the received signal is related to the magnitude of the transverse component, the signal in the coil decays as relaxation takes place.

Pulse timing parameters
The magnitude and timing of the RF pulses form the basis of MRI The two Pulse timing parameters are Pulse Repetition time TR Echo time TE

Pulse sequence A very simplified pulse sequence is a combination of RF pulses, signals and intervening periods of recovery as shown. RF Pulse RF Pulse RF Pulse TR TR TE TE TE

A pulse sequence consists of several components.
1. The repetition time (TR): This is the time from the application of one RF pulse to the application of the next RF pulse and is measured in milliseconds (ms). The TR determines the amount of relaxation that is allowed to occur between the end of one RF pulse and the application of the next. Therefore TR determines the amount of T1 relaxation that has occurred

2. The Echo time (TE) (TE) is the time from the application of the RF pulse to the Peak of the signal induced in the coil and is also measured in ms. The TE determines how much decay of transverse magnetization is allowed to occur before the signal is read. Therefore TE controls the amount of T2 relaxation that has occurred

Contrast of MR images The application of RF pulses at certain repetition times (TR) and The receiving of signals at pre-defined echo times (TE) produces contrast in MRI images.

Image Weighting & Contrast
T1 weighted ? T2 weighted ? Proton density ?

Contrast ? An image has contrast if there are areas of high signal (white on the image) as well as areas of low signal (dark on the image). Some areas have an intermediate signal (shades of grey in-between white and black))

The NMV & signal in different tissues
The NMV can be separated into the individual vectors of the tissues such as CSF, fat and muscle. A tissue has a high signal if it has a large transverse component of magnetization. The coil receives a high signal resulting bright area on the image A tissue has a low signal if it has a small transverse component of magnetization

If there is a small component of transverse magnetization the amplitude of the signal received by the coil is small resulting in a dark area on the image. Generally, the two extremes of contrast in MRI are fat and water. Fat & Water appear differently in MR images

Fat & Water The Larmor frequency of hydrogen in water is higher than hydrogen in fat . Hydrogen in fat recovers more rapidly along the longitudinal axis than water and loses transverse magnetization faster than In water. Subsequently fat and water appear differently in MR images

Fat & Water B0 Fat vector Longitudinal components Water vector
Transverse components of magnetization

Contrast mechanisms Intrinsic factors
Images obtain contrast mainly through the mechanisms of : T1 recovery T2 decay Proton or spin density. (proton density of a tissue is the number of protons per unit volume of that tissue)

T1 & T2 of Fat & Water Fat Water T1 short long T2 Short (= 80ms)
Long (= 200ms)

Recovery time constant -T1
T1 is the time it takes 63% of the longitudinal magnetization to recover in the tissue 100% 63% Signal intensity T1 Time

Time constant of decay – T2
100% T2 is the time it takes 63% of the transverse magnetization to be lost Signal intensity 37% T2 Time

Demonstration of T1 contrast
B0 Longitudinal components Longitudinal components flipped by next RF pulse 1st RF pulse Fat Water Fat water Recovery Transverse components

T1 contrast / weighted image
As the T1 time of fat is shorter than water, the fat vector realigns with B0 faster than that of water. The longitudinal component of magnetization of fat is therefore longer than water. After a certain TR the next RF pulse is applied.

The RF excitation pulse flips the longitudinal components of magnetization of both fat and water into the transverse plane. As there is more longitudinal magnetization in fat before the RF pulse, there is more transverse magnetization in fat after RF pulse. Fat therefore has a high signal and appears bright on a T1 contrast image

As there is less longitudinal magnetization in water before the RF pulse, there is less transverse magnetization in water after the RF pulse. Water therefore has a low signal and appears dark on a T1 contrast image. Such images are called T1 weighted images.

Demonstration of T1 contrast
B0 Longitudinal components Longitudinal components flipped by next RF pulse 1st RF pulse Fat Water Fat water Recovery Transverse components

Weighting? To demonstrate either; T1, proton density or T2 contrast,
Specific values of TR and TE are selected for a given pulse sequence. The selection of appropriate TR and TE weights an image so that one contrast mechanism predominates over the other two.

T1 weighting The contrast depends predominantly on the differences in the T1 times between fat and water and therefore all the tissues with intermediate signal as well Because TR controls how far each vector can recover before it is excited by the next RF pulse, to achieve T1 weighting; TR must be short enough so that neither fat no water has sufficient time to fully return to B0

Contrast between fat & water
T1 weighting Signal intensity No contrast between fat & water Short T1 fat Contrast between fat & water Long T1 water TR (ms) Short TR Long TR TR controls the amount of T1weighting For T1 weighting TR must be short

T2 contrast / weighted image
The T2 time of fat is shorter than that of water Therefore the transverse component of magnetization of fat decays faster. The magnitude of transverse component of magnetization of water is large. Water has a high signal and appears bright on a T2 contrast image.

However, the magnitude of transverse magnetization in fat is small.
Fat therefore has a low signal, and appears dark on a T2 contrast image. Such images are called T2 weighted images

Production of T2 contrast
Fat Decay of transverse magnetization Water Long T2 Short T2 small amount of dephasing Large amount of dephasing Small transverse component Large transverse component Produces low signal Produces high signal

T2 weighting Contrast predominately depends on the differences in the T2 times between fat and water (and therefore all the tissues with intermediate signal as well) The TE controls the amount of T2 decay that is allowed to occur before the signal is received. to achieve T2 weighting, the TE must be long enough to give both fat and water time to decay

T2 weighting TE controls the amount of T2 weighting
Signal intensity Small contrast difference between fat and water TE controls the amount of T2 weighting For T2 weighting TE must be long Long T2 water Short T2 fat Large contrast difference between fat and water Short TE Long TE TE (ms)

Proton density contrast
Proton density contrast refers to the differences in signal intensity between tissues which are a consequence of their relative number of protons per unit volume. To produce this contrast, the transverse component of magnetization must reflect these differences. Tissues with high proton density (e.g. brain) have a large transverse component of magnetization and therefore a high signal

They appear brighter on a proton density contrast image
Tissues with a low proton density (e.g. cortical bone) have a small transverse component of magnetization and therefore a low signal. They appear dark on a proton density contrast image. Proton density contrast is always present and depends on the patient and the area being examined. It is the basic MRI contrast

Proton density weighting
The difference in the numbers of protons per unit volume in the patient is the main determining factor in forming image contrast Proton density is always present to some extent To obtain proton density weighting T1 & T2 contrast must be diminished Long TR will diminish T1 and short TE will diminish T2

Summary For T1 weighting For T2 weighting For proton density weighting
To exaggerate T TR is SHORT To diminish T TE is SHORT For T2 weighting To exaggerate T TE is LONG To diminish T TR is LONG For proton density weighting To diminish T TE is SHORT

Typical values of TR and TE
Long TR ms Short TR – 750 ms Long TE ms + Short TE 20 – 25 ms

Summary…. Fat has a short T1 and T2 time
Water has a long T1 and T2 time To produce high signal, there must be a large component of magnetization in the transverse plane to induce a large signal in the coil To produce a low signal, there must be a small component of magnetization in the transverse plane to induce a small signal in the coil.

Summary … T1 weighted images are characterized by bright fat and dark water. T2 weighted images are characterized by bright water and dark fat. Proton density weighted images are characterized by Areas with high proton density are bright Areas with low proton density are dark

T1 weighted Image

T2 weighted image

Proton Density Weighted image

End