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THIS POWER POINT ILLUSTRATED THE SECOND CHAPTER OF THE FIFTH EDITION BOOK ( MRI IN PRACTICE )
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Image Contrast All are selected in scan protocol, Some of these parameters depend on pulse sequence. TR, TE and b value
T1 Recovery • Known as spin-lattice relaxation time (measure of how quickly (NMV) recovers to its ground state in the direction of B0). • T1 relaxation: exponential process. The length of the NMV for a spin echo sequence is given by the equation: Mzt = Mz(1-e-t/T1) where [SI=(1-e-t/T1)] • Occurs at different rates in different tissues (Table below ) typical T1 recovery times of brain tissue at T1.
T1 Relaxation • Mzt =Mz (1−e−t/T1) Therefore, • SI=(1−e−t/T1) Mzt is the amount of longitudinal magnetization at time t (ms) after the removal of the excitation pulse • Mz is full longitudinal magnetization T1 is the T1 recovery time (ms) and is the time taken to increase the longitudinal magnetization by a factor of e.
SI is the signal intensity in a tissue This equation plots the size of the recovering NMV as a function of time after the removal of the excitation pulse and the T1 recovery time. • When t=T1, 63% of the longitudinal magnetization recovers. When t=2×T1, 86% recovers and when t=3×T1, 95% recovers. It usually takes between 3 and 5 T1 recovery times for full recovery to occur
T2* decay Spin–spin interaction is inherent to the tissue. but dephasing also caused by inhomogeneities in the B0 field. Inhomogeneities are areas within the magnetic field that do not exactly match the external magnetic field strength. Some areas have a magnetic field strength slightly less than the main magnetic field (shown in purple in Figure follow), while other areas have a magnetic field strength slightly higher than the main magnetic field (shown in red in Figure slide #12)
Equation of T2 and T2* Decay 1/T2* = 1/T2 + 1/2γΔβ0 T2 and T2* are the tissues T2 and T2* relaxation times (ms) , γ is the gyromagnetic ratio (MHz/T), and ΔB0 is the variation in magnetic field (parts per million, ppm) This equation shows how T2 and T2* are related to each other. Poor field inhomogeneities result in T2* being much shorter than T2, and a fast decaying signal
Things to remember – relaxation • Relaxation is a general term refers to a loss of energy. In MRI, this is energy that is delivered to the spins via the RF excitation pulse and then lost once it is switched off • Spin lattice energy transfer is a relaxation process, spins give up the energy absorbed through RF excitation to the surrounding molecular lattice of the tissue. It causes T1 recovery • T2 decay an irreversible loss of phase coherence due to spin–spin interactions on an atomic and molecular level. causes of T2 decay • Pulse sequences are mechanisms that permit refocusing of spins so that images can be acquired with different types of contrast
Relaxation in different tissues Every tissue has a different affect on longitudinal (T1) and transverse (T2) relaxation.
Fat and water Fat tissue Water molecule
The magnitude of transverse magnetization vs amplitude of signal.
T1 recovery in fat. T1 recovery in water.
T1 Contrast • T1 contrast : Image contrast is derived from differences in the T1 recovery times of the tissues rather than any other mechanism • T1 contrast occur if vectors do not fully recover their longitudinal magnetization between each RF excitation pulse. • If the TR is longer than the relaxation times of the tissues, full recovery occurs in all tissues, and, therefore, it is not possible to produce an image that demonstrates contrast based on the differences in their T1 recovery times • T1 Fat shorter T1 water (mean) • T1 Fate hyper-intensity and T1 water hypo-intensity
T2 contrast • T2 contrast: Image contrast derived from differences in the T2 decay times of the tissues rather than any other mechanism. • TE long Increases signal (short TE no signal ) • T2 fat shorter than water fat decays faster than water T2 contrast generation.
Weighting • Intrinsic contrast parameters were listed before. • simultaneously affect image contrast. • obtain images of mixed appearance (very difficult to determine relative contribution of each intrinsic contrast parameter to the contrast observed). • To minimize this, extrinsic contrast parameters selected to weight image contrast toward one of the intrinsic contrast parameters and away from the others. • To demonstrate T1, T2, or proton density weighting, specific values of TR and TE are selected.
T1 weighting • contrast depends on differences in the T1 recovery times between fat and water (and all the tissues with intermediate T1 recovery times) • TR controls the amount of T1 contrast. • For T1 weighting, the TR must be short and the TE must also be short
T2 weighting • Contrast depends on the differences in the T2 decay times between fat and water (and all the tissues with intermediate T2 decay times). • TE controls the amount of T2 decay that occurs before signal is received. • To achieve T2 weighting, the TE must be long enough to give the vectors in both fat and water time to dephase. • If the TE is too short, the differences in their T2 decay times are not demonstrated.
T2-weighted images are used to image pathology because most pathology has a high water content (hyperintense). • TE controls the amount of T2 contrast. • For T2 weighting, the TE must be long and the TR must also be long.
Proton density weighting • A PD-weighted image is one where differences in the number of mobile hydrogen nuclei per unit volume of tissue main determining factor in forming image contrast. • To achieve PD weighting, the effects of T1 and T2 contrast are diminished so that proton density contrast dominates. • A long TR allows the vectors in both fat and water to fully recover their longitudinal magnetization and so diminishes T1 contrast. • A short TE does not give the vectors in fat or water time to dephase and so diminishes T2 contrast. • PD-weighted images are used to image anatomy and pathology.
the heat analogy • Turning the TR knob down turns the heat up on T1 contrast, i.e. T1 contrast increases. • Turning the TE knob up turns the heat up on T2 contrast, i.e. T2 contrast increases. • To weight an image, necessary turn the heat up on one intrinsic contrast parameter and heat down on the others
To turn the heat up on T1 contrast, the TR is short (TR knob down). • To turn the heat down on T2 contrast, the TE is short (TE knob down). T1 weighting and the heat analogy.
To turn the heat up on T2 contrast, the TE is long (TE knob up). • To turn the heat down on T1 contrast, the TR is long (TR knob up) T2 weighting and the heat analogy.
To turn the heat down on T1 contrast, the TR is long (TR knob up). • To turn the heat down on T2 contrast, the TE is short (TE knob down) Proton density weighting and the heat analogy
Learning tip: Relaxation definitions • It is very important to understand the differences between: • T1 recovery. • T1 recovery time • T1 contrast and. • T1 weighting. • and the equivalent distinctions in terms of T2 relaxation processes.
Other contrast mechanisms There are other techniques that are used to generate very specific image contrast: • Diffusion-weighted imaging (DWI) • Functional MRI (fMRI) • Magnetization transfer contrast (MTC). • Susceptibility-weighted imaging (SWI). • Contrast agents.
