Spin Echo Pulse Sequence PDF

10/16/24 INTRODUCTION To rephase mag moment of H+ nuclei & produces a signal/echo Series of RF pulses, gradient applications, and interval time...

10/16/24 INTRODUCTION To rephase mag moment of H+ nuclei & produces a signal/echo Series of RF pulses, gradient applications, and interval time periods Used to determine image weighting SPIN ECHO PULSE Image contrast relies on difference in T1 or T2 relaxation SEQUENCE time or PD between tissues 2 ways of rephasing the mag moments: - Using an additional 180° RF pulse; spin-echo pulse sequences (SE) - Using gradients; gradient-echo pulse sequences (GE/GRE) 2 1 2 SPIN ECHO PULSE SEQUENCE principle Characterized by RF rephasing: à 90° RF excitation pulse (commonly) flip the NMV into Mxy à NMV precesses in Mxy & inducing voltage in the receiver coil à After switch off 90° RF excitation pulse, a FID occurs à T2* dephasing from inhomogeneities in the B0 field occurs almost immediately, and signal decays to zero. à After a time (tau), another RF pulse(180° RF rephasing pulse) is applied to compensate for this dephasing and refocus or rephase the mag moments of H+ 3 4 3 4 1  10/16/24 RF REPHASING / REFOCUSING principle…cont T2* dephasing causes mag moments of H+ to dephase in Mxy àMag moments that slow down form the trailing edge àMag moments that speed up form the leading edge àStill in Mxy, 180° RF rephasing pulse is then applied & move the NMV through 180° (like flipping a pancake) àNOW; mag moments that formed the leading edge before, now form the trailing edge & vice versa àAs direction of precession remains the same, so the trailing edge begins to catch up with the leading edge àAt a specific time (TE), 2 edges are superimposed/are in phase :- max signal is induced àThis signal is called a spin-echo 5 6 5 6 In spin-echo pulse sequences:- T2* dephasing is eliminated by the 180° RF rephasing pulse T2 decay is negligible/ insignificant:- spin–spin interactions (randomly fluctuate) Tissues have more time to recover & decay, resulting in a different image weighting 180° RF refocusing 8 7 7 8 2  10/16/24 SPIN ECHO PULSE SEQUENCE parameters QUESTION TR: between 90° RF excitation pulse for each slice TE: between 90° RF excitation pulse & peak of the spin-echo TE is 2x tau & system applied the When will the system apply the 180° RF rephasing pulse by halving the TE 180° RF refocusing pulse if the TE is 10ms? Echo is symmetry:- mag moments of H+ gradually come into phase, signal gradually builds, reaching a peak at the TE & then, mag moments that are precessing rapidly soon overtake those that are precessing slowly, and dephasing occurs again:- loss of signal 9 10 9 10 CONVENTIONAL SE PULSE SEQUENCE SE with single echo Produce T1-w images by selecting 90° RF followed by 1or 2 180° RF a short TR and a short TE rephasing pulses One 180° RF rephasing pulse is Typically generate 1or 2 spin-echoes applied after the 90° RF :- generates a single spin-echo Contrast is mainly determined by the spin-echo Short TE à ensures 180° RF rephasing pulse & spin-echo occur BUT may influence by rephasing of early so little T2 decay occurs :- mag moments of H+ by negative minT2 decay times of the tissues polarity applications of the slice select and FE gradients Short TRà ensures fat and water & application of spoiler gradients vectors do not fully recover & at the end of each TR period differences in T1 recovery times dominate the spin-echo and its contrast 11 12 11 12 3  10/16/24 SE with two echoes TR is long (both echoes):- T1 recovery differences between the tissues are minimized 1st spin-echo is generated early by : short TE à little T2 decay occurs & min differences between the tissues (min T2 decay) à PD-w image 2nd spin-echo is generated by: long TE à significant amount of T2 decay occurs & differences in the T2 decay times of the tissues are max à T2-w image 13 14 13 14 TURBO / FAST SE PULSE SEQUENCE Scan time is ⏬ by modifying the phase matrix component:- àNo of PE steps is maintained so phase matrix is unchanged àBUT, no of PE steps per TR is ⏫ :- k-space is filled more efficiently & scan time ⏬ Performing >1 PE step & filling >1 line of k-space per TR Achieved by using several 180° RF rephasing pulses to produce several spin- echoes (echo train) àEach rephasing produces a spin-echo à& Different PE step is performed on this echo No of 180° RF rephasing pulses performed every TR:- àCorresponds to no of spin-echoes produced in the echo train & àNo of lines of k-space filled with data This number is called the turbo factor or echo train length (ETL) 15 16 15 16 4  10/16/24 FSE WEIGHTING 17 18 17 18 FSE weighting Based on effective TE Data collected near effective TE have more impact on image weighting: àfill the central lines of k-space àcontribute signal and contrast to the image Steep slopes of PE (less signal/low amplitude):- placed echoes away from the effective TE (outer lines of k-space) Shallow slopes of PE (max signal/high amplitude) :- placed echoes near the effective TE & centre lines of k-space 19 20 19 20 5  10/16/24 For T2 weighting: effective TE = 100 ms, TR = 4000 ms, ETL = 12 The shallowest PE steps are performed around 100 ms & effective TE is at or close to 100 ms The steepest PE are performed at very beginning & end of the echo train. Signal amplitude of these echoes is small Data from early and late spin-echoes contribute to image contrast, BUT their impact is significantly less than from spin-echoes whose TE is at or around the effective TE (=100ms). 21 22 21 22 Sagittal T2-w Coronal PD-w Axial T1-w 23 24 23 24 6  10/16/24 TUTORIAL INVERSION RECOVERY (IR) PULSE SEQUENCE principles Uses 180° RF inverting pulse to suppress signal from certain tissues Explain the following pulse sequences: à180° RF pulse is applied at the beginning when the NMV is aligned along B0 /Mz à 180° RF - Single-shot turbo spin echo pulse inverts the NMV through 180° & flip - Driven equilibrium NMV in the opposite direction to -B0/−Mz àWhen the RF inverting pulse is removed, the NMV relaxes back to B0 (T1 recovery processes) àAt a certain time-point during this recovery, 90° RF excitation pulse is applied and then switched off àThe resultant FID is then rephased by another 180° RF rephasing pulse to produce a spin- echo at time TE 25 26 25 26 INVERSION RECOVERY (IR) PULSE SEQUENCE principles Time from 180° RF inverting pulse to 90° RF excitation pulse:- TI (time from inversion) Image contrast depends primarily on TI àIf 90° RF excitation pulse is applied after the NMV has not full recovery, image contrast depends on the amount of longitudinal recovery of each vector & resultant image is T1-w TI àIf 90° RF excitation pulse is not applied until the NMV has reached full recovery, a PD-w is produced, as both TI fat and water are fully relaxed 27 28 27 28 7  10/16/24 IR uses IR parameters Used to produce heavily T1-w images to demonstrate anatomy TI is the most influential contrast controller 180° RF inverting pulse produces a large contrast difference between fat Medium TI values produce T1-w & water. Why? Long TI, image becomes more PD-w àFat & water vectors are completely saturate at the beginning of each TR; TR should always be long enough to allow full recovery of begin their recovery from full the NMV before application of the next RF inverting inversion pulse àAllows more time for differences in T1 recovery times between tissues to apparent àThus, produce heavier T1-w than conventional spin-echo 29 30 29 30 31 32 31 32 8  10/16/24 FAST IR PULSE SEQUENCE SHORT TAU INVERSION RECOVERY (STIR) PULSE SEQUENCE Uses TI to represent the time it 2 main seq: STIR and FLAIR takes the fat vector to recover 180° RF inverting pulse à TI à 90° RF excitation pulse à from full inversion to Mxy & thus train of 180° RF rephasing pulses to fill out multiple lines no longitudinal magnetization of k-space (Mz) associated with fat Fast IR is used in with T2-w to suppress signal from This is called the null point specific tissues, allowing water and pathology to return a As there is no longitudinal high signal component of fat when 90° RF excitation pulse is applied, there is no transverse component after excitation, and signal from fat is nulled 33 34 33 34 SHORT TAU INVERSION RECOVERY (STIR) PULSE SEQUENCE TI = 100 –175ms usually achieves fat suppression; varies slightly at different field strengths Use in MSK :- normal bone (contains fatty marrow, is suppressed) & lesions within bone such as bone bruising and tumors are seen more clearly Useful sequence for suppressing Coronal STIR fat in general MR imaging 35 36 35 36 9  10/16/24 FLUID ATTENUATED INVERSION RECOVERY (FLAIR) PULSE SEQUENCE FLUID ATTENUATED INVERSION RECOVERY (FLAIR) PULSE SEQUENCE Choose TI corresponding to the vector in CSF recovering from full inversion to the Used in brain and spine imaging:- periventricular and cord lesions more transverse plane (Mxy) à nulls signal from CSF clearly (high signal from CSF is nulled) Useful multiple sclerosis (MS) plaques, As there is no longitudinal component of CSF when the 90° RF excitation pulse is acute subarachnoid hemorrhage, and applied, there is no transverse component meningitis after excitation, and signal from CSF is Another modification in brain imaging is: nulled. àUse TI that corresponds to the null point Used to suppress high CSF signal in T2-w of white matter so pathology adjacent to CSF is seen more àAs no signal from white matter, so that clearly lesions within it appear much brighter TI = 1700–2200ms usually achieves CSF àTI is about 300 ms suppression (varies slightly at different field Axial T2-w FLAIR Coronal IR-FSE T2-2 with TI null the white matter strengths) àUseful for white matter lesions 37 38 37 38 39 40 39 40 10  10/16/24 TERIMA KASIH fsk. uitm.edu.my Fakulti Sains Kesihatan UITM @fskuitm FSK Media UiTM 41 11