T4 Pulse Seq SE Combine Class PDF

11/27/24 INTRODUCTION To rephase mag moment of H+ nuclei & produces a signal/echo Series of RF pulses, gradient applications, and interval time...

11/27/24 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  11/27/24 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  11/27/24 SPIN ECHO PULSE SEQUENCE parameters CONVENTIONAL SE PULSE SEQUENCE TR: between 90° RF excitation pulse for each slice TE: between 90° RF excitation 90° RF followed by 1or 2 180° RF pulse & peak of the spin-echo rephasing pulses TE is 2x tau & system applied the Typically generate 1or 2 spin-echoes 180° RF rephasing pulse by Contrast is mainly determined by the halving the TE spin-echo Echo is symmetry:- BUT may influence by rephasing of mag moments of H+ gradually come mag moments of H+ by negative into phase, signal gradually builds, polarity applications of the slice reaching a peak at the TE & select and FE gradients then, mag moments that are & application of spoiler gradients precessing rapidly soon overtake at the end of each TR period those that are precessing slowly, and dephasing occurs again:- loss of signal 9 10 9 10 SE with single echo SE with two echoes Produce T1-w images by selecting TR is long (both echoes):- T1 recovery a short TR and a short TE differences between the tissues are One 180° RF rephasing pulse is minimized applied after the 90° RF :- 1st spin-echo is generated early by : generates a single spin-echo short TE Short TE à ensures 180° RF à little T2 decay occurs & min differences rephasing pulse & spin-echo occur between the tissues (min T2 decay) early so little T2 decay occurs :- minT2 decay times of the tissues à PD-w image Short TRà ensures fat and water 2nd spin-echo is generated by: vectors do not fully recover & long TE differences in T1 recovery times à significant amount of T2 decay occurs & dominate the spin-echo and its differences in the T2 decay times of the contrast tissues are max à T2-w image 11 12 11 12 3  11/27/24 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  11/27/24 DISCUSS THE WEIGHTING IN FSE (EFFECTIVE TE) 17 18 17 18 19 20 19 20 5  11/27/24 TUTORIAL Explain the following pulse sequences: Sagittal T2-w - Single-shot turbo spin echo - Driven equilibrium Coronal PD-w Axial T1-w 21 22 21 22 INVERSION RECOVERY (IR) PULSE SEQUENCE principles INVERSION RECOVERY (IR) PULSE SEQUENCE principles Uses 180° RF inverting pulse to suppress signal from certain tissues Time from 180° RF inverting pulse to 90° RF excitation pulse:- TI (time from inversion) à180° RF pulse is applied at the beginning when Image contrast depends primarily on TI the NMV is aligned along B0 /Mz à 180° RF pulse inverts the NMV through 180° & flip àIf 90° RF excitation pulse is applied after the NMV has not full recovery, NMV in the opposite direction to -B0/−Mz image contrast depends on the àWhen the RF inverting pulse is removed, the amount of longitudinal recovery of NMV relaxes back to B0 (T1 recovery each vector & resultant image is T1-w processes) TI àIf 90° RF excitation pulse is not àAt a certain time-point during this recovery, applied until the NMV has reached full 90° RF excitation pulse is applied and then recovery, a PD-w is produced, as both switched off fat and water are fully relaxed àThe resultant FID is then rephased by another 180° RF rephasing pulse to produce a spin- echo at time TE 23 24 23 24 6  11/27/24 IR uses Used to produce heavily T1-w images to demonstrate anatomy 180° RF inverting pulse produces a large contrast difference between fat & water. Why? àFat & water vectors are completely saturate at the beginning of each TR; begin their recovery from full inversion àAllows more time for differences in T1 recovery times between tissues to TI apparent àThus, produce heavier T1-w than conventional spin-echo 25 26 25 26 IR parameters TI is the most influential contrast controller Medium TI values produce T1-w Long TI, image becomes more PD-w TR should always be long enough to allow full recovery of the NMV before application of the next RF inverting pulse 27 28 27 28 7  11/27/24 FAST IR PULSE SEQUENCE 2 main seq: STIR and FLAIR 180° RF inverting pulse à TI à 90° RF excitation pulse à train of 180° RF rephasing pulses to fill out multiple lines of k-space Fast IR is used in with T2-w to suppress signal from specific tissues, allowing water and pathology to return a high signal 29 30 29 30 SHORT TAU INVERSION RECOVERY (STIR) PULSE SEQUENCE SHORT TAU INVERSION RECOVERY (STIR) PULSE SEQUENCE Uses TI to represent the time it TI = 100 –175ms usually achieves takes the fat vector to recover fat suppression; varies slightly at from full inversion to Mxy & thus different field strengths no longitudinal magnetization Use in MSK :- normal bone (Mz) associated with fat (contains fatty marrow, is This is called the null point suppressed) & lesions within bone such as bone bruising and tumors As there is no longitudinal are seen more clearly component of fat when 90° RF excitation pulse is applied, there Useful sequence for suppressing is no transverse component after fat in general MR imaging excitation, and signal from fat is nulled 31 32 31 32 8  11/27/24 FLUID ATTENUATED INVERSION RECOVERY (FLAIR) PULSE SEQUENCE Choose TI corresponding to the vector in CSF recovering from full inversion to the transverse plane (Mxy) à nulls signal from CSF As there is no longitudinal component of CSF when the 90° RF excitation pulse is applied, there is no transverse component after excitation, and signal from CSF is nulled. Used to suppress high CSF signal in T2-w Coronal STIR so pathology adjacent to CSF is seen more clearly TI = 1700–2200ms usually achieves CSF suppression (varies slightly at different field Axial T2-w FLAIR strengths) 33 34 33 34 FLUID ATTENUATED INVERSION RECOVERY (FLAIR) PULSE SEQUENCE Used in brain and spine imaging:- periventricular and cord lesions more clearly (high signal from CSF is nulled) Useful multiple sclerosis (MS) plaques, acute subarachnoid hemorrhage, and meningitis Another modification in brain imaging is: àUse TI that corresponds to the null point of white matter àAs no signal from white matter, so that lesions within it appear much brighter àTI is about 300 ms Coronal IR-FSE T2-2 with TI null the white matter àUseful for white matter lesions 35 36 35 36 9  11/27/24 TERIMA KASIH fsk. uitm.edu.my Fakulti Sains Kesihatan UITM @fskuitm FSK Media UiTM 37 37 38 10