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Pulse sequences Categorization Spin echo • Conventional spin echo • Fast spin echo Inversion recovery Gradient echo • Coherent • Incoherent Steady state free precession Ultra-fast imaging Conventional spin echo Illustration The situation before the patient is placed inside the magnet The patient is now inside the bore of the magnet. The magnetization of the patient’s protons M0 is aligned along the Z axis The 900 pulse is now applied. M0 is now completely removed from the Z axis and lies along the Y axis Relaxation is now taking place. Spin-lattice (T1)relaxation caused the magnetization to re-grow along the Z axis. Spinspin relaxation causes the magnetization vectors to dephase (move apart) while still in the X-Y plane 1800 pulse is now given. All the vectors now point in the opposite direction. The magnetization vectors rephase (come together) in the X-Y plane. As they come together the echo is being formed The magnetization is now completely rephased in the X-Y plane and points along the Y axis. This causes the full height of the echo. The actual MRI signal is taken here. Relaxation continues to take place. The magnetic vectors again dephase in the X-Y plane while the regrowth along the Z axis continues Complete relaxation has taken place. There is no net vector in the X-Y plane, and the magnetization is full grown along the Z axis. This is identical to the situation before the 900 pulse was applied The coordinate system in relation to the magnet Short TE and short TR gives T1 weighted image Use two RF rephasing pulses generating two spin echoes to produce T2 and proton density weighting First echo has short TE and long TR – produce proton density weighting Second echo has a long TE and a long TR – produce T2 weighting uses Gold standard for most imaging May be used for every examintion T1 images useful for demonstrating anatomy – because high SNR With contrast enhancement T1 images show pathology T2 images also demonstrate pathology Diseased tissues are generally more oedematous and/or vascular. They have increased water content and, have a high signal on T2 images Parameters T1 weighting • Short TE 10-20 ms • Short TR 300 – 600 ms • Typical scan time 4-6 min Proton density/T2 • Short TE 20 ms/long TE 80 ms+ • Long TR 2000 ms+ • Typical scan time 7-15 min Advantages • Good image quality • Very versatile • True T2 weighting sensitive to pathology Disadvantages • Scan times relatively long Fast spin echo In contrast to conventional spin echo, fast spin echo applies a train of 1800 pulses per TR and different phase encoding steps are used. Each 1800 pulse produce an echo (proton density & T2) This drastically reduce the scan time The number of 1800 pulses in the train called the turbo factor or train length E.g. if in conventional SE 256 phase matrix and 1 NEX is used, the scan time is 256TR In FSE if the turbo factor is 16, the scan time is 256TR/16 =16TR Uses Useful in most clinical applications Central nervous system, pelvis, musculoskeletal regions Note • Fat remains bright on T2 Unless fat saturation techniques are used • Muscles appear darker in FSE images • Artefacts from metal implants is significantly reduced Parameters T1 weighting • • • • Short effective TE less than 20ms Short TR 300 – 600 ms Turbo factor 2-6 Typical scan time 30s to 1 min T2 weighting • • • • Long effective TE 100 ms Long TR 4000 ms+ Turbo factor 8-20 Typical scan time 2 min Advantages & Disadvantages Reduced scan time High resolution matrices and multiple NEX can be used Image quality improved Increase T2 information Some flow and motion effects increased Incompatible with some imaging options Fat bright on T2 Reduces magnetic susceptibility effect, so should not be used when haemorrhage is suspected Inversion Recovery Starts with a 1800 inversion pulse. This inverts NMV through 1800 into full saturation. When inverting pulse removed NMV begins to relax back to B0 A 900 excitation pulse is then applied at a time TI (Time from Inversion) from the 1800 inversion pulse TI TI determines the weighting & contrast Short TI gives T1 contrast Long TI gives proto density contrast After the 900 excitation pulse 1800 rephasing pulse is applied at a time TE This produces the spin echo TR is the time between each 1800 inverting pulse Uses conventionally used to produce heavily T1 weighted images to demonstrate anatomy & in contrast enhanced imaging Now more widely used in conjunction with fast spin echo to produce T2 weighted images Parameters STIR (short TI inversion recovery) Uses TI that corresponds to the time it takes fat to recover from full inversion to the transverse plane so that there is no longitudinal magnetization corresponding to fat. As a result the signal from fat is nulled. Used to achieve suppression of fat in T1 weighted images. TI 150 – 175 ms FLAIR (Fluid attenuated inversion Recovery) The signal from CSF is nulled by selecting a TI corresponding to the time of recovery of CSF from 180 to the transverse plane and there is no longitudinal magnetization present. Used to suppress signal from CSF in T2 weighted images TI - 1700 -2200 ms