Pulse sequences Categorization n Spin echo Conventional spin

Pulse sequences

Categorization n Spin echo • Conventional spin echo • Fast spin echo n n Inversion recovery Gradient echo • Coherent • Incoherent n n Steady state free precession Ultra-fast imaging

Conventional spin echo Illustration n The situation before the patient is placed inside the magnet

n The patient is now inside the bore of the magnet. The magnetization of the patient’s protons M 0 is aligned along the Z axis

n The 900 pulse is now applied. M 0 is now completely removed from the Z axis and lies along the Y axis

n Relaxation is now taking place. Spin-lattice (T 1)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

n 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

n 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.

n Relaxation continues to take place. The magnetic vectors again dephase in the X-Y plane while the regrowth along the Z axis continues

n 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 90 0 pulse was applied

n The coordinate system in relation to the magnet

n n Short TE and short TR gives T 1 weighted image Use two RF rephasing pulses generating two spin echoes to produce T 2 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 T 2 weighting

uses n n n Gold standard for most imaging May be used for every examintion T 1 images useful for demonstrating anatomy – because high SNR With contrast enhancement T 1 images show pathology T 2 images also demonstrate pathology Diseased tissues are generally more oedematous and/or vascular. They have increased water content and, have a high signal on T 2 images

Parameters n T 1 weighting • Short TE 10 -20 ms • Short TR 300 – 600 ms • Typical scan time 4 -6 min n Proton density/T 2 • Short TE 20 ms/long TE 80 ms+ • Long TR 2000 ms+ • Typical scan time 7 -15 min

n Advantages • Good image quality • Very versatile • True T 2 weighting sensitive to pathology n Disadvantages • Scan times relatively long

Fast spin echo n n 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 & T 2) This drastically reduce the scan time The number of 1800 pulses in the train called the turbo factor or train length

n n E. g. if in conventional SE 256 phase matrix and 1 NEX is used, the scan time is 256 TR In FSE if the turbo factor is 16, the scan time is 256 TR/16 =16 TR

Uses n n n Useful in most clinical applications Central nervous system, pelvis, musculoskeletal regions Note • Fat remains bright on T 2 n Unless fat saturation techniques are used • Muscles appear darker in FSE images • Artefacts from metal implants is significantly reduced

Parameters n T 1 weighting • • n Short effective TE less than 20 ms Short TR 300 – 600 ms Turbo factor 2 -6 Typical scan time 30 s to 1 min T 2 weighting • • Long effective TE 100 ms Long TR 4000 ms+ Turbo factor 8 -20 Typical scan time 2 min

Advantages & Disadvantages n n Reduced scan time High resolution matrices and multiple NEX can be used Image quality improved Increase T 2 information n n Some flow and motion effects increased Incompatible with some imaging options Fat bright on T 2 Reduces magnetic susceptibility effect, so should not be used when haemorrhage is suspected

Inversion Recovery n n Starts with a 1800 inversion pulse. This inverts NMV through 1800 into full saturation. When inverting pulse removed NMV begins to relax back to B 0 A 900 excitation pulse is then applied at a time TI (Time from Inversion) from the 1800 inversion pulse

TI

n n n TI determines the weighting & contrast Short TI gives T 1 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 n n conventionally used to produce heavily T 1 weighted images to demonstrate anatomy & in contrast enhanced imaging Now more widely used in conjunction with fast spin echo to produce T 2 weighted images

Parameters

STIR (short TI inversion recovery) n n 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 T 1 weighted images. TI 150 – 175 ms

FLAIR (Fluid attenuated inversion Recovery) n n n 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 T 2 weighted images TI - 1700 -2200 ms