Available Pulse Sequences

Available Pulse Sequences

The following pulse sequences are available to choose from (for pulse sequence descriptions please see separate document).

SEMS Spin Echo Multi Slice

GEMS Gradient Echo Multi Slice

FLASH Fast Low Angle Shot

TURBO FLASH Turbo Fast Low Angle Shot with 2d and 3d options

SPGR Spoiled Grass (Gradient Recalled Acquisition in the Steady State

RR-EPI Robarts Research Echo Planar Imaging (for centric ordered segmented EPI).

GE-MBEST Gradient Echo Modulus Blipped Echo planar Shot Technique

SE-MBEST Spin Echo Modulus Blipped Echo planar Single pulse Technique

FIELD MAP

SEMS

The Spin Echo Multi Slice (SEMS) pulse sequence shown below comprises of a 90 degree RF pulse followed by a 180 degree refocusing pulse both applied in the presence of a slice select gradient.

Proton density, (PD), T1 and T2 weighted images can be acquired by choosing the relevant TRs and TEs depending on the T1 and T2 values of the tissue under examination.

This pulse sequence can also be used with a 180 degree inverting pulse applied a TI time before the beginning of the pulse sequence in order to manipulate image contrast (Inversion Recovery option).

Pulse sequence diagram here

GEMS

The Gradient Echo Multi Slice (GEMS) sequence uses a variable, operator dependent, flip angle instead of a 90 degree pulse and a gradient instead of an RF pulse to rephase the FID. This pulse sequence can be used to acquire T2*, T1 and PD images. The flip angle in conjunction with the TR determines the saturation and therefore T1 weighting. The TE controls the amount of dephasing. To minimise T2* the TE needs to be short.

Pulse sequence diagram

STEADY STATE

The Steady State is a condition where the TR is shorter than the T1 and T2 times of the tissue. There is therefore no time for the transverse magnetisation to decay before the pulse sequence is repeated again. There is coexistence of both longitudinal and transverse magnetisation. The flip angle and the TR maintain the steady state. Flip angles of about 30 to 35 degrees in conjunction with a TR of 20 to 50ms are used. Most gradient echo sequences use the steady state as the shortest TR and scan time are achieved. When utilising the steady state the TR equals the TAU of the spin echo.

Coherent gradient echo pulse sequences use rewinder gradients to rephase the residual magnetisation so that it is in phase at the beginning of the next repetition (angiographic or myelographic effect). To maximise T2* use long TEs 15-25ms. To minimise T2* and produce more PD or T1 weighting use the shortest TE possible

Incoherent gradient echo pulse sequences use a variable flip angles and gradient rephasing to produce a gradient echo. The steady state is maintained so that the residual transverse magnetisation is left over from the previous repetition. These sequences then spoil or dephase this magnetisation so that its effect on image contrast is minimal. There are 2 ways to achieve spoiling: by digitised RF and by gradient spoiling.

FLASH

The Fast Low Angle Shot sequence, or FLASH, is a gradient echo sequence that utilises a semi-random spoiler gradient after the echo to spoil the steady state by causing a spatially dependent phase shift. This technique spoils the transverse steady state but there is still a longitudinal steady state that depends upon the T1 values and the flip angle.

TR, TE and flip angle can be manipulated. TR itself has little effect on contrast as long as the RF pulse is properly chosen (longer TRs will result in higher signal to noise but with longer scan time). As TR is decreased the optimal flip angle becomes smaller.

The TE determines the degree of T2* and sensitivity to motion. TE usually as short as possible. The flip angle has the largest effect on contrast. Smaller flip angles will produce predominately Proton Density weighted images. Larger flip angles produce more T1 weighting. A 90 degree flip produces a heavily T1 weighted image but very little signal.

Vasculature is well demonstrated as flowing blood is very bright.

Gradient spoiling is less effective than RF spoiling and so more T2* information is present in the signal.

Pulse sequence diagram

SPGR

The spoiled gradient recalled acquisition in steady state, or SPGR, uses semi-random changes in the phase of the RF pulses to produce a spatially independent phase shift.

should be chosen to optimise the brighter tissue, i.e. white matter

The optimal Flip Angle (the Ernst angle) can be determined depending on the T1 of the tissue and the TR through:

Cos FA = exp (-TR/T1)

The optimum Flip Angle increases with TR and decreases slightly with a longer T1.


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