Image Formation Pulse Sequences

pulse sequence

set of specifically timed instructions to the magnet telling it how images should look with regards to the tissue being sampled

Gradient echo

gradient echo sequence lacks _______ making it more susceptible to __________

180° refocusing RF pulse, magnetic field inhomogeneities.

why are Gre sequences fast

the lack of 180° refocusing pulses, the TR can be shorter, thus shortening scan time

In a gradient echo pulse sequence, the gradient coils are used to

refocus the protons and create the echo.

In order to produce an echo in a gradient echo pulse sequence,

a gradient field and an RF pulse are used.

Gradient echo sequences can yield ________ with influences caused by

either T1 or T2* characteristics, susceptibility, chemical shift, and inhomogeneities.

steady state occurs when

TR in a gradient echo is less than the T2 (or T2*), resulting in residual transverse magnetization at the time of the next excitation pulse

Gradient echo sequences requiring high signal from fluid

T2* gradient echo, steady-state gradient echo or coherent gradient echo

Gradient echo sequences requiring low signal from fluid

Incoherent gradient echo (or spoiled gradient echo) sequences

why do Incoherent gradient echo (or spoiled gradient echo) sequences give the contrast it does

removal of residual transverse magnetization before the next excitation

BTFE sequences are and are quipped with

variations of high T2 signal, balanced steady state sequences
equipped with slice, phase and frequency rewinder pulses, yielding images with mixed contrast T2/T1

BTFE sequences look like

image of high SNR with bright fluids, particularly useful in evaluation of cranial nerves

.

BTFE manufacturer names chart

Philips: BTFE

GE: Fiesta

Siemens: TrueFISP

Hitachi: BASG

Toshiba: True SSFP

spoiled gradient echo sequence does what

removes any residual transverse magnetization prior to the next excitation pulse

To reduce the steady state (T2*) effect what technique can be used

"spoiling"

"spoiling" technique does what

allow for fast scans with T1 contrast,

RF spoiling occurs when

additional RF pulses are used to "spoil away" the steady state effect (residual transverse magnetization) before the next excitation, yielding T1 contrast for dynamic imaging of organs or CE MRA sequences.

example of a T1 spoiled gradient echo sequence

SPGR Cor Oblique acquisition for a patient with history of seizures for evaluation of hippocampus

spin echo

Conventional spin echo sequences begin with

90° RF excitation pulse

what refocuses a decaying spin echo

The 180° RF pulse

Conventional spin echo techniques acquire ____ line(s) of k-space during each repetition (TR).

one

Fast spin echo techniques acquire ______ lines of kspace during each repetition.

multiple  (2 to 50+)

1/2 TE time (Tau)

In a spin echo sequence, the time between the 90° RF pulse and the 180° RF pulse

In a spin echo pulse sequence, an echo is produced from

a combination of two or more RF pulses

dual spin echo

In the pulse sequence diagram above, if there were 20 slices planned, 40 total slices would be reconstructed, with 2 slices per TE (TE1 and TE2) with differences in T2 weighting due to the differences in TE.

A dual echo spin echo sequence generates_____

two images for every slice,

in a dual spin echo with two images for every slice what are the characteristics

both have varying TE’s, but the same TR.

dual spin echoes generally acquired as

short TE (proton density weighted) and a long TE (T2 weighted) combination

Ex. In a dual contrast spin echo sequence with echo times of 25ms and 90ms, the second echo image has ______ contrast but ____ SNR than the first echo image.

more T2, lower SNR

fast spin echo

in FSE Scan time is reduced by ___________ because

a factor matching the number of echo train lengths (TSE Factor--Philips), utilized,
several echoes for each slice are collected during each TR period.

Each train of echoes in a fast spin echo sequence _____ scan time _____ SNR and _____ SAR absorption in the patient.

reduces, lowers, increases

Conventional spin echo techniques acquire

one line of k-space during each repetition (TR).

Fast spin echo techniques acquire

multiple (2 to 50+) lines of kspace during each repetition.

In a fast spin echo pulse sequence, if the TSE factor (echo train length) is increased by a factor of 3

the scan time will be three times faster.

In a fast spin echo sequence, ____ are used to______, thus reducing scan time by the number of echoes.

multiple echoes,  fill multiple lines of kspace for each TR period

In a Fast Spin Echo sequence, the effective TE are

the echoes that are encoded with a low amplitude phase encoding gradient

In a fast Spin Echo Sequence the effective TE occurs at

the line that is closest to the center of k-space (k0).

In a Fast Spin Echo sequence, increasing the # ETL can lead to

an increase in blurring.

in FSE the number of shots is calculated by

#Phase encodings / ETL

"shot" in a Fast Spin Echo pulse sequence

refers to the number of "nonredundant" RF excitations.

in a single shot sequence

the entire k-space matrix can be filled with a single excitation and a train of echoes matching the lines in k-space.

single shot sequence useful in

Abdominal MRI to reduce respiratory motion or to improve temporal resolution (speed)

In a Multi Shot sequence, ______ filled with each excitation, which can help 

only a portion or fraction of the k-space data 
increased spatial resolution (detail) opportunity

calculating the number of shots in a Fast Spin Echo sequence

Divide the # Phase Encodings by the Echo Train Length (ETL)

inversion recovery

inversion recovery

begins with a 180° RF pulse, followed by a 90° RF excitation pulse

IR first pulse designed to

null signal from fat or fluid, depending upon its length and proportional to field strength

TI (inversion time)

time interval between the 180° Inverting RF pulse and the 90° RF excitation pulse

types of inversion recovery sequences

STIR (short TI inversion recovery)
FLAIR (fluid attenuated inversion recovery)
3D-IR (T1 weighted inversion recovery)

If the desire is to null the signal from a specific tissue using an inversion recovery sequence, the inversion time (TI) selected should be

69% of the T1 relaxation time of that tissue

in a STIR sequence, the null point of fat in a 1.5Tesla magnet is

140-160 milliseconds

Short TAU inversion recovery (STIR) sequences are typically used for the evaluation of

compression fracture, lesions within retro-orbital fat, musculoskeletal contusions and fat suppression.

null the signal from fluid in a FLAIR sequence (CSF, for example) at 1.5Tesla, an Inversion Time (TI) of

2000ms would be used

T2 weighted FLAIR (fluid attenuated inversion recovery) sequences are typically used for evaluation of

periventricular white matter

SPGR stands for

SPoiled Gradient Recalled echo

SPoiled Gradient Recalled echo does what

the RF excitation pulse is phase shifted each time the RF is applied which prevents accumulation of the residual transverse magnetization effects throughout the acquisition, in effect, spoiling transverse magnetization

in SPGR if you combine iy with a short TE

reduce T2 effects and T1 effects will predominate the image contrast.

SPGR sequences by manufacturer

Siemens – FLASH (Fast Low Angle SHot , the original coined name for

the sequence upon development in 1985 in Germany)

GE – SPGR (SPoiled Gradient Recalled echo)

Philips – T1-FFE (Fast Field Echo)

Hitachi – SARGE (Steady state Acquisition Rewound Gradient Echo)

The repetition time (TR

is the time between two 90º RF excitation pulses

The echo time (TE)

he time between the 90º RF pulse and the peak of the signal in the receiver coil

Lengthening TR with a short TE in a spin echo sequence will

increase proton density weighting

As the TE is increased, the available number of slices is

decreased

longer TE means ______ slices can fit into the TR period

fewer

As the TR is increased, SNR is ____ and the available number of slices is ______

increased, increased

scan time in seconds

TR x Phase matrix x NEX = Scan time in milliseconds, ÷ 1000

scan time formula with ETL

TR x Matrix x NEX ÷ ETL

The timing of RF pulses in an MRI pulse sequence controls

image contrast.

Spin echo sequences contain _______, which _________, therefore making them the most _______ to inhomogeneity.

180° RF refocusing pulse(s)

which aid in correcting for local field inhomogeneities
insensitive

Reducing the flip angle yields images with less ________.

T1 information

The slice selection gradient is the determinant of

scan plane, slice thickness

using two ______ will create oblique scan plane

SSG

gradient that is on during the production of the echo

frequency encoding (readout) gradient

Echo-planar imaging capable of

acquiring an entire MR image in only a fraction of a second, and is the fastest sequence commonly available

fastest sequence commonly available

echo planar imaging

Spin echo pulses

90° RF followed by 180° RF

Fast spin echo pulses

90° RF followed by train of 180° RF pulses

Inversion recovery pulses

180° RF followed by 90°RF

Gradient echo pulses

~variable° RF followed by gradient to produce echo

A SE (spin echo) sequence can be best described as

a 90° pulse followed by a 180° pulse

A FSE (fast spin echo) sequence can be best described as

a "train" of spin echoes

An IR (inversion recovery) sequence can be best described as

a 180° pulse followed by a 90°/180° combination

An EPI (echo planar imaging) sequences can be best described as

a "train" of gradient echoes

450 TR; 30° flip angle combination would yield a

T2* weighted gradient echo.

45 TR; 90° flip angle combination would yield a

T1* weighted gradient echo.

Reducing the flip angle yields images with less

T1 information

Reducing the TE yields images with less

T2 information

Increasing the flip angle increases _____ up to

SNR, the Ernst angle.

what play a role in lengthening scan time in an MR pulse sequence

/\ TR
\/ the # ETL
/\ NEX
\/ the parallel imaging factor

what have an impact on shortening overall scan times in an MR pulse sequence

\/ TR
/\ the #ETL
\/ NEX,
enabling half-scan
/\ parallel imaging factor

what help to improve SNR in an MR image.

/\ TR
\/ the Phase Matrix
/\ the FOV
\/ (narrowing) the receive Bandwidth
/\ NEX/NSA
\/ the #ETL

what negatively impact SNR in an MR image

\/ in TR
\/ in Pixel Size
\/ in FOV
/\ (widening) of the receiver Bandwidth
\/ in NEX
/\ in #ETL