MRI Pulse Sequences

a series of RF and gradient pulses applied over several time periods

pulse sequence

2 ways rephasing can be accomplished

using a 180 degree refocusing pulse

or

using the magnetic gradients

_________ pulses are also used to spatially encode the information contained within the MR signals, and to rephase and dephase transverse magnetization during certain pulse sequences

gradient

rephasing is performed by the 180 degree rephasing pulse

spin echo pulse seqeunces

(conventional spin echo, fast spin echo)

rephasing is performed by the gradients

gradient echo pulse sequences

(coherent/incoherent gradient echo)

what type of pulse sequence is the gold standard in MR imaging and produces the standard in T1 and T2 contrast

Conventional spin echo pulse sequence

True or False

The slice select gradient is turned on whenever the RF energy is being transmitted

True

(This ensures that the only protons that are excited are the ones in the current slice that we are imaging, and not the rest of the anatomy around it)

The _________ gradient comes on with the echo, as it reads and encodes varying frequencies within the echo. It stays on throughout rephasing and dephasing.

readout

The ________ of the readout gradient determines the range of frequencies encoded in the echo, known as the receiver bandwidth

amplitude

Does a steeper readout gradient encode more or less frequencies than a shallow readout gradient?

steeper gradient readout = more frequencies

shallow gradient readout = less frequencies

The timing of the ________ encoding gradient is not as specific as the slice select and readout gradient

phase

When is the phase gradient usually turned on?

after the 90 degree excitation pulse and before the 180 degree rephasing pulse

phase dispersion is only allowed by the ______ encoding gradient since _______ is a unique identifier used to locate the position of the signal

phase

true or false

a shift in phase increases incremental values as time continues

true

A positive change in frequency followed by an _______ negative change in frequency will result in no shifting of the phase

equal

the protons on the positive side of the gradient are going to spin ________ than protons on the negative side

faster

If we want to cancel the phase shift created by the application of the slice select and readout gradients, we need what?

an equal but opposite pulse from those same gradients

If a gradeint of equal amplitude, but opposite polarity is turned on, the phase will _______, leaving a net phase of 0, a rephased signal.

This is how we rephase the signal in a ___________ pulse sequence

cancel

gradient echo

With the spin echo technique, utilizing a 180 degree refocusing pulse halfway between the initial 90 degree RF and TE, the effects of what are eliminated?

T2*

the time it takes for spins to rephase after the initiation of the 180 degree pulse

TE is twice tau

tau is 1/2 TE

After the application of the 90 degree excitation pulse, what is there?

FID (T2* dephasing)

Is FID ever rephased?

NO

Are spin echo and gradient echo ever rephased?

yes, ALWAYS

How are gradient and spin echo rephased?

either with a 180 RF pulse

or

180 degree RF rephasing pulse

what is used to compensate for FID dephasing?

180 degree rephasing pulse

in a spin echo pulse sequence, what governs the decay of the signal after the 180 degree pulse?

spin spin only (True T2)

What type of dephasing occurs before tissues have time to relax by T2 and T1?

T2*

a spin echo technique follows the 90 degree pulse with a 180 degree pulse.. for what?

to bring nuclei back into phase

1. 90 degree excitation pulse

2. T2* immediately causes dephasing

3. a voltage would be induced in the receiver coil but the FID signal is not collected.

4. a 180 degree rephasing pulse brings nuclei back into phase

5. The rephased signal induces a voltage in the receive coil at time TE. This is our MR signal

timing of the spin echo pulse sequence

What is the purpose of the 180 degree rephasing pulse?

to bring the transverse magnetization back to full phase coherence

what supresses T2*?

spin echo signal

The application of the 180 degree refocusing pulse causes the spins to what?

reverse polarity

A gradient of _______ amplitude and the _______ polarity is applied on either side of the 180 degree refocusing pulse, leaving a net phase of _____

same

same

0

a positive change occurs with the application of the gradient

a 180 degree rephasing pulse is applied

another equally positive gradient is applied

the end result is no phase shift

how we balance gradients for phase in spin echo pulse sequences

During a conventional spin echo pulse sequence, if a 256 phase matrix is selected, then _____ uniquely phase encoded echoes will be produced to fill the _____ lines of kspace.

Assuming one excitation is selected, than 256 TR's are needed to complete one image. What is unique about them? (answered on next slide)

256

256

The phase gradient steps its amplitude ______ each TR, giving a unique phase to the protons in every line of k-space

down

In conventional spin echo sequences, only ______ phase encoding step is applied per TR for each slice.

The resulting spin echo then fills only _____ line of k space per TR

1

1

What is the difference between spin echo pulse sequences and fast spin echo?

Both sequences still only produce one echo per TR, per image, and k space is still filled in the same way

what's different is that we are getting an additional echo, just one, at a different weighting (different contrast)

A dual contrast conventional SE sequence contains an extra _______ pulse and generates a ________ spin echo at a ______ TE

180

2nd

2nd

What type of pulse sequence does this process describe:

2 different k-space files are opened, and one line is filled with each TR. One echo is placed in the topmost line of one k-space file, the second echo is placed in the topmost line of the second kspace file. This same phase encoding amplitude is used for both echoes

dual contrast conventional Spin echo sequence

True or False

Dual contrast techniques can also be performed with a fast spin echo sequence

true

dual contrast conventional spin echo pulse sequences generally use a _______ TR.

By using a ______ TR, these sequences are good for generating what 2 types of contrast?

long

long

PD (Long TR, short TE)

T2 contrast (Long TR, Long TE)

In a dual contrast conventional spin echo pulse sequence, the first echo will be used to generate ______ images, and the second echo will be used for _______ images

1st echo = PD images

2nd echo = T2 images

The time from the 90 degree excitation pulse to the center of the first echo

TE for first echo

The time from the 90 degree excitation pulse to the center of the second echo

TE for the second echo

In order to generate the second echo, what must be applied half way between the center of the first echo?

the second 180 degree rephasing pulse

how is fast spin echo the same conventional spin echo?

they both use the initial 90 degree RF excitation pulse, followed by a 180 degree pulse to rephase the spins, we then get a spin echo at time TE

What is the difference between conventional spin echo and fast spin echo?

with FSE, our scan times are much shorter than conventional spin echo, this is because the echoes are collected more frequently and placed in k-space differently

Many echoes are collected per TR, instead of just one echo

what is this?

fast spin echo

Each echo is placed appropriately in k-space so that the desired image contrast is achieved

what is this?

fast spin echo

FSE sequences complete the data acquisition very ________ compared to conventional spin echo, while generating images with contrast __________ to a conventional spin echo sequence

quickly

similar

in conventional spin echo, how are echoes placed in k-space?

sequentially

only one echo for one slice

in fast spin echo, how are echoes collected?

multiple echoes are collected during a single repetition, for each slice

more echoes are collected per TR

In fast spin echo, ________ echoes are placed in k space in a single TR, which means ______ repetitions are required to fill the man lines of raw data

multiple

fewer

What type of sequences use a 180 degree rephasing pulse?

conventional spin echo

fast spin echo

The number of echoes collected

echo train length

when more than one 180 degree refocusing pulses is applied after a 90 degree excitation pulse, each successive echo is of _______ amplitude

less

the 180 degree pulse can rephase dephasing caused by inhomogeneities and other dephasing mechanisms, but can it rephase spin spin dephasing?

NO!!!!! each tissue is unique and we can't control this process

For conventional spin echo and fast spin echo, what determines how many spin echoes will be needed to fill kspace for each image in the sequence?

our phase matrix

what is effective TE?

the TE that will give us the contrast we desire

in both conventional spin echo and fast spin echo, when does the 180 degree pulse have to come on?

halfway between each subsequent echo

MR echoes are temporarily stored in the K-space until they are constructed into an image in the _______ ________ using the ________ _______

array processor

Fourier transform

each echo represents ______ phase encoded line of data and contains information about the ________ slice

one

entire

Information located at the ________ of k space contains high spatial frequencies, since this is where the amplitude of the phase encoding gradient is the _________

periphery (end)

greatest

high spatial frequencies correspond to what?

spatial resolution (aka edge defintion or line pairs per mm)

information located toward the center of k-space contains ______ spatial frequencies, since this is where the amplitude of the phase encoding gradient is at the _______

low

lowest

low spatial frequencies correspond to what?

contrast information

true or false

each and every echo contributes to the entire MR image, regardless of the type of sequence that is run

true

what happens if the center of k-space is removed?

we have sharp resolution, but zero contrast defintion

what happens if the periphery of k-space is removed?

we have contrast definition but no resolution

in conventional spin echo sequences, the first echo collected is place into the top most line of k space, the second echo collected is placed in the line immediately below that and so on...

This type of "mapping" means what?

early and late echoes contribute to the images resolution, while the central echoes (the echoes collected half way through the sequence) contribute to the image contrast

In fast spin echo, the echoes are mapped out differently.

The system places the echoes that occur near/at the ____________ in the central lines of k-space, so the resulting image displays the desired contrast.

The echoes not close to the __________ will be placed into the outer lines of k-space, both positive and negative

effective TE

in fast spin echo, if the scan parameters specify a TE of 90 ms, the echo which formed at ________ will be placed in the very center of k-space to provide the appropriate contrast information

90ms

True or False

In fast spin echo, each phase encoding step applies a different slope of the phase encoding gradient to phase shift the signal by a different amount each time

true

for fast spin echo:

very steep phase encoding slopes = _____ signal amplitude

very shallow phase encoding slopes = _______ signal amplitude

low

high

In fast spin echo, the system re-orders the echoes by using a ______ phase encoding gradient to produce echoes at or near the effective TE and placing them in the center of k-space

the other echoes utilize a ______ phase encoding slope and are placed away from the effective TE

shallow

Steep

on a fast spin echo T2 weighted image, fat and water are both ________

bright

why do fat and water both appear bright on FSE T2 images?

j coupling

what is J-coupling

precessional frequencies can vary within the same molecule because the spinning protons that are side by side with each other (coupled) can lose or gain magnetization from each other. This "coupling" causes increased signal loss (called j-coupling) on a spin echo sequence

true or false

rapid rephasing of fast spin echoes suppresses the j coupling dephasing (signal loss), resulting in brighter signals for fat and water on T2 weighted images

true

what does j-coupling mean for fast spin echo?

the rapid 180 degree rephasing pulse disrupts the signal loss originally caused by j-coupling. Since we do not have signal loss in fat on a FSE sequence, we get bright (strong) signal from fat, similar to the signal we get from water

what if we don't want to see fat as bright on T2FSE sequence?

use a fat suppression technique (STIR)

What type of sequence is an inversion recovery sequence?

FSE sequence that begins with an 180 degree inversion pulse

what does the inversion period do?

creates a large contrast difference between fat and water over time & this allows for differences in the t1 recovery times , producing heavy T1 weighting

an inversion recovery sequence ALWAYS begins with what?

a 180 degree rephasing pulse

the time from the first 180 degree RF inverting pulse to the 90 degree RF excitation pulse

inversion time (TI)

Do we use gadolinium for STIR (short TI inversion recovery)?

NO

(b/c contrast agents shorten the T1 time of perfused tissues and STIR sequences reduce the signal from short T1 tissues. If contrast was given, it would be suppressed and we wouldn't see it.

what is very useful in musculoskeletal imaging?

STIR

Because STIR surpasses fatty bone marrow, what can be seen more clearly?

bony lesions

fat has a very bright signal on MR images and can make it difficult to visualize pathology in fatty areas such as

orbits

abdomen

joints

TI & TR for FLAIR

TI = very long around 2000 ms and a TR of 6-10 seconds

Applications of FLAIR

visualizing MS plaques in the brain

metastatic disease of the brain and spine

In FSE, the SAR _________ significantly because of multiple 180 degree pulses, applied in quick succession

increases

How do we decrease the SAR?

Decreasing the rephasing angle to 120-160 degrees

which are faster:

spin echo or gradient echo?

gradient echo

differences between GRE & SE

GRE utilizes short TR's (regardless of what type of contrast we want to see)

no 180 RF pulse (gradients are used to rephase it)

echo is called gradient echo

what type of flip angles are used in GRE?

Flip angles of less than 90 degrees are used, so full recovery of longitudinal magnetization occurs faster. This allows for faster repetition of the sequence

what controls contrast in a conventional GRE pulse sequence?

flip angle

what is used to rephase the signal in a GRE pulse sequence

equal but opposite gradients

why are GRE pulse sequences faster?

hydrogen protons respond to magnetic fields faster than they do with RF energy and resonance , so they rephase faster than an RF pulse