MRI Pulse Sequences

Question 1

What is a pulse sequence?

Answer 1

A series of RF pulses and gradient events, fired to excite the spins and induce a signal in the reeive coil, in a short time (TR)

Question 2

What are the goals of imaging sequences?

Answer 2

• To generate an RF signal perpendicular to static magnetic field
• To generate good tissue contrast
• To minimize artifacts
• To maximize the SNR
• To acheive all the above, in the shortest possible scan time

Question 3

What is a Spin Echo?

Answer 3

The basic sequence (with good contrast)
-90-180 echo

Question 4

The function of a spin echo is to:

Answer 4

• generate T1, PD (proton density), and T2 weighted images
• minimizes artifacts

Question 5

List some advantages of spin echo imaging.

Answer 5

• High signal to noise (SNR)
• Least artifact prone sequence due to 180 refocusing pulse
• Contrast mechanisms accurate and easy to understand

Question 6

List some disadvantages of spin echo imaging.

Answer 6

• High SAR (how much RF you put into the patient) because of 90 degree and 180 degree RF pulses
• Long scan time due to single encoding per TR

Question 7

Scan time Equation

Answer 7

Scan time = TR x phase resolution x averages (nex)

Question 8

T1 weighted has a short/long TR & short/long TE.

Answer 8

short TR & short TE

Question 9

T2 weighted was a short/long TR and short/long TE.

Answer 9

long TR and long TE

Question 10

A PD weighted image has a short/long TR and short/long TE.

Answer 10

long TR and short TE

Question 11

In a T2 weighted image, fluid appears _.

Answer 11

bright

Question 12

What occurs during Fast/Tubo Spin Echo?

Answer 12

• Multiple 180 pulse is used
• For one excitation (90) there are many 180 pulses -> echoes
• Multiple phase encodes per TR
• Amplitude of echoes decrease as the time from 90 increases due to T2 decay

Question 13

List some advantages of fast/turbo spin echo.

Answer 13

• Much faster sequence than SE (spien echo)
• Has nearly the same contrast as that of SE
• Magnetic susceptibility effects are minimized
• Artifact due to metallic implants are reduced (inhomogeneity reduced)

Question 14

What is the scan time equation for fast/turbo spin echo?

Answer 14

Scan time = TR x phase resolution turbo factor x average (nex)

Question 15

List some disadvantages of fast/turbo spin echo.

Answer 15

• The weighting is not perfect due to multiple TEs -> it works better for T2 than T1 images
• Multiple 180 pulses make fat look bright on T2 weighted images
• Magnetic susceptibility is reduced so the sequences is not good for Hemorrhages or head trauma
• Image blurring might occur (SNR of the last echoes that are filling the k-space border could be very low)
• Very high SAR (due to multiple 180 pulses (high chance of burning)

Question 16

What is Single Shot Fast Spin Echo (SS FSE)?

Answer 16

Fills half of k-space in one TR! Large echo trains, uses half Fourier to predict the rest of the k-space; very fast but has low SNR

Question 17

What is a gradient echo?

Answer 17

the simplest sequence; also known as gradient recalled echo (GRE)

Question 18

What are the three main parameters of a gradient echo and what are its effects?

Answer 18

3 main parameters: TR, TE, & flip angle
* low SAR
* Artifact prone
* weak signal (as compared to SE)

Question 19

What is a partial flip angle?

Answer 19

flip/rotate a small amount of magnetization (no more than 90) = fast recovery

Question 20

T2 decay occurs between the _ and the _ gradients.

Answer 20

dephasing; rephasing

Question 21

Rephasing incompletely recovers the _.

Answer 21

signal

Question 22

Signal loss is greater with longer/shorter TEs

Answer 22

longer

Question 23

Gradient Echo imaging cannot recover signal losses from:

Answer 23

• magnetic field inhomogeneity
• magnetic susceptibility
• water-fat incoherence

Question 24

List some advantages of Gradient Echo.

Answer 24

• Faster imaging-can use shorter TR and short TEs than SE
• Low flip angle -> deposits less energy (more slices per TR than SE, decreases SAR)
• Compatible with 3D acquisitions

Question 25

List some disadvantages of gradient echo?

Answer 25

* Difficult to generate good T2 weighting * T2* relaxation influences contrast * Magnetic field inhomogeneity cause signal loss * - worse with increasing TE times * -susceptibility effects * - dephasing of water and fat photons

Question 26

What is an Inversion Pulse?

Answer 26

Initial 180 RF pulse which is followed by the excitation (90 pulse)

Question 27

What is Inversion T1 time?

Answer 27

time from inital 180 pulse to 90 pulse

Question 28

What are the advantages and disadvantages of inversion recovery (IR)?

Answer 28

Disadvantages: longer imaging time, fewer slices Advantages: better T1 contrast

Question 29

How is suppression achieved?

Answer 29

by sampling, as the longitudinal magnetization of the tissue (a) is at the zero crossing

Question 30

What does STIR stand for?

Answer 30

Short Tau Inversion Recovery

Question 31

What does STIR do?

Answer 31

Short Tau Inversion Recovery: nullifies fat signal; short T1; very useful in musculoskeletal and body imaging

Question 32

What does FLAIR stand for?

Answer 32

Fluid Attenuated Inversion Recovery

Question 33

What is FLAIR and what is it used for?

Answer 33

Fluid Attenuated Inversion Recovery: A turbo-spin echo sequence with IR; routinely used for brain imaging; long TR and T1, dark CSF; bright tumors, fat and bleeds; very sensitive to parameter change

Question 34

True or false. Water and fat resonate at slightly different frequencies.

Answer 34

True

Question 35

A gradient echo image used in-phase looks _ and out-of-phase looks _.

Answer 35

bright; black outline around organs

Question 36

Ernst Angle

Answer 36

A defined flip angle less than 90 degrees used to optimize contrast of a given tissue or fluid

Question 37

What is SPGR?

Answer 37

Spoiled (incoherent) Gradient Echo- spoiling destroys the accumulated transverse magnetization; it is done by an additional de-phasing gradient; it can also be done by RF (phase detection)

Question 38

What is the advantage of Spoiled Gradient Echo (SPGR)?

Answer 38

it maximizes T1 contrast

Question 39

What is FLASH?

Answer 39

Fast Low Angle Shot (RF or Gradient Spoiled)

Question 40

FLASH T1 Contrast has a short/long TE, short/long TR, large/small flip angle.

Answer 40

short TE, short TR, large flip angle

Question 41

Spoiled Gradient Echo T2* Contrast has short/long TE, short/long TR, and small/large flip angle.

Answer 41

long TE, long TR, small flip angle

Question 42

What is EPI?

Answer 42

Echo Planar Imaging- uses gradient echo | starts with 90 -> 180 -> gradient echo

Question 43

What are the advantages of EPI?

Answer 43

very fast sequence, all k-space in one TR, and 3D image in below a minute

Question 44

What are disadvantages of EPI?

Answer 44

low SNR, very noisy, and very susceptible to artifacts, specifically susceptibility

Question 45

Why are gradient echo images used?

Answer 45

due to short TR times

Question 46

In 3D imaging, an additional _ is added perpendicular to the imaging plane.

Answer 46

phase encoding gradient

Question 47

Partitions

Answer 47

planes of the 3D volume

Question 48

What is MPRAGE?

Answer 48

Magnetization Prepared Rapid Acquisition Gradient Echo- Fast 3D Gradient Echo with good T1 contrast that has an initial 180 inversion pulse followed by GRE sequence with rewinding gradient

Question 49

MPRAGE -In order to produce 3D images, a _ occurs in the slice selection direction

Answer 49

secondary phase encoding

Question 50

What are the different k-space fillings?

Answer 50

linear, centric, spiral, and propellar/blade

Question 51

Which k-space filling is less motion sensitive?

Answer 51

spiral

Question 52

What is parallel imaging?

Answer 52

using multi-channel coils

Question 53

Which k-space filling corrects for motion?

Answer 53

propeller/blade