Week 5 Flashcards
What is a pulse sequence in MRI?
The process of acquiring image data by disorienting net magnetization with RF pulses
What are the four types of traditional Pulse sequences?
- Spin echo (SE)
- Inversion recovery (IR)
- Gradient echo (GE)
- Echo planar imaging (EPI)
The pulse sequence is represented by a timing diagram that details the RF pulse and gradient variation events. What are the five events recorded in a pulse sequence timing diagram?
- RF transmission (RF-t)
- RF receive (RF-t)
- Slice select
- Readout
- Phase
What is Echo time (TE)?
The time it takes to complete an entire sequence
What is TE measured in?
Milliseconds (ms)
What interval denotes 1 full TE period?
From the 90° pulse all the way to the resulting echo
In the pulse sequence timing diagram, the interval from the 90° pulse to the 180° pulse would represent how much of a TE period?
Half of 1 TE period
What happens during the first event of the timing diagram, RF-t?
An RF pulse flips net magnetization into the transverse direction
What is another term for the RF transmit event?
Excitation event
The excitation event always begins with a 90° initial pulse, followed by 180° pulse. True or false?
False; it normally is, but could also be a flip angle
Once net magnetization is flipped during RF-t, what happens during the second event (RF-r)?
The signal created by the relaxing magnetization immediately begins to weaken.
The “Dephasing event” is another term used for which event?
The second event, RF-receive
Explain what free induction decay (FID) refers to.
The signal that’s produced by the protons during the relaxation/dephasing process.
*Remember, T2 relaxation and amplitude
When does the third event, slice select, take place?
Slice selection occurs during the first event, RT-t
The slice select event allows for a specific slice to be singled out in the body on the chosen plane (sagittal, coronal, axial). What is this phenomena known as?
Spatial localization
How many times does the fourth event occur?
- the readout gradient is active during RF-t, RF-r, and slice selection.
- Turns on again during the resultant echo
What does the readout gradient do while it’s active?
It’s samples the signal produced by the excitation event, dephasing event, and any subsequent echoes.
The readout gradient samples the signals produced. What is this process called?
Frequency encoding
When does the fifth event occur?
The phase gradient is active between the 90° and 180° pulse
What is the phase gradient determined by?
The number of phase encoding lines in a slice
What are the two types of spin echo pulse sequences?
- Conventional spin echo (CSE)
- Fast spin Echo (FSE)
Conventional spin echoes can be T1 weighted, T2 weighted, or proton density weighted. True or false?
True
Conventional spin echo pulse sequences are traditional and basic. When are they utilized?
Never, obsolete
How many echoes are contained per one single TE period in a CSE? How does this affect scan time?
One; very long scan times.
FSEs are significantly faster that CSEs. Why?
Because of their added echo train length.
Fast spin echo pulse sequences can be T1 or T2 weighted, but not proton density weighted. True or false?
False; similar to conventional, FSEs can be T1, T2, or PD weighted.
What makes an inversion recovery pulse sequence different than a spin echo sequence?
IR contains an additional 180° pulse before the initial 90° pulse
What does inversion time (TI) refer to?
The time between the pre-180° pulse to the 90° pulse during an inversion recovery pulse sequence
Approximately how long are inversion times during STIR imaging?
100-150 ms
Longer inversion times will result in the suppression of what tissue type?
Fat
When a certain tissue type is suppressed, how does its appearance change on the MRI image?
Suppressed tissues appear darker
If we recall the appearance of different tissues during T1 relaxation (week 4), what would happen to T1 contrast if the inversion time is increased? Why?
Decrease; Long TI times suppress fat, suppressed fat appears dark - just like water.
What is the purpose of the pre-180° pulse during IR?
The pre-pulse establishes T1 relaxation and contrast before the pulse sequence even begins.
What are the two types of inversion recovery Pulse sequences?
- Short time inversion recovery (STIR)
- Fluid attenuated inversion recovery (FLAIR)
What kind of technique is STIR imaging?
Fat suppression technique
IV Gadolinium contrast cannot be used during STIR imaging due to its appearance being too similar to fat on images. True or false?
True
Describe the characteristics of an image obtained via STIR
All anatomy with a high fat content, like bone, tendon, and muscle, will appear very dark. This results in water content appearing highlighted - blood, edema, and infection.
What type of technique is FLAIR imaging?
A cerebral spinal fluid (CSF) suppression technique
Approximately how long are inversion times in FLAIR imaging?
About 2000 ms
How do FLAIR imaging TIs and TRs compared to STIR?
FLAIR has longer TI and TR times.
Describe the characteristics of an image obtained via FLAIR.
All fluid in the image will be suppressed, resulting in enhancement of fat containing tissues and their potential pathology
What are the four types of gradient echo pulse sequences?
- Gradient recall echo (GRE)
- Spoiled gradient echo (SPGE)
- Steady state gradient echo (SSGE)
- Fast gradient recall echo (FGRE)
What 3 tissues would be best displayed during a GRE pulse sequence?
- Blood in the brain
- Ligaments
- Cartilage
What is the purpose of a spoiler pulse during a spoiler gradient echo pulse sequence?
To terminate any remaining magnetization
During an SPGE, when is the spoiler pulse emitted?
After the third event, readout gradient
What two things could spoiler pulse be?
- A strong radio frequency pulse
- A strong gradient pulse
What’s the difference between a radio frequency pulse and a gradient pulse?
An RF pulse changes the direction of hydrogen atoms in the body from the longitudinal to the transverse plane. A gradient pulse causes positive and negative variations to the magnetic field.
Which gradient echo pulse sequence preserves remaining magnetization by rephasing it?
Steady state gradient echo (SSGE)
How does rephasing preserve magnetization?
By allowing both the relaxed protons and the remaining magnetization to be present for the next RF flip angle.
What kind of images are created as a result of rephasing?
T2* weighted images
What three facts make GRE sequences unique from SE and IR?
- SE and IR use a 90° RF pulse to initiate the sequence (not counting IR’s pre-180°). GRE uses a smaller flip angle.
- GRE uses a gradient pulse for refocusing, not a 180° RF pulse.
- GR
What kind of pulse is used to result in a T1 weighted FGE?
A 180° RF pulse
What kind of pulse is used to result in a T2 weighted FGE?
A 90°-180°-90° RF pulse combo
Why are the RF pulses/pulse used before the initial flip angle during a fast gradient echo pulse sequence?
To establish different types of contrast: T1 weighted, or T2 weighted
At the end of an FGE pulse sequence, a spoiler is used to_______ the relaxation process
Accelerate
What are the three requirements for a FGE?
- Short TR.
- Short TE
- Small flip angle
Which type of gradient echo pulse sequence is the fastest and loudest technique?
Echo planar imaging (EPI)
EPI sequences can acquire larger amounts of gradient echoes per TR cycle. How does it achieve this?
By alternating the readout gradient and phase collection
This diagram displays which event during the pulse sequence?
First event; RF-transmit
This diagram displays which event during the pulse sequence?
Second; RF-receive
What event of the pulse sequence is displayed by the diagram?
Third event; slice select gradient
Which event of the pulse sequence is displayed by the diagram?
Fourth event; readout gradient
Which event of the pulse sequence is displayed by the diagram?
Fifth event; phase gradient
Which pulse sequence is displayed by the diagram?
Conventional spin echo
Which pulse sequence is displayed by the diagram?
Fast spin echo
Which inversion recovery pulse sequence is displayed by the diagram?
Short time inversion recovery; STIR
Which inversion recovery pulse sequences displayed by the diagram?
Fluid attenuated inversion recovery; FLAIR
Which of the four gradient echo pulse sequences is displayed by the diagram?
Gradient recall echo; GRE
Which of the four gradient echo pulse sequences is displayed by the diagram?
Spoiler gradient echo; SPGE
Which of the four gradient echo pulse sequences is displayed by the diagram?
Steady state gradient echo; SSGE
Which of the four gradient echo pulse sequences is displayed by the diagram?
Fast gradient echo; FGE
Which of the four pulse sequence types is displayed by the diagram?
Echo planar imaging; EPI
This is an example of an image acquired via what specific pulse sequence?
STIR
This is an example of an image acquired with what specific pulse sequence?
FLAIR