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02 - Image weighting and contrast
All clinical diagnostic images must demonstrate contrast between normal anatomical features and between anatomy and any pathology.
59
Medical
Professional
04/20/2015

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Cards

Term

_____________ contrast parameters are those that cannot be changed because they are inherent to the body's tissue.

 

 

21

Definition
Intrinsic
Term

_____________ contrast parameters are those that can be changed.

 

 

21

Definition
Extrinsic
Term

Some intrinsic contrast parameters are:

 

 

22

Definition
  • T1 recovery time
  • T2 decay time
  • Proton density
  • flow
  • apparent diffusion coefficient (ADC)
Term

Some extrinsic contrast parameters are:

 

 

22

Definition
  • TR
  • TE
  • flip angle
  • TI
  • turbo factor/echo train length
  • b value

 

Term

A tissue has a high large __________ component of coherent magnetization at time TE.

 

 

22

Definition
transverse
Term

T1 and T2 relaxation depend on three factors:

 

 

22-23

Definition
  • The inherent energy of the tissue
  • How closely packed the molecules are
  • How well the molecular tumbling rate matches the Larmor frequency of hydrogen.
Term

Generally, the two extremes of contrast in MRI are _______ and _______ .

 

 

23

Definition
fat, water
Term

Fat has a high/low inherent energy, slow/fast tumbling rate and can/cannot easily absorb energy into its lattice from hydrogen nuclei.

 

 

23-24

Definition
low, slow, can
Term

Water has a high/low inherent energy, matched/unmatched tumbling rate and can/cannot easily absorb energy into its lattice from hydrogen nuclei.

 

 

25

Definition
high, unmatched, cannot
Term

Fat has a short/long T1 and T2 time and appears light/dark on a T2 weighted image. 

 

 

24-27

Definition
short, dark
Term

Water has a short/long T1 and T2 time and appears dark on a T1/T2 - weighted image. 

 

 

25-26

Definition
long, T1
Term

At 1T, the T1 and T2 times for water is ______ ms.

 

 

28

Definition
2500
Term

To demonstrate either T1, proton density or T2 contrast, specific values of _____ and _____ are selected for a given pulse sequence.

 

 

29

Definition
TR and TE
Term

For T1 weighting, the TR must be short/long.

 

 

30

Definition
short
Term

For T2 weighting, the TE must be ______.

 

 

30

Definition
long
Term

To achieve proton density weighting, the effects of T1 and T2 contrast must be __________ so that proton density weighting can dominate.

 

 

30

Definition
diminished
Term

The TR knob controls the amount of _____ contrast.

 

 

31

Definition
T1
Term

The TE knob controls the amount of ______ contrast.

 

 

31

Definition
T2
Term

Turning the TR knob down turns the heat up/down on T1 contrast, meaning the contrast is increased/decreased.

 

 

31

Definition
up, increased
Term

Turning the TE knob up turns the heat up/down on T2 contrast, meaning the contrast is increased/decreased.

 

 

31

Definition
up, increased
Term

For T1 weighting, turn the heat up/down on T1 and up/down on T2. Both TR and TE should be long/short.

 

 

31

Definition
up, down, short
Term

For T2 weighting, turn the heat up/down on T1 and up/down on T2. Both TR and TE should be short/long.

 

 

31

Definition
down, up, long
Term

For PD weighting, turn the heat up/down on T1 and up/down on T2. The TR should be long/short and TE should be long/short.

 

 

31

Definition
down, down, long, short
Term

T2* decay is faster than T2 decay because of a combination of two effects:

 

 

31-34

Definition
  • T2 decay itself
  • dephasing due to magnetic field inhomogeneities
Term

Whenever the NMV is pushed beyond 90º it is said to be _____________.

 

 

33

Definition
partially saturated
Term

When the NMV is pushed to a full 180º it is said the be ___________.

 

 

33

Definition
fully saturated
Term

The ____________ (how far the excitation pulse moves the vectors via resonance) has a significant impact on saturation effects.

 

 

34

Definition
flip angle
Term

To measure relaxation times and produce an image with good contrast we need to regenerate the signal. There are two ways to do this:

 

 

34-37

Definition
  • by using an additional 180º RF pulse
  • by using gradients
Term

A __________ pulse sequence uses a 180º rephasing pulse to generate an echo.

 

 

37

Definition
spin echo
Term

A ____________ pulse sequence uses a gradient to regenerate an echo.

 

 

37

Definition
gradient echo
Term

______ is the time between each 90º excitation pulse for each slice.

 

 

40

Definition
TR
Term

______ is the time between the 90º excitation pulse and the peak of the spin echo.

 

 

40

Definition
TE
Term

The _____ is the time to rephase after the application of the 180º RF pulse and equals the time to dephase when the 90º RF pulse is withdrawn.

 

 

40

Definition
TAU
Term

A spin echo using only one echo can be used to produce a ________ weighted image if a short TR and TE are used.

 

 

40

Definition
T1
Term

A spin echo using two echoes can be used to produce both __________ and ________ weighted images in the TR time.

 

 

42

Definition
  • proton density
  • T2
Term

Typical values of TR and TE:

Long TR -

Short TR -

Long TE -

Short TE -

 

 

43

Definition
  • 2000 ms
  • 300-700 ms
  • 60 ms+
  • 10-25 ms
Term

A ___________ pulse sequence uses an RF excitation pulse that is variable, and therefore flips the NMV through any angle, not just 90º.

 

 

46

Definition
gradient echo
Term

____________ are generated by coils of wire situated within the bore of the magnet.

 

 

47

Definition
Magnetic field gradients
Term

The ______________ is the center of the bore of the magnet in all planes and remains at the field strength of the magnetic field.

 

 

 

47

Definition
magnetic isocenter
Term

The direction of the gradient, i.e. which end is greater than B0 and which is lower than B0, is called ________________.

 

 

47

Definition
polarity
Term

When a gradient is switched on, the magnetic field strength along its axis is _______ or ________.

 

 

48

Definition
sloped, graded
Term

As gradients cause nuclei to speed up or slow down, they can be used to either ________ or _________ their magnetic moments.

 

 

48

Definition
dephase, rephase
Term

Gradients that dephase are called __________.

 

 

49

Definition
spoilers
Term

Gradient echo pulse sequences are usually associated with much longer/shorter scan times than spin echo pulse sequences.

 

 

51

Definition
shorter
Term

The most important disadvantage of gradient echo pulse sequence is that there is no compensation for magnetic field ____________.

 

 

51

Definition
inhomogeneities
Term

In gradient echo sequences, the __________ is an extrinsic contrast parameter that is changed to affect image contrast. Its value, combined with the ________ determines whether T1 effects are maximized or minimized.

 

 

51

Definition
flip angle, TR
Term

A short _____ produces T1 weighting and never permits a T2 or proton density weighted image to be obtained.

 

 

52

Definition
TR
Term

To give gradient echo imaging more flexibility, the flip angle is usually reduced to _________.

 

 

52

Definition
less than 90º
Term

To obtain a T1 weighted image in gradient echo, the differences in the T1 times of the tissues are minimized/maximized and the differences in the T2 times of the tissues are minimized/maximized.

 

 

52

Definition
maximized, minimized
Term

To obtain a T2* weighted image in gradient echo, the differences in the T2* times of the tissues are minimized/maximized and the differences in the T1 times are minimized/maximized.

 

 

52

Definition
maximized, minimized
Term

To obtain a proton density weighted image in gradient echo, both T1 and T2* processes are minimized/maximized so that the differences in proton density of the tissues can be demonstrated.

 

 

52

Definition
minimized
Term

For T1 weighting using gradient echo, turn the heat up/down on T1 and the heat up/down on T2*.

 

 

54

Definition
up, down
Term

To turn the heat up on T1 contrast using gradient echo, the TR is short/long and the flip angle is low/high.

 

 

54

Definition
short, high
Term

To turn the heat down on T2* using gradient echo, the TE is short/long.

 

 

54

Definition
short
Term

For T2* weighting using gradient echo, turn the heat up/down on T2* and the heat up/down on T1.

 

 

54

Definition
up, down
Term

To turn the heat up on T2* using gradient echo, the TE is short/long.

 

 

54

Definition
long
Term

To turn the heat down on T1 contrast using gradient echo, the TR is short/long and the flip angle is low/high.

 

 

54

Definition
long, low
Term

For proton density weighting using gradient echo, turn the heat up/down on T1 and the heat up/down on T2*.

 

 

54

Definition
down, down
Term

Typical values in gradient echo imaging:

Long TR -

Short TR -

Short TE -

Long TE -

Low flip angles -

Large flip angles -

 

 

56

Definition
  • 100 ms+
  • less than 50 ms
  • 1-5 ms
  • 15-25 ms
  • 5-20º
  • 70º +
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