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Psychology - CN > Lecture 2 - MRI > Flashcards

Lecture 2 - MRI Flashcards

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1
Q

Strength of the static magnetic field is determined by…

A

the current flow through the coil

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2
Q

Larmor frequency of hydrogen is…

A

42 MHz/T

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3
Q

TR refers to

A

the time interval between successive excitation pulses

usually measured in seconds

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4
Q

Spatial encoding within a slice involves…

A

changing the magnetic field in the y direction and x direction using gradient coils

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5
Q

After RF pulse, magnetic signals from the H atoms are detected in the ___ plane

A

transverse (x,y) plane

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6
Q

T-1 weighted image is dependent on…

A

difference in the recovery of magnetic signal in the z direction

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7
Q

T-2 decay results from…

A

changes in the synchronization of the hydrogen ions as they rotate in the x,y plane

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8
Q

Slice selection involves the combined use of…

A

z gradient coil and an appropriate radio-frequency (RF) pulse

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9
Q

how to reduce resistance in the coils

A

superconductive magnets cooled by liquid helium

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10
Q

How do you increase the homogeneity of the magnet?

A

Increase length and decrease diametre of bore

Increase number of turns in coil

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11
Q

Has hydrogen a positive or negative charge?

A

positive

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12
Q

What is the speed of rotation (larmour frequency) dependent on?

A

the strength of the magnetic field

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13
Q

What frequency does the RF pulse have to be to knock the hydrogen atoms into the transverse plane?

A

Their larmor frequency (42MHz if a 1T magnet)

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14
Q

How many gradient coils are there?

A

3 - x,y,z

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15
Q

What is a bottom up, interleaved technique?

A

taking odd slices ascending up the brain, and even numbers on the way down

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16
Q

Which gradient do you manipulate to excite different slices in the axial plane?

A

z

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17
Q

What is phase encoding?

A

changing magnetic strength along the y plane

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18
Q

What is frequency encoding?

A

changing magnetic strength along the x plane

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19
Q

What kind of spatial information does the centre of k space give you?

A

course spatial information with a low spatial frequency

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20
Q

what kind of spatial information does the outer regions of k space give you?

A

finer detail spatial information

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21
Q

Where is the overall signal represented in k space?

A

in the centre

22
Q

How do you decode k space into image space?

A

Apply inverse Fourier transformation

23
Q

what is TE?

A

time from RF pulse being delivered to image being captured

24
Q

Proton Density Imaging

A

Hydrogen Density
TR Long, TE short
Measures the amount of water in different parts of the brain

25
Q

T1 contrast image

A

TR intermediate, TE short

26
Q

T2 contrast

A

TR long, TE intermediate

Used in functional MRI

27
Q

Echo-planar imaging

A

rapidly changing x and y gradients allows quick sampling of k space and gain image of whole brain in three seconds

28
Q

What are shimming coils?

A

Reduce the imhomogeneity of the magnet

Account for size and shape of PSs head

29
Q

Which direction is phase encoding applied to minimize the spread of artifacts into the brain?

A

In the left-right direction

30
Q

Where in k space are low and high frequencies? What about orientation?

A

Low frequencies in the centre
High frequencies in the surround
Different orientations round the clock

31
Q

K space and echo planar imaging?

A

Sample k-space in a linear zig-zag trajectory

32
Q

Spiral imaging and K-space

A

Sample k-space in a spiral trajectory

33
Q

What are field map corrections?

A

Correct for distortions and inhomogeneities in image

34
Q

Aunt Bessies MRI recipe

A

Put protons in strong magnet - these protons precess at specific frequency mainly in the longitudinal plane

Transmit RF pulse at larmor frequency - protons absorb this energy and flip to transverse plane

Stop RF transmission - protons return to original state
Longitudinal magnitization increases (T1)
Transverse magnitization decays (T2) due to spin-spin interaction

Measure RF frequency at appropriate point in time

Convert RF represented in k-space to representation of physical space

35
Q

Equal magnetic fields that point in different directions?

A

Cancel out

36
Q

If the magnetic field was increased along the z axis…

A

the RF frequency to excite the top of
the brain would be different from the
bottom of the brain

37
Q

If you wanted to select coronal slices, which direction would you vary the magnet in?

A

Y direction

38
Q

If you wanted to select sagittal slices, which direction would you vary the magnet in?

A

X direction

39
Q

If you wanted to select axial slices, which direction do you vary the magnet?

A

Z direction

40
Q

Vertical information in the image

is represented along _____ axis in K-space

A

x

41
Q

TE in the order of ms or s

A

MS

42
Q

TR in the order of ms or s

A

S

43
Q

Which is longer, T1 or T2?

A 
T1 longer  (seconds)
T2 shorter (milliseconds)
44
Q

Proton density imaging measures the density of what?

A

Hydrogen/water

45
Q

Why do we use a short TE for proton density imaging?

A

Because we are interested in the amount of signal in the transverse plane of hydrogen atoms

46
Q

T1 contrast is used for what type of image?

A

Structural image

shows differences in white matter, grey matter and ventricles

47
Q

T2 weighted image is used for what type of scans?

A

Clinical scans and fMRI

48
Q

T2 contrast: TE and TR?

A 
TR long (not interested in differences of T1) 
TE intermediate (because contrast in T2 is larger at that point)
49
Q

Why wouldn’t you use proton density?

A

For when there is the same amount of hydrogen in 2 different areas

50
Q

T1 relaxation for ____ matter is quicker than ____ and ____ matter

A

white
ventricles
grey

51
Q

If you are interested in T1, is TR or TE more important?

A

TR (intermediate)

52
Q

If you are interested in T2, is TR or TE more important?

A

TE (intermediate)

Psychology - CN (9 decks)

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