Lecture 2 - MRI

Question 1

Strength of the static magnetic field is determined by…

Answer 1

the current flow through the coil

Question 2

Larmor frequency of hydrogen is…

Answer 2

42 MHz/T

Question 3

TR refers to

Answer 3

the time interval between successive excitation pulses

usually measured in seconds

Question 4

Spatial encoding within a slice involves…

Answer 4

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

Question 5

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

Answer 5

transverse (x,y) plane

Question 6

T-1 weighted image is dependent on…

Answer 6

difference in the recovery of magnetic signal in the z direction

Question 7

T-2 decay results from…

Answer 7

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

Question 8

Slice selection involves the combined use of…

Answer 8

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

Question 9

how to reduce resistance in the coils

Answer 9

superconductive magnets cooled by liquid helium

Question 10

How do you increase the homogeneity of the magnet?

Answer 10

Increase length and decrease diametre of bore

Increase number of turns in coil

Question 11

Has hydrogen a positive or negative charge?

Answer 11

positive

Question 12

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

Answer 12

the strength of the magnetic field

Question 13

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

Answer 13

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

Question 14

How many gradient coils are there?

Answer 14

3 - x,y,z

Question 15

What is a bottom up, interleaved technique?

Answer 15

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

Question 16

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

Answer 16

z

Question 17

What is phase encoding?

Answer 17

changing magnetic strength along the y plane

Question 18

What is frequency encoding?

Answer 18

changing magnetic strength along the x plane

Question 19

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

Answer 19

course spatial information with a low spatial frequency

Question 20

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

Answer 20

finer detail spatial information

Question 21

Where is the overall signal represented in k space?

Answer 21

in the centre

Question 22

How do you decode k space into image space?

Answer 22

Apply inverse Fourier transformation

Question 23

what is TE?

Answer 23

time from RF pulse being delivered to image being captured

Question 24

Proton Density Imaging

Answer 24

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

Question 25

T1 contrast image

Answer 25

TR intermediate, TE short

Question 26

T2 contrast

Answer 26

TR long, TE intermediate Used in functional MRI

Question 27

Echo-planar imaging

Answer 27

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

Question 28

What are shimming coils?

Answer 28

Reduce the imhomogeneity of the magnet | Account for size and shape of PSs head

Question 29

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

Answer 29

In the left-right direction

Question 30

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

Answer 30

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

Question 31

K space and echo planar imaging?

Answer 31

Sample k-space in a linear zig-zag trajectory

Question 32

Spiral imaging and K-space

Answer 32

Sample k-space in a spiral trajectory

Question 33

What are field map corrections?

Answer 33

Correct for distortions and inhomogeneities in image

Question 34

Aunt Bessies MRI recipe

Answer 34

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

Question 35

Equal magnetic fields that point in different directions?

Answer 35

Cancel out

Question 36

If the magnetic field was increased along the z axis...

Answer 36

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

Question 37

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

Answer 37

Y direction

Question 38

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

Answer 38

X direction

Question 39

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

Answer 39

Z direction

Question 40

Vertical information in the image | is represented along _____ axis in K-space

Answer 40

x

Question 41

TE in the order of ms or s

Answer 41

MS

Question 42

TR in the order of ms or s

Answer 42

S

Question 43

Which is longer, T1 or T2?

Answer 43

``` T1 longer (seconds) T2 shorter (milliseconds) ```

Question 44

Proton density imaging measures the density of what?

Answer 44

Hydrogen/water

Question 45

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

Answer 45

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

Question 46

T1 contrast is used for what type of image?

Answer 46

Structural image | shows differences in white matter, grey matter and ventricles

Question 47

T2 weighted image is used for what type of scans?

Answer 47

Clinical scans and fMRI

Question 48

T2 contrast: TE and TR?

Answer 48

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

Question 49

Why wouldn't you use proton density?

Answer 49

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

Question 50

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

Answer 50

white ventricles grey

Question 51

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

Answer 51

TR (intermediate)

Question 52

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

Answer 52

TE (intermediate)