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Flashcards in The basics of MRI Deck (26):
1

What is the abbreviation for slice thickness in MRI?

Thickness (Thk)

2

What is the smallest information element of a picture?

Pixel

3

The tomographic image plane contains many small elements called?

Voxels

4

What two types of molecules are responsible for the image in MRI?

water and fat

5

What is meant by tomographic image?

It is an image of a thin slice

6

What is a decibel?

a logarithmic representation of the ratio between two quantities (dB = 10 log(P1/P2)) or

7

What would the graph of y = e^-(t/tau) look like?

An exponential decay

8

What would the graph of y = (1- e^-(t/tau)) look like

Exponential growth (opposite the decay curve)

9

How do researchers solve the problem of discovering where a pixel comes from since if you have a gradient along all three axes, different combinations of Bx+By+Bz can all equal a single value?

Use the gradient magnetic fields sequentially

10

How can you tell which slice the signal comes from?

In a gradient, only a thin slice will have resoncance at the frequency of the EM pulse

11

What does each dot in k space represent?

Each dot in k-space corresponds to a sine wave with a particular spatial frequency (wave number). dots farther away from the origin will have higher spatial frequencies, while those close to the center have lower frequencies. Brightness indicates the amplitude/ or brightness of the wave. Changes from the vertical and horizontal axes change the orientation of the wave.

12

What is frequency encoding? Draw what the MR signal would look like with and without frequency encoding.

You can change the speed of precession(frequency) at different points along the x-axis by applying an x gradient at the time of signal measurement. Without frequency encoding, the signal looks like a wave that is symmetrical about the x-axis and has a particular frequency. It decreases or relaxes logarithmically. With frequency encoding, it looks more like a slow AM envelope has been added to it. A fourier transform of a frequency encoded signal can separate out the frequency components which, since there is a gradient in the x axis become position coordinates.

13

Phase encoding

After protons are excited, we can change the speed of precession (frequency) at different points in space by applying a y-gradient. When the y-gradient is turned off, protons in the y-axis precess at the same frequency but at different phases. This is called phase encoding.

14

Each level of phase encoding makes signal sensitive to a different ___________ frequency

spatial.

15

What is T2* relaxation

T2* is the dephasing of transverse magnetization due to both microscopic molecular interactions (T2) and spatial variations of the external main field(tissue/bone/air/ interfaces) It has an exponential decay

16

How do you get to the BOLD signal from increased neural activity?

Increased neural activity induces increases in blood flow which means an increase in oxyhemoglobin, which apparently means an increase in T2* which means an increase in MR signal?

17

How does Echo Planar Imaging fill in k-space?

Usually, in a gradient echo sequence, we set one discreet level of the phase encode gradient after each RF pulse (-kmax to +kmax). In EPI, a single RF excitation is used to acquire all the lines in k-space.....

18

Why would you use EPI for BOLD MRI?

image contrast is determined by magnetic susceptibility, images can be acquired quickly and are heavily T2* weighted. Thus, EPI is ideal for rapidly tracking changes in T2* (susceptibility related contrast) over time.

19

Usually, how long is the TR for EPI?

2-4 seconds.

20

For EPI, what is TE?

The time between the echo pulse and the readout at the center of k-space. (At 3T usually about 20-30msec).

21

For EPI, how is spatial resolution determined?

Spatial resolution is determined by slice thickness, field of view and matrix size(I think this refers to k-space?)

22

For EPI how is voxel size determined?

Field of view / Matrix size

23

What is the partial volume effect and what does spatial resolution have to do with it.

I'm not entirely certain, but the slide has a white circle in a black square. If your resolution is too poor to see the circle in the square (for example if you divide the high contrast square into four regions and then take the average intensity of each region, you end up with four grey squares).

24

What are the advantages to higher resolution imaging?

less partial volume effects, better ability to discriminate different brain regions and less susceptibility artifacts.

25

What are the disadvantages to higher resolution imaging?

Takes longer to image, lower SNR/CNR, and more motion sensitivity (because you are more likely to move into an adjacent voxel instead of within a single voxel).

26

What is the typical resolution?

FOV = 220-240, matrix = 64X64, Thk = 4-6mm, which comes out to a voxel that is about 3.75 X 3.75 X 5mm (How does that make sense?)