The basics of MRI

Question 1

What is the abbreviation for slice thickness in MRI?

Answer 1

Thickness (Thk)

Question 2

What is the smallest information element of a picture?

Answer 2

Pixel

Question 3

The tomographic image plane contains many small elements called?

Answer 3

Voxels

Question 4

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

Answer 4

water and fat

Question 5

What is meant by tomographic image?

Answer 5

It is an image of a thin slice

Question 6

What is a decibel?

Answer 6

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

Question 7

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

Answer 7

An exponential decay

Question 8

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

Answer 8

Exponential growth (opposite the decay curve)

Question 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?

Answer 9

Use the gradient magnetic fields sequentially

Question 10

How can you tell which slice the signal comes from?

Answer 10

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

Question 11

What does each dot in k space represent?

Answer 11

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.

Question 12

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

Answer 12

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.

Question 13

Phase encoding

Answer 13

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.

Question 14

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

Answer 14

spatial.

Question 15

What is T2* relaxation

Answer 15

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

Question 16

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

Answer 16

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?

Question 17

How does Echo Planar Imaging fill in k-space?

Answer 17

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…..

Question 18

Why would you use EPI for BOLD MRI?

Answer 18

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.

Question 19

Usually, how long is the TR for EPI?

Answer 19

2-4 seconds.

Question 20

For EPI, what is TE?

Answer 20

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

Question 21

For EPI, how is spatial resolution determined?

Answer 21

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

Question 22

For EPI how is voxel size determined?

Answer 22

Field of view / Matrix size

Question 23

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

Answer 23

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).

Question 24

What are the advantages to higher resolution imaging?

Answer 24

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

Question 25

What are the disadvantages to higher resolution imaging?

Answer 25

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).

Question 26

What is the typical resolution?

Answer 26

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?)