Week 1: CT Image Display

Question 1

Linear Attenuation Coefficient

Answer 1

How much attenuation occurs from a certain material at a certain kV. HU created, water = 0.

Question 2

Hounsfield Units

Answer 2

Each pixel is assigned a HU after the raw data is plotted on the matrix. HU quantify the degree that structures attenuate the x-ray beam. HU can be compared across all scanners and do not change when WW/WL is manipulated.

Question 3

Volume Averaging

Answer 3

Many attenuation values are combined then represented in one, less accurate pixel

Question 4

Partial Volume Effect

Answer 4

Volume averaging could cause a partial volume effect where the number averaged significantly and the pixel is very inaccurate.

Question 5

Window Level (WL)

Answer 5

the mid point of a grey scale range, used to set the centre of the CT numbers displayed. Set to demonstrate tissue of interest. Low window level results in light image, high window level results in a dark image.

Question 6

Window Width (WW)

Answer 6

determines the total number of His that will be displayed

Question 7

Wide WW

Answer 7

low contrast (lots of greys), for tissues that vary greatly, for when you want to see many things (ex abdomens)

Question 8

Narrow WW

Answer 8

high contrast (fewer greys), for tissues that are very similar, for when you need to see small difference sin attenuation (ex brains)