CT- section 2

Question 1

what are the three distinct phases of a CT formation

Answer 1

1) data acquisition
2) image reconstruction
3) image display

Question 2

what is data acquisition

Answer 2

‘scanning phase’- produces raw data but not the image

Question 3

what is image reconstruction

Answer 3

processes the raw data converting it to image data

Question 4

what is image display

Answer 4

the image data is displayed on a CRT monitor. image manipulation may take place at this stage

Question 5

what happens during the scanning phase

Answer 5

a fan-shaped x-ray beam scans around the body, is attenuated by the patient anatomy and is detected by the detectors

Question 6

what 3 things does attenuation depend on

Answer 6

1) density
2) atomic number
3) thickness of a structure

Question 7

what is the linear attenuation coefficient

Answer 7

is the degree to which the x-ray intensity is reduced per thickness by the material it passes through

Question 8

how often does the DAS sample the detected x-ray beam

Answer 8

at regular time intervals

Question 9

DAS produces numerical measurements called

Answer 9

views

Question 10

what does each view produce

Answer 10

a line of data in the form of a graph called an attenuation profile.
This means that the amount the x-ray beam has been attenuated along its path is now represented numerically in the form of a graph. known as RAW DATA

Question 11

what happens during image reconstruction

Answer 11

the raw data is used to reconstruct an image in a process known as back projection

Question 12

what are the attenuation profiles for a particular cross sectional slice of anatomy projected onto

Answer 12

a matrix

Question 13

what is the one drawback of back projection

Answer 13

it causes streaks or artefacts on the image

Question 14

how is streaks/artefacts made by back projection minimised

Answer 14

the data is filtered before back projection takes place

Question 15

what is filtering

Answer 15

involves a number of complex mathematical steps, where algorithms are applied to raw data to remove the blurring seen in unfiltered back projection

Question 16

the more views used…

Answer 16

the more accurate representation of the original

Question 17

what is a matrix made up of

Answer 17

squares called pixels

Question 18

the pixels are actually a cube of data known as

Answer 18

a voxel

Question 19

each pixel/voxel will have a numerical value due to…

Answer 19

the back projection of the numerical data from the attenuation profiles produced during data acquisition

Question 20

the numerical value of each voxel is expressed in

Answer 20

Hounsfield Units (HU)

Question 21

what does the HU represent

Answer 21

the average density of the anatomical tissues within the voxel

Question 22

what are the HU units for water, bone and air respectively

Answer 22

water = 0 HU 
bone = +1000 HU 
air = -1000 Hu

Question 23

what does each HU value have

Answer 23

a corresponding shade of grey

Question 24

once the HU’s are displayed on the matrix on the monitor what does it create

Answer 24

an image

Question 25

what is this data then referred to as

Answer 25

image data

Question 26

what is window width (WW)

Answer 26

determines the range of HU that will be represented on the image.

Question 27

what does the window level select

Answer 27

the centre value of the window width

Question 28

what is the benefit of windowing

Answer 28

gives CT a very high contrast resolution. this is because a window can be set to display and make visible very small differences in tissue densities