CT intro 1

Question 1

what are the benefits of CT over conventional radiography?

Answer 1

1. body is a 3D object therfore imposition of a 3D subject onto a 2D plane is problamatic. many objects overlap each other and we can not tell them apart
2. limited differentiation of diffrent densitites: water and tissiue
3. especially in imaging skull
   
   1. bony density of the skull is problamatic since it attenuates x-ray
   2. brain can be viewed as a tissue bathed in cerebrospinal fluid (CSF)

Question 2

what is the main idea behind CT imaging?

Answer 2

data regarding density values of the area of interest is fed into a computer and by using a mathematical technique called F_iltered Back Projection_, spesific density values are projected into the image.

x-ray source passes through a slice of body and then detected by the row of detectors, density values of each slice is fed into the computer, then posprocessed by Filtered Back Projection, different density values are assigned with different shades of gray, which will be projected to computer and processed into an image.

Question 3

what is Hounsfield Unit?

HU

Answer 3

it is a value used to measure density on a CT scan, where water has the value of zero HU and the air has the value of a -1000 HU accordingly we can build a scope (window) by defining a central value (window level) and window width to study a specific scope trasnleted into shades of grays thus adjusting the contrast levels

Question 4

the higher the HU the —- appears on CT imaging,

Answer 4

brighter, therefore air is the darkest shade -1000 and bone with HU value, between + 400 +1000 is the brightest shade

Question 5

why do we use window width/level when viewing an image?

Answer 5

due to the limits of human’s eye in differentiating shades of gray, which is approximately 16 shades of gray therfore by means of defining window level and window width we can solve this limitation of observation of shades of grays. For example if we look at an image with the window with of 2000 HU, meaning a range of -1000 to 1000 HU, considering human’s eye visual limit: 2000/16= 125HU, human’s eye will be abe only to differentiate between radiodensities which have the discripency of 125 Hu. In other words, in a window width of 2000 HU and window level of 0, we can only observe that the shade of gray at 125 HU is diffrenet from 250 HU, however we can not see any diffrence between shades of gray of 125 HU and 130 HU.

Question 6

what are the key information that one needs to imiduately observe at CT image?

Answer 6

Orientation:

1. anterior
2. posterior
3. left
4. right

Recognition of Contrast

1. ​Contrast-assited image
2. non contrast image

Hounsfield Unit (HU)

1. adjusting window with/level therby optimizing contrast
2. radiodensity of objects imaged, HU is the unit of radiodensity, meaning how well x-ray is absorbed or passed though an anatomical object