CT

Question 1

What is pixel?

Answer 1

The picture element

•the more pixels means the smaller pixels and the higher resolution it is

Question 2

What is voxel?

Answer 2

The volume element

Question 3

Pros and cons of thinner slices

Answer 3

• Thinner slices means that you are less likely to get a mixture of tissues and having this could lead to a mixture of colours and confusion
• But this does lead to an increased dose for the patient

Question 4

What is filtered back projection

Answer 4

• Relatively easy for a computer to do
• When it finds pixels that are different it makes them more different
• this isn’t good for noise so this needs to happen at the source

Question 5

What are CT numbers

Answer 5

• They are determined by the attenuation within the voxel
• the baseline is water that has a CT number of 0
• Higher density/ higher atomic number materials have higher CT numbers
• Bone is middle hundreds to low thousands
• soft tissue rumours are low which means they can be hard to catch

Question 6

What are the two effects of windowing

Answer 6

• Change the type of tissue displayed

* change the brightness (shade of grey) relationship between tissues in the final image

Question 7

What do wide and narrow ranges of greys do

Answer 7

• A wide range would give a low contrast (grey)image

* A narrow range would produce a high contrast (black and white) image

Question 8

What is WL

Answer 8

WL is the tissue (CT number) that occupies the mid point of the available grey levels

Question 9

What is WW

Answer 9

The range of tissues that occupy the grey levels

The wider the WW the lower the contrast

Question 10

What is helical or spiral CT

Answer 10

• Where the table (linear) and the tube/ detectors (rotational) move at the same time
• there is a single arch and it’s a single slice system

Question 11

What is multislice CT

Answer 11

• There are several arcs and hundreds of detectors
• Instead of one slice per rotation there could be loads so much faster scanning
• combining helical and multi-slice leads to even faster scanning

Question 12

What are some disadvantages of CT

Answer 12

• They are expensive but as the cost has stayed the same and they are now a lot faster the cost per patient has dropped and there is a reduced cost per scan
• CT can have a high dose due to Filtered back projection and the issue with noise but using low dose CT based on different maths could resolve this
• Access used to be an issue but now they are much more available

Question 13

Why isn’t CT used in radiotherapy planning

Answer 13

• The tabled aren’t flat and this needs to be the case
• the gantry is too small
• MRI is better for assessing tumour margins