Computed Tomography I

Question 1

how many generations of CT scanner have been produced?

Answer 1

four

Question 2

which generation CT scanner do we use today? Why?

Answer 2

we use the third generation CT scanner nowadays, fourth was developed but increased patient dose so was abandoned

Question 3

what does CT involve?

Answer 3

sectional imaging of the patient in the axial plane

Question 4

name equipment required for CT?

Answer 4

X-ray tube, high voltage generator, collimator, detector array mounted on a rotating gantry

Question 5

what is the axis of rotation referred to in CT?

Answer 5

The Z-axis

Question 6

is there collimation in CT?

Answer 6

yes

Question 7

what is the aim of collimating in CT?

Answer 7

to produce a wide fan beam to cover the full cross section of the patient

Question 8

what does the gantry contain?

Answer 8

the x-ray tube, x-ray generator, detector

Question 9

can the gantry be tilted and what conditions would this be done?

Answer 9

gantry can be tilted, this is often done during brain CT

Question 10

one complete rotation of the gantry = ?

Answer 10

one complete rotation of the gantry = one complete data set

Question 11

what is used by the computer to reconstruct the images?

Answer 11

the complete data set

Question 12

what is the anode-cathode parallel to?

Answer 12

the anode-cathode axis is parallel to the Z-axis

Question 13

CT uses high ___ and __

Answer 13

CT uses high kVp and high mA

Question 14

what types of radiation does CT scanners have?

Answer 14

bremsstrahlung and characteristic radiation

Question 15

what cools the oil around the CT scanner and why?

Answer 15

the heat exchangers, this is because CT scanners have a high heat capacity

Question 16

what is the function of slip rings?

Answer 16

to provide electric current

Question 17

what is a feature of slip rings?

Answer 17

copper or graphite brushes that rub against the slip rings to produce electric current

Question 18

what type of x-ray beam do you need for CT to produce a reasonably monochromatic image for reconstruction?

Answer 18

you need a monochromatic beam!

Question 19

how does CT achieve a mono-energetic beam?

Answer 19

by using high filtration

Question 20

what type of filter is commonly used in CT?

Answer 20

Copper filters, removes low energy x-ray photons

Question 21

what is the purpose of the bow tie filter?

Answer 21

the bow tie filter equalises noise levels, allowing for varying thicknesses of the patient

Question 22

where is noise greatest in a CT image?

Answer 22

noise greatest at the centre of the imaged area and lowest at the periphery

Question 23

what does the bow tie ensure?

Answer 23

bow tie filter ensures that all detectors receive the same exposures

Question 24

what is the bow tie filter made of ?

Answer 24

teflon

Question 25

purpose of beam-shaping/bow tie filter?

Answer 25

the purpose of the bow tie filter is to reduce the dynamic range of signal recorded by the detector, reduce dose, normalise beam hardening, aid calibration

Question 26

what can be adjusted to produce a range of slice widths

Answer 26

the z-axis dimension of the fan beam

Question 27

what determines slice thickness?

Answer 27

collimator

Question 28

what sets the upper limit of slice thickness?

Answer 28

the detector width

Question 29

if kVp and mAs stay the same, what increases linearly with slice thickness

Answer 29

the number of photons increases linearly with slice thickness if kVp and mA stay the same

Question 30

what does the Slice Sensitivity Profile depend on?

Answer 30

the SSP depends on the finite size of the focal spot, penumbra of the collimator, use of multiple projections to produce image, helical scanning further increases SSP

Question 31

How many projections does a full rotation of the x-ray tube need

Answer 31

1000 projections

Question 32

desirable properties of detector

Answer 32

high DQE, small size, fast response, negligible dead time, stable, noise free response and a wide dynamic range

Question 33

what is involved in a helical CT scanner?

Answer 33

the patient is moved continuously through the scanner as the gantry rotates around the patient

Question 34

what is the scan pitch?

Answer 34

table movement per rotation / nominal slice thickness eg: patient moves 10mm/rotation, slice thickness is 2mm, therefore scan pitch is 2

Question 35

what is the effect of an increased scan pitch?

Answer 35

decrease scan time, decrease dose, decrease image quality due to Z-axis blurring