CT scanner

Question 1

What is an attenuation coefficient?

Answer 1

Measure of how easily a material can be penetrated by an X-ray beam

Question 2

Give an example of a material with a high attenuation coefficient.

Answer 2

Bone

Question 3

How does density of a material relate to attenuation coefficient?

Answer 3

The more dense a material the higher the attenuation coefficient.

Question 4

Summarise briefly the mode of action of a CT scanner

Answer 4

X-ray tube produces X-ray photons which penetrate the patient. Residual beams exit the patient and are detected by the detector array. The detector array can translate the residual beam energy into a measure of attenuation. This data is then transferred to form a digital image.

Question 5

What is the high voltage generator used for in the gantry?

Answer 5

Supplies a high voltage to the X-ray tube.

Question 6

Name 4 components found within a CT scanner gantry

Answer 6

1) X-ray tube
2) Detector
3) Collimators
4) High voltage generator

Question 7

What are individual components of a detector called?

Answer 7

Detector modules

Question 8

How are the detector modules arranged and what do they make up?

Answer 8

Arranged in rows and they make up the detector array

Question 9

What does the number of rows of a detector module usually tell us?

Answer 9

How many slices a CT scan produces.

Question 10

What is the name of the process that happens when a residual X-ray beam hits a detector module?

Answer 10

Scintillation

Question 11

Explain via scintillation how a digital signal is produced at the detector module?

Answer 11

1) X-ray photos hit the scintilating detectors in the detector modules
2) light photons are produced
3) Photodiodes measure the light produced
4) absorption of light energy creates an electrical signal which is converted to a digital signal.

Question 12

What is scintillation

Answer 12

This is a process where light is emitted by a material following absorption of ionising radiation.

Question 13

Name the two types of Collimators found in a CT scanner and what they do

Answer 13

Pre-patient - determine the beam width (usually set to cover the detector array)
Post patient - maintains beam uniformity by absorbing scattered photons

Question 14

What is the purpose of the BOWTIE filters?

Answer 14

These are found in front of the X-ray tube and ensure that the beam energies are homogenous before hitting the patient.

Question 15

Name the two types of acquisition model of a CT scan

Answer 15

1) Helical (most commonly used), X-ray beam is ON continuously as patient passes through machine.
2) Axial (step and shoot). Patient is moved through scanner in increments and X-ray beam is turned on to complete a slice and then turned off.

Question 16

What are two advantages of using a helical acquisition model over an axial one?

Answer 16

Due to the spiral nature of the scan around the body, parts of the body are double dosed which means more data is acquired resulting in a more accurate image.
It is also less time consuming.

Question 17

Give 4 examples of pre-processing parameters in CT

Answer 17

1) X-ray tube voltage (kV)
2) X-ray tube current (mA)
3)Rotation time (s)
4)Pitch

Question 18

What is the effect of increasing the voltage supplied to the X-ray tube?

Answer 18

higher voltage = more penetrative X-rays (this can sometimes reduce image quality)

Question 19

What effect does rotation time have on the patient dose.

Answer 19

A shorter rotation time means less dose to patient. Gantry spins quicker which means exposure to radiation is a shorter time.

Question 20

What effect does pitch have on the patient dose/image quality

Answer 20

A lower pitch covers more of the patient, delivering more dose but resulting in more data and thus better image quality.

Question 21

What is pitch expressed as?

Answer 21

Ratio

Question 22

What is the hounsfield unit?

Answer 22

Relative quantitative measurement of the radiodensity of a material relative to water.

Question 23

List 3 post-processing parameters

Answer 23

1)slice spacing/interval
2)reconstruction algorithm
3)window width/window level

Question 24

What two things need to be considered when administering contrast agents?

Answer 24

1) Kidney function
2) Allergies