Computed Tomography

Question 1

Describe attenuation.

Answer 1

The radio density of a material measured by how much stopping power it has on radiation through absorption and reflection

Question 2

How is attenuation measured?

Answer 2

-In hounsfield units
-By the different radiodensities of the different components of the body

Question 3

What are linear coefficients and how are they used in CT?

Answer 3

Values that are converted to Hounsfield units and produce a greyscale image. They are used to describe how much of the X-ray beam is attenuated by a material

Question 4

List the 5 components of a CT scanner and their functions.

Answer 4

-Gantry: houses the X-ray tube, detectors and generators
-X-ray Tube: rotates 360 degrees in the axial plane around the patient’s body
-Filters: improve efficiency of the X-ray beam
-Collimators: reduce patient dose and improve image quality
-Detectors: arranged in rotating rows in a continuous arch directly opposite the X-ray tube

Question 5

2nd generation CT scanners…

Answer 5

…have multiple detectors and a fon-shaped beam which makes it faster than a 1st generation scanner.

Question 6

What 4 things should a CT detector have?

Answer 6

-Dynamic range to allow the detector to respond to a wide range of beams
-A fast response
-Robust stability
-High efficiency so it can capture, absorb and convert X-ray photons into electrical signals

Question 7

What is the difference between 3rd gen and 4th gen CT scanners?

Answer 7

A 3rd-gen CT scanner has a rotating X-ray tube linked to an arch of detectors that can only rotate 180 degrees to each side of the patient.
Whereas 4th-gen CT scanners have detectors all around the gantry that allow the X-ray beam to rotate 360 degrees

Question 8

What do 1st generation CT scanners consist of?

Answer 8

A detector and a pencil-like X-ray beam that takes 5 minutes to capture a slice and 20 minutes to analyse data

Question 9

List the 3 features of the scanner that dissipate heat quickly.

Answer 9

-High-speed rotors
-Direct oil cooling of the anode
-Focal spot cooling algorithms

Question 10

What is the equation for attenuation?

Answer 10

tissue m - water m
______________________
water m x 100

Question 11

Windowing is..

Answer 11

…the process of changing CT numbers to manipulate the CT image greyscale by adjusting WW & WL

Question 12

Describe the functions of WW and WL.

Answer 12

-WW: controls the image contrast and highlighting
-WL: controls the image density

Question 13

Define the purpose of a CT detector.

Answer 13

To convert photons into electrical signals called analog data which is then converted to digital information for computer reconstruction.

Question 14

State the difference between Conventional data acquisition and Volume data acquisition.

Answer 14

Volume data acquisition is where the X-ray tube and detectors continuously rotate around the patient. Whereas conventional data acquisition the table moves slice by slice.

Question 15

Name 5 features of a multiple detector.

Answer 15

-Faster scanning times
-Takes multiple slices
-Optimises scanning times after contrast injection
-Reduced radiation times
-Acquire thick and thin slices over large volumes

Question 16

What are the features of a spiral CT scanner?

Answer 16

-Rotating slip rings which allow electricity to be conducted by brushes touching them
-Conduction that permits continuous X-ray tube and detector rotation without rotating the table

Question 17

What is spatial resolution?

Answer 17

The ability to tell apart structures that differ in density

Question 18

True or false?
Contrast resolution is the scanners ability to tell apart smaller differences in attenuation to tell apart smaller tissues.

Answer 18

True.

Question 19

Decreasing the FOV…

Answer 19

…increases spatial resolution and decreases SNR

Question 20

What happens when the mAs is decreased?

Answer 20

Spatial resolution increases, and SNR and radiation dose are decreased

Question 21

What does SNR compare?

Answer 21

The levels of desired signals on an image to the level of the background noise.

Question 22

True or false?
Increasing the matrix increases spatial resolution and radiation dose but decreases SNR.

Answer 22

False, increasing the matrix has no effect on the radiation dose.

Question 23

When the pitch is increased…

Answer 23

…the SNR, spatial resolution and radiation dose decreases

Question 24

SNR and radiation dose are increased when…

Answer 24

…kVp is increased and spatial resolution is decreased.

Question 25

Why is windowing important?

Answer 25

-A higher WL & WW results in a darker greyscale and viewing of dense bones
-Manipulates greyscale for more detail
-Lowering the WW & WL whitens the greyscale which allows for soft tissue viewing

Question 26

Are motion artifacts patient-based or physics-based?

Answer 26

Patient-based, it is photon starvation artifacts that are physics-based

Question 27

How can photon starvation artifacts be reduced?

Answer 27

-mA modulation
-Adaptive filtration
-Iterative reconstruction

Question 28

List 4 causes of patient-based motion artifacts.

Answer 28

-Patient
-Cardiac
-Bowel motion
-Respiratory

Question 29

Describe how ring artifacts occur.

Answer 29

A faulty detector, during its rotation around the patient, can produce a ring artifact of signal loss

Question 30

Name 5 ways radiation dose to the patient can be reduced.

Answer 30

-Increase the pitch
-Use automatic mA modulation
-Ensure examination is justified beforehand
-Reduce kVp
-Use short scan time
-Set right exposure factors to avoid repeat exams
-Ensure the patient is correctly positioned