section 2 computed tomography (CT)

Question 1

With regard to CT, describe the effect of decreasing the kVp on the following, assuming no other changes are made:

a) Signal to noise ratio

(1 mark)

b) Radiation dose

(1 mark

Answer 1

a) decreased SNR

b) decreased radiation dose

Question 2

With regard to CT, discuss what is meant by the following terms:

a) Hounsfield scale

(1 mark)

b) Hounsfield unit

(1 mark)

Answer 2

a) derived from the relative x-ray attenuation coefficients of tissue types compared to water
b) individual attenuation unit extending from 1000 to +1000 which are converted into grey scale or image intensity

Question 3

With regards to CT, explain the differences between the terms window width and window level:

Answer 3

-the window width is the range of CT numbers displayed as shades of grey
- it controls the image contrast
the window level determines the CT number in the centre of window width
- it controls image density

Question 4

With regard to CT, define the following terms and state the factors that affect them:

a) Spatial resolution

(3 marks)

b) Noise

(4 marks)

Answer 4

A)- spatial resolution is the ability to detect smaller objects in the image

• affected by pixel and
• slice thickness

B) - noise is defined as the standard deviation of signal in a homogeneous phantom

• affected by quantum mottle
• electronic noise
• noise introduced by the reconstruction process e.g. filtered back projection versus iterative reconstruction

Question 5

Describe the CT image appearances of a movement artefact and state FIVE methods to reduce this in CT imaging.

(5 marks)

Answer 5

• causes blurring, double images and long range streak artefacts on the image
  solution;
• faster scan times ( to reduce the length of time on the table) - particularly helpful in paediatric and geriatric patient (dementia/confused patients)
• consider the patient comfort ( pillows, pads to aid comfort)
• compliance and consider immobilisation or sedation
• for cardiac imaging use gated ECG to minimise movement artefacts from the heart and coincide with cardiac rhythm