section 2 computed tomography (CT) Flashcards
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
a) decreased SNR
b) decreased radiation dose
With regard to CT, discuss what is meant by the following terms:
a) Hounsfield scale
(1 mark)
b) Hounsfield unit
(1 mark)
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
With regards to CT, explain the differences between the terms window width and window level:
-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
With regard to CT, define the following terms and state the factors that affect them:
a) Spatial resolution
(3 marks)
b) Noise
(4 marks)
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
Describe the CT image appearances of a movement artefact and state FIVE methods to reduce this in CT imaging.
(5 marks)
- 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
