Factors affecting Ct images

What does a higher signal to noise ratio result in?

Higher quality image

What is kVp?

Peak voltage applied to the x-ray tube

How does kVp effect the SNR?

Increases SNR because more energetic x-rays penetrate deeper tissues

What is MAs?

Determines the total number of photons used

How does MAs effect SNR?

Higher SNR because more photons increase amount of signal

How does dose modulation work?

Optimises SNR by adjusting x-ray dose according to the area being imaged

How does slice thickness effect SNR?

Thicker slices collect more signals giving a higher SNR

Thinner slices collect more detail but reduce the amount of signal giving a lower SNR

What is pitch?

Ratio of the table movement per tube rotation to the total width of the x-ray beam

What is the equation which represents pitch?

Table movement per rotation/ beam width

How does low pitch effect the SNR?

Allows more overlap between slices, increasing signal and SNR

How does high pitch effect the SNR?

Reduced overlap leading to less signal to lower SNR

What are the three reconstruction algorithms?

Filtered back projection

Iterative reconstruction

Deep learning reconstruction

What are the effects of filtered back projection on the SNR?

Lower SNR and higher dose

What is the effect of iterative reconstruction on SNR?

Improves SNR by reducing noise and enhancing quality

What is rotation time?

How long it takes for the Ct tube to complete a full rotation around the patient

What is temporal resolution?

Time it takes for the slices to be taken

What are the two advantages of shorter rotation time?

Improves temporal resolution

Reduces motion artefacts

What are two disadvantages of shorter rotation time?

Increases noise due to less signal

Requires higher dose

What are the two disadvantages of longer rotation time?

Causes blurring or artefacts

Less suitable for trauma imaging

What is the advantage of the longer rotation time?

More photons collected, more signal improves SNR

Why do larger patients increase noise production?

X-rays penetrate more tissue, reducing the signal reaching the detector, overlapping the lost signals from the anatomy outside of the detector field

What is the contrast to noise ratio?

Difference in hounsfield units between two areas divided by the noise level

What are two factors affecting the contrast to noise ratio?

Contrast agents

Imaging parameters