Seems as if many have problems and or concerns regarding dose and original recon thickness (recon 1).
Exposure/dose is always calculated from recon 1. Also, the exposure needed is calculated form the scout view so please make certian that patient centering within the gantry bore is a priority.
When the system is programmed to have recon 1 at a thin slice thickness such as 0.625mm and the NI is set high, it is the same dose/exposure as a thicker slice such as 2.5mm with a lower noise index.
The system calculates the amount mA needed, based on the NI for a programmed signal to noise ratio in the preprescribed axial image.
Think of this...(possible examples)
If a protocol is set up with a noise index of 28 and a slice thickness of 0.625mm, the resulting image should average a signal to noise ratio resulting in 28. If the same/original data set is reconstructed into 2.5mm the measured signal to noise ratio will be around 15. Same dose-same original scan but different signal to noise ratios. Remember that when slice thickness increases, image noise goes decreases if the same data set is reconstructed.
If a protocol is set up with a noise index of 15 and a slice thickness of 2.5mm, the resulting image should average a signal to noise ratio resulting in 15. If you took the same/original data set and reconstructed at 0.625mm and measured the signal to noise ratio it would result in an average of 28. Remember that when slice thickness decreases, image noise increases if the same data set is reconstructed .
Both examples have exactly the SAME dose
Please forgive me, but I am trying to recall noise index settings from home and they may not be exactly accurate but are close. If building protocols please refer to the owners manuals. As "Jack" says, please also follow ALARA.
Hope this helps!
Last edited on Fri Apr 27th, 2012 11:30 pm by FMLHCT