mA modulation Flashcards
what is the z-axis
the longitudinal axis of the patient
how does the tube rotate in CT
it rotates around the patient and the table translates through the gantry along the z-axis of the patient
what is image noise
the result of the random variation in pixel number due to ;
a) the random distribution of x-ray photons
b) imperfections in the imaging system
what does image noise cause
a reduction in image quality
noise reduces as
the intensity of the incoming signal is increased, but this is at the cost of a higher patient dose
noise is
inversely proportional to 1/ square root of number of photons
what is mA modulation
it is an automatic exposure control system within the CT system
using the initial scout view, what does the CT system calculate
the optimum mA for the patient anatomy that is included in each CT ‘slice’
what must be done to make sure that mA modulation is accurate
the patient must be positioned at the isocentre of the gravity
what two things should care be taken with to make sure mA modulation is correct
1) remove artefacts
2) position anatomy that is not required in the scan out of the scan field of view- this is so that the Ct system does not include arms/artefacts within the mA calculation as this would increase patient dose
what happens if the noise index is decreased
the amount of noise acceptable is reduced, therefore mA values may increase to reduce the noise within the image
what would be the effect of noise index being decreased on image quality and dose
image quality would be improved BUT patient dose would be increased
what are the 4 benefits of mA modulation
1) CT exposure factors are tailored for each individual patient
2) Ct exposure factors are tailored for differing anatomy within the patient, throughout the scan
3) patient dose may be reduced
4) image quality optimised throughout the scan
