# C.8.15.3.8.1 Exposure Time in ms Attribute (Informative)

An example of how the value of Exposure Time in ms (0018,9328) can be calculated:

The Exposure Time T exposuretime of a volume shall be the time of the X-Ray radiation in position with this volume. It depends on the travel speed and the total collimation width.

T exposuretime of the volume represented by a distinguished frame of a Spiral Scan can be calculated as follows:

T exposuretime = N x S / V travelspeed

CT pitchfactor = DD / N x S

V travelspeed = DD / T revolutiontime

From this the exposure time can be derived:

T exposuretime = N x S x T revolutiontime / DD = T revolutiontime / CT pitchfactor

Legend:

DD = Table Feed per Rotation (0018,9310) in mm distance moved by the patient support in the z direction per 360 o revolution in Spiral scanning

S = collimation of a single slice in mm

N = number slices

N x S = Total Collimation Width (0018,9307) in mm

V travelspeed = Table Speed (0018,9309) in mm/second, travel speed of the patient table

T revolutiontime = Revolution time (0018,9305) in seconds

CT pitchfactor = Spiral Pitch Factor (0018,9311)