Dose Considerations and Radiation Protection Issues in Multislice CT

To obtain the average dose for a multiple slice CT scan performed over a larger body region, it is thus sufficient to measure the CTDI from a single rotation by acquiring the dose over the entire dose profile. The situation can calculate, for two CT series carried out over the same scan length with p=1(MASD=CTDI) and p<1(MASD=CTDI), respectively

In practice, CTDI measurement are usually performed with a pencil ionization chamber with an active length of 100 mm, which is positioned at center (CTDI _100,c) and at periphery (CTDI _100,p) of either a standard head or body CT dosimetry phantom. On the assumption that the dose decreases linearly with the radical position from the surface to the center of the phantom. The average dose given by weighted CTDI

CTDI_w =1/3 CTDI_100,c + 2/3 CTDI_100,p

Because the CTDI_w is directly proportional to electrical current-time product (Qel in mAs) chosen for the scan, it has to be measured for all combinations of tube potentials (u in kV) and slice collimations that can realized at specific type of scanner but only for affixed Qel value

According to revised IEC standard 60601244 , the dose quantity sisplayed at operator console of a CT system is the effective or volume CTDI :

CTDIvol=CTDIw/p

That takes the effect of overlap of slice profiles at the level of focal dose into account. The CTDI_vol is principal dose description in CT, reflecting not only the combined effect of the scan dose level, but also of scanner specific factors such as beam filtration, beam shaping filter , geometry and over beaming.

Besides CTDI_vol, the length L of the scan region that determines radiation exposure of patient undergoing a CT Procedure. There fore, the dose length product(DLP)

DLP = CTDI_vol L

(given in mGy.cm), is used as further operational dose quantity (source :   Protocols for Multislice CT (R. Brüning, A. Küttner, T. Flohr)2005: 19)