To explain the above, the concepts of CTDI and MSAD are needed.
While the MSAD is the average dose to the center of a series of scans, the CTDI is the dose to a single slice in the middle of a series of contiguous slices.
The CTDI can be measured by a single pencil ionization chamber measurement, and this measurement is used to calculate the MSAD.
When the SW equals the BI, the MSAD numerically equals the CTDI.
The most recent survey of the MSAD from 250 CT scanners in 26 states was published in 1992.
Rules for estimating dose (MSAD) in CT are provided in Table 5.
Table 5 Rules for Estimating Dose To the Patient (MSAD) in CT(*)
For a well-collimated system, MSAD increases only slightly as slice thickness decreases.
* Are the dose descriptors, the CTDI and the MSAD used to quantify the dose in conventional CT adequate for spiral/helical CT?
Are the 2 standard dose descriptors, the MSAD and the CTDI, valid for assessing the dose delivered to a patient during spiral/helical CT scanning?
To reduce the dose to the patient, one should consider ways to reduce the MSAD. There are 2 possibilities: