MSAD usually measured directly otherwise modifying CTDI by scan pitch. For example overlapped multi scan would have higher MSAD. 

For CT, multiple scan average dose (MSAD) is the dose from a multiple scan examination, averaged over one scan interval in the central portion of the multiple-scan dose profile. See Bauhs JA, Vrieze TJ, Primak AN, Bruesewitz MR, McCollough CH. CT dosimetry: comparison of measurement techniques and devices. Radiographics. 2008;28(1):245-53.

thanks a lot dear teachers

Johanset Orihuela : in your opinion " bias" means tending to overestimation in dosimetry ?if yes, why?

You may find useful  information about your question in this paper

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3079120/ 

•The multiple scan average dose (MSAD) is the standard for determining radiation dose in CT.

There are many definition for MSAD:

1-The MSAD is the dose to tissue that includes the dose attributable to scattered radiation emanating from all adjacent slices.

2-The MSAD is defined as the average dose, at a particular depth from the surface, resulting from a large series of CT slices.

#-The MSAD could be measured directly by placing a small exposure meter at a point in a dose phantom, taking a large series of CT scans of the phantom with the meter in the middle of the slices, and adding the doses from all slices

In clinical practice (scanning patients), only CTDIvol and DLP computed by the scanner are displayed on the console and recorded in the DICOM RDSR, so most non-physicist folks have never heard of MSAD. For acceptance testing (which is what you asked about), AAPM Report 39 (1993) has a lot of discussion of MSAD, but I don't think that IAEA or ImPACT or ACR–AAPM Technical Standard even mention it,

I quote from the McCollough et al article mentioned in an earlier comment: "In the early days of CT, direct measurement of the MSAD was a labor-intensive process ... the introduction of CTDI by Shope et al provided a much more practical method with which to estimate the MSAD".

See also the discussion in AAPM Reports 96 and 111.

https://www.aapm.org/pubs/reports/RPT_39.pdf

http://www-pub.iaea.org/MTCD/publications/PDF/Pub1393_web.pdf

http://www.impactscan.org/download/acceptancetesting.pdf

http://www.acr.org/~/media/1C44704824F84B78ACA900215CCB8420.pdf

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3079120/

http://www.aapm.org/pubs/reports/RPT_111.pdf

http://www.aapm.org/pubs/reports/RPT_96.pdf

Dear Dr David Clunie

thanks a lot, for your answer.

Dear Hoseinkhani

I have calculated the MSAD and CTDIvol.  Generally spoken, the CTDI will be similar to MSAD for slices more than 10. If the slices less than 10, the CTDI wil be greater than MSAD. Please review my article "Profile of CT scan output dose in axial and helical modes using convolution", Journal of Physics Conference Series, 694 (2016) 012034.

Dear Anam, Thanks so much for your answer.

I'll read your article,  it will be useful surely.

Good luck .