GRU Size-Specific Dose Estimator (SSDE) for
Pediatric and Adult CT Examiniations

CTDI vol (mGy)
Phantom (cm) 32 cm
16 cm

SSDE based on Patient Dimension(s)
AP (cm)
LAT (cm)
For SSDE Calculation based on Age (18 or under)
Scan Date / /
Birth Date / /


This page and its calculations are based on AAPM Report No. 204

  • The CTDI volume displayed on the CT scanner is the dose that would be measured in a uniform round lucite phantom of designated size using the specified scan parameters (kVp, mA, time, display field of view, pitch, etc). CTDI volume is NOT patient dose but can used in conjuction with information about patient size to estimate patient dose.
  • The scanner will provide CTDI volume (and phantom size) before a scan is done. It will also display this value on the dose report after the study is complete. The post-scan value of CTDI volume takes into account any phototiming during the procedure and may not match the pre-scan CTDI volume.
  • Patient size can be provided in one of two ways, either through actual patient dimensions or estimated by patient age (for patients 18 and younger). Patient dimensions can be obtained through actual caliper measurements, from CT scouts, or from axial scans. The AAPM protocol provides for defining patient size by designating only the AP dimension, only the lateral dimension, or both dimensions.
    • By entering patient dimensions from scouts, caliper measurements, or using date of birth, and using the pre-scan CTDI displayed on the scanner set-up screen, size specific patient dose can be estimated BEFORE scanning.
  • To use this page, first enter the CTDI volume and associated phantom size as displayed on the scanner (this can be done after the study is set up and before scanning). If one or both patient dimensions are known, they should be input and the left "calculate" button clicked. If dimensions are not known, the patients date of birth is input and the right "calculate" button clicked.
  • The AAPM points out that the SSDE value calculated is an estimate based on several research studies using both phantoms and computer simulations. Inaccuracies of this calculation are thought to be about 20%.

George David
Associate Professor of Radiology
Department of Diagnostic, Therapeutic, and Interventional Radiology
Georgia Regents University