Visualization and comparison of CT dose distribution between axial and helical acquisitions.

BACKGROUND

It is challenging to assess the accuracy of volume CT Dose Index (CTDIvol ) when the axial scan modes corresponding to a helical scan protocol are not available. An alternative approach was proposed to directly measure C T D I v o l H $CTDI_{vol}^H$ using helical acquisitions and relatively small differences (< 20%) from CTDIvol were observed.

PURPOSE: To visually demonstrate the 3D dose distribution for both axial and helical CT acquisitions and quantitively compare C T D I v o l H $CTDI_{vol}^H$ and CTDIvol .

METHODS

3D dose distribution within the standard CTDI phantoms (16 and 32 cm diameter) from a single CT projection, Dp (x,y,z) was first generated using Monte Carlo simulation (GEANT4) with 9×108 photons per combination of tube voltage (80-140 kV), collimation width (1-8 cm), and z-axis location of the central ray of the x-ray beam, with a spatial resolution of 1 mm3 . These dose distributions from one single projection were analytically ensembled to simulate 3D dose volumes DA (x,y,z) and DH (x,y,z) for axial and helical scans, respectively, with different helical pitches (0.3-2) and scan lengths (100-150 mm). 2D planar dose distributions were obtained by integrating the inside 100 mm of the dose volumes. CTDIvol and C T D I v o l H $CTDI_{vol}^H\;$ were calculated using the planar dose data at corresponding pencil chamber locations and the percentage differences (PD) were reported.

RESULTS

High spatial resolution 3D CT dose volumes were generated and visualized. PDs between C T D I v o l H $CTDI_{vol}^H$ and CTDIvol had strong dependency on scan length and peripheral chamber locations, with subtle dependency on collimation width and pitch. PDs were mostly within the range of ± 3% for a scan length of 150 mm with four peripheral chamber locations.

CONCLUSIONS

With a scan length covering the entire phantom length, C T D I v o l H $CTDI_{vol}^H$ directly measured from helical scans can serve as an alternative to CTDIvol only if all four peripheral locations were measured.