Exhibit Hall | Forum 1
Purpose: Optimizing CT protocols is challenging in the presence of dose modulation because of the unknown CT dose index (CTDI_vol). The task is more difficult when both the noise index (NI) and the third-generation iterative reconstruction (ASIR-V) are prospectively coupled to dose determination. It is of clinical interest to determine a relationship between CTDI_vol and NI, ASIR-V fraction, and patient size for the purpose of establishing CT protocol parameters yielding acceptable dose.
Methods: Dose modulation data was collected on a GE Revolution 256-slice scanner utilizing a Mercury phantom. This phantom has five disks of distinct diameter from 16 – 36 cm and conical segments joining the disks for a smooth mA-modulation transition. The modulated parameters were determined by performing a scout scan and then setting the scan range for each diameter independently over the full mA range. The fixed parameters were 120 kVp, pitch of .984, and a collimation of 40 mm with a primary slice width of 2.5 mm. The CTDI_vol was recorded for a range of NI (10-24) and ASIR-V blending values (0%-100%) per diameter. The relationship between CTDI_vol, NI, and ASIR-V for each diameter was determined using Origin Pro 2020. Parameter combinations resulting in maximum or minimum mA were rejected from the fit.
Results: For each diameter, the CTDI_vol was fitted with a 2nd order polynomial of ASIR-V multiplied by a power law of NI. The power law exponent varied from -2.9 (36 cm) to -2.0 (16 cm) with an R-Square of approximately 0.99. The dose rate was 0.7 mGy/mAs.
Conclusion: The fitted relationship determines the CT dose index for given vales of NI and ASIR-V blending for a range of patient sizes for this model scanner. The results can be of direct help for selecting parameters in CT protocol development.