Author: Harold Y Hu, Yanle Hu, Shuai Leng, Maryam Sadeghian, Joe Swicklik 👨🔬
Affiliation: Mayo Clinic Arizona, Basis Scottsdale, Mayo Clinic 🌍
Purpose: Radiotherapy CT simulation often requires two scans: a non-contrast scan for dose calculation and a contrast-enhanced scan for target delineation. Photon-counting-detector (PCD) CT allows the generation of virtual monoenergetic imaging (VMI) at various energy levels from a single acquisition. In this work, we investigated the use of VMIs at low and high energies to support target delineation and dose calculation in a single scan.
Methods: An electron density phantom with four soft tissue inserts and two contrast inserts (2.0 and 4.0 mg/mL iodine) was scanned on a PCD-CT using an abdomen protocol (120kVp, 330mAs, 0.6pitch). Multiple image sets were reconstructed from the same CT acquisition, including a single energy (SE) PCD-CT (i.e., counting all photons) and 16 VMIs from 40keV to 190keV at 10keV intervals. 40keV VMI had enhanced contrast visualization, making it suitable for target delineation, whereas 190keV VMI had no contrast enhancement, making it suitable for dose calculation. Contrast to noise ratio (CNR) was quantified between contrast and base inserts for both 40keV VMI and SE PCD-CT. For dose calculation, errors in relative electron density (RED) estimation were quantified for both contrast inserts, using calibration curves established from soft tissue inserts for both 190keV VMI and SE PCD-CT.
Results: Compared to SE PCD-CT, VMI at 40keV significantly improved CNR (10.4 and 21.4 vs 4.8 and 9.7), and VMI at 190keV substantially reduced RED estimation errors (0.4% and 0.5% vs. 5.2% and 9.9%) for the two contrast inserts (2.0 and 4.0mg/mL Iodine).
Conclusion: Using the VMI approach, a single PCD-CT acquisition would be sufficient to support both target delineation (40keV VMI) and dose calculation (190keV VMI) for radiotherapy, effectively reducing the number of CT acquisitions and associated imaging dose.