RESUMO
PURPOSE: To study the image effects of the time-wise dynamic aspect of intravenous contrast agents to enable contrast-enhanced cone-beam CT (CE- CBCT) localization of liver lesions for stereotactic body radiation therapy (SBRT). METHODS: A model was developed to study dynamic IV contrast agents using static phantoms and to derive optimum parameters for CE- CBCT imaging. Ten samples containing iodine at 0-5 mg/mL were prepared in cylindrical tubes, corresponding roughly to 0-100 HU as measured by 120 kV helical CT imaging. Each sample was imaged separately in a tissue- equivalent phantom, yielding ten datasets (roughly 650 projections each) corresponding to these static CBCT images. To reconstruct images of dynamic contrast concentrations, the CBCT 2D projections were re- assembled to match the expected amount of contrast at different points in time. This model was applied to published hepatic contrast enhancement curves, and optimum imaging and contrast injection parameters were derived. RESULTS: A signal-to-noise ratio (SNR) decrease of 25%-75% in dynamic CE-CBCT images from ideal CT of samples with a 20-100 HU difference from water was observed in the un-optimized scans. This demonstrates the difficulty of CE-CBCT, and was noticed even in geometries that minimize or eliminate x-ray scatter, detector glare, and motion. Using our model, we found parameters for iodine injection, CBCT scanning, and injection/scanning timing which optimize contrast enhancement, and a 100% SNR increase with respect to the un-optimized scans was achieved. CONCLUSIONS: The effect of IV contrast is severely degraded in CBCT, and optimization of image and timing parameters is crucial for improved CE-CBCT imaging for target localization. CBCT has very low temporal resolution, and the pharmacokinetics of IV contrast must be carefully considered in order to apply this technique to localize liver lesions for SBRT. This model will be used to establish the feasibility of CE- CBCT for routine localization of liver lesions.
RESUMO
PURPOSE: We have clinically observed that larger rotational setup errors are more prominent in pediatric patients who received radiation therapy for brain tumors. In this work, we quantitatively evaluated the daily setup corrections in pitch, roll, and yaw axes for children who received intracranial radiation therapy under x-ray image guidance. METHODS: Daily localization data of 43 patients between the ages of 10 months and 21.9 years were analyzed in this study. Patients were immobilized with thermoplastic mask during treatments, and 2D orthogonal x-ray images wereacquired for setup corrections before each treatment. Rotational setup corrections in pitch, roll, and yaw axes were extracted from 873 treatment fractions, and were analyzed for the whole group of patients and for two age groups: < 5 and = 5 years old. RESULTS: The mean values for the pitch corrections were 1.91° and 1.65° (p:0.02), roll corrections were 1.37° and 1° (p<0.001), and yaw corrections were 1.93° and 1.47° (p<0.001), respectively. For patients < 5 years, 21.7% of treatments had pitch corrections more than 3°, versus 15.6% of treatments required pitch corrections more than 3° for patients >= 5 years. Similarly, 10.6% of roll corrections and 20.9% of yaw corrections were more than 3° for patients < 5 years. On the other hand, 2.1% of roll and 13.8% of yaw corrections were more than 3° for patients = 5 years old. CONCLUSIONS: Data indicate that children less than 5 years old are more prone to rotational setup errors during intracranial radiation therapy. This can be attributed to reduced efficacy of immobilization devices due to smaller and rounder anatomicalfeatures of pediatric patients, and challenges in setup while the patient is under anesthesia. The role of daily image guidance and rotational setup corrections becomes important to ensure target coverage, especially for children < 5 years old.