Room: Room 202
Purpose: Patient size-specific dose estimate (SSDE) calculations depend on the accuracy of size surrogates. AAPM Report 220 charges that to obtain accurate size measurements from CT localizers, a magnification correction is required. Previously reported magnification corrections are overly simplistic, assuming the patient is a flat line, and are based on a similar triangles correction that includes the distance the patient is off set from iso-center. Here we present a new patient-model based correction making less assumptions than previously reported approaches.
Methods: Our model-base magnification correction assumes an elliptical patient with anterior-posterior (AP) and lateral (LAT) cross-sections. We parameterize the problem by modeling a line emanating from the source to where it first grazes the patient and terminates onto the detector plane. The main difference between our approach and previously reported corrections is that we account for the grazing of x-rays with the patient at a plane other than the plane going through the center of the patient. We validated our model by acquiring localizer images of elliptical phantoms at different table heights (ranging from centered to 11 cm mis-centered). We compared un-corrected â€œvendorâ€? measurements to corrected versions previously reported in the literature and our model based method. We compared absolute AP and LAT distance values and SSDE normalized dose coefficient (NDC).
Results: The model-base method provides consistent accurate results (less than 1.8% error for AP/LAT and 1.2% error for SSDE NDC) for all positions and patient sizes. Similar triangle based methods show a maximum error for AP/LAT and SSDE NDC of 7.5% and 5.2% while uncorrected â€œvendorâ€? maximum errors were 30.9 % and 17.0%, respectively.
Conclusion: Our new model-base method outperforms simple geometric magnification correction approaches. By modeling a patientâ€™s cross-section and beam geometry we demonstrate SSDE NDC correction factor improves from 17.0 % (vendor correction) to 1.2%.
Funding Support, Disclosures, and Conflict of Interest: TPS consultant GE Healthcare, provides CT Protocols to GE Healthcare, MAB of iMALOGIX, co-owner LiteRay Medical LLC, founder ProtocolShare.org