Room: Exhibit Hall | Forum 9
Purpose: Patient mis-centering increases patient surface dose and image noise in CT. In busy imaging centers patient miscentering occurs frequently when technologists cannot afford to spend sufficient time for patient positioning. Bowtie filters were designed to spatially shape the x-ray intensity with maximum intensity through thickest part of the patient and reduced x-ray as patient attenuation decreases. A properly positioned patient has their center of mass at the isocenter of the CT gantry. In this note we introduce methodology to accurately determine the patientâ€™s miscentering in the y-direction of the CT scanner using CT localizers acquired prior to CT scanning.
Methods: The offset is defined as the difference in the patient table to center of rotation and half of the patientâ€™s anterior-posterior (AP). We acquired AP and lateral (LAT) CT localizers of 32 cm CTDIvol, 21.5 cm cylinder, 3 different size elliptical phantoms, and CIRS abdomen and chest phantoms. A model-base magnification correction to ACR DIR extraction, developed in our laboratory, was applied to the AP and LAT dimensions. Table height was retrieved from DICOM metadata. The offset was calculated using AP measurements from both model-base and CT axial images.
Results: We show excellent agreement between the model-base and CT axial approach. For the elliptical phantoms the maximum error in the offset is 8.4 % with a mean of 0.9 %. For the CIRS phantoms the maximum error in the offset is 14.0 % with a mean of 0.4 %.
Conclusion: We compared the model-based method to the patientâ€™s center offset from the center of rotation calculated using the actual AP dimension of all the phantoms and they show excellent agreement. The maximum errors seen were for table heights where the phantom is 150 mm off-center. This method may be implemented in the clinic for patient or phantom centering.