Simulated low dose cone beam CT (CBCT) in paediatric patients
Session type: Oral
Theme: Diagnosis and therapy
Daily imaging for image-guided radiotherapy (IGRT) is more reluctantly applied to children due to concerns over imaging dose. Simulating low dose CBCT is useful for testing the limits of dose reduction whilst maintaining sufficient image quality, allowing development of optimised paediatric imaging protocols. Lower imaging dose means that children can undergo more frequent imaging, allowing reduced treatment margins and more accurate delivery. This study simulates very low dose CBCT, assessing the effect on image quality and registration accuracy.
Low dose CBCT was simulated in 9 paediatric patients with various tumour sites (Craniospinal, Abdomen/lungs). Gaussian noise of the correct magnitude according to the desired simulated exposure settings was added to the original projection images. Initial exposure settings were 100kV/16mA/10ms and 120kV/20mA/16ms, simulated down to 5mA/5ms and 5mA/8ms respectively. In brain imaging this approximates to a reduction in dose to eyes from 1.94mGy to 0.30mGy. Scans were reconstructed using the modified projection images. Quality of the simulated images was assessed visually and by testing registration accuracy, comparing the couch correction to that of the initial full exposure images. One patient's registration was performed using the grey value algorithm, all others using bone registration.100kV/16mA/10ms input120kV/20mA/16ms inputSimulated exposure%initial doseSimulated exposure%initial dose10mA/10ms65.5%10mA/16ms50%8mA/10ms50%10mA/10ms31.3%8mA/5ms25%10mA/8ms25%5mA/5ms15.6%5mA/8ms12.5%
Visual image quality worsened in the simulated low dose images with more noise artefacts and reduced soft tissue contrast, however bony anatomy remained clearly visible. Registration remained accurate even at lowest simulated exposures. The largest registration discrepancy compared to the original scan was 1.1mm; all others were below 0.62mm.
Simulating low dose CBCT shows that imaging dose for children can be significantly reduced to 12-15% original dose by lowering exposure settings, whilst maintaining acceptable image quality and registration accuracy for IGRT.