Dose Evaluation of Head CT Scans Using Phantom for Optimization Protocols

Abstract

Computed Tomography (CT) scans promote a higher dose deposition than conventional radiology exams. These tests contribute significantly to the increase in the patient and collective dose, being a public health concern worldwide. There is a great need to improve protocols to seek lower doses while maintaining the diagnostic image quality. The development of phantoms allows the testing of different acquisition protocols. For this, the phantoms must present an absorption characteristic of the X-ray beam similar to the represented patient. In this study, two CT head phantoms were used, the standard head phantom and another with smaller volume. The objects are cylinders with 16 cm (standard adult) and 12 cm in diameter and 15 cm in length, made of polymethyl-methacrylate (PMMA). Tests of acquisition protocols were performed on a GE CT scanner, LightSpeed VCT model with 64 channels. The central slice of the phantoms was irradiated successively, and using a pencil ionization chamber, measurements of CT air kerma index in PMMA (C k,PMMA,100 ) were performed. From these results, the CT Dose Index values weighted and volumetric (CTDI w , CTDI vol ) were obtained for 10 cm scans of the central region of the head phantoms, in helical mode. The scans were performed using a voltage of 120 kV value and load (mA.s). Dose values varied from 17.52 mGy to 24.83 mGy. The highest recorded dose value was 24.83 mGy for the smaller head phantom and 19.96 mGy for the standard head phantom. The results obtained allowed to evaluate that, for the generation of images with the same diagnostic objective, the volumetric dose index showed a higher dose value in the 12 cm phantom, corresponding to a head with less volume, compared to the value measured in the standard head phantom.