The association of dosimetric quantities from computed tomography with operational factors: basis for optimization strategies

Abstract

Clinical Computed Tomography (CT) imaging is supported by a patient - technology - observers system. Such system involves dosimetric quantities associated with image quality descriptors, where operational factors are predictors. Knowledge of quantitative association between CT dosimetric and image quality quantities with systemic factors, provides the basis to devise scanner-specific optimization strategies. Kerma indexes were measured with a pencil ionization chamber free in air C _a,100 and in phantom C _pmma,x (x changes into c and p for center and periphery respectively). Polymethyl Methacrylate (PMMA) standard phantoms were used (diameters of 16 and 32 cm). Several operational factors of a Siemens Sensation 64 Cardiac were considered: estimated spectrums, tube potential F ₈ (80 - 140 kV), tube current x time product F ₁ (40 - 350 mAs) and total collimation at isocenter F ₃ (2,7 - 19,2 mm). The water equivalent radius R _w , an important factor for patient Size Specific Dose Estimators (SSDE), was estimated by taking into account the spectrums in each phantom. Average pixel noise was measured from Regions of Interest (ROIs) in water phantoms with radius of 2,5; 3; 6; 8 and 11,5 cm. A linear association was found between C _pmma,p and C _pmma,c . A dose reduction of C _pmma,c = 2 mGy per tube rotation can be obtained from data analysis (head mode), with F ₁ = 50 mAs, F ₃ = 19,2 mm, resulting in average pixel noise of 20 Hounsfield Units (HU). Knowledge of noise association with C _pmma,c provides a straightforward tool for quantitative optimization, considering a systemic approach, which includes patient - technology - observer factors.