Effect of pitch on CT image quality based on SNR evaluation using in house phantom

The purpose of this study was aimed to evaluate the symmetrical effect of pitch factor to SNR (Signal to Noise Ratio) in thorax phantom CT images. Thorax CT phantom was designed to contain cork and cylindrical objects with several sizes located symmetrically respect to Y-axis. Phantom size is 20 cm diameter, 22 cm length, and the objects simulated vessels filled with contrast medium (diameter A: 2mm, B: 1.7 mm, C: 1 mm, respectively). The distance between two objects: A: 6 cm, B: 8 cm, and C: 12 cm. Image acquisitions were performed:130 kVp, tube rotation 0.6s, 70 mAs, and pitch values were 0.5, 0.8, 1.0, 1.5, 1.8 and 2.0. CT images represented by 9 axial images with a slice thickness of 5 mm. The difference of SNRs (Δ): a value of ΔA, in general high at initial slice and fall significantly up to the third slice and then followed by a nearly constant value. ΔB at initial slice trend decreased with increasing slice images. ΔC at all pitch initially shown zero value and increase to maximum up to second slice (not more than 20 units) and tend to constant after at sixth slice. Effect of all pitch values on symmetrical object images was influenced by the distance between two corresponding objects and slice order. Generally at the initial slice, the SNR difference between two corresponding objects (Δ) relatively high compared with the following order slice. Therefore, in clinical practice, it is recommended to give additional FOV at least 1.5 cm at the initial slice interest.