Influence of beam pitch and tube current on lung nodule 64- MDCT - chest protocol : A phantom study

Abstract

Background : Multi-Detector Computed Tomography (MDCT) is a powerful modality to detect small pulmonary nodules while the benefits of CT exceed the harmful effects of radiation exposure in patients; thus, CT doses should be kept as low as reasonably achievable. The way to reduce radiation dose is based on modification of
scanning parameters.

Objective : The purpose of this study is to investigate the radiation dose and image quality affected by beam pitch and tube current for chest CT scan using a lung man phantom with lung nodules.

Designs : Experimental study (in vitro).

Setting : Department of Radiology, King Chulalongkorn Memorial hospital, Bangkok.

Material and Method : A lung man chest phantom with various simulated nodules was scanned by 64-MDCT with standard and high beam pitch technique and varying tube current (40-300 mA). The quantitative image quality was investigated by the Contrast to Noise Ratio (CNR). The qualitative image quality was assessed by two reviewers,
using five -point scale for nodules detectability. The volume CT dose index (CTDIvol) were recorded to evaluate the radiation dose for standard beam pitch technique and high beam pitch technique of lung nodules CT scan.

Results : When the tube current, mA, was reduced from 300 to 40, the CTDIvol decreased from 12.3 to 1.60 mGy for standard beam pitch technique and from 8.60 to 1.18 for high beam pitch technique. Also, comparing the standard beam pitch technique and high beam pitch technique on various tube current (mA), the average radiation dose reduced by 30%. The average CNR decreased when high beam pitch was used. The scoring on image quality by two reviewers was in good agreement.

Conclusions : Beam pitch and tube current is an important factor for CT chest image quality. The high beam pitch CT showed a low radiation dose and short scan time when compared to the standard beam pitch with diagnostic image quality maintained.