Assessment the effects of scanning and technical parameters on the detection of pulmonary nodules at different computed tomography systems

Computed tomography (CT) is the most sensitive imaging technique for the detection of pulmonary nodules and metastases, and may reveal peripheral nodules as small as 2 mm in diameter. Detection of metastases would influence treatment. CT is also useful for planning biopsy or surgical resection of putative metastases. It is therefore important to optimize the CT scanning protocol, with the goal of maximizing the sensitivity and specificity of detecting pulmonary nodules while minimizing patient dose to ionizing radiation. The chest phantom that we used made of clear transparent plastic that has inside shape and dimensions of a sheep thorax to hold simulated nodules-lung preparation of a sheep. The number of lesions per lung ranged from 40 to 50; that sized 1mm to 5 mm at two categories; calcified and soft tissue (myocardium tissue).the approximate sites of implantation and the sizes of the lesions were documented. These maps later served as references for nodule detection. To simulate the radiation-absorbing qualities, Urine bags are filled with solution 15grm iodine/lit water (20mlit urografin %76 per one liter water). The phantoms are inflated through tracheal tubes during the experiment. The CT scans are obtained by using three systems at same technical and exposure parameters. Radiologist reported the images. Probability of nodules detection at three CT scan systems for slice thickness, 2 mm in comparing with slice thicknss,10mm and also 5mm; differed significantly (p<0.05). Although the use of thinner sections for nodule detection at conventional or spiral CT scans without volume rendering, can lower specificity due to the fact that vessels in the lung may become indistinguishable from nodules on the very thin sections. According to high contrast and air filled tissue content in lungs and solid content of nodules decreasing “mAS” (mili ampere seconds), and low dose CT is acceptable for nodule detection. For better nodule detection, continuous imaging with thinner sections, without gap, faster scanning and volume rendering is necessary. Between different CT scan systems, spiral CT and multi-detector, multi-slice spiral CT scan systems; according to continuous and rapid scanning must be choose for nodule detection. Low dose CT with decreasing “mAS” is acceptable for nodule detection.