胸部低剂量CT图像噪声和伪影分析

Image noise and artifact in chest low-dose CT

目的 分析胸部低剂量CT扫描图像噪声与伪影的影响因素及分布特点。方法应用组织等效胸部模型置于Philips Brilliance64层CT机以常规剂量（管电压120kV，管电流250mAs）和低剂量（管电压120kV，管电流50、30和21mAs）分别扫描，测量、记录模型各部位CT值、CT值噪声标准差（SD），分别行方差分析。对200例肺结节患者以30或21mAs剂量组行小范围低剂量扫描，通过卡方检验分析不同剂量扫描影像噪声和伪影严重程度与患者性别、体型的关系及在肺部的分布特点。结果不同扫描剂量条件下测量模型各部位CT值差异均无统计学意义：肺-777．3～-758．2HU（F=0．992，P〉0．05），胸壁107．9～111．3HU（F=2．044，P〉0．05），椎骨835．6—875．3HU（F=1．453，P〉0．05）；而CT值SD差异有统计学意义：肺9．5～29．0HU（F=108．7，P〈0．01），胸壁10．1～32．4HU（F=84．3，P〈0．01），椎骨19．2～57．1HU（F=30．6，P〈0．01），且随电流降低而增加。临床患者低剂量扫描显示，不同性别组图像噪声和伪影严重程度（男性无或轻微者74例，严重者17例；女性无或轻微者81例，严重者28例）差异无统计学意义（x^2=2．294，P〉0．05），不同体型组的体质量指数[（BMI）〈18．5组无或轻微者29例，严重者2例；18．5≤BMI〈24．0组无或轻微者120例，严重者13例；BMI／〉24．0组无或轻微者6例，严重者30例]差异有统计学意义（x^2=128．274，P〈0．01）。低剂量扫描图像的噪声和伪影在上肺野（无或轻微者80例，严重者38例；X^2=18．918，P〈0．01）、肺野后部（无或轻微者89例，严重者33例；X^2=6．760，P〈0．05）较严重。结论低剂量CT扫描图像噪声增加，噪声和伪影在肺野后、上部较严重，可能和骨骼分布有关。应根据受检者BMI调整扫描方案（mAs值），做到扫描方案个体化。

Objective To analyze the image noise and artifact of low-dose chest CT scanning and the distribution pattern. Methods A chest phantom equivalent to human tissue was scanned by 64 slices spiral scanner at standard dose （250 mAs） and low-dose （50, 30,and 21 mAs） respectively, HU in sites of the phantom and SD of which was recorded. 200 patients with pulmonary nodules were scanned at 30 or 21 mAs for minimal length. The relationship between severity of noise and artifact in chest low-dose CT scanning and gender or body mass index （ BMI ） of the patients, as well as the distribution of noise and artifact was evaluated. Results There was no statistical difference between the HU in sites of the phantom： lung （-777.3--758.2 HU, F=0.992, P〈0.01）, chest wall （107.9-111.3 HU, F=2.044, P 〉 0. 05） , vertebra （835.6-875.3 HU, F = 1. 453, P 〉 0. 05 ）, while the SD of which was of statistical signification： lung （9. 5-29.0 HU, F = 108.7, P 〈 0. 01 ）, chest wall （ 10. 1-32. 4 HU, F = 84. 3, P 〈0. 01）, vertebra （19. 2-57. 1 HU, F = 30. 6, P 〈0. 01 ）,the SD increased with the decrease of the tube current. There was no statistical difference between male （in which 74 cases no or mild, 17 cases severe ） and female （ 81 cases no or mild, and 28 cases severe） in image noise and artifact in low-dose images （ X^2 = 2. 294, P 〉0. 05 ）, and significant difference between groups of different BMI（ in BMI 〈 18.5 group, 29 cases no or mild,2 cases severe, in group of 18.5≤BMI 〈24. 0, 120 cases no or mild, 13 cases severe, and in group of BMI t〉24. 0, 6 cases no or mild, 30 cases severe, X^2 = 128. 274, P 〈 0. 01 ）. The noise and artifact was greater in the upper （80 cases no or mild, 38 cases severe, X2 = 18. 918, P 〈0. 01 ） and dorsal field （89 cases no or mild, 33 cases severe, X2 = 6. 760, P 〈 0.05 ）. Conclusions The image noise and artifact was significant in low-dose CT, especially in the dorsal and upper field of the lung, which might be attributed to the distribution of skeleton in the chest. It was recommended that scanning protocol （mAs value） be individualized adjusted in according to the patients BMI.