Application Study Of Sinogram-affirmed Iterative Reconstruction On Low Dose Chest CT

Part1Application of different level iterative reconstruction of routine dose chest CT on image quality and noiseBackground:CT is the most commonly used inspection method of chest disorders. However, along with increasing clinical application of chest CT examination, the radiation exposure that associated with CT has been raised. Accordingly, CT examination should follow the ALARA (As Low As Reasonably As Achievable) principle. The main challenge of CT is to keep radiation dose to a minimum, while still obtaining better diagnostic image quality that enough to meet the needs of diagnosis. Reducing the radiation dose to the greatest extent is the focus of CT technology research. In order to reduce CT radiation dose, the relevant researchers have developed a lot of new image acquisitions, such as selecting weight-adapted protocols, using automatic tube voltage and tube current modulation systems, and large scan pitch technology, etc.Using traditional FBP reconstruction algorithm, there was a trade-off between the downside of radiation dose and image noise. The reduction of CT radiation dose will lead to reduction of image noise and image quality, this kind of trade-off relationship limits the further reduction of radiation dose of CT. This trade-off limits the minimum radiation dose required for a specific diagnostic application. Recently, the iterative reconstruction techniques were another option to save dose. The use of the iterative reconstruction can overcome the limit of filter back projection algorithm between radiation dose and image quality. Some studies about iterative reconstruction were introduced to improve image quality, enhance image resolution and lower image noise. Many CT manufacturers have launched different iterative reconstruction technologies that were applied in clinical in the past few years. Sinogram-affirmed iterative reconstruction (SAFIRE) is one of recently introduced iterative reconstruction algorithm, which is aimed at overcoming the constraints of conventional FBP by decoupling the relationship between spatial resolution and image noise. SAFIRE estimates the local noise content in each image pixel by analysis the raw data contributing to this pixel, and removed it from the current image data set. SAFIRE can significantly reduce the image noise and reduce artifacts, effectively improve the quality of the image, which could further reduce the radiation dose.Purpose:To evaluate the application of different level of SAFIRE reconstruction technique of routine dose chest CT examination on subjective image quality and image noise, SNR (signal noise ratio) and compared image quality with conventional FBP reconstruction image, to explore the iterative reconstruction level of best image quality and the lowest noise.Material and Methods:The routine dose chest CT examinations of30patients were performed on the dual source flash128-slice MDCT system (SOMATOM Definition Flash, Siemens Healthcare, Germany). All were unenhanced CT examinations. The scopes of CT examinations were from the thoracic inlet to the diaphragm during a breath hold after inspiration. Using two tubes with single energy mode, with the following parameters as collimation:64×2×0.6mm with z-flying focal spot; CARE Dose4D; pitch:1.2; rotation time:0.5s; section thickness and inter slice spacing:5mm. The initial lung and mediastinal images reconstructed with FBP algorithm and SAFIRE1-5algorithm respectively. In each data set, the objective assessment of noise was obtained by measuring the standard deviation of the measured Hounsfield units (HU) within circular regions of interest (ROIs) on axial images by a radiologist with5years experience in CT. The signal attenuation was measured on the same time, and the signal-to-noise ratio (SNR) was calculated. Subjective image quality was rated in5point Likert scale by two radiologists with12and16years experience in CT, respectively, who blinded to reconstruction technique. On each series of mediastinal and lung images, subjective noise was rated using a5-point Likert scale according to the severity of image noise, quality of contour delineation, and general image impression.(1=excellent;2=good;3=moderate;4= fair;5=poor). The final scores of subjective image quality were decided by consultation by two radiologists.Result:Noises of SAFIRE1-5were all lower than those of FBP, and SNR values were all higher than those of FBP. As SAFIRE iterative reconstruction strength increased, gradually reduce the image noise, improving the image SNR. The comparison of CT values has no significant statistical difference. SAFIRE2-4reconstruction image have the highest subjective score.Conclusion:On chest CT examination, compared with conventional filter back projection (FBP) reconstruction algorithm, the application of iterative reconstruction technique (SAFIRE1-5) can significantly improve image quality and reduce the image noise, improve signal to noise ratio. Using SAFIRE2-4can get the best subjective image quality. Part2Comparison of SAFIRE versus FBP reconstruction of low-dose chest CT on image noise and image qualityBackground:Computed tomography (CT) is widely used to bring the increase of radiation dose. Low dose technique became the hot topics in the study of CT technology. CT radiation dose reduction would bring increasing of image noise used conventional FBP reconstruction. This trade-off limits the further reduction of CT dose. Iterative reconstruction can effectively reduce the noise in the reconstruction of the images, can be solved the trade-off limits in traditional filter back projection of FBP algorithm. In recent years, along with the technology development of computer, the time of iterative reconstruction was shortened to clinical acceptable range. Compared to FBP algorithm, the iterative reconstruction algorithm can significantly reduce the scanning dose of selectively remove the noise of the image at the same time, in order to gain a better image quality. SAFIRE reconstruction is one of the latest iterative reconstruction algorithms that based on the original data.Objective:Routine dose chest computed tomography (CT) reconstructed with filtered back projection (FBP). Low dose chest computed tomography (CT) reconstructed with FBP and with raw data-based iterative reconstruction. To compare image noise, image SNR, subjective image quality and lesion conspicuity, and to evaluate the ability of SAFIRE reconstruction technology that reduce the scanning radiation dose and find the best scan protocol.Methods:87consecutive patients (46male, mean age54.54±16.12, mean BMI24.58±4.07) referred for initial chest CT with full-dose examinations (mean DLP183.37±44.13mGy.cm) and follow-up chest CT with half-dose examinations (mean DLP91.08±23.81mGy.cm). The full-dose protocol reconstructed with FBP, half-dose protocol reconstructed with FBP and Sinogram-affirmed iterative reconstruction (SAFIRE) respectively. Noise and signal-to-noise (SNR) were compared using a paired Student t-test, subjective image quality and lesion conspicuity was compared using wilcoxon signed ranks test. Results:Actual radiation dose of follow-up CT was about50%(49.26%±2.62) of routine dose protocol. Compared to full-dose images with FBP, there was no significantly difference in half-dose images with SAFIRE in the objective noise (Ascending aorta:P=0.38, Descending aorta:P=0.70, Trachea on mediastinal images: P=0.37) and SNR (Ascending aorta:P=0.14, Descending aorta:P=0.72, Trachea on mediastinal images:P=0.06) on mediastinal images; Noise was significantly lower (P<0.001) and SNR was significantly higher (P<0.001) in half-dose images with SAFIRE on lung images; Noise was significantly higher (P<0.001) and SNR was significantly lower (P<0.001) in half-dose images with FBP. Subjective image noise and lesion conspicuity were both similar on mediastinal images (p=0.317) and lung images (p=0.614) of half-dose SAFIRE images versus full-dose FBP images. Subjective image quality were both significantly lower on mediastinal images (p<0.001) and lung images (p<0.001) of half-dose FBP images versus full-dose FBP images. The conspicuity of some lesion was significantly lower (ground-glass opacitie, p<0.0001; ill-defined micronodule, p<0.0001; lung cyst, p<0.0001; emphysematous lesion, p=0.003) on half-dose FBP versus full-dose FBP images.Conclusion:Compared to full-dose CT images reconstruction with the conventionally FBP algorithm, SAFIRE with three iterations could provide similar or better image quality on at50%less dose. Half-dose FBP images provide poor image quality at50%less dose.