Prospective-ECG Triggered High-pitch Spiral Scan Of CT Pulmonary Angiography:Image Quality And Radiation Dose Using Reduced Tube Voltage And Iterative Reconstruction

Objective:(a) To compare image quality between iterative reconstruction (IR) andfiltered back projection (FBP) algorithm in high-pitch spiral scan of CT pulmonaryangiography (CTPA).(b) To assess the image quality and radiation dose of high-pitchspiral scan of CTPA using reduced tube voltage and IR, in comparison with CT usingroutine tube voltage and FBP reconstruction.Methods:70consecutive patients who had suspected pulmonary embolism underwentCTPA using reduced tube voltage according to body mass index (BMI):(a)80kV forBMI <25kg/m2(group A1,35subjects),(b)100kV for BMI≥25kg/m2(group B1,35subjects). For comparison, the two sex-age-and BMI-matched groups using routinetube voltage and FBP reconstruction were selected:(a)100kV for BMI <25kg/m2(group A2,35subjects),(b)120kV for BMI≥25kg/m2(group B2,35subjects). CTPAusing prospective-ECG triggered high-pitch spiral scan mode was performed on a2ndgeneration dual source CT system. Acquisition parameters were as follows:2×64×0.6mm detector collimation,280ms gantry rotation time,3.4pitch and320mAs perrotation tube current time product. High-pitch spiral scan was prospectively triggered at60%of the R-R interval. Images were reconstructed with FBP and IR algorithm.Parameters for image reconstruction include a slice thickness of0.75mm, increment of0.7mm, and a medium soft convolution kernel (I26f for IR and B26f for FBP). Contrastmedium enhancement was achieved by injecting60ml of iodinated contrast materialand injected at5-6ml/s through an18-G intravenous antecubital catheter by using a dual-syringe injector. The image noise, signal-to-noise ratio (SNR) and contrast-to-noiseratio (CNR) were compared respectively between IR and FBP reconstruction in eachgroup. The image noise, SNR, CNR, image quality and radiation dose were comparedrespectively in each matched groups (A1and A2; B1and B2).Results:(a) In the each group, there was no significant difference in CT attenuation ofpulmonary artery between two reconstruction algorithms (P>0.05). The image noisewas significantly lower with IR than with FBP (P <0.01), and the SNR and CNR weresignificantly higher with IR than with FBP (P <0.01). The image quality scores weresignificantly lower with IR than with FBP (P <0.05).(b) In patients with a BMI <25kg/m2, the CT attenuation of pulmonary artery was significantly higher with reducedtube voltage and IR (group A1) than with routine tube voltage and FBP (group A2)(682.9±111.2HU vs.456.8±79.9HU, P <0.01). The image noise was significantlyhigher in group A1than in group A2(42.1±8.3HU vs.28.5±5.3HU, P <0.01). Nosignificant differences in SNR (16.6±3.2vs.16.9±3.6, P>0.05) and CNR (14.9±3.0vs.14.5±3.5, P>0.05) were seen between two groups. There were no significantdifferences in image quality scores between two groups (1.2±0.5vs.1.3±0.4, P>0.05). The effective radiation dose of group A1was52.44%lower than that of group A2(0.68±0.07mSv vs.1.43±0.12mSv, P <0.01).(c) In patients with a BMI≥25kg/m2,the CT attenuation of pulmonary artery was significantly higher with reduced tubevoltage and IR (group B1) than with routine tube voltage and FBP (group B2)(478.0±108.7HU vs.376.4±80.4HU, P <0.01). The image noise was similar between twogroups (26.5±4.3HU vs.24.9±3.7HU, P>0.05). The SNR and CNR weresignificantly higher in group B1than in group B2(17.6±3.8vs.15.4±3.6,14.8±3.5vs.12.5±3.3, P <0.05). There were no significant differences in image quality scoresbetween two groups (1.2±0.5vs.1.3±0.5, P>0.05). The effective radiation dose ofgroup B1was38.36%lower than that of group B2(1.43±0.13mSv vs.2.32±0.21mSv,P <0.01).Conclusion:(a) Compared with FBP reconstruction, IR can reduce image noise andimprove image quality in high-pitch spiral scan of CTPA using reduced tube voltage orroutine tube voltage.(b) In patients with a BMI <25kg/m2, high-pitch spiral scan ofCTPA using80kV and IR can provide equivalent coronary quality, compared with CTusing100kV and FBP reconstruction, that decreased approximately52%of theradiation dose.(c) In patients with a BMI≥25kg/m2, high-pitch spiral scan of CTPA using100kV and IR can provide equivalent coronary quality, compared with CT using120kV and FBP reconstruction, that decreased approximately38%of the radiationdose.