背景:高管电流向光子计数探测器(PCD)产生高通量的X射线,这可能会导致并发光子形成的脉冲堆积,这可能会导致计数损失和能量分辨率下降。
目的:为了评估高通量临床光子计数CT(PCCT)系统的性能,可能受到脉冲堆积效应的影响,在任务通用图像质量度量方面。
方法:在商用PCCT扫描仪上扫描临床体模(NAEOTOMAlpha,西门子)在120kV下,在十四个不同的管电流水平(40-1000mA)下,旋转时间为0.25s,间距为1。剂量水平对应于CTDIvol(32cm体模)0.79-19.8mGy。使用QIR-off模式(非迭代重建算法)重建CT正弦图,Br44内核,体素尺寸为0.4102×0.4102×3mm3$0.4102\\times0.4102\\times3{\\mathrm{\\mm}}^3$。imQuest,使用基于MATLAB的开源软件包来计算噪声功率谱(NPS),任务传递函数(TTF),对比噪声比(CNR),和CT数量根据AAPM任务组233计量。
结果:TTF的50%截止频率(f50)在所有插入件的所有较高管电流中基本保持恒定,即聚乙烯,骨头,空气,和丙烯酸。使用最低的两个数据点(40和80mA),噪声大小与管电流之间的预期关系被确定为噪声α$\\propto\\$mA-0.47。测得的噪声幅度比最高管电流下的期望值高11.1%。随着管电流从40mA增加到1000mA,NPS(fav)的平均频率从0.32下降到0.29mm-1。在任何插入件的CT数测量中都没有观察到相当大的影响;但是,随着管电流的增加,空气和骨骼的CT数几乎单调变化。对于所有插入件,绝对CNR单调增加;然而,在所有管电流中,测量的CNRs和预期的CNRs之间的差异约为-6%至12%。
结论:增加管电流并不影响空间分辨率,但轻微影响临床PCCT系统的CT数量和噪声测量。然而,仅在20厘米小体模上使用临床无关的管电流时,效果才相当可观。在一般临床实践中,自动曝光控制技术用于减少探测器上的通量变化,这减轻了由于高计数率而导致的检测器饱和的机会。观察到的影响可能是由于脉冲堆积,系统的信号相关过滤,或重建算法中的非线性。总之,在光子计数CT系统中使用的探测器的死时间较短,使得由于脉冲堆积而导致的计数损失可以忽略不计,或脉冲堆积对常规临床实践中的临床光子计数CT系统的图像质量影响不大,因为可能对该系统进行校正。
BACKGROUND: High tube current generates a high flux of x-rays to photon counting detectors (PCDs) that can potentially result in the piling up of pulses formed by concurrent photons, which can cause count loss and energy resolution degradation.
OBJECTIVE: To evaluate the performance of clinical photon-counting CT (PCCT) systems in high flux, potentially influenced by pulse pileup effects, in terms of task-generic image quality metrics.
METHODS: A clinical phantom was scanned on a commercial PCCT scanner (NAEOTOM Alpha, Siemens) at 120 kV under fourteen different tube current levels (40-1000 mA) with a rotation time of 0.25 s and a pitch of 1. The dose levels corresponded to CTDIvol (32 cm phantom) of 0.79-19.8 mGy. CT sinograms were reconstructed using QIR-off mode (noniterative reconstruction algorithm), Br44 kernel, and a voxel size of 0.4102 × 0.4102 × 3 mm 3 $0.4102 \\times 0.4102 \\times 3{\\mathrm{\\ mm}}^3$ . imQuest, an open-source MATLAB-based software package was used to calculate noise power spectrum (NPS), task transfer function (TTF), contrast-to-noise ratio (CNR), and CT number according to AAPM Task Group 233 metrology.
RESULTS: The 50% cut-off frequency of TTF (f50 ) remained mostly constant across all higher tube currents for all inserts, namely polyethylene, bone, air, and acrylic. Using the lowest two data points (40 and 80 mA), the expected relationship between noise magnitude and tube current was determined to be noise ∝ $ \\propto \\ $ mA-0.47 . The measured noise magnitude were up to 11.1% higher than the expected value at the highest tube current. The average frequency of NPS (fav ) decreased from 0.32 to 0.29 mm-1 as tube current increased from 40 to 1000 mA. No considerable effects were observed in CT number measurement of any insert; however, CT numbers for air and bone changed almost monotonically as tube current increased. Absolute CNR increased monotonically for all inserts; however, the difference between measured and expected CNRs were approximately -6% to 12% across all tube currents.
CONCLUSIONS: Increasing tube currents did not affect the spatial resolution, but slightly affected the CT number and noise measurements of the clinical PCCT system. However, the effects were only considerable at clinically irrelevant tube currents used on a small 20-cm phantom. In general clinical practices, automatic exposure control techniques are used to decrease the variation of flux on the detector, which alleviates the chances of detector saturation due to high count rates. The observed effects could be due to pulse pileup, signal-dependent filtration of the system, or nonlinearities in the reconstruction algorithm. In conclusion, either the deadtime of the detector used in the photon-counting CT system is shorter such that count losses due to pulse pileup are negligible, or pulse pileup has inconsiderable effects on the image quality of clinical photon-counting CT systems in routine clinical practice due to possible corrections applied on the system.