SPECT骨断层重建的滤波反投影法和有序最大期望值法的比较

目的:比较SPECT骨断层图像重建的有序最大期望值法(OSEM)与滤波反投影法(FBP)的图像效果.方法:32例患者进行SPECT骨断层显像,采集部位以脊柱为主.给药(30mCi 99mTc-MDP)3～4h后进行断层图像采集.由1位技师对图像数据分别进行OSEM和FBP(包括Metz窗函数和Butterworth窗函数)图像重建.由3位核医学医师独立双盲阅片.结果:OSEM放射条纹状伪影远远少于FBP,图像分辨率和图像清晰度也优于FBP.结论:骨断层重建方法OSEM法优于FBP法.     

滤波反投影法和OSEM重建图像测量心功能参数的比较

目的比较静息门控心肌显像滤波反投影法（FBP）和OSEM重建图像后用定量门控心肌断层显像（QGS）、四维模型心肌断层显像（4D—MSPECT）、爱莫瑞心脏工具箱（ECToolbox）软件测量的心功能参数。方法临床疑诊或确诊冠心病患者144例,均行^99Tc^m-MIBI静息门控心肌SPECT显像,所有患者均用FBP和OSEM重建图像,用QGS、4D—MSPECT、ECToolbox软件计算心功能参数LVEF,EDV和ESV,采用Bland—Altman法检验2种重建方法的一致性,配对t检验方法检验心功能参数差异,相关性分析用直线回归分析。结果FBP和OSEM重建测量的心功能参数一致性和相关性好（r均〉0．93,P均〈0．001）。QGS软件FBP重建测得的EDV低于OSEM重建测得的EDV,其他2种软件为FBP高于OSEM[QGS：（82．2±39．1）ml和（83．5±40．8）ml,t=-2．53,P〈0．05;4D—MSPECT：（93．5±46．9）ml和（88．8±45．2）ml,t=5．95,P〈0．01;ECToolbox：（106．4±51．1）ml和（100．8±49．0）ml,t=3．99,P〈0．01]。对于ESV,4D-MSPECT软件FBP测量值高于OSEM[（37．5±41．4）ml和（34．8±37．6）ml,t=3．92,P〈0．01]。QGS软件FBP测得的LVEF低于OSEM测得的LVEF[（62．1±16．9）％和（63．1±16．1）％,t=-3．14,P〈0．01]。ECToolbox软件FBP测得的LVEF高于用OSEM测得的LVEF[（74．1±18．8）％和（71．3±17．1）％,t=5．28,P〈0．01]。结论2种重建方法所测量的心功能参数虽然相关性和一致性很好,但某些参数值差异有统计学意义。     

探测距离对SPECT断层分辨率的影响

目的探讨探测距离对SPECT断层分辨率的影响.方法采用内置不同直径的圆柱体(冷区)的自制黑箱模型.设定不同探测距离对模型进行360°采集,并对所采集的20组原始资料进行重建处理,观察重建后模型图像.结果配置低能高分辨准直器,探头旋转半径≤18cm时,及配置低能通用准直器,探头旋转半径为15cm时,重建结果可分辨出直径10mm柱型模型,余重建结果均达不到此分辨效果.结论探测距离愈小,分辨率愈高;反之,则分辨率下降.     

IQ·SPECT、碲锌镉探测器SPECT和传统SPECT在心肌灌注显像中的应用分析

IQ·SPECT与碲锌镉（CZT）探测器SPECT是心肌灌注显像新技术,与传统SPECT相比,具有灵敏度和空间分辨率高、采集时间短、注射显像剂剂量少及辐射剂量低等优点。与传统SPECT不同（低能高分辨率平行孔准直器）,IQ·SPECT使用SMART-ROOM准直器及心脏为焦点的采集模式,提高了灵敏度;CZT探测器SPECT的探测器为半导体,取代了传统NaI晶体探测器,提高了能量分辨率、空间分辨率、灵敏度及信噪比等,并可定量分析心肌血流储备。笔者主要对三者的成像原理、性能参数及临床应用进行综述。     

不同采集矩阵与函数重建对SPECT影像质量影响

目的:研究SPECT显像中不同采集矩阵与函数后重建对图像质量影响.方法:分析不同采集矩阵在窄气介质中的系统分辨率.系统分析22种不同函数后重建对平面显像图像的影响.通过Jaszezak模型研究小同采集矩阵与函数重建下的图像质量,Metz、Butterworth、Generalized Hanning、有序最大期望值法(OSEM)函数重建后.阿加做Chang's AC与不加做Chang's AC对脑断层图像的影响结果:在平面与断层显像中,高矩阵采集图像可以通过后重建转换为与低矩阵图像相刚的效果,而低矩阵图像却不能通过后重建转换与高矩阵图像相同的效果.不同矩阵采集时间差肄不明显.高矩阵采集图像经过设当函数重建后所获影像结果最优.结论:使用高矩阵采集条件获得高空间分辨率影像,选用合适的重建函数,获得种类更多的图像效果,效果更佳的影像质量,有助于帮助临床诊断.     

FBP和OSEM对正常人脑内多巴胺转运体分布半定量值影响的研究

目的 研究不同PET重建方法对正常人脑内多巴胺转运体（DAT）分布半定量值的影响。方法 将2014年3月至2015年6月期间41例健康受试者（正常人）的11C-甲基-N-2β-甲基酯-3β-（4-氟-苯基）托烷（11C-CFT）PET图像分别进行滤波反投影法（FBP）和有序子集最大期望值迭代法（OSEM）重建,自动勾画尾状核、壳核前部和后部等感兴趣脑区,以缺乏DAT分布的顶枕皮质作为参考区,按公式计算DAT分布的半定量值。DAT分布半定量值的组间比较采用配对t检验,相关性分析采用Pearson相关分析法。结果 基于OSEM重建的PET图像中DAT分布半定量值分别为:尾状核（1.77~2.15）、壳核前部（2.17~2.39）、壳核后部（1.71~2.06）;基于FBP重建的PET图像中DAT分布半定量值分别为:尾状核（1.68~2.10）、壳核前部（2.07~2.37）、壳核后部（1.62~1.96）。基于OSEM重建的PET图像中双侧尾状核、壳核前部及后部的DAT分布半定量值均显著高于基于FBP重建的PET图像,且两者间的差异均有统计学意义（t=9.658~15.859,均P=0.000）。Pearson相关性分析结果显示,基于OSEM与FBP重建的PET图像的DAT分布半定量值在双侧尾状核、壳核前部及后部均呈显著正相关（R2=0.907~0.951,均P=0.000）。基于OSEM与FBP重建的PET图像中双侧尾状核、壳核前部及后部DAT分布半定量值均呈现随年龄增长逐渐减低的趋势。结论 不同PET重建方法获得的正常人脑内DAT分布半定量值存在显著差异,在多中心或纵向研究中需要保持PET图像重建方法的一致性。     

低剂量脑CT灌注的迭代重建研究

【摘要】目的：探讨迭代重建（IR）技术对低剂量脑CT灌注（CTP）图像质量的影响。方法：43例缺血性脑血管病患者行低剂量脑CTP,CTP原始图像分别经IR和滤波反投影（FBP）重建得到两组图像,比较两组图像客观质量及灌注参数图的主观质量。结果：对经IR及FBP重建的灌注参数图（CBV、CBF、MTT、TTP）进行评分并比较,两组各灌注参数图脑灰白质分界评分P值分别为0.317、0.157、0.257、0.083,图像伪影严重程度评分P值分别为0.083、0.046、<0.001、0.011。经IR重建的CTP原始图像客观质量及灌注参数图的图像均匀性、总体图像质量均优于FBP重建,且差异均有统计学意义（P<0.05）。结论：低剂量脑CT灌注迭代重建能明显降低患者所接受的辐射剂量,改善图像质量,且具有较高的诊断效能。     

评价迭代重建法与滤过反投影法在冠状动脉CTA检查中的价值

目的：对行冠状动脉CTA检查的患者原始数据应用低剂量自适应迭代重建技术（AIDR ）与滤过反投影法（FBP ）两种方法的图像进行比较,旨在了解迭代重建算法的图像质量及其临床应用价值。方法：26例临床疑似稳定性心绞痛患者采用320排CT行冠状动脉CTA检查。利用AIDR 重建的常规X线剂量方案扫描,对采集得到的原始数据分别用AIDR和FBP两种方法进行图像重建。测量升主动脉开口、右冠状动脉近段、左冠状动脉主干、右冠状动脉中段、左前降支近段及左回旋支近段的CT值、背景SD并计算分析SNR、CNR值。比较两者的图像质量。结果：26例患者都在一次心动周期完成冠状动脉检查。在血管各节段测得的平均CT值差异无统计学意义（P＞0．05）,背景SD值的差异有统计学意义（P＜0．001）,计算得到的SNR、CNR差异有统计学意义（P＜0．001）。两组图像质量的主观评分差异有统计学意义,AIDR （3．73±0．49）、FBP（3．59±0．61）（P＜0．01）。结论：应用AIDR重建法的图像质量优于FBP 重建法,迭代重建技术可明显降低图像噪声,改善影像质量并具有降低辐射剂量的作用。     

Karl迭代技术降低16层CT胸部扫描辐射剂量的水模和临床研究

目的：探讨Karl迭代重建技术在16层CT胸部检查中的应用价值。方法：水模研究：采用标准模体,管电压120 kV,管电流分别为150、136、123、109、95、83、68、55和41 mAs,使用滤波反投影（FBP）或Karl迭代技术进行图像重建,测量和比较不同条件下图像噪声和对比噪声比（CNR）。临床研究：将100例进行胸部 CT 体检者随机分为2组,对照组（A组）采用标准剂量（120 kV、150 mAs）进行扫描并采用FBP法进行重建;低剂量组采用120 kV、83 mAs进行图像采集,并分别采用Karl技术（B组）和FBP法（C组）进行图像重建。对三组中的图像噪声、信号噪声比（SNR）、对比噪声比、图像质量主观评分、容积CT剂量指数（CTDIvol）和有效剂量（ED）等指标进行比较。结果：在水模研究中,当管电流为95和83 mAs时,辐射剂量分别为标准剂量的60％和50％,且使用Karl重建技术时图像噪声、CNR与使用标准剂量、FBP 法重建的图像间差异无统计学意义（P＞0．05）。在临床研究中,低剂量组的有效剂量为（2．38±0．26）mSv,较标准剂量组的（4．87±0．37）mSv减少51．1％（P＜0．05）。B 组的图像噪声、CNR、SNR、主观评分与 A 组比较,差异均无统计学意义（P＞0．05）;而B组的SNR、CNR及主观评分均高于C组,两组间差异有统计学意义（P＜0．05）。结论：在胸部 CT 平扫中,使用Karl迭代重建技术能在降低辐射剂量后有效地保证图像质量,优于FBP技术,具有较好的临床应用价值。     

乳房X线摄影术； 患者定位； 乳腺疾病

【摘要】目的：对行冠状动脉CTA检查的患者原始数据应用低剂量自适应迭代重建技术（AIDR）与滤过反投影法（FBP）两种方法的图像进行比较,旨在了解迭代重建算法的图像质量及其临床应用价值。方法：26例临床疑似稳定性心绞痛患者采用320排CT行冠状动脉CTA检查。利用AIDR 重建的常规X线剂量方案扫描,对采集得到的原始数据分别用AIDR和FBP两种方法进行图像重建。测量升主动脉开口、右冠状动脉近段、左冠状动脉主干、右冠状动脉中段、左前降支近段及左回旋支近段的CT值、背景SD并计算分析SNR、CNR值。比较两者的图像质量。结果：26例患者都在一次心动周期完成冠状动脉检查。在血管各节段测得的平均CT值差异无统计学意义（P＞0.05）,背景SD值的差异有统计学意义（P＜0.001）,计算得到的SNR、CNR差异有统计学意义（P＜0.001）。两组图像质量的主观评分差异有统计学意义,AIDR (3.73±0.49)、FBP（3.59±0.61）（P＜0.01）。结论：应用AIDR重建法的图像质量优于FBP重建法,迭代重建技术可明显降低图像噪声,改善影像质量并具有降低辐射剂量的作用。     

Evaluation of reconstruction techniques for lung single photon emission tomography: a Monte Carlo study

BACKGROUND: In studies of the distribution of lung function, the image quality of lung single photon emission computed tomography (SPECT) is important and one factor influencing it is the reconstruction algorithm. AIM: To systematically evaluate ordered subsets expectation maximization (OSEM) and compare it with filtered back-projection (FBP) for lung SPECT with Tc. METHODS: The evaluation of the number of iterations used in OSEM was based on the image quality parameter contrast. The comparison between OSEM and FBP was based on trade-off plots between statistical noise and spatial resolution for different filter parameters, collimators and count-levels. A Monte Carlo technique was used to simulate SPECT studies of a digital thorax phantom containing two sets of activity: one with a homogeneous activity distribution within the lungs and the other with superposed high- and low-activity objects. Statistical noise in the reconstructed images was calculated as the coefficient of variation (CV) and spatial resolution as full width at half-maximum (FWHM). RESULTS: For the configuration studied, the OSEM reconstruction in combination with post-filtering should be used in lung SPECT studies with at least 60 MLEM equivalent iterations. Compared to FBP the spatial resolution was improved by about 1 mm. For a constant level of CV, a four-fold increase in count level resulted in an increased resolution of about 2 mm. Spatial resolution and cut-off frequency depends on what value of noise in the image is acceptable also increased by using a low-energy, high-resolution collimator for CV values above 3%. The choice of noise-reducing filter and cut-off frequency depends on what value of noise in the image is acceptable.     

A comparison of the uniformity requirements for SPECT image reconstruction using FBP and OSEM techniques

OBJECTIVE: Gamma camera nonuniformity can result in the presence of ring artifacts in reconstructed SPECT images. The objective of this study is to compare the relationship between ring artifact magnitude and image noise in tomographic images reconstructed using FBP and OSEM. METHODS: A cylindrical phantom was filled with water and (99m)TC: Seven tomographic acquisitions were performed, with total counts per acquisition ranging from 1.5 Mcts to 100 MCTS: All acquisitions were reconstructed using both FBP and OSEM. Ring artifacts were generated in the transaxial data by introducing defects at a given location in each planar image. The modified acquisitions were again reconstructed using both FBP and OSEM. The ring artifacts were isolated by the subtraction of the uncorrupted datasets from the corrupted datasets. The magnitude of the ring artifacts in the corrupted reconstructions was measured and compared to the mean counts and noise level in the uncorrupted data. RESULTS: Ring magnitude in OSEM-reconstructed images is approximately one third that of FBP images. However, there is a corresponding reduction in image noise with OSEM and the ratio of ring magnitude-to-image noise was relatively similar for both OSEM and FBP. Rings generated with OSEM fell off more rapidly with distance from the image center, and reached a plateau at a higher magnitude at large distances. The visibility of rings with OSEM relative to FBP will depend on the location of the causative defect in the planar data and the number of iterations performed with OSEM. Differences between the 2 algorithms are subtle. CONCLUSION: Our results would indicate that the uniformity requirements for SPECT are similar for FBP and OSEM reconstruction algorithms.     

Preliminary evaluation of image reconstruction by ordered-subset expectation maximization in thallium-201 lung scintigraphy

OBJECTIVES: We observed whether clearer tumor delineation and greater tumor to non-tumor (T/N) count ratios could be obtained using an iterative ordered-subsets expectation maximization (OSEM) algorithm than conventional filtered-back projection algorithm (FBP) in the image reconstruction of thallium-201 (201Tl) lung scintigraphy. METHODS: In 29 patients with lung cancer and phantom studies, tomograms were reconstructed using FBP and OSEM algorithms, with and without a prefilter (Butterworth filter: BW), whose cut-off frequencies were 0.10 cycles/pixel for FBP and 0.10 and 0.17 cycles/pixel for OSEM. Visual interpretation and tumor to non-tumor (T/N) count ratios were obtained and compared. RESULTS: Without a prefilter, T/N ratios from OSEM and FBP were 1.89 +/- 0.31 (early) and 2.00 +/- 0.54 (late) for OSEM, 1.90 +/- 0.33 (early) and 2.05 +/- 0.59 (late) for FBP, respectively. The OSEM reconstruction without prefiltering showed clearer tumor contours than FBP without a prefilter. Incorporation of BW showed visually low-noised images but decreased T/N ratios in both reconstructions with BW (0.10 cycles/pixel). No greater T/N ratios were obtained by OSEM than FBP, with or without prefiltering. With BW with a cut-off frequency of 0.17 cycles/pixel, the same T/N ratios as those without BW were obtained. The tumor model sized 0.9 cm in the phantom study was invisible in both OSEM and FBP reconstructions without a prefilter, but visible with a prefilter. The influence of prefiltering on T/N ratios was also observed in phantom studies. CONCLUSIONS: Visually improved tumor delineation could be obtained in OSEM reconstruction without a prefilter as compared to FBP reconstruction without a prefilter for tumors greater than 2 cm. Prefiltering should be incorporated into OSEM reconstruction in diagnosing small tumors. However, the influence of prefilter (BW) setting on semi-quantitative interpretation needs further discussion.     

Preliminary evaluation of image reconstruction by ordered-subset expectation maximization in thallium-201 lung scintigraphy

Objectives: We observed whether clearer tumor delineation and greater tumor to non-tumor (T/N) count ratios could be obtained using an iterative ordered-subsets expectation maximization (OSEM) algorithm than conventional filtered-back projection algorithm (FBP) in the image reconstruction of thallium-201 (²⁰¹Tl) lung scintigraphy.Methods: In 29 patients with lung cancer and phantom studies, tomograms were reconstructed using FBP and OSEM algorithms, with and without a prefilter (Butterworth filter: BW), whose cut-off frequencies were 0.10 cycles/pixel for FBP and 0.10 and 0.17 cycles/pixel for OSEM. Visual interpretation and tumor to non-tumor (T/N) count ratios were obtained and compared.Results: Without a prefilter, T/N ratios from OSEM and FBP were 1.89±0.31 (early) and 2.00±0.54 (late) for OSEM, 1.90±0.33 (early) and 2.05±0.59 (late) for FBP, respectively. The OSEM reconstruction without prefiltering showed clearer tumor contours than FBP without a prefilter. Incorporation of BW showed visually lownoised images but decreased T/N ratios in both reconstructions with BW (0.10 cycles/pixel). No greater T/N ratios were obtained by OSEM than FBP, with or without prefiltering. With BW with a cut-off frequency of 0.17 cycles/pixel, the same T/N ratios as those without BW were obtained. The tumor model sized 0.9 cm in the phantom study was invisible in both OSEM and FBP reconstructions without a prefilter, but visible with a prefilter. The influence of prefiltering on T/N ratios was also observed in phantom studies.Conclusions: Visually improved tumor delineation could be obtained in OSEM reconstruction without a prefilter as compared to FBP reconstruction without a prefilter for tumors greater than 2 cm. Prefiltering should be incorporated into OSEM reconstruction in diagnosing small tumors. However, the influence of prefilter (BW) setting on semi-quantitative interpretation needs further discussion.     

Performance evaluation of OSEM reconstruction algorithm incorporating three-dimensional distance-dependent resolution compensation for brain SPECT: A simulation study

Iterative reconstruction techniques such as an ordered subsets-expectation maximization (OSEM) algorithm can easily incorporated various physical models of attenuation or scatter. We implemented OSEM reconstruction algorithm incorporating compensation for distance-dependent blurring due to the collimator in SPECT. The algorithm was examined by computer simulation to estimate the accuracy for brain perfusion study. METHODS: The detector response was assumed to be a two-dimensional Gauss function and the width of the function varied linearly with the source-to-detector distance. The attenuation compensation (AC) was also included. To investigate the properties of the algorithm, we performed computer simulations with the point source and digital brain phantoms. In the point source phantom, the uniformity of FWHM for the radial, tangential and longitudinal directions was evaluated on the reconstruction image. As for the brain phantom, quantitative accuracy was estimated by comparing the reconstructed images with the true image by the mean square error (MSE) and the ratio of gray and white matter counts (G/W). Both noise free and noisy simulations were examined. RESULTS: In the point source simulation, FWHM in radial, tangential and longitudinal directions were 14.7, 14.7 and 15.0 mm at the image center and were 15.9, 9.83 and 10.6 mm at a distance of 15 cm from the center by using FBP, respectively. On the other hand, they were 8.12, 8.12 and 7.83 mm at the image center, and were 7.45, 7.44 and 7.01 mm at 15 cm from the center by OSEM with distance-dependent resolution compensation (DRC). An isotropic and stationary resolution was obtained at any location by OSEM with DRC. The spatial resolution was also improved about 6.5 mm by OSEM with DRC at the image center. In the brain phantom simulation, the blurring at the edge of the brain structure was eliminated by using OSEM with both DRC and AC. The G/W was 2.95 and 2.68 for noise free and noisy cases, respectively, when no compensation was performed. But the values for G/W without and with noise became 3.45 and 3.21 with AC only and were improved to 3.75 and 3.71 with both AC and DRC. The G/W approached the true value (4.00) by using OSEM with both AC and DRC even when there was statistical noise. CONCLUSION: In conclusion, OSEM reconstruction including the distance-dependent resolution compensation algorithm was reasonably successful in achieving isotropic and stationary resolution and improving the quantitative accuracy for brain perfusion SPECT.     

Comparison of image reconstruction algorithms in myocardial perfusion scintigraphy

The purpose of this study was to compare the clinical utility of two image reconstruction algorithms in myocardial perfusion SPECT (single-photon emission computed tomography): filtered back-projection (FBP) and ordered subset expectation maximization (OSEM). A rest/stress one-day protocol with 99mTc-MIBI or tetrofosmin was performed on 102 consecutive patients who underwent coronary angiography. After SPECT data acquisition, images were reconstructed with FBP and OSEM algorithms. We assessed diagnostic performance (sensitivity, specificity and accuracy) in detecting coronary artery stenosis and evaluated regional tracer uptake with a 4-point scoring system. Although there were no significant differences in diagnostic performance between FBP and OSEM reconstruction, the OSEM method yielded higher uptake in the RCA area than the FBP method by reducing the count-loss artifact due to hepatic uptake of the tracers. In addition, regional uptake in the LCX area was significantly lower in the OSEM image than in the FBP image; this phenomenon was observed mainly in patients with coronary stenosis and/or infarction in the LCX territory. In conclusion, OSEM and FBP offered comparable diagnostic performance in stress myocardial perfusion SPECT. The OSEM method contributed to reduction of the count-loss artifact in inferior and posterior walls and to easy recognition of hypoperfusion in the LCX area.     

Comparative analysis of iterative reconstruction algorithms with resolution recovery for cardiac SPECT studies. A multi-center phantom study

Background

This investigation used image data generated by a physical phantom over a wide range of count statistics to evaluate the effectiveness of several of the newer commercially available SPECT reconstruction iterative algorithms (IRR) in improving perfusion defect contrast and spatial resolution, while controlling image noise.

Methods

A cardiac phantom was imaged using four different gamma cameras over a wide range of counts statistics (from 6 to 0.8 Mcounts). Images were reconstructed with FBP, OSEM, and the IRR available on site. IRR were applied without corrections (IRR NC), with attenuation correction (IRR AC), scatter correction (IRR SC), and attenuation + scatter corrections (IRR SCAC). Four image performance indices related to spatial resolution, contrast, and image noise were analyzed.

Results

IRR NC always determined significant improvements in all indices in comparison to FBP or OSEM. Improvements were emphasized with IRR SC and IRR SCAC. Count reduction from 6 to 1.5 Mcounts did not impair the performances of any of the considered indices.

Conclusions

This is the first study comparing the relative performance of different, commercially available, IRR software, over a wide range of count statistics; the additional effect of scatter and attenuation corrections, alone or in combination, was also evaluated. Our results confirm that IRR algorithms produce substantial benefits with respect to conventional FBP or OSEM reconstruction methods, as assessed through different figures of merit, in particular when SC and/or SCAC are also included.     

Evaluation of reconstruction techniques in regional cerebral blood flow SPECT using trade-off plots: a Monte Carlo study

BACKGROUND AND AIM: The image quality of single photon emission computed tomography (SPECT) depends on the reconstruction algorithm used. The purpose of the present study was to evaluate parameters in ordered subset expectation maximization (OSEM) and to compare systematically with filtered back-projection (FBP) for reconstruction of regional cerebral blood flow (rCBF) SPECT, incorporating attenuation and scatter correction. METHODS: The evaluation was based on the trade-off between contrast recovery and statistical noise using different sizes of subsets, number of iterations and filter parameters. Monte Carlo simulated SPECT studies of a digital human brain phantom were used. The contrast recovery was calculated as measured contrast divided by true contrast. Statistical noise in the reconstructed images was calculated as the coefficient of variation in pixel values. RESULTS: A constant contrast level was reached above 195 equivalent maximum likelihood expectation maximization iterations. The choice of subset size was not crucial as long as there were > or = 2 projections per subset. The OSEM reconstruction was found to give 5-14% higher contrast recovery than FBP for all clinically relevant noise levels in rCBF SPECT. The Butterworth filter, power 6, achieved the highest stable contrast recovery level at all clinically relevant noise levels. The cut-off frequency should be chosen according to the noise level accepted in the image. CONCLUSION: Trade-off plots are shown to be a practical way of deciding the number of iterations and subset size for the OSEM reconstruction and can be used for other examination types in nuclear medicine.     

Optimization of iterative reconstruction parameters with attenuation correction,scatter correction and resolution recovery in myocardial perfusion SPECT/CT

Objective

The aim of this study was to characterize the optimal reconstruction parameters for ordered-subset expectation maximization (OSEM) with attenuation correction, scatter correction, and depth-dependent resolution recovery (OSEM_ACSCRR). We assessed the optimal parameters for OSEM_ACSCRR in an anthropomorphic torso phantom study, and evaluated the validity of the reconstruction parameters in the groups of normal volunteers and patients with abnormal perfusion.

Methods

Images of the anthropomorphic torso phantom, 9 normal volunteers and 7 patients undergoing myocardial perfusion SPECT were acquired with a SPECT/CT scanner. SPECT data comprised a 64 × 64 matrix with an acquisition pixel size of 6.6 mm. A normalized mean square error (NMSE) of the phantom image was calculated to determine both optimal OSEM update and a full width at half maximum (FWHM) of Gaussian filter. We validated the myocardial count, contrast and noise characteristic for clinical subjects derived from OSEM_ACSCRR processing. OSEM with depth-dependent resolution recovery (OSEM_RR) and filtered back projection (FBP) were simultaneously performed to compare OSEM_ACSCRR.

Results

The combination of OSEM_ACSCRR with 90–120 OSEM updates and Gaussian filter with 13.2–14.85 mm FWHM yielded low NMSE value in the phantom study. When we used OSEM_ACSCRR with 120 updates and Gaussian filter with 13.2 mm FWHM in the normal volunteers, myocardial contrast showed significantly higher value than that derived from 120 updates and 14.85 mm FWHM. OSEM_ACSCRR with the combination of 90–120 OSEM updates and 14.85 mm FWHM produced lowest % root mean square (RMS) noise. Regarding the defect contrast of patients with abnormal perfusion, OSEM_ACSCRR with the combination of 90–120 OSEM updates and 13.2 mm FWHM produced significantly higher value than that derived from 90–120 OSEM updates and 14.85 mm FWHM. OSEM_ACSCRR was superior to FBP for the % RMS noise (8.52 ± 1.08 vs. 9.55 ± 1.71, p = 0.02) and defect contrast (0.368 ± 0.061 vs. 0.327 ± 0.052, p = 0.01), respectively.

Conclusions

Clinically optimized the number of OSEM updates and FWHM of Gaussian filter were (1) 120 updates and 13.2 mm, and (2) 90–120 updates and 14.85 mm on the OSEM_ACSCRR processing, respectively. Further assessment may be required to determine the optimal iterative reconstruction parameters in a larger patient population.     

Noise reduction in oncology FDG PET images by iterative reconstruction: a quantitative assessment.

Tumor detection depends on the contrast between tumor activity and background activity and on the image noise in these 2 regions. The lower the image noise, the easier the tumor detection. Tumor activity contrast is determined by physiology. Noise, however, is affected by many factors, including the choice of reconstruction algorithm. Previous simulation and phantom measurements indicated that the ordered-subset expectation maximization (OSEM) algorithm may produce less noisy images than does the usual filtered backprojection (FBP) method, at equivalent resolution. To see if this prediction would hold in actual clinical situations, we quantified noise in clinical images reconstructed with both OSEM and FBP. METHODS: Three patients (2 with colon cancer, 1 with breast cancer) were imaged with FDG PET using a "gated replicate" technique that permitted accurate measurement of noise at each pixel. Each static image was acquired as a gated image sequence, using a pulse generator with a 1-s period, yielding 40 replicate images over the 10- to 15-min imaging time. The images were or were not precorrected for attenuation and were reconstructed with both FBP and OSEM at comparable resolution. From these data, images of pixel mean, SD, and signal-to-noise ratio (S/N) could be produced, reflecting only noise caused by the statistical fluctuations in the emission process. RESULTS: Noise did not vary greatly over each FBP image, even when image intensity varied greatly from one region to the next, causing S/N to be worse in low-activity regions than in high-activity regions. In contrast, OSEM had high noise in hot regions and low noise in cold regions. OSEM had a much better S/N than did FBP in cold regions of the image, such as the lungs (in the attenuation-corrected images), where improvements in S/N averaged 160%. Improvements with OSEM were less dramatic in hotter areas such as the liver (averaging 25% improvement in the attenuation-corrected images). In very hot tumors, FBP actually produced higher S/Ns than did OSEM. CONCLUSION: We conclude that OSEM reconstruction can significantly reduce image noise, especially in relatively low-count regions. OSEM reconstruction failed to improve S/N in very hot tumors, in which S/N may already be adequate for tumor detection.