PET成像中部分容积效应的成因及影响因素

部分容积效应(Partial volume effect,PVE)是影响医学影像设备图像分析的重要因素.PET成像系统有限的分辨率使PVE更加明显,造成大病灶图像边缘模糊,小病灶图像模糊暗淡;在定量分析上使标准化摄取值(SUV)降低,影响对病灶性质的判定及放化疗等治疗效果的评估.本文对PVE的成因、影响因素、实际影响进行初步的探讨. 1 PVE的成因 PVE是由两个效应共同作用下形成的,即点源扩展及组织分数效应.点源扩展是PET分辨率较低造成的[1],组织分数效应主要和PET采集图像信号的方式有关[2],下面进行详细的叙述.     

选择一体化PET/MR小病灶图像重建方案

目的 观察不同重建条件组合对不同大小病灶图像质量的影响，提出一体化PET/MR对于小病灶的最佳图像重建方案。方法 采用GE SIGNA TOF PET/MR系统对符合国际电工协会（IEC）推荐标准的PET图像质量（IQ）发射体模进行扫描，并对列表模式PET原始数据按照以下条件分别组合进行重建：有序子集最大期望值（迭代子集：28；迭代次数：1~9次）；重建矩阵（128×128，192×192，256×256）；高斯低通滤波器，半高宽（FWHM）（1~6 mm）；单独使用飞行时间（TOF）技术；单独使用点扩散函数（PSF）；同时使用TOF和PSF（TOF+PSF）；不使用TOF和PSF（NOTOF+NOPSF）。分析不同重建条件下图像对比度恢复（CR）、背景变化率（BV）和信噪比（SNR），评估图像质量。结果 10 mm和13 mm小球在OSEM（3次迭代，28个子集）、192×192矩阵、2 mm高斯滤波半高宽、TOF+PSF重建组合下信噪比最高，分别为13.31%、21.73%，13.31%、21.73%，25.74%、35.80%和26.25%、46.01%。结论 3次迭代、28个子集、192×192矩阵、TOF+PSF为本研究条件下一体化PET/MR对于模拟小病灶的最佳图像重建方案。     

PET系统分辨力及影响因素的实验研究

目的研究不同条件下,PET系统的分辨力.方法采用椭圆柱分辨力模型,测定ECAT EXACT HR PET系统2D采集模式下,10个空间位置处、17种总活度,两种重建条件(①:FBP,Hanning,截止频率0.4;②:OSEM,Gaussian,核2 mm后滤波)时的系统分辨力.结果系统分辨力不随视野中总活度而变化;但与空间位置相关,中心区最好,距中心16mm处的径向分辨力下降近2 mm;由条件二重建的图像中心区分辨力为4.52 mm,高于条件一重建的图像(7.51 mm),空间各点处两种条件重建图像的分辨力相差约3 mm.结论 PET系统分辨力与活度无关,但空间分布不均,且受重建条件影响较大.     

屏气采集配合ultraHD重建技术改善PET/CT检查肺部代谢灶融合不良

目的比较吸气后屏气采集配合ultra HD重建法较传统自由呼吸法在改善肺部病灶PET/CT检查受呼吸运动而导致融合不良的作用大小。方法制定入组条件:患者常规PET/CT检查CT图像可见明确占位性病灶，PET有明确与之对应的高代谢灶，患者有相应的CT需屏气采集检查史且屏气效果良好，可以对技师的屏气指令做出良好配合。按上述入组条件依次抽取2019年1~6月在某三甲医院PET/CT中心检查的患者60例，将患者常规采集所得图像作为对照组；常规检查结束后立即行肺部高代谢灶单床位吸气后屏气采集配合ultra HD重建，将所得图像作为观察组。比较对照组与观察组图像的融合质量、平均标准率摄取值（SUV_avg）、40%的肿瘤代谢体积（MTV_40%），病灶与肝血池SUV_max的靶本比（T/B_max），并从成像原理的基础上加以解释分析。结果对照组融合良好17例占28.33%，观察组融合良好58例占96.67%；SUV_avg观察组为8.28±2.45、对照组为6.84±2.58，观察组明显高于对照组（χ2=10.50，P<0.05）；MTV_40%观察组为5.61±4.40、对照组为7.70±5.39，观察组明显低于对照组（χ2=5.37，P<0.05），T/B_max观察组为6.29±2.39、对照组为4.87±1.78，观察组明显高于对照组（χ2=13.71，P<0.05）。结论屏气采集配合ultra HD重建法得到的图像较传统法所得图像，融合良好占比更高；SUV_avg、MTV_40%，T/B_max测量值受部分容积效应和移动边界扩大效应的影响更小；加之更加精确的PSF在重建过程中的运用，使上述定量指标更加精确，值得肺部疾病患者临床PET/CT采集中借鉴并有针对性的酌情使用。      

不同重建算法及有序子集最大期望值法重建参数对SPECT/CT定量计算结果的影响

目的 探讨三维有序子集最大期望值法（3D-OSEM）、二维有序子集最大期望值法（2D-OSEM）、滤波反投影（FBP）重建图像及OSEM迭代次数和子集数对SPECT/CT定量计算结果的影响。方法 分别对Jaszczak圆筒模型、NEMA/IEC体模进行SPECT/CT断层显像,并进行CT衰减校正和散射校正以及3D-OSEM图像重建。根据Jaszczak圆筒模型重建图像获得特定采集及重建条件下的系统容积灵敏度（cpm/kBq）,计算IEC体模内不同大小放射性热区的绝对放射性活度（kBq/ml;绝对定量值）,并与真实活度值进行比较,计算定量误差。改变图像重建算法（2D-OSEM、FBP）和OSEM重建参数（迭代次数和子集数）对IEC体模进行图像重建,比较不同重建算法及OSEM迭代次数和子集数对定量误差的影响。结果 采用3D-OSEM、2D-OSEM、FBP重建图像时,各球体定量误差均随球体容积的减小而明显增大（r=-0.831、-0.831、-0.826,P均<0.05）。3种重建算法间不同容积球体定量误差差异有统计学意义（F=8.850,P<0.05）,其中,3D-OSEM小于2D-OSEM重建（P<0.05）,2D-OSEM小于FBP重建（P<0.05）。各不同容积球体定量误差均随迭代次数增加而减小（r=-0.721、-0.681、-0.691、-0.711、-0.845、-0.893,P均<0.05）,随子集数的增加而减小（r=-0.670、-0.694、-0.717、-0.852、-0.956、-0.998,P均<0.05）。结论 基于CT衰减校正和散射校正的3D-OSEM重建定量精确性明显优于2D-OSEM、FBP重建,选择合适的OSEM重建参数有利于提高SPECT/CT定量精确性。     

PET/CT融合图像诊断甲状腺癌——与CT、SPECT或PET的比较研究

目的通过与CT、SPECT或PET的对比研究,探讨PET/CT诊断甲状腺癌的价值.方法收集经随访、手术或活检、病理证实的甲状腺癌12例,CT检查12例,SPECT检查5例,PET检查7例,PET/CT检查12例,分别观察、记录病灶数目、位置、大小、核素分布、钙化等.结果12例共有病灶19个,CT显示病灶13个,SPECT静态显像发现病灶6个,灌注显像全部病灶血流灌注增加,PET发现放射性浓聚影9个,PET/CT见到异常高代谢区19个.结论CT图像能清晰显示病变形态、密度;核医学图像可提供病灶代谢信息;PET/CT图像可同时观察病灶解剖与功能信息,提高了定位、定性诊断能力.     

重建算法对SPECT空间分辨力的影响

目的 探讨重建算法对SPECT断层成像分辨力的影响。方法 采用Siemens Symbia T SPECT机,对长轴分布5条线源的椭圆柱分辨力模型进行成像。选择Hann和Butterworth 2种滤波反投影法（FBP）,每种窗函数各选择3种截止频率;Flash 3D和有序子集最大期望值（OSEM）法2种迭代法,各选择5种平滑函数;分别对图像进行断层重建。以线源半高宽（FWHM）代表重建图像的分辨力,计算5条线源6个层面上的径向、切向和轴向FWHM,并行多元线性回归分析。结果 FBP重建中,FWHM随窗函数截止频率增大而变小;同一截止频率下,Butterworth重建图像的FWHM小于Hann。迭代法中,FWHM随平滑函数核宽度增大而变大;Flash 3D和OSEM法重建图像径向及切向FWHM差异无统计学意义,而轴向Flash 3D图像的FWHM显著小于OSEM。结论 重建算法及参数设定对断层图像分辨力的影响较大,应根据临床需要和经验适当选择。     

18F-FDG PET/CT诊断多发性骨髓瘤

目的 探讨18F-FDG PET/CT对多发性骨髓瘤(MM)的诊断价值.方法 MM患者20例,均于治疗前进行18F-FDG PET/CT全身显像.所有PET、CT及PET/CT融合图像均通过融合软件进行帧对帧对比分析.肿瘤病灶根据穿刺活组织病理学检查、多种影像学检查和临床随访结果 诊断.结果 19例18F-FDG PET/CT检出阳性病灶(95.00%),SUV为2.81±0.98(1.30～6.00), 共检出病灶303处,其中PET有222处表现为18F-FDG摄取增高(73.27%),CT检出268处灶状溶骨性改变(88.45%).PET和CT同时检出的病灶为187处(61.72%);PET检出而同机CT未检出的病灶35处(11.55%);同机CT检出而PET未检出病灶81处(26.73%).结论 18F-FDG PET/CT对于MM的诊断及评价全身累及范围具有一定价值.     

^18F-脱氧葡萄糖PET对非恶性肿瘤摄取的分析与识别

目的 探讨PET表现特殊的非肿瘤性异常放射性摄取的原因,提出鉴别诊断方法.方法 对2004年8月～2008年8月18F-脱氧葡萄糖(18F-FDG)PET全身显像表现为局部明显异常放射性摄取,经病理或临床随访证实为非恶性肿瘤98例的PET、CT图像进行综合分析,总结鉴别诊断要点.结果 本组PET图像均显示有明显异常的局限性放射性摄取,绝大多数浓聚灶标准化摄取值(SUV)最大值超过2.5,其中2例曾误诊为恶性肿瘤.98例中证实为感染与炎症病变46例,生理性摄取28例,良性病变24例.结论 在18F-FDGPET全身显像中,非恶性病变放射性摄取在PET、CT图像以及临床上各有特点,易误诊,应予以高度重视.     

同期~(18)F-FDG PET/CT及MRI对于前列腺癌的初步对照研究

目的对比18氟-氟代脱氧葡萄糖正电子发射断层成像/计算机断层成像(18F-FDG PET/CT)及同期MRI图像,初步探讨18F-FDG PET/CT在前列腺癌的诊断及评价中的价值。方法选取15例经手术或穿刺活检取得病理结果,且在1个月内同时行MRI及18F-FDG PET/CT的前列腺癌患者(其中9例未经治疗),对其图像、随访结果进行回顾性分析。结果前列腺原发病灶在18F-FDG PET/CT图像上可表现为放射性摄取异常增高,平均最大标准化摄取值(SUVmax)5.7(3~10.4):治疗前患者18F-FDG PET/CT及MRI的阳性率分别为89%和100%,治疗后患者分别为20%和80%。对于前列腺外的复发或转移灶,18F-FDG PET/CT和MRI的阳性率分别为73%和27%。对于全部入组患者,18F-FDG PET/CT及MRI的总阳性率均为93%。对于前列腺邻近器官的浸润,18F-FDG PET/CT发现1例,MRI发现2例。结论18F-FDG PET/CT能够较好地显示未经治疗的前列腺癌原发病灶,肿瘤治疗会影响其FDG摄取。对前列腺癌淋巴结、骨骼、软组织等部位的转移灶显示率18F-FDG PET/CT优于MRI;但对肿瘤周围浸润的显示,MRI更具优势。     

Assessment of the impact of model-based scatter correction on [18F]-FDG 3D brain PET in healthy subjects using statistical parametric mapping

It is recognized that scatter correction can supply more accurate absolute quantification, and that iterative reconstruction results in better noise properties and significantly reduces streak artefacts; however, it is not entirely clear whether they produce significant changes in [18F]-FDG distribution of reconstructed 3D brain PET images relative to not scatter corrected images and analytic reconstruction procedures. The current study assesses the effect of model-based scatter correction using the single-scatter simulation algorithm and iterative reconstruction in 3D brain PET studies, using statistical parametric mapping (SPM) analysis. The study population consisted of 14 healthy volunteers (6 males, 8 females; age 63-80 years). PET images were reconstructed using an analytic 3DRP reprojection algorithm with (SC) and without explicit scatter correction (NSC), as well as using an iterative ordered subset-expectation maximization (OSEM) algorithm. Calculated attenuation correction was performed assuming uniform attenuation (mu = 0.096 cm(-1)) for brain tissues when data are precorrected for scatter. The broad-beam attenuation coefficient (mu = 0.06 cm(-1)) determined from phantom studies was applied to NSC images. The images were coregistered and normalized using the default [15O]-H2O template supplied with SPM99 and an [18F]-FDG template. A t statistic image for the contrast condition effect was then constructed. The contrast comparing SC to NSC images suggest that regional brain metabolic activity decreases significantly in the frontal gyri, in addition to the middle temporal and postcentral gyri. On the other hand, activity increases in the cerebellum, thalamus, insula, brainstem, temporal lobe, and the frontal cortex. No significant changes were detected when comparing images reconstructed using analytic and iterative algorithms. It is concluded that, for some cerebral areas, significant differences in [18F]-FDG distribution arise when images are reconstructed with and without explicit SC. This needs to be considered when interpreting [18F]-FDG 3D brain PET images after applying SC.     

Ultrafast Ultrasound Imaging for Super-Resolution Preclinical Cardiac PET

Purpose

Physiological motion and partial volume effect (PVE) significantly degrade the quality of cardiac positron emission tomography (PET) images in the fast-beating hearts of rodents. Several Super-resolution (SR) techniques using a priori anatomical information have been proposed to correct motion and PVE in PET images. Ultrasound is ideally suited to capture real-time high-resolution cine images of rodent hearts. Here, we evaluated an ultrasound-based SR method using simultaneously acquired and co-registered PET-CT-Ultrafast Ultrasound Imaging (UUI) of the beating heart in closed-chest rodents.

Procedures

The method was tested with numerical and animal data (n?=?2) acquired with the non-invasive hybrid imaging system PETRUS that acquires simultaneously PET, CT, and UUI.

Results

We showed that ultrasound-based SR drastically enhances the quality of PET images of the beating rodent heart. For the simulations, the deviations between expected and mean reconstructed values were 2 % after applying SR. For the experimental data, when using Ultrasound-based SR correction, contrast was improved by a factor of two, signal-to-noise ratio by 11 %, and spatial resolution by 56 % (~?0.88 mm) with respect to static PET. As a consequence, the metabolic defect following an acute cardiac ischemia was delineated with much higher anatomical precision.

Conclusions

Our results provided a proof-of-concept that image quality of cardiac PET in fast-beating rodent hearts can be significantly improved by ultrasound-based SR, a portable low-cost technique. Improved PET imaging of the rodent heart may allow new explorations of physiological and pathological situations related with cardiac metabolism.

     

18F-FDG PET/CT综合分析法在胰腺癌诊断中的价值

目的探讨利用CT良性征象校正FDG假阳性的PET/CT综合分析方法在胰腺癌诊断中的价值。方法胰腺原发良恶性疾患40例,其中胰腺癌27例(手术6例,临床随访21例),急、慢性胰腺炎13例(腹腔镜2例,临床随访11例)。分别用单纯SUV测量法和PET与CT相结合的综合分析法分析,比较两种方法诊断胰腺恶性肿瘤的敏感性、特异性及准确性。SUV测量法以病灶SUVmax值大于2.5为恶性病变诊断指征,反之为良性。PET/CT综合分析法以PET为主导,CT具有否决权,利用PET/CT融合图像的精确对位,当浓聚热点相应CT表现为明显的良性病变征象时,无论FDG浓聚程度如何,均诊断为良性病变。结果综合分析法对胰腺癌诊断的敏感性、特异性、准确性都是100%;单纯SUV测量法敏感性、特异性、准确性分别为96.3%、76.9%、90.0%。结论CT的良性征象,尤其是炎性渗出征象,对FDG假阳性有校正作用,通过这种校正能够提高PET/CT在胰腺癌诊断的特异性和准确性。     

Functional and structural synergy for resolution recovery and partial volume correction in brain PET

Purpose: Positron Emission Tomography (PET) has the unique capability of measuring brain function but its clinical potential is affected by low resolution and lack of morphological detail. Here we propose and evaluate a wavelet synergistic approach that combines functional and structural information from a number of sources (CT, MRI and anatomical probabilistic atlases) for the accurate quantitative recovery of radioactivity concentration in PET images. When the method is combined with anatomical probabilistic atlases, the outcome is a functional volume corrected for partial volume effects.Methods: The proposed method is based on the multiresolution property of the wavelet transform. First, the target PET image and the corresponding anatomical image (CT/MRI/atlas-based segmented MRI) are decomposed into several resolution elements. Secondly, high-resolution components of the PET image are replaced, in part, with those of the anatomical image after appropriate scaling. The amount of structural input is weighted by the relative high frequency signal content of the two modalities. The method was validated on a digital Zubal phantom and clinical data to evaluate its quantitative potential.Results: Simulation studies showed the expected relationship between functional recovery and the amount of correct structural detail provided, with perfect recovery achieved when true images were used as anatomical reference. The use of T1-MRI images brought significant improvements in PET image resolution. However improvements were maximized when atlas-based segmented images as anatomical references were used; these results were replicated in clinical data sets.Conclusion: The synergistic use of functional and structural data, and the incorporation of anatomical probabilistic information in particular, generates morphologically corrected PET images of exquisite quality.     

Influence of Co-57 and CT Transmission Measurements on the Quantification Accuracy and Partial Volume Effect of a Small Animal PET Scanner

Purpose

Non-invasive in vivo positron emission tomography (PET) provides high detection sensitivity in the nano- to picomolar range and in addition to other advantages, the possibility to absolutely quantify the acquired data. The present study focuses on the comparison of transmission data acquired with an X-ray computed tomography (CT) scanner or a Co-57 source for the Inveon small animal PET scanner (Siemens Healthcare, Knoxville, TN, USA), as well as determines their influences on the quantification accuracy and partial volume effect (PVE). A special focus included the impact of the performed calibration on the quantification accuracy.

Procedures

Phantom measurements were carried out to determine the quantification accuracy, the influence of the object size on the quantification, and the PVE for different sphere sizes, along the field of view and for different contrast ratios.

Results

An influence of the emission activity on the Co-57 transmission measurements was discovered (deviations up to 24.06 % measured to true activity), whereas no influence of the emission activity on the CT attenuation correction was identified (deviations <3 % for measured to true activity). The quantification accuracy was substantially influenced by the applied calibration factor and by the object size. The PVE demonstrated a dependency on the sphere size, the position within the field of view, the reconstruction and correction algorithms and the count statistics. Depending on the reconstruction algorithm, only ～30–40 % of the true activity within a small sphere could be resolved. The iterative 3D reconstruction algorithms uncovered substantially increased recovery values compared to the analytical and 2D iterative reconstruction algorithms (up to 70.46 % and 80.82 % recovery for the smallest and largest sphere using iterative 3D reconstruction algorithms). The transmission measurement (CT or Co-57 source) to correct for attenuation did not severely influence the PVE.

Conclusions

The analysis of the quantification accuracy and the PVE revealed an influence of the object size, the reconstruction algorithm and the applied corrections. Particularly, the influence of the emission activity during the transmission measurement performed with a Co-57 source must be considered. To receive comparable results, also among different scanner configurations, standardization of the acquisition (imaging parameters, as well as applied reconstruction and correction protocols) is necessary.

     

基于MR模板的PET/MRI衰减校正方法

目的 设计并对比基于MRI的衰减校正模板对PET图像的衰减校正效果,寻找衰减校正效果最佳模板。方法 采集30名志愿者的¹⁸F-FDG PET图像,根据性别或不同头部尺寸创建MRI模板,包括女性组（fmg）、男性组（mmg）、小尺寸头部组（smg）、大尺寸头部组（bmg）和基础组（gmg）,与PET透射模板结合后获得个体化衰减图,并最终用于¹⁸F-FDG PET图像的衰减校正;测量并对比经不同模板校正的PET图像ROI内的放射性活度值和与原始PET图像相比的相对偏差值。结果 女性受试者经衰减校正的PET图像中,其ROI内放射性活度的最小差值为4 Bq/ml,来源于fmg与gmg模板间,而男性受试者放射性活度的最小差值为1 Bq/ml,来源于mmg与fmg模板间。经5个不同模板校正后与经PET透射扫描所获得的衰减数据之间均存在差异,相对偏差值范围为-6.31%~6.32%。gmg模板与其他4个模板的相对偏差值范围为-3.44%~5.34%。结论 基于MR模板的衰减校正方法对于受试者个体不存在明显的性别差异,一般情况下,gmg模板即可满足要求。     

Impact of Resolution Recovery in Quantitative 99mTc SPECT/CT Cardiac Phantom Studies

BackgroundThe aim of this study was to validate the quantitative accuracy of single photon emission computed tomography with computed tomography (SPECT/CT) images in cardiac phantom studies. The study was performed by assessing the effect of resolution recovery (RR) when using half-time of imaging acquisition in cardiac phantom.MethodsThe SPECT/CT images of the anthropomorphic phantom with a cardiac insert, liver, lung, and spine were acquired using the GE Discovery (NM/CT 670) SPECT/CT system. Different concentration activity ratios for different organ (10:10:1:0, cardiac:liver:background:lung) regions were acquired by using full- and half-time protocols for 64³ and 128³ voxel sizes that were reconstructed using filtered back projection (FBP) method and 3D ordered subset expectation maximization (3D-OSEM). Attenuation correction and scatter correction were applied to both reconstructions, whereas the RR only can be applied for 3D-OSEM. The data were analyzed and reported in terms of absolute recovery coefficient percentage between the cardiac insert and background activity concentration. Another parameter used to assess the quantitative accuracy for defect region was the relative error percentage.ResultsThe result of recovery coefficient percentage shows that the 3D-OSEM reconstruction with the RR gives the highest percentage estimation accuracy of 70% of activity recovery in the cardiac phantom wall compared with FBP (10.6%). The relative error percentage for reconstructed SPECT/CT images using 3D-OSEM reconstruction with RR shows the least error compared with FBP (21% vs. 45.1%) both in the full-and half-time acquisition of images with a larger number of matrix size used.Conclusions3D-OSEM reconstruction with the RR is beneficial in giving better quantitative evaluation with a good resolution myocardial perfusion image. To accomplish this, a larger matrix size is required for 3D-OSEM reconstruction with the RR and it demonstrated an improvement in image resolution and increased quantitative accuracy of the final reconstructed SPECT/CT images.     

Development and evaluation of an automated atlas-based image analysis method for microPET studies of the rat brain

An automated method for placement of 3D rat brain atlas-derived volumes of interest (VOIs) onto PET studies has been designed and evaluated. VOIs representing major structures of the rat brain were defined on a set of digitized cryosectioned images of the rat brain. For VOI placement, each PET study was registered with a synthetic PET target constructed from the VOI template. Registration was accomplished with an automated algorithm that maximized the mutual information content of the image volumes. The accuracy and precision of this method for VOI placement was determined using datasets from PET studies of the striatal dopamine and hippocampal serotonin systems. Each evaluated PET study could be registered to at least one synthetic PET target without obvious failure. Registration was critically dependent upon the initial position of the PET study relative to the synthetic PET target, but not dependent on the amount of synthetic PET target smoothing. An evaluation algorithm showed that resultant radioactivity concentration measurements of selected brain structures had errors=2% due to misalignment with the corresponding VOI. Further, radioligand binding values calculated from these measurements were found to be more precise than those calculated from measurements obtained with manually drawn regions of interest (ROIs). Overall, evaluation results demonstrated that this atlas-derived VOI method can be used to obtain unbiased measurements of radioactivity concentration from PET studies. Its automated features, and applicability to different radioligands and brain regions, will facilitate quantitative rat brain PET assessment procedures.