PET/MR联合成像的应用

PET／MRI 双模态成像整合了 PET 提供的人体生理代谢、分子信息和 MRI 提供的功能及解剖形态信息,实现了 PET 及 MR 成像设备的优势互补,对神经、心血管、肿瘤等疾病的研究及临床诊断具有重要意义,是将功能性成像和解剖性图像相结合的一种成像,提供更多分子、形态与功能信息,已开始应用于临床诊疗和科研工作中。本文将从 PET／MRI 的发展,在肿瘤、心血管和神经系统成像等方面的临床应用进行综述,并探讨其面临的挑战和前景。     

PET/MR在心血管疾病中的应用进展

PET/MR作为新型的多模态成像技术,集合了MRI高软组织对比度、多序列、多参数、可定量和PET多分子探针显像的高灵敏度的优势,实现了PET分子功能影像与具有精细解剖结构和组织特征的MRI影像同步扫描,在多种心血管疾病中的应用和研究越来越广泛。笔者总结PET/MR心血管成像的优点和局限性,并探讨其在临床中的主要应用。     

基于UTE序列和CT的衰减校正方法在脑部PET应用比较

目的通过与CT衰减校正方法的比较,探讨超短回波时间（UTE）序列作为脑部PET衰减校正方法的准确性。方法随机选取10名先后接受PET/CT和PET/MR脑部检查的受检者,基于MRI-UTE和CT方法对PET/MR采集的PET数据进行衰减校正,获得PETUTE与PETCT图像。基于阈值分割法对UET、CT衰减校正图（μ map）行体素分割,使用配对t检验比较两者在颅脑骨、脑实质及空气的差异;并用统计参数图分析PETUTE、PETCT的区别。结果UTE_μ map和CT_μ map在颅脑骨（t=-23.45,P < 0.05）和空气（t=5.29,P < 0.05）中的差异有统计学意义,差别分别为-64.8%±8.7%和74.8%±44.7%,在脑实质中的差异无统计学意义（t=-1.24,P>0.05）,差别为-1.5%±3.9%;且PETUTE脑实质相对于PETCT在越接近颅骨和空气区代谢低估越大。结论UTE序列能够准确评估脑实质的μ map,但对于接近颅骨和空气区的局部脑实质存在一定程度PET定量分析的低估。     

创伤性脑梗死的MR与PET成像研究进展

随着磁共振成像技术的快速发展,功能磁共振成像技术在创伤性脑梗死诊断方面的作用日益增强,尤其在半暗带的诊断方面获得很大进步.同时在创伤性脑梗死的病因和治疗方面也取得了进展.     

创伤性脑梗死的MR与PET成像研究进展

随着磁共振成像技术的快速发展,功能磁共振成像技术在创伤性脑梗死诊断方面的作用日益增强,尤其在半暗带的诊断方面获得很大进步。同时在创伤性脑梗死的病因和治疗方面也取得了进展。     

PET显像的散射校正和衰减校正

PET显像探测器所能记录到的真符合计数只占很小比例。源于体内的湮灭辐射使光子只有少部分能沿着初始的发射方向到达探测器(部分成为计数)，大部分光子对在体内穿行过程中(其中的1个或2个)或被吸收、或形成康普顿散射，这种现象称为衰减。散射光子的去向大致有4种：被进一步散射或(和)组织吸收、逸出探测野(field of view，FOV)、     

DWI联合PET/CT对转移性骨肿瘤的影像学表现分析

目的 分析转移性骨肿瘤患者单独应用MRDWI、PET/CT以及二者联合应用的影像学诊断价值。方法 选取145例本院接诊的转移性骨肿瘤患者,采取MRDWI、PET/CT单独应用以及两种技术联合应用的诊断方案,对比不同b值下MRDWI技术诊断转移性骨肿瘤的差异,对比不同诊断方案的准确率以及诊断效能指标。结果 1)相比于b=50 s/mm²,b=400 s/mm²、b=800 s/mm²时的信号质量指数、信噪比、信号强度均明显更低,且b=400 s/mm²低于b=800 s/mm²时的数值(P<0.05);2)实质区在b=800 s/mm²时的ADC值明显低于坏死区(P<0.05);3)病理诊断结果显示145例患者共有150个病灶,混合型骨转移、成骨型骨转移病灶分别为110个、40个,MRDWI联合PET/CT检测以及MRDWI单独检测不同转移性骨肿瘤病灶的准确率均明显高于PET/CT单独检测,且联合检测高于MRDWI单独检测准确率,MRDWI的检测...     

18F-FDG PET/MR在肝转移瘤诊断中的价值

目的:探讨 18F-脱氧葡萄糖(FDG)PET/MR在肝转移瘤诊断中的应用价值。 方法:回顾性分析2020年1月至2020年10月间于瑞金医院接受PET/MR与上腹部增强CT检查的可疑肝转移瘤患者75例[男46例,女29例;年龄(58.9±14.3)岁],分别基于病灶和患者计算并比较增强CT、PE...     

手掌腱鞘巨细胞瘤18F-FDG PET/CT显像1例

患者女,51岁。发现右手肿物5年,逐渐缓慢生长,伴有压痛。MRI显示右手第3掌指关节周围见不规则软组织团块影,呈分叶状,T₁WI呈等低信号,T₂WI呈混杂高信号,边缘可见线样低信号,邻近第3掌骨及指骨骨质形态欠规整,可见斑片状T₁WI低信号、T₂WI高信号,DWI局部扩散受限,增强呈明显不均匀强化,边缘线样低信号未见明显强化(图1)。     

Evaluation of Dixon Sequence on Hybrid PET/MR Compared with Contrast-Enhanced PET/CT for PET-Positive Lesions

Purpose

Hybrid positron emission tomography and magnetic resonance (PET/MR) imaging performs a two-point Dixon MR sequence for attenuation correction. However, MR data in hybrid PET/MR should provide anatomic and morphologic information as well as an attenuation map. We evaluated the Dixon sequence of hybrid PET/MR for anatomic correlation of PET-positive lesions compared with contrast-enhanced PET/computed tomography (CT) in patients with oncologic diseases.

Methods

Twelve patients underwent a single injection, dual imaging protocol. PET/CT was performed with an intravenous contrast agent (85 ± 13 min after ¹⁸F-FDG injection of 403 ± 45 MBq) and then (125 ± 19 min after injection) PET/MR was performed. Attenuation correction and anatomic allocation of PET were performed using contrast-enhanced CT for PET/CT and Dixon MR sequence for hybrid PET/MR. The Dixon MR sequence and contrast-enhanced CT were compared for anatomic correlation of PET-positive lesions (scoring scale ranging from 0 to 3 for visual ratings). Additionally, standardized uptake values (SUVs) for the detected lesions were assessed for quantitative comparison.

Results

Both hybrid PET/MR and contrast-enhanced PET/CT identified 55 lesions with increased FDG uptake in ten patients. In total, 28 lymph nodes, 11 bone lesions, 3 dermal nodules, 3 pleural thickening lesions, 2 thyroid nodules, 1 pancreas, 1 liver, 1 ovary, 1 uterus, 1 breast, 1 soft tissue and 2 lung lesions were present. The best performance was observed for anatomic correlation of PET findings by the contrast-enhanced CT scans (contrast-enhanced CT, 2.64 ± 0.70; in-phase, 1.29 ± 1.01; opposed-phase, 1.29 ± 1.15; water-weighted, 1.71 ± 1.07; fat weighted, 0.56 ± 1.03). A significant difference was observed between the scores obtained from the contrast-enhanced CT and all four coregistered Dixon MR images. Quantitative evaluation revealed a high correlation between the SUVs measured with hybrid PET/MR (SUVmean, 2.63 ± 1.62; SUVmax, 4.30 ± 2.88) and contrast-enhanced PET/CT (SUVmean, 3.88 ± 2.30; SUVmax, 6.53 ± 4.04) in PET-positive lesions (SUVmean, ρ = 0.93; SUVmax, ρ = 0.95), although hybrid PET/MR presented a decrease of SUVs compared with contrast-enhanced PET/CT (mean reduction; SUVmean, 32.44 ± 15.64 %; SUVmax, 35.16 ± 12.59 %).

Conclusions

Despite different attenuation correction approaches, the SUV of PET-positive lesions correlated well between hybrid PET/MR and contrast-enhanced PET/CT. However Dixon MR images acquired for attenuation correction were insufficient to provide anatomic information of PET images because of low spatial resolution. Thus, additional MR sequence with fast and higher resolution may be necessary for anatomic information.     

PET/CT和PET/MR在黑色素瘤中的临床应用及新进展

黑色素瘤易转移、复发率高,已经成为严重危及人民健康的恶性肿瘤之一,早期诊断和准确分期对预后及远期生存十分关键。18F-FDG PET/CT作为一项集PET与CT于一体的成像模式,被广泛地应用于包括黑色素瘤等在内的恶性肿瘤的诊断、分期及疗效的评估。随着一体化PET/MR成像系统研发的成功,多模态成像技术向前迈进了一大步,实现了真正意义上的数据同步采集。笔者就18F-FDG PET/CT及PET/MR在黑色素瘤分期、复发和疗效评价中的研究现状进行综述,介绍了多模态成像技术在黑色素瘤中的新进展。     

A hybrid method of attenuation correction for positron emission tomography brain studies

A hybrid method for attenuation correction (HAC) in positron emission tomography (PET) brain studies is proposed. The technique requires the acquisition of two short (1 min) transmission scans immediately before or after the emission study, with the patient and the head fixation system in place and after removing the patient from the scanner with the head fixation system alone. The method combines a uniform map of attenuation coefficients for the patient's head with measured attenuation coefficients for the head fixation system to generate a hybrid attenuation map. The HAC method was calibrated on 30 PET cerebral studies for comparison with the conventional measured attenuation correction method by ROI analysis. Average differences of less than 3% were found for cortical and subcortical regions. The HAC technique is particularly suitable in a PET clinical environment, allowing a reduction of the total study time, greater comfort for patients and an increase in patient throughput.     

Attenuation compensation in cerebral 3D PET: effect of the attenuation map on absolute and relative quantitation

It is generally well accepted that transmission (TX)-based non-uniform attenuation correction can supply more accurate absolute quantification; however, whether it provides additional benefits in routine clinical diagnosis based on qualitative interpretation of 3D brain positron emission tomography (PET) images is still the subject of debate. The aim of this study was to compare the effect of the two major classes of method for determining the attenuation map, i.e. uniform versus non-uniform, using clinical studies based on qualitative assessment as well as absolute and relative quantitative volume of interest-based analysis. We investigated the effect of six different methods for determining the patient-specific attenuation map. The first method, referred to as the uniform fit-ellipse method (UFEM), approximates the outline of the head by an ellipse assuming a constant linear attenuation factor (=0.096 cm^–1) for soft tissue. The second, referred to as the automated contour detection method (ACDM), estimates the outline of the head from the emission sinogram. Attenuation of the skull is accounted for by assuming a constant uniform skull thickness (0.45 cm) within the estimated shape and the correct value (0.151 cm^–1) is used. The usual measured transmission method using caesium-137 single-photon sources was used without (MTM) and with segmentation of the TX data (STM). These techniques were finally compared with the segmented magnetic resonance imaging method (SMM) and an implementation of the inferring attenuation distributions method (IADM) based on the digital Zubal head atlas. Several image quality parameters were compared, including absolute and relative quantification indexes, and the correlation between them was checked. The qualitative evaluation showed no significant differences between the different attenuation correction techniques as assessed by expert physicians, with the exception of ACDM, which generated artefacts in the upper edges of the head. The mean squared error between the different attenuation maps was also larger when using this latter method owing to the fact that the current implementation of the method significantly overestimated the head contours on the external slices. Correlation between the mean regional cerebral glucose metabolism (rCGM) values obtained with the various attenuation correction methods and those obtained with the gold standard (MTM) was good, except in the case of ACDM (R ²=0.54). The STM and SMM methods showed the best correlation (R ²=0.90) and the regression lines agreed well with the line of identity. Relative differences in mean rCGM values were in general less than 8%. Nevertheless, ANOVA results showed statistically significant differences between the different methods for some regions of the brain. It is concluded that the attenuation map influences both absolute and relative quantitation in cerebral 3D PET. Transmission-less attenuation correction results in a reduced radiation dose and makes a dramatic difference in acquisition time, allowing increased patient throughput.     

Attenuation correction in cardiac PET/CT with three different CT protocols: a comparison with conventional PET

Purpose CT-based attenuation correction may influence cardiac PET owing to its higher susceptibility to misalignment compared with conventional ⁶⁸Ge transmission scans. The aims of this study were to evaluate whether CT attenuation correction leads to changes in tracer distribution compared with conventional cardiac PET and to determine a suitable CT protocol. Methods A total of 27 patients underwent PET/CT and subsequently a PET scan. Twenty patients received a low-dose CT (LDCT group; 120 kV, 26 mA, 8-s scan time), seven patients a slow CT (SCT group; 120 kV, 99 mA, 46-s scan time) and ten patients an ultra-low-dose CT (ULDCT group; 80 kV, 13 mA, 5-s scan time) as the transmission scan in PET/CT. Polar maps were divided into 17 segments and regression analysis was computed in every scan pair (CT attenuation corrected–⁶⁸Ge attenuation corrected). Correlation coefficient (r), the slope (s) and the offset (os) of the regression line were determined. Visual assessment of misalignment between the transmission and emission data was performed. The effective dose of the different transmission scans was calculated. Results Overall, there was a moderate correlation between the mean values measured in all segments on PET/CT and on PET when using LDCT (r=0.78, p<0.0001), SCT (r=0.79, p<0.0001) and ULDCT (r=0.82, p<0.0001). No differences were observed when comparing the scores assigned in the visual misalignment assessment in the three groups (p=0.12). The differences between the results from the regression analysis observed in the respective groups were not statistically significant (Kruskal-Wallis p=0.11 for r, p=0.67 for s and p=0.27 for os). The effective dose was lowest for the ULDCT. Conclusion Our study shows that CT-based attenuation correction is feasible for cardiac PET imaging. The results indicate that ultra-low-dose CT is the preferable choice for transmission scanning.     

Clinical significance of apical thinning after attenuation correction

BACKGROUND: Apical thinning and other image changes at the apex have been described after attenuation correction of myocardial perfusion single photon emission computed tomography (SPECT) studies, but their clinical significance is unknown. METHODS AND RESULTS: We studied 102 subjects from a multicenter trial of attenuation correction, 46 with angiographic coronary artery disease and 56 normal subjects. We graded the presence and magnitude (on a 4-point scale) of apical thinning (decrease in wall thickness, best assessed in the vertical long-axis view) in both noncorrected and attenuation-corrected images. In attenuation-corrected images, apical thinning of any degree was present in 78% of the abnormal patients and 63% of the normal subjects (P = not significant [NS]). However, moderate or severe apical thinning was present in 30% of the abnormal patients compared with 5% of the normal subjects (relative risk = 2.2, P <.001). In noncorrected images, apical thinning of any degree was present in 87% of the abnormal patients and 71% of the normal subjects (P = NS). However, moderate or severe apical thinning was present in 28% of the abnormal patients compared with 4% of the normal subjects (relative risk = 2.3, P <.001). CONCLUSION: The presence of mild apical thinning is common in both noncorrected and attenuation-corrected SPECT images and does not imply coronary artery disease. Moderate or severe apical thinning is 7 times more common in patients than in normal subjects, but it is relatively uncommon and thus is not a generally useful clinical tool.     

PET/CT和MRI在多发性骨髓瘤中的应用与进展

多发性骨髓瘤（MM）是一种恶性B细胞肿瘤,其主要累及骨髓,部分也可出现髓外浸润。影像学检查是全面评估MM不可或缺的手段,X射线与CT对病灶的检出率低、定量能力差。PET/CT与MRI对MM的评估具有较大的价值,特别是18F-FDG及其他新型分子探针,全身扩散加权成像、动态增强MRI及水脂分离等MRI新技术。笔者就PET/CT及MRI在MM中的应用与进展进行综述。     

Measured attenuation correction methods

Accurate attenuation correction is a prerequisite for the determination of exact local radioactivity concentrations in positron emission tomography. Attenuation correction factors range from 4–5 in brain studies to 50–100 in whole body measurements. This report gives an overview of the different methods of determining the attenuation correction factors by transmission measurements using an external positron emitting source. The long-lived generator nuclide⁶⁸Ge/⁶⁸Ga is commonly used for this purpose. The additional patient dose from the transmission source is usually a small fraction of the dose due to the subsequent emission measurement. Ring-shaped transmission sources as well as rotating point or line sources are employed in modern positron tomographs. By masking a rotating line or point source, random and scattered events in the transmission scans can be effectively suppressed. The problems of measured attenuation correction are discussed: transmission/emission mismatch, random and scattered event contamination, counting statistics, transmission/emission scatter compensation, transmission scan after administration of activity to the patient. By using a double masking technique simultaneous emission and transmission scans become feasible.This article was presented at the 1st EEC workshop on accuracy determination in PET, January 19–20th. 1989 Pisa, Italy (COMAC-BME Concerted Project Characterization and Standardization of PET Instrumentation)     

PET recognition of pulmonary metastases on PET/CT imaging: impact of attenuation-corrected and non-attenuation-corrected PET images

Purpose The aims of this study were to assess the performance of FDG PET at PET/CT imaging for the detection of pulmonary metastases and to evaluate differences in lesion detectability on attenuation-corrected (AC) and non-attenuation corrected (NAC) PET images.Methods The institutional PET/CT database was searched for patients with pulmonary metastases of 3–60 mm in diameter. Ninety-two patients with 438 metastases to the lungs were included in the study. The primary tumours were 33 malignant melanomas, 12 carcinomas of unknown primary, 11 colorectal carcinomas, eight differentiated thyroid carcinomas, seven aggressive non-Hodgkins lymphomas, six head and neck cancers, three breast cancers, two prostate cancers and ten others. Lesion detectability was visually compared between PET and CT and between AC and NAC PET images using a five-point scale.Results Of the 438 pulmonary metastases, 174 were detected with FDG PET (39.7%), six of them on NAC images only (not significant). Visual scores were higher on NAC images in 41.4% and equal in 54.6% of lesions. The sensitivity of FDG PET increased significantly from 0.405 for metastases of 5–7 mm in diameter to 0.784 for lesions of 8–10 mm and to 0.935 for lesions measuring 11–29 mm in diameter. No metastases smaller than 5 mm in diameter were seen on PET images.Conclusion FDG PET/CT is useful for the assessment of pulmonary metastases. The frequency of lesion detection is similar for AC and NAC PET images. A reduced sensitivity of FDG PET has to be considered for lesions smaller than 11 mm in diameter.