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PET/MR作为多模态影像设备,优势之一是用PET技术定量分析放射性药物在活体组织内的代谢过程。然而对PET图像定量分析准确性影响较为严重的一个因素是所探测的γ光子在穿透人体时会被组织吸收而衰减,因此确定组织的衰减系数从而对γ光子的衰减进行校正成为PET显像中的关键技术之一[1]。在PET/CT显像中,是利用CT扫描直接获取人体组织的衰减系数图(μ map),然后将低能级宽能谱的X线(40~140 kV)衰减系数转换成高能级单一能量的511 keV γ光子的衰减系数[2]。然而在PET/MR显像中,MRI信号与组织对γ光子的衰减强度无关,因此无法直接用于PET的衰减校正[3]。
目前在PET/MR显像中,基于MRI的衰减校正方法有图集法和分割法。后者通过MRI信号强度把人体分割成不同的组织类型,然后分配其已知的衰减系数。这种分割法最大的挑战是难以得到骨骼的MRI信号,因为其T2弛豫时间非常短,很难在MRI图像上区分骨骼与空气,而二者又是衰减作用差异最大的。超短回波时间(ultrashort echo time,UTE)序列可以获得短T2的组织信号,如骨、肌腱和韧带,可以区分头部的颅骨、脑实质和空气,已用于PET/MR脑部显像的衰减校正[4-5]。
本研究的主要目的是基于UTE序列和CT的μ map对PET/MR采集的PET数据衰减校正,评价不同μ map得到的PET图像和定量比较两者μ map的差别,旨在了解PET/MR是否能够提供准确的定量诊断信息。
基于UTE序列和CT的衰减校正方法在脑部PET应用比较
Comparison of ultrashort echo time-based and CT attenuation correction in the application of PET brain
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摘要:
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。 -
关键词:
- 正电子发射断层显像术 /
- 磁共振成像 /
- 体层摄影术,X线计算机 /
- 脑 /
- 衰减校正 /
- 超短回波时间
Abstract:ObjectiveTo compare the accuracy of MR-ultrashort echo time(UTE) attenuation correction (AC) methods with that of CT AC methods in brain PET examination. MethodsTen patients who underwent the brain 18F-FDG examination of PET/CT and PET/MR were selected randomly. The PET data were attenuation corrected by MRI-UTE and CT methods, and PETUTE and PETCT images were obtained. With the threshold segmentation method, the UET and CT attenuation correction maps(μ Maps) were segmented, and their differences in the neurocranium, brain tissue, and air were compared using paired t test. The difference between PETUTE and PETCT was analyzed by the statistical parametric map. ResultsA significant difference was found between the UTE_μ maps and CT_μ maps in the neurocranium (t=-23.45, P < 0.05) and air(t=5.29, P < 0.05). The difference rates were -64.8%±8.7% and 74.8%±44.7%, respectively. No significant difference(-1.5%±3.9%) in the brain tissue was found between the two maps (t=-1.24, P>0.05). The closer the brain was to the skull and the air, the more metabolism was reduced. ConclusionsThe UTE sequence can accurately evaluate the μ map of brain tissue, but there was a certain underestimation of PET quantitative analysis in areas close to skull and air area. -
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