MRI-Based Attenuation Correction for PET/MRI Using Ultrashort Echo Time Sequences
MRI-Based Attenuation Correction for PET/MRI Using Ultrashort Echo Time Sequences
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DOI：
10.2967/jnumed.109.065425
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发表时间：
2010-05-01
影响因子：
9.3
通讯作者：
Vandenberghe, Stefaan
中科院分区：
文献类型：
--
作者：
Keereman, Vincent;Fierens, Yves;Vandenberghe, Stefaan
PET/MRI中的挑战之一是导出衰减图以校正PET图像的衰减。已经提出了用于从MR图像导出衰减图的不同方法。由于传统MRI序列获得的图像上皮质骨的低信号强度使得难以检测这种组织类型，因此这些方法依赖于某种解剖学前提来预测衰减图，从而提出了当患者可能表现出解剖学异常时这些方法是否可用于临床的问题。研究方法：我们建议使用横向弛豫率（从超短回波时间序列采集的图像中导出）将体素分类为3种组织类别（骨骼、软组织或空气）中的1种，而无需对患者解剖结构做出任何假设。每个体素被分配对应于其组织类别的线性衰减系数。参考CT扫描用于确定所提出的方法的逐体素精度。使用考虑到衰减校正的非局部效应的方法来评估基于MRI的衰减校正的总体准确性。结果：作为概念验证，猪的头部被用作成像的体模。新方法在90%的体素中产生了正确的组织分类。还处理了五个人脑PET/CT和MRI数据集，与CT衍生的衰减图相比，逐体素性能略差。PET数据集重建使用分割的MRI衰减图与新的方法，得到的图像进行了比较，分割CT为基础的衰减校正。大脑中发现的平均误差约为5%。结论：使用来自超短回波时间MR图像的横向弛豫率图来估计衰减图的可行性在体模和临床脑数据上显示。结果表明，新的方法，与基于CT的衰减校正相比，产生临床上可接受的误差。所提出的方法不对患者解剖结构做出任何假设，因此也可以用于存在解剖结构异常的情况。
One of the challenges in PET/MRI is the derivation of an attenuation map to correct the PET image for attenuation. Different methods have been suggested for deriving the attenuation map from an MR image. Because the low signal intensity of cortical bone on images acquired with conventional MRI sequences makes it difficult to detect this tissue type, these methods rely on some sort of anatomic precondition to predict the attenuation map, raising the question of whether these methods will be usable in the clinic when patients may exhibit anatomic abnormalities. Methods: We propose the use of the transverse relaxation rate, derived from images acquired with an ultrashort echo time sequence to classify the voxels into 1 of 3 tissue classes (bone, soft tissue, or air), without making any assumptions on patient anatomy. Each voxel is assigned a linear attenuation coefficient corresponding to its tissue class. A reference CT scan is used to determine the voxel-by-voxel accuracy of the proposed method. The overall accuracy of the MRI-based attenuation correction is evaluated using a method that takes into account the nonlocal effects of attenuation correction. Results: As a proof of concept, the head of a pig was used as a phantom for imaging. The new method yielded a correct tissue classification in 90% of the voxels. Five human brain PET/CT and MRI datasets were also processed, yielding slightly worse voxel-by-voxel performance, compared to a CT-derived attenuation map. The PET datasets were reconstructed using the segmented MRI attenuation map derived with the new method, and the resulting images were compared with segmented CT-based attenuation correction. An average error of around 5% was found in the brain. Conclusion: The feasibility of using the transverse relaxation rate map derived from ultrashort echo time MR images for the estimation of the attenuation map was shown on phantom and clinical brain data. The results indicate that the new method, compared with CT-based attenuation correction, yields clinically acceptable errors. The proposed method does not make any assumptions about patient anatomy and could therefore also be used in cases in which anatomic abnormalities are present.