Lundman Josef A, Johansson Adam, Olofsson Jörgen, Axelsson Jan, Larsson Anne, Nyholm Tufve
Department of Radiation Sciences, Umeå University, Umeå, Sweden.
Department of Radiation Sciences, Umeå University, Umeå, Sweden; Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, United States.
Phys Med. 2017 Mar;35:1-6. doi: 10.1016/j.ejmp.2017.02.019. Epub 2017 Mar 7.
Attenuation correction is a requirement for quantification of the activity distribution in PET. The need to base attenuation correction on MRI instead of CT has arisen with the introduction of integrated PET/MRI systems. The aim was to describe the effect of residual gradient field nonlinearity distortions on PET attenuation correction.
MRI distortions caused by gradient field nonlinearity were simulated in CT images used for attenuation correction in PET reconstructions. The simulations yielded radial distortion of up to ±2.3mm at 15cm from the scanner isocentre for distortion corrected images. The mean radial distortion of uncorrected images were 6.3mm at the same distance. Reconstructions of PET data were performed using the distortion corrected images as well as the images where no correction had been applied.
The mean relative difference in reconstructed PET uptake intensity due to incomplete distortion correction was less than ±5%. The magnitude of this difference varied between patients and the size of the distortions remaining after distortion correction.
Radial distortions of 2mm at 15cm radius from the scanner isocentre lead to PET attenuation correction errors smaller than 5%. Keeping the gradient field nonlinearity distortions below this limit can be a reasonable goal for MRI systems used for attenuation correction in PET for quantification purposes. A higher geometrical accuracy may, however, be warranted for quantification of peripheral lesions. These distortions can, e.g., be controlled at acceptance testing and subsequent quality assurance intervals.
衰减校正是正电子发射断层扫描（PET）中对活性分布进行定量分析的一项要求。随着一体化PET/MRI系统的引入，基于磁共振成像（MRI）而非计算机断层扫描（CT）进行衰减校正的需求应运而生。本研究旨在描述残余梯度场非线性畸变对PET衰减校正的影响。
在用于PET重建衰减校正的CT图像中模拟由梯度场非线性引起的MRI畸变。对于畸变校正图像，在距扫描仪等中心15厘米处产生的径向畸变高达±2.3毫米。在相同距离处，未校正图像的平均径向畸变是6.3毫米。使用畸变校正图像以及未应用校正的图像进行PET数据重建。
由于畸变校正不完全导致的重建PET摄取强度的平均相对差异小于±5%。这种差异的大小因患者以及畸变校正后残留畸变的大小而异。
在距扫描仪等中心半径15厘米处2毫米的径向畸变导致PET衰减校正误差小于5%。对于用于PET定量分析衰减校正的MRI系统而言，将梯度场非线性畸变控制在这一限度以下可能是一个合理目标。然而，对于外周病变的定量分析，可能需要更高的几何精度。例如，这些畸变可在验收测试及后续质量保证期间加以控制。
