Kotasidis Fotis A, Zaidi Habib
Division of Nuclear Medicine and Molecular Imaging, Geneva University Hospital, CH-1211 Geneva, Switzerland and Wolfson Molecular Imaging Centre, MAHSC, University of Manchester, Manchester M20 3LJ , United Kingdom.
Division of Nuclear Medicine and Molecular Imaging, Geneva University Hospital, CH-1211 Geneva, Switzerland; Geneva Neuroscience Centre, Geneva University, CH-1205 Geneva, Switzerland; and Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, 9700 RB, The Netherlands.
Med Phys. 2014 Jun;41(6):062501. doi: 10.1118/1.4875689.
The Ingenuity time-of-flight (TF) PET/MR is a recently developed hybrid scanner combining the molecular imaging capabilities of PET with the excellent soft tissue contrast of MRI. It is becoming common practice to characterize the system's point spread function (PSF) and understand its variation under spatial transformations to guide clinical studies and potentially use it within resolution recovery image reconstruction algorithms. Furthermore, due to the system's utilization of overlapping and spherical symmetric Kaiser-Bessel basis functions during image reconstruction, its image space PSF and reconstructed spatial resolution could be affected by the selection of the basis function parameters. Hence, a detailed investigation into the multidimensional basis function parameter space is needed to evaluate the impact of these parameters on spatial resolution.
Using an array of 12 × 7 printed point sources, along with a custom made phantom, and with the MR magnet on, the system's spatially variant image-based PSF was characterized in detail. Moreover, basis function parameters were systematically varied during reconstruction (list-mode TF OSEM) to evaluate their impact on the reconstructed resolution and the image space PSF. Following the spatial resolution optimization, phantom, and clinical studies were subsequently reconstructed using representative basis function parameters.
Based on the analysis and under standard basis function parameters, the axial and tangential components of the PSF were found to be almost invariant under spatial transformations (~4 mm) while the radial component varied modestly from 4 to 6.7 mm. Using a systematic investigation into the basis function parameter space, the spatial resolution was found to degrade for basis functions with a large radius and small shape parameter. However, it was found that optimizing the spatial resolution in the reconstructed PET images, while having a good basis function superposition and keeping the image representation error to a minimum, is feasible, with the parameter combination range depending upon the scanner's intrinsic resolution characteristics.
Using the printed point source array as a MR compatible methodology for experimentally measuring the scanner's PSF, the system's spatially variant resolution properties were successfully evaluated in image space. Overall the PET subsystem exhibits excellent resolution characteristics mainly due to the fact that the raw data are not under-sampled/rebinned, enabling the spatial resolution to be dictated by the scanner's intrinsic resolution and the image reconstruction parameters. Due to the impact of these parameters on the resolution properties of the reconstructed images, the image space PSF varies both under spatial transformations and due to basis function parameter selection. Nonetheless, for a range of basis function parameters, the image space PSF remains unaffected, with the range depending on the scanner's intrinsic resolution properties.
灵犀飞行时间（TF）PET/MR是一种最近开发的混合扫描仪，它将PET的分子成像能力与MRI出色的软组织对比度相结合。表征该系统的点扩散函数（PSF）并了解其在空间变换下的变化，以指导临床研究并可能在分辨率恢复图像重建算法中使用它，正成为一种常见的做法。此外，由于该系统在图像重建过程中使用了重叠且球对称的凯泽 - 贝塞尔基函数，其图像空间PSF和重建的空间分辨率可能会受到基函数参数选择的影响。因此，需要对多维基函数参数空间进行详细研究，以评估这些参数对空间分辨率的影响。
使用一个12×7的印刷点源阵列以及一个定制的体模，并在开启MR磁体的情况下，详细表征了该系统基于图像的空间可变PSF。此外，在重建过程中（列表模式TF OSEM）系统地改变基函数参数，以评估它们对重建分辨率和图像空间PSF的影响。在空间分辨率优化之后，使用代表性的基函数参数随后重建了体模和临床研究图像。
基于分析并在标准基函数参数下，发现PSF的轴向和切向分量在空间变换（约4毫米）下几乎不变，而径向分量从4毫米到6.7毫米有适度变化。通过对基函数参数空间的系统研究，发现对于半径大且形状参数小的基函数，空间分辨率会下降。然而，发现在重建的PET图像中优化空间分辨率，同时具有良好的基函数叠加并将图像表示误差降至最低是可行的，参数组合范围取决于扫描仪的固有分辨率特性。
使用印刷点源阵列作为一种与MR兼容的方法来实验测量扫描仪的PSF，成功地在图像空间中评估了该系统的空间可变分辨率特性。总体而言，PET子系统表现出出色的分辨率特性，主要是因为原始数据没有欠采样/重分箱，使得空间分辨率由扫描仪的固有分辨率和图像重建参数决定。由于这些参数对重建图像分辨率特性的影响，图像空间PSF在空间变换下以及由于基函数参数选择都会发生变化。尽管如此，对于一系列基函数参数，图像空间PSF保持不变，其范围取决于扫描仪的固有分辨率特性。
