Farag Adam, Thompson R Terry, Thiessen Jonathan D, Prato Frank S, Théberge Jean
Lawson Health Research Institute, Imaging Division, London, Ontario, Canada.
Department of Medical Biophysics, Western University, London, Ontario, Canada.
EJNMMI Phys. 2021 Mar 8;8(1):24. doi: 10.1186/s40658-021-00368-5.
Accurate quantification of radioactivity, measured by an integrated positron emission tomography (PET) and magnetic resonance imaging (MRI) system, is still a challenge. One aspect of such a challenge is to correct for the hardware attenuation, such as the patient table and radio frequency (RF) resonators. For PET/MRI systems, computed tomography (CT) is commonly used to produce hardware attenuation correction (AC) maps, by converting Hounsfield units (HU) to a linear attenuation coefficients (LAC) map at the PET energy level 511 keV, using a bilinear model. The model does not address beam hardening, nor higher density materials, which can lead to inaccurate corrections.
In this study, we introduce a transmission-based (TX-based) AC technique with a static Germanium-68 (Ge-68) transmission source to generate hardware AC maps using the PET/MRI system itself, without the need for PET or medical CT scanners. The AC TX-based maps were generated for a homogeneous cylinder, made of acrylic as a validator. The technique thereafter was applied to the patient table and posterior part of an RF-phased array used in cardiovascular PET/MRI imaging. The proposed TX-based, and the CT-based, hardware maps were used in reconstructing PET images of one cardiac patient, and the results were analysed and compared.
The LAC derived by the TX-based method for the acrylic cylinder is estimated to be 0.10851 ± 0.00380 cm compared to the 0.10698 ± 0.00321 cm theoretical value reported in the literature. The PET photon counts were reduced by 8.7 ± 1.1% with the patient table, at the region used in cardiac scans, while the CT-based map, used for correction, over-estimated counts by 4.3 ± 1.3%. Reconstructed in vivo images using TX-based AC hardware maps have shown 4.1 ± 0.9% mean difference compared to those reconstructed images using CT-based AC.
The LAC of the acrylic cylinder measurements using the TX-based technique was in agreement with those in the literature confirming the validity of the technique. The over-estimation of photon counts caused by the CT-based model used for the patient table was improved by the TX-based technique. Therefore, TX-based AC of hardware using the PET/MRI system itself is possible and can produce more accurate images when compared to the CT-based hardware AC in cardiac PET images.
通过集成正电子发射断层扫描（PET）和磁共振成像（MRI）系统来准确量化放射性仍然是一项挑战。此类挑战的一个方面是校正硬件衰减，例如患者检查床和射频（RF）谐振器。对于PET/MRI系统，计算机断层扫描（CT）通常用于生成硬件衰减校正（AC）图，通过使用双线性模型将亨氏单位（HU）转换为PET能量水平511keV时的线性衰减系数（LAC）图。该模型未考虑束硬化和更高密度的材料，这可能导致校正不准确。
在本研究中，我们引入了一种基于透射（TX）的AC技术，使用静态锗 - 68（Ge - 68）透射源，利用PET/MRI系统自身生成硬件AC图，无需PET或医学CT扫描仪。基于TX生成的AC图是针对由丙烯酸制成的均匀圆柱体生成的，用作验证器。此后，该技术应用于心血管PET/MRI成像中使用的患者检查床和RF相控阵的后部。所提出的基于TX和基于CT的硬件图用于重建一名心脏病患者的PET图像，并对结果进行分析和比较。
基于TX的方法得出的丙烯酸圆柱体的LAC估计为0.10851±0.00380cm，而文献报道的理论值为0.10698±0.00321cm。在心脏扫描所用区域，患者检查床使PET光子计数减少了8.7±1.1%，而用于校正的基于CT的图高估了计数4.3±1.3%。与使用基于CT的AC重建的图像相比，使用基于TX的AC硬件图重建的体内图像平均差异为4.1±0.9%。
使用基于TX的技术对丙烯酸圆柱体测量的LAC与文献中的结果一致，证实了该技术的有效性。基于TX的技术改善了用于患者检查床的基于CT的模型导致的光子计数高估问题。因此，利用PET/MRI系统自身对硬件进行基于TX的AC是可行的，并且与心脏PET图像中基于CT的硬件AC相比，可以产生更准确的图像。
