Evaluation of precalculated attenuation correction map for preclinical cardiac PET/MR using a ¹H/²³Na surface coil

In preclinical PET/MR, attenuation correction (AC) uses a mix of pre-calculated attenuation maps accounting for animal cradles and organ segmentation obtained from whole-body MR using a volume coil. The development of X-nuclei such as ²³Na or ³¹P may benefit from high sensitivity surface coils, which could lead to inaccurate PET quantification. In this study, we evaluated the benefit of a pre-calculated attenuation map including a dedicated cradle enclosing a surface coil for ¹⁸F-FDG PET imaging.

We developed a 3D-printed cradle (DC) embedding a 20 mm surface coil made of PLA (polylactic acid), coated with a thin cap of epoxy. An attenuation map was generated using computed tomography and integrated into PET reconstruction. To validate AC, we compared PET images to those obtained using a conventional cradle and a volume coil (CC). Various image quality metrics were evaluated in various phantoms including a NEMA NU-4 (recovery coefficients (RC), uniformity (%STD), spillover ratio (SOR)), a homogenous phantom (slice and inter-slice uniformity) and a resolution phantom (spatial resolution). Finally, cardiac ¹⁸F-FDG PET images acquired with the 2 cradles were compared in Sprague-Dawley rats.

RC obtained using DC and CC configurations were not significantly different. However, the %STD was significantly increased with the DC (5.07 ± 0.18) vs. CC (3.29 ± 0.53, p = 0.0002) leading to a decreased CNR with DC. The SOR was similar between the two cradles. In homogenous phantom, there was a non-significant 1.58% underestimation of the PET signal near the surface coil and 5% overestimation on the opposite side when using the DC vs. CC. Uniformity between slice was significantly higher in DC than in CC (3.15 ± 1.2% for DC vs. 2.24 ± 0.7% for CC, p = 0.02) while uniformity inter-slice was similar in DC and CC (2.25 ± 0.83% for DC vs. 1.93 ± 0.44% for CC, p = 0.2). Spatial resolution was similar between DC and CC (axial: 1.80 vs. 1.63 mm, tangential: 1.62 vs. 1.79 mm and radial resolution: 1.53 vs. 1.76 mm for DC and CC respectively). In-vivo, cardiac standardized uptake values were similar between the two cradles (3.55 ± 0.89 and 3.59 ± 1.02 for DC and CC respectively, p = 0.81).

Pre-calculated attenuation map using a standardized positioning of MR surface coil provided similar ¹⁸F-FDG PET images compared to a conventional PET/MR system with a volume coil, allowing its usage for combined PET and X-nuclei MR.