Establishment of a sequential dual tracer ¹⁸F-NaF/¹⁸F-FDG PET protocol for imaging the equine foot

Mathieu Spriet, Pablo Espinosa-Mur, Xuezhu Zhang, Eric Berg, Julien Bec, Scott Katzman, Larry Galuppo

Background: This study reports on using dual tracer PET imaging for the equine foot. The authors explain that 18 F-NaF and 18 F-FDG are two radiotracers that can detect bone and soft tissue lesions, respectively, and that combining them in a single scan could provide more complete information. However, they also acknowledge that there could be a loss of information due to the overlap of the two tracers.

Study: The aim of the study was to compare dual and single tracer images and to establish the preferred tracer order of injection and timing for a sequential dual-tracer approach. The authors hypothesized that dual 18 F-NaF/ 18 F-FDG scan would fail to identify some areas of uptake recognized on separate 18 F-NaF and 18 F-FDG scans, and that it would be feasible to collect both pertinent 18 F-FDG and 18F-NaF data under a single anesthetic episode in horses.

Methods: The study was a prospective, methods comparison, exploratory study performed on six horses from a research herd with foot lameness. The horses were imaged under general anesthesia twice, at least one week apart, with 18 F-NaF PET, 18 F-FDG PET, dual 18 F-NaF/ 18 F-FDG PET, and CT. Two different scenarios for the order of tracer injection were tested. The PET images were graded by a veterinary radiologist using a subjective 3-level grading system for 18 F-NaF and 18 F-FDG uptake in 18 and 32 regions of interest, respectively. Kappa-weighted statistics, sensitivity, and specificity were used to assess the agreement between the dual and single tracer scans.

Results: The results showed that the dual and single tracer scans had substantial and moderate agreement for 18 F-NaF and 18 F-FDG uptake detection, respectively. The sensitivity and specificity of the dual scan for 18 F-NaF uptake were 0.77 and 0.98, respectively, and for 18 F-FDG uptake were 0.5 and 0.98, respectively. The false negative cases for 18 F-NaF mostly involved areas of mild uptake, while the false negative cases for 18 F-FDG mostly involved tendon lesions adjacent to bone lesions. The authors found that the optimal protocol for dual tracer imaging was to inject 18 F-NaF prior to anesthesia, acquire 18 F-NaF data, then inject 18 F-FDG and start acquisition of dual tracer PET data 10 min later.

Limitations: The main limitation of the study was the small number of included horses, and the fact that only one horse was injected with 18 F-NaF under anesthesia. The authors also acknowledged that the horses had chronic lesions and were not in training, which could affect the tracer uptake. Furthermore, the study did not use subtraction between the dual and single tracer scans, which could improve the detection of the second tracer. The study also did not measure the radiation exposure to staff or the horses.

Conclusions: The authors concluded that the sequential dual tracer approach was a pertinent technique to optimize the PET data gained from a single anesthetic episode. They recommended that 18 F-NaF be injected first prior to anesthesia induction, and 18 F-FDG be injected after 18 F-NaF imaging under general anesthesia. They also suggested that this protocol should be further validated in a larger clinical study.

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