Standardized uptake values and attenuation correction in 18 F-sodium fluoride PET of the equine foot and fetlock.

Maximal standardized uptake values (SUVmax ) are commonly used for the interpretation of PET studies. Limited information regarding the SUVmax of 18 F-NaF PET in horses is currently available in the literature. The goals of this retrospective secondary analysis study were to provide reference values for 18 F-NaF SUVmax in the equine distal extremity and assess the effect of attenuation correction. Nonattenuation corrected (NAC) and CT-based attenuation corrected (CTAC) SUVmax were obtained from 19 feet and 19 fetlocks. Twenty regions of interest (ROIs) were defined for the foot and 22 for the fetlock. Areas presenting abnormal uptake were excluded. The overall NAC and CTAC SUVmax were 3.6 +/- 1.5 (mean +/- sd) and 5.0 +/- 1.8 for the feet and 2.9 +/- 1.1 and 3.8 +/- 1.4 for the fetlocks, respectively. The 3 ROIs showing the highest attenuation correction were the navicular center (83.4%), navicular flexor surface (74.9%) and distal phalanx flexor surface (81.3%), whereas attenuation correction was only 5.2% at the dorsal aspect of the proximal phalanx. Significant SUVmax differences were observed between the different ROIs (P < 0.0001), with the toe (CTAC SUVmax 7.7 +/- 3.7), dorsal (7.5 +/- 1.9) and central (6.1 +/- 2.2) ROIs of the distal phalanx being significantly higher than those of the other areas. This study demonstrates that attenuation correction affects SUVmax in the equine distal extremity and should be performed if CT data are available. However, as the maximal attenuation correction results in less than doubling the signal intensity, nonattenuation corrected images likely remain relevant for subjective clinical interpretation.