CT characterisation of the feline os penis

The aim of this study was to investigate the frequency of identification and to describe the CT features of the os penis in cats without genitourinary disorders.

CT studies from cats that underwent an abdominal or pelvic examination between October 2013 and May 2019 were reviewed retrospectively. Cats with no signs of urinary disease and with the external genitalia included in the scan were recruited. Length, width, height and pre- and post-contrast attenuation values of the os penis in soft tissue and bone algorithms were measured independently by two observers.

Twenty-three cats met the inclusion criteria. A cylindrical bone-attenuating structure inside the glans penis compatible with the os penis was visible in 20/23 (87%) cats. Mean length, width and height values were 3.48 mm × 1.41 mm × 1.37 mm in the soft tissue algorithm, and 3.26 mm × 1.15 mm × 1.06 mm in bone algorithm. The size of the os penis was not significantly different in neutered vs intact cats, but it was significantly larger in soft tissue vs bone algorithm. Age and body weight did not influence os penis size. Pre-contrast mean ± SD attenuation was 216.7 ± 69.5 Hounsfield units (HU) for soft tissue and 320.1 ± 135.9 HU for bone algorithms. Post-contrast attenuation was 289.1 ± 68.8 HU for soft tissue and 383.4 ± 130.9 HU for bone algorithms. A significant correlation between the attenuation in bone algorithm with body weight was noted, where the os penis was less attenuating with increased body weight (pre-contrast: r = -0.479; P = 0.038).

The feline os penis is commonly seen on CT images, being more frequently detected than on radiographs. Its presence should not be mistaken for uroliths in the penile urethra.