Bottom line: In temperate regions, Staphylococcus aureus is the causative agent in more than 75% of cases. The infection is thought to be preceded by bacteremia rather than being introduced via direct injury to the skin and muscle. Although it can affect immunocompetent persons, immunosuppressed patients are at greater risk. Quadriceps, gluteal, and iliopsoas muscles are common sites for the infection.
Agarwal V, et al. Pyomyositis. Neuroimag Clin N Am. 2011; 21:975-983.
Overview of the condition and discusses preference of MRI for investigating suspected pyomyositis.