In the early phase of HO, three phase bone scanning demonstrates increased uptake of osteotropic radionucleotides. Bone scanning has proven to be more sensitive than plain radiography in detecting early HO. Neurogenic HO becomes evident on plain radiography approximately 2 to 6 weeks after diagnosis using the three phase bone scan (Orzel et al. 1985; Freed et al. 1982). However, bone scans have lower specificity than radiography (Freed et al. 1982). CT or MRI scanning may be a useful tool when considering surgery as it allows for better visualization of the heterotopic bone (Amendola et al. 1983). Some studies have looked into diagnosing HO through elevations of biochemical markers such as alkaline phosphatase (Singh et al. 2003; Tibone et al. 1978) and creatine phosphokinase (Singh et al. 2003; Welch et al. 1973; Rossier et al. 1973). The predictive value of alkaline phosphatase has not been validated (Singh et al. 2003; Welch et al. 1973; Rossier et al. 1973), while there is conflicting evidence of an association of HO with increased serum creatine phosphokinase levels (Singh et al. 2003; Welch et al. 1973). Schurch et al. (1997) studied individuals with acute SCI and found increases in the 24 hour prostaglandin E₂ (PGE₂) urinary excretion a valid indicator of early HO formation.