Roentgenographic bony changes in osteomyelitis take 10 days to 2 weeks to develop; therefore, in the early stages, one can only infer the diagnosis from soft tissue changes. Osteomyelitis manifests in deep soft tissue edema as does pyomyositis, while superficial cellulitis demonstrates findings reflecting superficial edema (Fig. 2.1). Because of this, deep edema should suggest the presence of osteomyelitis or pyomyositis until proven otherwise.

After 10–14 days, bony destruction is seen and can manifest as an acute destructive process or a low-grade smoldering problem (low-grade osteomyelitis), which frequently mimics entities such as Ewing’s sarcoma and eosinophilic granuloma [1]. More aggressive permeative and “moth-eaten” changes are seen with the former, while with the latter, more reactive bone healing or sclerosis is seen. In addition, although osteomyelitis usually is a metaphyseal problem, it can involve the epiphysis [2, 3]. All of these variations are demonstrated in Fig. 2.2.

If one suspects the presence of osteomyelitis clinically but plain film changes do not support the diagnosis, one should turn to nuclear scintigraphy or MR. With nuclear scintigraphy, the findings are nonspecific but helpful. It should be remembered, however, that the pyrophosphate bone scan measures reactive (healing) bone activity and not bone destruction or the development of a purulent exudate. It is for this reason that in some cases imaging with gallium- or indium-labeled white blood cells [4] may be required to determine whether an infection is present. Finally it should be remembered that in many cases, the technetium bone scan, in the very early stages of the infection, may appear falsely normal [5]. The reason for this is that in such early stages of infection (osteomyelitis), there is so much congestion of the bone marrow by the developing infection that blood flow, and hence the delivery of isotope to the bone (marrow), is impaired. Usually within 24–48 h the bone scan becomes positive. Because of this, if one can make the presumptive diagnosis of osteomyelitis on the basis of clinical and plain film roentgenographic (deep edema) findings, one should assume that such infection is present and begin therapy. The bone scan, delayed a day or two, then will become positive [6].

Ultrasonography also has been used to detect osteomyelitis [7], but the changes demonstrable with ultrasound are usually late changes. They consist of purulent exudate elevating the periosteum from the cortex of the involved bone (Fig. 2.3). They are not very easy to demonstrate, and so overall, MR is becoming more and more popular for the evaluation of occult osteomyelitis where the normal high signal of marrow fat on T1-weighted images is lost, and on the proton-density and T2-weighted images, increased signal is seen (Fig. 2.4).