SPECT/CT imaging of bone

The skeleton is a metabolically active organ that undergoes continuous remodelling throughout life. Normal healthy bones are dynamic structures with inherent basal levels of turnover sustained by balanced osteoblastic and osteoclastic activity. Increased bone turnover (ie osteoblastic activity) is the central mechanism of attempts at bone healing in case of any insult, whether it is benign or malignant. Radionuclide 99mTc-MDP is a tracer that gets adsorbed into bone in proportion to bone turnover and thus provides an “osteoblastic map” of the entire skeleton. 99mTc-MDP planar bone scintigraphy has historically been employed in regular staging and re-staging of many cancers as well as for answering specific questions in orthopaedics. Many malignancies, eg breast, prostate, lung, thyroid and renal cell carcinomas, have a tendency to metastasize to bone. Most bone metastases, even when they are predominantly osteolytic, show some degree of increased bone turnover, thereby making them appear as “hotspots” compared to the background skeletal uptake. Similar increased bone turnover can be seen in other non-malignant causes of bone insult, eg infection, degenerative disease, fractures etc. This makes bone scintigraphy a sensitive modality for detection of all such lesions and disease processes. The major limitation of this functional technique has been its limited specificity. Single photon emission computerised tomography (SPECT) imaging improves the sensitivity and accuracy of prediction of skeletal lesions by virtue of increased lesion contrast and provision of tomographic information. However, a substantial number of skeletal lesions remain equivocal, due to limited spatial resolution of the technique and lack of precise anatomical localisation. Integrated SPECT/CT systems have become available recently, which provide co-registered functional and structural data allowing precise anatomical localisation and characterisation of the underlying disease process by virtue of morphological CT characteristics. It is important to note that accurate diagnosis of both benign as well as malignant disease is equally important for patient management. In many cases, such interpretation can be binary to subsequent patient management. Routine use of SPECT/CT has a radiation burden which may be deemed as unjustifiable, although there are some early data to support this approach. Due consideration is required for appropriate use of this technology, with additional steps being taken to reduce patient dose to as low as is practicable. A normal tracer distribution on planar bone scan usually makes the use of SPECT/CT unnecessary. Although in many cases the correct diagnosis can be derived from planar bone scans, SPECT/CT is necessary to make the correct diagnosis in case of undefined lesions. We have used the following criteria at our institution for the last few years to add limited low dose SPECT/CT where required to improve diagnostic accuracy (figure 1).