Normal Variants and Artifacts
Recalling the expression “all is not gold that glitters,” increased uptake is not necessarily pathological. Indeed, normal bone scintigraphs are often altered or modified by physiological or anatomical factors, imitating pathology (Howarth et al. 1996). F
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Normal Variants and Artifacts Myung-Hee Sohn and Yong-Whee Bahk
Recalling the expression “all is not gold that glitters,” increased uptake is not necessarily pathological. Indeed, normal bone scintigraphs are often altered or modified by physiological or anatomical factors, imitating pathology (Howarth et al. 1996). For example, the tuberosity of the humerus and proximal tibia, to which the deltoid and quadriceps muscles are attached, respectively, may accumulate tracer intensely due to constant physical stress. Tracer uptake is normally increased in growing bones, either locally in the physes (Figs. 4.13 and 4.15) or generally in the entire skeleton, and also in anatomical variants. Modest tracer uptake may be observed in secondary ossification centers that have recently been fused (Fig. 4.31a). A number of artifacts can be produced by various technical factors related to kit preparation or intravenous injection of radiopharmaceuticals, attenuation of gamma rays, or the function of the gamma camera and computer systems (Hung et al. 1996; Forstrom et al. 1996; O’Connor 1996). Fortunately, artifacts possess characteristic features of their own so that their identification is straightforward in most instances. Conversely, however, it is also true that symptom-free bone lesions or congenital anomalies can exist, imposing diagnostic problems. As is well known, the tracer not accumulated by bone is excreted through the kidneys, occasionally disclosing unexpected renal or bladder diseases. It is not uncommon that the tracer retained in the renal calices, ureters, and bladder simulates a disease in the bone. Radioactivity emitted from soft-tissue lesions is another source of mimicry of bone lesions (Gray and Krawnow 1996). Thus, many normal variants and artifacts can lead to erroneous diagnosis unless one becomes thoroughly familiar with them. This chapter describes some more common variants and artifacts that have a strong resemblance to real pathology. The artifacts related to the gamma camera and computer systems are beyond the scope of this book.
5.1
Normal Variants
5.1.1 Skull The appearance and intensity of tracer uptake within the skull vary because the count rate is comparatively low and radioactivity distribution is usually uneven. A higher count rate is commonly observed in the peripheries of the skull due to curvature and perpendicular or tangential aligning of the scan detector. Accordingly, calvarial uptake may be diffusely or locally not uniform even in the absence of hyperostosis. Diffuse increased calvarial uptake related to postmenopausal osteoporosis is a rather common finding in elderly women (Senda and Itoh 1987) (Fig. 5.1a, b). Experience dictates that radiographic correlation is a necessity to accurately distinguish increased tracer uptake of simple cranial hyperostosis (Fig. 5.3) (removal) from pathological uptake of polycythemic cranial hyperostosis, iatrogenic osteoporosis, Paget’s disease, and renal osteodystrophy with osteomalacia (Fig. 5.1c, d). Tracer uptake in normal sutures may simula
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