Skeletal SPECT/CT a review

Authors: Torsten Kuwert

Publish Date: 2014/12/02

Volume: 2, Issue: 6, Pages: 505-517

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Abstract

Skeletal scintigraphy is one of the most frequent in vivo procedures in the field of nuclear medicine Visualizing bone metabolism it exhibits a fairly high sensitivity to detect skeletal lesions but has limitations in terms of specificity and spatial resolution even when singlephoton emission computed tomography SPECT is used Combining SPECT with Xray computed tomography helps overcome these limitations This has in particular been shown when diagnosing bone involvement in malignant tumors Emerging evidence indicates the benefit of hybrid imaging for bone scintigraphy during the workup of painful conditions affecting the back and the extremities Methodological advances holding considerable promise for further improving its value are the quantitation of skeletal tracer uptake in absolute units as well as multimodal image reconstruction techniques that have recently become available for use in clinical routineThe quantification of bone metabolism was among the first applications of tracers in biology The letter to Nature published in 1935 by George de Hevesy which earned him the Nobel Prize in 1943 described the use of radioactive phosphorus to investigate bone metabolism in rats 1 De Hevesy concluded from his results that “the formation of bones is a dynamic process” thus laying the foundation for many approaches to the study of bone metabolism that nuclear medicine developed over the following decadesThe 99mTclabeled polyphosphonates that are used today were introduced approximately 40 years ago 2 Scintigraphic images acquired early after their intravenous injection provide information on perfusion and floridity of skeletal lesions for a general review see 3 Scintigraphy performed several hours after tracer injection therefore allows insight into bone metabolism or more specifically osteoblastic activity since the polyphosphonates are adsorbed on freshly built bone tissue 4 5 Initially bone scintigrams were planar images acquired either as spot views or as wholebody images Due to the sensitivity of this examination to detect osseous lesions skeletal scintigraphy has been widely used as a screening tool eg for staging malignant disease In the late 1980s singlephoton emission computed tomography SPECT became widely available allowing threedimensional visualization of the distribution of radioactivity within the human body This technology considerably improved the diagnostic accuracy of bone scintigraphy by allowing better localization of areas exhibiting pathological tracer uptake for a review see 6 Nevertheless due to limitations in the spatial resolution of skeletal SPECT which ranges from ca 8 to 10 mm in the reconstructed images the specificity of skeletal scintigraphy is still limited This is true particularly when it is compared to radiological techniques such as Xray computerized tomography CT or magnetic resonance imaging MRI the latter technique also allowing visualization of softtissue structures associated with bone such as tendons ligaments and cartilage that elude bone scintigraphy for obvious methodological reasonsApproximately 12 years ago the first hybrid system integrating a SPECT camera with a CT scanner into a single gantry became commercially available for a review see 7 The CT component of this system utilized a lowdose nonspiral CT scanner whose images lacked diagnostic quality but allowed fairly precise localization of SPECT foci of abnormal tracer uptake CT diagnosis of gross morphological abnormalities and attenuation correction of the SPECT images as well Since then these technologies have considerably advanced with current SPECT/CT systems offering a wide array of diagnostic quality multislice spiral CT scanners for review see 8This article reviews scientific evidence on the utility of SPECT/CT for bone scintigraphy following intravenous injection of Tc99mlabeled polyphosphonates As regards its value in imaging osteomyelitis with tracers concentrating in infectious and inflammatory lesions the reader is referred to the review article specifically covering this topic published in this issue of Clinical and Translational Imaging 9Using the terms SPECT/CT SPECTCT and skeletal we conducted an electronic search of the PubMed database without language restrictions The list of articles generated was augmented by retrieving further pertinent publications from the reference lists of the papers found in PubMed Due to heterogeneities in study design we did not perform metaanalyses or any evidencebased quality assessment of the available evidenceThe SPECT portion of a SPECT/CT examination is in principle no different from SPECT performed using a standalone system For skeletal CT the intravenous injection of contrast medium is not usually necessary With the hybrid systems featuring a multislice spiral CT scanner a CT examination of the skeleton of full diagnostic quality is possible at least in principle A literature survey shows however that even when such CT scanners are available lowdose CT protocols with tube current–time products ranging from 15 to 60 mAs are generally used 10 Furthermore and contrary to the practice encountered in radiology in a significant proportion of the studies so far published the CT field of view is restricted to the region of the body harboring abnormalities of tracer uptake visible on the planar scintigraphic images With this socalled SPECTguided lowdose CT as introduced by Römer et al 11 the average radiation doses delivered to the patient are reduced between 2 and 3 mSv in most cases and thus correspond to doses incurred by patients having planar radiographs or nondiagnostic CT scans with SPECT/CT cameras not equipped with a spiral CT

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