Review ArticleAxial QCT: Clinical Applications and New Developments
Introduction
Quantitative computed tomography (QCT) was one of the first quantitative imaging techniques in musculoskeletal imaging. It was introduced at the end of the 1970s (1), and in the 1980s, a number of studies were performed establishing QCT as one of the standard methods to measure bone mineral density (BMD) 2, 3, 4, 5, 6. Although dual-energy X-ray absorptiometry (DXA) is currently the most frequently used technique to measure BMD, QCT still is an important alternative to DXA, which is superior to DXA for certain indications because of the fact that it provides volumetric BMD (vBMD) measurements of the trabecular bone and better morphological assessment of the lumbar spine and hip. QCT may be used in situations where the availability of DXA is restricted, and it is not infrequently used as a problem-solving technique for a number of clinical indications.
More recently, practice guidelines for the use of QCT were developed by the American College of Radiology (ACR), which were revised in 2013, and permit definition of osteopenia and osteoporosis based on absolute BMD measurements (7). In the last decade, a number of important new developments and clinical studies have further highlighted the important role of QCT. These include advanced technologies to measure bone strength based on finite element modeling (FEM) of QCT data sets (8) and the use of standard nonenhanced and contrast-enhanced computed tomography (CT) studies to measure BMD and assess osteoporotic fracture deformities 9, 10, 11.
Figure 1 shows the increasing number of publications on QCT over the last decade illustrating the renewed interest in this technology. Given these new developments, the goals of this review article are to present current state-of-the-art axial QCT technologies including their radiation exposure doses, compare QCT and DXA, outline clinical indications for QCT, and give an overview of novel developments in QCT technologies and clinical applications.
Section snippets
Standard QCT Techniques
QCT may be performed on any CT scanner with the use of a calibration phantom and dedicated analysis software. The patient is examined in the supine position, lying on the phantom, usually with a water- or gel-filled cushion interposed between phantom and patient to avoid artifacts because of air gaps. Calibration phantoms are required to transform the attenuation measured in Hounsfield units into BMD (mg hydroxyapatite/mL). The patient and phantom are examined at the same time, which is defined
Axial QCT vs DXA
In contrast to DXA, QCT allows separate estimation of trabecular and cortical BMD and also provides a volumetric density in mg/cm3, rather than the areal density (mg/cm2) of DXA. Because trabecular bone has a substantially higher metabolic turnover, it is more sensitive to changes in BMD than DXA (16). QCT is also less susceptible to degenerative changes of the spine than DXA. Osteophytes and facet joint degeneration as well as soft tissue calcifications (in particular of aortic calcification)
Clinical Indications for Axial QCT
Axial QCT may either serve as a problem-solving tool if DXA is available or it may be the primary technique for BMD measurements if a dedicated DXA scanner is not available. If QCT is used as a problem-solving tool, there are 4 important clinical indications (Table 2).
Advanced QCT Technologies and New Clinical Applications
Multiple novel and innovative applications of MD-CT technology to the study of osteoporosis and fracture risk have been described: (1) standard abdominal and pelvic clinical CT has been used to measure BMD with and without calibration phantoms 9, 10, 41, 42, 43, (2) reformations of standard CT studies were applied to identify osteoporotic fracture deformities 44, 45, 46, and (3) bone structure in MD-CT scans have been analyzed using finite element analysis to measure bone strength and quality
Acknowledgments
We thank Alan D. Brett, PhD, for providing a thorough review of the manuscript combined with new information and publications on axial QCT.
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