Document downloaded from http://www.elsevier.es, day 22/01/2015. This copy is for personal use. Any transmission of this document by any media or format is strictly prohibited. Radiología. 2012;54(5):410---423 www.elsevier.es/rx UPDATE IN RADIOLOGY Dual energy X-ray absorptimetry: Fundamentals, methodology, ଝ and clinical applications ∗ R.M. Lorente Ramos , J. Azpeitia Armán, N. Arévalo Galeano, A. Munoz˜ Hernández, J.M. García Gómez, J. Gredilla Molinero Unidad Central de Radiodiagnóstico de la CAM, Hospital Infanta Leonor, Madrid, Spain Received 15 March 2011; accepted 27 September 2011 KEYWORDS Abstract Dual-energy X-ray absorptiometry (DXA; DEXA) is the technique of choice to diag- Dual-energy X-ray nose osteoporosis and to monitor the response to treatment. It is also useful for measuring absorptiometry; body composition. In recent years, new applications have been developed, including vertebral Densitometry; morphometry through the study of the lateral spine, prosthesis integration in orthopedics, and DXA; lipodystrophy in HIV+ patients, although its use in these cases is not well established. DXA DEXA; densitometry is accurate and precise. It is essential to optimize each step of the diagnostic Osteoporosis; process, taking care to ensure the best acquisition, image analysis, and interpretation of the Bone mineral density; results. Thus, to obtain the greatest utility from DXA, radiologists need to know the technique, Body composition its indications, and its pitfalls. This article reviews the fundamentals, modalities, methods, and clinical applications of DXA. © 2011 SERAM. Published by Elsevier España, S.L. All rights reserved. PALABRAS CLAVE Absorciometría con rayos X de doble energía. Fundamentos, metodología Absorciometría con y aplicaciones clínicas rayos X de doble energía; Resumen La absorciometría con rayos X de doble energía (DXA o DEXA) es la técnica de elec- Densitometría; ción para diagnosticar la osteoporosis y monitorizar la respuesta al tratamiento. Además, es útil DXA; para estudiar la composición corporal. En los últimos anos˜ han surgido nuevas aplicaciones como DEXA; la morfometría vertebral, estudiando la columna en visión lateral, la integración de prótesis Osteoporosis; en ortopedia, o la lipodistrofia en los pacientes con infección por VIH, aunque su utilización en Densidad mineral estos casos no está bien consolidada. En el estudio de la osteoporosis, densitometría es precisa ósea; y exacta. Para ello, es imprescindible optimizar cada etapa del proceso diagnóstico, cuidando la Composición corporal adquisición, el análisis de imágenes y la interpretación de los resultados. Por ello, para obtener la máxima utilidad para el clínico y el paciente, el radiólogo debe conocer la técnica, sus ଝ Please cite this article as: Lorente Ramos RM, et al. Absorciometría con rayos X de doble energía. Fundamentos, metodología y aplicaciones clínicas. Radiología. 2012;54:410---23. ∗ Corresponding author. E-mail address: [email protected] (R.M. Lorente Ramos). 2173-5107/$ – see front matter © 2011 SERAM. Published by Elsevier España, S.L. All rights reserved. Document downloaded from http://www.elsevier.es, day 22/01/2015. This copy is for personal use. Any transmission of this document by any media or format is strictly prohibited. Dual energy X-ray absorptimetry: Fundamentals, methodology, and clinical applications 411 indicaciones y las dificultades. El objetivo de este artículo es revisar la DXA, haciendo hincapié en sus fundamentos, modalidades, metodología y aplicaciones clínicas. © 2011 SERAM. Publicado por Elsevier España, S.L. Todos los derechos reservados. Introduction the analysis algorithm discriminate bone from soft tissue in 10 a variable way. The main modalities of DXA in clinical practice are axial Dual X-ray absorptiometry (DXA), also known as densit- bone densitometry with stationary scan table, the modality ometry or dual energy X-ray absorptiometry (DEXA), can of choice to measure the BMD, and whole body densitometry, distinguish different body structures. Axial bone densito- used to assess body composition. metry of the lumbar spine and hip is the modality most commonly used in clinical practice. This technique is use- ful for measuring the bone mineral density (BMD), and from these data the risk of fracture can be estimated, therapeu- Bone densitometry with dual energy X-ray tic decisions can be taken and the response to treatment 1 absorptiometry can be assessed. On the other hand, DXA, the least known method for whole body imaging, allows us to assess the total body Historical perspective composition. This is particularly useful in patients with weight disorders secondary to endocrine diseases and in Axial DXA of the lumbar spine and hip is at present the 2 11 pediatric patients with delayed growth. Whole body DXA technique of choice to study osteoporosis, although other can also be useful to assess lipodystrophy associated with imaging techniques are potentially useful to assess and mea- 3 4 1,7,12 retroviral infections, in the monitoring of arthroplasties sure the bone structure and study its quality (Table 1). 5 or to determine the cardiovascular risk. Plain radiography is useful to assess bone structure, DXA is still little known among radiologists, who con- although it cannot measure BMD. Some authors have tried sider this technique to be more typical of other specialties. to apply digital radiography with dual energy to obtain an Moreover, DXA tends to be wrongly considered as a rou- estimated measurement of the BMD. tine and automated technique, unlikely to be optimized Quantitative computed tomography (QCT) of the lum- and not requiring a radiological report. This is far from the bar spine (central QCT) is performed using conventional truth. DXA, like any other diagnostic modality, requires an computed tomography (CT) systems. QCT of radius or tibia adequate indication, careful methodology and precise inter- (peripheral QCT) can be performed using less sophisti- pretation, which is only possible with appropriate training cated equipment. QCT provides volumetric acquisitions from and interaction between technicians and radiologists. which BMD can be estimated. Central QCT has advantages As a consequence, our aim is to examine the current sta- over DXA, since it allows us to differentiate between cortical tus of DXA, particularly emphasizing its fundamentals, main and trabecular bone, assess the geometry of the vertebrae, 3 modalities, methodology and clinical applications. and estimate the BMD volumetrically, expressed in g/cm . The disadvantages of central QCT are the radiation dose and 13 the lack of validated diagnostic criteria. High resolution magnetic resonance (MR) imaging may be Fundamentals and modalities of dual energy X-ray used for assessment of the trabecular structure of periph- 14 absorptiometry eral bones (calcaneus, distal radius and phalanx). The bone architecture studied using CT or MR, quantified in terms DXA is based on the variable absorption of X-ray by the dif- of scale, shape, anisotropy and connectivity, allows for the ferent body components and uses high and low energy X-ray assessment of bone strength without considering the BMD. photons. Depending on the equipment used, these photons Advanced MR techniques such as diffusion, perfusion and 6 can be obtained using two mechanisms. In some cases, the spectroscopy will most likely provide useful additional infor- generator emits alternating radiation of high (140 kVp) and mation in the future. low (70---100 kVp) kilovoltage while moving across the sur- Quantitative ultrasound (QUS) is used for measuring BMD face of the body to be examined. In others, the generator in the peripheral skeleton, generally at the calcaneus. emits a constant beam while a rare-earth filter separates Photonic absorptiometry with iodine-125 (I-125) was ini- high energy (70 keV) from low energy (40 keV) photons. tially used to study the peripheral skeleton (radius and The available DXA systems include different types of calcaneus). It was subsequently replaced by dual pho- hardware (filters, collimators, detectors) and software tonic absorptiometry that uses gadolinium-153 and may be 7 (analysis algorithms). The X-ray source can emit a pencil- employed to study the axial skeleton (hip, spine and whole 15 beam (pinhole collimator), which is registered by a single skeleton). detector, or a fan beam (slit collimator), which is registered These modalities were later on substituted by X-ray based 8 by a multiple detector. The latter system reduces the acqui- technology, initially by plain X-ray and subsequently by DXA, 9 sition time and improves image quality. At the same time, which allowed for the measurement of the axial skeleton. Document downloaded from http://www.elsevier.es, day 22/01/2015. This copy is for personal use. Any transmission of this document by any media or format is strictly prohibited. 412 R.M. Lorente Ramos et al. Table 1 Densitometry modalities. X-ray Plain X-ray X-ray QCT Quantitative computed tomography X-ray (CT) MR Magnetic resonance MR QUS Quantitative ultrasound Ultrasound SPA Single photon absorptiometry Radioisotope 125-I DPA Dual photon absorptiometry Radioisotope Gd-153 SXA Single X-ray absorptiometry X-ray DXA/DEXA Dual X-ray absorptiometry/Dual energy X-ray absorptiometry X-ray Equipment Foundation (NOF), osteoporosis affects 10 million Ameri- cans, but another 34 million are at risk of developing the Peripheral DXA performed with portable units (such as condition. It is estimated that