Energy X-ray Absorptimetry: Fundamentals, Methodology, And .
oaded 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---423www.elsevier.es/rxUPDATE IN RADIOLOGYDual energy X-ray absorptimetry: Fundamentals, methodology,and clinical applications夽R.M. Lorente Ramos , J. Azpeitia Armán, N. Arévalo Galeano, A. Muñoz Hernández,J.M. García Gómez, J. Gredilla MolineroUnidad Central de Radiodiagnóstico de la CAM, Hospital Infanta Leonor, Madrid, SpainReceived 15 March 2011; accepted 27 September 2011KEYWORDSDual-energy osis;Bone mineral density;Body compositionPALABRAS CLAVEAbsorciometría conrayos X de ;Densidad mineralósea;Composición corporalAbstract Dual-energy X-ray absorptiometry (DXA; DEXA) is the technique of choice to diagnose osteoporosis and to monitor the response to treatment. It is also useful for measuringbody composition. In recent years, new applications have been developed, including vertebralmorphometry through the study of the lateral spine, prosthesis integration in orthopedics, andlipodystrophy in HIV patients, although its use in these cases is not well established. DXAdensitometry is accurate and precise. It is essential to optimize each step of the diagnosticprocess, taking care to ensure the best acquisition, image analysis, and interpretation of theresults. Thus, to obtain the greatest utility from DXA, radiologists need to know the technique,its indications, and its pitfalls. This article reviews the fundamentals, modalities, methods, andclinical applications of DXA. 2011 SERAM. Published by Elsevier España, S.L. All rights reserved.Absorciometría con rayos X de doble energía. Fundamentos, metodologíay aplicaciones clínicasResumen La absorciometría con rayos X de doble energía (DXA o DEXA) es la técnica de elección para diagnosticar la osteoporosis y monitorizar la respuesta al tratamiento. Además, es útilpara estudiar la composición corporal. En los últimos años han surgido nuevas aplicaciones comola morfometría vertebral, estudiando la columna en visión lateral, la integración de prótesisen ortopedia, o la lipodistrofia en los pacientes con infección por VIH, aunque su utilización enestos casos no está bien consolidada. En el estudio de la osteoporosis, densitometría es precisay exacta. Para ello, es imprescindible optimizar cada etapa del proceso diagnóstico, cuidando laadquisición, el análisis de imágenes y la interpretación de los resultados. Por ello, para obtenerla máxima utilidad para el clínico y el paciente, el radiólogo debe conocer la técnica, sus夽 Please cite this article as: Lorente Ramos RM, et al. Absorciometría con rayos X de doble energía. Fundamentos, metodologíay aplicaciones clínicas. Radiología. 2012;54:410---23. Corresponding author.E-mail address: rosa.lorenteramos@salud.madrid.org (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 applications411indicaciones y las dificultades. El objetivo de este artículo es revisar la DXA, haciendo hincapiéen sus fundamentos, modalidades, metodología y aplicaciones clínicas. 2011 SERAM. Publicado por Elsevier España, S.L. Todos los derechos reservados.IntroductionDual X-ray absorptiometry (DXA), also known as densitometry or dual energy X-ray absorptiometry (DEXA), candistinguish different body structures. Axial bone densitometry of the lumbar spine and hip is the modality mostcommonly used in clinical practice. This technique is useful for measuring the bone mineral density (BMD), and fromthese data the risk of fracture can be estimated, therapeutic decisions can be taken and the response to treatmentcan be assessed.1On the other hand, DXA, the least known method forwhole body imaging, allows us to assess the total bodycomposition. This is particularly useful in patients withweight disorders secondary to endocrine diseases and inpediatric patients with delayed growth.2 Whole body DXAcan also be useful to assess lipodystrophy associated withretroviral infections,3 in the monitoring of arthroplasties4or to determine the cardiovascular risk.5DXA is still little known among radiologists, who consider this technique to be more typical of other specialties.Moreover, DXA tends to be wrongly considered as a routine and automated technique, unlikely to be optimizedand not requiring a radiological report. This is far from thetruth. DXA, like any other diagnostic modality, requires anadequate indication, careful methodology and precise interpretation, which is only possible with appropriate trainingand interaction between technicians and radiologists.As a consequence, our aim is to examine the current status of DXA, particularly emphasizing its fundamentals, mainmodalities, methodology and clinical applications.Fundamentals and modalities of dual energy X-rayabsorptiometryDXA is based on the variable absorption of X-ray by the different body components and uses high and low energy X-rayphotons. Depending on the equipment used, these photonscan be obtained using two mechanisms.6 In some cases, thegenerator emits alternating radiation of high (140 kVp) andlow (70---100 kVp) kilovoltage while moving across the surface of the body to be examined. In others, the generatoremits a constant beam while a rare-earth filter separateshigh energy (70 keV) from low energy (40 keV) photons.The available DXA systems include different types ofhardware (filters, collimators, detectors) and software(analysis algorithms).7 The X-ray source can emit a pencilbeam (pinhole collimator), which is registered by a singledetector, or a fan beam (slit collimator), which is registeredby a multiple detector.8 The latter system reduces the acquisition time and improves image quality.9 At the same time,the analysis algorithm discriminate bone from soft tissue ina variable way.10The main modalities of DXA in clinical practice are axialbone densitometry with stationary scan table, the modalityof choice to measure the BMD, and whole body densitometry,used to assess body composition.Bone densitometry with dual energy X-rayabsorptiometryHistorical perspectiveAxial DXA of the lumbar spine and hip is at present thetechnique of choice to study osteoporosis,11 although otherimaging techniques are potentially useful to assess and measure the bone structure and study its quality1,7,12 (Table 1).Plain radiography is useful to assess bone structure,although it cannot measure BMD. Some authors have triedto apply digital radiography with dual energy to obtain anestimated measurement of the BMD.Quantitative computed tomography (QCT) of the lumbar spine (central QCT) is performed using conventionalcomputed tomography (CT) systems. QCT of radius or tibia(peripheral QCT) can be performed using less sophisticated equipment. QCT provides volumetric acquisitions fromwhich BMD can be estimated. Central QCT has advantagesover DXA, since it allows us to differentiate between corticaland trabecular bone, assess the geometry of the vertebrae,and estimate the BMD volumetrically, expressed in g/cm3 .The disadvantages of central QCT are the radiation dose andthe lack of validated diagnostic criteria.13High resolution magnetic resonance (MR) imaging may beused for assessment of the trabecular structure of peripheral bones (calcaneus, distal radius and phalanx).14 The bonearchitecture studied using CT or MR, quantified in termsof scale, shape, anisotropy and connectivity, allows for theassessment of bone strength without considering the BMD.Advanced MR techniques such as diffusion, perfusion andspectroscopy will most likely provide useful additional information in the future.Quantitative ultrasound (QUS) is used for measuring BMDin the peripheral skeleton, generally at the calcaneus.Photonic absorptiometry with iodine-125 (I-125) was initially used to study the peripheral skeleton (radius andcalcaneus). It was subsequently replaced by dual photonic absorptiometry that uses gadolinium-153 and may beemployed to study the axial skeleton (hip, spine and wholeskeleton).15These modalities were later on substituted by X-ray basedtechnology, initially by plain X-ray and subsequently by DXA,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.412Table 1R.M. Lorente Ramos et al.Densitometry modalities.X-rayQCTMRQUSSPADPASXADXA/DEXAPlain X-rayQuantitative computed tomographyMagnetic resonanceQuantitative ultrasoundSingle photon absorptiometryDual photon absorptiometrySingle X-ray absorptiometryDual X-ray absorptiometry/Dual energy X-ray absorptiometryEquipmentPeripheral DXA performed with portable units (such asAccuDXA ) focuses on the study of phalanxes. It is not veryaccurate, but its cost is also low.1 AccuDXA can be used toselect patients likely to be assessed with central DXA16 onstationary table or as a substitute where central DXA is notavailable.17Axial DXA of the lumbar spine and femur (central DXA) isthe preferred technique to measure BMD because of its goodresolution and reliability, rapid acquisition and little radiation. Several devices are commercially available (Lunar,Hologic, Norland) with different characteristics. This factmakes it advisable to perform the follow up of each patientalways in the same unit. The accuracy of bone densitometryperformed on a DXA system with stationary table is high,with a margin of error of 1---2%.18X-rayX-ray (CT)MRUltrasoundRadioisotope 125-IRadioisotope Gd-153X-rayX-rayFoundation (NOF), osteoporosis affects 10 million Americans, but another 34 million are at risk of developing thecondition. It is estimated that approximately 50% of womenover the age of 50 will suffer an osteoporic fracture during their lives.22 In Europe, the International OsteoporosisFoundation (IOF)23 collects data and promotes initiatives inevery country. In Spain, approximately two million womenhave osteoporosis (26.1% prevalence in women 50 years).24In 1994, the WHO introduced the measurement of BMDwith DXA as the reference standard to quantify osteoporosis. Based on a study performed on postmenopausal whitewomen that revealed a correlation between BMD and risk offracture,20 osteoporosis was defined as a ‘‘T-score of 2.5’’.Other values of reference for potentially useful parameterswere also determined (Table 3).25 Thus, axial DXA becamethe reference standard for osteoporosis.MethodologyIndicationsThe main use of DXA is the diagnosis of osteoporosis. Itmay also predict the risk of fractures, help determine thetreatment, and monitor the response to treatment.1 Itsindications are set out in the official guidelines of the International Society for Clinical Densitometry, which are revisedevery two years (Table 2).19,20Osteoporosis is the ‘‘reduction in bone mass and increaseof bone fragility which in turn increases the risk of fracture’’.21 Osteoporosis is a common, often silent diseasethat can lead to an increased risk of fractures, sometimesatraumatic. Osteoporosis poses a serious problem to public health because of its prevalence and the cost associatedwith its comorbidity. According to the National OsteoporosisTable 2Indications for bone densitometry.FemalesMalesBoth sexesOlder than 65 yearsYounger than 65 years (postmenopausalor perimenopausal)Older than 70 yearsYounger than 70 years with risk factorsof fractureUnexplained fractureDiseases or chronic treatmentsAny patient to whom possible treatmentis considered and to monitor theresponse to treatmentThe methodology of bone densitometry with axial DXArequires optimization and careful execution. The importance of each step to obtain good results must behighlighted.PreparationIn order to properly plan the study, detailed patient information is required, and both the medical report provided bythe referring physician as well as the preliminary questionnaire completed in the diagnostic center. The request formmust include the indications for the study, so the areas to beexamined can be determined. Bone diseases that might alterthe shape or density of the bone, such as osteopetrosis orankylosing spondylitis, need to be ruled out. Previous fractures or joint replacements that might alter the planningmust also be ruled out. Contraindications to DXA includepregnancy, recent ( 5 days) oral administration of a contrast agent, and a recent ( 2 days) isotopic study.26 Thepatient does not require any specific preparation except forthe removal of any metal items located on the body area tobe imaged.Areas of studyFor most adult patients, the DXA examination should includethe lumbar spine and proximal femur; the forearm can alsobe included when hip or spine cannot be measured.19 Inchildren and adolescents (younger than 20 years), the measurement is only performed in the lumbar spine.27,28 The
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 applicationsTable 3413Parameters evaluated in dual energy X-ray eoporosisBMIA/G ratioBone mineral contentBone mineral densityStandard deviationDifference in number of standard deviations between the mean BMD value of thepatient and the mean of a young adult reference population of the same sexDifference in number of standard deviations between the mean BMD value of thepatient and the mean of a reference population of the same race, sex and ageT-score between 1 and 2.5T-score 2.5Body mass indexRatio of android and gynoid A/G pelvic fatfinal result of the densitometry would be the lowest valueof the two regions studied.The postero-anterior (PA) scan of the lumbar spineincludes the vertebral bodies of L1---L4, from which a meanBMD of these four vertebrae is obtained. Vertebrae withchanges due to fracture or focal lesions will be excluded. Toassess the lumbar spine, at least two evaluable vertebraeare required.The study of the femur can indistinctly be performed onthe right or left hip, although it is useful to become usedto studying always the same side. Hips with changes dueto fracture, focal lesions or replacements will be excluded.The total proximal femur or the femoral neck are used,whichever is lowest.The study of the non-dominant forearm is performedwhen hip or spine cannot be measured (in order to havea second measurable region), to obese patients (to bypasstechnical difficulties), and to patients suffering from hyperparathyroidism (since forearm bones change before the axialskeleton).Patient positioningOptimization of the patient’s position on the DXA table isessential. Incorrect positioning is one of the main causes oferrors in the estimation of BMD.29 In the PA study of the lumbar spine, the patient lies supine on the table with the legsflexed over a support pad that reduces the lumbar lordosisand approaches the spine to the table (Fig. 1).For the study of the hip, the patient lies supine with legsslightly in abduction in order to maintain the femoral axisstraight and in internal rotation (15---30 ), in a way that thelesser trochanter is not visible on the image (Fig. 2).In the study of the forearm, the patient is seated, sideagainst the table with the arm resting on the tabletop withthe palm of the hand downwards patients sit beside thetable and with their forearm resting on it, with the handin pronati
ofDXA, particularly emphasizing its fundamentals, main modalities, methodology and clinical applications. Fundamentals and modalities of dual energy X-ray absorptiometry DXA is based on the variable absorption of X-ray by the dif-ferent body components and uses high and low energy X-ray ph
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