View source for Dual-energy X-ray absorptiometry

{{Short description|Diagnostic test for bone mineral density testing}}
{{Redirect|DXA}}
{{Infobox interventions 
| Name       = Dual-energy X-ray absorptiometry 
| Image      = DEXA_scanner_in_use_ALSPAC.jpg
| OPS301     = {{OPS301|3-900}} 
| OtherCodes =
}}
'''Dual-energy X-ray absorptiometry''' ('''DXA''' or '''DEXA'''; also BMD test, bone density test, bone densitometry, p-DEXA<ref>{{cite web|publisher= U.S. National Library of Medicine|url= https://medlineplus.gov/ency/article/007197.htm|title= Bone mineral density test}}</ref>) is a means of measuring [[bone mineral density]] (BMD) with [[Spectral imaging (radiography)|spectral imaging]]. Two [[X-ray]] beams, with different [[energy level]]s, are aimed at the patient's [[bone]]s. When [[soft tissue]] absorption is subtracted, the [[bone density|bone mineral density]] (BMD) can be determined from the absorption of each beam by bone. Dual-energy X-ray absorptiometry is the most widely used and most thoroughly studied bone density measurement technology.

The DXA scan is typically used to diagnose and follow [[osteoporosis]], as contrasted to the nuclear [[bone scan]], which is sensitive to certain metabolic diseases of bones in which bones are trying to heal from infections, fractures, or tumors. It is also sometimes used to assess [[body composition]].

==Physics==
Soft tissue and bone have different [[Attenuation|attenuation coefficients]] to X-rays. A single X-ray beam passing through the body is attenuated by both soft tissue and bone, and it is not possible to determine from a single beam how much attenuation is attributable to the bone. However, attenuation coefficients vary with the energy of the X-rays, and, crucially, the ratio of the attenuation coefficients also varies. DXA uses two energies of X-ray. The difference in total absorption between the two can be used, by suitable weighting, to subtract out the absorption by soft tissue, leaving just the absorption by bone, which is related to bone density.

One type of DXA scanner uses a [[cerium]] filter with a [[tube voltage]] of 80 [[Volt|kV]], resulting in effective photon energies of about 40 and 70 [[electron volt|keV]].<ref name="nos">{{cite web|url=https://nos.org.uk/media/1725/02_physical_principles.pdf|title=Physical Principles and Measurement Accuracy of Bone Densitometry|website=[[National Osteoporosis Society]]|access-date=2018-02-05|archive-date=2018-02-06|archive-url=https://web.archive.org/web/20180206131501/https://nos.org.uk/media/1725/02_physical_principles.pdf|url-status=dead}}</ref> Another type of DXA scanner uses a [[samarium]] filter with a tube voltage of 100 kV, which produces effective energies of 47 and 80 keV.<ref name="nos" /> Also, the tube voltage can be continuously switched between a low (for example 70 kV) and high (for example 140 kV) value in synchronism with the frequency of the electrical mains, resulting in effective energies alternating between 45 and 100 keV.<ref name="nos" />

The combination of [[dual X-ray absorptiometry and laser]] uses the laser to measure the thickness of the region scanned, allowing for varying proportions of lean soft tissue and [[adipose tissue]] within the soft tissue to be controlled for and improving the accuracy.

==Bone density measurement==
[[File:Blausen 0095 BoneDensitometryScan.png|thumbnail]]

===Indications===
{{main|Bone density#Testing}}
The [[U.S. Preventive Services Task Force]] recommends that women over the age of 65 should get a DXA scan.<ref name="ahrq">{{cite web|url=http://www.uspreventiveservicestaskforce.org/uspstf/uspsoste.htm|title=Screening for Osteoporosis|publisher=[[U.S. Preventive Services Task Force]]|work=uspreventiveservicestaskforce.org|date=January 2011|access-date=20 August 2012|archive-url=https://web.archive.org/web/20130530202927/http://www.uspreventiveservicestaskforce.org/uspstf/uspsoste.htm|archive-date=30 May 2013|url-status=dead}}</ref> The age when men should be tested is uncertain,<ref name="ahrq"/> but some sources recommend age 70.<ref name="AAFP">{{Citation|author1=American Academy of Family Physicians|author1-link=American Academy of Family Physicians|title=Five Things Physicians and Patients Should Question|publisher=[[American Academy of Family Physicians]]|work=Choosing Wisely: an initiative of the [[ABIM Foundation]]|url=http://choosingwisely.org/wp-content/uploads/2012/04/5things_12_factsheet_Amer_Acad_Fam_Phys.pdf|access-date=August 14, 2012|url-status=dead|archive-url=https://web.archive.org/web/20120624075453/http://choosingwisely.org/wp-content/uploads/2012/04/5things_12_factsheet_Amer_Acad_Fam_Phys.pdf|archive-date=June 24, 2012}}</ref> At-risk women should consider getting a scan when their risk is equal to that of a normal 65-year-old woman.

A person's risk can be estimated with the University of Sheffield's [[FRAX]] calculator, which takes into account prior fragility fracture, use of [[glucocorticoids]], heavy smoking, excess alcohol intake, rheumatoid arthritis, history of parental hip fracture, chronic renal and liver disease, chronic respiratory disease, long-term use of phenobarbital or phenytoin, celiac disease, inflammatory bowel disease, and other clinical risk factors.<ref name="ahrq" />

===Scoring===
[[File:Morbus Fabry DXA 01.jpg|thumb|DEXA assessment of bone mineral density of the femoral neck (A) and the lumbar spine (B): T scores of - 4.2 and - 4.3 were found at the hip (A) and lumbar spine (B), respectively in a 53-year-old male patient affected with [[Fabry disease]].]]
The World Health Organization has defined the following categories based on bone density in white women:

{|class="wikitable"
|-|Osteoporosis||T-score less than -2.5
|-
|Severe (established) osteoporosis||A T-score more than -2.5 standard deviations below the young adult female reference mean in the presence of one or more fragility fractures.
|}
Bone densities are often given to patients as a T score or a Z score. A T score tells the patient what their bone mineral density is in comparison to a young adult of the same gender with peak bone mineral density. A normal T score is -1.0 and above, low bone density is between -1.0 and -2.5, and osteoporosis is -2.5 and lower. A Z score is just a comparison of what a patient's bone mineral density is in comparison to the average bone mineral density of a male or female of their age and weight.

The WHO committee did not have enough data to create definitions for men or nonwhite women.<ref>{{cite web|url=http://courses.washington.edu/bonephys/opbmd.html#tz|title=Bone densitometry|publisher=Courses.washington.edu|access-date=2013-05-22}}</ref>
 
Special considerations are involved in the use of DXA to assess bone mass in children. Specifically, comparing the bone mineral density of children to the reference data of adults (to calculate a T-score) underestimates the BMD of children, because children have less bone mass than fully developed adults. This would lead to an over-diagnosis of [[osteopenia]] for children. To avoid an overestimation of bone mineral deficits, BMD scores are commonly compared to reference data for the same gender and age (by calculating a [[Z-score]]).

Also, there are other variables in addition to age that are suggested to confound the interpretation of BMD as measured by DXA. One important confounding variable is bone size. DXA has been shown to overestimate the bone mineral density of taller subjects and underestimate the bone mineral density of smaller subjects. This error is due to the way DXA calculates BMD. In DXA, bone mineral content (measured as the attenuation of the X-ray by the bones being scanned) is divided by the area (also measured by the machine) of the site being scanned.

Because DXA calculates BMD using area (aBMD: areal bone mineral density), it is not an accurate measurement of true bone mineral density, which is [[mass]] divided by a [[volume]]. To distinguish DXA BMD from [[volumetric]] bone-mineral density, researchers sometimes refer to DXA BMD as an areal bone mineral density (aBMD). The confounding effect of differences in bone size is due to the missing depth value in the calculation of bone mineral density. Despite DXA technology's problems with estimating volume, it is still a fairly accurate measure of bone mineral content. Methods to correct for this shortcoming include the calculation of a volume that is approximated from the projected area measure by DXA. DXA BMD results adjusted in this manner are referred to as the bone mineral apparent density (BMAD) and are a [[ratio]] of the bone mineral content versus a [[cuboidal]] estimation of the volume of bone. Like the results for aBMD, BMAD results do not accurately represent true bone mineral density, since they use approximations of the bone's volume. BMAD is used primarily for research purposes and is not yet used in clinical settings.

Other imaging technologies such as [[quantitative computed tomography]] (QCT) are capable of measuring the bone's volume, and are, therefore, not susceptible to the confounding effect of bone-size in the way that DXA results are susceptible.

It is important for patients to get repeat BMD measurements done on the same machine each time, or at least a machine from the same manufacturer. Error between machines, or trying to convert measurements from one manufacturer's standard to another can introduce errors large enough to wipe out the sensitivity of the measurements.{{Citation needed|date=September 2009}}

Taking [[strontium]] supplements causes DEXA to overestimate bone mineral density, because strontium is a stronger absorber of X-rays than calcium.<ref name="pmid17289524">{{cite journal|vauthors=Blake GM, Fogelman I|title=Effect of bone strontium on BMD measurements|journal=J Clin Densitom|volume=10|issue=1|pages=34–8|date=2007|pmid=17289524|doi=10.1016/j.jocd.2006.10.004 }}</ref>

DXA can also be used to measure the [[trabecular bone score]].<ref>{{cite web |title=New tools to predict fracture risk - Mayo Clinic |url=https://www.mayoclinic.org/medical-professionals/endocrinology/news/new-tools-to-predict-fracture-risk/mac-20430573 |website=www.mayoclinic.org |access-date=17 November 2025}}</ref><ref>{{cite journal |last1=Hans |first1=Didier |last2=Goertzen |first2=Andrew L |last3=Krieg |first3=Marc-Antoine |last4=Leslie |first4=William D |title=Bone microarchitecture assessed by TBS predicts osteoporotic fractures independent of bone density: The manitoba study |journal=Journal of Bone and Mineral Research |date=1 November 2011 |volume=26 |issue=11 |pages=2762–2769 |doi=10.1002/jbmr.499 |pmid=21887701 }}</ref>

===Clinical practice in pediatrics===
DXA is, by far, the most widely used technique for bone mineral density measurements, since it is relatively inexpensive, accessible, easy to use, and provides an accurate estimation of bone mineral density in adults.<ref name="pmid9518226">{{cite journal|vauthors=Gilsanz V|title=Bone density in children: a review of the available techniques and indications|journal=Eur J Radiol|volume=26|issue=2|pages=177–82|date=January 1998|pmid=9518226|doi=10.1016/S0720-048X(97)00093-4 }}</ref>

The official position of the [[International Society for Clinical Densitometry]] (ISCD) is that a patient be tested for BMD if they have a condition that could precipitate bone loss, are to be prescribed pharmaceuticals known to cause bone loss, or are being treated and need reqiore monitoring. The ISCD states that there is no clearly understood correlation between BMD and the risk of a child's sustaining a fracture; the diagnosis of osteoporosis in children cannot be made on the basis of densitometry criteria. T-scores are prohibited with children and should not even appear on DXA reports. Thus, the WHO classification of osteoporosis and osteopenia in adults cannot be applied to children, but Z-scores can be used to assist diagnosis.<ref>{{cite web|url=http://www.iscd.org/visitors/positions/OfficialPositionsText.cfm|title=2007 ISCD Official Positions|access-date=2012-05-24|url-status=dead|archive-url=https://web.archive.org/web/20120509172203/http://www.iscd.org/Visitors/positions/OfficialPositionsText.cfm|archive-date=2012-05-09}}</ref>

Some clinics may routinely carry out DXA scans on pediatric patients with conditions such as nutritional [[rickets]], [[Lupus erythematosus|lupus]], and [[Turner syndrome]].<ref name="pmid16715219">{{cite journal|vauthors=Binkovitz LA, Henwood MJ|title=Pediatric DXA: technique and interpretation|journal=Pediatr Radiol|volume=37|issue=1|pages=21–31|date=January 2007|pmid=16715219|pmc=1764599|doi=10.1007/s00247-006-0153-y }}</ref> DXA has been demonstrated to measure skeletal maturity<ref name="pmid14615883">{{cite journal|vauthors=Płudowski P, Lebiedowski M, Lorenc RS|title=Evaluation of the possibility to assess bone age on the basis of DXA derived hand scans-preliminary results|journal=Osteoporos Int|volume=15|issue=4|pages=317–22|date=April 2004|pmid=14615883|doi=10.1007/s00198-003-1545-6  |s2cid=26672947}}</ref> and body fat composition<ref name="pmid11517118">{{cite journal|vauthors=Sung RY, Lau P, Yu CW, Lam PK, Nelson EA|title=Measurement of body fat using leg to leg bioimpedance|journal=Arch. Dis. Child.|volume=85|issue=3|pages=263–7|date=September 2001|pmid=11517118|pmc=1718893|doi=10.1136/adc.85.3.263}}</ref> and has been used to evaluate the effects of pharmaceutical therapy.<ref name="pmid15695604">{{cite journal|vauthors=Barnes C, Newall F, Ignjatovic V, Wong P, Cameron F, Jones G, Monagle P|title=Reduced bone density in children on long-term warfarin|journal=Pediatr. Res.|volume=57|issue=4|pages=578–81|date=April 2005|pmid=15695604|doi=10.1203/01.PDR.0000155943.07244.04 |doi-access=free}}</ref> It may also aid pediatricians in diagnosing and monitoring treatment of disorders of bone mass acquisition in childhood.<ref name="pmid12244017">{{cite journal|vauthors=van der Sluis IM, de Ridder MA, Boot AM, Krenning EP, de Muinck Keizer-Schrama SM|title=Reference data for bone density and body composition measured with dual energy x ray absorptiometry in white children and young adults|journal=Arch. Dis. Child.|volume=87|issue=4|pages=341–7; discussion 341–7|date=October 2002|pmid=12244017|pmc=1763043|doi=10.1136/adc.87.4.341 }}</ref>

However, it seems that DXA is still in its early days in pediatrics, and there are widely acknowledged limitations and disadvantages with DXA. A view exists<ref name=Picaud>{{cite journal|vauthors=Picaud JC, Duboeuf F, Vey-Marty V, Delams P, Claris O, Salle BL, Rigo J|title=First all-solid pediatric phantom for dual X-ray absorptiometry measurements in infants|journal=J Clin Densitom|volume=6|issue=1|pages=17–23|date=2003|pmid=12665698|doi=10.1385/JCD:6:1:17  }}</ref> that DXA scans for diagnostic purposes should not even be performed outside specialist centers, and, if a scan is done outside one of these centers, it should not be interpreted without consultation with an expert in the field.<ref name=Picaud/> Furthermore, most of the pharmaceuticals given to adults with low bone mass can be given to children only in strictly monitored clinical trials.

Whole-body [[calcium]] measured by DXA has been validated in adults using [[in-vivo]] [[neutron activation]] of total body calcium<ref name=Margulies>{{cite journal|vauthors=Margulies L, Horlick M, Thornton JC, Wang J, Ioannidou E, Heymsfield SB|title=Reproducibility of pediatric whole body bone and body composition measures by dual-energy X-ray absorptiometry using the GE Lunar Prodigy|journal=J Clin Densitom|volume=8|issue=3|pages=298–304|date=2005|pmid=16055960|doi=10.1385/JCD:8:3:298 }}</ref><ref name=Horlick>{{cite journal|vauthors=Horlick M, Thornton J, Wang J, Levine LS, Fedun B, Pierson RN|title=Bone mineral in prepubertal children: gender and ethnicity|journal=J. Bone Miner. Res.|volume=15|issue=7|pages=1393–7|date=July 2000|pmid=10893689|doi=10.1359/jbmr.2000.15.7.1393  |s2cid=24475001}}</ref> but this is not suitable for paediatric subjects and studies have been carried out on paediatric-sized animals.<ref name=Margulies/><ref name=Horlick/>

==Body composition measurement==
[[File:Picture1 fs.png|thumb|DXA Fat shadow of an obese individual]]
DXA scans can also be used to measure total [[body composition]] and [[Body fat percentage|fat content]] with a high degree of accuracy comparable to [[hydrostatic weighing]] with a few important caveats.<ref name="pmid15345728">{{cite journal|vauthors=St-Onge MP, Wang J, Shen W, Wang Z, Allison DB, Heshka S, Pierson RN, Heymsfield SB|title=Dual-energy x-ray absorptiometry-measured lean soft tissue mass: differing relation to body cell mass across the adult life span|journal=J. Gerontol. A Biol. Sci. Med. Sci.|volume=59|issue=8|pages=796–800|date=August 2004|pmid=15345728|doi=10.1093/gerona/59.8.B796 |doi-access=free}}</ref>{{Specify|What caveats?|date=February 2017}} From the DXA scans, a low resolution "fat shadow" image can also be generated, which gives an overall impression of fat distribution throughout the body.<ref name=":0">{{cite journal|vauthors=Meral R, Ryan BJ, Malandrino N, Jalal A, Neidert AH, Muniyappa R, Akıncı B, Horowitz JF, Brown RJ, Oral EA|title="Fat Shadows" From DXA for the Qualitative Assessment of Lipodystrophy: When a Picture Is Worth a Thousand Numbers|journal=Diabetes Care|volume=41|issue=10|pages=2255–2258|date=October 2018|pmid=30237235|pmc=6150431|doi=10.2337/dc18-0978 }}</ref> It has been suggested that, while very accurately measuring minerals and lean soft tissue (LST), DXA may provide skewed results due to its method of indirectly calculating fat mass by subtracting it from the LST and/or body cell mass (BCM) that DXA actually measures.<ref name="pmid16448570">{{cite journal|vauthors=Manninen AH|title=Very-low-carbohydrate diets and preservation of muscle mass|journal=Nutr Metab (Lond)|volume=3|article-number=9|date=January 2006|pmid=16448570|pmc=1373635|doi=10.1186/1743-7075-3-9  |doi-access=free }}</ref>

DXA scans have been suggested as useful tools to diagnose conditions with an abnormal fat distribution, such as [[familial partial lipodystrophy]].<ref name="pmid28199729">{{cite journal|vauthors=Ajluni N, Meral R, Neidert AH, Brady GF, Buras E, McKenna B, DiPaola F, Chenevert TL, Horowitz JF, Buggs-Saxton C, Rupani AR, Thomas PE, Tayeh MK, Innis JW, Omary MB, Conjeevaram H, Oral EA|title=Spectrum of disease associated with partial lipodystrophy: lessons from a trial cohort|journal=Clin. Endocrinol. (Oxf)|volume=86|issue=5|pages=698–707|date=May 2017|pmid=28199729|pmc=5395301|doi=10.1111/cen.13311  }}</ref><ref name="pmid27473102">{{cite journal|vauthors=Guillín-Amarelle C, Sánchez-Iglesias S, Castro-Pais A, Rodriguez-Cañete L, Ordóñez-Mayán L, Pazos M, González-Méndez B, Rodríguez-García S, Casanueva FF, Fernández-Marmiesse A, Araújo-Vilar D|title=Type 1 familial partial lipodystrophy: understanding the Köbberling syndrome|journal=Endocrine|volume=54|issue=2|pages=411–421|date=November 2016|pmid=27473102|doi=10.1007/s12020-016-1002-x|s2cid=19689303|issn=1559-0100|hdl=10347/32433|hdl-access=free}}</ref><ref name=":0" /> They are also used to assess adiposity in children, especially to conduct clinical research.<ref name="pmid24842797">{{cite journal|vauthors=Kakinami L, Henderson M, Chiolero A, Cole TJ, Paradis G|title=Identifying the best body mass index metric to assess adiposity change in children|journal=Arch. Dis. Child.|volume=99|issue=11|pages=1020–4|date=November 2014|pmid=24842797|pmc=4215345|doi=10.1136/archdischild-2013-305163 }}</ref>
[[File:DXA Fat shadow.png|thumb|DXA fat shadow of a child with rare congenital generalized lipodystrophy]]

==Radiation exposure==
DXA uses X-rays to measure bone mineral density. The [[radiation]] dose of DEXA systems is small,<ref name="Radiology1">{{cite web|url=http://www.radiologyinfo.org/en/safety/index.cfm?pg=sfty_xray#part3|title=Patient Safety - Radiation Dose in X-Ray and CT Exams|website=RadiologyInfo.org|publisher=Radiological Society of North America |date=2012-04-25|access-date=2013-05-22}}</ref> as low as 0.001 [[Sievert|mSv]], much less than a standard chest or dental x-ray.<ref name="Radiology2">{{cite web|url=https://www.radiologyinfo.org/en/info.cfm?pg=dexa&bhcp=1|title=Bone Densitometry (DEXA, DXA)|author=<!--Not stated-->|website=RadiologyInfo.org|publisher=Radiological Society of North America|access-date=December 8, 2018|archive-date=June 16, 2018|archive-url=https://web.archive.org/web/20180616080359/https://www.radiologyinfo.org/en/info.cfm?pg=dexa&bhcp=1|url-status=dead}}</ref><ref>{{Cite web|url=https://www.radiologyinfo.org/en/info.cfm?pg=safety-xray|title=Patient Safety - Radiation Dose in X-Ray and CT Exams|last=Radiology (ACR)|first=Radiological Society of North America (RSNA) and American College of|website=www.radiologyinfo.org|language=en|access-date=2019-03-12}}</ref> However, the dose delivered by older DEXA radiation sources (that used [[radioisotope]]s rather than [[x-ray generator]]s) could  be as high as 35 mGy,<ref name="pmid8148566">{{cite journal|vauthors=Lewis MK, Blake GM, Fogelman I|title=Patient dose in dual x-ray absorptiometry|journal=Osteoporos Int|volume=4|issue=1|pages=11–5|date=January 1994|pmid=8148566 |doi=10.1007/BF02352255|s2cid=6225880}}</ref><ref name="pmid9224663">{{cite journal|vauthors=Blake GM, Fogelman I|title=Technical principles of dual energy x-ray absorptiometry|journal=Semin Nucl Med|volume=27|issue=3|pages=210–28|date=July 1997|pmid=9224663 |doi=10.1016/S0001-2998(97)80025-6}}</ref><ref name="pmid10028639">{{cite journal|vauthors=Njeh CF, Fuerst T, Hans D, Blake GM, Genant HK|title=Radiation exposure in bone mineral density assessment|journal=Appl Radiat Isot|volume=50|issue=1|pages=215–36|date=January 1999|pmid=10028639  |doi=10.1016/S0969-8043(98)00026-8}}</ref> considered a significant dose by [[X-ray#Adverse effects|radiological health]] standards.

==Regulation==
===United States===
The regulation of DXA operators varies widely. DXA is not regulated like other radiation-based imaging techniques due to its low dosage. Each US state differs regarding the certifications needed to operate a DXA machine. [[California]], for example, requires coursework and a state-run test, whereas [[Maryland]] has no requirements for DXA technicians. Many states require a training course and certificate from the International Society of Clinical Densitometry (ISCD).

===Australia===
In Australia, regulations differ according to the state or territory. For example, in Victoria, an individual performing DXA scans is required to complete an approved course in safe use of bone mineral densitometers.<ref>{{cite web |title=Bone mineral densitometer operators |url=https://www2.health.vic.gov.au/public-health/radiation/licensing/use-licences-employees/sector-specific-information/medical/bone-mineral-densitometer-operators |website=Health Department (Government of Victoria) |publisher=health.vic |access-date=11 October 2021}}</ref> In NSW and QLD a DXA technician only requires prior study in science, nursing or other related undergraduate study. The Environmental Protection Agency (EPA) oversees licensing of technicians, however, this is far from rigorous and regulation is non-existent.{{cn|date=July 2024}}

== References ==

{{reflist|30em}}

== External links ==
{{commons category|Dual-energy X-ray absorptiometry}}
* [http://www.osteoporosisadvice.com/testing-bone-density.php Non-invasive testing of bone density explained]
* [http://www.radiologyinfo.org/content/dexa.htm Information for patients], from [[Radiological Society of North America|RSNA]]
* [http://courses.washington.edu/bonephys/opbmd.html#tz Bone Densitometry explained]

{{Operations and other procedures on the musculoskeletal system}}
{{Medical imaging}}

{{DEFAULTSORT:Dual-Energy X-Ray Absorptiometry}}
[[Category:Diagnostic endocrinology]]
[[Category:Medical imaging]]
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[[es:Densitometría osea]]
[[Category:X-ray imaging techniques]]