Dual energy X-ray absorptiometry

Dual energy X-ray absorptiometry (DXA, previously DEXA) is a means of measuring bone mineral density (BMD). Two X-ray beams with differing energy levels are aimed at the patient's bones. When soft tissue absorption is subtracted out, the BMD can be determined from the absorption of each beam by bone. DXA is the most widely used and most thoroughly studied bone density measurement technology. A T-score of -2.5 or less is indicative of osteoporosis.

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DXA scans can also be used to measure total body fat content, which is useful for athletes, models and health-conscious people.

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) will underestimate the BMD of children, because children have less bone mass than fully developed adults. This would lead to an overdiagnosis 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 which 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 in which 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. In order 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. It should be noted that 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 which 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 aBMD, BMAD results do not accurately represent true bone mineral density, since they use approximations of the bone's volume. Other imaging technologies such as Computed Quantitative Computer Tomography (QCT) (see pQCT Peripheral quantitative computed tomography) 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.

BMAD is used primarily for research purposes and is not used in clinical settings, yet.

DXA uses X-rays to assess bone mineral density. However, the radiation dose is approximately 1/10th that of a standard chest X-ray.^[1]

The quality of DXA operators varies widely. DXA is not regulated like other radiation based imaging techniques because of its low dosage. Each state has a different policy as to what certifications are 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). Because BMD testing with DXA is very susceptible to operator error (it is not fool-proof) it is important to find out what qualifies the technician to operate the machine.

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.

Current clinical practice in paediatrics

DXA is, by far, the most widely used technique for bone measurements since it is considered; cheap, accessible, easy to use, and being able to provide accurate and precise quantitation of bone mass in adults"[1]. Could there be this potential in Paediatrics? Could there be the same applications of DXA in Paediatrics? If not, why not?

The Official position of the ISCD is that a patient may be tested for BMD if; he suffers from a condition which could precipitate bone loss or is going to be prescribed pharmaceuticals known to cause bone loss or he is being treated and needs to be monitored. The full list is contained “ISCD official position” and points 6-7 could be applied to children.

There is no clearly understood correlation between BMD and the risk of a child suffering a fracture and the diagnosis of Osteoporosis in Children cannot be made using the basis of a densitometry criteria. T-scores are prohibited with children and should not even appear on DXA reports, and thus, the WHO classification of Osteoporosis and Osteopenia in adults cannot be applied to children but Z-scores can be used to assist diagnosis. The full table can be found in the appendix.

There are clinics where DXA may be carried out routinely on Paediatric Patients with conditions such as nutritional rickets, lupus and Turner Syndrome[2]. DXA has been demonstrated to measure skeletal maturity[3] and body fat composition[4], has been used to evaluate the effects of pharmaceutical therapy[5] and there is some evidence that when used with one patient over time, it may give meaningful information[6] and thus paediatricians may be able to recognise and treat disorders of bone mass acquisition in childhood[7].

However it seems that DXA is still in its early days in Paediatrics and there are widely acknowledged limitations and disadvantages with DXA. A view exists[8] that DXA scans for diagnostic purposes should not even be performed outside Specialist centres and if a scan is done outside one of these centres, it should not be interpreted without consultation with an expert in the field[8]. Furthermore, most of the pharmaceuticals that are 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[9;10] but this is not suitable for paediatric subjects and studies have been carried out on paediatric-sized animals[9;10]

Reference List for paediatrics

[1.] Gilsanz V, Eur J Radiol 1998, 26 177-182. [2.] Larry A.Binkovitz, Maria J.Henwood, Pediatr Radiol. 2006. [3.] Pludowski P, Lebiedowski M, Lorenc RS, Osteoporos Int 2004, 14 317-322. [4.] T Sung et. al, Arch Dis Child 2001, 85 263-287. [5.] Chris Barnes et. al. Pediatric Research 2005, 52 578-581. [6.] M.Thearle et. al. J Clin Endocrinol Metab. 2007, 85 2122-2126. [7.] van der Sluis et al. Arch Dis Child 2002, 87 341-347. [8.] Picaud et al. J Clin Densitom 2003, 6 17-23. [9.] Margulies et al, J Clin Densitom 205, 8 298-304. [10.] Horlick et al. J Bone Miner Res 2000, 15 1393-1397.

References