By Benjamin Chan and Dr Vince Vardhanabhuti
The one-size-fits-all approach of medicine is evolving into personalised medicine with custom treatment and prevention plans for each individual, based on one’s unique factors, e.g. body composition (BC) and genetic make-up. The driving forces for this evolution are plenty, but not least the rising awareness that we are all made different, facilitated by technological advancements that make personal healthcare accessible and increasingly affordable. Notably, medical imaging has allowed physicians and patients to visualise and quantify BC parameters for disease diagnosis, risk assessment, and prevention. In the emerging field of longevity, multiple BC parameters, such as fat mass, muscle mass and bone density, have been found to correlate with disease and mortality risk [check our other blog post for correlates between BC parameters and mortality risk]. While computed tomography (CT) and magnetic resonance imaging (MRI) have conventionally been gold standards in measuring fat and lean mass and distribution, there is an expanding body of literature on applying dual-energy x-ray absorptiometry (DEXA) scans in clinical practice for measuring BC parameters, which is more accessible and affordable. This article will give an overview of DEXA scans and make a case for why their clinical popularisation should be encouraged and embraced.
What is DEXA?
DEXA scans emit low-dose X-ray beams of two energy levels and detect the attenuation of beams corresponding to each X-ray energy after passing through a subject’s body. The attenuation of X-ray intensity varies with the density and thickness of the body’s structure. By calculating the ratio of attenuation between the two energy levels, DEXA can differentiate between three types of structures that each X-ray beam has passed through – bone mineral density, fat and lean tissue mass. The output of the scan includes whole-body values for each of the three parameters, as well as values for individual regions, e.g., specifically in limbs. The image of BC distribution can also be produced.
Now you may be wondering about how a DEXA scan is done? Practically speaking, DEXA scans are quick and painless. Subjects are asked to lie down on their back with the face looking at the ceiling. The DEXA X-ray arm is positioned above the subject and moves progressively along the subject’s body as it takes the scans. A DEXA report is then produced for the physician to interpret. Currently, DEXA scans are the most common technique to measure bone density and overall BC.
How does DEXA compare with other imaging techniques?
The radiation exposure of DEXA scans is extremely low – between 0.001-0.004mSv, equivalent to half the daily background radiation exposure of an average person – far lower than a chest- x-ray which is typically around 0.1mSv. Compared to CT or MRI scans, DEXA scans are much quicker (around 5 minutes with subject set-up) and readily available. These factors make DEXA more comfortable and accessible than other scans, e.g., CT and MRI scans, which are much more expensive, limited and may be claustrophobic to some people. Conventionally, CT and MRI are the gold standards for measuring BC parameters as both techniques produce cross-sectional images and allow a compartmental analysis of BC around the body. CT and MRI scan measurements of fat correspond to anatomical compartments; the software calculates BC parameters based on the dimensions and density of anatomical compartments in the body. On the other hand, DEXA scans produce a rougher image of BC distribution and measurement of fats as a chemical type, instead of an anatomical compartment.
In general, DEXA scan measurements agree much with those of CT and MRI, producing a good correlation with those values. Comparing with CT in Asian populations, which have high visceral fats: Research by a group in 2013 found around 95% correlation between fat measured by CT scans and DEXA scans in the Asian Chinese population [1]. Another group study published in 2015 replicated the results in an Asian Korean population and found similar agreements between DEXA and CT scans [2]. Another investigator also found a 94% correlation for whole-body muscle mass between MRI and DEXA [3], which was further confirmed in paediatric populations by a separate study in 2017 [4]. Considering the reliability and accessibility of DEXA, there is a general consensus on using DEXA for clinical settings in measuring BC while reserving CT and MRI for academic research gold standards, e.g., the European Working Group on Sarcopenia in Older People recommends using DEXA for detecting low skeletal muscle mass in clinical practice [5].
Nonetheless, as with other imaging techniques, DEXA has its caveats that need to be made aware of. For instance, there is a non-negligible variation between DEXA measurements of scans by different manufacturers. These variations may arise as a result of hardware differences, e.g., different X-ray dosages, or software differences in image analysis. While this can be overcome by using the same machine in repeated measurements of the same subject, in case homogeneity cannot be ensured, threshold values need to be defined universally based on the values of prevalent DEXA manufacturers. In fact, this caveat is being addressed with increasing literature produced by the manufacturers and third parties on threshold values for specific DEXA scan models, e.g., GE-Lunar is a commonly used model for measuring fat mass and has been applied to different demographics, e.g. American [6] and Australian [7], to produce reference values for each population groups.
Moreover, trivial precautions of DEXA scans include subject positioning and preparation. As with all imaging techniques, there is a standard position for subjects in DEXA scans – the official position suggested by the International Society for Clinical Densitometry is to face up, lie down, upper limbs align with the body, palms down, and neutral feet [8]. Technicians should advise subjects clearly. Moreover, subject preparation is crucial and is a large source of variation. All metallic items must be removed to prevent interference with the X-ray. Furthermore, water and food intake need to be controlled prior to the DEXA scan. DEXA software calculates lean mass by subtracting measured fat mass from total mass. Any undigested food or excess water intake will increase the calculated lean mass. As such, subjects ought to be instructed well and made aware of preparations before the scan. Foreseeably, popularising DEXA use will naturally lead to the familiarisation of standard protocols.
The potential of DEXA
The accessibility of DEXA scans makes it extremely easy to obtain BC measurements. [Check Blog post #2 for correlates between BC parameters and mortality risk] DEXA-derived BC parameters have been found to correlate with mortality risk from conditions like obesity and heart failure. These values empower physicians and subjects to produce individualised lifestyle and medication changes to mitigate any known risk of mortality. Moving beyond the individual level, population-wide predictions may even be more accurate with DEXA measurements. For instance, conventional anthropometric techniques of hip and waist circumference underestimate the association of type 2 diabetes and cardiovascular disease with body fat, when compared to DEXA [9]. Despite the need for more research to verify DEXA-made predictions, we already know the parameters it measures have immense medical potential – making DEXA more accessible in the clinical setting seems an inevitable future for personalised and preventative medicine where quantifying biomarkers is the key to longevity.
References:
Lin, H., Yan, H., Rao, S., Xia, M., Zhou, Q., Xu, H., Rothney, M. P., Xia, Y., Wacker, W. K., Ergun, D. L., Zeng, M., & Gao, X. (2013). Quantification of visceral adipose tissue using lunar dual-energy X-ray absorptiometry in Asian Chinese. Obesity (Silver Spring, Md.), 21(10), 2112–2117. https://doi.org/10.1002/oby.20325
Choi, Y. J., Seo, Y. K., Lee, E. J., & Chung, Y. S. (2015). Quantification of visceral fat using dual-energy x-ray absorptiometry and its reliability according to the amount of visceral fat in Korean adults. Journal of clinical densitometry : the official journal of the International Society for Clinical Densitometry, 18(2), 192–197. https://doi.org/10.1016/j.jocd.2015.02.001
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Cruz-Jentoft, A. J., Bahat, G., Bauer, J., Boirie, Y., Bruyère, O., Cederholm, T., Cooper, C., Landi, F., Rolland, Y., Sayer, A. A., Schneider, S. M., Sieber, C. C., Topinkova, E., Vandewoude, M., Visser, M., Zamboni, M., & Writing Group for the European Working Group on Sarcopenia in Older People 2 (EWGSOP2), and the Extended Group for EWGSOP2 (2019). Sarcopenia: revised European consensus on definition and diagnosis. Age and ageing, 48(1), 16–31. https://doi.org/10.1093/ageing/afy169
Staynor, J.M.D., Smith, M.K., Donnelly, C.J. et al. DXA reference values and anthropometric screening for visceral obesity in Western Australian adults. Sci Rep 10, 18731 (2020). https://doi.org/10.1038/s41598-020-73631-x
Hirsch, K. R., Blue, M., Trexler, E. T., & Smith-Ryan, A. E. (2019). Visceral adipose tissue normative values in adults from the United States using GE Lunar iDXA. Clinical physiology and functional imaging, 39(6), 407–414. https://doi.org/10.1111/cpf.12593
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