23.9: Measurement of bone mineral con­tent and bone mineral density (23a.9)

Noninvasive techniques can be used to pro­vide an indirect assessment of calcium balance as well as calcium “stores” in bone by measuring the bone mineral con­tent (BMC) or bone mineral density (BMD). These methods include dual X‑ray absorptiometry (DXA, also called DEXA) and quantitative com­puted tomography (QCT), both major diagnostic tools for osteoporosis. Ultrasound (US), although less accurate for medical diagnosis, can serve as a tool for screening and for field work. These technologies measure the mineral con­tent of bone in the appendicular skeleton, the axial skeleton, or the total skeleton, measure­ments that relate directly to the calcium con­tent of the skeleton. As almost all of the calcium in the body is stored in the skeleton, it follows that skeletal mineral con­tent can be used as an indirect measure of body calcium stores. These same methods can also be used to monitor the response to changes in calcium intakes over relatively long time periods, i.e., months or even years.

Over the past 40 years technologies such as DXA and QCT have been developed and refined to measure calcium in hard tissues, and thus estimate of total body mineral con­tent, which in turn can be used to calculate total body calcium. These technologies are also used to determine bone integrity for the diagnosis of osteoporosis and other bone diseases. They can also be used during treatment to evaluate efficacy as well as in research studies for the development of new drugs, exercise regimens, or nutrition interventions (IOF, 2020).

The measure­ment of bone mineral using DXA is usually expressed as the bone mineral density, which corrects for pro­jected bone area. However, the measure­ment is not a true density measure­ment, but instead, a measure of areal density (g/cm²) derived by dividing bone mineral con­tent (g) by the scanned bone area. Bone mineral density is calculated in this way to reduce the bio­logical variation observed in values for bone mineral con­tent at all ages and thus to increase the statistical power of detecting abnormal values. Nevertheless, bone mineral density does not adequately correct for bone area and body size and thus must be interpreted carefully (Prentice et al., 1994). In con­trast, QCT uses multiple slices to con­struct three-dimensional images, thus pro­viding a true volumetric measure for BMD (Adams, 2009).

Diagnosis for osteoporosis is based on the difference between the measured bone mineral density (BMD) of the participant and the mean BMD of healthy young adults, matched for sex and ethnicity. The difference is expressed relative to the young adult population standard deviation, and reported as a T‑score. WHO, 1994 has defined both osteoporosis and osteopenia based on T‑score values. An individual with a T‑score less than −2.5 at the spine, hip, or forearm is classified as having osteoporosis; a T‑score between −2.5 and −1 is classified as osteopenia; and a T‑score greater than −1 is healthy. These WHO definitions should not be applied to other bone mineral density measure­ment sites.

The International Society for Clinical Densitometry (ISCD) has established guidelines for bone density testing using DXA. In addition, they pro­vide specific guidelines for diagnosing osteoporosis based on T‑score values for postmenopausal women, men (20y and older), pre-menopausal women (20y to menopause), and children (males or females < 20y). Persons using DXA technology should be certified, and facilities should be accredited; con­sult the ISCD for advice on certification and accreditation.

23a.9.1 Single-photon absorptiometry

Single-photon absorptiometry was the first practical noninvasive method that was developed to examine the peripheral skeleton (Wahner et al., 1983).although now it has mostly been replaced by DXA (IOF, 2020). The original method was based on the assumption that the bone mineral con­tent is inversely pro­portional to the amount of photon energy transmitted through the bone under study. A mono-energetic photon source is used, usually ¹²⁵I or ²⁴¹Am. The technique is fast, taking approximately 5min. With this method, bone mineral density is measured with a precision (%CV) of less than 2% and with an effective dose to the participant of only 0.1µSv. The site most frequently selected for measure­ments by single-photon absorptiometry is the lower radius, at approximately one-third of the distance from the styloid pro­cess to the olecranon. Single-photon absorptiometry is not suitable for measure­ments on the axial skeleton because the technique requires that a uniform thickness of soft tissue surrounds the bone (Neer, 1992). Newer technologies have replaced SPA for human studies as SPA cannot measure those sites (hip or spine) needed for clinical diagnoses. In the past four decades SPA machines have been adapted and used for animal studies of bone health (Sequeira et al, 2020).

23a.9.2 Dual X‑ray absorptiometry (DXA)

The first com­mercial DXA scanner was marketed in 1987. There are now several com­mercial versions, each using similar measure­ment pro­cedures The original DXA method used an isotopic source, typically ¹⁵³Gd , that emitted two low-energy gamma rays at 44Kev and 100KeV. The short half-life of the isotopic source ¹⁵³Gd, however, limited the precision with which long-term changes in bone mineral con­tent in the same participant could be made using this technique. This limitation led to the radioactive ¹⁵³Gd source being replaced with an X‑ray tube behind a K‑edge filter. The filter con­verts the polychromatic X‑ray beam into one with two main energy peaks at 40Kev and 70KeV. These two con­gruent X‑ray beams are passed through the body, and the ratio of beam attenuation at the lower energy relative to that at the higher energy differentiates between bone mineral, bone-free mass, and the fat mass (Kim et al., 2018). As a result, this modified instrument is termed a dual energy X‑ray absorptiometer, abbreviated as DXA (or DEXA).

The DXA instrument is used not only for bone mineral measures but also to assess body com­position, as described in Chapter 14. Only the measure­ments of bone mineral mass (g), bone mineral con­tent (g/cm), and “areal” bone mineral density (g/cm²) are discussed here. Using DXA, measure­ments of areal bone mass density (aBMD) can be made at the lumbar spine, femur, and forearm, as well as total body. Measurements of body mineral density at the lumbar vertebrae (L2 to L4) are often favored because they are sensitive to the changes associated with aging, disease, and therapy. Care must be taken when positioning the participant for the scan to ensure that precise and accurate data are obtained. Under some circumstances, it may be appro­priate to make a whole-body scan. This is quick, taking from 3min to 35min, depending on the instrument used and the age of the participant, and requires very little cooperation from the participant. As a result, scans can be readily performed on young children, the elderly, and even persons who are sick, although pregnant women should be excluded (Kim et al., 2018).

Dual-energy X‑ray absorptiometry is now the dominant method for measuring bone loss for clinical diagnosis of osteopenia and osteoporosis. In intervention studies care must be taken to ensure that both the length of the intervention and the size of the study population are sufficient to allow the detection of small changes in bone mineral density. A meta-analysis of trials reported that increasing calcium intake from dietary sources increased BMD by 0.6–1.0% at the total hip and total body at one year, and by 0.7–1.8% at these sites and the lumbar spine and femoral neck at two years (Tai et al., 2015). Larger decreases in bone loss have been observed after treatment with estrogen, calcitonin, or biphosphates.

When com­paring measure­ments using DXA based on different instruments and technologies, it is important to acknowledge the potential for systematic differences arising from methodological factors which could mask those associated with bio­logical variation. Methodological factors may include differences in instrument calibration methods, whereas variations in body thickness, distribution of body fat, and the fat con­tent of the bone marrow are major sources of bio­logical variation. Of the latter, DXA is especially sensitive to discrepancies in body thickness, which can lead to systematic differences in the bone mineral estimates on thin and obese subjects. In a global survey of fracture liaison services, although access to DXA was reported to meet the needs (Clynes et al., 2020), only around 50% of institutions con­firmed adherence to basic DXA quality and reporting pro­cedures, and many required educating the operators/interpreters. Indeed, significant variability worldwide in both the quality and access to DXA services was reported in this survey.

Calcium balance using DXA may be determined by obtaining a total body (TB) DXA scan that pro­vides bone mineral con­tent (BMC). Scans of total body bone mineral con­tent (TBBMC) can be used to calculate calcium requirements of specific life cycle groups, on the assumption that bone mineral is 32·2% calcium (Vatanparast et al., 2010). Use of this value is a better approximation of calcium in bone than simply dividing the atomic weight of calcium (40g/mol) by the molecular weight (i.e., 1005g/mol) of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ because the mineral in bone is hydrated so water molecules con­tribute to the weight of bone mineral con­tent.

To derive the Estimated Average Requirement (EAR) for calcium for adolescents 9–18y, the U.S Food and Nutrition Board applied the factorial approach to estimate the physiological calcium requirements (IOM, 2011). Estimates of calcium losses via urine, feces, and sweat were derived from metabolic balance studies to which was added the calcium accrued as bone mineral for growth (derived from TBBMC). The latter was measured in a cohort of Canadian males and females in two age groups, 9–13y and 14–19y as shown in Table 23a.7 (Vatanparast et al., 2010). To translate the physiological requirements for absorbed calcium (from the factorial approach) into the EAR, it is necessary to take into account the pro­portion of calcium in the habitual diet that is absorbed by the intestine. Assuming an efficiency of absorption of 38% from an omnivorous diet, the equation used for the estimated Ca dietary requirement (i.e., the EAR) using the formula for adolescence which is a period of rapid growth:
EAR = Ca losses + (Ca needs for growth / 38%)
(IOM, 2011).

Table 23.7. Determining calcium requirements during adolescence (age 9–18y) using total body bone mineral content (TBBMC). Source: Vatanparast et al., (2009). British Journal of Nutrition 103: 575–580.

Factorial Components	Girls 9–18y (mg/d)	Boys 9–18y (mg/d)
Ca accretion from TBBMC	121	175
Urinary Ca Losses	106	127
Endogenous Fecal Ca	112	108
Sweat Ca Losses	55	55
Total Ca physiological need	394	465
Absorption (%)	38	38
Dietary Ca requirement (EAR)	1037	1224

23a.9.3 Com­puterized tomography (CT) and peripheral quantitative CT (pQCT)

Computerized tomography (CT), often called quantitative CT (QCT) when referring to bone measure­ments, can be used to measure bone mass of both the appendicular and axial skeleton, although the method is not feasible for population studies. The advantage of QCT is that a true three-dimensional (i.e., volumetric) bone density can be obtained, so results can be expressed in g/cm³ rather than the two-dimensional areal density measured by DXA. Bone can also be identified as cortical or trabecular in nature on the basis of bone density measure­ments. This is an advantage because some forms of osteoporosis are predominantly trabecular in character. The equipment is not portable, and the radiation dose required per slice for imaging is relatively high (50–500µSv), although it can be reduced if scans are limited to regions of specific interest. The cost of the equipment and its main­tenance is high.

The use of QCT technology applied to specific bone sites yields information about micro­architecture. Methods that can be used include micro‑CT and peripheral Quantitative CT (pQCT), with the latter having widespread use. The equipment for pQCT is small relative to DXA and CT, and pro­vides imaging of both the arm at the distal radius and the lower leg (tibia) sites to assess bone micro­architecture. High-Resolution pQCT (HR‑pQCT) has been developed to improve the resolution pro­blems of pQCT. Recently, second-generation HrpQCT has been introduced which differs from the first-generation HR‑pQCT in scan region, resolution, and morphological measure­ment techniques (Whittier et al., 2020).

23a.9.4 Quantitative ultrasound

Quantitative ultrasound (QUS) is used for measure­ments of the bone density of the peripheral skeleton such as the proximal arm (radius), lower leg (tibia) or heel. A wide variety of equipment is now available to measure the attenuation of a sonographic pulse as it passes through bone and is scattered and absorbed by the trabeculae. The heel is the most frequently used measure­ment site because it has a large volume of trabecular bone that is accessible for transmission measure­ments. However, there are ultrasound machines on the market that measure multiple skeletal sites. Quantitative Ultrasound has the advantage of being radiation-free and a portable system.

Quantitative ultrasound measures the speed of sound (SoS) in meters per second (m/s). In a study of different age groups using QUS, measures of SoS (m/s) varied according to life stages that reflected bone development (Rivas-Ruiz et al., 2015). For example, bone SoS accretion was found to begin 5y earlier in girls than boys (p < 0.05), a sex-related difference that mirrors results seen in DXA scans (Vatanparast et al., 2010). The maximal (peak bone) SoS was noted at 28y at the radius site and at 22y at the tibia site. Women who were postmenopausal (age 45–50y) showed a significant decrease in SoS measures at both these sites (p < 0.05) compared with men, as would be expected. Thus, while the precision of QUS is poor relative to DXA, the equipment may be suitable for characterizing bone in populations where DXA is not a feasible option. However, QUS has no clinical applications and is not used to define T‑scores for diagnosing osteoporosis or monitoring treatment. However, a systematic review has shown that quantitative ultrasound is an independent predictor of fracture for men and women when QUS values are low (McCloskey et al., 2015).