“There are now over 50,000 bone densitometers in the world. I doubt if more than 50 radiologists in the world know who invented the instrument,” declared the late John R Cameron, a pioneering medical physicist from Wisconsin, US, and the father of the modern dual-energy x-ray absorptiometry (DXA) scan – today’s established standard for measuring bone mineral density (BMD). 

“I learned from Dr Lester Paul, chair of radiology at the University of Wisconsin, that there was no method to detect early osteoporosis and many women were dying each year from broken hips. I invented bone densitometry in the early 1960s. It was of little interest at first because there was no known treatment. My invention was used to evaluate the different possible therapies,” he explained in an interview with Radiology Malaysia in 2004, a year before his death. “I have the satisfaction of knowing I did something useful for society.”

Osteoporosis in Ireland

In Ireland, more than 300,000 people aged 50 years and over may have osteoporosis, with this number set to double in the next 20 years in line with increasing life expectancy. There are now five times as many fractures each year due to osteoporosis as there were in the 1960s. Lifetime risk of osteoporotic fracture lies within the range of 40-50% in women and 13-22% in men, and approximately 30% of patients will die within a year due to causes related to the fracture.

DXA is the gold standard for estimation of BMD and is central to the diagnosis and assessment of osteoporosis. It is also valuable in tracking the effects of treatment for osteoporosis and other conditions that cause bone loss, as well as assessing an individual’s risk for developing fractures. The Irish Osteoporosis Society advises that all persons presenting with a fragility fracture after 50 years of age or menopause should be considered as possibly osteoporotic. The measurement of BMD is recommended, in addition to a detailed history of the fracture occurrence, physical examination and evaluation for other fractures. DXA is an enhanced form of x-ray technology, providing a quick and painless imaging technique that measures the transmission through the body of low-dose radiographic beams of two different photon energies. One energy peak is absorbed mainly by soft tissue and the other by bone. The soft tissue amount can be subtracted from the total and what remains is a patient’s BMD. 

When John Cameron introduced bone densitometry in 1963 he used single photon absorptiometry (SPA) to measure peripheral BMD, usually in the forearm. To correct for overlying soft tissue, the anatomical site in which BMD was measured had to be surrounded either by water or water equivalent mouldable material, with an additional correction for adipose tissue. In 1965 came the addition of a dual energy source for measuring regions of interest that could not be easily submerged in or surrounded by water. Although both of these techniques were widely used and provided many valuable clinical and research data, they had limitations. The photon energy was generated by radionuclides, which resulted in a degrading energy source, resolution limits and long scan times (20-40 minutes). This eventually led to the transition to DXA in 1987. Using x-rays as the energy source increased resolution, precision and dramatically reduced scan time to six minutes. Later improvements, replacing rectilinear scanning with array or fan beam, further decreased scan times to one or two minutes.

Types of DXA

There are basically two types of DXA equipment: a central device and a peripheral device. Central scanners allow measurement of BMD of the hip and spine, which are considered to be the best sites for monitoring overall BMD, and for predicting fracture. Typically located in hospitals and large medical centres, central devices consist of a large, flat table and an ‘arm’ suspended overhead. The patient lies down on the machine for 5-15 minutes while the moving arm of the machine passes over them, taking an image of their spine and hips.

Central DXA is non-invasive and painless, consisting of a very low-dose x-ray approximately 10% the radiation of a standard chest x-ray (CXR). That’s less than the radiation from an airplane trip and equivalent to two hours of direct sunshine. Central scanners can be subdivided according to the shape of the scanning x-ray beam. These include pencil beam, narrow-angle fan beam, wide-angle fan beam and flash beam scanners. Moving from pencil beam to flash beam, in general the scan time decreases, but the patient dose and cost increases. New developments in DXA technology enable patients to have a scan of their spine, if appropriate, at the same time as they are having their BMD measured. This technique, known as lateral vertebral assessment (LVA), obtains a single view of the spine (usually T5-L5) where most vertebral fractures occur. This way, doctors can evaluate for vertebral fractures in patients on corticosteroids, with height loss or undiagnosed back pain at the time of their DXA scan.

The second form of DXA is the peripheral scanner, which is a small portable device that measures BMD in the forearm, hand or heel. These can be used for screening to estimate overall fracture risk, but they are less accurate than the gold-standard central DXA and cannot be used for monitoring therapy. While there is interest in the use of these relatively low-cost, smaller DXA systems for assessing the peripheral skeleton, measurements generated from these devices cannot be interpreted using the World Health Organization definition of osteoporosis in terms of the T-score and Z-score.

How to score 

The DXA scanner measures bone mineral content in grams and the area in cm². These numbers are then used to calculate the bone density, an areal density, in g/cm². The bone density data are reported as T-scores and Z-scores. WHO criteria define a normal T-score value as within one standard deviation (SD) of the mean BMD value in a healthy young adult. Values lying farther from the mean are stratified as follows: 

• T-score of –1 to –2.5 SD indicates osteopenia
• T-score of less than –2.5 SD indicates osteoporosis
• T-score of less than –2.5 SD with fragility fracture(s) indicates severe osteoporosis.

Osteoporosis is diagnosed when the T-score is less than or equal to -2.5; the lower the score, the worse the bone mineral density is. For each SD reduction in BMD, the relative fracture risk is increased 1.5 to three times. This diagnostic classification should not be applied to premenopausal women, men younger than 50 years or children. Instead, Z-scores adjusted for ethnicity or race should be used, with values of –2.0 SD or lower defined as ‘below the expected range for age’ and those above –2.0 SD being ‘within the expected range for age’. DXA measurements are usually performed in the context of a comprehensive fracture risk assessment. 

The WHO introduced a fracture risk assessment tool (FRAX) in 2008, which estimates the 10-year probability of hip fracture or major osteoporotic fractures combined (hip, clinical spine, proximal humerus or forearm) for an untreated patient (aged 40-90 years) using clinical risk factors for fracture and femoral neck BMD (g/cm²). The easily obtainable clinical data are age, sex, height, weight, history of previous fractures, parental hip fracture history, smoking, steroid use, rheumatoid arthritis, secondary osteoporosis and alcohol consumption. Contraindications to a DXA scan include pregnancy; if the patient has a metal implant in the spine or hip, or if they have any metal body piercing; and if a patient has had an investigation using contrasts material recently, eg. barium meal, there needs to be one week between the tests. Quantitative CT can also be used to measure BMD using a standard x-ray CT scanner with a calibration standard to convert Hounsfield units of the CT image to bone mineral density values. 

Prof William Torreggiani, consultant radiologist at Tallaght Hospital, told Modern Medicine: “In recent years, the use of quantitative CT in measuring bone density has increased and has some advantages over DXA in that no errors due to spinal degenerative changes or aortic calcification occur and it also gives more information on bone morphometry. However, due to lower radiation dose, cost and access, DXA remained the predominant screening tool.”

Inequitable access 

Research undertaken in 2006 showed that in Ireland there are 61 DXA scanners (59 locations). Based on the population of each hospital network, the DXA coverage ranged from 1/46,000 to 1/122,000 population. International guidelines indicate that a minimum of 1,000 DXA scans per 100,000 population would be required, leading the HSE to conclude in its 2008 Strategy to Prevent Falls and Fractures in Ireland’s Ageing Population that the current availability of DXA scans in Ireland is sufficient to meet needs. 

The current service, however, is not provided on an equitable basis, concluded the study authors. Over half of these DXA scanners operate in the private health sector where scans can be provided on request. The waiting times for DXA scans in the public health sector is up to 20 weeks. The Irish Osteoporosis Society website currently lists a total of 70 DXA bone density scanning units in 17 counties. Approximately 20 are based in public hospitals. 

The HSE’s 2008 strategy stated that GPs should be facilitated in directly referring patients for DXA scanning using agreed guidelines, which could improve overall detection of osteoporosis and osteopaenia in the community. This recommendation is supported by findings of a 2008 evaluation of the GP direct access DXA scanning service in HSE Dublin Mid-Leinster Region, which also highlighted the inequitable access in Ireland, with direct access on GP referral largely limited to those who attend private facilities for their scans. The study authors identified a high positive detection rate of osteoporosis and osteopaenia (80% of those referred for scans), indicating appropriateness of the GP referral process, high levels of satisfaction with the new service and evidence that patients are receiving appropriate advice.