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THE FUNDAMENTALS OF ...

Bone Densitometry Robert Dondelinger

As we age, a number of things happen to our bodies, some more obvious than others. We look in the mirror and see the gray hair, the wrinkles, and the other visible signs of our aging. Unfortunately, it is what we cannot see that really hurts us. Neither gender can see the calcium leaching from their bones, but may experience the effect of the brittleness of osteoporosis. This metabolic bone disorder affecting from 25 to 44 million people in the United States alone is the cause of more than 1.3 million bone fractures. Treating these fractures cost patients billions of dollars each year. Given our aging population, occurrence of this disease can only increase and become more pervasive as the years continue. Osteoporosis is most commonly found in postmenopausal women, but it can be found in men and juveniles as well. By age 70, twice as many women are diagnosed with osteoporosis than men. Bones contain cross-linked collagen, calcium, and phosphorous, which makes them hard, dense, and strong. Osteoporosis is a Latin term literally meaning “porous bone” and is characterized by a loss of calcium hydroxyapatite and collagen matrix. This loss causes them to become less dense, more fragile, and more easily broken. The loss of collagen crosslinking cannot be reversed, but the calcium hydroxyapatite can be replaced with calcium supplements, through weight-bearing exercise, and sometimes with other medications. Healthcare

practitioners perform both the initial diagnosis and monitor the progress of their treatment by measuring the density of the bone using modified imaging technology.

Current Technology

About the Author Robert Dondelinger, CBET-E, MS, is the senior medical logistician at the U.S. Military Entrance Processing Command in North Chicago, IL. E-mail: robert. [email protected]

Three technologies are capable of providing quantitative measurements of bone density to both detect osteoporosis and to assess treatment efficacy. The first, considered the gold standard for bone densitometry, is the dual-energy X-ray absorptiometry or DXA (known as “DEXA” until 2003). The second is bone ultrasonometry, formerly known as ultrasonic bone densitometry. The third, and least common, is a software application for a computed tomography (CT) scanner. All three methods are considered noninvasive Healthcare practitioners perform and painless, as well as both the initial diagnosis and monitor relatively inexpensive. the progress of their treatment by The first two depend on specialized equipment, measuring the density of the bone which is the subject of using modified imaging technology. this article. Quantitative CT will not be included because it is a software application and represents a costly and, some argue, an inappropriate use of a CT while the results are no better than DXA, the gold standard. Dual-Energy X-ray Absorptiometry (DXA) Although soft tissue—muscle, fat, and connective tissue such as ligaments and Biomedical Instrumentation & Technology July/August 2014

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Bone densitometry is considered noninvasive and painless, as well as relatively inexpensive.

tendon tissue—is not well visualized with X-rays, they do cast a shadow on the film because they absorb and attenuate some of the X-ray beam. Fortunately, both bone and soft tissue attenuation is nonlinear— it varies with the X-ray energy level and is directly proportional to bone density and content. That is, the denser the bone, the more it attenuates the beam. Knowing that the attenuation levels of bone and soft tissue are both nonlinear and proportional to their

Whole-body systems make bone density measurements of the spine and hips, while peripheral systems use the heel, wrist, or finger to determine bone density. Peripheral systems, because of the smaller body parts they scan, are generally smaller than whole-body systems and oftentimes mobile. density allows the assessment of bone mineral density (BMD) and bone mineral content (BMC) without interference from soft tissue. DXA systems can be categorized as either whole body or peripheral systems

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and both rely on the generation of X-rays by a vacuum tube to provide the energy necessary to assess bone density. Whole-body systems make bone density measurements of the spine and hips, while peripheral systems use the heel, wrist, or finger to determine bone density. Peripheral systems, because of the smaller body parts they scan, are generally smaller than whole body systems and oftentimes mobile. DXA systems use one of two methods to generate X-rays of two different energy levels. The first method applies alternating pulses of high and low voltage to an X-ray tube. This causes the tube to produce X-rays of two distinctly different energy levels. The other method uses a single potential source but employs a K-edge or compound filter to split the output into high and low energy levels. These X-rays are collimated down to a pencil-thin beam (first-generation systems) or a broad beam (second-generation systems) that scans bones. Because of the beam shape, first-generation systems require multiple passes over the bones while second-generation systems require only one pass. On the opposite side of the body part of interest and across from the X-ray source, is either a single detector (first generation) or an array of detectors (second generation) that are similar to those found on X-ray phototimers. These detectors contain filtration to make them energy sensitive, causing them to discriminate between the high and low levels of radiation generated by the alternating pulses of high and low voltage supplied to the X-ray tube. A dual-channel analyzer amplifies the output from both detectors. Both bone and soft tissue absorb energy from higherintensity X-rays. However, only the soft tissue absorbs most of the low energy radiation, its level is subtracted from the high-energy radiation level to determine the amount of X-rays actually absorbed by just the bone. The system reports this calculated level of BMD in grams per square centimeter (g/cm2) and BMC as grams per centimeter (g/cm). Later systems go a step further, comparing the patients’ readings to statistical norms and reporting their bone density as the number of standard deviations above or (usually) below the reference adult population.

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Ultrasonic Bone Densitometry Ultrasonic bone densitometry or ultrasonometry measurement systems are inexpensive, simple, and quick but not as accurate as DXA. Although the bones of the fingers, toes, or the leg could be used, ultrasonometers usually measure the density of the patient’s heel. They are similar to traditional diagnostic ultrasound units, but these specialized systems employ a pair of transducers operating between 200 and 600 kHz, with each one placed on opposite sides of the patient’s heel, measurements of bone density are made using several principles of ultrasonography. The first is measurement of bone width based upon the sound waves passing through the changing density of soft tissue, bone, and soft tissue again. The second is based upon the attenuation rate of bone tissue itself, which is dependent upon its density—the denser the bone, the higher the attenuation.

Knowing the width of the bone and the amount of attenuation that occurs to the sound waves as they pass through the bone, the bone density can be calculated. This measurement is known as broadband ultrasonic attenuation or BUA and is expressed as decibels per centimeter per megahertz. Other measurements and determinations made by ultrasonic systems include bone stiffness BMD. As with all ultrasound probes, some coupling medium is required to ensure sound penetration of and below the skin. Ultrasonometry systems either use ultrasonic gel or immerse the transducers and the patient’s foot in a water bath to achieve this coupling.

How to Manage Bone Densitometry Systems DXA systems should be managed as any other X-ray–generating system, to include

Origin and Evolution Prior to the introduction of the first dedicated bone densitometry unit in the 1970s, physicians primarily used one of two methods to assess bone density. One was the totally subjective reading of X-rays based upon individual radiologist experience and the presence or absence of certain markers, such as the fine spicules, called “trabecula, that form a network in cancellous bone.” While the absence of trabecula may mean 30% to 50% bone loss, it is far from precise. Clearly, a better way to assess bone density was needed. This better way appeared to be nuclear medicine, the other way of assessing bone density. Originating in the 1960s, nuclear medicine employed single photon absorptiometry using iodine-125, which emitted 35 keV gamma rays. Basically, nuclear medicine studies require the injection of a radiopharmaceutical, waiting while the radioactive chemical localizes to specific cell receptors in the area of interest, then scanning the patient with a gamma camera to measure the level and form an image of the area of interest of the gathered radioactive. While this method was more precise than even subjective readings using markers, it is relatively expensive and was both time consuming and cumbersome for the patient. Another 1960s technology, quantitative ultrasound, was employed in an

attempt to accurately assess bone density, but it never achieved the accuracy level of nuclear medicine and even today only approaches the accuracy of DXA. Research by the University of Wisconsin in the late 1960s and early 1970s led to the successful commercial launch of a stand-alone bone densitometer based around conventional X-ray technology. Thus the single photon absorptiometer was born. Although it required the patient to sit with his or her arm immobilized for about 10 minutes, it was still faster, more accurate than a nuclear medicine study, and more accurate than either subjective film reading or ultrasonography of the time. Further development by a Massachusetts firm led to the late 1980s release of a dual-energy X-ray absorptiometry (DEXA) unit. This unit did not require immobilization of a limb. Instead, the patient reclined on a table while the unit moved above and (unbeknownst to the patient) below them scanning the pelvic and lower spine areas, reducing the scan time to just a few minutes. Around 2000, this same manufacturer introduced broad or fan-beam technology units. This increased the amount of information that was captured, providing both bone mass and additional features in a single pass.

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maintaining detailed records of periodic services, such as preventive maintenance, calibration, and electrical safety testing, as well as remedial and modification work orders. Likewise, ultrasonic bone densitometry systems should be maintained like other ultrasound systems. A “first call” annual service contract should be favorably considered if the contract price includes periodic software updates along with the required maintenance services.

DXA systems should be managed as any other X-ray– generating system, to include maintaining detailed records of periodic services, such as preventive maintenance, calibration, and electrical safety testing, as well as remedial and modification work orders. Regulations Both DXA and ultrasonometry systems are listed as radiation producing devices. There are no specific regulations covering bone densitometry beyond the requirement to conform to the regulations promulgated by the U.S. Food and Drug Administration (FDA) for sale and use of radiation producing medical devices in the United States. Additionally, some states, such as California, have their own regulations regarding radiation producing medical devices. Other authorities, such as the European Community, have similar standards, regulations, and requirements.

highest standards of practice and the scanner operates at its peak performance. Frequent transducer testing using an ultrasonometry phantom is key to ensuring proper scanner operation. Paying attention to these two points will minimize risk.

Troubleshooting There are no particularly unique troubleshooting issues with either the DXA or the ultrasonometry systems. Oftentimes, actionable maintenance issues will be revealed when the radiology technician performs their periodic QA/QC phantom checks, so these must be performed in accordance with the recommended schedule. DXA systems have the same problems as do other X-ray systems. Ultrasonic systems have the same problems with probe damage and breakage of the small wires in the probe cable as do their generalpurpose diagnostic ultrasonic cousins. As with any other diagnostic ultrasonic system, ultrasonometers are sensitive to proper positioning of the transducer assembly. The more proficient the technician, the better the technique, hence the better the accuracy of the system.

Training and Equipment

A good general-purpose medical electronics toolkit is mandatory to perform even minimal work on complex medical equipment, and bone densitometry units are no exception. Special phantoms are required to maintain these units at their performance Risk Management Issues peak. Although device-specific training is All risk management issues that exist for useful for performing repairs, a biomed radiographic X-ray systems apply to DXA specializing in X-ray maintenance can systems. Leaded walls are required for rooms perform routine scheduled services and containing these units. Operators need to be repairs to the pure X-ray side. However, since knowledgeable and DXA units basically aware of scatter and are an X-ray unit with A good general-purpose medical secondary radiation, special sensors and and both patients and specialized software, electronics toolkit is mandatory staff must wear leaded maintainers need to be to perform even minimal work protective aprons, proficient on both the on complex medical equipment, gonad, and thyroid X-ray portion and the and bone densitometry units shields. computer portion. Ultrasonic units Likewise, the are no exception. appear to be the most concepts of ultrasonic risk free. Strict quality bone densitometry assurance procedures must be in place to systems are based upon the underlying ensure both the sonographer maintains the general ultrasound principles, and these 298

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dedicated systems devices have much in common with them. Model-specific training is virtually essential for full service of these units. Like their DXA counterparts, special phantoms are required by in-house service personnel, and much can be done without specific training based upon knowledge of and experience with general diagnostic ultrasonic equipment. The level of additional training and experience needed for repairing these devices depends heavily on the quality of the manufacturer’s service literature and service software availability. Manufacturer training is highly recommended if both available and reasonably priced.

Future Development DXA is the gold standard for the measurement of bone density. DXA is used for research, diagnosis, and to assess the efficacy of treatment of both osteoporosis and osteopenia because it can detect density changes as small as 1%. Currently available units are generally second generation since they are improved designs that provide faster scan speeds, provide accurate results despite patient motion, improved precision, and provide on-board tracking of BMC changes over time. Future improvements in DXA systems promise provide automatic detection of bone-implant interfaces and measurement of patient responses to implants. They also will be used to assess the performance of hip implants and to provide evidence of changes in bone formation after hip implantation surgery. The ability to provide post-operative information will be a major new application of DXA. n

• Cloe A. How Does a DEXA Scan Work? Available at www.livestrong.com/article/78401-dexa-scanwork/-basics of how DXA works. Accessed April 27, 2014. • WebMD. DEXA Scan (Dual X-ray Absorptiometry) to Measure Bone Health. Available at www.webmd.com/osteoporosis/ guide/dexa-scan. Accessed April 27, 2014. • National Institutes of Health. Bone Mineral Density Scans. Available at www.ncbi.nlm.nih. gov/pmc/articles/PMC1124002/. Accessed April 27, 2014. • Tao W, Clarke W. Hologic Bone Densitometry and the Evolution of DXA. Available at www. hologic.com/data/Evolution-of-DXA.pdf. Accessed April 27, 2014.

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Resources • ECRI Institute. Healthcare Product Comparison System. Densitometers, Bone, Dual-Energy Absorptiometry; Ultrasonometers, Bone. Subscription service available at https://www. ecri.org/Products/Pages/hpcs.aspx. Accessed April 27, 2014. • Massachusetts General Hospital. History of the MGH Bone Density Center. Available at www2. massgeneral.org/bonedensity/history.html insight into bone density history. Accessed April 27, 2014.

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