Bone Densitometry



Bone Densitometry


Sharon R. Wartenbee, RT(R)(BD), CBDT, FASRT



Learning Objectives


At the conclusion of this chapter, you will be able to:


• Define the types of osteoporosis and risk factors


• State the diagnostic criteria for osteoporosis as defined by the World Health Organization


• Understand the function and types of bone


• Summarize the bone remodeling cycle


• List the standards for bone health


• Understand the basic statistical concepts of densitometry


• Apply the concepts of measuring bone mineral density (BMD) and reporting patient results as it relates to a T-score and Z-score


• Understand the properties of the x-ray beam, including quality (kilovolts peak), quantity (milliamperes), and time (seconds)


• Perform daily computer operation and electronic file management


• Describe the various types of dual-energy x-ray absorptiometry systems (DXA), including pencil beam and fan (array) beam


• Perform equipment quality control procedures using calibration and phantom methods, and troubleshoot problems


• Determine quality in bone mineral densitometry using precision and accuracy


• Describe the concept of FRAX and vertebral fracture assessment (VFA)


• Apply the fundamentals of radiation safety for the patient and the operator using the principle of ALARA (As Low As Reasonably Achievable)


• Describe the units used to measure absorbed dose (gray) and effective dose (sievert)


• Utilize the correct anatomy, positioning, acquisition, and analysis for spine, proximal femur, and forearm scanning; identify the common problems of serial scans as described



Bone densitometry encompasses the art and science of measuring the bone mineral content and density of specific skeletal sites or the whole body. The bone measurement values are used to assess bone strength, diagnose diseases associated with low bone density (especially osteoporosis), monitor the effects of therapy for such diseases, and predict the risk of future fractures.


The most versatile and widely used method of performing bone densitometry is dual-energy x-ray absorptiometry (DXA). This technique, considered the gold standard, has the advantages of low radiation dose, wide availability, ease of use, short scan time, high-resolution images, good precision, and stable calibration.


All bone densitometry operators must possess, utilize, and maintain knowledge in the core competency areas of radiation protection, patient care, history taking, basic computer operation, scanner quality control, patient positioning, scan acquisition, scan analysis, and proper record keeping and documentation. Consistent positioning and scan acquisition are the fundamentals of precise and accurate results, which can affect the treatment plan for the patient.



DXA versus conventional radiography


X-ray machines from different manufacturers are operated in essentially the same manner and produce identical images. This is not the case with DXA equipment. There are three major DXA manufacturers in the United States: GE Lunar Corp., Hologic Inc., and Norland Corp. (Fig. 26-1). The operator must be educated to operate the specific scanner model in his or her facility. The numeric bone density results obtained on machines from different manufacturers cannot be compared without proper standardization. This chapter presents general information on scan positioning and analysis, but the manufacturers’ specific procedures must be used when actual scans are performed.



DXA can be conceptualized as a subtraction technique. To quantitate bone mineral density (BMD), it is necessary to eliminate the contributions of soft tissue and measure the x-ray attenuation of bone alone. This is accomplished by scanning at two different x-ray photon energies (thus the term dual-energy x-ray) and mathematically manipulating the recorded signal to take advantage of the differing attenuation properties of soft tissue and bone at the two energies (Fig. 26-2). The density of the isolated bone is calculated based on the principle that denser, more mineralized bone attenuates (absorbs) more x-rays. It is essential to have adequate amounts of artifact-free soft tissue to help ensure the reliability of the bone density results.



Bone densitometry results are computed by proprietary software from the x-ray attenuation pattern striking the detector, not from the scan image. DXA scans provide images only for the purpose of confirming correct positioning of the patient and correct placement of the regions of interest (ROIs). Therefore the images may not be used for diagnosis, and any medical conditions apparent on the image must be followed up by appropriate diagnostic tests. The interpreting physicians must be skilled in interpreting the clinical and statistical aspects of the numeric density results and relating them to the specific patient history obtained by the operator.


Bone densitometry differs from diagnostic radiology in that good image quality, which can tolerate variability in technique, is not the ultimate goal. The goal is accurate and precise quantitative measurement by the scanner software, which requires stable equipment and careful, consistent work from the operator.



Osteoporosis


Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue. As a result, the bones are at increased risk for fragility fractures. It is estimated that 10 million Americans have osteoporosis, with 80% (8 million) of those being women. Another 34 million Americans have osteopenia or low bone mass, which puts them at risk of developing osteoporosis in the future (Fig. 26-3). Persons with osteoporosis may experience decreased quality of life from the pain, deformity, and disability of fragility fractures (especially at the hip and spine) and increased risk of morbidity and mortality, especially from hip fractures. In the United States, annual medical costs for osteoporosis, including hospitalization for osteoporotic hip fractures, were $19 billion in 2005, and the cost is increasing. By 2025, it is expected to be $25.3 billion. Controllable and uncontrollable risk factors are shown in Box 26-1. Patients with a normal rate of bone loss may still develop osteoporosis if their peak bone mass is low.





Primary and Secondary Osteoporosis


Primary osteoporosis can be type I (postmenopausal) and/or type II (senile or age-related). Type I osteoporosis arises when bone resorption exceeds bone formation due to estrogen deprivation in women. Type II osteoporosis occurs in aging men and women due to a decreased ability to build bone.


Secondary osteoporosis is osteoporosis caused by other conditions. Common causes of secondary osteoporosis include hyperparathyroidism, gonadal insufficiency (including estrogen deficiency in women and hypogonadism in men), osteomalacia (rickets in children), rheumatoid arthritis, anorexia nervosa, gastrectomy, adult sprue (hypersensitivity to gluten [wheat protein]), multiple myeloma, and use of corticosteroids, heparin, or anticonvulsants or excessive thyroid hormone treatment.


Several prescription medications arrest bone loss and may increase bone mass. These include traditional estrogen or hormone replacement therapies and bisphosphonates (which have antiresorptive properties), as well as selective estrogen receptor modulators (SERMs), salmon calcitonin, and parathyroid hormone (which stimulate bone formation). Other therapies are in clinical trials and may be available in the future.


Laboratory tests for biochemical markers of bone turnover may be used in conjunction with DXA to determine the need for or the effectiveness of therapy.



Bone Biology and Remodeling


The skeleton serves several purposes. It supports the body and protects vital organs so that movement, communication, and life processes can be carried on. It also manufactures red blood cells, and it stores the minerals that are necessary for life, including calcium and phosphate.


The two basic types of bone are cortical (or compact) and trabecular (or cancellous). Cortical bone forms the dense, compact outer shell of all bones, as well as the shafts of the long bones. It supports weight, resists bending and twisting, and accounts for about 80% of the skeletal mass. Trabecular bone is the delicate, latticework structure within bones that adds strength without excessive weight and accounts for 20% of the skeletal mass. It supports compressive loading in the spine, hip, and os calcis. It is also found at the ends of long bones, such as the distal radius.


Bone is constantly going through a remodeling process in which old bone is replaced with new bone. Bone-destroying cells called osteoclasts break down and remove old bone, leaving pits. This part of the process is called resorption. Bone-building cells called osteoblasts fill the pits with new bone. This process is called formation (Fig. 26-4).



Bone mass increases in youth until peak bone mass is reached at about 30 to 35 years of age. This is followed by a stable period in middle age. A decrease in bone mass becomes pronounced in women at menopause because of the loss of bone-preserving estrogen during this time; this decrease occurs at a somewhat later age in men (after 70 years).


Bone health requires adequate calcium and vitamin D intake and absorption. Calcium is a mineral that is essential for life, yet the majority of Americans do not get adequate calcium on a daily basis. Calcium plays an important role in building stronger, denser bones early in life and keeping bones strong and healthy later in life. About 99% of the calcium in our bodies is found in our bones and teeth. In addition to building and maintaining healthy bones, calcium allows blood to clot, nerves to send messages, muscles to contract, and other body functions to occur. Each day, our bodies lose calcium through skin, nails, hair, sweat, urine, and feces. The human body cannot produce calcium on its own. That is why it is important to obtain enough calcium through the foods we eat. When the diet does not provide enough calcium for the body’s needs, calcium is taken from the bones.


Vitamin D plays an important role in protecting bones. The body requires vitamin D to absorb calcium. Children need vitamin D to build strong bones, and adults need it to keep bones strong and healthy. Studies show that patients with low levels of vitamin D have lower bone density or bone mass and are more likely to break bones when they are older. Vitamin D deficiency is becoming more widespread nationally and internationally. It can cause a disease known as osteomalacia in which the bones become soft. In children, this condition is known as rickets.




Physical and mathematical principles of DXA


X-rays are produced in a vacuum tube when high-speed electrons suddenly decelerate at the tube target. The electrons are liberated by heating the tungsten filament and are accelerated by high voltage from a step-up transformer.


The properties of the x-ray beam are described by three measures:



• Quality—Voltage is measured at the peak of the electric cycle. When the voltage across the x-ray tube is measured, the units are often stated as kilovolts peak, abbreviated kVp. The terms kV and kVp are used interchangeably in radiography, with kVp being the preferred term. The voltage applied to the x-ray tube controls the speed of the electrons and the penetrating power of the x-ray photons produced.


• Quantity—Milliamperage (mA) is a measure of the rate of current flow across the x-ray tube, that is, the number of electrons flowing from filament to target each second. Milliamperage controls the volume of x-ray production and thus also the rate of exposure. It controls the intensity of the x-ray beam.


• Duration or time—Exposure time refers to the length of time that the x-rays are turned on. It is the duration of the x-ray exposure. Exposure time is measured in units of seconds.


The measurement of bone density requires separation of the x-ray attenuating effects of soft tissue and bone. The mass attenuation coefficients of soft tissue and bone differ and also depend on the energy of the x-ray photons. The use of two different photon energies (as in a dual-energy x-ray) optimizes the difference between soft tissue and bone. GE Lunar and Norland machines use a different method of producing the two energies than do Hologic machines.


GE Lunar and Norland scanners use a rare earth filtered x-ray source. The primary x-ray beam is passed through selected rare earth filters to produce a spectrum with peaks near 40 and 70 kiloelectron volt (keV) compared with the usual continuous spectrum with one peak near 50 keV. Sophisticated pulse-counting detectors are used to separate and measure the low- and high-energy photons. Calibration must be performed externally by scanning a calibration phantom on a regular basis.


The Hologic scanners use an energy-switching filter system that synchronously switches the x-ray potential between 100 and 140 kVp. This produces a primary beam with two photon energies with peaks near 40 and 80 keV. The energy-switching system continuously calibrates the beam by passing it through a calibration wheel or drum containing three sectors for an open air gap, a soft tissue equivalent, and a bone equivalent. Each sector is divided so that it can differentiate and measure the low- and high-energy photons. This permits the use of a relatively simple current-integrating detector that does not have to separate the photons.


Before the x-ray beam reaches the patient, it is collimated either into a narrow pencil beam using a pinhole collimator or into a fan (array) beam using a slit collimator.


Low- and high-energy x-rays are attenuated differently within each patient. This produces a unique attenuation pattern at the detector that is transmitted electronically to the scanner software. Mathematical computations are then performed that subtract the soft tissue signals, thereby producing a profile of the bone. Proprietary bone edge detection algorithms are next applied, and a two-dimensional area is calculated. The average BMD is calculated for all unit areas, and finally the bone mineral content (BMC) is calculated as BMC, BMD, and area. Thus the three bone densitometry parameters reported on the DXA printouts include area in square centimeters, BMC in grams, and BMD in grams per square centimeter. BMD is based on a two-dimensional area, not a three-dimensional volume, which makes DXA a projectional, or areal, technique.



Pencil-Beam and Array-Beam (Fan-Beam) Systems


The original DXA scanners employed a pencil-beam collimation system. In this system, a circular pinhole x-ray collimator produces a narrow (or pencil-beam) stream of x-ray photons that is received by a single detector. The pencil beam of x-rays moves in a serpentine (also called rectilinear or raster) fashion across or along the length of the body (Fig. 26-5). The pencil-beam system may still be in use but is no longer manufactured.



The array-beam system (also called fan-beam) has a narrow “slit” x-ray collimator and a multielement detector. The scanning motion is reduced to only one direction, which greatly reduces scan time and permits posteroanterior (PA) and lateral lumbar spine scans to be performed. The array-beam system introduces geometric magnification and a slight geometric distortion at the outer edges. Consequently, careful centering of the region of interest is necessary to avoid parallax. The software takes into account the known degree of magnification and produces an estimated BMC and estimated area.



Statistical measures in bone densitometry


Three statistics are particularly important in bone densitometry: mean, standard deviation (SD), and percent coefficient of variation (%CV). The mean, commonly called the average, is the sum of the data values divided by the number of values. The SD is a measure of variability that indicates the spread of the data values around their mean. It takes into account the average distance of the data values from the mean. The smaller the average distance or the spread, the smaller the SD. In bone densitometry, a smaller SD is better. The %CV is a statistic that allows the comparison of variability between different data sets, whether or not they have the same mean. A smaller %CV means less variability and is preferred in bone densitometry. The %CV is calculated using the following equation:


%CV=(SD/Mean)×100


image

Bone densitometry requires accurate and precise quantitative measurement by the scanner software. Two important performance measures in bone densitometry are accuracy and precision. Accuracy relates to the ability of the system to measure the true value of an object. Precision relates to the ability of the system to reproduce the same (but not necessarily accurate) results in repeat measurements of the same object.


Target shooting may be used to illustrate this point. In Fig. 26-6, A, the archer is precise but not accurate. In Fig. 26-6, B, the archer is accurate but not precise. Finally, in Fig. 26-6, C, the archer is both precise and accurate.



In bone densitometry practice, accuracy is most important at baseline when the original diagnosis of osteoporosis is made. Accuracy is determined primarily by the calibration of the scanner, which is set and maintained by the manufacturer. Preventive maintenance once or twice a year is recommended. Precision is followed closely because it is relatively easy to determine and is the most important performance measure in following a patient’s BMD over time. Precision can be measured in vitro (in an inanimate object [e.g., phantom]) or in vivo (in a live body). Precision is commonly expressed as %CV, and a smaller value indicates better precision.


In vitro precision is the cornerstone of the quality control systems built into the scanners to detect drifts or shifts (variations) in calibration. Each manufacturer provides a unique phantom for this purpose.


In vivo precision has two main aspects in bone densitometry:



The primary factors affecting precision include the following:



Mar 7, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Bone Densitometry
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