Chapter 9

Medical Imaging in the Diagnosis

of Osteoporosis and Estimation of the Individual

Bone Fracture Risk

Mark A. Haidekker and Geoff Dougherty

Abstract Osteoporosis is a degenerative disease of the bone. In an advanced state, bone weakened by osteoporosis may fracture spontaneously with debilitating consequences. Beginning osteoporosis can be treated with exercise and calcium/vitamin D supplement, whereas osteoclast-inhibiting drugs are used in advanced stages. Choosing the proper treatment requires accurate diagnosis of the degree of osteoporosis. The most commonly used measurement of bone mineral content or bone mineral density provides a general orientation, but is insufficient as a predictor for load fractures or spontaneous fractures. There is wide agreement that the averaging nature of the density measurement does not take into account the microarchitectural deterioration, and imaging methods that provide a prediction of the load-bearing quality of the trabecular network are actively investigated. Studies have shown that X-ray projection images, computed tomography (CT) images, and magnetic resonance images (MRI) contain texture information that relates to the trabecular density and connectivity. In this chapter, image analysis methods are presented which allow to quantify the degree of microarchitectural deterioration of trabecular bone and have the potential to predict the load-bearing capability of bone.

9.1 Introduction

Osteoporosis is defined as a skeletal disorder characterized by compromised bone strength predisposing a person to an increased risk of fracture. Bone strength primarily reflects the integration of bone density and bone quality [1].

The official definition of osteoporosis further specify bone density as referring to specify mineral content and bone quality as referring to architecture, turnover, damage accumulation, and mineralization [1]. Bone density peaks at an age between

M.A. Haidekker ( )

University of Georgia, Faculty of Engineering, Athens, GA 30602, Georgia e-mail: [e-mail]

G. Dougherty (ed.), Medical Image Processing: Techniques and Applications, Biological

193

and Medical Physics, Biomedical Engineering, DOI 10.1007/978-1-4419-9779-1 9, © Springer Science+Business Media, LLC 2011

194	M.A. Haidekker and G. Dougherty

20 and 30 and declines as people age. Hormonal changes, most notably menopause, accelerate this decline. For the purpose of diagnosis, individual bone density is commonly compared to an age-matched reference collective. The World Health Organization defines osteopenia as a loss of bone density to one standard deviation below the age-matched mean (T-score of −1) and osteoporosis as a loss of bone density to below 2.5 standard deviations (T-score of −2.5). The major health concern is the risk of fracture. The relationship between reduced bone density and the incidence of fractures is well known [2–5].

Bone loss can be slowed or prevented. A diet rich in calcium and vitamin D, or dietary supplements thereof, reduce the risk of osteopenia and osteoporosis [6]. Strength-building exercise stimulates bone formation (see [7] for a critical review). Whereas calcium intake and exercise primarily improve the baseline, patients with a low T-score need to be treated with drugs that reduce bone deterioration, such as calcitonin or bisphosphonates.

The primary goal of the diagnostic procedures is to assess the degree of bone loss for a decision on possible treatment. Whereas calcium and vitamin D supplementation are widely recommended, the type and vigorousness of a possible exercise regimen strongly depends on the degree of bone deterioration. The use of drugs also depends on the diagnosis. In advanced stages of bone deterioration it is, therefore, crucial to establish the individual fracture risk.

Presently, the diagnostic process most commonly involves the measurement of bone density (see Sect. 9.2). However, bone deterioration that leads to osteopenia and osteoporosis is a complex process [2, 8] that affects bone microarchitecture. In fact, early studies show that osteoporosis is associated with a deterioration of the complex three-dimensional network of trabeculae, which form the weight-bearing component of spongy bone [9]. There is a discrepancy between the relatively low bone density gain of around 1% by exercise [10] and the strong reduction of fracture incidence [11]. The benefits of exercise clearly include improved muscular strength, dexterity, and range of motion, thus directly contributing to a lower incidence of falls, accidents, or fracture-causing motions. Conversely, treatment with fluorides has been shown to strongly increase bone density while not decreasing [12] or even increasing fracture incidence [13]. Similarly, observations have been made for drugs that enhance bone formation. Moreover, bone density has been shown to strongly overlap between patients with and without osteoporosis-related fractures. Clearly, bone density alone is not a sufficiently specific predictor of the individual fracture risk [14].

Bone is heterogeneous and biomechanically complex. Fracture-prone sites, such as vertebrae, wrist, femoral head, and calcaneus are composed of spongy bone, which is a three-dimensional strut-like network of trabeculae, and the surrounding cortical shell, which is composed of compact bone. Both parts contribute to the weight-bearing capacity of bone. The loss of bone density reflects both the deterioration of the cortical shell and thinning of the trabeculae in spongy bone. An early study by Rockoff et al. found that the compact bone of the cortical shell carried between 45% and 75% of the total mechanical load, and that the weightbearing contribution of the cortical shell increased with decreasing ash content [15].