Chapter 19: Bone Densitometry

Detailed Chapter Overview

Chapter 19 provides a comprehensive and detailed examination of bone densitometry, a specialized quantitative imaging field focused on the measurement of bone mineral density (BMD). This chapter is critically important in the context of public health, as it addresses the diagnosis and management of osteoporosis, a widespread and silent disease that leads to an increased risk of debilitating fragility fractures. The central theme of the chapter is the role of Dual-Energy X-ray Absorptiometry (DXA) as the undisputed gold standard for measuring BMD. The text delves deeply into the underlying principles of bone biology, explaining the continuous process of bone remodeling and how an imbalance in this process leads to osteoporosis. It meticulously explains the physics of DXA technology, detailing how the use of two distinct x-ray energy levels allows for the accurate isolation and measurement of bone mineral content, separate from surrounding soft tissue. A significant portion of the chapter is dedicated to the interpretation of DXA results, providing a thorough explanation of T-scores and Z-scores, the statistical tools used to classify a patient's bone density and predict their future fracture risk. The chapter provides exacting, step-by-step instructions for positioning the lumbar spine and proximal femur, the primary sites for diagnostic measurement. For every aspect of the procedure, from patient preparation to quality control, rigorous protocols are established to ensure precision and accuracy, which are paramount for monitoring changes in a patient's bone density over time.

In-Depth Study Guide

Understanding Bone Health and Osteoporosis

Bone Remodeling and Bone Mass

• Bone Remodeling: Bone is a dynamic, living tissue that is constantly undergoing a process of removal and replacement known as remodeling. This process involves two types of cells: **osteoclasts**, which break down and resorb old bone, and **osteoblasts**, which build new bone.
• Peak Bone Mass: Individuals build bone mass throughout childhood and adolescence, reaching their "peak bone mass"—the maximum bone density and strength—in their late 20s or early 30s. After this point, the rate of bone resorption gradually begins to exceed the rate of bone formation, leading to a slow decline in bone mass.

Osteoporosis: The "Silent Disease"

• Definition: Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture. Essentially, the bones become weak, brittle, and porous.
• Clinical Significance: It is called the "silent disease" because there are typically no symptoms until a fracture occurs. These **fragility fractures** can happen from minimal trauma, such as a fall from standing height or even from a strong sneeze. Common sites for osteoporotic fractures include the hip, spine (vertebral compression fractures), and wrist.
• Risk Factors: Major risk factors include being female, advancing age, post-menopausal status (due to estrogen deficiency), family history, low calcium/vitamin D intake, a sedentary lifestyle, smoking, and excessive alcohol consumption.
• Osteopenia: This is a precursor to osteoporosis, indicating low bone mass that is not yet severe enough to be classified as osteoporosis. Identifying and treating osteopenia can prevent the progression to full-blown osteoporosis.

The Technology of Bone Densitometry: DXA

Principles of Dual-Energy X-ray Absorptiometry (DXA)

• The Gold Standard: DXA is the most widely used and validated method for measuring bone mineral density (BMD).
• The "Dual-Energy" Concept: The core of DXA technology is the use of two different x-ray energy beams (a high-energy beam and a low-energy beam). Soft tissue and bone attenuate these two beams differently.
  • Soft tissue absorption is roughly the same for both beams.
  • Bone, being much denser, attenuates the low-energy beam significantly more than the high-energy beam.
• Calculation of BMD: The scanner measures the amount of each x-ray beam that passes through the patient. By analyzing the difference in attenuation between the high and low energy beams, the computer software can mathematically subtract the contribution of the soft tissue, leaving a highly accurate measurement of the mineral content of the bone. The result is typically expressed in grams per square centimeter (g/cm²).
• Advantages of DXA: It is highly precise and accurate, has a very low radiation dose (significantly lower than a chest x-ray), and is a quick, non-invasive procedure.

Understanding the DXA Report: T-scores and Z-scores

The raw BMD value (g/cm²) is not easily interpretable on its own. Therefore, it is converted into standardized scores for diagnosis.

The T-score: The Primary Diagnostic Tool

• Definition: The T-score compares a patient's measured BMD to the **average peak bone mass of a healthy, young-adult, sex-matched reference population**. It is expressed in units of standard deviations (SD).
• Interpretation: A T-score of 0 means the patient's bone density is equal to that of a healthy young adult. A negative score means their density is lower; a positive score means it is higher.
• World Health Organization (WHO) Diagnostic Criteria:
  • Normal: T-score of -1.0 or higher.
  • Osteopenia (Low Bone Mass): T-score between -1.0 and -2.5.
  • Osteoporosis: T-score of -2.5 or lower.

The Z-score: Comparison to Peers

• Definition: The Z-score compares a patient's measured BMD to the **average BMD of a person of the same age, sex, and ethnicity**.
• Clinical Use: The Z-score is not used to diagnose osteoporosis in post-menopausal women or men over 50. Its primary use is for evaluating pre-menopausal women, men under 50, and children. A very low Z-score (e.g., -2.0 or lower) in these populations may suggest that something other than normal aging is causing bone loss, prompting further investigation for secondary causes of osteoporosis.

DXA Scanning Procedures and Positioning

Accuracy and precision are paramount in DXA. Small errors in positioning can lead to significant errors in BMD measurement, affecting diagnosis and treatment decisions.

1. Lumbar Spine (AP Projection)

• Purpose: To measure the BMD of the L1-L4 vertebral bodies.
• Positioning: The patient lies supine on the scanner table. The technologist places a large, square positioning block under the patient's lower legs. This serves two critical functions: it flexes the hips and knees, which **flattens the natural lumbar lordosis**, making the vertebral bodies more parallel to the tabletop and opening the intervertebral disk spaces for easier analysis by the software.
• Evaluation Criteria: The scan must include the L1 through L4 vertebrae, centered in the image. The spine should be straight with no rotation. The iliac crests should be visible at the bottom of the image.

2. Proximal Femur (Hip)

• Purpose: To measure the BMD of the femoral neck and total hip region, which are common sites of osteoporotic fractures.
• Positioning: The patient lies supine. The key to this position is **internally rotating the entire leg 15 to 25 degrees**. This rotation overcomes the natural anteversion of the femoral neck, placing it in a true AP profile and making it appear longer and more parallel to the detector. A special positioning device (a triangular block with straps for the foot) is used to maintain this precise degree of internal rotation.
• Evaluation Criteria: The femoral neck should be seen in its full length without foreshortening. The lesser trochanter should be minimally visible or not visible at all, confirming correct internal rotation. The greater trochanter should be seen in profile.