Science of DEXA

Question 1

What is Osteoporosis

Answer 1

A health condition that weakens bones, making them fragile and more susceptible to fractures

Question 2

Risk factors of Osteoporosis (4)

Answer 2

Menopausal related deficiency in oestrogen
Calcium deficiency
lifestyle
Genetic predisposition

Question 3

Osteoporosis on x-ray (3)

Answer 3

Impression of decreased bone density
Loss of mass on bony trabeculae (inner) and not the cortex (outer)
Compression fractures to vertebral bodies can be detected

Question 4

What is DEXA (5)

Answer 4

Dual-energy x-ray absorptiometry
Bone is isolated from soft tissue attenuation by scanning with 2 different x-ray energies: a fan beam and pencil beams
The denser the bone, the fewer x-ray photons transmitted
Information based on attenuation extracted and computed for data analysis
Photons transmitted are measured against pre-determined standards for young healthy adults

Question 5

Data analysis in DEXA (2)

Answer 5

Includes a T-score and Z-score

Question 6

What is a T Score

Answer 6

an indication of the number of standard deviation between the measurement and the standard data.
Indicates fracture risk

Question 7

What is a Z Score

Answer 7

compares the individual measurement to a data mean for a similarly aged population.
Indicates need for further evaluation of reasons for osteoporosis

Question 8

How DEXA works (3)

Answer 8

• X-ray photons of two different energies from 2 different sources in an alternating fashion at set frequencies are directed toward the patient lying below a detector
• Pencil beam, Fan beam technology require the linear displacement of the detector which moves backward and forward under the patient’s body via a C-arm assembly.
• Results are derived from the photons hitting the detectors, not from the scan image. In DEXA the image is only used to confirm patient position

Question 9

Limitations and issues with DEXA scanning (4)

Answer 9

• Object to image receptor distance: affects size distortion on the resulting image
• DEXA is of limited use in people with spinal deformity or those who have had previous spinal surgery/artefacts
• Very low resolution: cannot be used for diagnostic apart from bone mineral density (BMD) measurements
• Does not determine the reason for low BMD

Question 10

Advantages of DEXA (6)

Answer 10

• Inexpensive
• Improving reliablity
• Small radiation dose
• Stable calibration
• Sensitive and accurate in measurements of subtle changes in BMD over time or following drug therapy
• Can predict risk of future fractures

Question 11

Disadvantage of DEXA (4)

Answer 11

• Low resolution images with single parameter investigation
• 2D images
• Cannot distinguish between corticle and trabecular bone
• Manufacturer inconsistencies

Question 12

Future developments (6)

Answer 12

• Mainly new application coupled with advances in technology:
• Abdominal aorta calcification assessment
• Fractuer risk assessment - vertebral fracture assessment
• Computer aided fracture assessment
• Body composition
• Automated hip implant analysis