Bone
Mineral Densitometry (BMD) is the method used to quantify the mass of bone in the body.
Patients who have been referred by their doctors for a BMD scan are usually being assessed
for osteoporosis. Osteoporosis is a common bone disease which makes bone fragile and easy
to fracture. Future risk of fracture can be predicted for through a BMD measurement and
steps can then be taken to prevent them from happening.
Measurement
techniques include Single Photon Absorptiometry (SPA), Dual Photon Absorptiometry (DPA),
Dual energy X-ray Absorptiometry (DEXA), Quantitative Computed Tomography (QCT) and
Quantitative Ultrasound (QUS).
Quantitative
Computed Tomography (QCT) provides a cross-sectional or 3-dimensional image from which the
bone is measured directly, independent of the surrounding soft tissue. Dual photon
absorptiometry (DPA), measures the bone by computing the difference in absorption of
low-energy photons and high energy photons by the mixture of soft tissue and bone in the
path of the beam, and can generate a 2-dimensional image for localization of the bone.
Single photon absorptiometry (SPA) computes bone mineral from the increased absorption of
the beam as it passes from a constant thickness of soft tissue or water bag into the bone.
DEXA has evolved from SPA and DPA, both of which use radioactive sources (eg. Gd-153), while
DEXA employs X-rays.
Non absorptiometric methods such as ultrasound of bone (QUS) do not measure bone density
directly, but give alternative information about properties of bone such as the speed of
sound (SOS) and the broad band ultrasound attenuation (BUA). In some cases these values
are linearly combined to form a Stiffness Index.
The
devices used to measure bone density are generally known as ‘bone
densitometers’. The bone densitometers used in the Nuclear Medicine Department at
Liverpool Hospital is the DPX-IQ (Lunar) which uses the Dual Energy X-Ray Absorptiometry
(DXA) technique, and the Achilles+ (Lunar) Heel Densitometer which uses the Quantitative
Ultrasound (QUS) technique.
No preparation is required.
Usually two sites are measured on the DEXA: the lumbar spine and the hip.
All metallic objects are removed first, eg, belts, zippers, etc.
The patient lies on a scanning bed in a position determined by the operator.
Low energy x-rays are then passed through the patient’s body in a rectilinear fashion. The detectors in
the arm of the scanning bed move across the patient and measure the outgoing x-rays. The patient is relaxed and still throughout the scan.
One site is measured using the QUS - the heel / calcaneus.
The patient is required to insert their foot into a warm water bath. Ultrasound waves are then
transmitted through the heel of the foot via fixed transducers. The movement of the foot is restricted throughout the measurement by a shim.
The BMD measurement is statistically compared to a group of normal, young adults (T-Score) and to an age,
weight and ethnically matched group (Z-Score). These scores are then used to indicate (or
exclude) osteoporosis or osteopenia in the patient.
The World Health Organisation (WHO) defines these scores as:
- Normal: T-Score > -1 S.D. (standard deviation)
- Osteopenia: -2.5 S.D. < T-Score < 1 S.D.
- Osteoporosis: T-Score < -2.5 S.D.
- Severe Osteoporosis: T-Score < -2.5 S.D. with fragility fracture(s)
