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How does it work?
In bone mineral density (BMD)
scanning, the energy of x ray beams that are passed through bones is
absorbed, and what is not absorbed is detected on the other side of
the body. The more dense the bones (from greater mineral content),
the more energy is absorbed, and the less energy detected.
The radiation energy per
pixel ("picture element") is detected and converted into an "areal density"
measured in g/cm2. The number of pixels in the area is
summed, then the amount of bone in each pixel is calculated. This
allows a bone density to be calculated for the specific bone in
question.
Basic BMD measurement
requires the patient to be "on the table" for about 2 minutes, undressed to
light clothing, and with no metal piercings. Navel piercings can be a
problem, because they cover the vertebra L4, which is a common site
to scan.
In modern scanners, x
ray beams of two different energies are used (dual x ray absorptiometry),
giving a dose up to that of a chest x ray. If the whole
skeleton is being examined a higher dose is received because of the
greater area exposed (about 1.5 chest x rays). Two energies
allow an estimate to be made for soft tissue absorption separately
from that of the bones.
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Values for bone density may
be quoted as g/cm2 or converted into values related
to the average female (or male) peak bone mass or to the bone mass
related to the patient's age. These are T scores and Z scores, and
involve the following calculations.
In women, the peak lumbar spine density is maximal in the fourth decade of life. Osteoporosis is diagnosed if the T score is over -2.5 according to the WHO (1994).1 Z scores may be used to monitor long term follow up of treatment.
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Calibrations for average
bone densities are often based on a database of the upper femur called the
NHANES database.2
The x rays are
directed anterior-posterior or vice versa, depending on the instrument. There is
little deflection of beam to the operator, but with faster fan beam
machines deflection is greater. Fan beam and pencil beam machines can
scan laterally around the side of a patient, which is useful for
measuring the bone density of the lumbar spine.
In elderly people, where
the lowest ribs often cover L2 and the iliac crests cover L4, L3 may be the only
vertebra that can be scanned in the lateral position.
For femoral examinations,
the measurement most frequently used at present is known as the "total upper
femur." This is an easy measurement for technicians to reproduce, and
includes the femoral neck, trochanteric region, and inter
trochanteric region.
Femurs are examined with
patients lying flat on their back with toes together and a heel separation of
23 cm. 
The spine is examined with
patients lying on their backs and their knees flexed over a block at right
angles to flatten out the lumbar lordosis. The maximum weight on a
scanning table is 136 kg.
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Future machines will be
able to send the data directly from a scan into the clinic for diagnostic or
treatment decisions to be made.
Abi Berger
BMJ
References
| 1. | Kanis JA, Melton 3rd LJ, Christiansen C, Johnston CC, Khaltaev N. The diagnosis of osteoporosis. J Bone Miner Res 1994; 9: 1137-1141[Medline]. |
| 2. | Looker AC, Wahner HW, Dunn WL, Calvo MS, Harris TB, Heyse SP, et al. Updated data on proximal femur bone mineral levels of US adults. Osteoporos Int 1998; 8: 468-489[Medline]. |
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