RE: bone density, principle and practice -- long reply
Teri wrote: "One of my post-docs is interested in studying bone density.
Besides using radiography, what are some other methods (and possible
references) for this?
Thank you, as always!"
Manager Histology Core Facility
Stowers Institute for Medical Research
1000 E. 50th St.
Kansas City, Missouri 64110
Teri and interested others:
Bone density can be measured using an instrument and computer program
collectively referred to as Dual-Energy X-ray Absorptivity (DEXA for
short).The principle is that x-rays deflect from bone (and other radiodense
objects) in patterns specific to their density. DEXA units apply two x-ray
exposures (delivered by fan-beam or pencil-beam technology) sequentially to
an object and records the pattern of scatter as black (no scatter) or
degrees of white (scattered) pixels in a computer generated image. The DEXA
unit then adjusts the x-ray intensity and repeats this dual-exposure three
more times. The computer collects the images, overlays them, calculates the
varing density across the object pixel by pixel and displays a summary
image. The computer also calculates the relative density of the object
compared to a standard object you calibrate the instrument with. The
displayed image includes:
Bone mineral density (BMD) units are grams/cm2: a value calculated by
expressing the Bone mineral content (BMC) per object area in squared
Bone mineral content (BMC) units are grams: a computer calculated value
based entirely on the comparison of the collected scatter patterns directly
and individually to the standard calibration object. This value is expressed
as grams of mineral content. It's accuracy is dependent on the quality of
the calibration and the calibration object.
Area (cm2): Two definitions; one fixed the other operator defined.
Fixed or Total Area: The area of the entire beam path in squared
centimeters. A value fixed for each instrument by the physical size of its
Region of Interest Area: Some DEXA programs allow the operator to
select a region of interest (ROI) in the object to examine more closely. The
operator selects the pixel area for use in the calculation of BMD by drawing
a box around the portion of the object they wish more detail on. The
Computer calculates the mineral content of this defined pixel area against
the values for the calibration standard and calculates the BMD based on the
mineral content for that portion of the total pixel grid included in the
defined region. The accuracy of the BMD calculated for the ROI is dependent
then on the operator's accuracy and precision in drawing the pixel box.
There are several manufacturers for DEXA units. Our's in a Lunar Piximus
sized for accuracy in measuring BMD in mice. We also measure rats, rabbits
and other small objects using this instrument. Larger units are available
for measurements on large animals and human subjects. Your post-doc will
have to investigate the availibility of using an instrument nearby or send
the samples to a facility to do the measurements on a contract basis. We
operate such a facility as do others.
Hope this helps, Donna Montague, M.S.
Physiology/Biophysics and Orthopaedic Surgery
University of Arkansas for Medical Sciences
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