X-ray or gamma ray systems or devices – Specific application – Fluorescence
Patent
1996-05-13
1998-06-30
Font, Frank G.
X-ray or gamma ray systems or devices
Specific application
Fluorescence
378 53, 378 54, G01N 2302, G01N 2306
Patent
active
057745200
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Because the probability of bone fracture is directly and sensitively dependent on bone density, physicians and radiologists require non-invasive measurement in vivo of the bone density of patients, in order to estimate bone strength, assess and evaluate the presence of osteoporosis (lower than normal bone tissue per unit volume), osteopomalacia (lower than normal bone mineral content), to predict future bone fracture risk, and to evaluate the efficacy of various drug or other bone therapy regimens. Current non-invasive techniques, such as traditional radiographic views, or more quantitative radiographic absorptiometric methods, including dual X-ray absorptiometry (DEXA), single- or dual- photon absorptiometry (SPA or DPA) using X-rays and low-energy gamma-rays, or single- or dual X-ray quantitative computed tomography(QCT, PQCT, or CT), provide some measure of in vivo bone mineral content, bone thickness, or estimates of bone density, but yield only limited information of dubious accuracy, inconsistency among techniques, and non-standardized results.
Present, non-invasive devices that estimate bone qualities (bone mineral content, bone density, bone thickness) in vivo use X-rays or low-energy (equal to or less than about 100-keV) gamma-rays to scan the body part of interest. The fraction of such rays absorbed is assumed to correlate with the density of bone and tissue in the region scanned. However, the absorption of these low energy rays depends not only on the density of the material present in the region scanned, but also on the effective atomic number of the constituents of these entities, the distribution of the different types of tissue (bone, fat, soft tissue, marrow, etc.), and the exact energy of the impinging radiation. These variables affect absorption in ways that cannot be readily disentangled using presently available absorptiometric devices, and, thus, have an adverse effect on the quality of the data.
Since low-energy photons are used in all present devices, absorptivity depends on both the density and elemental composition of each substance which forms a part of the object scanned, and can be dependent as well upon the exact order in which tissues of the various types are juxtaposed along the path of the photons employed in these scans. This multiple dependency requires that physical models (phantoms) predicting and representing the distribution of bone, fat, and muscle tissue be used as test samples in order to properly calibrate the instrument used to attempt to determine the actual in vivo distribution of bone, fat, and muscle tissue in the object region scanned.
Obviously, the phantom used may not actually mesh with reality for any given patient. Indeed, a good deal of research and development is being devoted to attempts to provide sets of phantoms which might bear a much closer correspondence to in vivo reality and which might serve to better cross-calibrate these various types of scanning instruments.
The low-energy X-rays and gamma-rays currently used in these devices are polychromatic; none utilizes a source of mono-energetic photons; some of these photon rays are made up of a relatively broad and continuous distribution of energies. As polychromatic radiation penetrates an object, the lower-energy components of these rays are preferentially absorbed relative to their higher-energy components, effectively increasing the average energy of the ray-beam as it passes through the object. This effect is progressively accentuated the lower is the average energy of the initial polychromatic radiation, and is particularly severe for the average initial energies of radiation employed in present absorptiometric devices. This change in average energy or the spectral distribution of each of these polychromatic rays is known as beam-hardening. Beam-hardening results in a given thickness of a particular tissue scanned deeper within an object (further from the photon source) presenting a lower absorptivity to the beam-hardened rays than would an identical thickness of
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Font Frank G.
Stafira Michael P.
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