X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
1999-01-19
2001-08-28
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S056000
Reexamination Certificate
active
06282258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of measuring bone mineral density (BMD) with x-ray fluoroscopic imaging equipment, and in particular to methods of measuring BMD in the limbs or extremities of a human or other animal with so-called mini C-arm x-ray fluoroscopic imaging systems, as well as to apparatus therefor.
2. Description of the Prior Art
In present-day medical practice, x-ray based systems are widely employed for bone densitometry (including measurement of BMD) , e.g. to diagnose, evaluate and/or monitor osteoporosis. Apparatus and procedures for x-ray bone densitometry are described, for example, in U.S. Pat. Nos. 4,947,414 and 5,040,199, the complete disclosures of which are incorporated herein by this reference. X-ray bone densitometry has heretofore typically used a scanning pencil beam or fan beam of x-rays and a point detector or a linear array of point detectors, although the aforementioned U.S. Pat. No. 5,040,199 also describes use of a cone-shaped beam impinging on an x-ray image intensifier to produce an optical image that is viewed by a television camera. In general, known x-ray bone densitometers are dedicated systems of large weight and bulk that must be fixedly installed at permanent, stationary locations, occupying substantial area, although a system has been proposed that can fold so as to pass through a typical hospital room door. Pending U.S. patent application No. 08/484,568, filed Jun. 7, 1995 and assigned to the same assignee as the present application, describes and claims beam flattening in a fan-beam x-ray bone densitometer, and pending U.S. patent application No. 08/484,484, also filed Jun. 7, 1995, and assigned to the same assignee as the present application, describes and claims scanning sequences (with a fan beam) that include dual energy scans for BMD and single energy scans for imaging; both of these pending applications are incorporated herein by this reference.
As distinguished from densitometers, x-ray fluoroscopic imaging systems provide images of bone and tissue similar to conventional film x-ray shadowgrams but produced by conversion of an incident x-ray pattern to a “live” enhanced (intensified) optical image that can be displayed on a video monitor directly, i.e., essentially contemporaneously with the irradiation of the patient's body or body portion being imaged; the term “fluoroscopic imaging” is used herein to designate such provision of directly video-displayed x-ray images. In some x-ray fluoroscopic imaging systems, the entire system is carried on an easily movable cart and the x-ray source and detector are mounted on a rotatable mini C-arm dimensioned for examining smaller body parts such as the extremities (wrists, ankles, etc.) of a human patient. An imaging device, including an image intensifier, suitable for use in such a system is described in U.S. Pat. No. 4,142,101, the complete disclosure of which is incorporated herein by this reference; one illustrative example of a currently commercially available mini C-arm x-ray fluoroscopic imaging system is that currently sold under the trade name “FluoroScan III” by FluoroScan Imaging Systems, Inc., of Northbrook, Ill., a subsidiary of the assignee of the present application. Mini C-arm x-ray fluoroscopic imaging systems are economical in space, conveniently movable (as within a hospital, clinic or physician's office) to a desired temporary location of use, and offer superior safety (owing to low levels of electric current utilization and reduced exposure of personnel to scatter radiation) as well as ease of positioning the source and detector relative to a patient's extremity for imaging.
SUMMARY OF THE INVENTION
The present invention, in one aspect, contemplates the provision of a method of measuring BMD with an x-ray fluoroscopic imaging system that includes an x-ray source for emitting a volume x-ray beam (such as a cone beam or a pyramid beam, as opposed to pencil beams and fan beams) propagating along a beam path, a detector on which the beam impinges over a two-dimensional area representing a field of view, the detector being spaced from the source for enabling a bone-containing human or other animal body portion to be interposed therebetween in the path so as to be irradiated by the beam and generating output data representative thereof, the output data being a 2-dimensional electronic representation of the aforesaid area containing sufficient information for display on a video monitor as a visible representation of the image. As used with a system of this type, the method of the invention comprises the steps of interposing, between the source and the detector in the path of the beam, a human body portion containing bone having a selected region of which the BMD is to be measured and a sample of bone of predetermined BMD; operating the system, while the body portion and sample are thus positioned, to generate output data, representative of one or more images of the body portion bone and sample, sufficient for calculation of the BMD of the selected region of the body portion bone; correcting the output data thus obtained for inherent variations in each of two orthogonal directions of the output data over the field of view to produce corrected data; and deriving from the corrected data a value representative of the BMD of the selected region of the body portion bone.
The term “sample of bone of predetermined BMD” as used herein includes bone-equivalent material, e.g. synthetic material, for instance material of a type such as is currently used in some bone densitometry systems. In some instances an additional sample of calibration material such as tissue or tissue-equivalent material may also be interposed in the beam path.
In the method of the invention, the correcting step preferably comprises subtracting, from the initial output data, correction output data generated by the detector, representative of one or more 2-dimensional images produced by irradiating the field of view with the beam under conditions of known x-ray attenuation in the beam path. For instance, the correction output data may be generated under the condition that only air is present in the beam path between the source and the detector. In currently preferred embodiments of the method of the invention, however, the correction output data generated by the detector are representative of one or more 2-dimensional images produced by irradiating, with the beam, an object interposed between the source and the detector and having a known x-ray attenuation in the beam path in each of two orthogonal directions which are transverse to the beam path. Conveniently, this object may be a homogeneous object of uniform x-ray thickness in the beam path.
As a further feature of the invention, in currently preferred embodiments, the selected region of the body portion bone and the sample are disposed side by side within the beam path so as to be respectively represented by separate areas in an image displayed on a video monitor as aforesaid from the detector output data.
A preferred procedure for performing the step of operating the system to generate output data includes operating the system to generate first output data representative of at least one image of the body portion and sample produced while the source is emitting an x-ray beam at a first x-ray energy level (L) and second output data representative of at least a second image of the body portion and sample produced while the source is emitting an x-ray beam at a second x-ray energy level (H) higher than the first energy level (L). In these embodiments, the correction step preferably includes subtracting, from the first output data, first correction output data representative of at least one image of a known object interposed between the source and the detector, e.g. an object having uniform x-ray attenuation, produced while the source is emitting an x-ray beam at the first energy level (L) and subtracting, from the second output data, second correction output data representative of at least
Quinn Vincent E.
Shepherd John A.
Stein Jay A.
Weinstein Joel
Weiss Howard P.
Cooper & Dunham LLP
Hologic Inc.
Porta David P.
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