X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
1999-03-02
2001-11-20
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S054000
Reexamination Certificate
active
06320931
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
A low cost tabletop x-ray bone densitometer capable of measuring bone density in the human body.
2. Description of the Prior Art
Bone density has been directly associated with bone strength and the risk for non-traumatic fractures. Early detection of low bone mass and the application of appropriate therapies is of significant medical value. The ability to monitor therapy effectiveness by detection of small changes in bone density is also of value. Because of the vast need for diagnosis and the high cost of existing devices, there is an unmet need for a low cost bone densitometer with adequate sensitivity for widespread screening of patients at risk for osteoporosis. The immediate readout, low cost, ease of use, and method of calibration provide important advances to the prior art. This invention has the potential to readily provide bone densitometry tests to millions of patients who currently do not, or can not, afford the more involved and costly exams.
Various non-invasive methods for bone density measurements have been developed. These include Quantitative Computed Tomography (QCT), Single Photon and Dual Photon Absorptiometry (SPA and DPA), Dual Energy X-ray Absorptiometry (DXA), peripheral dual energy x-ray absorptiometry (pDXA) and Radiographic Absorptiometry or Micro-densitometry (RA). All of these techniques utilize the difference in x-ray or gamma ray attenuation of bone and soft tissue components. Use of dual energy methods improves on the ability to separate overlying soft tissues from bone in the measurement. The different techniques are largely separated based on the particular bone to be measured and the quantity of surrounding soft tissue. For example, QCT measures the central bone of the spine, which is surrounded by much tissue, while pDXA typically measures the radius, which has little overlying tissue. Osteoporosis is believed to be a systemic disease process, but it is well known that different regions of the skeleton lose bone at different rates.
The QCT method uses a x-ray CT Scanner to generate a cross sectional slice through three to four lumbar vertebra. Typically a bone and tissue equivalent phantom is scanned simultaneously with the patient to provide calibration.
With the SPA and DPA methods, x-rays from a radioactive source such as Gd153 are employed as the source. These methods allow measurements at both the extremities and central skeleton sites and, in the case of DPA, uses dual energy techniques. These techniques have largely been replaced with the newer DXA devices due to the long exam times and need to periodically replace the isotope source.
DXA and pDXA have become widely used in clinical practice. The use of filtered x-ray sources, in place of radioactive isotopes, has improved precision, exam speed and long term reliability. DXA techniques use a scanning x-ray source in a rectilinear fashion to cover the target body part. An image of the body is created showing regions of bone and soft tissue and requires software to find the bone images and edges of bone for measurement. In the DXA approach, the x-ray tube and a point imaging detector are scanned over the target region in a roster fashion resulting in the point-by-point transmission of the beam through the bone and surrounding soft tissue. Point source or more recently fan beam geometries and line detector arrays are used requiring some significant time to complete the mechanical scan. In addition, a mechanical structure is required to reproducibility move the x-ray source and x-ray detectors over the body. The measurement Region of Interest (ROI) is determined in software by finding bone regions. The x-ray source output must be maintained within close limits during the scan, placing stringent electrical stability requirements on both the high voltage power supply and the x-ray tube current throughout the scan. In addition, electrical analysis of pulse height and energy separation is required in the x-ray pulses used to create the images. DXA nevertheless provides bone mineral density, (“BMD”), measurements throughout the skeleton system with low radiation dose and good precision. Whole body DXA devices are expensive and generally require a room size facility for their operation. In response to these limitations, pDXA (peripheral DXA) devices have been developed more recently which are smaller and less expensive. The measurement target is usually the distal radius, although the calcaneous is also measured. One challenge of pDXA devices is the need to reproduce the measured region of interest (ROI) precisely, to maintain high precision in follow up clinical studies. Although of smaller size and lower cost, pDXA devices also use rectilinear scanning techniques as in whole body DXA devices, thus requiring a more complex mechanical design and associated expense. PDXA devices also create an image of the target bone requiring software to define the measurement ROI. PDXA devices require similar controls as the larger DXA machines, on the power supply and detector response over the entire scan time. Scanning pDXA devices remain relatively expensive for widespread clinical use in primary care physician offices.
In recent years, the older technique of microdensitometry has been revived as radiographic absorptiometry (RA). A representative system is marketed by COMPUMED, which provides a mail order service to analyze the x-ray films and provide clinical reports. With RA, a finger bone is exposed simultaneously with a reference aluminum step wedge to x-rays. The image is recorded with direct exposure x-ray film and processed in a standard film processor. The developed films are then scanned with a photo densitometry to record the density patterns of both the target bone and the aluminum step wedge. The aluminum step wedge becomes the calibration phantom for reference to the bone absorption. Although the initial cost to use the service is low (cost of the aluminum step wedge and x-ray film), this approach has several major drawbacks. First, the user must have a complete x-ray system, x-ray generator and x-ray tube, available for use as well as a film processor. These are relatively expensive devices, and are not available in the vast majority of primary care physician offices. The scanning photo densitometer to read the films must have acceptable performance and reliability, thus adding additional costs. Film processors can vary in performance, sometimes producing streaks and artifacts, which need to be accounted for at the film scanning stage. The exposed and processed films must be mailed to a central processing facility, necessitating important delays in obtaining the final clinical results. The use of aluminum to calibrate for bone density is far from optimal. Although aluminum is close to bone in physical density, if differs in atomic composition, and is not an adequate reference for bone for highly consistent results. Human bone is composed largely of calcium hydroxyapatite in the presence of soft tissue components, blood and fat. The x-ray attenuations of these tissues are dependent on their physical density, their effective atomic numbers, and the energy of the x-ray beam. The x-ray beam spectrum produced by x-ray systems is dependent upon many variables, including the primary kilo voltage applied to the tube, its waveform, inherent and added filtration, x-ray tube target angle, x-ray tube aging and target changes with over exposures, and in some cases, the quality of the line voltage and its stability. In short, x-ray systems from different manufacturers, and in use at different clinics, produce different x-ray beam spectra. These differences in beam energy are important for highly precise quantitative measurements. We have shown with earlier measurements that aluminum may have limitations for accurate calibrations of bone across beam energy changes which may occur with different x-ray systems in use at various radiographic clinics.
The radiation beam in bone densitometers is typically collimated to the desired region of interest before
Image Analysis, Inc.
Knobbe Martens Olson & Bear LLP
Porta David P.
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