X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1999-10-13
2001-09-04
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S101000, C378S102000, C378S106000, C250S367000, C250S370090, C250S370110
Reexamination Certificate
active
06285740
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to x-ray systems for determining bone mineral density and for other applications, and to methods using such systems.
RELATED ART
Precision bone mineral densitometry is important for the early detection of osteoporosis and the prediction of future bone fracture risk. Bone mineral loss is associated with aging and is more rapid in post-menopausal women. In addition, bone mineral loss is accelerated during long-term bed rest and in the weightless environment of space.
Prior clinical studies indicate that the association of measured bone density with osteoporosis and bone fracture is more significant for the major weight-bearing axial sites (lumbar spine and proximal femur) than for extremity sites (hand, radius, and calcareous). The bone mineral content is correlated with the vertebral strength determine in vitro. Clinical studies also indicate that the preferred site for bone mineral densitometry may be the lumbar spine, and the preferred view is lateral rather than anterior-posterior (AP). The lateral view of the vertebrae is not obscured by the posterior vertebral elements. This permits the isolation of the trabecular bone region that is most susceptible to mineral loss. However, soft tissue absorption can affect lateral projection measurements to a greater extent than AP projection measurements.
It has been estimated that a measured bone mineral density (BMD) that is one standard deviation (approximately 2%) below the average for a control population implies a significantly higher risk of a future bone fracture. Thus, any bone densitometry technique should have at least 2% precision and absolute accuracy.
In many clinical settings, it is important to determine BMD with a short patient observation time. In addition, since BMD measurements may be repeated over a period of time, the exposure to the patient should be as low as possible for each observation. Although ultrasonic and magnetic resonance methods are convenient and have no x-ray exposure, such methods do not have the required accuracy at the present time.
The rather high precision and accuracy required for bone densitometry has resulted in the development of x-ray radiographic absorption techniques. Bone densitometry apparatus for the hand, radius, and calcareous have been developed. However, these measurements are not as significant as axial site measurements for diagnosing osteoporosis and predicting fracture risk.
Considering some of the techniques that have been used, quantitative computed tomography (QCT) x-ray scanning techniques produce three-dimensional images of skeletal regions. This permits the elimination of soft tissue attenuation and a precise determination of the BMD of interior trabecular bone regions. However, QCT techniques are not suitable for repeated clinical measurements because of the high patient dose (surface dose ~100 mrem), the long patient positioning and immobilization time (tens of minutes), and the relatively high cost of the apparatus.
Dual-energy x-ray absorptiometry (DEXA) projection scanning units have been developed by Hologic Inc. (Waltham, Mass.) and Lunar Corp. (Madison, Wis.). Using dual-energy calibration and subtraction algorithms, the attenuation by soft tissue is effectively eliminated from the measurement, and the BMD of axial sites can be precisely and accurately determined. In the case of lumbar spine and proximal femur measurements, clinical studies indicate a typical precision of 1-3% and an absolute accuracy of 4-15%. The patient surface dose is typically of order 10 mrem while the patient positioning and observation time is approximately 10 minutes.
Lumbar vertebrae have been characterized in vitro by ash mass, metrology, and bone densitometry techniques. The directly measured quantities have been compared to the equivalent quantities determined in vitro by non-invasive techniques. The measured ash mass (mineral content excluding water, soft tissue, and other combustibles) is in the range 5-15 g. The ash mass is equivalent to the bone mineral content (BMC). The volume measured by metrology is in the range 30-50 cm
3
, and this quantity may be determined by QCT techniques. The projected area measured by metrology is in the range 11-17 cm
2
, depending on the viewing direction, and can be determined by radiographic projection techniques. The ash mass to volume values are in the range 0.20-0.35 g/cm
3
, and this quantity is equivalent to the bone mineral volumetric density determined by QCT. The ash mass to projected area values are in the range 0.4-1.2 g/cm
2
. This quantity is equivalent to the bone mineral areal density that is determined by most DEXA instruments, and this quantity is usually referred to as the bone mineral density (BMD).
The primary goal for a DEXA instrument is to determine the BMD to a precision of 2%. In addition, the absolute BMD, typically in the range 0.4-1.2 g/cm
2
, should be determined to an accuracy of 2%. The primary difficulty is to account for the soft tissue areal density, along the same line of sight, with values up to 20 to 30 g/cm
2
in the case of the lumbar spine.
In general, a dual-energy x-ray distribution useful in BMD measurements may be enhanced by switching the source voltage (developed by Hologic Inc.), changing the filtration (developed by Lunar Corp.), or a combination of these two techniques. Previous studies that were based on x-ray tube loading, x-ray quantum noise, and patient exposure established that x-ray energies in the two ranges 40-60 keV and 80-130 keV are suitable for dual-energy x-ray absorptiometry. Most DEXA systems are limited by x-ray quantum noise, and other noise sources (such as phosphor, detector, and electronics) are smaller. As indicated above, patient exposure is of primary concern, although the exposure provided by DEXA systems is usually lower than the exposure from other radiology procedures and from the natural background.
SUMMARY OF THE INVENTION
In accordance with the invention, a system is provided which employs a pulsed, portable hard and soft x-ray source which is useful in medical imaging in general, and BMD measurements in particular, as well as in flash x-ray absorptiometry. As discussed below, the x-ray source produces a single very short (10 to 200 nanosecond) pulse, and this is of obvious advantage in treating patients, particularly as compared with systems such as those described above which involve substantially longer patient exposure times. Further, the loading on the x-ray tube is minimal during the single x-ray pulse, thereby permitting operation of the x-ray tube at higher voltage and power levels than is possible with conventional x-ray tubes. This is important in patient diagnosis because a higher x-ray number density reaches the associated detector, thereby improving the x-ray quantum signal to noise ratio and reducing the dose absorbed by the patient.
According to one aspect of the invention, a dual-energy x-ray system is provided which comprises: a dual energy x-ray source, comprising a Marx generator and a field emission x-ray tube driven by the Marx generator, for producing a single x-ray pulse providing x-ray energy of a first value early in the pulse and x-ray energy of a second, lower value later in the pulse so as to provide a dual energy level x-ray distribution comprising hard and soft x-rays; and a detector system, having dual x-ray energy discrimination properties, for receiving said pulse and for discriminating between the dual x-ray energy levels of the pulse.
Preferably, the detector system comprises, in tandem, a soft x-ray detector, a hard x-ray detector and an inter-detector filter disposed between the soft and hard x-ray detectors for filtering soft x-rays passing through the soft x-ray detector.
A pre-filter at the x-ray tube output is preferably provided for filtering x-rays produced by the x-ray tube to enhance the dual level x-ray energy distribution, harden x-ray fluence and reduce the x-ray dosage to an object receiving the x-ray pulse.
In one preferred embodiment, the soft x-ray detector comprises
Boyer Craig N.
Holland Glenn E.
Seely John F.
Ferrett Sally A.
Ho Allen C.
Karasek John J.
Kim Robert H.
The United States of America as represented by the Secretary of
LandOfFree
Dual energy x-ray densitometry apparatus and method using... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dual energy x-ray densitometry apparatus and method using..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dual energy x-ray densitometry apparatus and method using... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2532944