Radiant energy – Calibration or standardization methods
Reexamination Certificate
2001-11-15
2004-10-26
Hannaher, Constantine (Department: 2878)
Radiant energy
Calibration or standardization methods
C250S370080
Reexamination Certificate
active
06809311
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved methodology of discriminating between desired and undesired readings or events measured by radiation detectors. Specifically, the method is intended for use in radiation detectors having non-Gaussian energy response functions.
BACKGROUND OF THE INVENTION
Gamma ray cameras are relatively well known devices used to detect gamma ray emissions from radioactive decay. A known gamma ray camera described in U.S. Pat. No. 3,011,057 for RADIATION IMAGE DEVICE, hereby incorporated by reference, uses a single sodium iodide (“NaI”) scintillation detector crystal to detect gamma ray emissions. The detector crystal is positioned to receive a portion of the gamma ray emissions from the radioactive decay. When a gamma ray strikes and is absorbed in the detector crystal, the energy of the gamma ray is converted into a large number of scintillation photons that emanate from the point of the gamma ray's absorption in the detector. A photomultiplier tube, optically coupled to the detector crystal, detects a fraction of these scintillation photons and produces an electronic signal that is proportional to the number of detected incident scintillation photons. The gamma ray camera typically has several photomultiplier tubes placed in different positions, with the signals from the different photomultiplier tubes being combined to provide an indication of the positions and energies of detected gamma rays.
Gamma ray cameras are frequently used in nuclear medical imaging to record pictures of a radiation field emanating from a subject's body. The gamma rays originate from a decay of a radioactive tracer that has been introduced into the subject's body. The radioactive tracer, such as
99m
Tc, is a pharmaceutical compound to which a gamma ray emitting nuclide has been attached and which undergoes some physiological process of interest after introduction into the body. For example, the tracer may accumulate in areas of high blood flow, thereby pinpointing areas of physiological activity.
Only substantially collimated gamma rays reach the gamma ray camera because a collimator usually is placed between the source of radiation and the scintillator. Of these collimated gamma rays, the gamma ray camera only detects the small fraction of gamma rays that impinge a detector, such as the above-described NaI detector crystal and photomultiplier tube assembly. From these detected gamma rays, the gamma ray camera typically selects a desired sample of gamma rays based upon their measured energy according to a process described in greater detail below. The gamma ray camera then records in an image memory the number of gamma events detected at each of a number of spatial locations corresponding to regions in the patient's body from which the gamma rays emanated. The data in the image memory is then used to form an image that corresponds to the distribution of the detected gamma events, thereby providing an image of the organ, tissue or body region of interest.
The quality of the gamma ray camera's image of the distribution of the radioactive tracer is dependent on the sensitivity of the detectors in the camera. As the sensitivity of the detector to gamma rays is increased, more gamma rays are detected and incorporated into the image, because the overall detection system sensitivity, combined with the activity of radioactive tracer presence in the body, determines the number of detected gamma ray events. Creating an image from a small set of gamma ray events may result in an inaccurate image because the small sample may not accurately represent the true distribution of the radioactive tracer within the subject's body. In particular, a reduction in the number of detected events, or “counts,” results in increased fractional statistical uncertainty because the emission and subsequent detection of gamma-rays are both random, stochastic processes. As a result, for any single gamma ray detector, the standard deviation in the number of counts in that detector increases, as a fraction or as a percentage, as the number of counts decreases. Therefore, the image would be more noisy in terms of image quality as a result of decreasing the total number of counts contributing to the image.
However, increasing the total number of counts by the inclusion of false events may degrade the image produced by the camera. One source of false events is the scattering of some of the gamma rays between the point of emission and exiting the patient's body. These scattered gamma rays do not provide an accurate indication of the distribution of the radioactive tracer within the patient's body because the scattering changes the gamma ray's direction and the measured energy. As a result, the inclusion of too many scattered gamma rays in the image memory causes the gamma ray camera to produce an inaccurate image with poor contrast.
Gamma ray detectors, therefore, seek to strike a balance between scattered and unscattered gamma rays without a loss of sensitivity. Typically, this result is accomplished by evaluating the energy levels of the detected gamma ray events and selecting those gamma ray events that fall within a preset “window” or range of acceptable energy levels. This technique is based on the assumption that gamma rays of abnormally low or high energy levels have been altered prior to detection or did not originate from the radioactive tracer of interest and, accordingly, do not provide reliable indications of the rays' direction of origination. The unscattered gamma ray events tend to fall within the window, whereas the scattered gamma ray events tend to fall outside the window. While a small number of scattered gamma rays would also fall within this energy window, these detected scattered gamma rays would be greatly outnumbered by the unscattered events. The gamma ray camera may vary the energy window as needed to achieve a desired result; for example, a narrower energy window may continue to accept a large number of unscattered gamma ray events while excluding more scattered events.
Frequently, the energy resolution of the gamma ray detector, such as the above-described NaI detector, can be characterized by a Gaussian distribution. The energy resolution of the gamma ray detector is then described by the full-width at half-maximum (“FWHM”) of the Gaussian shaped photopeak.
As explained above, gamma ray cameras generally use energy windows to discriminate between scattered gamma rays and gamma rays that reached the detector without interaction. The use of the energy window that defines a range of acceptable energy events is fairly effective in gamma ray cameras having Gaussian energy response functions because a large percentage of the desired gamma ray events are concentrated within a small range of energy levels. For instance, with a monoenergetic gamma ray emitting isotope in the absence of any scattered gamma rays, an energy window of approximately twice the width of the FWHM centered over the photopeak would select on the order of 99% of the total unscattered events.
However, the use of an energy window has the unavoidable disadvantage of excluding some of the desired gamma ray events while including some of the undesired gamma ray events, regardless of the width of the energy window. It is therefore the goal of the present invention to present an improved methodology to differentiate between the desired and undesired gamma ray events, without decreasing the sensitivity of the detector.
U.S. Pat. No. 5,561,297 for SCATTER CORRECTING GAMMA CAMERA, incorporated by reference herein in its entirety, discloses a method that improves the quality of an image by reducing the inclusion of scattered gamma rays into an image. The method measures the response of the imaging detector to substantially unscattered gamma rays, fitting a calibration function to the detector response to the unscattered gamma rays for each spatial location. The method then fits the response of the imaging detector to a field of both scattered and unscatter
Burckhardt Darrell D.
Engdahl John C.
Hannaher Constantine
Moran Timothy
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