Reduction of image artifacts caused by Compton scatter

A method of producing practically Compton free images of a patient with a gamma ray imaging system, said method comprising the steps of:

• determining a trial function, said step of determining a trial function comprising the steps of:
• multiplying a known system energy spread function by an unknown count number due to unscattered photons,
• convolving at least one order of energy distribution of the scattered photons derived from the probability the physical interaction of the photons, as determined by physical relativistic Compton characteristics of the scattered photons and the known system energy spread function to provide a system dependant set of Compton fit functions,
• multiplying the system dependant Compton fit functions by unknown numbers corresponding to counts of scatteres photons,
• measuring locally an energy spectrum of the patient that includes total counts due to scattered and to unscattered photons,
• locally fitting the trial function to the measured energy spectrum to determine the unknown count number due to unscattered photons, and
• using the count number due to the unscattered photons to provide the practically Compton free images.

A system for reducing the contribution of Compton scattered photons on an image produced by a gamma ray imaging system, said system comprising:

• means for detecting as counts photons impinging on a gamma ray detector at X,Y locations of impingement responsive to gamma radiation from a source,
• means for determining an X,Y location for each detected count according to the location of impingement on the detector.
• means for measuring the energy of the impinging photons,
• a plurality of memory matrices comprised of energy windows that together span an energy range, each of said windows of said matrices extending over a portion of said energy range,
• means for inserting each detected count in a window of a memory matrix of said plurality of memory matrices according to the measured energy of the impinging photon, said matrices each located at spatial X,Y coordinates, said detected counts being inserted at the X,Y coordinate location according to the location of the impingement on the detector,
• said memory matrices accumulating counts at each of the X,Y coordinate locations.
• means for constructing a measured energy spectrum at each X,Y coordinate location using the accumulated counts per window of said matrix per location, said measured energy spectrum including counts produced by scattered and unscattered photons,
• means for obtaining the counts per X,Y locations produced by only the unscattered photons,
• said means for obtaining the counts per X,Y locations produced by only unscattered photons including:
• means for calculating energy distributions of one or more orders of Compton scattered photons using physical relativisic Compton characteristics,
• means for convolving the energy distribution of the one or more orders of Compton scattered photons with the energy spread function of the system to obtain system dependant Compton energy distributions,
• means for constructing a trial function comprising the system energy spread function multiplied by an unknown coefficient of the unscattered photons plus unknown coefficients of the scattered photons multiplied by the system dependant Compton energy distribution,
• means for locally fitting the trial function to the measured energy spectrum to obtain the count of the unscattered photons by solving for the unknown coefficient of the unscattered photons, and
• means for using the count of the unscattered photon to produce a practically Compton free image.

A gamma ray imaging system that reduces the contribution of scattered Compton photons on produced images, said system comprising:

• means for detecting as count photons impinging at X,Y coordinate locations in a gamma ray detector responsive to gamma radiation from a source,
• means for determining an X,Y coordinate location per each detected count according to the location of the impingement of the photons on the detector,
• means for measuring the energy of the impinging photons,
• a plurality of memory matrices each having compartments corresponding to a plurality of energy windows, said plurality of energy windows spanning an energy range,
• means for placing each detected count in one of the plurality of compartments according to the measured energy of the impinging photon, said memory matrices each located at X,Y coordinates, said matrices each being digital images and said X,Y coordinates corresponding to the location of the impingement of the photon on the detector and to a pixel in the images,
• means for providing a measured energy spectrum per X,Y location using the counts accumulated in each of the X,Y coordinate locations of the matrices, said measured energy spectrum including counts produced by both scattered and unscattered photons.
• means for calculating an energy distribution of Compton multi-scatter components using physical relativistic Compton characteristics, where multi-scatter includes at least first order scatterings,
• means for convolving the calculated energy distributions of the Compton multi-scatter photons with the system energy spread function to obtain a system dependant energy distribution of the multi-scatter photons with unknown local coefficients,
• means for constructing a trial function including:
• means for summing the convolved multi-scatter photon energy distribution with unknown local coefficients for the at least first order scatterings and the contribution of the system energy spread function with unknown local coefficients,
• means for locally fitting the trial function to the measured energy spectrum to solve for the unknown local coefficients and thus determining the counts per location of the unscattered photons, and
• means for using the counts of the unscattered photons per pixel to provide a Compton free image.

A system of producing practically Compton free images of a patient with a gamma ray imaging system, said system comprising:

• means for determining a trial function, said means for determining a trial function comprising:
• means for multiplying a known system energy spread function by an unknown count number due to unscattered photons,
• means for convolving at least a first order distribution of the scattered photons derived from the probability of physical interaction of the photons and the known system energy spread function to provide a system dependent Compton energy distribution,
• means for multiplying the system dependent Compton energy distribution by an unknown number corresponding to counts of scattered photons,
• means for measuring an energy spectrum of the patient that includes total counts due to scattered and to unscattered photons,
• means for fitting the trial function to the measured energy spectrum to determine the unknown count number due to unscattered photons, and
• means for using the count number due to the unscattered photons to produce the practically Compton free images.