Method of eliminating effect of afterglow on radiation image read out of photostimulable phosphor screen
View Patent ↗Method of eliminating the effect of afterglow on a radiation image read out of a photostimulable phosphor screen. For each pixel the amount of afterglow generated by previously scanned pixels in the same line of pixels is determined and subtracted from the digital signal representation of that pixel.
1. A method of eliminating the deteriorating effect of afterglow in a radiation image of a patient that has been stored in a photostimulable phosphor screen comprising:
line-wise scanning a phosphor screen with a deflected laser beam so as to stimulate a recorded phosphor radiation image of a patient;
converting, by a photomultiplier, image-wise modulated light emitted by said phosphor screen upon stimulation into an electric signal representation, wherein the electric signal representation includes parts corresponding to an instantaneous pixel value and an afterglow of previously scanned pixels;
determining, by the image processor, for each pixel of the electric signal representation, the amount of afterglow generated by all previously scanned pixels in the same line of pixels prior to an instantaneous pixel, wherein said amount of afterglow is equal to Σe −t/tau ·f(t) , where tau is the time constant of the afterglow, f(t) is the amount of emitted light as a function of time during scanning, and Σ is the summation for all pixels in the same line that were scanned prior to the instantaneous pixel;
multiplying, by an image processor, for each pixel of the electric signal representation, each determined amount of afterglow with a constant amplitude (Amp) value, wherein the constant amplitude value is an optimal value obtained through a fitting procedure applied to a preset amplitude value;
subtracting, by the image processor, for each pixel of the electric signal representation, said determined amount of afterglow multiplied by said constant amplitude (Amp) value from Σe −t/tau ·f(t) Σ
Afterglow
(
i
+
1
)
=
Pixel
(
i
+
1
)
*
tau
+
afterglow
(
i
)
1
+
tau
,
the instantaneous pixel value.
2. A method according to claim 1 , wherein said afterglow values are determined at the same temperature as the temperature at which said phosphor screen is scanned.
3. A method according to claim 1 , wherein said electric signal is filtered with one or more low pass filters of the first order, having characteristics so that the response of the one or more filters to a Dirac function has a time constant, tau, and an amplitude, Amp, that correspond with the time constant and the intensity of the afterglow of the phosphor, to obtain a low pass filtered signal, and wherein said low pass filtered signal is subtracted from said electric signal.
4. A filtering system configured to eliminate the deteriorating effect of afterglow in a radiation image of a patient that has been stored in a photostimulable phosphor screen comprising:
a scanner configured to line-wise scan a phosphor screen with a deflected laser beam so as to stimulate a recorded phosphor radiation image of a patient;
a photomultiplier configured to convert an image-wise modulated light emitted by said phosphor screen upon stimulation into an electric signal representation, wherein the electric signal representation includes parts corresponding to an instantaneous pixel value and an afterglow of previously scanned pixels;
an image processor configured to, for each pixel of the electric signal representation, determine the amount of afterglow generated by previously scanned pixels in the same line of pixels prior to an instantaneous pixel, wherein said amount of afterglow in pixel (i+1) prior to the instantaneous pixel is determined as
Afterglow
(
i
+
1
)
=
Pixel
(
i
+
1
)
*
tau
+
afterglow
(
i
)
1
+
tau
,
where tau is the time constant of the afterglow;
an image processor configured to multiply, for each pixel of the electric signal representation, each determined amount of afterglow with a constant amplitude (Amp) value, wherein the constant amplitude value is an optimal value obtained through a fitting procedure applied to a preset amplitude value;
the image processor configured to, for each pixel of the electric signal representation, subtract said determined amount of afterglow multiplied by said constant amplitude (Amp) value from Σe −t/tau ·f(t) Σ
Afterglow
(
i
+
1
)
=
Pixel
(
i
+
1
)
*
tau
+
afterglow
(
i
)
1
+
tau
the instantaneous pixel value.
5. A system according to claim 4 , wherein said afterglow values are determined at the same temperature as the temperature at which said phosphor screen is scanned.
6. A system according to claim 4 , wherein said filtering system comprises one or more low pass filters of the first order, having characteristics so that the response of the one or more filters to a Dirac function has a time constant, tau, and an amplitude, Amp, that correspond with the time constant and the intensity of the afterglow of the phosphor, to obtain a low pass filtered signal, and wherein said low pass filtered signal is subtracted from said electric signal.