CN104184473A - Method and device for carrying out non-linear correction on TDC - Google Patents

Abstract

The invention provides a method and device for carrying out non-linear correction on a TDC. The method comprises the following steps: a) placing a radioactive source in a scanning visual field of a detector and carrying out initialization zeroing on detector and electronics time migration; b) detecting a radiation event and recording output times of each TDC channel to obtain counting distribution of each TDC channel; c) selecting one TDC channel as a reference channel so as to enable the sum of output times of the TDC channels arranged at the two sides of the selected channel to be equal; and d) setting time for the reference channel and calculating the time of each TDC channel according to the time of the reference channel. The device can realize the method for carrying out non-linear correction on the TDC. By utilizing the method in the invention, the output accuracy of the TDC can be effectively improved; and meanwhile, in the correction process, externally-provided signal sources are not needed, and the operation is simple.

Description

TDC is carried out to the method and apparatus of gamma correction

Technical field

The present invention relates to time figure switch technology field, relate in particular to a kind of method and apparatus that the time-to-digit converter for radiation detector (Time-to-Digital Converter, TDC) is carried out to gamma correction.

Background technology

Time figure switch technology is based upon R.Nutt and has proposed on delay-line structure basis in nineteen sixty-eight.So-called delay line is comprised of one group of delay cell, when this organizes delay cell propagation delay in theory, equates, and be all τ.And the measurement in time interval realization of sampling of the propagation on delay line by twice pulse signal, its basic structure is as shown in Figure 1.In Fig. 1, the delay cell that whole piece delay line is τ by a group delaing time forms, and each delay cell coordinates a trigger, and the delay cell is here level triggers, rather than edging trigger; Trigger is just edging trigger on the contrary, rather than level triggers.When the time interval, T started, Start is uprised by low, for rising edge, then this rising edge is propagated to postpone τ in delay cell, until finish at interval, Stop is uprised by low, and all flip-flop states are all locked, this group trigger output Q forms the output of similar and a thermometer structure, and reading can be expressed as time interval length thus.

The outstanding advantages of time figure conversion (TDC) technology based on Nutt structure is simple in structure, can realize monolithic integrated, can on ASIC, realize, and also on monolithic FPGA, realizes.While realizing on ASIC, can reach and other temporal resolution of the same level of delay lock loop, but its temporal resolution can produce the drift in certain limit with the change of temperature.Its shortcoming is the delay that the temporal resolution of measurement is limited to used delay cell, and be uncontrollable the time of delay of delay cell.Its source of error mainly comprises following four aspects: the one, and quantization error, i.e. the time of a delay cell, what reduce quantification error band is the increase of delay cell; The 2nd, delay line is integrated non-linear, owing to can not accomplishing that in integrating process each postpones list without in full accord, causes the time of delay of each delay cell unequal, externally shows as nonlinear effect, and the method for rectification has the method for average, vector method etc.; The 3rd, change at random, is changed and is caused by self temperature of delay cell and supply power voltage; The 4th, time jitter, comprises the shake of clock and the time jitter of delay cell trigger signal switch.

For by the integrated non-linear measure error causing of delay line, need to carry out gamma correction to TDC, at J.Kalisz, " Review of methods for time interval measurements with picosecond resolution, " Metrologis, vol.41, pp.17-32, a kind of method of TDC being carried out to gamma correction is disclosed in 2004, the method needs delay time generator (Delay generators), for generation of two time delayed signals accurately, between two signals, amount of delay is known, input TDC, calculate the relation between input variable and output variable, thereby obtain the mapping relations of time and TDC passage.At High-Resolution Multi-Channel FPGA-TDC Architectures, Eugen Bayer, another kind of method of TDC being carried out to gamma correction is disclosed in Nikolaus Kurz, utilize time that signal propagates in wire and the relation (1cm=50ps) of line length, manufacture different line lengths and obtain different time delays.

Summary of the invention

The object of this invention is to provide a kind of method of TDC being carried out to gamma correction, without outside, provide signal source, can realize the nonlinearity erron of TDC is proofreaied and correct, simply effective.

For achieving the above object, the invention provides a kind of method that TDC is carried out to gamma correction, comprise the following steps: a) in the scan vision of detector, place radioactive source, and the time migration initialization of detector and electronics is made zero; B) probe radiation event, records the output number of times of each TDC passage, to obtain the count distribution of TDC passage; C) select a TDC passage as with reference to passage, the output number of times sum that makes to be arranged in the TDC passage of described reference channel both sides equates; D) for described reference channel, is set a time, and the time of each TDC passage of the Time Calculation based on described reference channel.

Further, described steps d) in, the time of each TDC passage of the Time Calculation based on described reference channel, calculating formula is:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.

Further, further comprising the steps of: in response to detection event, by described calculating formula, to calculate the time that TDC passage is corresponding.

Further, further comprising the steps of: by described calculating formula, calculated the mapping relations of time and TDC passage, the mapping table of rise time and TDC passage in response to detection event, is searched the time that TDC passage is corresponding that obtains in described mapping table.

Further, described steps d) in, the time that described reference channel is set is t represents the TDC clock cycle.

Further, described step b) in, when meeting the following conditions, probe radiation event, the step that records the output number of times of each TDC passage stops: for effective passage, the average output number of times of the TDC passage recording is greater than threshold value.

For achieving the above object, the present invention also provides a kind of device that TDC is carried out to gamma correction, comprises with lower unit:

Counting unit, for when the probe radiation event, records the output number of times of each TDC passage, to obtain the count distribution of TDC passage;

Selected cell, for selecting a TDC passage as with reference to passage, the output number of times sum that makes to be arranged in the TDC passage of described reference channel both sides equates;

Computing unit, is used to described reference channel that a time is set, and the time of each TDC passage of the Time Calculation based on described reference channel.

Further, in described computing unit, according to following calculating formula, calculate the time of each TDC passage:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.

Further, in described computing unit, for time of described reference channel setting is t represents the TDC clock cycle.

Further, described computing unit calculates the mapping relations of time and TDC passage by following calculating formula, rise time and TDC channel map, in response to detection event, in described mapping table, search the time that TDC passage is corresponding that obtains, described calculating formula is:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.

The method and apparatus that TDC is carried out to gamma correction provided by the invention, utilize the method for statistics, obtain the count distribution of TDC passage, and the gamma correction of the count distribution of the TDC passage based on obtaining realization to TDC, the accuracy of TDC output can effectively be improved; In trimming process, without outside, provide signal source simultaneously, simple to operate.

Accompanying drawing explanation:

Fig. 1 is the delay-line structure schematic diagram in a kind of existing TDC;

Fig. 2 is the nonlinear function schematic diagram between time and TDC passage;

Fig. 3 is that the present invention carries out the schematic flow sheet of the method for gamma correction to TDC;

Fig. 4 is the count distribution histogram of the TDC passage in the embodiment of the present invention;

Fig. 5 is that the present invention carries out the structural representation of the device of gamma correction to TDC.

Embodiment:

For above-mentioned purpose of the present invention, feature and advantage can be become apparent more, below in conjunction with drawings and Examples, the specific embodiment of the present invention is described in detail.

Embodiment mono-

The method that the present invention carries out gamma correction to TDC is mainly used in radiation detection field, comprise and be applied to PET scanner, and be applied to utilize radiation to transmit and radiopharmaceutic other nuclear medicine instrument, for example single photon emission computerized tomography,SPECT imager, plane X radial imaging instrument, radio astronomy, for example, for detector (, Cerenkov radiation, synchrotron light, colorimetric method detector etc.) of high energy particle etc.To specifically with reference to present invention is described above, should be appreciated that below, the present invention can also be applied to other radiation detector pattern, and the system and method that uses radiation detector.

As illustrated examples, positron emission tomography (Positron emi ssion tomography, PET) is described.In positron emission computerized tomography (PET), radiopharmaceutical is injected into imaging object, and wherein radiopharmaceutic radioactive decay event produces positron.Each positron and electron interaction are to produce the positron-electron annihilation event of gamma (γ) ray that sends two reverse directions.Use concurrent detection circuit, around the annular array of the radiation detector of imaging object, survey the concurrent reverse direction gamma ray event corresponding with positron-electron annihilation.Connecting the line of response of two concurrent detections and the position of positron-electron annihilation event intersects.Such line of response is similar to data for projection and can be rebuilt to produce two dimension or 3-D view.In time-in-flight pet (TOF-PET), the little time difference between the detection of two concurrent gamma-rays events of use positions the annihilation event along line of response.The timestamp (t imestamp) that time-to-digit converter (TDC) output is associated with each probe radiation event.This timestamp is used for determining concurrent to the line of response with corresponding by concurrent detection circuit, and also by flight time measurement circuit, is used.

When TDC exports the timestamp being associated with each probe radiation event, need to be to being proofreaied and correct by the integrated non-linear measure error causing of delay line.Please refer to Fig. 3, according to method of the present invention, TDC carried out to gamma correction:

Execution step S301 places radioactive source, and the time migration initialization of detector and electronics is made zero in the scan vision of detector;

Execution step S302, probe radiation event, records the output number of times of each TDC passage, to obtain the count distribution of TDC passage;

Radioactive source is placed in the scan vision of PET detector, radiation event time engrave at random and be uniformly distributed.In response to the radiation event detecting, the timestamp that TDC output is associated with each radiation event, records the number of times of each TDC passage output, and accumulative total obtains the count distribution of TDC passage.Preferably, the count distribution that represents TDC passage with histogram.In ideal conditions, do not have error to occur, the histogram obtaining should be uniform distribution, and the number of times of each TDC passage output equates.But because the time of delay of each delay cell of TDC is unequal, the wide difference in JiTDC road, causes the output number of times of each TDC passage different, finally obtains histogram as shown in Figure 4.In Fig. 4, abscissa represents TDC passage, is designated as i=0,1,2,3 .., and i is the numbering for each TDC channel definition, ordinate represents the output number of times of each TDC passage of record, is designated as N _i.It should be noted that, in TDC timing circuit in the present embodiment, we are numbered each TDC passage successively according to signal propagation direction, are designated as successively i=0,1,2,3 ...

Radiation event time engrave at random and be uniformly distributed, and the output number of times of the TDC of record is abundant, can ignore the impact of time measurement shake on result, histogram shown in Fig. 4 can be used to estimate the nonlinearity erron of TDC passage so, can think wide being directly proportional in radiation event Shuo Yu TDC road that each TDC passage is corresponding, distribution by radiation event number on each TDC passage, can obtain the wide nonlinear quantitative relation in TDC road.It should be noted that, for ignoring the impact of time measurement shake on result, need the counting of TDC passage abundant, be preferably and when meeting the following conditions, stop counting: for effective passage, the average output number of times of the TDC passage recording is greater than predetermined threshold value, as being 1000 by threshold value setting.

The method of above-mentioned utilization statistics, the count distribution of acquisition TDC passage, estimates the wide non-linear quantitative relation in TDC road.Continue to carry out following steps, complete the gamma correction to TDC passage.

Execution step S303, selects a TDC passage as with reference to passage, and the output number of times sum of TDC passage that makes to be arranged in described reference channel both sides is equal;

Execution step S304, for described reference channel arranges a time, and the time of each TDC passage of the Time Calculation based on described reference channel.

The count distribution of the TDC passage based on obtaining selects a passage as with reference to passage from all passages of TDC, and a time value is set for this reference channel.The TDC passage that is selected as reference channel need to meet some requirements, and is specially: the output number of times sum that is arranged in the TDC passage of reference channel both sides equates.The described reference channel both sides that are arranged in refer to the position relationship in physical significance, and in TDC timing circuit, all TDC passages are arranged in order along signal propagation path, to being positioned at the output number of times of the TDC passage on reference channel both sides, carry out respectively read group total.The time value of TDC output is relative value, for selected reference channel, is arranging after a time value, and the time of other TDC passages can arrange with respect to the time of this reference channel.Be preferably, the count distribution of the time based on described reference channel and the TDC passage obtaining, the time of each TDC passage of recursive calculation, calculating formula is:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.Be preferably, the time of described reference channel is set

Through calculating, finally obtain the mapping relations T (i) of time and TDC passage, i=0,1,2,3 ...Carry out PET when scanning carrying out the object to be imaged, utilize time of obtaining and the mapping relations of TDC passage, can be obtained by TDC passage the time of radiation event, specific implementation can be for utilizing above-mentioned calculating formula to calculate time value; Also can be for be calculated in advance the mapping relations T (i) of time and TDC passage, i=0,1 by above-mentioned calculating formula, 2,3 .., the mapping table of rise time and TDC passage in response to detection event, is searched the time that TDC passage is corresponding that obtains in described mapping table.

Method of TDC being carried out to gamma correction of the present invention, utilize the method for statistics, obtain the count distribution of TDC passage, and the count distribution of the TDC passage based on obtaining realizes the gamma correction to TDC, can effectively improve the accuracy of TDC output, and in trimming process, without outside, provide signal source, simple to operate.

Embodiment bis-

Corresponding above-mentioned method of TDC being carried out to gamma correction, the present invention also provides a kind of device that TDC is carried out to gamma correction, comprises with lower unit:

Counting unit 501, for when the probe radiation event, records the output number of times of each TDC passage, to obtain the count distribution of TDC passage;

Selected cell 502, for selecting a TDC passage as with reference to passage, the output number of times sum that makes to be arranged in the TDC passage of described reference channel both sides equates;

Computing unit 503, is used to described reference channel that a time is set, and the time of each TDC passage of the Time Calculation based on described reference channel.

A kind of preferred implementation of above-mentioned computing unit is: the time that calculates each TDC passage according to following calculating formula:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.Preferably, the time for described reference channel setting is t represents the TDC clock cycle.

The another kind of preferred implementation of above-mentioned computing unit is: the mapping relations that calculate time and TDC passage by following calculating formula, rise time and TDC channel map, in response to detection event, in described mapping table, search the time that TDC passage is corresponding that obtains, described calculating formula is:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.

In the present embodiment, the described concrete enforcement of device that TDC is carried out to gamma correction can, with reference to described in the present embodiment, TDC being carried out the enforcement of the method for gamma correction, not repeat them here.

Although the present invention discloses as above with preferred embodiment; so it is not in order to limit the present invention, any those skilled in the art, without departing from the spirit and scope of the present invention; when doing a little modification and perfect, so protection scope of the present invention is worked as with being as the criterion that claims were defined.

Claims (10)

1. TDC is carried out to a method for gamma correction, it is characterized in that, comprise the following steps:

A) in the scan vision of detector, place radioactive source, and the time migration initialization of detector and electronics is made zero;

B) probe radiation event, records the output number of times of each TDC passage, to obtain the count distribution of TDC passage;

C) select a TDC passage as with reference to passage, the output number of times sum that makes to be arranged in the TDC passage of selected passage both sides equates;

D) for described reference channel, is set a time, and the time of each TDC passage of the Time Calculation based on described reference channel.

2. the method for claim 1, is characterized in that, described steps d) in, the time of each TDC passage of the Time Calculation based on described reference channel, calculating formula is:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.

3. method as claimed in claim 2, is characterized in that, further comprising the steps of: in response to detection event, by described calculating formula, calculate the time that TDC passage is corresponding.

4. method as claimed in claim 3, it is characterized in that, further comprising the steps of: by described calculating formula, to be calculated the mapping relations of time and TDC passage, the mapping table of rise time and TDC passage, in response to detection event, in described mapping table, search the time that TDC passage is corresponding that obtains.

5. the method for claim 1, is characterized in that, described steps d) in, the time that described reference channel is set is t represents the TDC clock cycle.

6. the method for claim 1, is characterized in that, described step b) in, when meeting the following conditions, probe radiation event, the step that records the output number of times of each TDC passage stops: for effective passage, the average output number of times of the TDC passage recording is greater than threshold value.

7. TDC is carried out to a device for gamma correction, it is characterized in that, comprise with lower unit:

Counting unit, for when the probe radiation event, records the output number of times of each TDC passage, to obtain the count distribution of TDC passage;

Selected cell, for selecting a TDC passage as with reference to passage, the output number of times sum that makes to be arranged in the TDC passage of described reference channel both sides equates;

Computing unit, is used to described reference channel that a time is set, and the time of each TDC passage of the Time Calculation based on described reference channel.

8. device as claimed in claim 7, is characterized in that, in described computing unit, calculates the time of each TDC passage according to following calculating formula:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.

9. device as claimed in claim 7, is characterized in that, in described computing unit, for time of described reference channel setting is t represents the TDC clock cycle.

10. device as claimed in claim 7, it is characterized in that, described computing unit calculates the mapping relations of time and TDC passage by following calculating formula, rise time and TDC channel map, in response to detection event, in described mapping table, search the time that TDC passage is corresponding that obtains, described calculating formula is:

$T_{i-1}=T_i-N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c-1,c-2,c-3,...,0;$

$T_{i+1}=T_i+N_i\×\left(\frac{T}{\mathrm{\Σ}{N}_i}\right),i=c,c+1,c+2,c+3,...$

Wherein, T _ithe time that represents the TDC passage that sequence number is i, c represents the sequence number of reference channel, N _ithe output number of times that represents the TDC passage that sequence number is i, T represents the TDC clock cycle.