Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2001-09-28
2004-10-12
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C702S189000
Reexamination Certificate
active
06803579
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to PET scanners generally and specifically to a method and apparatus for increasing the counting efficiency of a digital time stamping PET scanner by eliminating counting error due to the “picket fence” effect.
Positrons are positively charged electrons which are emitted by radionuclides which have been prepared using a cyclotron or other device. The radionuclides most often employed in diagnostic imaging are fluorine-18, carbon-11, nitrogen-13 and oxygen-15. Radionuclides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances such as glucose or carbon dioxide. One common use for radiopharmaceuticals is in the medical imaging field.
To use a radiopharmaceutical in imaging, the radiopharmaceutical is injected into a patient and accumulates in an organ, vessel or the like, which is to be imaged. It is known that specific radiopharmaceuticals become concentrated within certain organs or, in the case of a vessel, that specific radiopharmeceuticals will not be absorbed by a vessel wall. The process of concentrating often involves processes such as glucose metabolism, fatty acid metabolism and protein synthesis. Hereinafter, in the interest of simplifying this explanation, an organ to be imaged will be referred to generally as an “organ of interest” and prior art and the invention will be described with respect to a hypothetical organ of interest. After a radiopharmaceutical becomes concentrated within an organ of interest and while the radionuclides decay, the radionuclides emit positrons. The positrons travel a very short distance before they encounter an electron and, when the positron encounters an electron, the positron is annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to medical imaging and particularly to medical imaging using photon emission tomography (PET). First, each gamma ray has an energy of essentially 511 keV upon annihilation. Second, the two gamma rays are directed in substantially opposite directions.
In PET imaging, if the general locations of annihilations can be identified in three dimensions, the shape of an organ of interest can be reconstructed for observation. To detect annihilation locations, a PET scanner is employed. An exemplary PET scanner includes a plurality of detector modules and a processor which, among other things, includes coincidence detection circuitry. An exemplary detector module includes six adjacent detector units. An exemplary detector unit includes an array of crystals (e.g. 36) and a plurality of photo-multiplier tubes (PMTs). The crystal array is located adjacent to the PMT detecting surfaces. When a photon impacts a crystal, the crystal generates light which is detected by the PMTs. The PMT signal intensities are combined and the combined signal is compared to a threshold energy level (e.g. 100 keV). When the combined signal is above the threshold, an event detection pulse (EDP) is generated which is provided to the processor coincidence circuitry. Other hardware determines which crystal generated the light (i.e. absorbed the photon).
The coincidence circuitry identifies essentially simultaneous EDP pairs which correspond to crystals which are generally on opposite sides of the imaging area. Thus, simultaneous pulse pair indicates that an annihilation has occurred on a straight line between an associated pair of crystals. Over an acquisition period of a few minutes, millions of annihilations are recorded, each annihilation associated with a unique crystal pair. After an acquisition period, recorded annihilation data is used via any of several different well known procedures to construct a three dimensional image of the organ of interest.
A PET scanner may test the energy level before or after testing for coincidence timing and the coincidence timing test may be either analog or digital. In a typical analog coincidence circuit the duration of a timing signal is set to a pre-determined value (e.g. W/2 where W is a time period corresponding to a coincidence window). The timing signals from the detector units are then combined using conventional AND logic gate which produces an output only when two timing pulses overlap (i.e. two consecutive pulses are within +/−W/2).
In a typical digital coincidence circuit each EDP timing signal is compared to a master clock signal in a time to digital converter (TDC) and a time stamp digital value is provided for the EDP. The time stamp digital value from the TDC corresponds to the time lapsed between the previous master clock pulse and the EDP. For instance, in one exemplary system a master clock cycle may be 250 nanoseconds and the TDC may be capable of further dividing each master cycle into 192 separate sub-periods. For the purposes of this explanation a master clock cycle of 250 nanoseconds and further division of each cycle into 192 time stamps will be assumed although other cycle divisions and stamp divisions are completed. After each master clock cycle the time stamps corresponding to all EDPs detected during the completed master clock cycle (i.e. the stamps which occurred between the preceding two master clock pulses) are compared. EDPs which have time stamp differences between the time stamps of smaller than +/−W/2 are identified as coincidence pairs.
During an acquisition period there are several sources of annihilation detection error. One source of error in systems that include digital coincidence circuitry is referred to as the “picket fence effect”. To this end, as indicated above, event detection pulses are generated relative to a master clock cycle and thereafter all time stamps corresponding to pulses that occurred during the master clock cycle are compared to identify coincidence pairs. In this type of system, EDPs that occur either near the beginning or the end of a master clock cycle may have a matching coincidence event that falls into either a previous or a subsequent master clock cycle. Coincidence pairs including EDPs that “straddle” two master clock cycles are effectively lost as the coincidence circuitry has no way to associate the two EDPs with a single annihilation event. In some cases event losses due to the picket fence effect have accounted for as much as 1% of the total possible signal. The amount of loss depends on the width of the event time stamp and period of the master clock cycle. In the case of wide time stamp and short clock cycle, this loss can be several percent.
The picket fence phenomenon can best be understood by example and, to this end, refer to
FIG. 4
where a timing diagram
98
illustrates the end and the beginning of consecutive leading and following master clock cycles, respectively, along with exemplary EDPs. The end of the leading cycle as illustrated includes time stamps
186
through
191
while the beginning of the following cycle includes time stamps
0
through
5
. The EDPs that have time stamps during the leading cycle are identified by downwardly directed arrows while the EDPs that have time stamps during the following cycle are identified by upwardly directed arrows. Six exemplary EDPs
1
l,
21
,
3
l,
4
l,
5
l and
6
l are illustrated with EDPs
1
l,
2
l and
3
l occurring during the leading clock cycle and having time stamps
186
,
188
and
190
, respectively, while EDPs
4
l,
5
l and
6
l occur during the following clock cycle and having time stamps
0
,
2
and
5
.
For this example, assume that the EDPs
1
l,
2
l,
3
l,
4
l,
5
l and
6
l correspond to three separate annihilation events. In addition, assume a coincidence window W period corresponding to 12 consecutive time stamps. In this case, half the coincidence window (i.e., W/2) corresponds to six time stamp periods and therefore, any two EDPs having time stamps within 6 time stamp periods of each other should be conside
McDaniel David Leo
Williams John Jay
General Electric Company
Hannaher Constantine
Horton Carl B.
Lee Shun
Quarles & Brady LLP
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