The positron emission tomography scanner is a nuclear diagnostic imaging system which utilizes a positron emitter and finds widespread applications such as cancer diagnosis or molecular imaging.
A positron emitter is an isotope, such as 18F, which is unstable because of excess number of protons in the atomic nucleus against the number of neutrons, and which undergoes +β decay to emit a positron and a neutrino. The positron thus emitted is the antimatter counterpart of the electron. Accordingly, when the positron encounters an electron, they annihilate each other causing their mass to be entirely converted into energy. This energy is released in the form of annihilation radiation or high-energy electromagnetic wave. Since the law of conservation of momentum is obeyed before and after the pair annihilation, two annihilation radiations are principally emitted concurrently at an angle of about 180 degrees. In a strict sense, a single radiation or three or more radiations may also be emitted; however, they are less than 1% of the total radiations and thus can be ignored for imaging purposes. The respective energy of the two emitted radiations corresponds to the mass of one electron (or positron), i.e., approximately 511 keV.
The principle of positron imaging is based on the coincidence measurement of the two annihilation radiations. When a radiation of 511 keV is detected substantially simultaneously at two oppositely disposed radiation detectors, this indicates that a positron has undergone a pair annihilation on the straight line connecting the two radiation detectors. As shown in FIG. 1A, this information is collected with a number of radiation detectors 16 disposed around a subject 10, and reconstructed by the similar mathematical technique to X-ray CT. A tomographic video or static image is thus provided that approximates the distribution of positron emitters 12 in the subject 10. In the figure, reference numeral 18 denotes a bed.
Accordingly, the performance required of the radiation detector 16 is to be capable of measuring the position of incidence, energy, and incidence time of an annihilation radiation 14 with the highest accuracy. As used herein, the expression “substantially the same time (simultaneously)” refers to points in time approximately within 15 nanoseconds (nano denotes 10−9). These points in time may also fall within 10 nanoseconds or less or 5 nanoseconds or less when the radiation detector can determine time with higher accuracy. A frame of time (or time window) may be reduced in which two annihilation radiations are determined to be incident at the same time and a single pair of annihilation radiations resulting from one electron-positron-pair annihilation. This would reduce the possibility of erroneously combining a plurality of annihilation radiations resulting from separate pair annihilations, namely accidental coincidence, thereby improving measurement accuracy and signal to noise ratios. It should be noted that although the temporal resolution of each radiation detector can currently be improved up to about 0.3 nanoseconds, those time windows reduced accordingly but excessively would reject the counting of true combinations of annihilation radiations, and also cause the field of view covered by the scanner to be narrowed with its sensitivity reduced.
Suppose that an electric signal from the radiation detector 16 can be processed to determine the time of incidence of the annihilation radiation 14 generally within 15 nanoseconds or less. In such a case, as is known to those skilled in the art, time-of-flight (TOF) difference between annihilation radiations can be used to improve the signal to noise ratio of the positron emission tomography scanner, maintaining the time window not rejecting the counting of true combinations of annihilation radiations. As an example, when a pair annihilation occurs at the center of two oppositely disposed radiation detectors, the two annihilation radiations arrive at the radiation detectors at the same time. On the other hand, when a pair annihilation occurs at coordinates (spatial coordinates) closer to either one of the radiation detectors, an annihilation radiation arrives at the closer radiation detector earlier than at the other. That is, the difference in arrival time between the radiations arriving at the respective radiation detectors can be determined and thereby converted into the difference in distance between the spatial coordinates at which the pair annihilation has occurred and the respective radiation detectors. In a conventional PET scanner, shown in FIG. 1A, which does not make use of the time-of-flight difference, the information obtained from a pair of coincidence measurements provides a straight line containing the spatial coordinates on which the pair annihilation is supposed to have occurred. However, use of the time-of-flight difference as with a time-of-flight difference type PET (TOF-PET) scanner shown in FIG. 1B narrows the special coordinate uncertainty to a certain confined range on the straight line. The accuracy of this confinement depends on the temporal resolution of the scanner, so that as the accuracy of determination is increased, the amount of information regarding the location of the pair annihilation is increased thereby providing an improved signal to noise ratio (see W. W. Moses, IEEE Trans. Nucl. Sci., Vol. 50, No. 5, pp. 1325-1330, 2003).
Note that if the time of incidence of an annihilation radiation can be determined generally within 100 picoseconds or less (pico denotes 10−12), it can be expected that not only the signal to noise ratio but also the spatial resolution of tomographic video or static images will be improved.
The concept of the TOF-PET scanner that makes use of the time-of-flight difference. between annihilation radiations was already known in the 1980s (see T. Tomitani, IEEE Trans. Nucl. Sci., Vol. 28, No. 6, pp. 4582-4589, 1981). However, at that time, the level of technological sophistication was insufficient to improve signal to noise ratios due to the inadequate performance of the scintillator crystals used as radiation detection elements, radiation detectors, and circuits for processing electric signals from radiation detectors. Today, scintillator crystals with good response speeds have been developed, such as LSO (lutetium oxyorthosilicate doped with a trace amount of cerium) or LYSO (a mixed crystal of LSO and yttrium oxyorthosilicate doped with a trace amount of cerium). In addition, the timing performance of a photomultiplier tube (PMT) that is used as an optical detector for detecting scintillation light produced through an interaction with radiations has also been improved. Furthermore, the application-specific integrated circuit technology has also advanced. It has been thus recognized that the TOF-PET scanner that makes use of the time-of-flight difference between annihilation radiations offers a performance advantage over the conventional PET scanner in its signal to noise ratio. Accordingly, there is an increasing need for a radiation detector that has an advantageous temporal resolution. An improvement in signal to noise ratio makes it possible to reduce the acquisition time required for positron emission tomography and the amount of radioactive pharmaceutical dosed to a subject.
As shown in FIG. 2, a first cause of the error in detection time results from a difference in propagation speed in a scintillator crystal 22 between the annihilation radiation 14 and scintillation light 24. In the figure, reference numeral 20 denotes an optical detector such as a photomultiplier tube.
In the air as well as in the crystal, the flight speed of the annihilation radiation 14 is substantially the same as the speed of light c in a vacuum (299,800 km per second). In contrast, the scintillation light 24 travels at a speed of about c, in the air, but at a reduced speed of c/n in the crystal, where n is the refractive index of the crystal, which is typically greater than 1.0. To efficiently detect an annihilation radiation of 511 keV which has a great penetration power, a scintillator crystal 22, which is about a few centimeters in thickness, is typically employed.
As shown to the right of FIG. 2A, when the annihilation radiation 14 interacts with the scintillator crystal 22 near the top of the scintillator crystal 22, the scintillation light 24 has to travel longer through the scintillator crystal 22 to reach the optical detector 20. In contrast, as shown to the left of FIG. 2A, when an interaction occurs near the bottom of the scintillator crystal 22, the scintillation light 24 travels a correspondingly short distance to reach the optical detector 20. That is, as shown in FIG. 2B, an earlier apparent detection time is observed when the annihilation radiation 14 travels a longer distance through the scintillator crystal 22 still at a flight speed of c and is then converted immediately before the optical detector 20 into the scintillation light 24 traveling at a slower flight speed of c/n.
Assuming that when measuring a pair of annihilation radiations respectively with two radiation detectors, one optical detector detects scintillation light produced near the top of the scintillator crystal, whereas the other optical detector detects scintillation light produced near the bottom of the scintillator crystal. In such a case, the spatial coordinates of the pair annihilation estimated from the difference between the detection times are closer to the latter radiation detector than the actual location. Accordingly, a correction can be made to the error in detection time caused by the difference in propagation speed between the annihilation radiation and the scintillation light in the scintillator crystal, thereby providing information on time-of-flight difference with improved accuracy. It should be noted that for simplicity in illustration of the principle, FIG. 2A shows one typical scintillation photon emitted directly below per one pair annihilation radiation. However, in practice, several thousands to several tens of thousands of photons are emitted not only directly below but also in other directions. In addition, since some photons are absorbed on the boundary of the scintillator crystal or a reflective material or the like, not all photons reach the optical detector.
As illustrated in FIG. 3A, a second cause of the error in detection time results from a difference in path length in the scintillator crystal 22 along which the scintillation light 24 travels. Although part of the scintillation light 24 is directly incident upon the optical detector 20, generally half or more of the photons are reflected off on the upper surface or sides of the scintillator crystal 22 more than once and then incident upon the optical detector 20. For example, as shown to the right of FIG. 3A, a portion of the scintillation light 24 produced and emitted upwardly near the top of the scintillator crystal 22 arrives immediately at the top of the scintillator crystal 22, where it is reflected downwardly by a reflective material covering the upper surface of the scintillator crystal 22. In contrast, as shown to the left of FIG. 3A, the scintillation light 24 emitted upwardly near the bottom of the scintillator crystal 22 travels the length of the scintillator crystal 22 until it is reflected downwardly from the top of the scintillator crystal 22. Moreover, the scintillation light 24 emitted sideward at an angle travels a different propagation path depending on how it is reflected on a side of the scintillator crystal. Furthermore, when the scintillator crystal is arranged two- or three-dimensionally up/down, right/left, or forward/backward, the scintillation light 24 may take a more complex propagation path depending on how it is reflected or refracted between those scintillator crystals. In circumstances with a longer propagation path, the scintillation light 24 takes a longer time to reach the optical detector 20, thereby causing the time to be delayed at which it is determined that the annihilation radiation has been detected.
FIG. 3B shows the calculated results of a propagation path of scintillation light and its propagation time in a crystal block, where the crystal block has four layers to increase the detection accuracy of the location of an interaction between an annihilation radiation and a scintillator crystal. In this arrangement, one layer includes a square array of 32×32 LSO crystals each being 1.45 mm×1.45 mm×4.50 mm in size. To aid simplicity in illustrating the principle, the figure shows the relation between the number of photons reaching the optical detector and the elapse time, assuming that one crystal at the center of each layer is selected, and that at a reference time, one hundred thousand photons are emitted in random directions from the center of the respective crystals. Depending on the linear distance between the coordinates of the light emission in a scintillator crystal emitting light and the optical detector, the time when the first photons reaching the optical detector and the time at which photons are the largest in number are varied. Further, the time of second peak, which is caused by the reflection on the upper surface of the crystal, is varied depending on the distance. A correction can be made to the error in detection time caused in the above-described manner by the difference in propagation path length of scintillation light, thereby providing information regarding time-of-flight difference with improved accuracy.
A third cause of the error in detection time results from the difference in output waveform of an optical detector caused by a difference in propagation path. As can be seen from FIG. 3B which shows the distribution of times at which light (input) arrives at the optical detector, the time required from the arrival of the first photon from each crystal until the arrival of the greatest number of photons differ depending on the crystal. It is also evident from the shapes of the graphs that the number of photons tends to increase differently with time.
To most simply determine the time from the output waveform of the optical detector, a threshold value is first set in order to discriminate signals from noises, whereupon an output that exceeds the threshold value is defined as a signal so that the time at which the threshold value is exceeded is taken as the detection time. As shown in FIG. 3C, however, the threshold value is immediately exceeded after the arrival of the first photon when the output signal is comparatively high (e.g., the 4th layer), while when the output signal is comparatively low (e.g., the 1st layer), the threshold value is only exceeded near the time at which the output is at maximum. Thus, the time to be determined varies according to the signal magnitude. Accordingly, in practice, a further sophisticated determination method such as the constant fraction method is widely employed in order to avoid variations in the time irrespective of the magnitude of the output signal (see Radiation Detection and Measurement, 3rd Edition, p. 662, 2000, published by John and Wiley & Sons, Inc.).
However, although the constant fraction method can accommodate variations in the magnitude of output signals, it cannot accommodate variations in the waveform of output signals. Thus, the time to be determined varies depending on whether the signal rises sharply or gradually. Accordingly, a correction can be made to the error in detection time resulting from the difference in output waveform of an optical detector caused by the difference in propagation path of scintillation light, thereby providing information regarding time-of-flight difference with improved accuracy. This is also applicable not only to the constant fraction method but also other timing determination method such as the leading edge method. By way of example, a correction can be made in accordance with the gradient of the rise of a signal or also with a change in gradient.
It should be noted that a technique is already known which employs not the information regarding a three-dimensional location (emission location) but only the information regarding the location of emission in the direction of depth for a radiation detector as shown in FIG. 4 to make a correction to detection time (see T. Tsuda et al, IEEE Trans., Nucl. Sci., Vol. 53, No. 1, pp. 35-39, 2006). In the figure, reference numeral 40 denotes a radiation detector (also referred to as a DOI radiation detector) which is capable of obtaining information regarding the location of a depth of interaction (DOI). The radiation detector 40, which was suggested by the applicant in Japanese Patent Laid-Open Publication No. 2004-279057 (Patent Document 1), includes, e.g., a 256-channel position sensitive photomultiplier tube (PS-PMT) 21 and a 6×6 four-layered scintillator crystal block 23.
However, using only the information regarding the location of emission in the direction of depth may lead to an inaccurate correction. As shown in FIG. 2, if only one photon is emitted directly below, the location of emission in depth may be the sole cause of the error. However, in practice, a number of photons are reflected on the upper and side surfaces, and thus behave differently before they reach the optical detector depending on the effects of adjacent crystals and the distance to the sides of the crystal block. As already discussed, this may cause those errors to be produced which result from the difference in propagation path length of scintillation light and the difference in output waveform of an optical detector caused by the difference in propagation path of scintillation light.