Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2001-09-07
2003-07-15
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S363020
Reexamination Certificate
active
06593575
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is positron emission tomography (PET) scanners and other similar medical imaging systems, and particularly the event locator circuits or other circuits of PET scanners that are employed to determine the exact times at which photons are detected during PET scans.
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. These are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected in the patient and become involved in such processes as blood flow, fatty acid, glucose metabolism, and protein synthesis. As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation is characterized by two features which are pertinent to PET scanners—each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image is created by determining the number of such annihilations at each location within the field of view.
A typical PET scanner is cylindrical and includes a detector ring assembly composed of rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by detectors located on opposite sides of the patient. The number of such simultaneous events (coincidence events) indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. During an acquisition, coincidence events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the detector ring. These numbers are employed to reconstruct an image using well-known computed tomography techniques.
In order to accurately determine coincidence events and thereby obtain useful information for producing images, PET scanners require timing circuits that accurately identify and log the exact times at which photons are received at the detectors of the scanners. These circuits, which are often referred to as event locator circuits, typically include digital counters that count time periods based upon a digital clock, and digital counter latches that receive both the count signals from the counters and impulse signals from the detectors of the PET scanner whenever photons are detected. Based upon the count signals, the counter latches effectively time-stamp the impulse signals with times indicative of when the impulse signals are received, and output this information for use by the PET scanner in determining coincidence events.
As shown in
FIG. 1
(Prior Art), a timing circuit
10
for use in a conventional event locator circuit of a PET scanner includes a delay-line based counter
12
and an asynchronous counter latch
36
. The delay-line based counter
12
operates by providing a clock signal
14
from a clock
16
to a binary counter
18
and then to a series of analog delay lines
20
,
22
and
24
. The binary counter
18
, which is shown to be a 5-bit counter, counts the clock pulses from the clock
16
and outputs a 5-bit binary count signal
28
. A lowest bit
26
of the binary count signal
28
alternates at the frequency of the clock signal
14
, which in
FIG. 1
is shown to be a 40 MHz clock having a period of 25 nsec. The binary counter
18
is chosen to be a 5-bit counter in order to allow different times to be distinguished within up to 32 cycles of the clock signal
14
.
In order to measure time gradations at an even higher frequency than that of the clock signal
14
, the lowest bit
26
of the binary count signal
18
is additionally provided successively to the series of analog delay lines
20
-
24
, which in turn respectively output count signals
30
,
32
and
34
. The count signals
30
,
32
and
34
each take on the same values as the lowest bit
26
of the binary count signal
28
, except insofar as each respective count signal takes on the value of the lowest bit only after the passage of respective time delays. In the embodiment shown, in which there are three analog delay lines
20
-
24
, each delay line delays transmission of the lowest bit
26
of the binary count signal
28
by one quarter of the period of the clock, or about 2.5 nsec. Together with the lowest bit
26
of the binary count signal
28
, the count signals
30
-
34
output by the three analog delay lines
20
-
24
act as a four-bit Johnson-type counter in which the allowable states of the lowest bit of the binary count signal and the three count signals
30
-
34
are limited to 1000, 1100, 1110, 1111, 0111, 0011, 0001 and 0000. Therefore, by virtue of the analog delay lines
20
-
24
, three additional state changes occur in between each change in the lowest bit
26
, such that time intervals are measured at four times the clock frequency, or 100 MHz. The binary count signal
28
, together with the other count signals
30
-
34
, form an overall 8-bit count signal
54
.
The asynchronous counter latch
36
includes four output registers
38
,
40
,
42
and
44
that respectively receive the binary count signal
28
and the three additional count signals
30
-
34
from the binary counter
18
and the analog delay lines
20
-
24
. In particular, the first output register
38
is a 5-bit register capable of storing all 5 bits of the binary count signal
28
, while the other output registers
40
-
44
are single-bit registers capable of storing the individual bits of information of the respective single-bit count signals
30
-
34
. The four output registers
38
-
44
, which are typically D-type flip-flops, further receive and are clocked by an event detection signal
39
that is typically a digital signal provided from one of the acquisition circuits of the PET scanner. The event detection signal
39
typically switches temporarily from a low-level to a high-level whenever photons are received at one or more detectors associated with the particular acquisition circuit. Whenever the output registers
38
-
44
are clocked by a rising edge of the event detection signal
39
, the current values of the binary count signal
28
and the counts signals
30
-
34
are stored in the respective registers and also output by the registers as respective output signals
45
,
46
,
47
, and
48
. Together, the output signals
45
-
48
form an overall 8-bit output signal
49
that represents the times at which the event detection signal
39
switches and thus the times at which photons are received at the associated detectors of the PET scanner.
Referring additionally to
FIG. 2
(Prior Art), a timing diagram
50
shows exemplary operation of the timing circuit
10
of FIG.
1
. In particular, the clock signal
14
is shown to vary at a particular frequency, and this is the frequency at which the lowest bit
26
of the 5-bit binary count signal
28
is shown to vary. Additionally, the values of the respective count signals
30
,
32
and
34
are shown to follow that of the lowest bit
26
of the binary count signal
28
except insofar as each successive count signal is delayed with respect to the lowest bit by successive 90 degree phase intervals due to the analog delay lines
20
,
22
and
24
. For example, during a period
52
of the clock signal
14
in which the binary count signal
28
has a value of 00001, the count signal
30
only takes on a high-level value (e.g., a value of 1) one-quarter of the period of the clock signal
14
after the time at which the lowest bit
26
has already taken on a value of 1. Given such operation of the delay-line based counter
12
, the overall 8-bit count signal
54
based upon the binary count signal
28
and the count signals
30
-
34
is determined.
With this pr
Gabor Otilia
GE Medical Systems Global Technology Company LLC
Hannaher Constantine
Horton Carl
LandOfFree
System and method for ascribing times to events in a medical... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System and method for ascribing times to events in a medical..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System and method for ascribing times to events in a medical... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3018551