Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-11-15
2002-01-08
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S363040, C250S363090
Reexamination Certificate
active
06337481
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of medical diagnostic systems, such as imaging systems. More particularly, the invention relates to a technique for storing collected imaging data to a stand alone memory device during data acquisition so as to reduce the amount of memory required by a processor to acquire an entire set of imaging data.
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 (
18
F), carbon-11 (
11
C), nitrogen-13 (
13
N), and oxygen-15 (
15
O). 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 the 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 camera is employed. An exemplary PET camera includes a plurality of detectors and a processor which, among other things, includes coincidence detection circuitry. Each time a 511 keV photon impacts a detector, the detector generates an electronic signal or pulse which is provided to the processor coincidence circuitry.
The coincidence circuitry identifies essentially simultaneous pulse pairs which correspond to detectors which are generally on opposite sides of the imaging area. Thus, a simultaneous pulse pair indicates that an annihilation has occurred on a straight line between an associated pair of detectors. Over an acquisition period of a few minutes millions of annihilations are recorded, each annihilation associated with a particular detector 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.
PET cameras have been configured in many geometries. Because annihilation data has to be collected from essentially 360 degrees about an organ which is to be imaged, one popular PET camera configuration includes small detectors arranged to form an annular gantry about the imaging arc. In this case data from all required degrees can be collected at the same time, separated into data from different angles about the imaging area and then back projected as different profile type views to form the tomographic image. Unfortunately annular cameras require large numbers of detectors and therefore are extremely expensive which renders annular cameras unsuitable for many applications.
Referring to
FIG. 1
, another common PET camera configuration
10
includes first and second cameras
12
,
14
, respectively, each camera
12
,
14
including an impact surface
13
,
15
, respectively, for detecting impacting gamma rays. Each camera
12
and
14
is characterized by a width W across which hardware which can distinguish M different impact locations is arranged. To detect coincident gamma ray pairs, first and second cameras
12
and
14
are positioned a distance D apart and such that surfaces
13
and
15
oppose each other on opposite sides of an imaging area
16
and define a field of view (FOV). With the opposing camera configuration, instead of collecting tomographic data from all angels about imaging area
16
simultaneously as with an annular configuration, during an acquisition session, first and second cameras
12
and
14
are rotated (see arrows
18
,
20
) about imaging area
16
through approximately 180 degrees, the cameras maintained at different stop angles for short acquisition periods which together comprise the acquisition session.
For the purposes of this explanation the term “profile view” or simply “view” will be used to describe all annihilation data collected during a data acquisition period which emanates from the imaging area along parallel paths. At each camera position, cameras
12
and
14
collect annihilation data corresponding to several different profile views. A more detailed analysis of
FIG. 1
can be used to better understand profile views and how data corresponding to several views is collected at each camera position.
Referring to
FIG. 1
, an initial camera position angle &thgr;o is defined by a line between and perpendicular to impact surfaces
13
and
15
. During rotation, a stop angle &thgr;s is defined by the angle between the initial position angle &thgr;o and the instantaneous line between and perpendicular to impact surfaces
13
and
15
. While some systems operate with a continuously changing stop angle &thgr;s during data acquisition, unless indicated otherwise and in the interest of simplifying this explanation, it will be assumed that an exemplary system actually stops at different stop angles and only acquires data while stationary.
Referring still to
FIG. 1
, assuming cameras
12
and
14
are in the initial position illustrated so that stop angle &thgr;s is zero degrees, if an annihilation event occurs at the center of imaging area
16
as indicated by point
22
, the annihilation event may generate a corresponding gamma ray pair which emanates along virtually any path. However, with cameras
12
and
14
positioned as illustrated, cameras
12
and
14
can only collect generated gamma rays if the rays are directed within an angle range between a maximum negative flight path angle −&thgr;m and a maximum positive flight path angle +&thgr;m (and within a z-axis plane which is perpendicular to the illustration). In the interest of simplifying this explanation it will be assumed that cameras
12
and
14
are single dimensional (i.e. z=1) and, although range −&thgr;m through +&thgr;m may span several different ranges, it will be assumed that range −&thgr;m through +&thgr;m spans 30° (i.e. 15° on either side of an instantaneous stop angle &thgr;s).
Referring also to
FIG. 2
, assume the annihilation event at point
22
(i.e. a center of imaging area
16
) which is being studied generates gamma rays which are directed along the a flight path
50
which is parallel to initial position angle &thgr;o. In addition, assume that other annihilation events occur at other positions indicated at points
24
,
26
,
28
,
30
and
32
and that each of those events, like the event at point
22
, generates a pair of gamma rays which emanate along flight paths parallel to path
50
. Because all of the ray pairs in
FIG. 2
are parallel, the pairs together form a profile view of data, in essence indicating what the collected data
Bridge William J.
Stearns Charles W.
Della Penna Michael A.
Epps Georgia
Foley & Lardner
General Electric Company
Hanig Richard
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