Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2002-09-20
2004-05-25
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S36100C, C250S370080
Reexamination Certificate
active
06740881
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to radiant energy imaging devices with anisotropic signal sampling, and more particularly to imaging devices having hexagonal arrays of sensor units for imaging a radiation field.
2. Description of the Background Art
Imaging is the science of capturing images of a target object or process. Imaging may also include processing the captured images or image data in order to provide accurate and useful results. A final image or images may be used for any number of applications, such as, for example, medical imaging, manufacturing, astronomy, seismology, etc.
Imaging may employ any type of radiation in order to form an image. An image may be formed by detecting a response to radiation such as light emission, radioactive decay emission, magnetic waves, radio waves, electrons, etc. Radiographic imaging is the detection of radiation in order to form an image. By detecting the amount of radiation passing through or emanating from a test subject, the resultant image may give a representative view of the materials and construction of the test subject.
Gamma rays are a form of radiation that is emitted by excited atomic nuclei during the process of passing to a lower excitation state. Gamma radiation is capable of passing through soft tissue and bone. Gamma radiation may therefore be used for medical imaging, among other applications.
Gamma radiation for medical imaging usually involves a radiopharmaceutical, such as thallium or technetium, for example, that is administered to the patient. The radiopharmaceutical travels through the patient's body, and may be chosen to be absorbed or retained by an organ of interest. The radiopharmaceutical generates a predictable emission of gamma rays through the patient's body that can be detected and used to create an image. This may include imaging areas of the body or imaging specific organs. For example, a radiographic imaging device may be used to capture images of the heart, including real time images.
A radiographic imaging device may be used to detect radiation emanating from the patient and may be used to form an image or images for viewing and diagnosis. The radiographic imaging device may be a device such as a gamma or gamma ray camera, also referred to as a scintillation camera or an Anger camera. The radiographic imaging device allows a doctor to perform a diagnosis on a patient in a non-invasive manner and additionally may allow the doctor to observe organ function. In addition, the radiographic imaging device may be used for other imaging functions.
FIG. 1
shows a typical gamma camera or gamma camera component unit, including a scintillation crystal
105
having an emission face
108
, a sensor
112
or a sensor array, and a processing apparatus
116
.
The scintillation crystal
105
is typically a thallium doped sodium iodide crystal that generates photons when impinged upon by radiation, such as gamma radiation (i.e., it scintillates). This scintillation process converts the gamma radiation into light photons, which can be more easily detected. The photons emerge from the emission face
108
and may be detected by the sensor
112
or sensor array.
The sensor
112
may be, for example, a photon detection sensor such as a photomultiplier tube. The sensor
112
receives the photons generated by the scintillation crystal
105
and converts them into a representative electronic signal. A typical photomultiplier tube
112
may include a semi-transparent photocathode, a focusing grid, dynodes, and an anode (not shown). Multiple sensors
112
may be used to form a sensor array in a radiographic imaging device.
The processing apparatus
116
receives the electronic signals from the sensor
112
or sensor array, may amplify and filter the signals, and processes them to form an image.
FIG. 2
is a block diagram of a scintillation crystal
105
and an associated array of sensors
112
, as used in a gamma camera, for example. Each sensor
112
has an output which consists of output current signal that may be amplified, filtered and processed to generate an image. The scintillation crystal
105
emits light photons when a radiation photon such as a gamma particle impinges upon the crystal and is absorbed by the crystal, which is referred to as an “event.” For example, the event
122
may emit from the point of gamma-ray interaction with the crystal a plurality of photons that may be received by one or more of the sensors
112
. It should be understood that the event
122
may be any type of event, including a source of photons, a source of rays due to a radioactive decay, a source of electrons or protons, a source of electromagnetic waves, etc.
FIG. 3
shows a histogram created by photon counts from the sensors
112
during an event
122
. A histogram may be thought of as a visual representation of the contents of a series of storage bins that count photons as they are received and categorize them according to the location of the sensors along a predefined direction. The histogram therefore creates a wave form peak. The peak may indicate the approximate center of the event
122
(e.g., a centroid). The centroid may be determined from one or more sensor outputs and may be used to create an image composed of one or more event centroids. Similar considerations apply to the Y-centroid.
FIG. 4
shows an array of sensors
112
and the event
122
. Due to the difference in spacing and distribution along the X axis and along the Y axis, as is reflected in the distances D
X
and D
Y
, it is highly desirable that some correction or adjustment be made to the sensor readings in order to accurately determine the centroid of the event
122
.
The figure shows an example of a hexagonal sensor array used for imaging a large area. Typically, rows of the contiguous sensors
112
are aligned in the direction of a first Cartesian axis of the scintillation crystal
105
, such as an X-axis, for example. However, if the sensors
112
are grouped into an array in a square grid fashion, gaps are left between the individual sensors
112
, as the sensors
112
are typically round. Therefore, in order to pack the sensors
112
together as densely as possible, they are typically formed into a grid having a substantially hexagonal arrangement (as shown) in order to minimize gaps between the sensors
112
.
FIG. 5
shows the event
122
and the sensors
112
that are affected by the event
122
. From
FIG. 5
it can be seen that in the X dimension, there are approximately five samples (columns of sensors) within the area of the event
122
. However, in the Y dimension, there are only approximately three samples (rows of sensors) within the area of the event
122
. Because the sensors
112
are hexagonally packed as shown in
FIG. 4
, the histogram in the X-axis direction will have more bins than the Y-axis histogram (see FIG.
6
), and therefore resolution and linearity along the Y-axis will suffer in comparison to the resolution and linearity along the X-axis.
Early gamma cameras utilized scintillation crystals approximately 1 centimeter (cm) in thickness. For such crystals, the difference in X-axis and Y-axis sampling resolution produces only minor differences in image resolution and linearity characteristics. Recently, however, the crystal thickness has been increased to more than 2.5 cm in order to facilitate detection of high energy radiation. The extra crystal thickness is needed because the high energy radiation penetrates farther into the crystal, while the lower energy radiation does not penetrate very far. The lower energy radiation therefore causes events
122
that are farther from the emission face
108
, and the resulting photons may spread more within the scintillation crystal
105
. Consequently, the extra crystal thickness increases the disparity between the X and Y direction image resolution/linearity performance.
In the prior art, a positioning algorithm is used to compute a centroid (or center of received radiation or received signal) for each of the X- and Y
Hannaher Constantine
Moran Timothy
Siemens Medical Solutions USA
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