EP1709463A1

EP1709463A1 - Asymmetric axial filter for pet imaging systems

Info

Publication number: EP1709463A1
Application number: EP05702588A
Authority: EP
Inventors: Daniel Gagnon; Jeffrey A. Kolthammer
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2004-01-15
Filing date: 2005-01-05
Publication date: 2006-10-11
Also published as: CN100498380C; JP2007523322A; CN1910475A; WO2005071439A1

Abstract

A radiographic imaging system (10) includes an array of detectors (16) for detecting emission radiation emitted from a subject (14). The detectors (16) are arranged around a circular bore (18), defining a field of view of the imaging system (10). End shields (40) are disposed at an entrance and at an exit of the axial field-of-view (18) defining a subject receiving aperture (46). One or more septa (50), partially covering the circumference, shields the detectors (16) from the radiation originating outside of the axial field-of-view (18) as well as the body scattered radiation. Septa (50) are spread out sparsely across the field of view such that a ratio of the area shielded by the septa (50) to the field of view is negligible. The patient aperture (46) is increased without compromising the 3D imaging.

Description

ASYMMETRIC AXIAL FILTER FOR PET IMAGING SYSTEMS

DESCRIPTION The present invention relates to the diagnostic imaging systems and methods. It finds particular application in conjunction with the Positron Emission

Tomography (PET) scanners and will be described with particular reference thereto. It will be appreciated that the invention is also applicable to other radiological scanners and the like. PET is a valuable patient imaging technique employing positron emitting compounds. PET provides specific metabolic information about tissues that conventional scanners such as CT and MRI can not provide. Typically, PET scanners include a substantially circular bore that is surrounded by an array of detectors which detect concurrent energy events. Prior to the scan, the patient is injected with a positron emitting radioisotope which is taken up by cells. When a positron emits from a radioisotope, it combines with an electron to produce an annihilation reaction, in which the pair's mass is converted into energy. The energy is dispersed in the form of two 511 kev gamma rays or photons, traveling in 180 degrees opposite directions. When two detectors "see" 511 kev photons from the annihilation event concurrently or within nanoseconds of each other, the detectors register a coincidence along the line between the detector points - a line of response (LOR). The PET system draws lines of responses between each detection pair, registering coincidence events during the scan. When the scan is completed, areas with more intersecting lines indicate more concentrated areas of radioactivity. The system uses this information to reconstruct a three dimensional image of radioisotope distribution in the body. The scanner accepts photons from anywhere from the field of view, and, in addition, accepts photons originating outside of field of view that travel into the field of view. The photons originating outside of the field of view do not contain useful information for image reconstruction. Typically, the detectors are shielded from out-of the- field-of view events by flange lead shields at the entrance and exit of the PET scanner bore. The flange extends from the outer periphery of the bore toward the central axis of the bore and leaves a circular patient aperture of about 50-60cm in diameter. Generally, it is desirable to have a bigger patient aperture, about 70-80cm, since the smaller aperture presents a problem when the larger patients do not fit comfortably through it. One solution is to increase the shielding diameter to about 70cm and keep the detector diameter the same, at about 80-90cm. However, the studies have shown that the image degradation occurs to a degree that is gauged not acceptable. Another solution is to increase the detector diameter to about 100cm while increasing the shielding aperture to about 70cm. By increasing the diameter of the detector ring, the out- of-the-field-of-view activity is restricted and remains at the level of systems currently in use. However, this approach involves more costs as the detector diameter (hence the number of detectors) increases, and contributes to overall reduced sensitivity. Yet other approach, employed in some PET scanners, is to install annular anti-scatter septa, e.g. lead plates between each scintillation crystal element. About 15-24 annular septa are spaced along the whole axial field of view, allowing the detectors to receive only truly collimated events. Then again, this approach (known as 2D detection) is deficient as it limits the detection of some sought after within-the-field-of-view events as well as the undesirable out-of-field-of-view events. To perform 3D imaging, the septa blades might be made removable or retractable allowing to operate either in the truly collimated 2D mode or the 3D mode. The drawback of this design is that the detectors have differently specified sensitivity for each of the two modes of operation. Further, movable or retractable septa add mechanical complexity and labor. There is a need for the cost-effective method and apparatus that would permit a use of a bigger patient aperture while not compromising the image quality. The present invention provides a new and improved imaging apparatus and method which overcomes the above-referenced problems and others.

In accordance with one aspect of the present invention, a radiographic imaging system is disclosed. A detecting means, which is arranged around a circular bore, defining a field of view of the imaging system, detects emission radiation emitted from a subject. One or more circumferentially extending septa shields the detecting means from the emission radiation originating outside of the bore, which septa are spread out sparsely across the field of view. In accordance with another aspect of the present invention, a method of a 3D radiographic imaging is disclosed. Emission radiation emitted from the subject is detected with the detecting means of a PET scanner, which detecting means is arranged around a circular bore defining a field of view of the imaging system. The detecting means is shielded by one or more circumferentially extending septa shields from the emission radiation originating outside of the bore, which septa are spread out sparsely across the field of view. Lines of response are calculated with a calculating circuit. An image representation is reconstructed with a reconstruction processor. At least a portion of the image representation is displayed on a display. One advantage of the present invention resides in effective anti-scatter filtering that allows increasing the patient aperture without compromising 3D imaging. Another advantage of the present invention resides in effective anti-scatter filtering that keeps the impact on the sensitivity of the detectors to a minimum.. Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. FIGURE 1 is a diagrammatic illustration of a diagnostic imaging system; FIGURE 2 is a diagrammatic illustration of a circular subject receiving aperture with a 180 degree septa; FIGURE 3 is a diagrammatic illustration of a portion of a diagnostic imaging system with a 180 degree septa viewed transverse to FIGURE 2; FIGURE 4 is a diagrammatic illustration of a circular subject receiving aperture with a 360 degree septa; FIGURE 5 is a diagrammatic illustration of a non-circular subject receiving aperture with a sectional septa; FIGURE 6 is a diagrammatic illustration of a circular subject receiving aperture with a limited arc septa and a transmission radiation source; FIGURE 7 is a diagrammatic illustration of a non-circular subject receiving aperture with a sectional septa including a transmission radiation source.

With reference to FIGURE 1, an imaging system 10 includes a subject support means 12, such as a table or couch, which supports a subject 14 being imaged. The subject 14 is injected with one or more radioisotopes to induce positron emission. A circularly cylindrical, annular array of detectors 16 is arranged around a bore 18 of a PET scanner 20 that defines an axial field-of-view. When the detectors may have planar faces, the detector array 16 may be an octagon or other regular polygon that approximates a circle. Typically, individual detector elements have a radiation receiving face on the order of 1 cm² or less. The detector elements are preferably mounted in planar sub-arrays that are mounted end-to-end to define the detector array 16. Other types of detectors are also contemplated and again preferably have a resolution of 1 cm or better. The subject support 12 is advanced and retracted to achieve the desired positioning of the subject 14 within an examination region 22 defined by the bore 18, e.g. with the region of interest centered in the field of view (FOV) of the detector array. Radiation events detected by detectors 16 are collected by a line of response (LOR) calculating circuit 24. The LOR calculator 24 includes a coincidence detector 26 that determines when two events are within a preselected temporal window of being simultaneous. From the position of the detectors 16 and the position within each detector, at which the coincident radiation was received, a ray between the radiation detection points is calculated by line extrapolator 28. The acquired LOR data are preferably stored in a data memory or buffer 30. A data reconstruction processor 32 reconstructs an electronic image representation from the LOR data stored in data memory 30 and stores the resultant image representation in an image memory 34. Portions of the stored image representation are retrieved by an image processor 36 and converted to an appropriate format for display on a monitor 38, such as a video, CCD, active matrix, plasma, or other monitor. Of course, a color printer or other output device may also be used to present the data in a convenient format. With continuing reference to FIGURE 1 and further reference to FIGURES

2-4, radiation end shields 40 are mounted at an entrance 42 and an exit 44 of the circular bore 18 to define a receiving area or entrance aperture 46 of the PET scanner. An anti- scatter filter or septa blades or plates 50 is disposed over at least a section of a circumference of the bore 18 . The anti-scatter filter 50 preferably includes two fixed septa extending about 2.5-3.5mm each in axial direction, e.g., in the direction along the central axis of the bore 18. The septa 50 are equally spaced within the field-of-view, e.g., the septa spaced the same distance d from each other as from the end shields 40 denoting boundaries of the field-of-view. Only the events pairs with both ends of the lines of response in the field-of-view and with a preselected angular criteria are accepted by a use of electronic collimation. The septa 50 are manufactured from lead, tungsten, or other high density (high-Z) shielding material. The ratio of a shielded area of the detectors 16 to the field-of- view is negligible and does not affect the geometry of the scanner or sensitivity of detectors. E.g., if the two plates 50 are 3mm each and the field-of-view is 18cm, the ratio is 1 to 30. Preferably, depending on the field-of-view, the number and thickness of the plates 50 are selected to block the 51 IkeV radiation coming from different angles to optimize the goal of keeping the sensitivity of the detector high while blocking the outside incidental rays as much as possible. The number of plates 50 and each plate's thickness might change depending on the parameters of the imaging system, such as field-of-view, size of outside shielding, size of the detector, and others. The plates 50 are installed substantially perpendicular with respect to the surface of the detectors 16, with a tolerance of 5-10 degrees or less to restrict shadowing on the detectors. Raising the plates about 4-5mm above the detectors improves angular acceptance. In the embodiment of FIGURE 2, each filter 50 spans 180 degrees at an upper half 60 of the bore 18 circumference. 180 degree shielding effectively blocks stray radiation, at the same time blocking much less useful radiation from reaching the detector 16. Preferably, the couch 12 includes couch shields 62 which are disposed underneath the couch 12 to enhance blocking the out-of-bore radiation from reaching the detectors 16. In the embodiment of FIGURE 4, the filter 50 extends full 360 degrees shielding the entire ring of the detectors 16. With reference to FIGURE 5, the end shields 40 define a non-circular aperture 46. Preferably, the non-circular aperture 46 is an ellipse with a larger diameter Dl or major axis along a horizontal axis parallel to the axis drawn through the shorter dimension of couch 12, and a smaller dimension D2 or minor axis along the vertical axis perpendicular to the couch 12. The aperture 46 is sized such that a nominally sized subject centered in the aperture is generally equidistant from the end shield 40 in all directions. The septa 50 spans two separate 90 degree sections centered along the major axis of the ellipse. Preferably, the filter 50 includes two blades 3.5mm thick. Preferably, the couch 12 includes couch shields 62 which are disposed underneath the couch 12 attached to the couch or the end shield to enhance blocking the out-of-bore radiation from reaching the detectors 16. In a preferred embodiment, the lower surface of the end shield 40 conforms to the shape of the bottom of the couch 12. With reference to FIGURES 6 and 7, the imaging system 10 includes a transmission radiation source 70 disposed inside or between the septa 50 forming a transmission radiation source/filter assembly 72. The transmission radiation source 70 transmits the radiation across the examination region 22 to an unobstructed part of the detector 16 which is exposed to the radiation. A motor means 74 rotates the source/filter assembly 72 around the examination region 22 to acquire the projections. The data for reconstruction transmission radiation preferably includes a radioisotope of an energy near 511 kev, but sufficiently different that it can be separated from the radiopharmaceutical radiation on the basis of the energy z of the photon peaks. The reconstruction processor processes the transmission radiation to reconstruct a 3D radiation image representation indicative of the transmission radiation absorbed by the subject 14. The transmission radiation is used to correct the reconstructed emission radiation image representation in the injected radiopharmaceuticals, e.g., for radiation absorbed by bones. In the embodiment of FIGURE 6, the filter 50 spans a variable section of the bore 18 circumference, depending on the radiation source angle λ. Of course, it is also contemplated that the filter has a constant angle such as 180 or 360 degrees. In the embodiment of FIGURE 7, the filter 50 spans two fixed 90 degree sections centered along the major axis of the elliptical aperture 46 which do not rotate with the source. As another option, the source rotates 180+λ around the bore and the septa span the 180-λ that is not irradiated. The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

CLAIMS Having thus described the preferred embodiments, the invention is now claimed to be: 1. A radiographic imaging system (10) comprising: a means (16) for detecting emission radiation emitted from a region of a subject (14) in an examination region (18), the detecting means (16) arranged around a circular bore (18) and defining an axial field-of-view of the imaging system (10); and one or more circumferentially extending septa (50) for shielding the detecting means (16) from the emission radiation originating outside or away from a center of the axial field-of-view, which septa (50) are axially spread sparsely across the field of view.

2. The system as set forth in claim 1, wherein the septa (50) displaced from the detecting means (16) with a defined air gap to minimize an area of the detecting means shielded from the radiation by the septa (50).

3. The system as set forth in claim 2, wherein a ratio of the area shielded by the septa to the field of view is or smaller than 1 to 25.

4. The system as set forth in claim 1, further including: end shields (40) for shielding the detecting means (16) from the emission radiation originating outside or away from the center of the axial field-of-view, which end shields (40) are positioned at an entrance (42) and an exit (44) of the bore (18), at least the end shield or the entrance shield defining a subject receiving aperture (46).

5. The system as set forth in claim 4, wherein the septa (50) include: a pair of curved plates disposed between the end shields (40) and each other, each plate being from 2.5 to 3.5mm thick in an axial direction of the bore (18).

6. The system as set forth in claim 4, wherein the subject receiving aperture (46) is circular and the septa (50) extend circumferentially along an upper portion (60) of circumference of the bore (18).

7. The system as set forth in claim 6, wherein the septa (50) extend circumferentially around the top half of the bore (18).

8. The system as set forth in claim 4, wherein the subject (14) is received in the bore (18) on a subject support (12) and further including: a subject support shield (62) disposed underneath a portion of the subject support (12) to enhance the blocking of the radiation originating outside or away from the center of the axial field-of-view.

9. The system as set forth in claim 4, further including: a transmission radiation source (70) which emits the transmission radiation, which transmission radiation source (70) is integrated within the septa (50); and a means (74) for rotating the transmission radiation source and the septa concurrently around the bore (18).

10. The system as set forth in claim 9, wherein the transmission radiation source (70) transmits the radiation at a radiation source angle λ and the septa (50) span a circumferential section of the bore (18) not radiated by the radiation source (70).

11. The system as set forth in claim 4, wherein the septa (50) extend 360° circumferentially around the bore (18).

12. The system as set forth in claim 4, wherein the subject receiving aperture (46) is non-circular.

13. The system as set forth in claim 12, wherein the subject receiving aperture (46) has a horizontal major axis and a vertical minor axis and the septa (50) span two separate arc segments each centered on the major axis.

14. The system as set forth in claim 13, wherein the arc segments spanned by each septa (50) is substantially 90°.

15. The system as set forth in claim 4, wherein the detecting means (16) includes a multiplicity of radiation detectors disposed in a cylindrical array to define the subject receiving bore (18), and the cepta (50) include a fraction as many radiation shielding plates as radiation detectors spreading axially along the bore (18).

16. The system as set forth in claim 15, wherein two axially spread plates each extend half of circumference of the bore (18).

17. The system as set forth in claim 16, wherein each plate extends a quarter of the circumference of the bore (18).

18. The system as set forth in claim 15, wherein the subject receiving aperture (46) is elongated along a major axis relative to a minor axis and the septa (50) extend in arc segments which intersect the minor axis.

19. The system as set forth in claim 1, wherein the imaging system (10) includes a PET scanner (20).

20. The system as set forth in claim 1, further including: a line of response calculating circuit (24) which calculates lines of response for detected concurrent events; a reconstruction processor (32) which reconstructs the detected radiation into a volumetric image representation; an image memory (34) for storing the resultant volumetric image representation; and a display (38) for displaying at least a portion of the image representation.

21. A method of a 3D radiographic imaging comprising: detecting emission radiation emitted from the subject (14) with the detecting means (16) of the scanner (20) of claim 19; calculating lines of response with a calculating circuit (24); reconstructing an image representation with a reconstruction processor (32); and displaying at least a portion of the image representation on a display (38).