Stationary computed tomography system and method

Short designation: Stationary computer tomography system and method.

The invention generally relates to the field of computed tomography imaging systems. More particularly, the invention relates to geometries and configurations for the stationary computed tomography systems in which a detector and a distributed source element are fixedly positioned in a scanner of the imaging system.

Many applications exist for computed tomography imaging systems. Developed over recent decades, such imaging systems provide a powerful tool for imaging internal features of subjects of interest, which are typically presented as slices and volumes. In general, the systems consist of a source that directs radiation through the subject of interest to a detector. While the source may be any type of radiation capable of penetrating the subject of interest, special attention is given in the description to an X-ray source. The X-ray source and detector are mounted on a rotating gantry in traditional systems and are rotated at a relatively high rotational speed, such as on the order of two revolutions per second, although higher and lower speeds are also employed. Measurements of the intensity of the radiation incident on the detectors are made during rotation in many locations and the results thereof are stored for later analysis and processing. The systems then calculate useful reconstructed images by processing acquired results from intensity measurements, allowing determination of the location of features in the subject. Although variants of this basic design have been proposed and are currently in use, current technologies employ source and detector rotation with a selection of acquisition windows and special processing of the data to improve clarity of the reconstructed images.

While such devices have proven to be extremely useful in identifying features of interest within a subject, they are limited by the need to rotate the source and detector elements. X-ray sources include tyt027506 = pisch conventional X-ray tubes, which are somewhat heavy and must be energized and cooled during rotation. Also, the detectors become increasingly bulky, due to the spanned field of view of the high resolution imaging system, and multiple rows are required to acquire significant amounts of data during an examination. Circuits associated with the detectors must also be rotated to perform the data acquisition and initial operation.

To produce a carefully balanced energy and data transmission link system along with the mechanisms to remove the heat generated during operation requires technical skill.

There is a need today for improved system designs in computed tomography. In particular, there is a significant need for a design that can alleviate rotational loads, or even eliminate the need for joint rotation of system components. There is a particular need for systems which can generate high quality images while reducing the mechanical, electrical, thermal and other problems associated with rotation of a source and detector.

The invention provides new geometries and configurations for computed tomography (CT) systems designed to meet such needs. The technique can be applied in a wide range of application environments, including medical diagnostic contexts, third party inspection, parcel and baggage handling applications, and so on. The techniques allow distributed X-ray sources and detectors to be incorporated into a system without the need to rotate the sources or detectors.

Various useful configurations for distributed sources and detectors are provided, some of which can be used in conjunction with each other to improve image data quality, improve completeness of acquired data for reconstruction purposes, provide coverage for excellent reconstructed images, and so on. The technique can be applied with a variety of emitters or sources operating by different techniques, which emitters or sources can be specifically and uniquely addressed to produce beams of radiation for imaging studies. Technology can also

10275963 are used with a wide variety of detector configurations, with many of the geometries based on detector elements of conventional construction. While certain elements of the system, such as collimators or other elements, can be rotated, the present technique allows principal elements, i.e., the source and detector, to be kept essentially stationary during survey sequences.

FIG. 1 is a schematic representation of an example of a stationary CT system according to aspects of the present art;

Fig. 2 is a schematic representation of an example of a distributed source for use in a system of the type shown in Fig. 1;

Fig. 3 is a schematic representation of a portion of a detector for use with the system in Fig. 1;

Fig. 4 is a schematic representation of a first embodiment of a stationary CT configuration including a ring source and a ring detector;

Fig. 5 is a further configuration employing a ring source, a ring detector and line sources;

Fig. 6 is an alternative configuration employing a ring source between two ring detectors;

FIG. 7 is a schematic representation of an alternate configuration, incorporating a ring source between two ring detectors with line.

applies sources;

Fig. 8 is a schematic representation of an alternative configuration employing a pair of ring sources with a ring detector;

FIG. 9 is a schematic representation of an alternative configuration employing a pair of ring sources and line sources with a ring detector;

Fig. 10 is a schematic representation of an alternate configuration employing a pair of partial ring sources with a ring detector;

FIG. 11 is a schematic representation of an alternative configuration employing partial ring sources and line sources with a ring detector;

Fig. 12 is a schematic representation of an alternative configuration employing an array of ring sources and a ring detector;

FIG. 13 is a schematic representation of an alternative configuration employing an array of ring sources and line sources with a ring detector;

Fig. 14 is a schematic representation of an alternative configuration employing an array of ring sources with an array of ring detectors;

Figure 15 is a schematic representation of an alternative configuration employing an array of ring sources and line sources with an array of ring detectors;

FIG. 16 is a schematic representation of alternative configurations employing one or more partial ring sources, with line sources, and a partial ring detector;

Fig. 17 is a schematic representation of an alternative configuration employing an array of line / arc sources and a ring detector;

Fig. 18 is a schematic representation of an alternative configuration employing a partial ring source and a partial ring detector in a recessed configuration;

FIG. 19 is a schematic representation of an alternative configuration employing a partial ring source and line / arc sources, some of which partially overlap a partial ring detector;

Fig. 20 is a schematic representation of an alternative configuration employing a ring source and ring detector;

Fig. 21 is a schematic representation of an alternate configuration employing a single turn spiral ring source and a ring detector;

Fig. 22 is a schematic representation of an alternative configuration employing a multi-turn spiral ring source and a ring detector;

Fig. 23 is a schematic representation of an alternative configuration employing a ring source, the ring source configured to direct radiation through openings in a ring detector;

Fig. 24 is a schematic representation of an alternative configuration employing a ring source, the ring source configured to direct radiation through slots in a ring detector;

Fig. 25 is a schematic representation of an alternative configuration employing a tilted ring source and ring detector;

Fig. 26 is a schematic representation of an alternate configuration employing a further configuration for a tilted ring source and ring detector;

Fig. 27 is a schematic representation of an alternative configuration employing two tilted ring wells and a ring detector;

Fig. 28 is a schematic representation of an alternate configuration, which also employs two tilted ring wells and a ring detector;

Fig. 29 is a schematic representation of an alternate configuration employing a combination of partial ring sources and ring detectors in complementary recessed configurations;

Fig. 30 is a schematic representation of an alternative configuration employing a ring source and ring detector device for telescope operation or a combined interaction; and Fig. 31 is a schematic representation of an alternative configuration employing a combination of ring and partial ring sources and a ring detector.

Reference is now made first to FIG. 1 of the drawings, which illustrates a stationary computed tomography (CT) system, generally designated by reference numeral 10.

The CT system 10 includes a scanner 12, which is formed of a support structure and which internally contains one or more stationary and distributed X-ray sources (not shown in Fig. 1) and one or more stationary digital detectors (not shown in Fig. 1). 1) as described in detail below. The scanner is arranged to receive a table 14 or other support for a patient or more generally a subject to be scanned. The table can be moved through an opening in the scanner to suitably position the subject in an imaging volume or plane being scanned during imaging sequences.

The system further includes a radiation source controller 16, a table controller 18 and a data acquisition controller 20, all of which operate under the control of a system controller 22. The radiation source controller 16 controls timing for X-ray discharges conducted from points around the scanner 12 to a detector segment on an opposite side thereof, as shown below.

1027586=

- 6 explained. In current stationary CT devices, the radiation source controller 16 can trigger one or more emitters in a distributed X-ray source at any point in time to create multiple acquisitions of emitted X-ray intensity data,

For example, in certain arrangements, the X-ray source controller 16 can trigger the emission of radiation in arrays to collect adjacent or non-adjacent acquisitions of X-ray emitted intensity radiation around the scanner. Many such measurement results can be collected in a survey sequence, and the data acquisition controller 20 coupled to the detector elements receives signals from detector elements and processed signals for storage and later image reconstruction. The table controller 18 then serves to suitably position the table and subject in a plane in which the radiation is emitted or in the present context generally within a volume to be imaged. The table can be moved between two imaging series or during certain imaging series, depending on the imaging protocol used.

The system controller 22 generally controls the operation of the radiation source controller 16, the table controller 18, and the data acquisition controller 20. Thus, the system controller 22 can cause the radiation source controller 16 to trigger the emission of X-rays as well as coordinate such emissions during imaging sequences defined by the system controller. The system controller can also control the motion of the stage in coordination with such emission to collect radiated X-ray intensity measurement data from volumes of particular interest or in different imaging modes, such as helical modes. The system controller 22 also receives the data acquired by the data acquisition controller 20 and coordinates the storage and processing of the data.

It should be kept in mind that the controllers, and essentially various circuits described herein, may be defined by hardware circuits, firmware or software. For example, the particular imaging sequence protocols will generally be defined by code executed by the system controllers. In addition, initial processing, conditioning, filtering, and other operations to be performed on the X-ray intensity data transmitted by the scanner may be performed in one or more of the components shown in FIG. Like

0275967 described below, for example, detector elements will produce analog signals representative of charge depletion in photodiodes positioned at locations corresponding to pixels of the acquisition detector. Such analog signals are converted into digital signals by electronics in the scanner and sent to the data acquisition controller 20. At this point, partial processing can take place and the signals are eventually sent to the system controller for further filtering and processing.

The system controller 22 is also coupled to an operator interface 24 and to one or more memory devices 26. The operator interface may be an integral part of the system control and will generally include an operator workstation for initiating imaging sequences, controlling such sequences, and manipulating data acquired during imaging sequences. The memory devices 26 may be included in the imaging system or may be partially or wholly remote from the system. Thus, the memory devices 26 may include local magnetic or optical memory, or local or remote image data repositories for reconstruction. In addition, the memory devices may be configured to receive raw, partially edited, or fully edited data for reconstruction.

The system controller 22 or operator interface 24, or remote systems and workstations, may include software for image editing and reconstruction. As will be appreciated by those of skill in the art, such processing of CT data can be performed by a number of mathematical algorithms and techniques. For example, conventional filtered back-projection techniques can be used to process and reconstruct the data acquired by the imaging system. Other techniques and techniques used in conjunction with filtered back projection can also be used. A remote link 28 may be included in the system for transmitting data from the imaging system to such remote processing stations or memory devices.

The scanner 12 of the stationary CT system 10 preferably includes one or more distributed X-ray sources as well as one or more digital detectors for receiving radiation and processing corresponding signals to produce data. FIG. 2 shows part of an example of a distributed X-ray source of the type,

1027598 which can be used in the stationary CT system. As shown in FIG. 2, in an exemplary embodiment, the X-ray distributed source 30 may include an array of electron beam emitters 32 coupled to a radiation source controller 16 shown in FIG. 1 and triggered by the source controller during scanner operation. The electron beam emitters 32 are positioned adjacent to a target 34. Upon triggering by the source controller, the electron beam emitters 32 can emit electron beams 36 to the target 34. Target 34, which may be, for example, a rail or element of woolly frame, emits X-rays, as indicated by reference numeral 38, upon impact of the electron beams. The X-ray source can operate in a reflection or transmission mode. In the figure shown in fig. 2 The reflection mode shown, it is intended that the X-rays are produced substantially on the same side of the target as where the electrons impact. In transmission mode, the X-rays are produced on the opposite side of the target. The X-ray beams 38 are then directed to a collimator 40, which collimator is generally opaque to the X-ray, but includes apertures 42. The apertures 42 can be of a fixed size or adjustable. The apertures 42 allow a portion of the X-ray beams to pass through the collimator to form collimated beams 44, which collimated beams will be directed through the subject of interest to the imaging volume of the scanner and detector units on an opposite side of the scanner. will hit.

A number of alternative configurations for emitters or distributed sources may, of course, be provided. In addition, the individual X-ray sources in the distributed source can emit different types and shapes of X-ray beams. These may include, for example, fan beams, cone beams and beams of different cross-sectional geometries. In addition, the different components included in the distributed X-ray source can also vary. For example, in one embodiment, a cold cathode emitter is provided, which emitter will be included in a vacuum housing. A stationary anode is then placed in the housing and remote from the emitter. This type of device generally corresponds to the schematic illustration of Fig. 2. Other materials, configurations, and operating principles for the distributed source may, of course, be employed. The emission devices can be one of a kind

10275969 available electron emission devices are, for example, thermionic emitters, carbon-based emitters, photo-emitters, ferroelectric emitters, laser diodes, monolithic semiconductors, etc.

As explained in detail below, the present stationary CT techniques are based on the use of a number of distributed and controllable X-ray sources. In addition, the distributed radiation sources can be connected to a single enclosure or tube or to a plurality of tubes designed to cooperate. Some of the source configurations described below are arcuate or annular in order to be positionable around the aperture in the scanner. Other sources are linear in configuration to extend along the image forming volume in the z direction in terms of the conventional CT nomenclature. The individual sources can be controlled independently and individually, so that radiation from each of the sources can be triggered at times during the imaging sequence as defined by the imaging protocol. In other configurations, the sources are controllable in logical groups, for example, pairs or triples of emitters may be connected together by wires. If desired, more than one source can be triggered simultaneously at any point in time, or the sources can be triggered in specific sequences to mimic rotation of a portal or in any desired sequence around the rendering volume or plane.

A number of detector elements form one or more detectors, which receive the radiation emitted by the distributed sources. FIG. 3 shows a portion of a detector that can be used for the present purposes. Each detector can consist of detector elements of varying resolution to suit a particular imaging application. The detector arrangement may be generally similar to detectors used in conventional rotary CT systems, but is extended around a greater portion or all of the interior surface of the scanner. Special configurations for the detector or detectors are summarized below. Generally, however, the detector 46 includes an array of detector elements 48 and associated sig35 retouching circuitry 50. Each detector element can contain an array of photodiodes and associated thin film transistors. The X-rays hitting the detectors are converted into lower energy photons by a scintillator and these photons strike the photodiodes. A charge maintained across the photodiodes is thus depleted

The transistors can be controlled to recharge the photodiodes and thus measure the charge depletion. By sequentially measuring the charge depletion in the different photodiodes, each corresponding to a pixel in the collected data for each acquisition, data is collected that encodes the emitted radiation at each of the pixel locations. This data is processed by the signal processing circuitry 50, which will generally convert the analog depletion signals to digital values, perform necessary filtering, and send the acquired data to the processing circuitry of the imaging system, as described above. While the detector has been described in terms of a scintillator-based energy integrator, detectors with direct conversion, photon count, and energy discrimination are also suitable.

In the detector, a plurality of detector elements 48 may be interconnected to define many rows and columns of pixels. As described below, the detector configurations of the present art position detector elements opposite independently controllable distributed X-ray sources to collect a large number of view acquisitions for image reconstruction.

As will be recognized by those skilled in the art, reconstruction techniques in CT systems vary in their use of acquired data and in their techniques and assumptions for image reconstruction. In the present art, it has been found that a number of geometries are available for high speed and efficient operation of a stationary CT system, which data provides for accurate image reconstruction. FIG. 4-31 show examples of geometries and configurations for distributed sources and for detectors which are stationary in the CT scanner, but which can be used with conventional or improved image processing and image reconstruction algorithms.

Many of the current contemplated geometries include 360 ​​° sources and / or detectors. However, some of these concepts can be reduced to less expensive and similarly or generally satisfactory configurations by eliminating part of the entire ring in these structures. For example, depending on the image reconstruction technique employed, detectors extending through an angle of 180 ° plus the fan angle of the emitted X-ray beams may be sufficient for excellent data collection and image reconstruction. In addition, for mechanical reasons, there may be a gap between sources and detectors at transition planes. Such gaps can be accounted for by additional measurements from sources and / or detectors, which take into account the missing data. In addition, the configurations described below are provided for both axial and helical scan modes. However, depending on the particular application, certain configurations of these modes may be more suitable than others, such as the axial mode for medical applications and the helical modes for applications such as baggage sensing. Finally, the sources and detectors described in the configurations below can have different diameters, sizes, extensions, and so on. In addition, the sources and detectors can be linear sections or planar sections, approximating the configurations explained below.

In a first configuration shown in Fig. 4, a ring detector 52 includes a plurality of detector elements, such as those shown in Fig. 3, which extend generally completely around an imaging volume. A distributed source in the form of a ring source 54 is positioned adjacent to the ring detector 52 and includes a plurality of individually controllable sources or emitters, as described above. The sources can be triggered by the system controller to emit radiation to the detector generally located in a diametrically opposite location, which radiation passes through the subject of interest, is attenuated by characteristics of the subject of interest, and strikes the detector for data collection.

The configuration of Fig. 5 includes a ring detector 52 of the type shown in Fig. 4, along with a ring source 54. The device also includes a pair of line sources along the Z direction, as indicated by line sources 56. The single ring source is positioned on one side of the detector and the line sources 56 extend from the ring source to add to the completeness of the data collected by the system. The ring detector 52 may contain gaps to accommodate the line sources 56. While such line35 sources can increase the complexity of data collection operations, they are believed to potentially improve the mathematical completeness of the collected data acquisitions. Although two such line sources are shown in FIG. 5, this one can

10275Θ6 configuration contain a single line source or more than two line sources.

The configuration of Fig. 6 includes a pair of ring detectors 52, between which detectors 52 a ring source 54 is positioned. This configuration is believed to be particularly attractive to low dose efficiency and may be particularly useful in the detection of certain substances of interest, such as explosives.

The configuration of FIG. 7 includes a pair of ring detectors 52, a ring source 54 and one or more line sources 56. The ring source is positioned between the ring detectors, and the line sources extend generally parallel to the ring detector. Gaps may be provided in the ring detectors to accommodate the line sources. The line sources are added to the configuration to measure additional data for mathematical completeness of the acquired data, as in the case of the device of Fig. 5.

FIG. 8 shows a further alternative embodiment, in which two ring sources 54 are positioned in a flanking arrangement on either side of a ring detector 52. It is believed that this arrangement can provide reasonable data completeness and that missing areas of interest around the center plane of the scanner can be reduced. In addition, the detector can be made smaller than in the case of the previous device for the same axial coverage at the center of the field of view of the imaging system. In addition, scattering can become significant.

reduced due to the use of a smaller cone angle for the sources. A higher data acquisition sampling rate can,^:in this arrangement are required if the same number of views are to be maintained for each ring source.

The configuration of FIG. 9 includes a ring detector 52 flanked by a pair of ring sources 54 and including one or more line sources 56. As before, the line sources provide a means of acquiring additional data, which adds to the mathematical completeness of the collective data acquisition, and the ring sources provide the advantages of the configuration of Fig. 8.

The configuration of FIG. 10 is based on a combination of partial ring sources 58 flanking a ring detector 52 of the type described above. The partial ring sources are shown as being two in number, although other numbers of partial ring sources may be included, and these may be positioned on alternate sides of the ring detector. It is assumed that the configuration of Fig. 10 reduces the cost of the resources as compared to full-ring sources, although somewhat less mathematically complete data may result. To provide partial ring sources on alternate sides of the ring detector, equal numbers of partial ring sources should be used, with better results expected from two partial ring sources and lesser results expected from four partial ring sources.

The configuration of FIG. 11 is similar to that of FIG. 10, although the apparatus further includes line sources 56 in combination with the partial ring sources 58. As in the previous cases of using line sources 56, gaps may be provided in the ring detector 52 to allow radiation to be emitted from the line sources along the Z axis. The line sources along the Z axis contribute to the mathematical completeness of the collective data acquisition. In the illustration of Figure 11, two or four such partial ring sources may be used, although this number may vary to larger numbers.

The configuration of Fig. 12 employs a ring detector as in the foregoing arrangement flanked by pairs of ring sources 54. In this embodiment, two such ring sources are provided immediately adjacent to the ring detector 52 and additional ring sources are located at remote locations on either side thereof. eats. It is envisaged that the outer rings can provide redundant data when used in conjunction with the inner ring. Similarly, the configuration of Fig. 13 uses ring sources and a ring detector in conjunction with a pair of line sources 56 in the Z direction. As before, it is envisioned that the line sources 56 can be helpful in providing more complete data for image reconstruction.

The configurations of Figures 14 and 15 are analogous to those of Figures 12 and 13, although multiple ring detectors are shown positioned between the spaced apart ring sources. As will be appreciated by those of skill in the art, the configurations of Fig. 14 and Fig. 15 result in smaller cone angles for the emitted radiation for given subject coverage areas. the Z direction compared to the configurations in Fig. 8 and Fig.

9. If desired, the detectors 52 of the device of FIG. 14 and FIG. 15 may have different resolutions, with a central detector having a higher resolution than the outer detectors. The arrangement of FIG. 15 is similar to that of FIG. 14 but adds line detectors along the axis to improve the mathematical completeness of the acquired data.

Depending on the application, one or more ring sources 54 and one or more ring detectors 52 in Fig. 14 may be used for a particular imaging protocol. Various data acquisition schemes are provided in which the resolution of the detectors can be configured depending on the number of ring detectors 52 and ring sources 54 used - minimizing the electronics required in the data acquisition system for the detector. The device of fig. 14 using configurations are applicable to the device of Fig. 15; however, additional data is acquired using available line sources 56.

The configurations of Figure 16 include at least one partial ring source 58 in conjunction with a partial ring detector 60. The devices may also include one or more line sources 56 along the Z axis. Devices as shown in Figure 16 provide coverage of relevant portions of the imaging volume, such as 180 ° of the volume through the detector and 180 ° of the volume through the source. Advantages of such devices include the fact that the source and detector can be placed in the same general imaging plane. However, data may be somewhat incomplete in some devices.

The configuration of FIG. 17 includes a generally annular detector 52 and oblique arcuate partial-annular sources 62. The sources are arranged to emit radiation through the detector, which detector may be of slots or apertures for receiving the sources therein. to provide. The resulting structure provides a generally arcuate profile, a linear profile, a sinusoid, a tilted circle, or two tilted circles. The design can be extremely flexible from the point of view of a mathematical completeness of the data acquisition.

FIG. 18 shows an exemplary configuration including a modified partial ring detector indicated by reference numeral 64. A partial ring source 58 is positioned to extend into recesses 66 of the detector. The device of fig.

allows the source and detector both to cover more than 180 ° of the imaging volume. In a preferred configuration, the source covers at least an angular range of 180 ”plus the fan angle of the radiation emitted by the independently controlled and distributed sources.

The configuration of Fig. 19 is similar to that of Fig. 18, but allows the coverage of an additional volume through the combination of multiple partial ring sources 58 with line sources along the Z axis, as indicated by the reference numeral 56. The composite The source may then cover an angular range of greater than 180 ° of the imaging volume, and preferably an angular range of 180® plus a fan angle of the radiation or more. The stationary detector is then a partial ring 60, as described above. It can also cover an angular range of more than 180 ° of the imaging volume.

The configuration of Figure 20 is somewhat similar to that of Figure 6. The configuration consists of a ring detector 52 and a ring source 54. However, in the arrangement of Figure 20, the detector is provided as a single unit without a central gap. The central detector elements will be physically arranged to enable the distributed source 54 to emit radiation through the detector on one side to be detected by detector elements on an opposite side of the device.

The configuration of Figure 21 includes a spiral source 68 located adjacent to a spiral detector 70. The source and detector may generally be of similar configurations to the configurations described above, but as shown in FIG.

Spiral arrangement shown allows scanning in a manner corresponding to conventional spiral volumetric acquisition. Accordingly, the apparatus of Fig. 21 can be used to obtain data acquisition results similar to the results obtainable through the configuration of Fig. 6 but with table translation. The configuration of fig. 21 enables such acquisition without missing portions or slices of the imaging volume.

The configuration of Fig. 22 is similar to that of Fig. 21, but herein the spiral source 68 forms a number of turns around the imaging volume, as does the adjacent spiral detector 70. The arrangement of Fig. 22 provides relatively complete data except at the edges of the coil and except in the gaps between the source and the detector.

In another configuration shown in FIG. 23, an annular source 54 containing a plurality of independently controllable emitting sources represented by dots in FIG. 23 is disposed around a ring detector 52. The resulting arrangement is similar to that of FIG. 6. However, in the device of Figure 23, the detector is provided with apertures 72 through which the source can transmit radiation. The arrangement allows additional data to be collected between the locations at which the distributed source transmits through the detector.

In a slightly different configuration shown in Fig. 24, a ring source 54 at least partially surrounds a ring detector, which is shown as containing a plurality of segments 74. The segments are separated from each other by slots or openings 76. In practice, one or more such ring sources can be used. The source emits radiation through slots 76, which may be oriented generally along the Z axis.

FIG. 25-28 show exemplary configurations, which include annular sources indicated by reference numeral 78, which are used in conjunction with an annular detector 52. For example, in the arrangement of Fig. 25, the detector is arranged to provide space for emission of radiation in locations where the ring source intersects the detector, in a manner similar to the manner explained above with reference to Fig. 17. In the alternative arrangement of fig. 26 the ring detector 52 is used with eex | tilted ring source 78, but does not provide passage for radiation emitted through the detector. In the alternate configuration of Fig. 27, two tilted ring sources 78 are employed in a manner similar to that shown in Fig. 25 with a ring detector 52. In this embodiment, vias provide a permitted emission of radiation through the ring detector, as described above. with reference to FIG. 25. In the configuration of Fig. 28, two ring sources 78 are again employed, but no passage is provided for emission of radiation through the ring detector 52 in a manner similar to that described in Fig. 26 above. Although the wells are shown in Figures 25-28 as being annular, they may be truncated and made up of linear segments. Moreover

02 In FIGS. 26 and 28, the source sections whose emitted radiation is blocked by the detector can be omitted.

The arrangement of Fig. 29 is essentially a combination of two recessed detectors and corresponding source elements of the type shown above with reference to Fig. 18. That is, two recessed detectors 64 presenting recesses 66 for receiving a partial ring source 58 are combined end-to-end to provide a generally complete arrangement around the imaging volume. The device of Fig. 29 is shown

I is believed to provide benefits in terms of the mathematical completeness of the acquired data. In addition, line sources 56 extending along the Z axis can be added to the device to improve data completeness.

The device of FIG. 30 includes a pair of ring sources 54 connected to respective ring detectors 52. The pairs of sources and detectors, indicated in Figure 30 by reference numerals 80 and 82, have different diameters, which allows telescopic movement of one device within the other. In some respects, the device of Figure 30 is similar to the double ring 20 source device of Figure 3. 8, but in which the detector is split into two concentric parts to allow for adaptive Z-axis coverage. Such adaptive coverage can allow for lower doses and can reduce scattering in studies where only a small Z-axis coverage is desired. This device is not limited to pairs of ring sources and ring detectors, but may include two or more ring sources and ring detectors.

A further configuration shown in Fig. 31 includes a plurality of ring sources, two of which are shown in the figure, as indicated by the reference numeral 54, in combination with a partial ring source 58. A modified ring detector, indicated by the reference numeral 84 , is provided between the ring sources 54. The modified detector 84 includes an opening 86 for receiving the partial ring source 58. The device of FIG. 31, and devices of this type, which include a combination of ring 35 sources and partial ring sources, can provide more mathematical completeness in the acquired data than the devices in the previous configurations. To further improve the mathematical completeness of the measured data, line sources, which are located

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extending along the Z axis can be included in this configuration, however these line sources are not shown in FIG.

While the invention may be susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and described in detail herein. However, it will be understood that the invention is not intended to be limited to the particular shapes disclosed. Rather, the invention is intended to cover all modifications, equivalents and alternatives within the spirit and scope of the invention as defined by the following claims.

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