Phantom for spectral CT image system calibration

A phantom includes a housing enclosing an interior volume and having a plurality of passages formed therein, wherein each passage is fluidly isolated from the interior volume. First and second inserts are included and configured to be positioned in a first passage of the plurality of passages and include materials having a known material density. The material is selected from iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). The material of the inserts can be different materials or the same material at different densities.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to CT imaging and, more particularly, to a phantom for spectral CT image system calibration.

Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis that ultimately produces an image.

Generally, the x-ray source and the detector assembly are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. The detector assembly is typically made of a plurality of detector modules. Data representing the intensity of the received x-ray beam at each of the detector elements is collected across a range of gantry angles. The data are ultimately processed to form an image.

Conventional computed tomography (CT) systems emit an x-ray with a polychromatic spectrum. The x-ray attenuation of each material in the subject depends on the energy of the emitted x-ray. If CT projection data is acquired at multiple x-ray energy levels or spectra, the data contains additional information about the subject or object being imaged that is not contained within a conventional CT image. For example, spectral CT data can be used to produce a new image with x-ray attenuation coefficients equivalent to a chosen monochromatic energy. Such a monochromatic image includes image data where the intensity values of the voxels are assigned as if a CT image were created by collecting projection data from the subject with a monochromatic x-ray beam.

A principle objective of energy sensitive scanning is to obtain diagnostic CT images that enhance information (contrast separation, material specificity, etc.) within the image by utilizing two or more scans at different chromatic energy states. A number of techniques have been proposed to achieve energy sensitive scanning including acquiring two or more scans either (1) back-to-back sequentially in time where the scans require multiple rotations of the gantry around the subject or (2) interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials.

High frequency generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views. As a result, data for two or more energy sensitive scans may be obtained in a temporally interleaved fashion rather than two separate scans made several seconds apart as typically occurs with previous CT technology. The interleaved projection data may furthermore be registered so that the same path lengths are defined at each energy level using, for example, some form of interpolation. Spectral CT data facilitates better discrimination of tissues, making it easier to differentiate between materials such as tissues containing calcium and iodine, for example.

It is important that spectral CT system provide material density images that are accurate. Accordingly, spectral CT systems need to be calibrated to meet the accuracy specifications for different material images. Known calibration methods for the material domain include creating individual material phantoms for a variety of materials and separately analyzing each one. These individual material phantoms are generally created just prior to calibration and are discarded after calibration due since they cannot be stored. The phantom created for one material may vary from the phantom created for the same material at a different time or by a different technician. Additionally, it may be difficult to calibrate a spectral CT system for different patient sizes using such phantoms.

X-ray or CT phantoms for non-spectral CT imaging systems can be made to last a long time. Such phantoms typically comprise synthetic materials configured to mimic the x-ray attenuation of clinically relevant materials such as iodine, fat, water, calcium, and the like in Hounsfield units (HU). However, these synthetic material phantoms fail to accurately mimic the same materials in spectral CT imaging systems. For example, polytetrafluoroethylene (PTFE) or a similar material has been used in the image domain to simulate the HU range of calcium. In the material domain, PTFE fails to mimic calcium.

Therefore, it would be desirable to design a phantom for spectral CT that overcomes the aforementioned drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect, a phantom includes a housing enclosing an interior volume and having a plurality of passages formed therein, wherein each passage is fluidly isolated from the interior volume. A first insert is included and configured to be positioned in a first passage of the plurality of passages and comprising a first material having a known material density of one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A second insert is also included and configured to be positioned in a second passage of the plurality of passages and comprising a second material having a known material density of one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). The known material density of the second material is one of a different material density of a same material as the first material and a known material density of a different material than the first material.

In accordance with another aspect, an apparatus includes a housing having a pair of slots formed therein and having an interior volume hermetically sealed from the pair of slots. A first material insert is configured to be positioned in one of the pair of slots and comprising a known density of a first material selected from the group consisting of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A second material insert is configured to be positioned in another of the pair of slots and comprising a known density of a second material selected from the group consisting of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). The second material is one of a distinct material from the first material and a same material as the first material.

In accordance with yet another aspect, phantom for spectral CT imaging calibration includes an enclosure enclosing a volume and having a plurality of passages formed therein, wherein the volume is hermetically sealed from the plurality of passages. A first insert is configured to be positioned in a first passage of the plurality of passages and comprising a known density of a first material comprising one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A second insert is configured to be positioned in a second passage of the plurality of passages and comprising a known density of a second material different from the first material and comprising one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A shell having an opening therein is configured to receive the housing.

DETAILED DESCRIPTION

The operating environment of the invention is described with respect to a sixty-four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the invention is equally applicable for use with other multi-slice configurations. Moreover, the invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that the invention is equally applicable for the detection and conversion of other high frequency electromagnetic energy. The invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems.

In addition, certain embodiments of the present invention provide systems, methods, and computer instructions for analyzing multi-energy data, such as dual energy data, for example. Certain multi-energy data can be used in spectral imaging systems, such as photon counting systems, for example. Dual energy data, which is a type of multi-energy data, can be embodied in monochromatic images, material density images, and/or effective-Z images. While many of the embodiments described herein are discussed in connection with dual energy data, the embodiments are not limited to dual energy data and can be used in connection with other types of multi-energy data, as one skilled in the art will appreciate. Also, while many of the embodiments discussed herein discussed describe a region of interest that can be selected in an image, a volume of interest can also be selected in an image, as one skilled in the art will appreciate.

Referring toFIG. 1, a computed tomography (CT) imaging system10is shown as including a gantry12representative of a “third generation” CT scanner. Gantry12has an x-ray source14that projects a beam of x-rays toward a detector assembly or collimator16on the opposite side of the gantry12. Referring now toFIG. 2, detector assembly16is formed by a plurality of detectors18and data acquisition systems (DAS)20. The plurality of detectors18sense the projected x-rays22that pass through a medical patient24, and DAS20converts the data to digital signals for subsequent processing. Each detector18produces an analog electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient24. During a scan to acquire x-ray projection data, gantry12and the components mounted thereon rotate about a center of rotation26.

Rotation of gantry12and the operation of x-ray source14are governed by a control mechanism28of CT system10. Control mechanism28includes an x-ray controller30that provides power and timing signals to an x-ray source14and a gantry motor controller32that controls the rotational speed and position of gantry12. An image reconstructor34receives sampled and digitized x-ray data from DAS20and performs high speed reconstruction. The reconstructed image is applied as an input to a computer36which stores the image in a mass storage device38.

Computer36also receives commands and scanning parameters from an operator via console40that has some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated display42allows the operator to observe the reconstructed image and other data from computer36. The operator supplied commands and parameters are used by computer36to provide control signals and information to DAS20, x-ray controller30and gantry motor controller32. In addition, computer36operates a table motor controller44which controls a motorized table46to position patient24and gantry12. Particularly, table46moves patients24through a gantry opening48ofFIG. 1in whole or in part.

As shown inFIG. 3, detector assembly16includes rails50having collimating blades or plates52placed therebetween. Plates52are positioned to collimate x-rays22before such beams impinge upon, for instance, detector18ofFIG. 4positioned on detector assembly16. In one embodiment, detector assembly16includes 57 detectors18, each detector18having an array size of 64×22 of pixel elements54. As a result, detector assembly16has 64 rows and 912 columns (22×57 detectors) which allows 64 simultaneous slices of data to be collected with each rotation of gantry12.

Referring toFIG. 4, detector18includes DAS20, with each detector18including a number of detector elements54arranged in pack56. Detectors18include pins58positioned within pack56relative to detector elements54. Pack56is positioned on a backlit diode array60having a plurality of diodes62. Backlit diode array60is in turn positioned on multi-layer substrate64. Spacers66are positioned on multi-layer substrate64. Detector elements54are optically coupled to backlit diode array60, and backlit diode array60is in turn electrically coupled to multi-layer substrate64. Flex circuits68are attached to face70of multi-layer substrate64and to DAS20. Detectors18are positioned within detector assembly16by use of pins58.

In the operation of one embodiment, x-rays impinging within detector elements54generate photons which traverse pack56, thereby generating an analog signal which is detected on a diode within backlit diode array60. The analog signal generated is carried through multi-layer substrate64, through flex circuits68, to DAS20wherein the analog signal is converted to a digital signal.

Referring now toFIG. 5, a phantom72for spectral CT image calibration according to an embodiment of the invention is shown. Phantom72includes a housing or enclosure74having a hollow, interior volume76. In one embodiment, interior volume76is filled with water. A plurality of slots78-80formed in housing74allow the placement of material inserts86therein. Slots78-80may extend partially into or all the way through housing74. As illustrated, slots78extend through housing74from a first surface or side88to a second surface or side90, and slot80extends only partially into housing74from first side88toward, but not extending through, second side90. Interior volume76is hermetically sealed from the ambient environment and is fluidly isolated from each of the slots78-80.

Material inserts86are solid inserts that contain materials relevant to those materials typically found in imaging patients. For example, material inserts86may contain clinically relevant materials such as iodine, hydroxyapatite (HAP) or tricalcium phosphate (TCP), body fat or fatty plaque, sodium chloride (NaCl), or other biomarker materials such as gold (Au) or iron (Fe). These enumerated materials are not exhaustive of the plurality of materials that may be used, however, and embodiments of the invention are not limited to such.

The clinically relevant materials may be suspended and preserved in a matrix to form the solid insert86. The matrix may be, in an example, a polymer or epoxy matrix. In a preferred embodiment, the matrix is a neutral encapsulant that does not interfere or react with the suspended clinically relevant material. The polymer or epoxy matrix helps ensure that the clinically relevant material suspended therein does not lose its properties over time. Furthermore, the concentration (e.g., mg/cc) of the clinically relevant material in the solid insert86is known and can be accounted for during calibration.

In one embodiment, the solid material inserts86may be similarly sized so as to be interchangeable with one another and placed in any of the corresponding slots78to create a variety of different phantom combinations for calibration. Phantom72may thus be customized for particular calibration parameters. In another embodiment, the sizes of one or more of the inserts86may be different than other inserts86so as to create a larger or smaller quantity of respective clinically relevant material for calibration.

As shown inFIG. 6, a plurality of inserts92-104of a first combination are positioned in a phantom housing74according to one embodiment. Inserts92-104are positioned according to a first placement scheme. In this example, inserts92-94are calcium inserts having a known density of a calcium material such as HAP or TCP, insert96is a soft tissue insert having a known density of water and NaCl, and insert98is a fat/oil insert having a known density of fat or oil. Inserts100-104include, in this example, iodinated contrast in different known concentrations or densities. Calibration of a spectral CT imaging system using this phantom configuration includes simultaneously calibrating for four different materials and calibrating for three different known concentrations of one of the materials.

FIG. 7illustrates a placement of the combination of material inserts92-104in a second placement scheme according to another embodiment of the invention. As illustrated, inserts92-104are re-positioned with respect to that illustrated inFIG. 6. In addition to merely allowing for re-positioning material inserts92-104among the various slots78,80a different combination of inserts may be positioned in housing74. For example, it may be desirable to have only a single type of material insert positioned in housing74. In this case, one or more inserts of a single material type may be positioned in slots78,80. Alternatively, it may be desirable to fill each slot78,80with a combination of material inserts that includes a different material for each of the inserts.

A shell106having an opening108is also shown inFIG. 7positioned about housing74. Shell106is removable from housing74and attenuates x-rays to simulate a particular patient size. A plurality of shells106of various sizes and shapes may be configured to removably engage housing74to allow for calibrating the spectral CT imaging system based on a plurality of patient sizes.

Embodiments of the invention include any shape to housing74or material inserts92-104and passages78,80. That is, while the cross-sectional shapes of housing74or material inserts92-104and passages78,80are illustrated inFIGS. 5-7as circular cylinders, embodiments of the invention may include other elliptical or polygonal shapes. For example, as shown inFIG. 8, housing74, material inserts92-104, and passages78are shown as having an elliptical cross-sectional shape, while passage80and insert94are shown as having a square cross-sectional shape. It is contemplated that any of inserts92-104with their respective passages78,80as shown inFIG. 8may extend either only partially into or completely through housing74.

Embodiments of the invention allow for switching out the different material concentration modules/inserts for application specific uses including calibration. The phantom described herein allows for assessing the quantitative accuracy and quality of a spectral CT system for different materials at the same time.

While embodiments of the invention are described as being usable with spectral CT image systems, one skilled in the art will recognize that the embodiments of the invention described herein are also applicable to calibration of any imaging system based on x-ray detection. That is, embodiments of the invention described herein may also be used in an x-ray or CT system for calibration based on Hounsfield units.

Referring now toFIG. 9, package/baggage inspection system110includes a rotatable gantry112having an opening114therein through which packages or pieces of baggage may pass. The rotatable gantry112houses a high frequency electromagnetic energy source116as well as a detector assembly118having scintillator arrays comprised of scintillator cells similar to that shown inFIG. 3or4. A conveyor system120is also provided and includes a conveyor belt122supported by structure124to automatically and continuously pass packages or baggage pieces126through opening114to be scanned. Objects126are fed through opening114by conveyor belt122, imaging data is then acquired, and the conveyor belt122removes the packages126from opening114in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages126for explosives, knives, guns, contraband, etc.

Therefore, in accordance with one embodiment, a phantom includes a housing enclosing an interior volume and having a plurality of passages formed therein, wherein each passage is fluidly isolated from the interior volume. A first insert is included and configured to be positioned in a first passage of the plurality of passages and comprising a first material having a known material density of one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A second insert is also included and configured to be positioned in a second passage of the plurality of passages and comprising a second material having a known material density of one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). The known material density of the second material is one of a different material density of a same material as the first material and a known material density of a different material than the first material.

In accordance with another embodiment, an apparatus includes a housing having a pair of slots formed therein and having an interior volume hermetically sealed from the pair of slots. A first material insert is configured to be positioned in one of the pair of slots and comprising a known density of a first material selected from the group consisting of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A second material insert is configured to be positioned in another of the pair of slots and comprising a known density of a second material selected from the group consisting of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). The second material is one of a distinct material from the first material and a same material as the first material.

In accordance with yet another embodiment, phantom for spectral CT imaging calibration includes an enclosure enclosing a volume and having a plurality of passages formed therein, wherein the volume is hermetically sealed from the plurality of passages. A first insert is configured to be positioned in a first passage of the plurality of passages and comprising a known density of a first material comprising one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A second insert is configured to be positioned in a second passage of the plurality of passages and comprising a known density of a second material different from the first material and comprising one of iodine, hydroxyapatite (HAP), tricalcium phosphate (TCP), body fat, fatty plaque, sodium chloride (NaCl), gold (Au), and iron (Fe). A shell having an opening therein is configured to receive the housing.