WO2023282895A1

WO2023282895A1 - Attenuation characteristics for objects

Info

Publication number: WO2023282895A1
Application number: PCT/US2021/040705
Authority: WO
Inventors: Clara REMACHA CORBALAN; Virginia Palacios Camarero
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-07-07
Filing date: 2021-07-07
Publication date: 2023-01-12

Abstract

In an example, a method includes receiving an image indicative of attenuation of radiation by a first object. The method may include deriving object model data for use in generating a second object representing the first object in a radiation environment, using additive manufacturing. In some examples deriving the object model data includes deriving, from the image, a plurality of zones, each associated with a different radiation attenuation level, determining a radiation condition of the radiation environment, selecting, for each zone and based on the radiation condition, an additive manufacturing attribute comprising an additive manufacturing material and/or a material density for each zone, wherein the additive manufacturing attributes are associated with attenuation characteristics and deriving the object model data by combining selected additive manufacturing attributes to specify a plurality of regions of the second object, which, when the second object is generated, correspond to a geometrical configuration of the zones.

Description

ATTENUATION CHARACTERISTICS FOR OBJECTS

BACKGROUND

[0001] Additive manufacturing techniques may generate a three-dimensional object through the solidification of a build material, for example on a layer-by-layer basis. In examples of such techniques, build material may be supplied in a layer-wise manner and the solidification method may include heating the layers of build material to cause melting in selected regions. In other techniques, chemical solidification and/or binding methods may be used.

BRIEF DESCRIPTION OF DRAWINGS

[0002] Non-limiting examples will now be described with reference to the accompanying drawings, in which:

[0003] Figures 1A and 1B are examples of a method for obtaining object model data for use in generating an object in additive manufacturing;

[0004] Figures 2A and 2B are graphs showing examples of relationships between radiation energy and attenuation;

[0005] Figure 3A is an example of an apparatus for obtaining an image indicative of attenuation of radiation of an object;

[0006] Figure 3B is an example of an object generated by additive manufacturing;

[0007] Figure 4 is an example method of generating an object according to object model data by additive manufacturing;

[0008] Figure 5 is an example of an apparatus;

[0009] Figure 6 is another example of an apparatus; and

[0010] Figure 7 is an example machine-readable medium associated with a processor. DETAILED DESCRIPTION

[0011] Additive manufacturing techniques may generate a three-dimensional (3D) object through the solidification of a build material. In some examples, the build material is a powder-like granular material, which may for example be a plastic, ceramic or metal powder and the properties of generated objects may depend on the type of build material and the type of solidification mechanism used. Build material may be deposited, for example on a print bed and processed layer by layer, for example within a fabrication chamber. According to one example, a suitable build material may be Polyamide materials (e.g., PA12, PA11), Thermoplastic Polyurethane (TPU) materials, Thermoplastic Polyamide materials (TPA), Polypropylene (PP) and the like.

[0012] In some examples, selective solidification is achieved through directional application of energy, for example using a laser or electron beam which results in solidification of build material where the directional energy is applied. In other examples, at least one print agent may be selectively applied to the build material, and may be liquid when applied. For example, a fusing agent (also termed a ‘coalescence agent’ or ‘coalescing agent’) may be selectively distributed onto portions of a layer of build material in a pattern derived from data representing a slice of a 3D object to be generated (which may for example be determined from structural design data). The data may be derived from a digital or data model of the object, e.g. object model data provides a data, or virtual, model of an object to be generated. The fusing agent may have a composition which absorbs energy such that, when energy (for example, heat) is applied to the layer, the build material to which it has been applied heats up, coalesces and solidifies, upon cooling, to form a slice of the 3D object in accordance with the pattern. In other examples, coalescence may be achieved in some other manner. [0013] According to one example, a suitable fusing agent may be an ink-type formulation comprising carbon black. Such a fusing agent may comprise any or any combination of an infra-red light absorber, a near infra-red light absorber, a visible light absorber and a UV light absorber. Examples of fusing agents comprising visible light absorption enhancers are dye based colored ink and pigment based colored ink. [0014] In addition to a fusing agent, in some examples, a print agent may comprise a coalescence modifier agent, which acts to modify the effects of a fusing agent for example by reducing or increasing coalescence or to assist in producing a particular finish or appearance to an object, and such agents may therefore be termed detailing agents. In some examples, detailing agent may be used near edge surfaces of an object being printed to reduce coalescence. A coloring agent, for example comprising a dye or colorant, may in some examples be used as a fusing agent or a coalescence modifier agent, and/or as a print agent to provide a particular color for the object.

[0015] As noted above, additive manufacturing systems may generate objects based on structural design data. This may involve a designer determining a data model of an object to be generated, for example using a computer aided design (CAD) application. The model may define the solid portions of the object. To generate a 3D object from the model using an additive manufacturing system, the model data can be processed to define slices or parallel planes of the model. Each slice may define a portion of a respective layer of build material that is to be solidified or caused to coalesce by the additive manufacturing system.

[0016] Medical imaging and radiotherapy can be used to diagnose and treat patients by the application of radiation to a patient. Examples of medical imaging include x-ray imaging, CT (computed tomography), PET (positron emission tomography), fluoroscopy, MRI (magnetic resonance imaging) and ultrasound. As used herein, radiation refers to the transmission of energy in the form of waves, such as electromagnetic radiation or acoustic radiation. Some methods of medical imaging utilise non-ionising radiation such as MRI or ultrasound, whereas other methods use ionising radiation such as x-ray or CT. Radiation can also be used to treat patients in radiotherapy, for example using radionuclide therapy, brachytherapy, or external beam radiation therapy.

[0017] For the accuracy of the images, and therefore diagnoses, the apparatus used for medical imaging should be calibrated accurately. Moreover, as in some examples ionising radiation may be used in treatment of patients, the apparatus should be calibrated accurately, because applying an incorrect dose of radiation or applying radiation to the wrong part of the body may harm the patient.

[0018] Objects referred to as ‘phantoms’ may be used in calibration of medical imaging and radiotherapy apparatus. A phantom in this context is an object with a particular shape and attenuation properties which may be imaged or irradiated.

[0019] Attenuation is a measure of the loss of intensity of radiation as it passes through a material. For example, when a beam of x-rays is incident on a low attenuation material, such as soft body tissues (e.g. fat, muscle) a large portion of the x-ray radiation passes through the material, and a small portion is absorbed. In contrast, when a beam of x-rays is incident on a high attenuation material, such as hard body tissue (e.g. bone) a significant proportion of the radiation is absorbed. The amount of transmitted radiation is measured and compared with the amount of incident radiation to obtain the attenuation.

[0020] Attenuation is a property of an object and depends on the shape, size and attenuation coefficients of the object, wherein the attenuation coefficient is a property of the material. In particular the attenuation depends on the attenuation coefficient and the path length of the radiation through the object. The attenuation is linearly dependent on the path length through the material and the attenuation coefficient. The attenuation properties of a material may, for example, be measured using the Hounsfield scale (HU). In the Hounsfield scale, distilled water is defined to have a value of 0 HU and air is defined to have a value of -1000 HU. Attenuation measured on the Hounsfield scale can be obtained from the linear attenuation coefficient of a material according to the equation: HU = 1000 c -ίϋί ^. wherein u is the linear attenuation coefficient of the material, p_w is the linear attenuation coefficient of water and p_a is the linear attenuation coefficient of air. The linear attenuation coefficient is defined as m = — wherein F is

F dz the radiant flux and z is the path length of the beam.

[0021] Different types of body tissue may have different attenuation properties, therefore a phantom may comprise portions which have different attenuations to ensure the apparatus is calibrated across the range of attenuations of different tissue types. A phantom may be designed for use with a particular type or energy of radiation. For example, a phantom may be designed to have a particular attenuation when irradiated with x-rays or ultrasound. Some phantoms may be suitable for use with different types of apparatus, for example a phantom may be designed to be suitable for calibration of x-ray imaging apparatus and CT apparatus.

[0022] Some examples of phantoms may be designed to mimic the human body, or a portion thereof. Such phantoms are referred to as anthropomorphic phantoms and may be constructed from various materials which have similar attenuation to tissue of the human body. Other phantoms may not be designed to mimic the human body, for example simple geometry phantoms may comprise materials with different attenuations arranged in a relatively simple geometry. For example, they may comprise a cylinder having a particular attenuation with holes into which other modules (e.g. cylinders) having another particular attenuation can be inserted. These phantoms may be expensive to produce, especially anthropomorphic phantoms which often include complex geometries and manual assembly. [0023] Figure 1A is an example of a method, which may comprise a method for deriving object model data for use in generating an object in additive manufacturing. In this example the method is carried out at least in part by processing circuitry, which may comprise at least one processor.

[0024] The method comprises, in block 102, receiving (by processing circuitry) an image indicative of attenuation of radiation by a first object. The image may be an image obtained from any type of medical imaging apparatus, for example x-ray, CT, MRI, PET or ultrasound, which describes attenuation of radiation within an object. The image may be a two dimensional (2D) image (for example an x-ray image) in which the third dimension of the object is projected into a plane. The image may comprise a grid of pixels, wherein each pixel represents an attenuation value. In some examples the attenuation value of each pixel represents the measured total attenuation along a path from the radiation source to the location on the detector associated with the pixel. In other examples the image is a 3D image which provides a 3D representation of the attenuation of the object (for example CT or MRI). Such a 2D or 3D image may be used in generating a 3D model for use in additive manufacturing to generate a second object which represents the first object. In some examples the received image may be modified, for example to reduce the information therein. For example, the image may be simplified to remove features which are not intended to be included in the second object. For example, the image may be cropped so that the second object represents a portion of the first object. For example images of portions of the anatomy which are not relevant, or which are not of interest, may be removed. In some examples the image may be simplified, for example by reducing the resolution of the attenuation values or simplifying the shape of complex structures.

[0025] In some examples, the obtained image of the first object may be modified to introduce a feature, for example data indictive of abnormal tissue or the like. This may for example allow investigation of whether a certain type of condition can be imaged in a particular body type, or in a particular position within a body, when using a particular imaging protocol, which may in turn be used to improve such protocols.

[0026] A 3D image may comprise an array of voxels, each representing the attenuation at a particular location within the object. A 2D image may be obtained from the 3D image, either by extracting a slice of voxels from the 3D image, or by summing voxels along a dimension of the 3D image to project the 3D image onto a plane. In other examples, a 2D image may be acquired directly by imaging apparatus. [0027] In some examples, the radiation may be encoded in an image as a greyscale value. For example, the higher the attenuation of a material, the brighter it appears in a CT image. Bone may therefore appear white, whereas air in the lungs, with a relatively low attenuation, may appear black.

[0028] The method comprises, in block 104, and by processing circuitry, deriving object model data for use in generating a second object using additive manufacturing, wherein the second object is to represent the first object in a radiation environment. The object model data may define the size and shape of the second object. The object model data may for example comprise a Computer Aided Design (CAD) model, and/or may for example be a STereoLithographic (STL) data file or a 3D Manufacturing Format (3MF) data file. The object model data may comprise a representation of the second object, for example as a plurality of voxels or a mesh model. The object model data may also comprise a description of the intended attenuation of a portion of the second object or a material from which a portion of the object is to be formed.

[0029] Where a 3D image (or a series of 2D images) has been received in block 102, this may be processed to obtain data at a resolution of an intended 3D printer. For example, the images may have a separation of Ypm, whereas a 3D printing layer may have a height of Xpm. Where Y is smaller than X, images may be combined to provide an attenuation value for a 3D printing voxel. For example, if three images relate to one layer to be generated in additive manufacturing, the average attenuation value of three corresponding pixels in those images may be determined as an attenuation value for an additive manufacturing pixel. While the height of a layer has been considered here, the same could be true for a width or a depth of voxel, and the appropriate combination may depend on the angle of imaging and on the intended orientation of object generation. In other examples, where Y is larger than X, then each image may provide an attenuation value for more than one layer. In still further examples, in which the voxel dimensions in additive manufacturing are configurable, these may be configured based on the resolution of an image and/or the separation between images.

[0030] Figure 1B is an example of a method of deriving the object model data as described in block 104 of Figure 1A. The method comprises, in block 106, deriving, from the image, a plurality of zones, wherein each zone is associated with a different radiation attenuation level. A zone may be defined as a continuous region which has the same attenuation level. In other examples a zone may be defined as a continuous region which has attenuation values within a particular range of values. As noted above, in some examples, the attenuation level may be encoded as a greyscale value, and therefore deriving the zones may comprise identifying a continuous region which comprises greyscale values within a range.

[0031] In some examples a zone may be a non-continuous region with the same attenuation values, or attenuation values within a range of values. In some examples deriving the zones may comprise identifying the tissue types in the image and defining a continuous (or non-continuous) region of a particular tissue type to be a zone. In other examples deriving zones may comprise identifying an organ and identifying the organ as a zone. Identification of tissue types or organs may be achieved by use of attenuation thresholds, that is, a region may be identified as an organ if it comprises tissue with an attenuation in the range corresponding to the expected range of attenuations of that particular organ. For example, lung tissue may have an attenuation in the range of -950 HU to -550 HU, fat may have an attenuation in the range -100 HU to -80 HU and bone may have attenuation above 50 HU. In other examples, identifying a zone may comprise a manual input, for example a user may view an image and select a region of the image which is then identified as a zone. In other examples, identifying a zone may comprise use of a machine learning or artificial intelligence model, for example to identify particular organs. For example, a machine learning model may be trained using a data set comprising images with tagged organs. When trained, the model may then identify similar organs as zones.

[0032] The method comprises, in block 108, determining a radiation condition of the radiation environment. The radiation condition may include the radiation conditions which the second object is intended to be exposed to, and may be different to a radiation condition under which the image was obtained. For example, the radiation condition may include the type (for example x-ray, MRI, proton beam, ultrasound or CT) and/or the energy of the radiation the second object is intended to be exposed to. The radiation condition may be described in terms of the frequency of the radiation (for example x-rays have frequencies in the range 30x10¹⁵ Hz to 30x10¹⁸ Hz), the energy of the radiation (for example x-rays have energies in the range 124 eV to 124 keV) and/or the wavelength of the radiation (for example x-rays have a wavelength in the range 10 pm to 10 nm). In some examples the energy of the radiation may be characterised by a voltage applied to an apparatus used to generate the radiation, such as x-rays which may be generated by application of a voltage to an x-ray tube. For example voltages of 20 kV to 150kV may be used to generate diagnostic x-rays. The radiation condition may describe the specific type and energy of the radiation to be used, or it may describe the type of imaging or radiotherapy the second object is intended to be used with. In some examples, the second object may be intended to be used in several different radiation environments, for example both MRI and CT.

[0033] A certain tissue type or part of a body may be imaged or treated with a particular type and energy of radiation. Therefore, different radiation environments may be used to image or treat different body parts, for example when irradiating lungs, a different energy and/or type of radiation may be utilised compared to when skin is to be irradiated.

[0034] The method comprises, in block 110, selecting, for each zone and based on the radiation condition, an additive manufacturing attribute comprising at least one of an additive manufacturing material and a material density for each zone, wherein the additive manufacturing attributes are associated with attenuation characteristics.

[0035] In additive manufacturing, the attenuation of the generated object may be a function of the build material used (e.g. type of plastic, metal). Therefore, the additive manufacturing attribute of each zone may define the type of material to be used within a region of build material corresponding to that zone.

[0036] In some examples, the material density describes an amount of build material to be solidified, and/or whether unfused build material is to be encapsulated within a portion of the second object. For example, the material density specified as an additive manufacturing attribute may comprise the specification of a physical structure, which may be a microstructure, or ‘infill pattern’ which is be provided for the zone. For example, fusing agent may be deposited within a region in a lattice-like pattern, such that within the interior of the generated object there are portions of unfused build material, which may be removed in some examples when object generation is complete. The relative size of regions of voids (or retained ‘pockets’ of unfused build material) and regions of solidified build material within a region of build material corresponding to the zone will result in a particular attenuation for the zone. Therefore, the additive manufacturing attribute may also define a pattern of print agent within the interior of an object to provide such a microstructure, or lattice structure. For example, for zones associated with a higher attenuation, a greater proportion of the build material corresponding to the zone may be solidified. In some examples the attribute may comprise a representation of the internal structure of the object, for example it may define a lattice structure within the object. Thus, the microstructure may define a material density may be a density of solidified build material within the zone (e.g. a proportion of solidified to unsolidified build material). In some examples, the material density may be specified as an indication of whether a region of the second object is to be formed with a void, or is to encapsulate unsolidified build material.

[0037] The additive manufacturing attributes may be selected from a predetermined set of additive manufacturing attributes. The selection may comprise selecting from a look up table, database or other list of additive manufacturing attributes. The additive manufacturing attributes may be stored in association with an attenuation and radiation environment or radiation condition. This may therefore provide a ‘library’ of additive manufacturing attributes which result in a given attenuation in a given radiation environment. In order to produce a portion of an object with a particular attenuation in a particular radiation environment, the attenuation values and radiation condition of the intended radiation environment may be looked up and the corresponding additive manufacturing attributes selected.

[0038] The predetermined set of additive manufacturing attributes and the associated attenuation for a given radiation type or other condition may be obtained by generating objects using a variety of different additive manufacturing attributes (e.g. build material type, whether build material encapsulated in the object is solidified, or remains in a granular form, different geometries, different lattice structures, and the like). The generated objects may then be exposed to a variety of different radiation conditions and their attenuation measured. The measured attenuation may then be associated with the additive manufacturing attributes used when generating that object and the radiation condition(s) used when measuring the attenuation. Therefore, when an object (or portion of an object) is to be generated with a particular attenuation at a particular radiation condition, the predetermined set of additive manufacturing attributes and the associated radiation condition and attenuation may be looked up and the appropriate additive manufacturing condition selected based on the intended attenuation and radiation environment/conditions.

[0039] In this example, the method further comprises, in block 112, deriving the object model data by combining the selected additive manufacturing attributes associated with each zone to specify a plurality of regions of the second object, which, when the second object is generated, correspond to a geometrical configuration of the zones. In other words, in some examples, when an image is captured of the second object it is intended to match the image of the first object, both in terms of the relative attenuation (given the respective radiation environments in which the images are obtained) and the physical location of the zones within their respective bodies. The zones of the second object may have different attenuations by virtue of the additive manufacturing attributes. For example, a zone with high attenuation may be generated using build material with a higher attenuation coefficient or an internal structure with a high solidified proportion such as completely solidified internal build material. In contrast a zone with lower attenuation may be generated using a build material with a higher attenuation coefficient or an internal structure with a lower proportion of solidified material, such as a hollow structure, a structure comprising unfused build material and/or an internal lattice structure.

[0040] In some examples the additive manufacturing attributes comprise a build material type. Figures 2A and 2B show graphs which demonstrate the relationship between attenuation and radiation energy for example samples of four different materials. The horizontal axis of each graph indicates the energy of x-ray radiation in kilovolts (kV) and the vertical axis represents the attenuation of the material in Hounsfield units (HU). The measurements shown in Figure 2A were performed on a uniform solid object formed from the material, that is, an object without any internal voids or microstructure. The measurements shown in Figure 2B were performed on an object comprising an unsolidified interior (described as “hollow”), which comprised an interior of unsolidified powder build material encapsulated in an outer solidified ‘shell’. In this example, the attenuation was measured for cubes of a standard size. It may be noted that the attenuation is related to the path length of the radiation through the material under test, whereas the attenuation coefficient is a property of the material and therefore independent of the path length.

[0041] Taking PA12 as an example, Figure 2A demonstrates the relationship between x-ray energy and attenuation for solidified PA12. As can be seen from the graph, the attenuation of the material is less than zero and in the range -79 HU to -48 HU, and therefore less than the attenuation of water. The attenuation of PA12 is comparable to the attenuation of fat, which has an attenuation of -80 HU to -100 HU and therefore could be used to mimic fat in a phantom. The graph also shows that the attenuation of the material increases with increasing x-ray energy.

[0042] Taking PA12GB as another example, Figure 2A demonstrates the relationship between x-ray energy and attenuation for solidified PA12GB. As can be seen from the graph, the attenuation of the material is the range 375 HU to 476 HU which is greater than zero, and therefore greater than the attenuation of water. The attenuation of PA12GB is comparable to the attenuation of compact bone, which has an attenuation of over 300 HU, and therefore could be used to mimic compact bone in a phantom. The graph also shows that the attenuation of this material decreases with increasing x-ray energy, which is the opposite of the trend shown by the PA12 (and indeed the other materials under test).

[0043] PA12GB differs from PA12 in that it contains encapsulated glass beads in the powder, which increase the stiffness of PA12GB relative to PA12. The glass beads also alter the chemical composition of the material which results in the different attenuation characteristics, as can be seen in Figures 2A and 2B.

[0044] Other tissues may be mimicked in a phantom by using other materials or combinations of materials. For example solid TPU has a similar attenuation to blood (50 HU to 60 HU).

[0045] In some examples the additive manufacturing attributes comprise a physical structure of a portion of the second object. Figure 2B shows the attenuation of four different materials as a function of x-ray energy when the object is hollow, that is the interior of the object comprises encapsulated unfused build material. Therefore, a particular attenuation can be achieved by specifying whether the build material in the interior of an object, or a portion of an object, should be fused or unfused. In some examples the interior may be filled with air rather than unfused build material, for example a hole may be made in the exterior of the object and the unfused build material removed, to achieve an even lower attenuation.

[0046] The attenuation of various materials has been measured with solidified and unsolidified (powder) interiors over a range of x-ray energies. For example, over a range of 80 kV to 135 kV x-ray energies PA12 has a range of attenuations from -613.8 HU to -600.6 HU when it has an unsolidified interior and a range of attenuations from -79.2 HU to -48.5 HU when it has a solidified interior. Intermediate attenuations may also be achievable by partially solidifying the interior of an object, for example by generating a lattice structure in the interior of the object or otherwise partially fusing the interior of the object. Therefore, the additive manufacturing attribute may comprise a degree of fusing of the interior of an object, or a portion of an object.

[0047] In some examples the radiation condition includes at least a type of radiation or radiation energy. The graphs of Figures 2A and 2B demonstrate that different materials can have different attenuation properties and that the same material may have a different attenuation at a different radiation energy. In other words the attenuation of a material is a function of the energy of the radiation. The Figures also demonstrate that the relationship between radiation energy and attenuation can be different for different materials. For example, as noted above, the attenuation of PA12GB decreases with increasing radiation energy whereas the attenuation of PA12 increases with increasing radiation energy. Therefore, when a material is selected to achieve a particular attenuation, both the type and energy of radiation may be considered.

[0048] Figure 3A shows a first object 302 and apparatus 300 which may be used to obtain an image indicative of attenuation of radiation by the first object 302. The apparatus 300 comprises a source 304 which emits radiation 306. In some examples the source 304 is a source of x-rays, for example an x-ray tube, and the radiation 306 is x- ray radiation suitable for imaging patients. The radiation 306 is incident on the first object 302. Some of the radiation 306 passes through the object and is detected by the detector 308, which is located on the opposite side of the object 302 to the source 304. The detector 308 may be any means suitable for detecting the radiation, for example x- ray radiation may be detected using x-ray film, image plates or flat panel detectors. In other examples the source 304 may emit a different type of radiation, for example it may comprise a radioactive source such as ¹⁹²lr, ¹³⁷Cs or ⁶⁰Co to provide gamma rays, a transducer to produce ultrasound or a linear accelerator.

[0049] In the example, the first object 302 comprises three portions: an upper portion 310, a middle portion 312 and a lower portion 314. In this example the upper portion 310 has a low attenuation, the middle portion 312 has an intermediate attenuation and the lower portion 314 has a high attenuation. When the first object 302 is illuminated with radiation 306 from the source 304, some of the incident radiation passes through the first object 302 and is detected by the detector 308. A small portion of the radiation is absorbed by the upper portion 310 because it has a low attenuation. Therefore, a relatively large proportion of the radiation which passes through the upper portion 310 is detected by the detector 308. Conversely, a large portion of the radiation which is incident on the lower portion 314 is absorbed by the material, and therefore a relatively small proportion of the radiation which is incident on the lower portion 314 is not detected by the detector 308. In this way the detector 308 is able to measure the attenuation of different parts of the first object 302.

[0050] In some examples the detector 308 may be located on the same side of the object as the source 304, and rather than detecting transmitted waves, it detects reflected waves. For example, in ultrasound imaging, reflected ultrasound waves are detected by a detector 308 located on the same side, an often integrated with, the source 304. In such examples, the source 304 may be a piezoelectric transducer.

[0051] In other examples the source 304 and detector 308 may have different relative positions. For example, the source 304 may be placed inside the first object 302. Such a configuration may be used in, or to simulate, PET, which comprises injecting a radioactive tracer into a patient’s body.

[0052] An image 316 may be created based on the measured attenuations. In this example, in the image 316, the attenuation level is encoded in the greyscale value such that regions with high attenuation are represented by lighter shading and regions with lower attenuation are represented by darker shading.

[0053] The image 316 (which may be one of a plurality of images of the object 302) is then used as the basis for determining object model data for generating a second object, wherein the second object is to represent the object 302 in a radiation environment.

[0054] In this example, the image 316 is automatically partitioned into ‘zones’ wherein each zone is associated with a range of attenuation values. As noted above, in some examples, the image may for example be processed before it is divided into zones, for example reducing the information therein by reducing the resolution thereof, simplifying contours, removing portions thereof, and/or removing noise and/or artefacts.

[0055] Once an intended radiation condition of the radiation environment to which the second body is to be subjected is identified, an additive manufacturing attribute comprising at least one of an additive manufacturing material and a material density (e.g. microstructure (or infill pattern) and/or the specification of voids/unfused build material encapsulation) for each zone is selected, in this example from a look up table. This is then used to derive object model data by combining the selected additive manufacturing attributes associated with each zone to specify a plurality of regions of the second object, which, when the second object is generated, correspond to a geometrical configuration of the zones.

[0056] As described in relation to Figures 2A and 2B, the attenuation of the material may be affected by the physical or geometrical structure of the generated object. Therefore, in order to produce an object in additive manufacturing which simulates the attenuation of the first object 302, the physical structure (e.g. a lattice structure, or microstructure) of each zone may be selected based on the intended attenuation, and this structure may be specified in the object model data. Figure 3B shows a cross section of a second object 318 and shows the internal structure of the second object 318 wherein the second object 318 was generated using additive manufacturing and corresponds to the first object 302. In this second object 318, there are three portions: an upper portion 320, a middle portion 322 and a lower portion 324 which correspond to the upper, middle and lower portions 310, 312 and 314 of the first object 302 respectively. In this example the low attenuation of the upper portion 302 is obtained by forming the interior of the upper portion 320 from unsolidified build material. The outer faces of the upper portion 320 may be formed from solidified build material, while the interior remains unsolidified to maintain the low attenuation. In some examples, the unsolidified powder may remain encapsulated inside an object, whereas in other examples, to obtain a lower attenuation, a portion of the object may be completely hollow. In powder based additive manufacturing this may be achieved by forming a hole in an exterior surface of the object through which unsolidified powder may be removed.

[0057] The lower portion 324 of the second object 318 has a relatively high attenuation. This is achieved by solidifying the build material throughout the whole, or substantially the whole, interior of this portion.

[0058] In some examples the additive manufacturing attribute describes a physical structure, or microstructure, of a portion of the second object and the physical structure is a lattice structure, which specifies a proportion of the build material which is to be solidified. The middle portion 322 of the second object 318 has an intermediate attenuation which is achieved by specifying, in the object model data, a lattice structure in the interior of this portion. The lattice structure may describe a regular or irregular pattern of solidified build material, for example a geometric lattice of solidified material formed around voids or pockets of unfused build material. The lattice may be designed such that the lattice itself is not visible when the generated second object 318 is imaged. This may for example comprise designing a lattice in which the separation of portions to be solidified (for example, solid struts making up the lattice structure) is small, and in some examples, below an imaging resolution. In other examples, designing a lattice may comprise designing a lattice to have a consistent attenuation in an intended imaging direction. For example, the lattice may be designed such that, for each part of the middle portion, the imaging radiation may pass through an approximately equal amount of solidified and unsolidified material. [0059] In still other examples, a lattice may be selected based on the object being imaged. For example, some human tissue has characteristic structure. Breast tissue may for example have a varying density which may be perceptible in images. A lattice structure which may produce a similar effect, when imaged, as a given tissue type may be selected to represent that tissue type. However, selection of the lattice structure may also be dependent on the intended attenuation.

[0060] The relative proportion of the second portion 322 which comprises unsolidified build material (or is hollow) to solidified build material in the lattice structure may be selected to achieve a target attenuation value. In some examples the object model data comprises higher density lattice structures (i.e. a higher proportion of solidified material) in zones with higher radiation attenuation levels and lower density lattice structures (i.e. a lower proportion of solidified material) in zones with lower radiation attenuation levels. For example, the second object may comprise more than one portion which is provided with an internal lattice structure to achieve particular attenuation values. In such examples the different portions may comprise different lattice structures. For example, a denser lattice structure may provide a higher attenuation value and a lower density lattice (i.e. with more hollow space or unsolidified build material) may provide a lower attenuation.

[0061] Different types of tissue may be mimicked in this way by use of lattice structures. For example kidney and pancreas tissue may have attenuation in the range of 30 HU to 40 HU and 30 HU to 50 HU, respectively. These ranges of attenuations can be provided by designing different lattice structures and generating objects from the materials such as those described in relation to in Figures 2A and 2B.

[0062] In some examples the first object is a human or animal body, or a portion thereof, and the second object is a medical imaging phantom. An image may be obtained of the first object, for example as described in relation to Figure 3A. The medical imaging phantom may be intended to simulate a generic human or animal body, or a portion thereof. The phantom may be a generic phantom, in that it is intended to represent a body or a portion of a body without any identifying or unique features. In some examples, the obtained image 316 of the first object 302 may be modified to introduce an abnormality.

[0063] In some examples the phantom may be intended to represent a specific patient or a patient with a particular condition, such as a tumour, broken bone, cardiopathies, organ malfunctions or any trauma condition. In such examples the received image 316 comprises image data representing a plurality of tissue types, wherein at least one tissue type is an abnormal tissue and wherein deriving a plurality of zones comprises associating each zone with a tissue type. In such examples, the body which is imaged may have the condition. In such examples, the phantom may be used for treatment or diagnoses of the particular patient, for example in simulating application of radiation in radiotherapy, or for calibrating a particular imaging machine using the phantom as a known baseline. For example, this may help in more accurately tracking the progress of a condition or treatment, even when different imaging apparatus and/or technologies are used. In other examples, the phantoms may be generally used in calibration of apparatus used in medical imaging or radiotherapy.

[0064] In some examples the phantom may be used as a medical training model. Medical training models are objects which represent a body and are used in training for surgery. For example, a medical training model may represent a particular pathology and may be used to train or practice treating that pathology in surgery. Some types of surgery may be performed while being imaged, for example using x-ray imaging, CT imaging, MRI, ultrasound imaging or using any other suitable medical imaging. For example, vascular interventions may be performed under x-ray imaging. Therefore, a medical imaging phantom may be used for training for such surgery since it comprises attenuation properties intended to mimic the attenuation properties of the body. Medical imaging phantoms which are intended to be used as medical training models may be constructed so that their physical properties mimic those of a patient. For example, the materials may be selected to have similar properties to those of the relevant body tissue, for example a material may be selected which has a similar elasticity, flexibility, strength as a particular tissue (e.g. skin, muscle or fat). In such examples, selecting additive manufacturing attributes for each zone may be based on other intended physical properties, such as elasticity, flexibility or strength, in addition to the attenuation characteristics.

[0065] Figure 4 provides an example of the method of Figure 1A. As discussed in greater detail below, blocks 402 and 404 may provide an example of the method of blocks 102 and 104 described in relation to Figure 1A.

[0066] Block 406 comprises generating the second object in additive manufacturing according to the derived object model data. The second object may be a medical imaging phantom. [0067] In order to generate an object, print agent may be selectively deposited onto portions of build material. For example, a fusing agent may be deposited in areas which are intended to be solidified to generate the object and a detailing agent may be deposited in regions surrounding the regions intended to be solidified in order to prevent ‘over fusing’ or fusing in areas around the object. Additionally, the detailing agent may be deposited in regions intended to be solidified for temperature control.

[0068] Generating the second object using additive manufacturing may comprise obtaining data describing which portions of build material print agent is to be deposited upon, based on the derived object model data representing the second object to be generated. Print agent coverage amounts referred to herein may be the print agent coverage amounts to be deposited in these portions, for example a fusing agent coverage amount may refer to the fusing agent coverage amount which is to be deposited in a region intended to be solidified and a detailing agent coverage amount may refer to the detailing agent coverage amount which is to be deposited in a region which is not intended to be solidified. In some examples, detailing agent is deposited in proximity to regions in which fusing agent is applied, for example about the periphery of a layer of the object being formed, rather than all regions of a layer of build material which are not intended to be solidified. In some examples, detailing agent may be dispensed onto a region of build material which is intended to be solidified. The print agent coverage amounts may for example be defined as an area coverage, that is the volume of printing agent to be deposited per unit area, or as a percentage coverage, that is, the percentage of an area which is intended to be covered with the print agent. In some examples, it may be defined as a contone level. The locations to which print agent drops are applied and/or the amount and/or size of such drops may be determined according to an intended coverage, for example using halftoning techniques and the like.

[0069] In other examples, other additive manufacturing techniques may be used to generate the second object.

[0070] Block 408 comprises obtaining an image indicative of attenuation of radiation by the second object. The image of the second object may be obtained using the same type of imaging apparatus which was used to obtain the image of the first object. In other examples a different type of apparatus may be used, for example the image of the first object may be obtained using CT whereas the image of the second object may be obtained using x-ray imaging. In some examples, at least one attribute of the radiation used to image the second object is different from that used to image the first object. For example, at least one of the wavelength, type and/or energy may be different. In some examples, the second object may be imaged under a particular radiation condition(s) and may provide an image which may be similar to the image of the first object if the first object were to be imaged under that radiation condition(s) (albeit that the original object may have been imaged in a different radiation condition(s)).

[0071] In some examples the method further comprises calibrating the imaging apparatus used to obtain the image indicative of attenuation of radiation by the second object based on the obtained image. In other examples the image indicative of attenuation of the second object may be used in planning treatment for a patient.

[0072] Figure 5 is an example apparatus 500, which may be used in some additive manufacturing operations, for example in deriving object model data for use in additive manufacturing to generate a medical imaging phantom. The apparatus 500 comprises processing circuitry 502, the processing circuitry 502 comprising an image module 504, zone module 506, radiation condition module 508, a selection module 510 and an object model module 512. In some examples, the processing circuitry 502 may carry out any or any combination of blocks 102 to 112 of Figures 1A and 1B, or any combination of blocks 402 to 408 of Figure 4.

[0073] In this example, in use of the apparatus 500, the image module 504 is to obtain an image, or a set of images, of attenuation values of a first object. The image(s) may be obtained as described in relation to block 102 of Figure 1A or block 402 of Figure 4. In some examples the apparatus may further comprise a source and/or detector, for example as described in relation to Figure 3A. In such examples the image module 504 may control the source and/or detector to acquire the image. In some examples, the image module 504 may perform some image processing, for example to remove features which are not intended to be included in a second object to be generated. For example, the image(s) may be cropped so that the second object represents a portion of the first object. In some examples the image(s) may be simplified, for example by reducing the resolution of the attenuation values or simplifying the shape of complex structures.

[0074] In this example, in use of the apparatus 500, the zone module 506 derives, from the image, a plurality of zones, wherein each zone is associated with a different radiation attenuation level (which may include a range of attenuation values). Different attenuation levels may be associated with different types of tissue and/or organs. Therefore, at least one of the zones may be associated with a particular type of tissue or organ. The plurality of zones may be obtained as described in relation to block 106 of Figure 1B. [0075] In this example, in use of the apparatus 500, the radiation condition module 508 obtains a radiation condition of a radiation environment. The radiation environment may be an intended imaging environment, which may be different from the imaging environment in which the image(s) of attenuation values of the first object was obtained. The radiation condition may describe the type of radiation or the energy of the radiation. In some examples the radiation condition may describe a type of medical imaging. The radiation condition may be obtained as described in relation to block 108 of Figure 1B.

[0076] In this example, in use of the apparatus 500, the selection module 510 selects, for each zone and based on the radiation condition, an additive manufacturing attribute from a predetermined set of additive manufacturing attributes, wherein the additive manufacturing attributes are associated with attenuation characteristics. The predetermined set of additive manufacturing attributes may comprise a list of attributes associated with attenuation characteristics, and the attributes may be selected in order to provide the associated attenuation characteristics. For example, the additive manufacturing attributes may comprise the build material type and/or the physical structure (e.g. an internal lattice structure) which may be used to achieve a particular attenuation under radiation with a particular type of radiation at a particular energy. Therefore, selecting the additive manufacturing attribute(s) for each zone may comprise looking up, in a look up table or database, the radiation condition and intended attenuation characteristics to determine the additive manufacturing attributes to associate with that zone. In some examples selecting the additive manufacturing attributes may comprise interpolating between stored values of the additive manufacturing attributes. The additive manufacturing attributes may be selected as described in relation to block 110 of Figure 1 B.

[0077] In this example, in use of the apparatus 500, the object model module 512 derives object model data for use in generating a second object representing the first object in the radiation environment by combining the selected additive manufacturing attributes associated with each zone to specify a plurality of regions of the second object, which, when the second object is generated, correspond to a geometrical configuration of the zones. The object model data may be obtained as described in relation to block 112 of Figure 1B, and for example comprise a Computer Aided Design (CAD) model, and/or may for example be a STereoLithographic (STL) data file or a 3D Manufacturing Format (3MF) data file. [0078] In some examples the radiation condition of the radiation environment describes or comprises the type of radiation and/or the energy of the radiation.

[0079] In some examples the additive manufacturing attribute comprises at least one of: selection of a build material and/or specification of the internal structure, for example by generating a lattice or mesh structure comprising unsolidified build material and/or voids within the object.

[0080] In some examples, in use of the apparatus 500, the object model module 512 determines, for each zone, a mesh (i.e. lattice) structure based on the additive manufacturing attribute associated with that zone and derives object model data describing or modelling the determined structure in each zone. The structure may be a lattice structure as described in relation to Figure 3B.

[0081] Figure 6 shows an example of an apparatus 600 comprising processing circuitry 502 which includes the image module 504, the zone module 506, the radiation condition module 508, the selection module 510 and the object model module 512 of the apparatus of Figure 6. In addition, the apparatus 600 further comprises additive manufacturing apparatus 602. In use of the apparatus 600, the additive manufacturing apparatus 602 is to generate the second object based on the determined print agent coverage.

[0082] The additive manufacturing apparatus 602 may generate objects in a layer-wise manner by selectively solidifying portions of layers of build material. The selective solidification may in some examples be achieved by selectively applying print agents, for example through use of ‘inkjet’ liquid distribution technologies, and applying energy, for example heat, to each layer using the plurality of fusing energy sources. In some examples, control instructions are determined from the derived object model data modelling the second object. For example, the object model data may be ‘sliced’ into slices corresponding to each layer to be generated in additive manufacturing, the portions of the layer which are to be solidified may be identified within the slice, and control instructions generated therefrom. The control instructions may describe where print agent should be placed on a layer of build material in order to generate a layer of the object. For example print material coverage amounts may be determined as outlined above, and then the placement of drops of print agents may be determined using halftoning techniques or the like to provide a determined print agent coverage amount. [0083] In use of the apparatus 600, energy may be provided by fusing energy sources to cause the build material to which fusing agent has been applied to fuse. The additive manufacturing apparatus 600 may comprise other additional components not shown herein, for example a fabrication chamber, at least one print head for distributing print agents, a build material distribution system for providing layers of build material and the like.

[0084] The apparatus 500, 600 may, in some examples, carry out at least one of the blocks of Figure 1A, Figure 1B or Figure 4. In other examples, other types of additive manufacturing may be used, such as fused deposit modelling, directed energy techniques such as laser sintering, stereolithography, use of binding or curing agents, or the like.

[0085] Figure 7 shows an example of a tangible machine readable medium 702 in association with a processor 704. The machine readable medium 702 stores instructions 706 which, when executed by the processor 704 cause the processor to carry out actions.

[0086] In this example, the instructions 706 comprise instructions 708 to cause the processor 704 to derive, from an image indicative of attenuation of radiation of a first object obtained by imaging under a first radiation condition, a plurality of zones wherein each zone is associated with a different radiation attenuation level. The image may be obtained as described in relation to block 102 of Figure 1A or block 402 of Figure 4 and the plurality of zones may be derived as described in relation to block 106 of Figure 1B.

[0087] In this example, the instructions 706 comprise instructions 710 to obtain a second radiation condition. The second radiation condition may be obtained as described in relation to block 108 of Figure 1B, and may comprise a radiation type and/or energy level.

[0088] In this example, the instructions 706 comprise instructions 712 to select, for each zone and based on a second radiation condition, an additive manufacturing attribute from a predetermined set of additive manufacturing attributes, wherein the predetermined additive manufacturing attributes are associated with attenuation characteristics. The additive manufacturing attributes may be selected as described in relation to block 110 of Figure 1 B.

[0089] In this example, the instructions 706 comprise instructions 714 to derive object model data by combining the selected additive manufacturing attributes associated with each zone, wherein the object model data is for generating a second object using additive manufacturing, the second object representing the first object and wherein the second object is to be generated to have attenuation values corresponding to the attenuation values of the first object when the second object is imaged under the second radiation condition. In some examples, the second object may be imaged under the second radiation condition and may provide an image which may be similar to the image of the first object if the first object were to be imaged under the second radiation condition. The object model data may be obtained as described in relation to block 112 of Figure 1B.

[0090] In some examples, the instructions 706 comprise instructions 716 to determine control instructions, which when executed, instruct an additive manufacturing apparatus to generate a second object according to the object model data. In some examples the instructions 706 may further comprise instructions to execute the determined control instructions, thereby generating the second object according to the derived object model data, for example as described in relation to block 406 of Figure 4.

[0091] In some examples the instructions 706 may further comprise instructions to obtain an image indicative of attenuation of the generated second object, for example as described in relation to block 408 of Figure 4.

[0092] In some examples the additive manufacturing attribute associated with each zone comprises at least one of a type of additive manufacturing build material or physical lattice structure.

[0093] Examples in the present disclosure can be provided as methods, systems or machine-readable instructions, such as any combination of software, hardware, firmware or the like. Such machine-readable instructions may be included on a computer readable storage medium (including but not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0094] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each block in the flow charts and/or block diagrams, as well as combinations of the blocks in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0095] The machine-readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine-readable instructions. Thus, functional modules of the apparatus and devices (such as the image module 504, zone module 506, radiation condition module 508, selection module 510 and/or object model module 512) may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors.

[0096] Such machine-readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0097] Such machine-readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by block(s) in the flow charts and/or block diagrams.

[0098] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure.

[0099] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims.

[00100] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[00101] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.

Claims

1. A method comprising, by processing circuitry: receiving an image indicative of attenuation of radiation by a first object; and deriving object model data for use in generating a second object using additive manufacturing, wherein the second object is to represent the first object in a radiation environment; wherein deriving the object model data comprises: deriving, from the image, a plurality of zones, wherein each zone is associated with a different radiation attenuation level; determining a radiation condition of the radiation environment; selecting, for each zone and based on the radiation condition, an additive manufacturing attribute comprising at least one of an additive manufacturing material and a material density for each zone, wherein the additive manufacturing attributes are associated with attenuation characteristics; and deriving the object model data by combining the selected additive manufacturing attributes associated with each zone to specify a plurality of regions of the second object, which, when the second object is generated, correspond to a geometrical configuration of the zones.

2. A method as claimed in claim 1 wherein the additive manufacturing attributes comprise at least one of: a build material type; an indication of whether the second object is to encapsulate unfused build material; and a physical structure of a portion of the second object.

3. A method as claimed in claim 2 wherein the additive manufacturing attribute describing material density describes a physical structure of a portion of the second object and the physical structure is a lattice structure.

4. A method as claimed in claim 3 wherein the lattice structure describes a lattice having solid portions which are separated by less than an imaging resolution for imaging the second object in the second radiation environment.

5. A method as claimed in claim 1 wherein the radiation condition includes at least one of: a type of radiation; and a radiation energy.

6. A method as claimed in claim 1 wherein the first object is a human or animal body and the second object is a medical imaging phantom and/or a medical training model.

7. A method as claimed in claim 1 further comprising, by processing circuitry: processing the image to reduce the information therein; and partitioning the processed image into zones.

8. A method as claimed in claim 1 , further comprising: generating the second object in additive manufacturing according to the derived object model data.

9. A method as claimed in claim 8 further comprising: obtaining an image indicative of attenuation of radiation by the second object.

10. An apparatus comprising processing circuitry, the processing circuitry comprising: an image module to obtain an image of attenuation values of a first object; a zone module to derive, from the image, a plurality of zones, wherein each zone is associated with a different radiation attenuation level; a radiation condition module to obtain a radiation condition of a radiation environment; a selection module to select, for each zone and based on the radiation condition, an additive manufacturing attribute from a predetermined set of additive manufacturing attributes, wherein the additive manufacturing attributes are associated with attenuation characteristics; and an object model module to derive object model data for use in generating a second object representing the first object in the radiation environment by combining the selected additive manufacturing attributes associated with each zone to specify a plurality of regions of the second object, which, when the second object is generated, correspond to a geometrical configuration of the zones.

11. An apparatus as claimed in claim 10 wherein the radiation condition of the radiation environment describes the type of radiation and the energy of the radiation.

12. An apparatus as claimed in claim 10, wherein the additive manufacturing attribute comprises a proportion of the build material to be solidified; and the object model module is further to, for each zone, determine a mesh structure based on the additive manufacturing attribute associated with that zone and derive object model data comprising the determined structure in each zone.

13. An apparatus as claimed in claim 10, further comprising: an additive manufacturing apparatus to generate the second object based on the derived object model data.

14. A machine-readable medium comprising machine-readable instructions which, when executed by a processor, cause the processor to: derive, from an image indicative of attenuation of radiation of a first object obtained by imaging under a first radiation condition, a plurality of zones wherein each zone is associated with a different radiation attenuation level; obtain a second radiation condition comprising at least one of a radiation condition and a radiation energy; select, for each zone and based on a second radiation condition, an additive manufacturing attribute from a predetermined set of additive manufacturing attributes, wherein the predetermined additive manufacturing attributes are associated with attenuation characteristics; derive object model data by combining the selected additive manufacturing attributes associated with each zone, wherein the object model data is for generating a second object using additive manufacturing, the second object representing the first object and wherein the second object is to be generated to have attenuation values corresponding to the attenuation values of the first object when imaged under the second radiation condition.

15. A machine-readable medium as claimed in claim 14 wherein the additive manufacturing attribute associated with each zone comprises at least one of an additive manufacturing build material or physical lattice structure.