GB2504258A

GB2504258A - X-ray scanner phantom

Info

Publication number: GB2504258A
Application number: GB201207288A
Authority: GB
Inventors: Peter Andrew Hiles; Stephen Roger; Lynn Bateman
Original assignee: BETSI CADWALADR UNIVERSITY LOCAL HEALTH BOARD
Current assignee: BETSI CADWALADR UNIVERSITY LOCAL HEALTH BOARD
Priority date: 2012-04-26
Filing date: 2012-04-26
Publication date: 2014-01-29
Also published as: GB201207288D0

Abstract

An X-ray scanner phantom 110 includes a plurality of portions 112. Each portion is a three-dimensional shape having a width, a length and a depth. The width and/or the length of one portion differs from the width and/or the length of another portion. Each portion may be elliptical, and the plurality of portions may form a stepped arrangement. The phantom 110 may be constructed as a hollow shell, which may be filled with a fluid (e.g. water), or a solid (e.g. resin). The phantom 110 may also be constructed with individual portions connected together. An insert assembly 116 having a plurality of cylindrical inserts may be placed in the centre of the phantom. The cylinders may be formed of, or contain, materials to simulate tissue. A central cylinder (304, fig. 3A) may be acrylic and include a dosimeter. A support 106 for the phantom 110 may also be provided. The phantom is to test the automatic exposure control (AEC) systems of CT scanners or hybrid systems (e.g. SPECT/CT).

Description

X-ray Scanner Phantom The present invention relates to X-ray scanner phantoms and in particular to phantoms for use with Computed Tomography (CT) scanners.

From as early as 1977 physicists have been using specified test phantoms to assess the performance of X-ray CT scanners. Using standardised test objects, independent of the scanner manufacturer, has enabled comparisons to be made between different scanners. However, the actual design of such test tools has varied little from the original proposals of uniform cylinders containing various inserts which have a relatively short length in the z-axis (parallel to patient couch). In order to assess head and body modes, multiple test phantoms are required.

In the meantime the scanners themselves have altered significantly. In particular, the introduction of helical, multi-slice and cone beam CT has significantly increased the imaging beam size in the z-axis. The way in which the scanners are used has also changed, moving away from simple axial slice views to multiple reconstructions (including coronal and sagittal planes) and 3D images. CT imaging itself has expanded into other areas, including so called hybrid imaging (e.g. SPECT CT) and, in the form of cone beam CT, is available on interventional fluoroscopy equipment, radiotherapy linear accelerators and dental equipment.

Modern CT scanners are filled with a system which can automatically adjust the radiation exposure for the shape, size and attenuation of each patient.

Traditionally, testing of CT scanner functionality is carried out using conventional cylindrically uniform test objects. As these offer no variation to the attenuation of the X-ray beam they are unable to test the function of this installed device. Since the automatic exposure control (AEC) system is now used on the majority of patients undergoing CT scanning, failure to test the function of the AEC is to omit a major component of the imaging system. The aim of the AEC system is to automatically match the scanning parameters used for each examination to the requirements of each individual patient and the level of image quality required for evaluation of the resultant images. If these systems do not work as expected then patient imaging will be compromised. This could occur in two ways: either the patient is scanned using inappropriately low scanning parameters, leading to poor, potentially non-diagnostic images. In this instance the patient is likely to be re-scanned and the radiation dose delivered for the original scan is wasted. Another possibility is that excessive scanning parameters are chosen. In this instance the patient would receive a radiation dose which is greater than that required, with no additional benefit to the patient.

A known alternative to the traditional cylindrical phantom is a conical phantom. However, this has disadvantages because its shape varies rapidly along its length. CT scanners can use the data they acquire whilst the X-rays are passing, for example, from anterior to posterior of the patient in order to determine the requirements needed when the system has rotated 180 degrees.

Modern scanners use X-ray beams which are often over 3 cm wide along the z-axis (patient's length). The conical phantom's rapid variation in dimension/volume does not allow such pre-determination of requirements because they change so quickly over a short distance. Further, the conical phantom is constructed from polymethyl methacrylate (PMMA), which provides different radiation attenuation characteristics to those of real patients. Other disadvantages of the conical phantom include it having no capability to represent different tissues to allow optimisation for multiple tissue equivalents (e.g. settings good for bone may not be good for soft tissue), it does not allow dose measurements and also does not allow evaluation of the resolution of the CT system.

Embodiments of the present invention are intended to address at least some of the abovementioned problems. Embodiments of the phantom described herein allow effective testing of the AEC function of CT scanners.

Embodiments allow measurements of image quality and radiation dose to be made whilst forcing the CT system to adjust the exposure as it would for a patient. Embodiments are suitable for use in assessing the functionality of CT AEC systems, to allow comparison of different scanners and also for optimisation of scanning protocols. Embodiments can provide a single test phantom that allows image quality and dose to be assessed not only on modern CT scanners employing AEC, but also on hybrid imagers (e.g. SPECT/CT scanner) and cone beam applications.

According to a first aspect of the present invention there is provided an X-ray scanner phantom including or comprising: a plurality of portions, each said portion being a three-dimensional shape having a width, a length and a depth, wherein the width and/or the length of one said portion differs from the width and/or the length of another said portion.

The depth of a said portion may be defined by a continuous line, e.g. with a minimum dimension of 1 mm and typically 10 mm. In some embodiments, the depth is 100 mm. The depth of each said portion may be generally identical.

A said portion may have an elliptical wall, with the length of the portion being defined by a major axis of the ellipse, the width of the portion being defined by a minor axis of the ellipse and the depth of the portion being defined by a height of the elliptical wall. Edges or corners of the elliptical wall may be rounded. An interface between the portions of the phantom may have a blended radius.

In some embodiments, the width and the length of each said portion differs from the width and the length of all other said portions. The widths and the lengths of the portions may vary in a stepped manner between ends of the phantom. The widths and lengths of the portions may be selected so as to correspond with patient dimensions encountered clinically. Examples of the lengths can be in the region of around 250 -400 mm and the widths can be in the region of 140-220 mm.

At least one of the portions may be separately connected to the phantom.

A said portion may include an arrangement for separately connecting to another said portion. Alternatively, the portions may be formed using a single solid body or an at least partially hollow shell. The body or shell may be formed of rigid material with low radiation attenuation properties (i.e does not attenuate the radiation beam significantly). A thickness of the body or shell may be around 3 mm. The body or shell may be formed by a vacuum forming process.

The phantom may further include at least one elongate insert. A holder assembly may be provided for holding a plurality of said elongate inserts. The at least one elongate insert can extend through the depth of at least one (and typically all) of the portions in a direction of a main axis of the phantom. A said elongate insert may be formed from a material intended to simulate a human or animal tissue during X-ray exposure. The elongate inserts may be formed from materials selected from a set including polymethyl methacrylate, PMMA; polyethylene (UHMW PE); black acetal. A said elongate insert may be hollow or at least partially solid. A hollow said elongate insert may be fitted with air. A said hollow elongate insert may be formed of clear acrylic. A said elongate insert may be cylindrical and may have a diameter of around 20 -25 mm. A said elongate insert may include a formation, e.g. a bore, slot or trench, for receiving a wire, e.g. a copper or tungsten wire. A length of a said elongate insert may correspond to the depth of a said portion, or to sum of the depths of at least some, or all, of the portions in the phantom. A said elongate insert may include an arrangement for fixing to an attachment, e.g. a handle. At least one said elongate insert may comprise a refillable container. In use, the refillable container may contain an isotope or other fluid.

At least one said portion may be filled with water or a solid material intended to simulate human or animal tissue during X-ray exposure, e.g. SZ-50 or a suitable epoxy resin An outer surface of the phantom may include an indicator corresponding to a central axis of the phantom.

At least one said portion may include an arrangement for receiving a dosimeter or ion chamber. A sheath may be provided for at least part (e.g. an end portion) of the dosimeter/ion chamber.

According to a further aspect of the present invention there is provided a kit including a phantom substantially as described herein and at least one support. The support may include a surface shaped to engage with a surface of a said portion. The support may be dimensioned so that, in use, when the phantom is placed upon it a main axis of the phantom is substantially horizontal (or perpendicular to the scan plane). The support may have a curved lower surface for use on a curved couch of the X-ray scanner. The support may include at least one foot portion, the foot portion having: a curved surface that corresponds to at least pad of the curved lower surface of the support, and a lower surface of the foot being designed to rest the support on a level surface.

The foot portion may be connected to the support by means of a flexible strip.

The kit may further include a holder for the phantom and/or the support. The holder may include a set of wheels and a handle.

According to another aspect of the present invention there is provided a method of assessing at least one property of an X-ray scanner, the method including or comprising: positioning a phantom substantially as described herein within an X-ray scanner; exposing the phantom to X-rays using the X-ray scanner, and analysing at least one property of the phantom during, or following, the X-ray exposure.

The at least one property may include a radiation dose received by an insert fitted within the phantom.

The X-ray scanner may comprise a CT scanner or a hybrid imaging scanner, e.g. SPECT CT scanner.

Whilst the invention has been described above, it extends to any inventive combination of features set out above or in the following description.

Although illustrative embodiments of the invention are described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments. As such, many modifications and variations will be apparent to practitioners skilled in the art.

Furthermore, it is contemplated that a particular feature described either individually or as part of an embodiment can be combined with other individually described features, or parts of other embodiments, even if the other features and embodiments make no mention of the particular feature. Thus, the invention extends to such specific combinations not already described.

The invention may be performed in various ways, and, by way of example only, embodiments thereof will now be described, reference being made to the accompanying drawings in which: Figure 1 is a schematic illustration of a first embodiment of the phantom being positioned in a CT scanner; Figure 2A is a schematic plan view of the phantom; Figures 2B and 2C are sectional front and side views, respectively, through Figure 2A; Figures 3A and 3B are plan and front views, respectively, of an insert for the phantom; Figures 4A and 4B are plan and front views, respectively, of another embodiment of the phantom; Figure 5 is a sectional view of the phantom of Figures 4 with an ion chamber holder, and Figures 6A and 6B are sectional and end views, respectively, of an alternative chamber holder.

Figure 1 shows an example of a conventional CT scanner machine 100 having an aperture 102 through which a patient couch 104 is, in use, moved to perform the CT scan. Such machines are known and need not be described herein in detail. The Figure further shows a phantom support 106 lying on the couch and an example phantom 110 being positioned on the support/couch.

The example phantom 110 includes four portions 112A -112D, each having an elliptical outer wall (shown partially transparent in the Figure). There may be a flat elliptical base 114 on the end portion 112D. In the example phantom the portions are all formed from a single shell of rigid material, such as acrylonitrile butadiene styrene, ABS, and can be formed by a vacuum forming process, for example. The material from which the shell is formed will be selected so as to be reasonably durable, wipe-able (using common detergent/cleansing wipes) and able to withstand temperature fluctuations (e.g. between -10°C and +40°C). The external surface(s) may be marked in various ways, e.g. with a product logo and/or datum lines to reflect the central axes of the ellipses.

The shell is formed with a hollow volume and can be filled with a fluid, typically water, and so is a sealed unit (but may include at least one removable cap (not shown) for filling and emptying). Having a shell that is refillable allows the weight of the phantom to be minimised, assisting with transportation, etc. An insert assembly 116 is also fixed inside the central region of the phantom.

The support 106 for the phantom 110 comprises a body 118 having curved upper and lower surfaces. The concave upper surface is dimensioned so as to fit underneath end portion 11 2A so that the central axis of the phantom is substantially horizontal. The convex lower surface is dimensioned so that the support is substantially level when it is placed on a correspondingly curved surface, e.g. a couch 104 having a concave surface. Foot portions 120 are provided for when the support is to be placed on a flat surface. The foot portions are wedge-shaped and fit underneath the sides of the convex lower surface of the support. Each foot portion may be connected to an adjacent side surface of the body by means of a flexible strip 122. The flexible strips allow the foot portions to be set at different positions to help keep the phantom resting horizontally.

Referring to Figures 2A -2C, the construction of the phantom 110 can be seen in more detail. The lengths and widths of the four portions 112A -112D differ from each other (herein the "length" of a portion relates to the major axis of the elliptical shape and the "width" relates to the minor axis of the elliptical shape). Generally, the lengths and widths decrease in a stepped manner from the end of the phantom having the base 114 to the opposite end. Example lengths and widths for the portions are given below: Length Width Portion 112A 250 mm 140 mm Portion 112B 300 mm 170 mm Portion 11 2C 350 mm 200 mm Portion 11 2D 400 mm 220 mm The ratios of the ellipses may be 1.79, 1.73,1.77. 1.75 (largest to smallest) and are chosen so that the transition between ellipses is smooth, although it will be understood that variations are possible. The example dimensions were selected because they correspond to the dimensions of parts of a patient's body that are typically scanned, e.g. an adult shoulders (male and female), an adult's thorax, a paediatric trunk etc. It will be understood that the above dimensions are exemplary only and variations are possible, as can the number of portions. For instance, an additional portion having a depth of 100 mm and a length of 160 mm and a width of 130mm may be provided to simulate part of a child's body. The ellipse was chosen because it is the closest geometrical shape to the human anatomy.

The depth (i.e. the height of the elliptical wall) of each portion 112A - 112D in the example is 100 mm. Thus, the depth is defined by a continuous line having a minimum length of at least 1 mm and, more typically, a minimum length of at least 10 mm. The phantom therefore comprises several distinct portions, each having a different volume that can be used for testing radiation doses, etc, as will be described below. Although the depth of each portion is equal in the example phantom 110, it will be understood that in other cases the depth of at least one portion may vary from the depth of another portion. Further, although the lengths and widths of the portions vary in a stepped manner between the ends of the phantom in the example, it will be appreciated that this need not always be the case, e.g. in other embodiments a portion having a greater length and/or width may be positioned between other portions having relatively smaller lengths and/or widths.

The base 114 is a flat elliptical plate (which has a length of 440 mm, a width of 260 mm and a depth of 8 mm in the example) that can be attached to the end portion 112D by any suitable means, e.g. welding, adhesive bonding, etc. As best seen in Figures 2B and 20, the base includes a central aperture 202 through which the insert assembly 116 is fitted inside the phantom 110.

Dependant on the final structure, this base may not be required Referring to Figures 3A and 3B, the insert assembly 116 includes a pair of opposed circular plates 302A, 302B between which extend a set of elongate inserts. The plates can be formed of clear acrylic and have a thickness of around 9 mm. A set of bores 303 (arranged as a 2 by 2 grid, 70 mm apart, around the centre of the plate) are also present in the circular plate 302A.

Nuts/bolts (or other suitable fixing means) pass through these bores, and corresponding bores in the base plate 114, to fix the insert assembly 116 to the phantom 110. In embodiments where the phantom is to be filled, open/closure means may be provided, e.g. the outer two nuts/bolts in Figure 2B.

In the illustrated embodiment there is a central cylindrical insert 304 comprising a clear acrylic cylindrical tube with a diameter of around 25 mm.

This can be used to receive an ion chamber or dosimeter. Arranged adjacent the outer perimeter of the plates 302, at 900 intervals, are four further inserts 306A -306D. At least some of the inserts can be formed of (or contain) material(s) that are intended to simulate human or animal tissue when exposed to X-rays. In the example, insert 306A is a polyethylene (UHMW PE) cylinder having a diameter of 20 mm; insert 306B is a tube of clear acrylic having a diameter of 20 mm; insert 306C is a tube or clear acrylic having a diameter of 25 mm and insert 306D is a cylinder of black acetal having a diameter of 20 mm. In some cases, the elongate inserts are hollow and can have removable (at least at one end) cap(s) to enable it to be filled with any desired fluid, e.g. contrast material or radioactive material to allow use in hybrid imaging systems. This insert may also be filled with solid materials to mimic other human or animal tissues. The materials have been chosen to give values (CT numbers) within the range that is typically encountered in clinical images. Acrylic approximates to a level accepted as a generic soft tissue; the black acetal to trabecular/compact/cancellous bone, and the polyethylene to adipose tissue.

Two further inserts are also present in the illustrated example insert assembly 114. The first is insert 308A, which is a clear acrylic tube that contains a length of copper wire having a diameter of 0.2 mm. The central axis of insert 308A is offset by 22° from the central axis of insert 306B. The second further insert 308B is offset by 22° from the central axis of insert 306C. This can also comprise a clear acrylic tube that contains a length of copper wire having a diameter of 0.2 mm. In alternative embodiments, the tube containing the wire may be filled with solid water material, in which case wires of under 0.1 mm diameter can be used. The wire can be used to evaluate the resolution of the system. An image of the wire can be taken and either the full-width at half maximum is determined, or the evaluation is made to mathematically determine the system resolution from the point spread function/impulse response. It will be appreciated that the number, arrangement, dimensions and materials of the inserts are exemplary only and many variations are possible. The lengths of the inserts 306 and 308 generally correspond to the total depth of the phantom 110; however, the central insert 304 can be longer so that its end protrudes beyond the circular plate 302A.

Figures 4A and 4B illustrate another embodiment 410 of the phantom.

Rather than being filled with a fluid in use, this embodiment can be filled with a solid water material, which may be permanently contained within a rigid outer shell from manufacture, or constructed of a solid material chosen to simulate human or animal tissue. Details for one example suitable material, a known Urethane base resin called SZ-50 (including a comparison with water) are given

in the tables below:

______ Percentage by weight (sample WT/SR 6) -A Components: Epoxy CB4(80.48), Polyethylene (10.00) calcium carbonate (5.77), ______ _____________ PMS (3.75)# B Elements: H (8.09), C (67.22), N (2.40), 0 ______ _______________ (19.84), Ca (2.32), Cl (0.13) C Specific 1.015±0.02* _______ Gravity: _____________________________________ Unsieved PMS *Relative to water at 22°, which has a density of 997.8 kglm3 Soft tissue (SZ-50) material Elemental Composition Effective Electron Density Material Atomic Density g/cm3 Number xlOn23e/g H C N 0 Water 1.000 7.417 3.343 11.19 88.81 SZ-50 1.061 6.14 3.258 8.41 72.25 4.61 14.73 Another difference between the embodiment 410 and the phantom 110 is its modular design. Instead of the portions being formed by a single integral shell/body, at least one of the portions 412A -412D can be separable from the rest of the phantom 410. Surfaces of the portion(s) may include suitable interengaging formations, or other connecting means, for allowing the portions to be connected together (with zero/minimal air gaps between). In the example, the two portions 412A, 412B are fixed together and are detachable from the other two portions 412C, 412D, as shown by line 413. However, it will be understood that this can vary, e.g. all four (or more/less) portions may be connectable together in any order. In yet another alternative embodiment, the entire phantom may be formed of one piece of solid material.

It will also be noted that the upper edges/corners 415 of the portions are rounded, with blended radii being used in the interfaces 417 between portions.

This reduction/elimination of sharp edges/corners has advantages in terms of safety, reduced risk of damage, etc. This feature also provides benefits to the quality of image data acquired, very sudden changes in attenuation can cause image artefacts.

Instead of the insert assembly 116, the phantom 410 includes a set of bores for receiving elongate inserts. These bores extend through the entire depth of each portion (and are aligned in all portions). In the illustrated example, there is a central bore 434, with a set of four bores 436A -4360 arranged around it at 90° intervals. Again, it will be understood that the number, arrangement and dimensions of the bores can vary. Tubes/cylinders of material (e.g. the elongate inserts described above) can be inserted through these bores.

In alternative embodiments, materials, such as those used in the inserts described above, may be incorporated/fixed into the structure of the phantom rather than being provided as inserts.

Figure 5 is a cross-sectional view through the phantom 410 and the central bore 434. An ion chamber 502 has been inserted within the central bore.

An inner end holder 504 may be provided for the ion chamber, the holder including internal contours arranged to complement the shape of the end of at least one conventional ion chamber (e.g. a narrower portion for receiving a standard 0.6 cc ion chamber and a larger portion for a 3 cc ion chamber) and an outer surface configured to fit snugly within the central bore. The holder may be formed of clear acrylic or the like. A sliding sleeve 506 may be provided at one exposed end of the bore for assisting with insertion of the ion chamber straight through the bore. The sleeve may be formed of clear acrylic and includes a cylindrical portion with a flange 507 that abuts the outer surface of the phantom.

The ion chamber may be fitted within a cylindrical extension bar 508, formed of acetal or the like. The extension bar can include a straight slot (around 4 mm deep) 509 along at least part of its length to act as a channel for protecting the cable of the ion chamber. A distance scale indicator may be marked on the external surface of the bar (e.g. marked 0 when the bar is fully inserted into the phantom). At least one end of the bar may be threaded or the like for connection of an attachment. An example of a suitable attachment is a handle 510 (formed of, e.g,, acetal) may also be provided for the end of the extension bar to assist with pushing/pulling it. A case (not illustrated) may also be provided for the phantom (and, optionally, accessories, such as the elongate inserts, extension bar, etc). The case may include a set of wheels and a handle (possibly an extendable handle).

Figures 6A and SB show an alternative holder 600 that has been designed so that a standard ionisation chamber (0.6 cc or 3 cc) can be held in its central channel 601, such that the active volume of the chamber is located free in air. The left hand end 602 of the holder is inserted into the phantom and chamber is fixed into the right hand end 604. The holder can be formed of a material such as acrylic and rubber 0-rings may be provided to help hold the chamber in place. In use, the phantom is positioned on a couch such that the chamber, outside of the phantom, is located at the centre of the rotation of the CT scanner, free in air. This measurement can provide information on the x-ray beam width as well as information about the x-ray generator parameters. Set-up of the chamber to the centre of the scan plane is also simplified using this holder as there is no need for an extra tripod, boom arm or the like.

In use, the phantom 110/410 is set up with the desired inserts, etc, and positioned on the couch 104 of the CT scanner 102. The scanner is then activated as normal and the phantom is exposed to doses of X-rays. The ion chamber, or any other suitable dosimeter, in the phantom can be used to measure the dose of radiation received inside each portion 112/412, either in real-time whilst in situ in the scanner, or as a separate analysis operation after being removed from it. The phantom can additionally be used to test image quality parameters (e.g. image noise, Hounsfield Number of materials (CT number), spatial resolution).

Radiation doses will be obtained from the results of the ion chamber (other dosimeter). Variations in the scanning parameters set by the operator or selected by the system will reflect in the measurement of dose within the phantom. The CT scanner will give an indication of the dose (Computed Tomography Dose Index (CTDI) and likely Dose Length Product (DLP)). These values give an indication of the overall level of energy imparted to the patient during the scan. The CTDI value is defined using one of two standard phantoms (16cm and 32cm diameters to represent head and body). Measurements are made at the centre and at 1 cm inboard of the edge/periphery and combined to give CTDI = 1/3 dose at centre +2/3 dose at periphery. This gives an indication of the dose delivered during a single rotation and so, when combined with information on the length of the total scan and how spread-out the individual slices are, the DLP can be calculated (DLP = CTDI x scan length).

It may be possible to determine a conversion factor for these measurements if a dosimeter is used to obtain dose levels on the surface of each ellipse and combined with comparable measurements at the centre to allow comparison of actual measurement to those indicated by the system.

Image noise is usually quantified in terms of the standard deviation in the CT number in regions of interest on an image -the higher the standard deviation, the noisier the image and the harder it therefore becomes to distinguish separate elements in the image. However, generally, low image noise equates to high radiation dose.

CT images use a specific scale (Hounsfield scale), which means that the grey levels in the image are scaled such that water = 0 and air = -1000. The materials chosen allow determinations to be made regarding how well the system has dealt with different materials (c.f. tissues).

The embodiments described herein mean that it may be possible to use a single phantom to routinely test CT scanners. This has many benefits, including reducing the clinical down-time of the scanner for testing, and minimising the manual handling requirements inherent with traditional testing using a number of different phantoms. One embodiment of the phantom can be constructed in such a way that it is water filled and can be emptied and filled on site, whereas traditional phantoms are of solid construction and consequently heavy and cumbersome to transport. Another positive feature of the phantom is its relative manufacturing simplicity, which enables it to be produced and sold inexpensively. It can also reduce scanner down-time due to testing and allow easy, meaningful comparison of different scanners and protocols. Quality assurance procedures can be simplified. Use of the phantom can result in quality control, minimisation of radiation doses and can help facilitate diagnosis.

Claims

CLAIMS1. An X-ray scanner phantom (110) including: a plurality of portions (112), each said portion being a three-dimensional shape having a width, a length and a depth, wherein the width and/or the length of one said portion differs from the width and/or the length of another said portion.
2. A phantom according to claim 1, wherein at least one of the portions (112) is separately connected to the phantom (110).
3. A phantom according to claim 2, wherein a said portion (112B) includes an arrangement for separately connecting to another said portion (11 2C).
4. A phantom according to claim 1, wherein the portions (112) are formed using a single solid body or shell.
5. A phantom according to any one of the preceding claims, wherein at least one said portion (112) consists of a material intended to simulate human or animal tissues.
6. A phantom according to any one of claims 1 to 5, wherein the at least one said portion (112) comprises an at least partially hollow shell filled with a solid water material, e.g. SZ-50.
7. A phantom according to any one of the preceding claims, wherein a said portion (112) has an elliptical wall, with the length of the portion being defined by a major axis of the ellipse, the width of the portion being defined by a minor axis of the ellipse and the depth of the portion being defined by a height of the elliptical wall.
8. A phantom according to claim 7, wherein edges or corners of the elliptical wall are rounded.
9. A phantom according to claim 7 or 8, wherein an interface between the portions (112) of the phantom has a blended radius.
10. A phantom according to any one of the preceding claims, wherein the width and the length of each said portion (112) differs from the width and the length of all other said portions.
11. A phantom according to claim 10, wherein the widths and the lengths of the portions (112) vary in a stepped manner between ends of the phantom (110).
12. A phantom according to any one of the preceding claims, wherein the widths and lengths of the portions (112) are selected so as to correspond with patient dimensions encountered clinically.
13. A phantom according to claim 12, wherein the lengths are in a region of around 250-400 mm, and the widths are in a region of around 140-220 mm.
14. A phantom according to any one of the preceding claims, wherein the phantom further includes at least one elongate insert (304).
15. A phantom according to claim 14, including a holder assembly (116) for holding a plurality of said elongate inserts (304, 306).
16. A phantom according to claim 14 or 15, wherein the at least one elongate insert (304) extends through the depth of at least one, and typically all, of the portions (112) in a direction of a main axis of the phantom (110).
17. A phantom according to any one of claims 14 to 16, wherein a said elongate insert (304) is formed from, or contains, a material intended to simulate a human or animal tissue during X-ray exposure.
18. A phantom according to claim 17, wherein the elongate insert (304) is formed from a material selected from a set including: polymethyl methacrylate; polyethylene (UHMW RE); black acetal.
19. A phantom according to any one of claims 14 to 18, wherein a said elongate insert (304) is hollow or at least partially solid.
20. A phantom according to claim 19, wherein a hollow said elongate insert (306C) is filled with air.
21. A phantom according to claim 19 or 20, wherein a said hollow elongate insert (306C) is formed of clear acrylic.
22. A phantom according to any one of claims 14 to 21, wherein a said elongate insert (304) is cylindrical and has a diameter of around 20 -25 mm.
23. A phantom according to any one of claims 14 to 22, wherein a said elongate insert (308A) includes a formation, e.g. a bore, slot or trench, for receiving a wire, e.g. a copper or tungsten wire.
24. A phantom according to any one of claims 14 to 23, wherein a length of a said elongate insert (304) corresponds to a sum of the depths of all of the portions (112) of the phantom (110).
25. A phantom according to any one of claims 14 to 24, wherein a said elongate insert (304) comprises a refillable container.
26. A phantom according to claim 25, where, in use, the refillable container contains an isotope or other material.
27. A phantom according to any one of the preceding claims, wherein an outer surface of the phantom (110) includes an indicator corresponding to a central axis of the phantom.
28. A phantom according to any one of the preceding claims, wherein the depth of a said portion (112) is defined by a continuous line.
29. A phantom according to claim 28, wherein a said portion (112) has a minimum depth of at least 1 mm.
30. A phantom according to claim 29, wherein a said portion (112) has a minimum depth of at least 10 mm.
31. A phantom according to claim 30, wherein a said portion (112) has a minimum depth of 100 mm.
32. A phantom according to any one of the preceding claims, wherein the depth of each said portion (112) is generally identical.
33. A phantom according to any one of the preceding claims, wherein at least one said portion (412) includes an arrangement (434) for receiving a dosimeter or ion chamber (502).
34. A phantom according to claim 33, wherein a sheath (508) is provided for at least part, e.g. an end portion, of the dosimeter/ion chamber (502).
35. A kit including a phantom (110) according to any one of the preceding claims and at least one support (1 06).
36. A kit according to claim 35, wherein the support (106) includes a surface shaped to engage with a surface of a said portion (112) of the phantom (110).
37. A kit according to claim 34, wherein the support (106) is dimensioned so that, in use, when the phantom (110) is placed upon it, a main axis of the phantom is substantially horizontal, or perpendicular to a scan plane of the X-ray scanner (100).
38. A kit according to claim 37, wherein the support (106) has a curved lower surface for use on a curved couch (104) of the X-ray scanner (100).
39. A kit according to claim 38, wherein the support (106) includes at least one foot portion (120), the foot portion having: a curved surface that corresponds to at least part of the curved lower surface of the support, and wherein a lower surface of the foot is designed, in use, to rest the support on a level surface.
40. A kit according to claim 39, wherein the foot portion (120) is connected to the support (106) by means of a flexible strip (122).
41. A kit according to any one of claims 35 to 40, further including a holder for the phantom andlor the support.
42. A method of assessing at least one property of an X-ray scanner, the method including: positioning a phantom (110) according to any one of claims 1 to 34, within an X-ray scanner (100); exposing the phantom to X-rays using the X-ray scanner, and analysing at least one property of the phantom during, or following, the X-ray exposure.
43. A method according to claim 42, wherein the at least one property includes a radiation dose received by an insert (304) fitted within the phantom (110).
44. A method according to claim 42 or 43, wherein the X-ray scanner (100) comprises a CT scanner or a hybrid imaging scanner, e.g. SPECT CT scanner.
45. An X-ray scanner phantom substantially as descried herein and/or with reference to the accompanying drawings.
46. A method of assessing at least one property of an X-ray scanner substantially as descried herein and/or with reference to the accompanying drawings.