WO2022090742A1

WO2022090742A1 - Hadron energy determination

Info

Publication number: WO2022090742A1
Application number: PCT/GB2021/052826
Authority: WO
Inventors: Nigel Allinson; Michela ESPOSITO
Original assignee: University Of Lincoln
Priority date: 2020-10-31
Filing date: 2021-11-01
Publication date: 2022-05-05
Also published as: GB2604088A; GB202017288D0

Abstract

An energy determining apparatus (100) for determining the individual energy of one or more particles of hadron radiation (40) has a main axis (101) for aligning with a general direction of incident particles (40). The apparatus comprises a two-dimensional array of scintillator elements (41) configured to emit a quantity of light in response to energy of one or more incident particles of hadron radiation (40) passing through, or along, that scintillator element (41). Each of the scintillator elements (41) is an elongate element with a longitudinal axis extending parallel to the main axis (101). An array of detectors (44) convert the light from the scintillator elements (41) into electrical signals. A processor (115) is configured to determine a total quantity of light corresponding to energy of a single particle (40) based on outputs of the array of detectors (44).

Description

HADRON ENERGY DETERMINATION

BACKGROUND

There are three main recognised treatment methods for cancer: surgery, chemotherapy and radiotherapy. The most common form of radiotherapy involves the use of external beams of high-energy x-rays. A more recent form of radiotherapy utilises very high-energy beams of charged particles, notably protons. This is called Proton Beam Therapy (PBT).

The chief advantages of using protons, or other charged particles, is that they travel a finite distance into tissue. This distance is controllable by modifying the initial energy of the protons. FIGURE 1 shows how PBT can deliver a greater fraction of the dose to a target location (i.e. the tumour site) and significantly reduce radiation dose to surrounding healthy tissues and organs. The position of the Bragg peak, near the end of travel for the protons, is determined by the proton stopping power of the tissues encountered and the incident energy of the protons.

Before treatment can begin, information is acquired about the treatment site by imaging. Computed Tomography (CT) is an imaging technique which acquires information about a subject by performing a plurality of scans of a treatment site from different directions. This provides a plurality of cross-sectional images at different directions. These can be combined into a 3D data set. Currently, treatment is usually planned by CT imaging using x-rays. However, there are disadvantages with this approach.

The properties of x-rays and protons are different. They lose their energy in material in different ways. X-rays primarily lose their energy due to electron density in the material traversed. CTs are calibrated in terms of Hounsfield units (obtained from a linear transformation of the measured x-ray attenuation coefficients); while the relevant parameter for PBT is the relative stopping power. The conversion process introduces uncertainties. It has been recognised that more accurate results could be achieved by using proton CT imagery for treatment planning.

FIGURE 2 schematically shows an arrangement for recording a proton CT. As proton trajectories are non-linear due to multiple Coulomb scattering, it is necessary to track the paths of individual protons. This tracking demands strict requirements of the instrumentation system and the delivery of incident protons. There is a tracker 23 on a proximal side of the subject and a tracker 24 on the distal side of the subject. FIGURE 3 shows one of the trackers 23, 24 in more detail. The tracker 23, 24 comprises two position sensitive detectors 27. Each of the position sensitive detectors 27 can determine a position of incident radiation as an x-y coordinate. The two recorded coordinates (Xi, Yi) and (X₂, Y₂), provide an estimate of the trajectory 28 of the proton and of: (i) a position A on the subject’s surface where the proton entered (from tracker 23); and (ii) a position B on the subject’s surface where the proton exited (from tracker 24). The proton loses energy in passing through the subject. A Residual Energy-Resolving Detector (RERD) records, directly or indirectly, the remaining energy, ER, of the exiting proton. The RERD can take the form of a calorimeter. The initial energy of the proton, Ei, is known from the settings of the proton accelerator. Knowing both the initial energy Ei and the remaining energy ER, it is possible to determine the energy lost across the subject (E/ - R).

These triplets of data [A(x,y), B(x,y), (E/ - E )] are obtained for many thousands of protons for each orientation of the subject and the instrument. These triplets can be used to construct a radiograph in terms of proton relative stopping power. A proton CT can be created by combining a series of such radiographs acquired at different rotations of the subject and the instrument about the isocentre. There are several algorithms for CT reconstruction, such as filtered back-projection or iterative methods.

FIGURE 4 shows a known way of implementing the position sensitive detectors 27 used in the trackers 23, 24. The position sensitive detectors 27 comprises an array of crossed silicon strip detectors or crossed scintillator fibres. In the case of a silicon strip sensor a single proton will create excess charge. In the case of a scintillator fibre sensor a single proton will create excess light. The charge or light is detected at the end of each strip, thereby providing an x-y location. FIGURE 4(b) shows a scenario where two protons pass through the detector within one recording period. The two protons give rise to four possible locations, with Fi and F₂ being false events. The fraction of false events increases as the number of real events increase. An improved position sensitive detector with three or more sets of rotated strips is described in WO 2015/189601 A1.

FIGURE 5 shows two possible ways of implementing the RERD (FIGURE 2, 26). In FIGURE 5(a), the RERD is a calorimeter which records the total residual energy. The calorimeter can comprise a monolithic block 31 of scintillator plastic. The incident proton (30) loses energy as it penetrates the scintillator and so produces fluorescent light proportional to the energy lost. The light is collected by a detector 32 and converted to an electrical signal. As there is no spatial information as to where the light is generated, such a RERD can only reliably work for single protons. In FIGURE 5(b) the RERD is a range telescope where the energy is detected by determining the distance travelled through layers of calibrated material. A plurality of thin layers or plates 33 are arranged orthogonally to the primary direction of the incident proton beam. As the proton traverse the layers it will deposit energy, recorded as light or electronic charge, up until the Bragg Peak. By recording the layer number of the last detected signal and the proton stopping power of the layers, the residual energy can be calculated.

WO 2015/189601 A1 describes a range telescope which can provide spatial information. The range telescope comprises layers of absorber elements with an array of CMOS photodiode devices positioned between absorber elements. Strip detectors are also provided at some positions within the range telescope. A disadvantage of this approach is that the number of layers becomes excessive if a high energy resolution is required across the full range of expected proton energies. For example, the full energy range could be around 200 MeV and, for a resolution of 3 MeV (a typical requirement for quality proton CT reconstruction), this requires around 70 layers.

It is an aim of the present invention to address at least one disadvantage associated with the prior art.

SUMMARY OF THE INVENTION

There is provided an energy determining apparatus for determining the individual energy of one or more particles of hadron radiation, the apparatus having a main axis for aligning with a general direction of incident particles, the apparatus comprising: a two-dimensional array of scintillator elements configured to emit a quantity of light in response to energy of one or more incident particles of hadron radiation passing through or along that scintillator element, wherein each of the scintillator elements is an elongate element with a longitudinal axis extending parallel to the main axis; an array of detectors to convert the light from the scintillator elements into electrical signals; a processor configured to determine a total quantity of light corresponding to energy of a single particle based on outputs of the array of detectors.

Optionally, the processor is configured to determine a group of detector outputs corresponding to a single particle and to determine a total quantity of light corresponding to the single particle from the determined group of detector outputs when there is a plurality of substantially simultaneously incident particles.

Optionally, the processor is configured to determine an individual group of detector outputs corresponding to a single particle when there is a plurality of overlapping groups of detector outputs.

Optionally, the processor is configured to determine a start point of a group corresponding to a single particle from detector outputs and thereby to determine a position at which the single particle is first incident on the energy determining apparatus.

Optionally, the processor is configured to: determine an end point of a track from detector outputs, wherein a peak intensity value corresponds to a Bragg peak of a particle at the end of a group; determine the start point based on the end point of the group.

Optionally, each of the scintillator elements has a response time shorter than a repetition rate of incident particles. A rise time of the scintillator elements can be of the order of 100 ps.

Optionally, each of the scintillator elements has a length greater than an absorption depth of incident particles at maximum energy. Preferably, the length is at least 30 cm for 250 MeV protons.

Optionally, each of the scintillator elements has a core of a first material of a first refractive index and a cladding of a material of a second refractive index which is higher than the first refractive index.

Optionally, a downstream end of the array of scintillator elements is configured to directly output light to the array of detectors.

Optionally, there is at least one intermediate stage between a downstream end of the array of scintillator elements and the array of detectors. The at least one intermediate stage can comprise a gain stage. The at least one intermediate stage comprises an optical reduction stage with an input side having an input area and an output side having an output area, where the output area is less than the input area. Optionally, a number of detectors in the array of detectors is equal to a number of the scintillator elements in the two-dimensional array.

Optionally, a number of detectors in the array of detectors is less than a number of the scintillator elements in the two-dimensional array.

There is provided a computed tomography scanner apparatus comprising: a first beam tracker for positioning on an input side of a subject; a second beam tracker for positioning on an output side of a subject; an energy determining apparatus according to any one of the preceding claims, wherein the first beam tracker and the second beam tracker are configured to determine the position of hadron particles passing through the beam trackers and the energy determining apparatus is configured to determine individual residual energy of one or more particles after passing through the subject and the second beam tracker.

Optionally, the second beam tracker comprises a position sensitive detector configured to determine a position of a particle in a two-dimensional plane and the energy determining apparatus is configured to determine a position of a particle in a two-dimensional plane, and a processor is configured to determine an output vector of a particle from the position determined by the second beam tracker and the position determined by the energy determining apparatus.

There is provided a proton probe comprising: a beam tracker for positioning on an output side of a subject; an energy determining apparatus according to any one of claims 1 to 14, wherein the beam tracker is configured to determine the position of hadron particles passing through the beam tracker and the energy determining apparatus is configured to determine individual residual energy of one or more particles after passing through the subject and the beam tracker.

Optionally, the beam tracker comprises a position sensitive detector configured to determine a position of a particle in a two-dimensional plane and the energy determining apparatus is configured to determine a position of a particle in a two-dimensional plane, and a processor is configured to determine an output vector of a particle from the position determined by the second beam tracker and the position determined by the energy determining apparatus. There is provided an energy determining apparatus for determining the individual energy of one or more particles of hadron radiation, the apparatus having a main axis for aligning with a general direction of incident particles, the apparatus comprising: a two-dimensional array of scintillator elements configured to emit a quantity of light in response to energy of one or more incident particles of hadron radiation passing through or along that scintillator element, wherein each of the scintillator elements is an elongate element with a longitudinal axis extending parallel to the main axis; and an array of detectors to convert the light from the scintillator elements into electrical signals.

There is provided a method of determining the individual energy of one or more particles of hadron radiation, the apparatus having a main axis for aligning with a general direction of incident particles, the method comprising: providing a two-dimensional array of scintillator elements configured to emit a quantity of light in response to energy of one or more incident particles of hadron radiation passing through or along that scintillator element, wherein each of the scintillator elements is an elongate element with a longitudinal axis extending parallel to the main axis and an array of detectors to convert the light from the scintillator elements into electrical signals; determining a total quantity of light corresponding to energy of a single particle based on outputs of the array of detectors.

Optionally, the method comprises determining a group of detector outputs corresponding to a single particle and determining a total quantity of light corresponding to the single particle from the determined group of detector outputs when there is a plurality of substantially simultaneously incident particles.

Optionally, the method comprises determining individual groups of detector outputs corresponding to a single particle when there is a plurality of overlapping groups of detector outputs.

The method may also perform any of the processing functions or steps described in this specification.

One advantage of the energy-determining apparatus is that it can determine the energy of each of a plurality of particles which are incident on the apparatus during a particular time interval, such as a cyclotron period or the sampling period of the instrument. The apparatus is not limited to a single particle during a particular time interval. This improves accuracy of results and can make it possible to acquire higher quality data in a shorter overall time period. The array of scintillator elements and the array of detectors allow the apparatus to record a cluster or track formed by each particle as it is incident on and/or moves across the array of scintillator elements.

The individual tracks can then be separated, if required, to identify the energy of each incident particle. The apparatus is, in effect, a pixelated calorimeter.

The ability to determine energy per particle for multiple simultaneously incident particles reduces data acquisition time and reduces the radiation dose to the subject to acquire a useful data set. It can also allow an accelerator (i.e. the source of particles) to operate at a more efficient operating level.

Detecting light outputs from scintillator elements which are oriented parallel to a general direction of incident particles allows the apparatus to determine a higher range of energy values (i.e. lowest energy value to highest energy value) and a finer measurement resolution (i.e. spacing between determined energy values). This can also simplify the energy determining apparatus compared to a conventional range telescope with a large set of absorber layers and detectors between layers.

The term “particles of hadron radiation” includes protons and any other penetrating charged particles.

Embodiments of the invention may be understood with reference to the appended claims.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example, features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. For the avoidance of doubt, it is to be understood that features described with respect to one aspect of the invention may be included within any other aspect of the invention, alone or in appropriate combination with one or more other features.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which:

FIGURE 1 shows absorbed radiation dose versus tissue depth for x-rays and proton radiation;

FIGURE 2 shows a proton CT scanner;

FIGURE 3 shows use of position-sensitive detectors to determine a vector of a proton path;

FIGURE 4 shows conventional strip detectors as position-sensitive detectors;

FIGURE 5(a) shows a calorimeter and FIGURE 5(b) shows a range telescope for determining residual energy in a proton;

FIGURE 6 shows a pixelated calorimeter;

FIGURE 7 shows an end view of a pixelated calorimeter and a proton track;

FIGURE 8 shows operation of the pixelated calorimeter over a plurality of time intervals;

FIGURE 9 shows outputs of an array of scintillator elements of a pixelated calorimeter;

FIGURE 10 shows separation of contributions of individual protons;

FIGURE 11 shows a chain coding scheme;

FIGURE 12 shows an example of overlapping tracks recorded by a pixelated calorimeter and possible chain codes for the tracks;

FIGURE 13 shows the example of overlapping tracks of FIGURE 12 and other possible chain codes;

FIGURE 14 shows a differential chain coding scheme;

FIGURE 15 shows a mosaic of individual pixelated calorimeters;

FIGURE 16 shows an example of interfacing scintillator elements to a detector array;

FIGURE 17 shows an example of interfacing scintillator elements to a detector array via an optical taper;

FIGURE 18 shows a linear array of pixelated calorimeters and scanning of a pencil beam;

FIGURE 19 shows an example of a relationship between scintillator elements and detectors; FIGURE 20 shows another example of a relationship between scintillator elements and detectors;

FIGURE 21 shows a detailed view of the downstream end of a scintillator element and detectors;

FIGURE 22 shows a conventional beam tracker on a distal side of a subject and a conventional calorimeter;

FIGURE 23 shows a simplified beam tracker on a distal side of a subject and a pixelated calorimeter;

FIGURE 24 shows an example plot of light intensity values along a track;

FIGURE 25 shows an example of a proton CT scanner;

FIGURE 26 shows a proton probe;

FIGURE 27 shows a proton probe and outputs obtainable with a conventional calorimeter and a pixelated calorimeter;

FIGURE 28 shows use of proton probe;

FIGURE 29 illustrates the use of an arrangement of proton probes according to an embodiment of the present invention during a process of patient verification in which treatment beams are provided according to a planned treatment plan and characterised in turn in order to confirm that they are in agreement with the treatment plan; in the arrangement shown, a moderator element is provided in place of the patient in order to reduce the flux reaching the proton probe although the provision of this element is optional;

FIGURE 30 illustrates the arrangement of FIG. 29 in which a phantom replaces the moderator element of FIG. 29;

FIGURE 31 shows a method of processing outputs of the detector array;

FIGURE 32 shows another part of a method of processing outputs of the detector array;

FIGURE 33 shows an example of a detector and read-out circuitry for the calorimeter;

FIGURE 34 shows a charge pulse and operation of the comparator and counter of FIGURE 31 ;

FIGURE 35 shows a data array of energy values;

FIGURE 36 shows a processing apparatus for performing at least part of the processing functions of the energy-determining apparatus.

DETAILED DESCRIPTION

FIGURE 6 shows a side view of an energy-determining apparatus 100 for determining the individual energy of one or more particles of hadron radiation. The apparatus 100 can, for example, be used as part of a proton CT system for acquiring imaging information to plan treatment or as part of a proton probe. The apparatus 100 is positioned on the distal (output) side of a subject. The energy-determining apparatus 100 is also called a pixelated calorimeter in the following description.

The energy-determining apparatus 100 has a main axis 101 which is intended to be aligned with a general direction of incident particles. It will be appreciated that particles will arrive from a range of directions, but they will have a general direction determined by the source. Usually, a source and a detector are aligned with each other on opposite sides of a subject. The apparatus 100 comprises a two-dimensional array or bundle 110 of scintillator (light converting) elements 41. The array 110 has an input/upstream end 102 and an output/ downstream end 103. FIGURE 6 shows a side view of the array 110. FIGURE 7 shows an end view of the two-dimensional array 110. Each of the scintillator elements 41 is an elongate shaped element with a longitudinal axis extending parallel to the main axis 101 of the apparatus. The longitudinal axis of each of the scintillator elements 41 is perpendicular to an end face of the apparatus. The elongate shaped element can be a fibre, a bar, a strip, a tube or some other elongate element. In use, incident protons 40 enter the upstream end 102 of the array 110. Each of the scintillator elements 41 is configured to emit a quantity of fluorescent light in response to kinetic energy of one or more incident particles of hadron radiation passing through, or along, that scintillator element 41. The array or bundle 110 is a collection of individual scintillator elements 41 (e.g. optical fibre strands) which are assembled together so that the relative orientation of the individual elements/fibres is maintained throughout the length of the bundle. The result is that any pattern of illumination incident at the input/upstream end 102 of the bundle is maintained and ultimately emerges from the output/downstream end. This has the effect of preserving the input image.

The energy-determining apparatus 100 comprises an array of detectors 44 to convert the fluorescent light from the scintillator elements 41 into electrical signals. The array of detectors 44 is positioned at the downstream end of the array of scintillator elements 41. Sensor types include multi-anode photomultiplier tube (PMT) arrays or silicon pixel sensors, such as Complementary Metal Oxide Semiconductor (CMOS) active pixel sensors or hybrid detectors. A pre-storage gain element 43, such as an image intensifier or a multichannel plate, may be positioned between the downstream end of the scintillator elements 41 and the detectors 44. Some sensors have an elevated noise floor (equivalent to a few thousand visible photons) and can benefit from a pre-storage gain element (43). The electrical signals generated by the detectors 44 are read out by read-out circuitry 45, such as amplifiers and other signal conditioning elements) and converted into a form suitable for transmission to a processor 115, such as a host computer or a specialised processor dedicated to processing the detector data. The processor 115 is configured to determine a total quantity of light corresponding to energy of a single particle based on outputs of the array of detectors. Processor 115 may be local to the read-out circuitry 45, or remote to the read-out circuitry 45 and in communication with the read-out circuitry 45.

The energy-determining apparatus 100 may comprise a reflective layer 46 positioned on, or in front of, the upstream end of the array 110. For example, the reflective layer 46 may be coated onto upstream ends of the scintillator elements 41. The reflective layer 46 can be a thin layer which does not impede the progress of the protons. The reflective layer 46 may increase the light yield by minimising loss of light from the upstream ends of the scintillator elements 41.

In use, incident protons 40 enter the upstream end 102 of the array 110. As the protons lose energy along their trajectory, the scintillator material causes the emission of light (usually within the visible spectrum). The fluorescent light is guided along the elements by a process of total internal refection 42. An incident proton 40 may remain within a single scintillator element 41 , or may cross a plurality of the scintillator elements 41. When a proton crosses a plurality of the scintillator elements 41, light is emitted by each of the scintillator elements 41 that is crossed. The detectors 44 detect an intensity (amplitude) of generated light. The array of scintillator elements 41 is coherent so that a linear spatial relationship is maintained from the input side of the assembly and the input of the detector 44.

The apparatus 100 determines energy of individual protons that pass through a subject and the distal trackers. An advantage of the apparatus 100 is that it can determine an amount of energy (related to the quantity of light generated in the scintillator elements and detected by detectors 44) and a position (in the x-y plane) of the incident proton. The apparatus 100 has the capability to record the individual energy of multiple protons which arrive simultaneously, or which arrive at such small time differences that they cannot be distinguished from being simultaneous within the minimum sampling time of the system.

Advantageously, the scintillator material of the scintillator elements 41 quickly generates light in response to incident protons (i.e. has a fast rise time) and then the light level decays. Advantageously, the rise and decay time is much smaller than a pulse repetition rate of proton beams. Typically, the rise time is of the order of a few 100 ps. Rise time is generally taken as the time to transition from 10% to 90% of the peak value. The typical time period of protons from a cyclotron is 10 ns, which is of the order of 100 times slower than the rise time.

Plastic scintillators typically meet this requirement. Scintillator crystals normally have slower rise and decay times. The term "plastic scintillator" refers to a scintillator material in which the primary fluorescent emitter, called the fluor, is suspended in a solid polymer core. Typical core materials are polyvinyltoluene and polystyrene. Cores are usually clad with a higher refractive index material such as polymethylmethacrylate (PMMA), to enhance the total internal reflection. The fluors are large organic aromatic molecules. Only a small portion of the incident kinetic energy lost is converted in fluorescent energy. Typically, this is about 125 eV per scintillation photon, or about 8,000 photons per MeV.

The scintillator elements 41 have a length which is sufficient to accommodate the expected energy of the incident protons. The empirical Birks's law describes the light yield per path length as a function of the energy loss per path length for a particle traversing a scintillator (Birks JB. Scintillations from organic crystals: specific fluorescence and relative response to different radiations. Proceedings of the Physical Society. Section A. 1951 Oct 1;64(10):874). The relationship is:

where: L is the light yield;

S is the scintillation efficiency; dE/dx is the energy loss of the particle per unit path length; and ks is Birks's constant, which depends on the material, k_B is 0.126mm/MeV for polystyrene-based scintillators.

For use in PBT, where the typical maximum proton energy is 250 MeV, this translates to element lengths of about 300 - 350 mm. Plastic scintillator elements (referred to as bars) can be manufactured down to 5 mm cross-sectional width with a cross-sectional shape such as circular or square. Plastic scintillator elements having narrower cross-sectional widths can be manufactured using a similar technology. These are typically called scintillator fibres. Scintillator fibres can be made extremely thin (e.g. down to 10s of microns in cross-section) and comprise a plastic scintillator core and an external cladding. Scintillator bars are for all functional purposes comparable to scintillator fibres. The differences are due to manufacturing limitations: plastic scintillator bars cannot be fabricated with cross-sections smaller than a few mm while scintillator fibres can be manufactured down to 10s of microns. Both technologies together cover the range of dimensions that are required for the pixelated calorimeter.

FIGURE 7 shows an example of an array of scintillator elements 41 and a path of an incident proton across the array. In this example the scintillator elements 41 each have a circular cross-section, but other cross-sectional shapes are possible. When a proton crosses any medium it continuously deposits energy along its trajectory. The proton stops when its residual kinetic energy is zero (often termed its “range”). FIGURE 7 shows the end of the path at the Bragg peak 48. Charged particles undergo electronic interactions as they traverse a medium, losing energy in the process and slowing down. As the particle slows down, the density of ionisations induced in the medium increases. Ionisation density drops abruptly to zero beyond the Bragg peak (“range”), since all of the particle’s kinetic energy has been exhausted and it can be considered stationary. The rate at which the energy is lost along the proton trajectory is a function of the proton residual energy (i.e.“ fe ⁼ - )), and a lower energy proton will lose a higher amount of energy per unit length compared to a higher energy one. The function

can represented by the Bethe equation. For a particle with speed v, charge z (in multiples of the electron charge), and energy E, travelling a distance x into a target of electron number density n and mean excitation potential /, the relativistic version of the formula reads, in SI units:

where: c is the speed of light and s₀ the vacuum permittivity, = v/c, e is the electron charge and m_e is its rest mass.

Here, the electron density of the material can be calculated by:

where: p is the density of the material, Z its atomic number and A its relative atomic mass;

NA is the Avogadro number; and

M_u is the Molar mass constant. E /

The scintillator light output is proportional to for each element.

A summation of the light output from each activated scintillator element 49 is directly proportional to the total energy lost by the incident proton:

where: ER is the total energy of the incident proton; and h is the light output of the i^th activated element.

FIGURE 8 schematically shows operation of the apparatus 100. As illustrated here, multiple protons 51 arrive in bunches with the primary general direction of the proton being parallel to the longitudinal axes of the scintillator elements of the apparatus 100. The output 52 of the apparatus 100 is a list of the incident energy per proton. For example, in the time interval ATi there are protons with energy values: Proton 1 = 152 MeV; Proton 2 = 78 MeV; Proton 3 = 134 MeV and so on. Each energy value is the total energy per proton, obtained by combining outputs from multiple detector outputs of the apparatus 100. There is a list of determined energy values for the next time intervals AT2, AT3. The typical range of operating frequencies of a cyclotron (proton source) is 25 MHz to 100 MHz, where ATj= 1/(cyclotron frequency), with the active bunch time being typically about 4 ns.

An energy value per proton can be recorded along with an incident point on the calorimeter. A difference between the initial energy (set by the cyclotron) and the residual energy (determined by the calorimeter) gives the opacity or stopping power of the material that the proton passed through on its path.

The nature of a proton’s trajectory across the calorimeter will depend on several factors, such as: proton energy; angle of incidence to the face of the scintillator bundle; the type of scintillator material used in the scintillator elements; and dimensions of the scintillator elements. The trajectory of a proton may appear as a group of activated elements in the form of a cluster or as an elongated track of activated elements. FIGURE 9 shows some example trajectories for two different sizes of scintillator elements. FIGURE 9(a) shows scintillator elements having a 1 mm diameter and a trajectory with one incident proton and with two incident protons. FIGURE 9(b) shows scintillator elements having a much smaller 2 pm diameter and a trajectory with four incident protons. In general, resolution of trajectories improves as the diameter (or other cross-sectional dimension) of the scintillator elements is reduced. This makes it easier to resolve the trajectory of each proton arriving in the same time interval. As the diameter of each of the scintillator elements decreases there is a reduction in signal level (intensity) output by each of the scintillator elements, so there is a tradeoff between scintillator element dimension and signal intensity.

When more than one particle is incident on the array during the same time interval, the individual contributions of each particle may be recorded as separate groups (e.g. separate clusters or separate tracks) or as overlapping groups (e.g. overlapping clusters or overlapping tracks). A range of image processing algorithms can be used to identify and discriminate individual clusters or tracks. One possible approach is summarised in FIGURE 10. This uses binary morphology, after the proton tracks and clusters have been thresholded to produce a binary image or matrix. Clusters can then be identified and isolated. Binary masks are created (FIGURE 10(b)) which are used to identify which elements to sum to yield an individual proton energy (FIGURE 10(c)).

In the example of FIGURE 9(a) the trajectories of the two incident protons are separate, i.e. they do not overlap on the array. This simplifies determination of the energy per proton. In the example of FIGURE 9(b) the trajectories of the four incident protons overlap to some extent. Where trajectories cross, it is necessary to correctly isolate individual tracks and hence determine individual proton energy.

FIGURES 11-13 summarise one approach to skeletonise the binary image and calculate a chain-code for each elongated trajectory. FIGURE 11 shows the principle of the coding scheme. Starting at the central square, eight possible directions are labelled (0, 1, ... , 7). Each direction represents a possible direction of the next square (pixel) of the track.

FIGURES 12 and 13 each show data representing two overlapping tracks for protons output by the array. The ends of the tracks are represented by the letters A, B, C and D. There are two ways of interpreting this data to separate the individual track of each proton. FIGURE 12 shows a first way of interpreting the data, with a first track AB 53 and a second track 54. Using the coding scheme shown in FIGURE 11 , track AB can be represented by the chain code <0101010100101010> and track CD (54) can be represented by the chain code <0777606707>. The marked difference in the codes indicate the general direction of the trajectories. In general, the most likely complete trajectories are the smoothest combination of individual segments. FIGURE 13 shows a second way of interpreting the same pixel data. Track AC is represented by the new chain code <010101112334> and track DB is represented by the chain code <343201010>. The abrupt changes in chain code values indicate a sudden directional change.

FIGURE 14 shows a coding scheme for a differential chain code. This records the relative change in direction of the track and not the absolute direction. Starting at the central square, eight possible directions are labelled with values (-3, -2, -1, 0, +1, +2, +3). Each direction represents a possible direction of the next square (pixel) of the track. The lower values represent the most likely options for direction change and the higher values indicate the less likely options for direction change.

For the example of FIGURE 12, the differential chain code for the track AB = <0, +1 , -1, +1, - 1, +1 , -1, +1 , 0, 0, +1 , -1, +1 , -1, +1 , -1 > and for the track AC = <0, +1, -1, +1, -1, +1, -1 , +1, +1, +1 , +1, +1 , 0>. The sum of the individual differential chain code values provides a measure of smoothness of a track. In this example for track AB the sum is +1, and for track AC the sum is +4. The assumption is that tracks will be smooth over some distance, and given a choice of intersecting tracks, those that are the smoothest are taken as the correct ones. Applying this same differential chain coding scheme to FIGURE 13 gives higher corresponding sums of the differential chain code values. Therefore, in this example, tracks AB and CD are assumed to be the actual tracks for two protons. Including information on the intensity of the fluorescent light at each element can provide additional confirmation of the correct track choice. A variety of image processing algorithms, such as “snakes” or “active contours”, can be applied to this task.

Within a typical operating environment there can be other radiation types present in addition to protons. For example, x-rays and gamma rays. These will be detected by the apparatus 100 but can be eliminated by virtue of the intensity of the light produced (all secondary radiation will be at lower energies than the protons and the shape of the clusters produced.

The optical detectors 44 used in the apparatus 100 should advantageously be capable of providing very fast frame readout. Some examples of suitable detectors are multi-anode photomultipliers (MAPMT) and solid-state hybrid pixel sensors. MAPMTs have pixel sizes ranging from 2 mm to 8 mm square within arrays of typically 8 x 8 to 16 x 16 pixels. Hybrid sensors, comprising a bonding of a semiconductor sensor and a readout chip, have a total area of up to 2 cm square with pixel sizes between 25 to 100 pm, and active area currently limited to about 2 cm square. The aperture dimensions will increase with advances in processing technology. A full beam scan area, at the iso-centre, for a proton beam delivery system requires a large area, such as an area of 30 cm x 30 cm. One approach to achieve such a large area is to provide an array (a mosaic) of individual calorimeter devices. Each calorimeter device can be the same as the apparatus 100. FIGURE 15 shows a mosaic 55 of individual calorimeter devices 56, viewed from an end face. The array 55 of individual calorimeter devices 56 provides continuous coverage. Incident radiation can move between scintillator elements in adjacent calorimeters, in the same way that is shown in FIGURES 6 and 7 between adjacent scintillator elements. The apparatus can process outputs of the entire array of calorimeters and can, for example, determine tracks which start & end in different calorimeters.

FIGURE 16 schematically shows a calorimeter of the type shown in FIGURE 6, where the array of detectors 44 is implemented by a multi-anode PMT 57. A multi-anode PMT has a high sensitivity and low-noise floor, which can avoid the need for a pre-storage gain element (FIGURE 6, 43) between the downstream end of the scintillator elements 41 and the multianode PMT 57. The multi-anode PMT detector 57 may have an area which is matched to the downstream end of the array of scintillator elements 41. This avoids the need for any optical reduction (demagnifying) element.

FIGURE 17 schematically shows a side view of a calorimeter of the type shown in FIGURE 6, where the array of detectors 44 is implemented by hybrid detectors 59. A coherent fibre-optic taper 58 with a demagnifying ratio of, for example, between 2:1 to 5:1 is provided before the detectors 59. This can demagnify projection from the array of scintillator elements 41. FIGURE 17 shows a pre-storage gain element 43 between the downstream end of the array of scintillator elements 41 and the taper 58. This may be needed to compensate for the optical losses of light transmission through the taper and/or the relatively high noise floor of the hybrid detector (typically equivalent to 500 - 1 ,000 electrons rms).

In FIGURE 15 a large scan area is achieved by forming a two-dimensional array of individual calorimeters. It is possible to simplify this to a linear array of individual calorimeters. FIGURE 18 shows an example of a linear array 60A of individual calorimeters 60. Many current proton beam imaging systems use an electromagnetically scanned narrow or pencil beam. This means that it is possible to use a smaller, linear, detection area. This can considerably reduce the cost and complexity of a full-scan area calorimeter. A defocussed proton beam 61 at diagnostic flux levels is scanned across the subject. The proton beam has sufficient energy to pass entirely through the subject and exit the subject to reach the calorimeters 60. A typical scan velocity may be up to 10 ms^-1. During the scanning operation the proton beam is shifted a distance 62D between adjacent scan lines. The array 60A of calorimeters is shifted a corresponding distance 62D with respect to the subject between adjacent scan lines. This permits another line of the overall scan area to be scanned. Other than the additional time for this movement there will no increase in the radiation dose to the subject or adverse effects on the quality of the data acquisition.

The spatial resolution of the calorimeter is set by the dimensions of the “pixels” of the detector, or its magnified version if a demagnifying taper (FIGURE 17, 58) is used. If the calorimeter is not used to record the fine locations of protons then the spatial resolution can be fairly coarse as under diagnostic conditions the proton flux density is typically less than 5 protons per cyclotron period over the full beam cross-section.

FIGURES 19 and 20 show two possible relations between scintillator elements and detectors. In FIGURE 19 there is a 1:1 correspondence between scintillator elements and detectors. A downstream end of a scintillator element 63 outputs light to an individual detector 67. For example, the diameter or width of the scintillator element matches the anode size of a PMT detector. The scintillator elements 63 are registered (aligned) with the individual detectors 67. The proton (65) may track across several elements before it gets to its end of range. The proton only uses significant energy as it passes through the scintillator material, so small air gaps (66) between elements will not adversely affect energy measurements.

In FIGURE 20 there is an N:1 correspondence between scintillator elements and detectors. A downstream end of plurality (N) scintillator elements 68 outputs light to an individual detector 67. For example, the diameter or width of each scintillator element is much smaller than the anode size of a PMT detector (or the size of some other detector type).

FIGURE 21 shows an end of a scintillator element 69 and a detector 70 forming part of a detector array. Light emerging from the downstream end of the scintillator element 69 will disperse over a distance due to its numerical aperture, NA:

where: th and r>2 are the refractive indices of the core and cladding material respectively.

The distance, D, between the downstream end of the scintillator element and the detector 70 can be selected based on the spread of light due to the NA. In practical implementations of detector arrays there is dead space between active regions of individual detectors 70 in the array of detectors. An optical coupler 85 can be provided between the downstream end of the scintillator element and the detector 70 to maximise the transmission of light from the scintillator element to the detector. The optical coupler can have a refractive index which is the geometric mean of the refractive index Rh of the scintillator element and the refractive index RI2 of the detector, i.e. (Rh x RI2) .

FIGURE 22 schematically shows the output side of a conventional proton CT system. After the subject 72, there are two position sensitive detectors 71 offset along the path of the proton. As previously described with respect to FIGURE 3, the pair of position sensitive detectors provide a pair of coordinates (Xi, Y1), (X2, Y2). These coordinates can be used to determine the estimated vector 75 of the emergent proton 74 and hence the estimated exit position on the subject’s surface - indicated as point P. The calorimeter 73 is a conventional calorimeter without spatial indication of the position of the proton.

FIGURE 23 shows the output side of a proton CT system using a calorimeter 76 of the type described in this application. The calorimeter 76 can determine position of the emergent proton 74 due to the array of scintillator elements. This can allow replacement of one half of the distal tracker 71 of FIGURE 22. A single stage position sensitive detector 77 is shown. The single stage position sensitive detector 77 provides a first coordinate (Xi, Y1) of the path of the emergent proton 74. The calorimeter provides the second coordinate (X2, Y2) of the path of the emergent proton 74. A required resolution is in the order of 100 pm to 300 pm. The use of a fine-grained calorimeter reduces the cost and complexity of the overall system.

Referring again to FIGURE 7 and the examples of FIGURES 12 and 13, the calorimeter can record the track of the proton across the array of scintillator elements. The track has two end points. However, the calorimeter does not record which of these end points is the start point, i.e. the proton’s incident position on the calorimeter. This information can be determined by inspecting intensity values. FIGURE 24 shows an example track recorded by a calorimeter and light intensity values along the track. The upper part of FIGURE 24 shows the track 77 recorded by the calorimeter in the x-y plane. The lower part of FIGURE 24 shows light intensity values along the track 77. The light intensity values follow a Bragg curve. By analysing light values along the track 77, the highest values occur at end 78 of the track. Point 78 is determined to be the Bragg peak at the end of the track. The start point of the track is determined to be point 79 at the opposite end of the track 77. The start point 79 gives the incident position X of the proton on the calorimeter and has positional coordinates (x, y). Typically, an intensity value of the Bragg peak is between 4 or 5 times higher than an intensity value at a start point.

FIGURE 25 schematically shows a proton beam computerised or computed tomography (CT) scanner 200. The scanner 200 comprises a source 210 of protons (e.g. a cylcotron), a first beam tracker structure 220 on the input side of a scanning zone where a subject 205 is positioned, a second beam tracker 230 on the output side of the scanning zone and an energy determining apparatus 250 of the type described above. A controller 260 is connected to the beam trackers 220, 230 and the energy determining apparatus 250. A beam of protons 215 is projected towards the subject from the source 210. The beam of protons 215 passes through the first beam tracker structure 220 to the subject 205, emerges from the subject 205 and passes through the second beam tracker structure 230 and into the energy determining apparatus 250.

As described above, the beam tracker structures 220, 230 may each have a pair of mutually parallel beam position-sensitive detectors configured to detect a position at which the beam 215 passes through the detector. The controller 260 can determine a vector defining the path of travel of the beam into the subject. Similarly, the controller 260 can determine a vector defining the path of travel of the beam out of the subject.

The energy determining apparatus 250 is configured to measure the amount of energy contained in the beam entering the device 260. The controller 260 can determine an amount of proton energy absorbed by the subject (i.e. the dose) at a given location within the subject in a known manner. The energy determining apparatus 250 can determine energy for multiple protons within a measurement interval.

As described above with reference to FIGURE 23, the beam tracker 230 on the output side may be simplified, using the energy determining apparatus 250 to determine one position coordinate of the beam.

In the scanner 200 the subject 205 is rotated about an axis A through the subject. Proton intensity data is captured as a function of rotational position of the subject 205 about the A- axis, and the controller 260 can acquire a 3D dataset of the fraction of proton energy absorbed at a given 3D location within the subject 205. For imaging purposes, a large area has to be irradiated. The area scanned by current PBT systems is typically up to 40 cm square. The scanner 200 can be a 'broad beam' scanner in which the beam 215 is arranged to irradiate substantially continuously the area being imaged. Alternatively, with reference to FIGURE 18, the scanner 200 may operate in a scanned mode in which a narrow pencil like beam is scanned across the area being imaged. There are two possible ways of operating in the scanned mode. A first option is to perform a raster scan across a full scan area while the subject remains stationary. A second option is to perform a line scan and move the beam with respect to the subject between each line scan.

Since different tissues exhibit different absorption characteristics, the internal structure of the subject's anatomy can be determined from the 2D images (radiographs) and 3D datasets built up from the 2D images captured as a function of rotational position of the subject 205 about axis A.

It is to be understood that, knowing the incident energy of a proton, and tracking it through the apparatus so as to determine its residual energy following passage through the tissue, allows an absorbed dose of proton radiation to be calculated. Additionally, tracking the paths of individual protons over a range of incident angles allows the reconstruction of the three-dimensional volumetric CT image as described in further detail below. Measuring the energy of each proton and tracking the path of the proton so as to calculate where in the subject the proton lost its energy is important in some embodiments. This is because charged hadrons such as protons are typically relatively strongly scattered by the subject compared with X-rays. In contrast, in the case of X-ray CT scanner systems it is not necessary to measure the exit energy of each X-ray in order to generate a CT image of a subject.

The scanner 200 may operate in: a first mode in which a beam of particles is delivered through the subject to the energy determining apparatus 250 to acquire data about the subject, e.g. for treatment planning; and in a second mode in which a beam of particles is delivered to the subject for treatment.

PROTON PROBE

Another possible application of the pixelated calorimeter described above is in a proton probe. Background articles on proton probes are provided in the articles: “Proton range verification using a range probe: definition of concept and all initial analysis”, Mumot M et al., 2010, Phys. Med. Biol. 55, 4771-4782; and “Subject positioning verification for proton therapy using proton radiography”, Hammi A et al., Physics in Medicine & Biology, Dec 10 2018, 63(24):245009. A proton probe is a simpler form of instrumentation compared to a CT scanner. FIGURE 28 shows an example of a proton probe. Narrow proton beams 34, at diagnostic current levels, are directed, in pre-selected directions, through the subject 35 at an energy such that most protons exit the patient and enter a RERD/calorimeter 36 which records the residual energies of the protons. By comparing the measured residual depth dose with one simulated previously (e.g. based on Monte Carlo methods) on the planning CT of the patient, such a probe can provide useful information on in-vivo range. Proton probes have also been suggested as an aid to patient positioning within the treatment room. To date, experimental demonstrations have been limited to single-point detection. That is, recording the full cross-section of the proton beam. This has limited application to fairly homogeneous paths through the patient’s anatomy. A pixelated calorimeter as the RERD would permit finer and more detailed analysis of the path and provide improved guidance for treatment at the commencement of each session (radiotherapy fraction). In FIGURE 26 the proton beam 37 is directed through a particularly complex part of human anatomy, namely laterally through the front part of the head crossing the nasal cavities. FIGURE 27 contrasts the capabilities of a proton probe with a conventional calorimeter (output 38) and a proton probe with a pixelated calorimeter (output 39). A proton probe with a conventional calorimeter (output 38) can only record the mean proton stopping power along the path and averaged across the diameter of the calorimeter. A pixelated calorimeter of the type described above can provide a more detailed picture of the differing stopping power for smaller regions and can permit, in conjunction with the planning CT, a method to recover the relative stopping powers for the different tissue types. The pixellated image 39 is the result obtained after many (e.g. thousands of) accumulated protons.

FIGURE 28(a) shows an example of a 2D image through a subject which has been obtained by X-ray CT imaging. The image is segmented, i.e. decomposed, into different regions which represent different tissue types. The segmentation could either be performed manually or by automatic computer methods. This x-ray CT would normally be the planning CT. Along a selected trajectory 80, the percentages of tissue types can be identified. Prior to a treatment session, a proton probe 81 can be used to confirm subject position as previously proposed and in addition the same trajectory 82 as selected from the x-ray CT is selected and the overall Water Equivalent Thickness (WET) obtained from the planning CT. This situation is shown in FIGURE 28(b). Along the common trajectory, AB, the following relationship holds:

where: is the thickness of tissue type I encountered along the trajectory; and is the unknown Relative Stopping Power (RSP) for protons of the same tissue type.

By selecting further paths, a set of linear equations is obtained, that can be solved to find the values of the different RSPs. These experimental obtained RSP values can be compared with the estimates used in the treatment planning process and the treatment plan either confirmed or modified. If the tissue types are heterogeneous than the above relationship can be modified by including fractional percentages of tissue composition.

The probe is essentially the instrumental arrangement shown in FIGURE 23. The use of narrow, well-collimated proton beams can mean that there is no requirement for a proximal tracker. The whole probe would consist of one calorimeter module, 4 cm - 8 cm square and a crossed set of position sensitive detectors. The proton beam would typically be 1 to 4 cm in diameter. As the pixelated calorimeter and associated position sensitive detector can identify individual protons and their paths, a much finer resolution of RSPs is possible compared to earlier approaches.

The spatial capability of the calorimeter as well as identifying the individual energies of several protons at one time will improve the present art. Unlike a single point detector, the calorimeter can estimate the paths and energies of individual protons. This presents the opportunity to use a proton probe to confirm or modify treatment planning at the start of each treatment session.

It is to be understood that, in radiation therapy, it is important to ensure the safety of the patient and the integrity of the treatment. Quality assurance procedures are typically undertaken on a regular basis such as on a daily, weekly and monthly basis in order to ensure the safe and accurate operation of the treatment system. This is often achieved by the inclusion of a calibrated phantom in place of a patient. In addition, prior to commencing an individual patient’s treatment, the measurement of every patient field (beam direction) may be undertaken before the delivery of the first fraction, in a process that is termed ‘patient verification’. It is to be understood that these activities are typically time-consuming.

The use of a proton probe would benefit both activities by providing detailed information concerning proton energy and spot shape. For patient verification (FIGURE 29), the patient 41 is not present but treatment beams 40 are provided according to the planned treatment plan and characterised in turn in order to confirm that they are in agreement with the treatment plan. For the purpose of the patient verification process, in some methodologies according to the present invention the beam intensity, but not its other properties, can be reduced by changes in the beam delivery system. Alternatively, a uniform and calibrated moderator 42 may be provided at the expected location of the patient’s head during treatment in order to prevent saturating the proton probes 36. It is to be understood that by moderator is meant an element that has a known absorption characteristic for the hadron beams being used. It is to be understood that the moderator 42 employed will normally be of comparable size to the patient’s anatomy of interest, or at least sufficiently large and having a sufficiently high proton absorption characteristic to prevent saturation of the proton probes 36 during the patient verification process.

One element of the quality assurance procedures in some arrangements is the use of calibrated phantoms. Test beams 43 are directed at such a phantom 44, which is provided at the expected location of the patient’s head, as illustrated in FIGURE 30. These procedures allow the proton probe to be calibrated and hence act as a secondary standard. It is to be understood that a phantom is an element having one or more known protonstopping characteristics intended to approximate the corresponding characteristics of a patient’s anatomy.

FIGURE 31 shows a method of processing data from the array of detectors. This method can be performed, for example, by the processor 115 shown in FIGURE 6 or the processor 260 shown in FIGURE 25. At block 251 light intensity values are acquired from the array of detectors. At block 252 the method determines a group of pixels corresponding to an individual particle. In some cases, the contributions of individual particles will overlap. FIGURE 32 shows a process for separating groups of pixels. At block 253, the method determines total energy of each group of pixels per particle. At block 254 the method determines a start point of a group (e.g. track) representing a path of a particle. Block 254 can use intensity values to determine the end point (nearest the Bragg peak) and therefore the start point. The start point represents the point of incidence on the energy-determining apparatus. This can be used to determine an output vector of the particle.

FIGURE 32 shows another part of the method of processing data from the array of detectors. Block 255 determines if there are overlapping groups of intensity values, i.e. contributions of more than one particle which overlap to some extent. If there are no overlapping groups (e.g. just a single group, or separate groups) the method can proceed directly to block 253. At block 256 the method determines end points of tracks. This process was described earlier with reference to FIGURES 12 and 13. At block 257 the method determines the most likely separation of the groups/tracks, such as by using chain codes for each track between a start point and an end point. Once groups of contributions have been separated, the method proceeds to block 253 to determine the total energy of each group.

FIGURE 33 shows an example of a detector (44, FIGURE 6) and read-out circuitry 45 which can be used in any of the examples of the energy-determining apparatus 100 described in this specification. The apparatus shown in FIGURE 33 can be replicated for each detector and detector read out path. In this example the detector 261 is a multi-anode PMT. The detector 261 provides an output (in the form of a charge pulse) in response to incident light from a scintillator element. The PMT is biased by a high voltage supply 262. The charge pulse is capacitively coupled to a pre-amplifier 263, which can be a charge or transconductance amplifier. An output of the pre-amplifier 263 is provided to a pulse shaper 264. The pulse shaper 264 is configured to shape the pulse, e.g. by increasing the fall time of the pulse. The pulse shaper 264 can be an active filter. A comparator 265 receives the filtered pulse signal at a first input 265A. The comparator 265 receives a threshold signal value at a second input 265B. The threshold signal value is supplied by a programmable digital-to-analogue convertor (DAC) 266. The comparator outputs a control signal 267 to turn a counter 268 on and off. When the pulse value received at comparator input 265A is above the threshold signal value, the control signal 267 turns the counter 268 on. When the pulse value received at comparator input 265A is below the threshold signal value, the control signal 267 turns the counter 268 off. The counter 268 receives a fast clock input, i.e. a counter running at a rate which provides a desired measurement resolution. The counter accumulates a count total value 269 during a period between being turned on and off. This count total value 269 is indicative of intensity of the light received by the detector 261.

FIGURE 34 shows the general form of the charge pulse 271. Threshold value 272 is determined by the DAC 266. FIGURE 34 also shows the output 267 of the comparator 265 and operation of the counter 268. When the pulse value 271 is above the threshold level 272 the comparator output 267 is high. The comparator output 267 controls (i.e. gates) a counter 268. The clock increments at a rate with a time period t. Therefore, the number of counts equates to a time over threshold of A/ x t, where N is the number of counts. The amplitude of the charge pulse is proportional to the time over threshold measurement, so the recorded count is related to the proton energy lost in each scintillator element. It will be understood this is one possible way of implementing the detector and read-out circuitry. Other possible signal processing methods could be used. For example, the peak height of the charge pulse 271 can be measured. A constant-fraction discriminator can record the peak height of a pulse and this can be converted to a digital value by a sample-and-hold circuit and a fast analogue-to-digital converter (ADC). The purpose of the read-out circuitry is to convert the output of the detector into a form (e.g. a digital value representing an quantity of light received at the detector) which can be manipulated by the subsequent processing stage.

The energy-determining apparatus 100 determines a value for each element of the array of scintillator elements and detectors. FIGURE 35 shows a data array 280 of values stored in a memory of the energy-determining apparatus 100. Each element q(m,n) represents an amount of energy lost by a proton (or by multiple protons) in a scintillating element. As described above, there may be one detector (and therefore one value in the data array 280) per scintillator element, or the ratio might differ, with one data array element q(m,n) corresponding to several scintillator elements, or multiple data array elements q(m,n) corresponding to one scintillator element. As described above, elements of the data array which are determined to correspond to a single particle are summed together to give a total energy for that particle. It will be understood that the detectors measure light intensity. There is a relationship between the quantity of light detected by the detectors and energy of incident particles. The total energy value for a particle is obtained by converting the total light to energy through a calibration procedure, that is particles of known energy are incident on the instrument and hence a conversation factor between electrical signal produced and the particle energy deposited can be determined.

FIGURE 36 shows an example of a processing apparatus 300 which may implement at least part of the processing of the invention, such as the method of FIGURES 31 and 32. The processing apparatus 300 can be the processor 115 shown in FIGURE 6 or the processor 260 shown in FIGURE 25. Processing apparatus 300 comprises one or more processors 301 which may be any type of processor for executing instructions to control the operation of the device. The processor 301 is connected to other components of the device via one or more buses 306. Processor-executable instructions 303 may be provided using any data storage device or computer-readable media, such as memory 302. The processorexecutable instructions 303 comprise instructions for implementing the functionality of the described methods. The memory 302 is of any suitable type such as non-volatile memory, a magnetic or optical storage device. The processing apparatus 300 comprises input/output (I/O) interfaces 307. The I/O interfaces 307 can receive signals from other apparatus, such as electrical signals from the read-out circuitry 45 in FIGURE 6 and inputs from the positionsensitive detectors in FIGURE 25. The I/O interfaces 307 can output signals to other apparatus. The processing apparatus 200 connects to a user interface 208. Memory 302, or a separate memory, stores data used by the processor. This can include one or more of: light values 311 read from the array of detectors; data identifying determined clusters/tracks 312 representing individual particles; data 313 identifying start points and end points of tracks; total energy values 314 for individual particles.

In summary, the invention described, namely a pixelated calorimeter, provides an advance on existing calorimeters and range telescopes in proton CT instrumentation for the optimization of proton beam therapy in the treatment of cancer by allowing the accurate recording of the residual energy of protons. The ability to record the energies of multiple protons within one cyclotron clock period allows faster acquisition of data and the ability to operate within the normal range of clinical operating parameters. It also permits, through the use of a fine-grained calorimeter to simplify the complexity and cost of the overall system. Furthermore, the proposed invention can extend the capabilities of the emergent concept of proton probing by allowing much more detailed examination of individualised tissue-related RSPs at the time of treatment.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

28 CLAIMS:

1. An energy determining apparatus for determining the individual energy of one or more particles of hadron radiation, the apparatus having a main axis for aligning with a general direction of incident particles, the apparatus comprising: a two-dimensional array of scintillator elements configured to emit a quantity of light in response to energy of one or more incident particles of hadron radiation passing through or along that scintillator element, wherein each of the scintillator elements is an elongate element with a longitudinal axis extending parallel to the main axis; an array of detectors to convert the light from the scintillator elements into electrical signals; a processor configured to determine a total quantity of light corresponding to energy of a single particle based on outputs of the array of detectors.

2. An energy determining apparatus according to claim 1 wherein the processor is configured to determine a group of detector outputs corresponding to a single particle and to determine a total quantity of light corresponding to the single particle from the determined group of detector outputs when there is a plurality of substantially simultaneously incident particles.

3. An energy determining apparatus according to claim 1 or 2 wherein the processor is configured to determine an individual group of detector outputs corresponding to a single particle when there is a plurality of overlapping groups of detector outputs.

4. An energy determining apparatus according to any one of the preceding claims wherein the processor is configured to determine a start point of a group corresponding to a single particle from detector outputs and thereby to determine a position at which the single particle is first incident on the energy determining apparatus.

5. An energy determining apparatus according to claim 4 wherein the processor is configured to: determine an end point of a track from detector outputs, wherein a peak intensity value corresponds to a Bragg peak of a particle at the end of a group; determine the start point based on the end point of the group.

6. An energy determining apparatus according to any one of the preceding claims wherein each of the scintillator elements has a response time shorter than a repetition rate of incident particles.

7. An energy determining apparatus according to any one of the preceding claims wherein each of the scintillator elements has a length greater than an absorption depth of incident particles at maximum energy.

8. An energy determining apparatus according to any one of the preceding claims wherein each of the scintillator elements has a core of a first material of a first refractive index and a cladding of a material of a second refractive index which is higher than the first refractive index.

9. An energy determining apparatus according to any one of the preceding claims wherein a downstream end of the array of scintillator elements is configured to directly output light to the array of detectors.

10. An energy determining apparatus according to any one of claims 1 to 8 wherein there is at least one intermediate stage between a downstream end of the array of scintillator elements and the array of detectors.

11. An energy determining apparatus according to claim 10 wherein the at least one intermediate stage comprises a gain stage.

12. An energy determining apparatus according to claim 10 or 11 wherein the at least one intermediate stage comprises an optical reduction stage with an input side having an input area and an output side having an output area, where the output area is less than the input area.

13. An energy determining apparatus according to any one of the preceding claims wherein a number of detectors in the array of detectors is equal to a number of the scintillator elements in the two-dimensional array.

14. An energy determining apparatus according to any one of claims 1 to 12 wherein a number of detectors in the array of detectors is less than a number of the scintillator elements in the two-dimensional array.

15. A computed tomography scanner apparatus comprising: a first beam tracker for positioning on an input side of a subject; a second beam tracker for positioning on an output side of a subject; an energy determining apparatus according to any one of the preceding claims, wherein the first beam tracker and the second beam tracker are configured to determine the position of hadron particles passing through the beam trackers and the energy determining apparatus is configured to determine individual residual energy of one or more particles after passing through the subject and the second beam tracker.

16. A computed tomography scanner apparatus according to claim 15 wherein the second beam tracker comprises a position sensitive detector configured to determine a position of a particle in a two-dimensional plane and the energy determining apparatus is configured to determine a position of a particle in a two-dimensional plane, and a processor is configured to determine an output vector of a particle from the position determined by the second beam tracker and the position determined by the energy determining apparatus.

17. A proton probe comprising: a beam tracker for positioning on an output side of a subject; an energy determining apparatus according to any one of claims 1 to 14, wherein the beam tracker is configured to determine the position of hadron particles passing through the beam tracker and the energy determining apparatus is configured to determine individual residual energy of one or more particles after passing through the subject and the beam tracker.

18. A proton probe according to claim 17 wherein the beam tracker comprises a position sensitive detector configured to determine a position of a particle in a two-dimensional plane and the energy determining apparatus is configured to determine a position of a particle in a two-dimensional plane, and a processor is configured to determine an output vector of a particle from the position determined by the second beam tracker and the position determined by the energy determining apparatus.

19. A method of determining the individual energy of one or more particles of hadron radiation, the apparatus having a main axis for aligning with a general direction of incident particles, the method comprising: providing a two-dimensional array of scintillator elements configured to emit a quantity of light in response to energy of one or more incident particles of hadron radiation passing through or along that scintillator element, wherein each of the scintillator elements is an elongate element with a longitudinal axis extending parallel to the main axis and an array of detectors to convert the light from the scintillator elements into electrical signals; determining a total quantity of light corresponding to energy of a single particle based on outputs of the array of detectors.

20. A method according to claim 19 comprising determining a group of detector outputs corresponding to a single particle and determining a total quantity of light corresponding to the single particle from the determined group of detector outputs when there is a plurality of substantially simultaneously incident particles.

21. A method according to claim 19 or 20 comprising determining individual groups of detector outputs corresponding to a single particle when there is a plurality of overlapping groups of detector outputs.