WO1998038530A1

WO1998038530A1 - Measuring the energy output of x-ray sources

Info

Publication number: WO1998038530A1
Application number: PCT/GB1998/000465
Authority: WO
Inventors: David Didsbury; Keith Spanswick
Original assignee: Gammex-Rmi Limited
Priority date: 1997-02-28
Filing date: 1998-02-27
Publication date: 1998-09-03
Also published as: GB9704260D0; DE19882151T1; AU6299398A

Abstract

A detector head (10) for measuring the energy of x-rays emitted from a source. The head (10) comprises a linear array of photodiode detector devices (23) each positioned beneath a respective x-ray attenuation window (16). Preferably seven pairs of detector devices (23) are provided symmetrically arranged about an axis, each pair for detecting x-rays through an attenuating filter (16) of different thickness to the other pair.

Description

MEASURING THE ENERGY OUTPUT OF X-RAY SOURCES

The present invention relates to devices used for the measurement of x-radiation, and in particular to devices used for measuring the energy output of x-ray sources.

X-ray sources are widely used, for example in the field of medical physics, in particular for delivering controlled radiation doses to patients. It is important, particularly in such medical applications, for the maximum energy level of x-rays emitted by a source to be accurately calibrated since the energy level of the x-rays determines the depth of penetration of the x-ray photons.

The x-ray source output is generally measured in terms of the maximum potential reached during energization of the x-ray tube, ie. the kN peak (kVp) which corresponds to the highest energy x-ray photons emitted.

Rather than attempt to directly measure the very high voltages reached across the tube (eg. between 20 and 150 kV), existing kVp measurement devices deduce the voltage from the measured energy of the x-rays emitted. To measure the x-ray energy, such devices include two sets of x-ray sensitive detectors (eg. photodiodes) which are arranged, on a common plane, so that the "centre of detector area" of each of the detector sets are coincident, eg. each detector set has at least two axes of symmetry in the plane which pass through a centre point common to both sets. The first detector set is located behind an absorber material of a first thickness and the second detector set is located behind an absorber material of a second, different thickness. The ratio of radiation intensities received by the first and second sets as a result of different attenuation factors can then be used to deduce the kVp of the x-radiation. This is typically achieved by providing a memory-based look-up table or tables against which the two outputs are compared. The arranging of the detectors on a common plane is used to provide some independence of the orientation of the detector with respect to the cathode-anode axis of the x- ray tube.

There are a number of disadvantages with existing measurement systems of this type. The sensitivity of the detector photodiodes used is such that quite large areas of photodiode structures are required, and the output therefrom is integrated over a relatively long period of time. As a consequence of the relatively large area of photodiode, spatial resolution is compromised and thus a true peak x-ray output may be missed or the x-ray beam may fall only on a relatively small part of the detector surface area. As a consequence of relatively long integration period, temporal resolution is compromised and thus a transient peak x-ray output may be missed.

In addition, the deduction of true kVp output from the ratio of the two detector sets' outputs can be subject to large errors unless allowance is made for a number of other factors affecting the ratio, such as different energy ranges of output or different x-ray source types.

Many detectors, in fact, are only compatible with a single x-ray machine type or energy range, and their use with any other machine will produce erroneous readings. Other detectors, although not machine specific, deploy a number of different look-up tables, one for each machine type or energy range. Different look-up tables may be provided single phase x-ray machines and three phase machines. External settings on the detector must be made by the operator in order that the detector can determine which look-up table to use. Such manual settings are prone to cause errors by inadvertence or misunderstanding.

It is an object of the present invention to provide an x-ray measurement device which offers an improved spatial resolution.

It is a further object of the present invention to provide an x-ray measurement device which offers improved temporal resolution.

It is a further object of the present invention to provide an x-ray measurement device which offers improved accuracy in the measurement of kVp output.

It is a further object of the present invention to provide an x-ray measurement device which offers a reduced risk of error in operation during measurement of different types of x-ray sources.

In accordance with one aspect, the present invention provides an array of detector devices each positioned beneath a respective x-ray attenuation window, the array including at least a first pair of detector devices for detecting x-rays through respective first windows having a first level of attenuation, a second pair of detector devices for detecting x-rays through respective second windows having a second level of attenuation, and a third pair of detector devices for detecting x-rays through respective third windows having a third level of attenuation. Preferably the array is a linear array, of rectangular detector devices and respective windows separated by x-ray absorbing septa.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of an x-ray attenuator structure for positioning over a photodiode array;

Figure 2 shows a plan view of the attenuator structure of figure 1 ;

Figure 3 shows a side view of the attenuator structure of figure 1 ;

Figure 4 shows a schematic side view of the attenuator structure of figure 1 in position over a photodiode detector array;

Figures 5a and 5b show a schematic diagram of a signal processing circuit for use with the photodiode detector array of figure 4; and

Figure 6 shows a number of different configurations of attenuator structure as alternatives to that shown in figure 4.

With reference to figures 1 to 3 there is shown a detector head 10 comprising a linear array of rectangular windows 12, to 12,₄ which are framed on their long sides by thin septa 13 of absorbing material. In the preferred embodiment, the septa are formed from lead. As best seen in figures 3 and 4, each window 12_N contains a filter 16, to 16,₄ of different thickness of attenuating material such as copper or aluminium or a combination of both. In the preferred embodiment shown, the central two windows 12₇ and 12₈ correspond to filters of zero thickness, and therefore require no physical filter. In addition, since the central two window positions are of corresponding attenuation and adjacent to one another, no separating septum is required between windows 12₇ and 12₈, and thus they may in practice be fabricated as a single enlarged window which will be referred to herein as the reference window.

The remaining windows are arranged in corresponding pairs according to their distance from the reference window. In the preferred embodiment, window pair 12₆ and 12₉ each contain a filter of a first thickness of attenuating medium, window pair 12₅ and 12₁₀ each contain a filter of a second thickness of attenuating medium, window pair 12₄ and 12,, each contain a filter of a third thickness of attenuating medium, and so on to window pair 12, and 12,₄ which each contain a filter of a sixth thickness of attenuating medium.

As shown in figures 1 to 3, the filters 16_N may be formed from strips of copper with the central portion milled down to a required thickness and the edge portions 17 left upstanding to provide a separation support for the septa 13. The central, reference window 12_7>8 may be simply provided with block 18 to separate the adjacent septa as shown in figure 1.

For the presently preferred embodiment, the thicknesses of attenuating medium for each filter 16_N are given in Table 1 below. The attenuating medium is preferably copper, and for the thinnest filters, includes an additional layer of aluminium as shown. Other filter materials may be used. TABLE 1

The window filters 16_N and lead septa 13 are mounted onto support rails 14 and 15 of suitable material, preferably an x-ray absorbing material such as copper, like the filters. Between the support rails 14, 15 and beneath the window structure is defined a cavity 20 which is adapted to receive a detector device array 21 as shown schematically in figure 4.

The detector array consists of a linear array 21 of scintillation crystals 22, to 22,₄ bonded to a linear array of photodiodes 23, to 23 ,₄ with one scintillation crystal 22_N corresponding to one diode 23_N. Preferably, the scintillation crystals 22_N each comprise a single crystal of caesium iodide. Preferably, the photodiodes 23_N each comprise a silicon photodiode and may be fabricated on a single silicon substrate 24.

The scintillation crystals 22_N are surrounded by a reflective coating 30 except on the lower surface 25 which is directly bonded to the photodiode 23_N. Preferably, each machined single crystal caesium iodide scintillator is set accurately in a matrix of stabilised reflective epoxy resin

30 to provide a ladder-type structure which is bonded to the photodiode substrate to form the linear array 21 of detector devices. It has been determined that the use of Csl scintillator crystals and the reflective matrix provides sufficient improvement in sensitivity of the kVp detector devices that the greatly reduced surface area of detectors enable the construction of a practical array. It is also found that the outputs of the photodiodes

23_N do not need long integration times and can be sampled on an instantaneous basis providing real time measurement of kVp output.

The critical dimensions of two presently preferred arrangements of detector heads suitable for the detection of x-rays in the energy range 20 to 150 kVp are given below in Tables 2 and 3.

TABLE 2

Type 1

Overall Size: Length 23.9mm

Width 12.85mm

Active Area 23.9mm x 4.85mm

Pitch: 1.5mm (1.1mm copper and 0.4mm lead septa.)

Tolerances for copper filters 2.5mm filter +0.125mm 2.0mm filter ±0.100mm 1.0mm filter ±0.050mm 0.5mm filter ±0.025mm 0.25mm filter ±0.0125mm 0.15mm filter +0.010mm

Height of lead septas is 3.0mm.

Purity of copper: 99.9% oxygen free. Aluminium: standard industrial grade

TABLE 3

Type 2

Overall Size: Length 25.6mm

Width 14.0mm

Active Area 25.6mm x 4.86mm

Pitch: 1.5mm (1.1 mm copper and 0.4mm lead septa.)

Tolerances for copper filters 2.5mm filter +0.125mm 2.0mm filter ±0.100mm 1.0mm filter ±0.050mm 0.5mm filter ±0.025mm 0.25mm filter ±0.0125mm 0.15mm filter +0.0075mm

Height of lead septas: 3.0mm

Plurity of copper: 99/9% oxygen free. Aluminium: standard industrial grade The detector array 21 is mounted inside the cavity 20 between the support rails 14, 15.

In use, x-ray photons enter the top of the detector head structure and are either absorbed by the filters 16_N, the lead septa 13, or providing they have sufficient energy to pass through the respective filter, will enter a respective Csl scintillator crystal 22_N. Light is then generated in the crystal which is reflected by the resin 30 toward the respective photodiode 23_N. As a result, the photodiodes each generate current which is fed to a suitable analysis circuit as exemplified in figures 5a and 5b.

With reference to figures 5a and 5b, each corresponding pair of photodiodes 23_N are connected in parallel to the input of a respective preamplifier stage 40, to 40₇.

The geometrical configuration of the detector head has been optimised to provide a high spatial resolution at the same time as offering a low capacitance and high sensitivity providing high temporal resolution. The positioning of detector elements in opposite pairs provides good compensation for non-orthogonal presentation of the detector head to the x-ray source. A zero biased mode of operation provides excellent linearity and very low noise due to the almost complete elimination of leakage current. The first pre-amplifier 40, is connected to photodiodes 23₇ and 23_g which correspond to the reference window 12₇, 12₈. It will be recalled that this window has no filter above the scintillator and therefore no attenuation. The gain of the amplifier is set to a half, to match the attenuation of the first channel (pre-amplifier 40₂ connected to the photodiodes corresponding to windows 12₆ and 12₉) where the filter attenuation is approximately 50% . The outputs of photodiode pre-amplifiers 40_N are fed to respective attenuators 41 _N, 48_N to allow channel characterization with coarse and fine control respectively, preferably using computer controlled digital potentiometers 48_N. The channel characterization typically takes place on a one-off basis before installation of the filters 16_N during set up of the detector array, in order to compensate for any variability in the individual detector channels.

The attenuated output signal is buffered by buffer IC's 42, to 42₇ before being fed to respective precision unity gain differential amplifiers 43, to 43₆. It will be noted that the reference channel output from buffer amplifier 42, is subtracted from each of the other channels by each of the differential amplifiers 43, to 43₆ to reduce the dynamic range of the signal output.

The outputs of the respective differential amplifiers 43_N, together with the attenuated reference channel output, are passed, via ESD protection device to respective output buffers 45, to 45₇. The outputs of the output buffers 45_N are then passed, via ESD protection device 46 to an output connector 47, from where they are passed to suitable processing circuitry for sampling and comparison. The values obtained may then be located in a look-up table in order to deduce a suitable kVp value, which can be displayed on a suitable display device or provided to a further analysis system.

The provision of seven channels of attenuated / unattenuated data from a single, compact detector head enables far greater versatility in establishing kVp output of an x-ray source.

In one arrangement, the outputs may be compared in pairs, with the channels corresponding to windows 12_{6 9} and 12_{5 10} being used primarily when a low energy source is detected, the channels corresponding to windows 12₄ ,, and 12₃ ,₂ being used primarily when a medium energy source is detected, and the channel corresponding to windows 12^^ and 12_{1 14} being used primarily when a high energy source is detected.

In a preferred embodiment, the low energy channels would correspond to source energies between 20 and 38 kNp, the medium energy channels would correspond to source energies between 38 and 100 kVp, and the high energy channels would correspond to source energies between 100 and 150 kVp. Energy range selection is carried out automatically dependent upon the output levels determined from each channel.

In another embodiment, the simultaneous comparison of, for example, up to seven outputs of energy spectrum information (eg. six attenuated channels and one reference channel) can provide a far greater degree of precision in deducing the kVp of an x-ray source. Still further, it is found that the multiple outputs provide sufficient information on the full energy spectrum of the x-ray source being measured that it is possible automatically to determine the type or energy range of the particular x-ray source being measured and thereby automatically determine an appropriate look-up table without requiring manual adjustment. Look-up table values are determined empirically with reference to differing source types and energy ranges.

In addition, the simultaneous comparison of the six attenuated channels and reference channels enables the determination of the HNL (half value level) of intensity of the x-ray machine. This is normally routinely determined manually as part of a calibration exercise using completely different equipment to that which is used to determine kNp. The present invention enables the two different measurements to be determined using the same equipment.

The present detector head therefore offers a number of substantial advantages over prior art systems in greater functionality and enhanced performance. kVp output can be determined on an instantaneous basis, limited only be rise times of the sampling circuits of the order of 10 ns. Thus, the output is essentially real time. The determination of kVp values has been shown to be achieved to a resolution of at least 0.1 kVp. The autoranging facility permitted by the detector head greatly reduces risk of error and increases versatility.

Although the preferred configuration of filters 16_N is as shown in figure 4, it is possible to reconfigure the filters in different sequences such as illustrated in figure 7. As shown in some examples, the reference windows need not be situated in the centre positions, but could, for example, be situated at the ends of the array. Not all positions of a detector array need be used.

Claims

1. An x-ray detector head comprising an array of detector devices each positioned beneath a respective x-ray attenuation window, the array including at least a first pair of detector devices for detecting x-rays through respective first windows having a first level of attenuation, a second pair of detector devices for detecting x-rays through respective second windows having a second level of attenuation, and a third pair of detector devices for detecting x-rays through respective third windows having a third level of attenuation.

2. An x-ray detector head according to claim 1 in which the array of detector devices are provided as a linear array.

3. An x-ray detector head according to claim 2 in which said first windows have a zero level of attenuation.

4. An x-ray detector head according to claim 2 or claim 3 in which said second windows each comprise an elongate bar having a layer of aluminium and a layer of copper.

5. An x-ray detector head according to claim 2 or claim 3 in which said third windows each comprise an elongate bar of copper.

6. An x-ray detector head according to any one of claims 2 to 5 in which each window is separated from an adjacent window by a thin septa adapted to reduce cross-talk between adjacent detector devices.

7. An x-ray detector head according to claim 6 in which the windows are approximately 13 mm wide, and the septa thicknesses are between 0.2 and 2.0 mm.

8. An x-ray detector head according to any preceding claim including at least seven pairs of detector devices having respective window thicknesses ranging from zero to 2.5 mm thicknesses of copper.

9. An x-ray detector head according to claim 8 in which at least two detector devices include an additional thickness of between 0.15 and 2.5 mm of aluminium.

10. An x-ray detector head according to any one of claims 2 to 9 in which each detector device comprises a photodiode separated from the respective window by a scintillation crystal.

11. An x-ray detector head according to claim 10 in which the scintillation crystals are each surrounded by a reflective layer on all sides except the side adjacent to the respective photodiode.

12. An x-ray detector head according to claim 10 or claim 11 in which the scintillation crystals are formed as a ladder array of crystals set in a matrix of stabilised reflective epoxy resin.

13. An x-ray detector head according to claim 10, claim 11 or claim 12 in which the scintillation crystals are formed from single crystal caesium iodide.

14. An x-ray detector head according to any one of claims 10 to 13 in which the photodiodes comprise a linear array of silicon photodiodes on a common substrate.

15. An x-ray kNp meter comprising a detector head according to any preceding claim, and including signal processing circuitry comprising a number of channels corresponding with and respectively connected to the number of detector device pairs, and further including look-up table means for determining, from the outputs of selected ones of the channels, a kNp reading for an x-ray machine being monitored.

16. An x-ray kVp meter according to claim 15 further including selection means for determining, from said channel outputs, which of said channel outputs should be selected for determination of said kNp reading.

17. An x-ray detector substantially as described herein with reference to the accompanying drawings.