Control means for heat load in x-ray scanning apparatus

IS 2 333 331 T3

DESCRIPTION

Means of controlling the thermal load in an X-ray scanning apparatus.

The present invention relates to X-ray scanning in which X-rays are directed through an object from various positions around the object and the X-rays transmitted through the object are detected and used to develop an image of the object. This type of scan is called a computed tomography (CT) scan.

One method of CT scanning involves rotating an X-ray source around the object so that it directs X-rays through the object in different directions. Another method, for example, as disclosed in US4274005, involves placing several X-ray sources around the object and then driving the sources in turn so that the active source position scans around the object.

As the use of X-ray scanners, for example, in security applications, increases, there is a growing demand for scanners that work quickly and have a long service life.

Document FR 2 328 280 discloses an X-ray scanner having a plurality of electron sources arranged to generate X-rays from a plurality of positions around a body.

US 4 057 725 discloses an X-ray scanner that includes a multiplicity of radiators, and a multiplicity of separate detectors around an object that can be turned on in succession.

Accordingly, the present invention provides an X-ray imaging apparatus comprising a plurality of X-ray tubes, each tube providing a plurality of source positions therein, the source positions being spaced around a location of object and separated from each other by source separation, a plurality of X-ray sensors arranged to be spaced around the object location to detect X-rays emitted from the source positions passing through the object location, characterized by control means arranged to control the order in which the source positions are active in successive emission periods throughout a scan cycle, so that the average over the scan cycle of the smallest offset between an active source position in one emission period and an active source position in the subsequent period is greater than the source separation, and so that in each emission period an active source position is in a different tube from an active source position in the previous emission period.

This increase in the average spacing between successively active source positions helps to spread the thermal load at the X-ray source.

Preferably said smallest average offset is at least twice the source spacing. This can be achieved in the simplest manner by ensuring that the control means is arranged so that no active source position in any emission period is adjacent to an active source position in the next emission period.

The control means may be arranged so that in each emission period only one source position is active.

Alternatively, the control means may be arranged so that in each broadcast period a plurality of source positions are simultaneously active. This can reduce scan time and increase scan speed.

When the source positions are each arranged to produce X-rays to be detected by a corresponding group of sensors, the control means is preferably arranged so that in each emission period, there is no overlap between the groups of sensors for said plurality of font positions. This ensures that X-rays detected from each of the active sources simultaneously can be distinguished.

Preferably in each emission period at least half of the sensors are arranged to receive X-rays from the active source positions. More preferably at each emission period substantially all of the sensors are arranged to receive X-rays from the active source positions.

Preferably, the apparatus comprises a plurality of X-ray tubes, each providing a plurality of said source positions.

In this case the control means are preferably arranged so that in each emission period the active source position is in a different tube from the active source position in the previous emission period.

Conveniently only one source position is active in each emission period and the active source positions are provided in each of the tubes in turn.

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Preferably, within each tube, the order in which the source positions are active is arranged so that in each emission period no active source position is adjacent to an active source position in the previous emission period.

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

Figure 1 shows an X-ray emitter suitable for use with the invention;

Figure 2 is a diagram of an X-ray imaging system according to the invention including several emitting units as shown in Figure 1;

Figure 3 is an arrangement diagram of a first example of an X-ray imaging system according to the invention; and Figure 4 is an arrangement diagram of a second example of an X-ray imaging system according to the invention.

Referring to Figure 1, a multi-focus X-ray tube 10 comprises a ceramic former 12 and an emitter element 18 extending longitudinally between the sides 14,16 of the former. Several grid elements in the form of grid wires 20 are supported on the former 12 and extend over the gap between its two sides 14, 16 perpendicular to the emitter element 18, but in a plane that is parallel thereto. Several focusing elements in the form of focus wires 22 are supported in another plane on the opposite side of the grid wires from the emitter element. The focus wires 22 are parallel to the grid wires 20 and are spaced the same spacing from each other as the grid wires, with each focus wire 22 being aligned with a respective one of the grid wires 20.

The source 10 is enclosed in a housing 24 of an emitter unit 25 with the former 12 supported on the base 24a of the housing. The focus wires 22 are supported on two support rails 26a, 26b that extend parallel to the emitter element 18, and are spaced from the former 12, the support rails being mounted on the base 24a of the housing. Support rails 26a, 26b are electrically conductive so that all focus wires 22 are electrically connected to each other. One of the support rails 26a is connected to a connector 28 that projects through the base 24a of the housing to provide an electrical connection for the focus wires 22. Each of the grid wires 20 extends over one side 16 of the former and is connected to a respective electrical connector 30 that provides separate electrical connections for each of the grid wires 20.

An anode 32 is supported between the side walls 24b, 24c of the housing. The anode extends parallel to the emitter element 18. The grid and focus wires 20, 22 therefore extend between the emitter element 18 and the anode 32. An electrical connector 34 to the anode extends through the side wall 24b from the housing.

The emitter element 18 is supported at the ends of the former and is heated by means of an electrical current supplied thereto through additional connectors 36, 38 in the housing.

In order to produce an electron beam from one position, a pair of adjacent grid wires 20 can be connected to a draw potential that is positive with respect to element 18 while the remaining grid wires are connected to a blocking potential. which is negative with respect to element 18. By selecting which pair of wires 20 is used to extract electrons, the position of the electron beam can be chosen. Since the X-rays will be emitted from the anode 32 at a point where electrons collide with it, the position of the X-ray source can also be chosen by choosing the grid wire draw pair. The focusing elements 22 are all held at a positive potential with respect to the grid wires 20 so that electrons drawn between any pair of grid wires will also pass between, and will be focused by a corresponding pair of focusing elements 22.

Referring to Figure 2, an X-ray scanner 50 is configured with a conventional geometry and comprises an array of emitting units 25 arranged in an arc about a central X-scanner axis, and oriented to emit X-rays toward the axis of X scanner. A ring of sensors 52 is positioned within the emitters, directed inward toward the scanner axis. The sensors 52 and the emitting units 25 are offset relative to each other along the X-axis so that the X-rays emitted from the emitting units pass through the sensors closest to them, through the object, and are detected by several more sensors. away from them. The number of sensors 52 that will detect X-rays from each source depends on the width of the X-ray fan that is emitted from each source position in the tubes 25. The scanner is controlled by a control system that handles various functions represented by functional blocks in Figure 5. A system control block 54 controls and receives data from an image display unit 56, a ray tube control block. X 58 and an image reconstruction block 65. The X-ray tube control block 58 controls a focus control block 66 that controls the potentials of the focus wires 22 in each of the emitter units 25, a grating control block 67 that controls the potential of the individual grid wires 20 in each emitter unit 25, and a high voltage supply 68

ES 2 333 331 T3 that provides the power to the anode 32 of each of the emitting blocks and the power to the emitting elements

18. Image reconstruction block 65 controls and receives data from sensor control block 70 which in turn controls and receives data from sensors 52.

In operation, an object to be scanned is passed along the X-axis, and the X-ray beams are directed through the object from the X-ray tubes 25. In each scan cycle each source position in each tube 25 is used once, repeating the scan cycle as the object moves along the X axis. Each source position produces a fan of X-rays that after passing through the object are detected by various sensors 52. However, the order in which the tubes are used and the positions within the tubes is controlled as will be described below.

The order of X-ray emission from the source positions in the tubes 25 is chosen to minimize the thermal load on the X-ray tube. This is achieved by ordering the emissions so that each source position is non-adjacent, and thus both is separated from the previous and the subsequent one. This ordering applies both to the source positions within each tube 25, as well as to the tubes themselves. Therefore each source position is in a different tube than the one before and the next. In fact, the best heat load distribution is achieved if the source position cycles through all tubes, using one position from each tube, and then cycles through the tubes again using a different source position within each tube. The cycle is then repeated until all source positions in all tubes have been used once. This completes a scan cycle that can then be repeated.

Within each tube the source positions are taken in an order that propagates the heat load within the tube. This is achieved by ordering the source positions so that the distance between each source position and the next one in that tube, and the previous one in that tube are both maximized. First, therefore, if the number of source positions per tube allows, each source position in the tube must be non-adjacent to the next and previous one in that tube. Then, depending on the number of source positions, the ordering is chosen to distribute the heat load as much as possible.

For example, if as in a first example of an X-ray imaging system according to the invention shown in Figure 3, there are five X-ray tubes 60, 61, 62, 63, 64 numbered in the order in which are located 1, 2, 3, 4 and 5, and each can produce X-rays from 5 source positions 70, 71, 72, 73, also numbered in order along tube 60 as 1, 2, 3, 4 and 5, so the best ordering for the source positions within each tube is 1, 3, 5, 2, 4. The same sequence is also used to order the tubes to maximize the angular separation between successive emissions. This produces an emission order as follows, in which the source positions are numbered in order around object 75 starting from the left end of tube 60 at the left end of the row and counting towards the right end of tube 64 in the far right of the row.

<td>Tube</td><td>Font position in the tube</td><td>Font position global</td>

<td> 1</td><td> 1</td><td> 1</td>

<td> 3</td><td> 1</td><td> 11</td>

<td> 5</td><td> 1</td><td> 21</td>

<td> 2</td><td> 1</td><td> 6</td>

<td> 4</td><td> 1</td><td> 16</td>

<td> 1</td><td> 3</td><td> 3</td>

<td> 3</td><td> 3</td><td> 13</td>

<td> 5</td><td> 3</td><td> 23</td>

<td> 2</td><td> 3</td><td> 8</td>

<td> 4</td><td> 3</td><td> 18</td>

<td> 1</td><td> 5</td><td> 5</td>

<td> 3</td><td> 5</td><td> 15</td>

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<td> 5</td><td> 5</td><td> 25</td>

<td> 2</td><td> 5</td><td> 10</td>

<td> 4</td><td> 5</td><td> 20</td>

<td> 1</td><td> 2</td><td> 2</td>

<td> 3</td><td> 2</td><td> 12</td>

<td> 5</td><td> 2</td><td> 22</td>

<td> 2</td><td> 2</td><td> 7</td>

<td> 4</td><td> 2</td><td> 17</td>

<td> 1</td><td> 4</td><td> 4</td>

<td> 3</td><td> 4</td><td> 14</td>

<td> 5</td><td> 4</td><td> 24</td>

<td> 2</td><td> 4</td><td> 9</td>

<td> 4</td><td> 4</td><td> 19</td>

The same arrangement could be used with, for example, 25 source positions in a single tube that is shaped around object 75.

It should be appreciated that, for X-ray tubes with less than 5 source positions it is not possible to avoid the use of adjacent positions in subsequent emissions. However, for tubes with 5 or more source positions, this can be avoided.

Referring to Figure 4, in a second example of an X-ray imaging system according to the invention, a plurality of X-ray sources 80 are spaced about an X-axis, with a plurality of sensors 82 axially displaced from each other. fonts 80 as in the first embodiment. When one of the sources 80a emits an X-ray beam 84 it diverges, passes through the object 86 and reaches several of the sensors 82. The number of sensors 82 that will detect X-rays from each of the sources depends on the width of the X-ray beam which is a known quantity for any given system and can be quantified in terms of half an angle. This is the angle between the center of the beam and the edge of the beam.

When the sensors 82 that are needed to detect X-rays from each of the source positions 80 are known, source positions that can emit simultaneously can be selected, provided that no common detector is required. For example, if there are 24 source positions 80 and 24 sensors 82 and each source position requires 5 sensors, then four of the sensors 80a, 80b, 80c, 80d can be used simultaneously, spaced around the object at 90 ° intervals.

In practice the number of source and sensor positions is likely to be higher. To satisfy the Nyquist sampling theorem, it is necessary to match the number of source positions N._s with the number of sensors N_s width d required to cover the linear dimension of the object N_sd. This leads to the result

N = nNs / 2.

For example, an image in which N_s = 64 will require Ν_φ = 100 sampling points to satisfy the Nyquist sampling criterion.

It should be appreciated that the ordering of emission positions can be varied in many ways for any given number of emission positions and that the optimal ordering will also vary depending on the number of emission positions and the number of X-ray tubes.