GB2112260A - Patient-alignment devices for tomography systems - Google Patents

Abstract

The conventional single cylindrical lens of a laser alignment device is replaced with a plurality of smaller cylindrical lenses 58 which are placed in close side-by-side relationship so as to form an array which acts to spread the beam into a fan-shaped linear pattern for alignment use. The array provides essentially the same capabilities as a single lens but is not subject to the critical positioning requirements of such a conventional lens arrangement. <IMAGE>

Description

SPECIFICATION Patient-alignment devices for tomography systems This invention relates generally to patientalignment devices and, more particularly, to a device for properly positioning a patient on the table of a computerised tomography system.

Conventional computerised tomography systems include an upright gantry portion having a central opening formed therein and an adjacent, generally horizontally-disposed table on which the patient reclines for the longitudinal advancement into the central opening to facilitate the scanning process. Located within the gantry, in diametric opposition to a point at the centre of the central opening referred to as the iso-centre, are a radiation source and a detector. In order to effect a scan, at least one, and preferably both, of the source and detector are rotated about the patient when in the advanced position to thereby direct radiation from the source, through the patient and into the detector.As the rotation of the scan proceeds, a number of views are presented to the detector, and resultant signals are produced which are representative of the energy attenuation at each one of the views. The signals are then digitised and accumulated in a processor where certain algorithms are performed for the subsequent reconstruction of the image and display on a CRT monitor. The image is a cross-sectional view of the patient within the scan plane, i.e., generally perpendicular to the longitudinal axis of the central opening along which the patient is advanced. It is, of course, common practice to obtain oblique views through the patient by a tilting of the patient or of the gantry itself.

The thickness of the image slice is determined by the thickness (in the longitudinal direction) of the radiation fan beam and is on the order of 1 to 10 mm. Even though, during a normal diagnostic procedure, a number of adjacent scans are made, it is still very important to know the precise location of the patient with respect to the scan plane. This is important for two reasons. First, it is desirable to minimise the amount of radiation to which the patient is exposed, and, therefore, it is desirable to minimise the number of scans that are made. Second, and more important, is the desirability of scanning the precise region of interest within the patient's body. In order to accomplish this it is necessary to be able to precisely position the patient with respect to the scan plane.Similarly, in addition to this requirement for proper longitudinal or axial placement, it is also extremely desirable to maintain proper lateral and height placement of the patient with respect to the isocentre.

A common approach for obtaining patient alignment is to provide within the gantry a number of light sources which project in the direction of the isocentre such that a stationary light pattern appears on the patient and thereby provides an indication of whether or not the patient is in the proper position. The source of light used in this arrangement is often a laser beam which is diverged in a linear form by a cylindrical lens or the like.

A problem exists with the above approach in that the isocentre is well into the central opening such that, because of the restricted viewing angle, it is difficult to precisely apply the light patterns in order to obtain precise positioning. Accordingly, a known solution is to provide alignment lights in a more accessible position in advance of the isocentre such that an unrestricted view may be obtained. One such apparatus is shown and described in US Patent 4, 1 7,337, assigned to the assignee of the present invention. This arrangement is commonly referred to as an "external" alignment device as compared with the so-called "internal" alignment device which is in axial alignment with the isocentre.

Another type of external alignment light is that shown in US application serial no. 314,170, filed on 23 October 1981, and assigned to the assignee of the present invention. Here the external alignment lights are positioned within the gantry but still well in advance of the internal alignment lights in the scan plane. It should be mentioned that a preferred arrangement is to have both internal and external alignment lights for maximum flexibility in applying the scanning procedures.

In the prior art approach of placing a single cylindrical lens in the beam of a laser in such a manner as to spread the beam out into a fanshaped pattern, the relative positions of the beam and the lens are very critical. That is, not only does the axis of the lens have to be perpendicular to the axis of the laser beam, but the central axis of the laser beam has to be directed toward the central axis of the cylindrical lens in order that the beam diverges in the right direction. If the axis of the laser beam is offset from the central axis of the cylindrical lens, then the resulting projected line will be offset and therefore may not reach the patient. Of course, if the laser beam misses the cylindrical lens entirely, there will be no line projected, but rather a "bare", unspread laser beam which is projected out into the patient area.

This beam could possibly cause harm if it were intercepted by one of the patient's eyes.

One prior art approach which is used to protect patients from the possible projection of "bare" light is to place a cylindrical lens on either side of the primary lens so that if the laser beam is misdirected, it will be intercepted and refracted into a harmless line prior to reaching the patient.

But this approach will not solve the problem of the misdirected lines resulting from misalignment.

If the misalignment happens to be such that the central axis of the laser beam is directed precisely at the central axis of the adjacent cylindrical lens, then no change in the projected line will be observed. However, if the misalignment is such that the central axis of the beam falls anywhere between the central axes of the two cylindrical lenses, then the line will shift from its expected location and might miss the patient entirely.

The critical positioning requirements mentioned above must not only be attended to during the initial installation of an alignment system but must also be maintained during the life of the system. It sometimes becomes especially difficult to maintain alignment when certain optical components must be moved during normal maintenance and repair of the Xray system.

It is therefore an object of the present invention to provide an improved patient-alignment system in which the likelihood of misalignment between various components of the alignment system is reduced.

Briefly, according to one embodiment of the invention, the single cylindrical lens element of a conventional alignment device is replaced with a plurality of smaller lenses which are closely packed in side-by-side relationship to form an array of lenses which is relatively insensitive to precise positioning requirements. The size of the individual lenses is such that no matter where the collimated light beam (preferably a laser beam) hits the array, a plurality of the lenses will always be within the beam. Each one will then refract a portion of the laser beam into a fan-shaped profile, and the combined effect of all of these profiles will then result in a linear light pattern of sufficient intensity and length to meet the needs of the alignment function.

In one form, the lens array is made up of a plurality of plastic fibres which are formed into a ribbon which can be easily installed in the device.

By way of example only, an embodiment of the invention will now be described in more detail with reference to the accompanying drawings in which: Figure 1 is a perspective view of a computerised tomography system showing the locations of patient alignment devices embodying the present invention; Figure 2 is a front view of the gantry portion of the tomography system showing the locations of the patient alignment devices embodying the present invention; Figure 3 is a side view of a computerised tomography system with patient alignment devices embodying the present invention; Figure 4 is a schematic illustration of the alignment lights in a typical computerised tomography system; Figure 5 is a partial sectional view of a sagittal alignment light device embodying the present invention; Figures 6a and 6b are front and side views of the lens portion of the device of Fig. 5; and, Figure 7 is a schematic illustration of how a scan line is generated by the refraction of a laser beam with alignment device of Figure 5.

Referring now to the drawings, wherein like numerals correspond to like elements throughout, reference is initially made to Figs. 1, 2 and 3. A patient-alignment device is shown generally at 10 as installed in a computerised tomography system comprising a gantry 11 and a patient-support table 15. The gantry 11 includes a cover or shroud 12 which is generally rectangular in form at its outer boundaries and which has a central opening or tunnel 13 formed along a longitudinal axis 14. The shroud tunnel 13 is made up of converging portion 16, cylindrical portion 17 and the diverging portion 18, with the shape being determined by the need for easily positioning and observing the patient within the tunnel during an examination.Housed within the gantry shroud 12 are an X-ray source 19 and a detector 21, both of which are mounted on a detector plate 22 on opposite sides of the tunnel, such that their common plane, which will hereinafter be referred to as the scanning plane 23, intersects the tunnel longitudinal axis 14 at a point commonly referred to as the isocentre 24. The X-ray source 19 is adapted to emit a thin fan-shaped beam of X-rays along the scanning plane 23, through the isocentre 24 and to the detector 21.

The patient table 1 5 is disposed in front of the gantry 11 and includes a translatable cradle which is adapted to support the patient in a generally horizontal position. A drive means is provided within the table to translate the cradle and patient along the longitudinal axis to a position in the scan plane 23.

During the X-ray examination, the patient is exposed to the fan beam of radiation from the source 19, and the detector 21 is operable to measure the attenuation of X-ray energy by the patient. The detector 21 is typically made up of a plurality of cells with each cell capable of making its own independent measurement related to its sector of the fan beam within a scan plane. The responsive signals from all of the detector cells, at an instant of patient exposure, constitutes a view of the patient through the scan line.

During an examination scan, the source 19 and detector 21 are rotated by way of the detector plate 22 around the patient to obtain a number of views. The data from all of the views is then combined by appropriate algorithms so as to reconstruct an image of the slice through the patient at the scan plane in a well-known manner.

It should be mentioned that, while the present invention is being described in terms of use with a so-called third generation scanner, wherein both the source and detector are rotated about the patient, it could just as well be used with other types of scanners such as, for example, a socalled fourth generation scanner wherein either, but not both, of the source or detector is rotated about the patient.

In order to facilitate scans in oblique planes, provision is made to tilt the gantry 1 1 within its base to either side of the vertical plane such that the scanning plane 23 makes an oblique angle with the longitudinal axis 14. A typical mechanism for effecting this tilting function is shown and described in US patent no. 4 093 860 assigned to the assignee of the present invention.

It is known that to successfully scan the anatomy of interest, the patient must be precisely positioned in relation to the scan plane 23. This may be done when the patient is in the advanced position. One way in which this is accomplished is by way of so-called internal alignment lights located in the gantry and used for projecting a light pattern on the patient to indicate the position of the patient with respect to the scanning plane. Also, it is well known that the use of such internal alignment lights can often be difficult in practice because of the relative inaccessibility to the operator of the patient once the patient is in the advanced position.

An alternative or complementary approach is to selectively align the patient with respect to an external reference indicator located in a more accessible position outside of the tunnel, and then to advance the patient cradle and supported patient a distance equal to the longitudinal distance between the reference indicator and the scanning plane. In the present invention, the reference indicator is provided by so-called external alignment lights mounted within the gantry but located well forward of the scanning plane, such that the operator can easily visualise the projection of the light patterns on the patient when in the staged position. A description of both the internal and external alignment lights will now be given.

The axial alignment lights are shown in Figs.2, 3 and 4 comprise the external axial alignment lights 29 and 31 and the internal axial alignment lights 32 and 33. The external axial alignment lights 29 and 31 project complementary fanshaped beams from positions inside the gantry toward the patient by way of cutouts or windows as shown at 30. The internal lights 32 and 33 are located in the scan plane and project their complementary beams within that scan plane through a transparent hoop 34 made from a material such as Mylar (RTM). The function of the axial alignment lights is, of course, to facilitate the proper axial placement of the patient along the longitudinal axis or, said in another way, to assist in the selection of the slice that is to be imaged.

To aid in the selection of the proper height to which the patient is placed, the coronal alignment lights 36 and 37 are located in the horizontal plane of the longitudinal axis 14 so as to project a horizontal line onto each side of the patient. As shown in Figs. 1 and 4, a single pair of lights serves to provide both the internal and external coronal patterns, with the light for the external being projected through a window 38 having the form of a truncated sector, and the light beam for the internal coronal being projected through the Mylar (RTM) hoop 34. It should be mentioned that, whereas the axial and sagittal alignment lines tilt with the gantry, the coronal lines must remain in a horizontal disposition and therefore it is necessary to provide a means which will maintain this horizontal disposition regardless of gantry tilt.This feature will be more fully discussed hereinafter.

The sagittal alignment light 39 is adapted to project a fan beam downwardly on the patient as shown in Fig. 4 and serves as both the external and internal reference means.

Having discussed the various locations and functions of the several types of alignment lights used in the CT system, some discussion will now be given to the design and installation features related to the generation of the light patterns which are projected by those alignment devices. It will be recognised that the well-known approach of using cylindrical lenses can be used in combination with a laser to obtain any of the lines described hereinabove. However, in such a conventional arrangement, the positioning of the cylindrical lens within the beam path is very critical as described above. Normally, this is not a problem since the alignment devices are installed in the factory at the time of assembly, and, therefore, the required precision can be obtained.

However, if the operation or maintenance of the system is such that the laser and lens elements could become misaligned, then the condition of misplaced lines will present itself. One type of alignment light arrangement which is susceptible to such misalignment condition is a sagittal alignment light arrangement as described below.

In order to facilitate the easy access to the various components within the gantry for purposes of repair and maintanance, the gantry shroud 12 is hinged at 41, such that a door portion 42 can be easily opened to provide the necessary access. As mentioned above, a single sagittal light 39 serves for both the internal and the external alignment purposes. Accordingly, the point from which the light pattern fans out is well forward in the gantry as shown at 39 in Fig. 3. A pod 43 (see Fig. 1) is attached to the gantry door portion 42 and contains the lens device which causes the laser beam to be projected downwardly in a linear pattern as shown in Fig. 4.

As part of the sagittal light assembly, a laser 44 is mounted to the detector plate 22 with its beam directed forwardly, or generally parallel to the longitudinal axis 14. It should be mentioned here that in order to use the alignment lights, it is necessary to first place the detector plate 22 in such a rotational position that the tube is in the downward position as shown in Fig. 4. When in that position, the alignment light devices will all be in their proper position as indicated in Figure 4.

With the laser for the sagittal light now being in its proper upward position as mounted on the detector plate 22 with a bracket 46 as shown in Fig. 5, its laser beam is projected forwardly toward the shroud door portion 42. Near the door 42, a bracket 47, whch forms an extension of the bracket 46, holds a mirror 48 disposed at a 45Q angle so as to reflect the laser beam generally downward. Attached to the door 42 in a position directly below the bracket 47 is a bracket 49 which holds the lens 51 in the path of the projected laser beam so as to spread the beam along the longitudinal plane as shown.

It will be recognised that any misalignment between the laser 44 and the mirror 48 or lens 51 will cause the laser beam to be misdirected from the centre of the cylindrical rod portion of the lens 51. This misalignment may result from the laser 44 not coming back into the exact desired position when the detector plate 22 is rotated or, more likely, it may occur if either the mirror 48 is moved with respect to the laser or the lens 51 is not brought back into the exact desired position after the door 42 has been opened and closed.

In order to overcome this possible misalignment problem, the multiple lens device as shown in Fig. 6 is used in place of a single lens device of the prior art. The lens device comprises a frame 52 having a pair of holes 53 and 54 for mounting it to the bracket 49 and a central opening 56 through which the laser beam can project. An indentation 57 is formed in one side of the frame 52 for receiving a plurality of small cylindrical lenses 58 which are closely packed in parallel relationship so as to essentially fill the bottom of the indentation 57. The cylindrical lenses 58 may be made of any transparent material, such as glass or the like. A preferred material is a transparent fibre which is on the order of .010" (250 microns) in diameter.These individual fibres may be held together in their parallel positions so as to form a ribbon which is easily installed in the frame 52. The lenses 58 can be secured within the indentation 57 by any suitable means, such as with an epoxy adhesive.

Referring now to Fig. 7, one can see that the size of the cylindrical lenses 58 are sufficiently small so that the laser beam 55 projects simultaneously on a plurality of the lenses 58. In this way, that portion of the laser beam 55 which is directed to the centre of any one particular lens 58 will be spread out as shown to form a thin projected line. The portion of the beam 55 which falls on either side of that central position, but still falls within that particular lens, will also be refracted out into that plane so as to reinforce the projected line resulting from the central ray. Each one of the individual lenses then functions in this way. Light projected from each one acts to reinforce the projected light from the others. The result is a projection which is very similar to that resulting from a single lens. The intensity of the light projection is not quite as great as that resulting from a large single lens which is very accurately positioned within the beam, but this slight loss is more than offset by the elimination of the need for accurate positioning. That is, the laser beam 55 can be moved from side to side on the array of lenses as shown in Fig. 6 without affecting the overall appearance or position of the line which is formed from the projected light pattern.

Claims (10)

Claims

1. In a computerised tomography system having a gantry with a central opening and into which a patient is placed for scanning, an alignment device for positioning a patient with respect to the central opening comprising: a source for projecting a beam of collimated light and lens means disposed in a position for refracting said beam into a diverging linear pattern for use in aligning a patient with respect to the central opening, the lens means comprising a plurality of cylindrical lenses disposed in close parallel relationship and being sufficiently small in diameter such that the diameter of the beam spans a plurality of lenses at any one time.

2. An alignment device as set forth in claim 1 wherein said lens means comprises a plurality of transparent fibres which are fastened at their ends to a supporting frame.

3. An alignment device as set forth in claim 2 wherein said plurality of fibres are interconnected to form a unitary ribbon.

4. An alignment device as set forth in claim 1 wherein the number of lenses that are spanned by the diameter of the beam is in the range of 3 to 6.

5. An alignment device as set forth in claim 1 wherein said source comprises a laser.

6. A reference pattern projector for selectively positioning a patient with respect to the isocentre of a tomographic scanner comprising (a) an energy source projecting a beam of light in a first direction and (b) lens means disposed in a position for refracting said beam into a diverging linear pattern for use in aligning a patient with respect to the isocentre, the lens means comprising a plurality of cylindrical lenses disposed in close parallel relationship and being sufficiently small in diameter such that the diameter of the beam spans a plurality of lenses at any one time.

7. A reference pattern projector as set forth in claim 6 wherein said lens means comprises a plurality of transparent fibres which are mutually supported in the frame.

8. A patient-alignment apparatus for positioning a staged patient for subsequent longitudinal advancement into a cylindrical gantry opening of an X-ray scanner having an X-ray source and detector means for scanning the patient to produce a tomographic image in the plane of a scan comprising a light source for projecting the light beam toward the patient and lens means disposed in a position for refracting said beam in a diverging linear pattern that is projected on the patient as a reference line in a plane which corresponds with the scan plane when the patient is subsequently advanced, said lens means comprising a plurality of cylindrical lenses disposed in close parallel relationship and being sufficiently small in diameter such that the diameter of the beam spans a plurality of lenses at any one time.

9. A patient-alignment apparatus as set forth in claim 8 wherein said lens means comprises a plurality of transparent fibres which are supported at their ends by a frame.

10. A patient-alignment device substantially as herein described with reference to the accompanying drawings.