EP0224726A2

EP0224726A2 - Attenuator plate for use with X-ray apparatus

Info

Publication number: EP0224726A2
Application number: EP86115149A
Authority: EP
Inventors: Michael Leslie Cocklin
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1985-12-02
Filing date: 1986-10-31
Publication date: 1987-06-10
Also published as: EP0224726A3; GB8529615D0; JPS62133398A

Abstract

A plate (10, 12) for insertion between a source of X-rays (16) and a patient (19) under examination has an X-ray transmission factor which varies across the surface of the plate in at least one of two orthogonal directions to provide a large number of alternate local maxima and minima of the transmission factor. The X-ray transmission factor at the minima are sufficiently greater than zero that primary X-rays are transmitted to a substantial extent through the entire area of the plate.

Description

This invention relates to an X-ray attenuator plate and X-ray apparatus incorporating such a plate.
X-rays are the most common means of non-invasively investigating the internal human anatomy in medical diagnosis. The X-ray tube produces X-rays that pass through the patient and typically produce a shadow on film. This shadow is due to the differential attenuation of the X-rays by the different structures in the body. Attenuation of the X-rays is either by complete absorption or by scattering of the X-rays. The former process gives rise to the PRIMARY radiation exiting the patient and the most useful image information. The latter process results in SCATTERED radiation which produces a general fog on the radiograph, essentially unrelated to the true image information. Since the early days of radiography, attempts have been made to reduce the effects of this scattered radiation on the quality of the recorded image, see E E Christensen et al, An Introduction to the Physics of Diagnostic Radiology, Lea & Febiger, Philadelphia, 1978, pp 5-9 and 71-74).
The most common technique for the removal of scattered radiation effects from diagnostic radiographs is the use of an anti-scatter grid, see above reference, pp 89-91. This consists of a grid of lead bars with the spaces between the bars filled with some radiolucent material. The lead bars have an X-ray transmission factor that is effectively zero, while that of the radiolucent material is close to one (ie most X-rays are allowed to pass). In this specification, the X-ray transmission factor of an object is defined as the fraction, between 0 and 1, of incident X-ray flux which passes through the object.
The grid is placed between the patient and the X-ray detector. X-rays that are scattered by the patient usually impinge on the grid at an angle to their original direction and are absorbed by the lead bars. Unscattered (primary) X-rays, however, have the same direction as they had when entering the patient, and so can pass freely between the lead bars.
The problem with the current technique is that a significant proportion of the primary X-rays also impinge directly onto the lead bars, and are therefore absorbed along with the scattered X-rays. Hence, while anti- scatter grids do stop a large part of the scattered radiation from reaching the detector, this is done at the cost of stopping many of the primary X-rays. This means that higher X-ray exposures (2-4 times) have to be used to obtain good quality X-ray images.
Accordingly, it is an object of the invention to provide an attenuator plate useful in X-ray imaging which permits the reduction of the effect of scattered radiation without requiring significantly higher patient dosages.
To this end the present invention provides an attenuator plate for insertion between a source of X-rays and a patient under examination, the plate having an X-ray transmission factor which varies across the surface of the plate in at least one of two orthogonal directions to provide a large number of alternate local maxima and minima, the X-ray transmission factor at the minima being sufficiently greater than zero that primary X-rays are transmitted to a substantial extent through the entire area of the plate.
The invention further comprises an X-ray apparatus comprising a source of X-rays of given peak energy, a plate having a transmission factor for the X-rays which varies across the surface of the plate in at least one of two orthogonal directions to provide a large number of alternate local maxima and minima, the X-ray transmission factor at the minima being sufficiently greater than zero that primary X-rays are transmitted to a substantial extent through the entire area of the plate, and means for recording an image of the X-rays from the source after passage through the plate and a patient in that order.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a plate according to a first embodiment of the invention,
Figure 2 is a cross-sectional view of the plate of Figure 1,
Figure 3 is a cross-sectional view of a plate according to a second embodiment of the invention, and
Figure 4 is a schematic diagram of an X-ray apparatus using a plate as aforesaid.

In effect the invention comprises a special plate of non-uniform X-ray transmission factor which 'codes' the X-ray beam before it enters the patient. Knowledge of the coded X-ray input may be used to calculate and remove the scattered radiation contribution from the recorded image, after the X-ray image has been converted directly or indirectly into digital form for processing. Such conversions are currently done in Computed Tomography (brain and body) Scanners. For simplicity of processing the maxima and minima in the X-ray transmission factor preferably occur with a fixed period across the plate in one or both directions, and the transitions from one to the other may take place in distinct steps or smoothly such as sinusoidally.
The average X-ray transmission factor of the plate as a whole is typically in the region of 0.7 to 0.9, with the X-ray transmission factor at the minima about 0.5 to 0.6 and that at the maxima close to 1. The distance, ie the fixed period, between adjacent maxima (and minima) is typically from 0.4 mm up to about 2.0 mm depending upon the processing techniques used. Placing this plate between the X-ray tube and the patient results in an X-ray field of non-uniform intensity entering the patient. This is the coded X-ray beam.
The plate must cover the full area of the recorded X-ray field. If it is placed close to the X-ray tube, its size may be as small as a few centimetres on a side. However, the rate of variation of X-ray transmission factor across the plate would then have to be very high, leading to manufacturing problems. At the other extreme, the plate may be placed close to the patient so the required size is effectively the size of the X-ray detector (eg films for chest radiography are about the largest generally available and are typically 43 cm × 35 cm). Putting the plate close to the patient relaxes the manufacturing constraints, but means that spurious X-rays produced by the non-uniformity of the plate itself may not be dissipated before the X-ray beam enters the patient.
Plate position would therefore have to be some compromise between the above requirements. A reasonable choice for chest radiography, for example, is about 50 cm in front of the patient, giving a plate about 30 cm on each side.
The required rate of variation of the X-ray transmission factor edge-to-edge across the plate is determined by the position of the plate. If the plate is in the above position, it will be about 100 cm from the film or other image recording means. If the distance between the X-ray tube and the film is about 300 cm, this gives a plate magnification at the film of about 1.5. Given that the radiographic image is to be digitised, the upper limit on the rate of variation is determined by the elemental size of the digital array (the pixel size). At the film plane this may be expected to be in the region of 0.2 to 0.4 mm. Taking 0.3 mm as a guide, this means that significant changes in the plate (ie changes from a maximum to a minimum) do not have to occur in less than about 0.3/1.5 or 0.2 mm. The use of good interpolation techniques (see theory to follow) may allow this requirement to be relaxed by a factor of up to about 5, ie changes at 1 mm distances, without significant loss in image quality. Thus the distance, ie the fixed period, between adjacent maxima (or minima) is typically from 0.4 mm up to about 2.0 mm depending upon the processing techniques used.
The varying X-ray transmission of the plate may be achieved by using one material and varying its thickness or density across the surface of the plate, by using a combination of materials of the same thickness with different X-ray attenuation characteristics, or by a combination of these two techniques.
An example of the first technique is an aluminium plate 10 (Figure 1) provided with closely spaced parallel ridges 11 on one side so that its thickness fluctuates smoothly and periodically (preferably sinusoidally) between about 2 mm to 7 mm from one edge to the opposite edge (see Figure 2). Such a plate will have a smoothly varying X-ray transmission factor.
An example of the second technique is a plate 12 (Figure 3) of uniform thickness of about 5 mm consisting, across the surface of the plate, of alternate thin parallel strips of aluminium 13 and perspex 14 separated by thin strips of magnesium 15, aluminium being a material that attenuates X-rays quite a lot, perspex one that only attenuates a little, and magnesium one that has an attenuation in between. Such a plate will have a stepped X-ray transmission factor.
Using the latter technique, ie using thin parallel strips of materials of different X-ray attenuations, these would have the same width (eg about 0.2 - 0.5 mm) in the direction across the surface of the plate from one edge to the other, and the same thickness (eg about 5 mm) in the direction of the X-ray beam, and be layered together in a way similar to the conventional anti-scatter grid. For the purposes of comparison, the lead strips in conventional grids are about 0.05 mm wide, with the radiolucent material between them about 0.4 mm wide.
The peak X-ray energies used in medical diagnosis typically lie in the range from 50 to 150 KeV, and the invention is useful throughout that range.
It will be appreciated that the above describes the construction of plates whose X-ray transmission factor varies in one direction only. A variation in two orthogonal directions can be achieved by constructing two such plates and placing them together with the stripes or ridges of one plate at right angles to the stripes or ridges of the other plate, the transmission factors of the two plates being half that of the single plate.
Compared to the conventional anti-scatter grid, the plate according to the invention has the following advantages:

1. The plate has a relatively small amplitude variation in X-ray transmission across its surface compared to the sharp all or nothing change in transmission of the prior grid.
2. It is placed between the X-ray tube and the patient, not between the patient and the detector.
3. It is used to code the X-ray beam entering the patient, rather than physically stopping scattered X-rays.
4. Removal of the scatter effect is by computation rather than absorption by lead.
5. The use of the plate does not require any increased X-ray exposure to the patient.

It will be appreciated that a detailed example of an X-ray apparatus according to the invention need not be explicitly described, since it may comprise a standard X-ray source 16 (Figure 4) emitting X-rays 17 of a given peak energy lying within the above range, an image recording means 18, and a plate 10 or 12 as described above mounted in any convenient manner between the two such that X-rays can pass through a patient 19 after passing through the plate.
The theory of the use of the non-uniform transmission plate is as follows, for the case where the maxima and minima have a fixed period across the plate.
It is assumed in the following that the plate is so constructed that there are areas (occurring periodically, and aligned with the pixels of the digital array) having an X-ray transmission close to the average of the plate as a whole. In addition, there are other areas (similarly periodic and pixel aligned) having X-ray transmissions somewhat higher and lower than this average. This assumption can be made valid for the case of the layered plate construction, and is sufficiently valid for a plate of smoothly varying X-ray transmission. A smoothly varying plate more closely follows the constant scattering assumption used in the theory below, but small steps (in space and X-ray transmission) do not violate this to any significant extent.
Let the plate have a transmission factor of k at any point (0<k<1) with an average transmission of K (k=1 means all X-rays are transmitted, k=0 means no X-rays are transmitted through the plate). Range of k will typically be from about 0.6 to about 1, with an average value K of about 0.8. This means that the X-ray tube exposure will be some 25% higher than normal, but there is no change to the exposure to the patient.
The measured exposure at some point A where k ≠ K may be written as:
I_a= I₀ke^-x + S (1)
where I₀ is the exposure incident to the plate, S the scattered radiation and x the attenuation of the patient at that point. It is desired to measure x, the true physical signal being imaged.
For each such point A, an estimate is made of the exposure that would have been measured if the plate had been uniform with transmission factor K. This would be:
I_b = I₀Ke^-x + S (2)
where it is assumed that the scattered radiation would be the same. This is because the scattered radiation is dependent on the average attenuation of the body surrounding the measured point, which is the same in both cases. The fact that the plate transmission factor varies slowly (ie no abrupt transition from 0 to 1) also aids this assumption.
The value of I_b is obtained by interpolation from the measured exposures at the points immediately surrounding point A where the transmission factor k = K. There will typically be a square or two rows of such points in the vicinity of each point A. The assumption here is that the frequency of the periodic pattern is high enough to make a good estimate (eg 1 mm distance between the maxima or minima of the X-ray transmission values).
Subtraction of the estimated value from the measured value (ie (2) from (1)) gives:
I_a - I_b = I₀e^-x(k - K)
Hence:
e^-x = (I_a - I_b)/I₀(k - K))
So that:
x = log [ (k-K)/(I_a-I_b) ] +logI₀ (3)
This then gives the required attenuation value x in terms of the measured and estimated exposures I_a and I_b, the transmission factors of the plate k and K and the incident exposure I₀, for each point A where k ≠ K. The values of x for those points where k = K are then obtained by interpolation of these calculated values.
Accordingly, the complete sequence of steps to obtain all the required values is given below:

1. Take an X-ray exposure with the plate in place, but with no patient and store the resulting digital image.
2. Divide the value at each point (pixel) in the image by the maximum value in the image (ie corresponding to those points where k is a maximum).This gives an image of the fractional transmission through the plate - ie the k values.
3. Calculate the average transmission K, as the average of this image.
4. Identify all those points R where the pixel value is within some tolerance value of this average, ie where k = K ± e. The tolerance value is chosen so that the set of points R form regular patterns through the image.
5. Repeat the exposure with the patient in place and store the digital image of exposure values.
6. For each point A in the patient image, where the corresponding k value does not equal K ± e and the measured exposure value is I_a, obtain the interpolated value I_b from the measured values at the set of points R surrounding the point A.
7. Calculate at each point A:
log[(k-K)/(I_a-I_b)] (4)
From (3) above this is a measure of:
x - logI₀ (5)
8. At points A not in the shadow of the patient, x will be zero (no attenuation). These points can be identified as the minimum points in the set of values calculated using (4). This minimum value will therefore be the value -logI₀. The magnitude of this value is therefore added to the result of the calculation in (4) to obtain the value of x for each point A, as indicated by (5).
9. The values of x for all the points R where k = K ± e are then obtained by interpolation of the calculated values at the points A.

Claims

1. An attenuator plate for insertion between a source of X-rays and a patient under examination, the plate having an X-ray transmission factor which varies across the surface of the plate in at least one of two orthogonal directions to provide a large number of alternate local maxima and minima, the X-ray transmission factor at the minima being sufficiently greater than zero that primary X-rays are transmitted to a substantial extent through the entire area of the plate.

2. A plate according to claim 1, wherein the X-ray transmission factor varies in two orthogonal directions.

3. A plate according to claim 1 or 2, wherein the X-ray transmission factor varies from 0.5 to 1.0.

4. A plate according to claim 1, 2 or 3, wherein the maxima occur at a fixed period of from 0.4 mm to 2.0 mm in the or each direction.

5. An X-ray apparatus comprising a source of X-rays of given peak energy, a plate having a transmission factor for the X-rays which varies across the surface of the plate in at least one of two orthogonal directions to provide a large number of alternate local maxima and minima, the X-ray transmission factor at the minima being sufficiently greater than zero that primary X-rays are transmitted to a substantial extent through the entire area of the plate, and means for recording an image of the X-rays from the source after passage through the plate and a patient in that order.

6. An apparatus according to claim 5, wherein the X-ray transmission factor of the plate varies in two orthogonal directions.

7. An apparatus according to claim 5 or 6, wherein the X-ray transmission factor varies from 0.5 to 1.0.

8. An apparatus according to claim 5, 6 or 7, wherein the maxima occur at a fixed period of from 0.4 mm to 2.0 mm in the or each direction.

9. A plate or apparatus according to any preceding claim, wherein the peak X-ray energy is from 50 to 150 KeV.