Image analysis – Applications – Biomedical applications
Reexamination Certificate
1998-11-12
2001-12-25
Johns, Andrew W. (Department: 3621)
Image analysis
Applications
Biomedical applications
Reexamination Certificate
active
06333991
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the analysis of radiographic images.
BACKGROUND ART
Linear accelerators have been in use for a significant time for the treatment of severe medical conditions such as cancer. The principle of operation of these devices is essentially that electromagnetic radiation of sufficiently high energy (greater than 1 MeV) causes the death and/or destruction of living cells in its path. If therefore a beam of high energy photons or electrons is directed to a cancerous area, the cancer will be destroyed.
It is clear that this treatment has the potential to cause harm to the patient if the radiation is incident on healthy tissue. It is therefore common practice to collimate the beam in order to limit its spatial extent and therefore limit the amount of tissue, healthy or otherwise, upon which it is incident. Collimators to this effect are known, and include collimators of variable size and shape which are capable of selectively blocking the radiation beam at its edges thereby to produce a beam whose shape corresponds to that of the tumour.
In this situation, it is clearly necessary to ensure that the positioning of the patient is accurate. This is generally done by shining one or more collimated light sources on the patient, such as low energy lasers. When the patient is first treated, ink marks are placed on the patient's skin and these are subsequently used to align the patient relative to the lasers. This is able to provide a positioning accuracy of approximately 2-5 mm.
As a result, the shaped radiation beam needs to be formed to the approximate outline shape of the tumour plus a margin in each direction to allow for positioning error. This results in a significant volume of tissue around the tumour which is needlessly irradiated. If the patient could be positioned more accurately beneath the beam, then this margin around the tumour could be reduced thereby limiting the damage caused to the patient's healthy tissue.
It is known to prepare a “Portal image” derived from the radiation transmitted through the patient and incident on a photographic plate or other detector. However, the absorption co-efficients of bone, tissue etc. at the energies normally used for radiotherapy are very similar and therefore a Portal image shows very little detail.
SUMMARY OF THE INVENTION
The present invention provides a method of electronically analysing the Portal image which is able to distil from the initially indistinct image information which is relevant to the treatment and to the positioning of the patient. In its first general stage, the method of the present invention determines whether or not the radiation beam is correctly set up. In its second general stage, the present invention determines whether or not the patient is positioned correctly in the radiation beam. Clearly, in any practical use of the invention, it will be preferred that both stages are employed. The description of this Application should therefore be read accordingly. However, the Application relates to the two analysis stages independently and the following summary of the invention should likewise be read accordingly.
In its first general stage, therefore, the present invention relates to a method of analysing a radiographic image comprising the following steps;
(a) preparing a reference image;
(b) preparing a sample image;
(c) preparing a reference radiation field image and a sample radiation field image from the reference image and the sample image respectively by, in order, (i) defining a threshold value and (ii) setting pixels within the respective images to a dark or a bright state if darker or brighter than the threshold value;
(d) optionally, comparing the reference radiation field image and the sample radiation field image to determine at least one of their relative rotation, translation and scaling;
(e) subtracting one of the radiation field images from the other thereby to obtain a difference image;
(f) inspecting the difference image to ascertain changes in the radiation field between preparation of the reference image and the sample image.
This method therefore provides a first warning as to whether the radiation fields are appropriate. It may generate an abnormal result if, for example, the collimator is incorrectly set.
Step (d) would be unnecessary if, for example, the image capture apparatus was normally stable enough to preclude all rotation, translation or scaling.
The threshold value is preferably calculated by determining the pixel grey scale histogram of one or both images and selecting a value between two peaks of the histogram. The value is preferably approximately half-way between the peaks, for example within 10% of the mid-point.
The dark state is preferably a zero brightness, and the bright state is preferably a maximum brightness, for example 1.
A suitable method of inspecting the difference image is by counting the total number of pixels in either the dark or the bright state. If this total number is above a preset threshold, or less than a preset threshold if the pixel type being counted is the predominant pixel within the image, then this can be considered an error value.
It is preferred if an alarm is generated if inspection shows a significant difference between the images. The difference can be measured as greater or less than a preset number or percentage of the total number of pixels. This preset number can be fixed by the system or set in accordance with the practice and technique of an individual hospital.
It is to be expected that the difference image will contain a line of single pixels at the boundary of the field, due to limitations in the calculation. This can be removed by a suitable filter, if desired. The remaining difference pixels will correspond to an area of field mismatch which can be compared to the preset number.
In its second general stage, the present invention proposes a method of distilling from the Portal images sufficient information regarding the patient's internal structure to allow determination of the patient positioning within the radiation field.
The present invention therefore provides, in its second general stage, a method of analysing a radiographic image comprising the following steps:
(a) preparing a reference image;
(b) preparing a sample image;
(c) filtering at least the sample image to remove low frequency variations;
(d) selecting a region of interest (ROI) within the reference and sample images;
(e) correlating the ROI of the reference image with the ROI of the sample image;
(f) determining the relative displacements of the reference image and the sample image.
The region of interest (ROI) may be a selected field of the image. Alternatively, it can be the entire image, although it is particularly preferred if the edge of the image is avoided. This is because the edge generally includes details of the radiation field edge, which tends to be either very bright or very dark. Thus, such areas of the image tend to predominate in the correlation step. It is therefore preferred if a border is eliminated by the steps of shrinking the radiation field image generated in the first general stage. This shrunken image can, optionally, be thresholded and multiplied with the sample and/or reference image of the second general stage. Shrinking a bit map image can easily be carried out by setting the pixel to a dark state unless all four neighbours are bright. To shrink an image by more than one pixel, the process can either be repeated, or can be carried out by examining pixels a set distance away in each direction. In the present invention, it is preferred if that distance is between 15 and 30 pixels.
It is naturally preferred if this aspect of the invention is carried out in combination with the first aspect, and the output result of this aspect is the calculated difference between the radiation field displacement and the image displacement. This will reveal the displacement of the patient relative to the radiation field, which is of course the value of interest. As noted above, it may be
Kreuder Frank
Schreiber Bernd
Elekta AB
Johns Andrew W.
Kinney & Lange , P.A.
Nakhjavan Shervin
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