T 0990/98 30-04-2002

Reasons for the Decision

1. The appeal is admissible.

2. Amendments

All the modifications introduced in the claims are clear and fairly supported by the application as filed. Therefore, they are not open to formal objection.

3. Main request

The substantive merits of independent claim 1 may not be investigated since the independent claim 2 is anyway unallowable for the reasons set out below (see T 162/88, 9.7.1990, unpublished).

3.1. The subject-matter of claim 2 is based on the embodiment illustrated by Figure 3, which differs from that one of Figure 2 by the incorporation of two filtering units 9, 10 placed at the respective inputs of the second subtraction processing unit 8.

Like Figure 2, the second subtraction processing unit 8, generally performs subtraction between the low energy X-ray image 1 and the first processed image 11 of a first body material (here soft tissue), which itself results from a first subtraction processing 7 between the low energy X-ray image 1 and the high energy X-ray image 2. This arrangement provides for obtaining a second processed image 12 of a second body material (here bone) with low noise. Since, as generally known (cf. patent specification, page 2, lines 47 to 50 and page 3, lines 42 to 45) in the image by the high energy X-ray the contrast is low and the signal-to-noise ratio (SNR) is much deteriorated, by performing a second subtraction processing 8 between the first processed image 11 and the image 1 from the low energy X-ray, a second processed image 12 having high frequency components can be obtained without increasing noise, ie having better quality (cf. page 3, lines 46 to 50).

This result is still improved by the arrangement of Figure 3 in which a pair of filtering elements is provided, namely one space filter processing unit 10 (low-pass filter) to suppress the high space frequency components of the first processed image 11 (ie essentially noise components of high frequency) and one space filter processing unit 9 (high-pass filter) to enhance the high space frequency components contained in the low energy X-ray image 1 (ie the energy level at which the noise components are not so high, as previously mentioned) (cf. from page 4, line 56 to page 5, line 1).

3.2. The state of the art coming closest to the invention is referred to in the application as filed with reference to Figure 1 and partly to Figure 2.

Having regard to Figure 1 and related text (cf. patent specification page 2, lines 17 to 30) an X-ray image processing device is known, comprising means for obtaining both low and high energy image information which is a logarithmic conversion image information of a penetration image of an object radiated by low and high energy X-ray, respectively, and means for subtracting the low energy image 1 from the high energy image 2 to output a first processed image 5 or 6. The use of the energy subtraction method or dual energy projection radiography is clearly acknowledged as belonging to the state of the art. Therefore, it is known that X-ray images of different energies are subjected to logarithmic conversion and then subjected to differentiation or subtraction between each other (cf. page 2, lines 17 to 21).

Having regard to Figure 2, it is clearly stated (cf. page 3, lines 29 to 33) that all illustrated components starting from the production of the X-ray energies up to the obtention of the low and high energy images 1 and 2 are conventional to persons skilled in the art, including the arrangement for producing logarithmically converted X-ray image information of the radiated object (cf. page 3, lines 26 to 28). Moreover, it has to be mentioned that the known subtracting means 3 of Figure 1 for obtaining the first processed image 5, are again to be seen in Figures 2 and 3 under the reference signs 7 and 11, respectively.

Therefore, all the precharacterising features of claim 2 are known from the background of the invention.

3.3. Also Document E5 discloses the features mentioned above yet in more general terms, stating that (cf. Abstract and Introduction, page 122) a variety of medical imaging systems are known, in which a number of independent measurements using different X-ray energies (cf. dual-energy CT, page 124, right column) are combined to provide a selective (subtraction) image, ie using measurements made of different X-ray energy spectra to eliminate and/or enhance different body materials (eg soft tissues or bones). These subtraction studies provide images of improved conspicuity where undesired structures are eliminated.

Document E5 observes however that subtraction operations in such sytems result in a degradation of the SNR (signal-to-noise ratio), as compared to the individual measurements. The object of document E5 is, therefore, to introduce a novel filtering system that significantly improves the SNR of the selective image. Therefore, like the present patent, this state of the art aims at improving the picture quality.

According to the solution as generally disclosed in document E5 (cf. Abstract) the low spatial frequencies are derived from the selective image and the high frequencies from a non-selective combination of the measurements which have a greater SNR. More specifically (cf. Introduction, last paragraph), corresponding features of claim 2 being added into brackets for the sake of comparison, in E5 the basis algorithm involves combining (substraction processing 8) the low frequency components of the selective image (first image 11) with the high frequency components of the non-selective, high SNR image (low energy image 1) in order to provide an improved SNR selective image (second image 12). Since each component of the final image (second image 12), ie the low-pass filtered (filter 10) selective image (first image 11) and the high-pass filtered (filter 9) non-selective image (low energy image 1) comprises relatively low noise, the resultant image similarly has a reduced noise.

The principle of this solution corresponds exactly to that of the claimed solution as explained above (point 3.1) which solves the same technical problem. Consequently the subject-matter of independent claim 2 according to the main request does not involve an inventive step vis a vis the teaching of document E5, having regard to the general knowledge of the skilled person as recited in the patent itself. The requirements of Article 56 EPC are, therefore, not met.

4. First auxiliary request.

Claim 1 according to the first auxiliary request differs from claim 2 according to the main request in that the "first image (11)" is a "soft tissue" image, in that the "second image (12)" is an image of the "bones" and in that the "high frequency enhance image" is "low energy".

The last difference is implicitly disclosed by document E5, in which it is specified (cf. Introduction, last paragraph) that the combination processing involved in the filtering system implies the use of high frequency components of the non-selective, high SNR image. Since it is generally admitted (cf. point 3.1 above and patent specification, page 2, lines 47 to 50) that the X-ray image which offers the better SNR is precisely the low energy image, said "non-selective, high SNR image" referred to in document E5 must necessarily be the low energy image.

Document E5 discloses further (cf Description, page 122 and Applications, page 124, left column) that a selective image (S) is formed by an array of measurements at low and high energies where the weights ki (coefficients k1 to k4 in the patent) are chosen to select desired image components and reject undesired image components. In other words, the selection or the elimination of one body material (soft tissues or bones) in order to provide a first image depends only on the choice of these coefficients, which was already known from the background of the patent specification (cf. page 2, lines 28 to 34 and page 4, lines 43 to 45). As a matter of fact, in the system of two equations (1) and (2), one image can be eliminated by using appropriate coefficients and by subjecting both X-ray images to a substraction processing.

The sequence of first providing a processed image of soft tissues and then a processed image of bones is only a matter of choice depending on the objectives of the practitioner and the patient's demand. As is further mentioned in document E5 (cf page 124, right column) in dual-energy CT, all projections are taken at two energies and then processed to reconstruct images of two basis components. The selective (S) images can be either of these combinations. Therefore, in the Board's judgement, there is no invention in outputting soft tissues as a first image and as a second or final image the bones. In any case, the essential functions of the arrangement, ie two successive substraction processes and double-filtering steps applied before and after the first substraction processing, respectively, remain the same.

Moreover, whatever the nature of the second image may be, the double-filtering system disclosed in document E5 (cf page 123, paragraph bridging the two columns) also provides for preserving fine structures, for example by adjusting ("threshold") the low-pass filtered version (Sl) of the selective image (S) (cf. page 122, bottom of the right column).

It results therefrom that the subject-matter of claim 1 according to the first auxiliary request does not involve an inventive step vis a vis the disclosure of document E5 in combination with the general knowledge as stated in the background of the patent specification.

5. Second auxiliary request.

Claim 1 according to the second auxiliary request differs from the previous auxiliary request in that its subject-matter is drafted in a one-part form and by the incorporation of features related to the use of an X-ray tube of different energies and an X-ray sensor having a function of energy discrimination.

While these features are fairly supported by the patent specification, they have, however, no connection with the problem as originally stated and its solution, which is essentially based on successive substraction processes associated with double-filtering as explained above. Further, the incorporated features are all parts of known devices at the disposal of a person skilled in the art, either as a basis equipment as mentioned in above point 3.2 or as alternative equipment (cf. patent, page 6, lines 3 to 7).

Therefore, the incorporated features fail to add anything inventive to the claimed subject-matter, contrary to the requirements of Article 56 EPC.