GB2273357A

GB2273357A - Non-invasive medical scanning

Info

Publication number: GB2273357A
Application number: GB9225690A
Authority: GB
Inventors: Nicholas John Wald
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-12-09
Filing date: 1992-12-09
Publication date: 1994-06-15
Also published as: CA2151280A1; WO1994014132A1; GB9225690D0; AU5573694A

Abstract

A non-invasive medical scanning apparatus for generating an image of a fetus comprises means for detecting a quantitative measure relating to the fetal image; means responsive to the detected quantitative measure for generating a numerical value indicative of the shape of the fetal image; and means for detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality. The apparatus is particularly used to detect abnormality in the shape of the fetal head which may be indicative of spina bifida or of the nuchal fold which may be indicative of Down's Syndrome.

Description

NON-INVASIVE MEDICAL SCANNING This invention relates to non-invasive medical scanning.

Non-invasive medical scanning, such as ultrasound scanning, is used in the imaging of the interior of the human body. In the case of ultrasound scanning, ultrasonic vibrations generated by a hand-held transducer are transmitted into the body through the skin, and are reflected back to the transducer from tissues of different densities within the body. As described in the book "Physics and Instrumentation of Diagnostic Medical Ultrasound" (P. Fish, John Wiley & Sons, 1990), an image of a cross-section through the body can be built up by analysing the relative amplitudes and delays of the reflected vibrations.

One particular use which has been made of ultrasound scanning is the examination of the unborn fetus in the mother's uterus. A skilled operator can determine the orientation, gestational age and general condition of the fetus from its ultrasound image. Also, some fetal abnormalities, such as cardiac or renal anomalies, can be diagnosed by the operator. A difficulty is that some fetal abnormalities which would be evident from the ultrasound image may be overlooked because they occur so rarely that an operator would lack sufficient experience to be able readily to recognise those abnormalities.

This invention provides a non-invasive medical scanning apparatus for generating an image of an unborn fetus, the apparatus comprising: means for detecting a quantitative measure relating to the fetal image; means responsive to the detected quantitative measure for generating a numerical value indicative of the shape of the fetal image; and means for detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.

The invention addresses the problem that some rare fetal abnormalities may be evident from the shape of an image of the fetus generated by non-invasive scanning, but the recognition by an operator of these abnormalities may be difficult (through lack of operator experience) and time consuming.

As an example, the condition of spina bifida is indicated by a deformation of the head of an unborn fetus. This deformation of the fetal head (known as the "lemon" sign) shows up on an ultrasound scan of the unborn fetus, but the fact that spina bifida occurs in only about one in 1000 term births means that (a) the scanning operator would have to, on average, examine 1000 ultrasound images to detect one case of spina bifida; and (b) many operators would only be likely to see one or two cases of spina bifida per year, and so would not develop the experience to be able to detect the deformation.

The invention solves this problem by generating a numerical value indicative of the shape of a scan of, for example, the fetal head, and then detecting a possible fetal abnormality when the numerical value lies within a predetermined range. This process can be performed automatically on each scan of an unborn fetus, and can be used to generate an alarm indication (such as an indicator light or message) to prompt further medical investigations such as more detailed or more expert imaging of the fetus (particularly the spine in the case of spina bifida detection) and possibly an amniocentesis.

The invention is applicable to the non-invasive detection of various fetal abnormalities which show up as deformations of a fetal scan. For example, the dimensions of the nuchal fold of the fetus could be detected, with a large thickness being indicative of the condition of Down's Syndrome.

The quantitative measure may be, for example, the curvature of a part of the fetal body, the thickness of the nuchal fold, the diameter or circumference of the head, or another measure. The quantitative measure can be compared with a normal value expected for a fetus of that gestational age, or alternatively two or more such measures can be combined to generate a ratio which can then be compared with an expected value (which again may be dependent on the gestational age).

Preferably the invention should be employed during the earlier stages of pregnancy, at which stages termination of the pregnancy is possible.

In a preferred embodiment the apparatus comprises means for detecting an outline of the fetal image and means for detecting the lengths of two or more predetermined axes of the outline, and the generating means is responsive to the two or more detected lengths. In other words, a numerical value is generated, the calculation of which uses the two or more detected lengths, either alone or in combination with other values.

Preferably ultrasound scanning is employed, although the invention is applicable to other types of non-invasive imaging or scanning such as magnetic resonance imaging.

The means for detecting the outline can be embodied as a light pen or user-controlled cursor, with which the user can trace the outline of e.g. the fetal head. However, in a preferred embodiment the means for detecting the outline comprises a threshold detecting means operable to detect a predetermined luminance level of the scanned image.

In a preferred embodiment the apparatus comprises means for detecting the principal (i.e. major and minor) axes of the outline of a cross section of the fetal head. In this context, the major axis corresponds to the occipito-frontal measurement and the minor axis corresponds to the biparietal diameter of the fetal head. Various measurements can then be made with respect to the principal axes.

Preferably the principal axes are detected by detecting the longest bisector of the outline (this being the major axis) and then the longest axis at 900 to the major axis (this being the minor axis).

Various measurements can be used to detect the condition of spina bifida. In one preferred embodiment the apparatus comprises means for detecting the width of the fetal head at a position of 80% along the major axis of the outline, and for generating the numerical value by dividing that width by the sum of the lengths of the major and minor axes. In another preferred embodiment the apparatus comprises means for detecting the lengths of axes extending from the centre of the major axis and at a predetermined angle (such as 400) to the major axis, and for generating the numerical value by dividing the sum of those lengths by the sum of the lengths of the major and minor axes.

Preferably the predetermined range of the numerical value comprises values less than a predetermined cut-off value.

Viewed from a second aspect this invention provides a method of non-invasive medical scanning, the method comprising the steps of: generating an image of an unborn fetus; detecting a quantitative measure relating to the fetal image; generating a numerical value indicative of the shape of the fetal image in response to the quantitative measure; and detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.

Viewed from a third aspect this invention provides image processing apparatus for processing a non-invasively scanned image of an unborn fetus, the apparatus comprising: means for detecting a quantitative measure relating to the fetal image; means responsive to the detected quantitative measure for generating a numerical value indicative of the shape of the fetal image; and means for detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.

Viewed from a fourth aspect this invention provides a method of processing a non-invasively scanned image of an unborn fetus, the method comprising the steps of: detecting a quantitative measure relating to the fetal image; generating a numerical value indicative of the shape of the fetal image in response to the quantitative measure; and detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram of an ultrasound scanning apparatus; Figure 2 represents an ultrasound scan of the head of a fetus suffering from spina bifida; Figure 3 represents the ultrasound scan of a fetus not suffering from spina bifida; Figure 4 is a schematic diagram showing the operation of an outline detector; Figures 5 and 6 are schematic diagrams of two further embodiments of an outline detector; and Figures 7 and 8 are schematic diagrams showing measurements made with respect to the principal axis of a fetal head scan to detect possible fetal abnormalities.

Figure 1 is a schematic diagram of an ultrasound scanning apparatus. A hand-held ultrasound probe 10 comprises an ultrasound transmitter 20 and an ultrasound receiver 30. A scan controller 40 generates control signals to control the ultrasound transmitter, and also receives a signal indicative of the echoes received by the ultrasound receiver 30. An analogue to digital converter (ADC) 45 is employed to convert the echo signal from the ultrasound receiver 30 from analogue to digital form, so that the digitised echo signal can be processed, manipulated and stored digitally.

The digital echo signal is supplied from the scan controller 40 to an image processor 50 in which basic image processing operations such as image contrast enhancement or "freeze-frame" are performed.

The image processor 50 is responsive to user control, so that the operator of the ultrasound scanning apparatus can vary the image processing parameters to achieve the best possible images under the circumstances of a particular ultrasound scan. The image processor 50 also formats the processed digital echo signal for storage in a video memory 60.

The video memory 60 is of conventional form, in that its contents are read out continually in synchronism with the line scanning of a video display device 70 such as a cathode ray tube (CRT). In addition to this continual reading of the video memory 60, data can be written into the video memory 60 by the image processor 50 and can be read from the video memory 60 by an outline detector 80.

The outline detector 80 operates in one of a number of ways (to be described below) to detect the outline of the scan of a fetal head.

This scan is generally elliptical in shape, and data representing the outline are passed by the outline detector to a principal axis detector 90. The principal axis detector 90 detects the major axis by detecting the longest bisector of the fetal head outline. The minor axis of the fetal head outline is detected by the principal axis detector detecting the longest axis at 900 to the major axis.

Data representing the fetal head outline and the principal axes of the outline are passed to a shape detector 100 which analyses the fetal head outline in a manner described below, to generate a numerical value indicative of an aspect of the shape of the fetal head outline.

If this numerical value is less than a specified cut-off value, an alarm signal (such as an indicator light or a message displayed on the display device 70) indicative of a possible fetal abnormality is generated by the shape detector 100. When the alarm signal is generated, the operator of the scanning apparatus can initiate further medical tests such as amniocentesis or can refer the patient to a more qualified colleague for further examination.

Figure 2 represents an ultrasound scan 110 of the head of a fetus suffering from spina bifida. The scan represents a cross section through the fetal head in a plane at 900 to the longitudinal axis of the fetus. Figure 3 represents a similar scan of the head of a fetus not suffering from spina bifida. The significant differences between the scan 110 and the scan 130, which differences are indicative of the spina bifida abnormality, are the slight depressions 140 in the spina bifida scan. These depressions 140 give the head a characteristic "lemon" shape, and so are known as the "lemon" sign.

Figure 4 is a schematic diagram showing the operation of one embodiment of the outline detector 80. Typically the scan generated by an ultrasound scanning apparatus will not comprise sharp, well defined outlines. On the contrary, the edges of the fetal head are represented by a broad outline 140. In order that measurements may be made on the shape of the fetal head outline, a nominal outline 150 is generated from the broad outline 140 of the scan. This may be performed, for example, by a simple thresholding technique, described in the book "Fundamentals of Digital Image Processing" (A.K. Jain, Prentice-Hall International, 1989). According to this technique, the nominal outline 150 is generated by detecting a predetermined luminance level in the broad outline 140.If the image of the fetal head is positioned on the screen of the video display device 70 so that the outline of the head surrounds a predetermined point 160 on the screen (which may be marked with a graticule), the outline detector 80 can determine which parts of the scanned image corresponds to the fetal head outline by detecting the intersection of the nominal outline 150 with construction lines 170 radially extending from the point 160.

Figures 5 and 6 are schematic diagrams of two further embodiments of the outline detector. In each of Figures 5 and 6, the fetal head outline is indicated to the outline detector by the user tracing the outline on the screen of the video display device 70. In the embodiment shown in Figure 5, the user employs a light pen 180, connected to the outline detector 80', to trace the fetal head outline, whereas in Figure 6 the user employs a mouse-or key-driven cursor generated by the outline detector 80" to trace the fetal head outline.

Figures 7 and 8 are schematic diagrams showing examples of measurements which may be made with respect to the principal axis or axes of the fetal head outline, which measurements may then be used by the shape detector 100 to detect a possible fetal abnormality. In Figures 7 and 8, the length of the major axis of the fetal head outline is denoted by "1", and the length of the minor axis of the fetal head outline is denoted by "h".

Referring to the annotations on Figure 7, a numerical value n representing the shape of the fetal head outline is calculated as follows:

In the case of Figure 8, a similar numerical value m representing the fetal head shape is calculated as follows:

The shape detector compares the numerical values n or m with a predetermined cut-off value. If the respective value is below the cutoff value, the alarm signal is generated. The cut-off value is selected by balancing the aim of detecting the greatest possible proportion of cases of spina-bifida (maximising the detection rate) against the requirement of avoiding too high a false positive rate.

Suitable cut-off values determined in trials of the embodiment are listed below:

Numerical Value Cut-Off Value Detection Rate False Positive (Z) Rate (%) dl + d2 < 0.275 32 (23/71) O (0/20) I+h < 0.290 55 (#9/71) io (2/20) s, + s, (8=35") < 0.458 65 (46/71) o (0/20) < 0.466 78 (55/71) 10 (2/20) sl + s2 (0=400) < 0.445 59 (42/71) O (0/20) l+h < 0.448 68 (48/71) 10 (2/20)

Claims

CLAIMS 1. Non-invasive medical scanning apparatus for generating an image of a fetus, the apparatus comprising: means for detecting a quantitative measure relating to the fetal image; means responsive to the detected quantitative measure for generating a numerical value indicative of the shape of the fetal image; and means for detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.
2. Apparatus according to claim 1, comprising means for generating an alarm signal in response to a detection that the numerical value lies in the predetermined range.
3. Apparatus according to claim 2, comprising an indicator light operable in response to generation of the alarm signal.
4. Apparatus according to claim 2, comprising means for displaying a warning message in response to generation of the alarm signal.
5. Apparatus according to any one of the preceding claims, comprising: means for detecting an outline of the fetal image; and means for detecting the lengths of two or more predetermined axes of the outline, the generating means being responsive to the two or more detected lengths.
6. Apparatus according to claim 5, in which the means for detecting the outline comprises a threshold detecting means operable to detect a predetermined luminance level of the scanned image.
7. Apparatus according to any one of the preceding claims, in which the image is an image of a fetal head.
8. Apparatus according to claim 7, comprising means for detecting the thickness of the nuchal fold of the fetal head.
9. Apparatus according to claim 7, comprising means for detecting the principal axes of the outline of a cross section of the fetal head.
10. Apparatus according to claim 9, comprising: means for detecting the width of the fetal head at a position of 80% along the major axis of the outline; and means for generating the numerical value by dividing the width by the sum of the lengths of the major and minor axes of the outline.
11. Apparatus according to claim 9, comprising: means for detecting the lengths of axes extending from the centre of the major axis and at a predetermined angle to the major axis; and means for generating the numerical value by dividing the sum of the lengths of the axes extending from the centre of the major axis and at a predetermined angle to the major axis, by the sum of the lengths of the major and minor axes of the outline.
12. Apparatus according to any one of the preceding claims, comprising means for generating an ultrasound image of the fetus.
13. Apparatus according to any one of the preceding claims, in which the predetermined range of the numerical value comprises values less than a predetermined cut-off value.
14. A method of non-invasive medical scanning, the method comprising the steps of: generating an image of a fetus; detecting a quantitative measure relating to the fetal image; generating a numerical value indicative of the shape of the fetal image in response to the quantitative measure; and detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.
15. Image processing apparatus for processing a non-invasively scanned image of a fetus, the apparatus comprising: means for detecting a quantitative measure relating to the fetal image; means responsive to the detected quantitative measure for generating a numerical value indicative of the shape of the fetal image; and means for detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.
16. A method of processing a non-invasively scanned image of a fetus, the method comprising the steps of: detecting a quantitative measure relating to the fetal image; generating a numerical value indicative of the shape of the fetal image in response to the quantitative measure; and detecting whether the numerical value lies in a predetermined range indicative of a possible fetal abnormality.
17. Non-invasive medical scanning apparatus substantially as hereinbefore described with reference to the accompanying drawings.
18. A method of non-invasive medical scanning, the method being substantially as hereinbefore described with reference to the accompanying drawings.
19. Image processing apparatus substantially as hereinbefore described with reference to the accompanying drawings.
20. A method of image processing, the method being substantially as hereinbefore described with reference to the accompanying drawings.