WO2006000738A1

WO2006000738A1 - Lesion boundary detection

Info

Publication number: WO2006000738A1
Application number: PCT/GB2005/001841
Authority: WO
Inventors: Jamshid Dehmeshki
Original assignee: Medicsight Plc.
Priority date: 2004-06-23
Filing date: 2005-05-13
Publication date: 2006-01-05
Also published as: KR20070083467A; AU2005256785A1; EP1771822A1; CA2571720A1; JP2008503294A; JP5055115B2

Abstract

A method of detecting the junction between a lesion and a wall in a CT scan image, comprises determining the boundary (B) of the wall to an internal space (L), identifying critical points (c1, c2) along the boundary, and selecting one critical point at either side of the lesion as a junction point between the wall and the lesion. The critical points may be points of maximum local curvature and points of transition between straight and curved sections of the boundary. The critical points mad be selected by receiving first and second seed points (pl, p2) at either side of the lesion; moving the seed points to the boundary if they are not already located on the boundary; and finding the closest critical points to the seed points. The seed points may be determined by displacing the determined junction points (j1, j2) from an adjacent slice of the image into the current slice.

Description

Lesion Boundary Detection Field of the Invention The present invention relates to a method of detecting the junction of a lesion with a wall, such as a lung nodule attached to the pleura or a polyp attached to the colon wall, in an image previously scanned from a human or animal body, particularly but not exclusively in a computed tomography (CT) image. The invention encompasses software and apparatus for carrying out the method. Background of the Invention Detection of suspicious lesions in the early stages of cancer can be considered the most effective way to improve survival. Lung nodule detection and polyp detection are some of the more challenging tasks in medical imaging. Computer-assisted techniques have been proposed to identify regions of interest containing a nodule within a CT scan image, to segment the nodule from surrounding objects such as blood vessels or the lung wall, to calculate physical characteristics of the nodule, and/or to provide an automated diagnosis of the nodule. Fully automated techniques perform all of these steps without intervention by a radiologist, but one or more of these steps may require input from the radiologist, in which case the method may be described as semi-automated. Detection of the size or extent of a lesion is important for accurate diagnosis, but it is difficult to detect the extent of a lung nodule attached to the pleura, or to separate a nodule from the pleura, because of their similar intensity in CT scans. Likewise, it is difficult to detect the boundary between a polyp and the colon wall. Patent publications US-A-2003/0099384 and US-A-2003/0099389 disclose methods of detecting pleural nodules using morpho logical closing for small nodules, and a deformable surface model for nodules larger than the structural element used for morphological closing. Patent publication WO 03/010102 and the article 'Lung Nodule Detection on Thoracic Computed Tomography Images: Preliminary Evaluation of a Computer-aided Diagnostic System', Gurcan M et. al, Med. Phys. 29 (11), November 2002, pp. 2552- 2558, disclose a method of detecting pleural nodules using a local indentation search next to the lung pleura by finding a pair of points on a closed contour along the boundary of the lung where the ratio of the distance between the points along the boundary is greater than the straight line distance between the two points by more than a predetermined threshold. Statement of the Invention According to the present invention, there is provided a method of detecting the junction between a lesion and a wall in a scan image, comprising determining the boundary of the wall to an internal space, identifying critical points along the boundary, and selecting one critical point at either side of the lesion as a junction point between the wall and the lesion. The critical points may be points of maximum local curvature and/or points of transition between straight and curved sections of the boundary. The critical points may be selected by receiving first and second seed points, at either side of the lesion; moving the seed points to the boundary if they are not already located on the boundary; and finding the closest critical points to the seed points. The seed points may be determined by displacing the selected junction points from an adjacent slice of the image into the current slice. An advantage of this method is that the points of contact between the lesion and the pleura can be determined accurately in three dimensions, and the extent of the lesion can therefore be determined more precisely. The location of the junction points maybe fine tuned using various techniques, and may in some circumstances not coincide precisely with the determined critical points. The present invention is preferably implemented using a computer, and extends to software for carrying out the method.

Brief Description of the Drawings Figure 1 is a schematic diagram showing a CT scanner and a remote computer for processing image data from the scanner; Figure 2 is a flowchart of an algorithm in an embodiment of the invention; Figures 3a to 3c show respectively a single slice of a CT scan image of a lung, the lung area as obtained by segmentation, and the boundary of the lung area; Figures 4a to 4c show critical points detected on the lung boundary with three different thresholds; Figure 5 shows seed points at either side of the nodule; Figure 6 illustrates a method of moving the seed points from the pleura to the boundary; Figure 7 illustrates a method of moving the seed points from the lung space to the boundary; Figure 8 illustrates a method of moving the seed points to the closest critical points; Figure 9 illustrates a method of determining the best critical point when they are approximately equidistant; Figure 10 shows an example where the lung is divided into multiple boundaries; Figure 11 shows where a join between the boundaries is determined; Figure 12 is a graph of width against distance on a line midway between the two boundaries, showing how the join is determined; Figures 13a to 13d show a method of correcting overshot edges of a nodule; Figures 14a to 14d show a method of correcting overshot edges of a nodule where the detected edges lie beyond the nodule; Figure 15 shows the detected extent of the nodule; Figure 16 illustrates an alternative method for detecting the extent of the nodule; Figure 17 illustrates mapping of junction points to seed points in the next slice; and Figure 18 shows the detected extent of the nodule in a sequential series of slices.

Detailed Description of the Embodiments

CT Image Each embodiment is performed on series of CT image slices obtained from a CT scan of a human or animal patient. Each slice is a 2-dimensional digital grey-scale image of the x-ray absorption of the scanned area. The properties of the slice depend on the CT scanner used; for example, a high-resolution multi-slice CT scanner may produce images with a resolution of 0.5-0.6 mm/pixel in the x and y directions (i.e. in the plane of the slice). Each pixel may have 32-bit greyscale resolution. The intensity value of each pixel is normally expressed in Hounsfield units (HU). Sequential slices may be separated by a constant distance along the z direction (i.e. the scan separation axis); for example, by a distance of between 0.75-2.5 mm. Hence, the scan image is a three-dimensional (3D) grey scale image, with an overall size depending on the area and number of slices scanned. The present invention is not restricted to any specific scanning technique, and is applicable to electron beam computed tomography (EBCT), multi-detector or spiral scans or any technique which produces as output a 2D or 3D image representing X-ray absorption. As shown in Figure 1, the scan image is created by a computer 4 which receives scan data from a scanner 2 and constructs the scan image. The scan image is saved as an electronic file or a series of files which are stored on a storage medium 6, such as a fixed or removable disc. The scan image may be processed by the computer 4 to identify the extent of a lung nodule, or the scan image may be transferred to another computer 8 which runs software for processing the image as described below. The image processing software may be stored on a carrier, such as a removable disc, or downloaded over a network.

Nodules Attached to Lung Wall A specific embodiment is designed for detection of the boundary between a nodule and a lung wall. Peripheral pulmonary nodules often exhibit some degree of attachment to the pleural surface (on the periphery of the lung, compressed against the external boundary of the thorax). The nodules share a significant amount of their surface with the pleura. For this reason, the delineation of the boundary between pleura and nodule is a difficult task. This complexity is reduced by talcing two seed points as input on the periphery of the lung wall at opposite ends of the nodule. The seed points may be selected by a user, such as a radiologist, or determined automatically. The theory and implementation of the boundary delineation method is described below. In a first embodiment, the method comprises the following steps described in outline with reference to Figure 2, each of which is described in detail below: 1) Input the seed points (step 12) 2) Apply coarse segmentation to separate the lung from the surrounding tissue and determine the boundary (step 14); 3) Determine critical points on the contour (step 16); 4) For each slice (starting from the starting slice): a. If not at the starting slice and either of the current points are in the tissue, move the current points to the boundary (step 18). If this process fails, then terminate the process. b. If either point is in the lung space, move it towards the other point until it hits the boundary (step 20) - terminate if failed; c. Find closest critical points along the contour to the current two points (step 22); d. Fine tune the points (step 24) e. Get outline of nodule (step 26) in the current slice; f. Check the overlap with the nodule in the previous slice (step 28) - terminate if no overlap, g. Map the two points to the next slice (step 30) 5) Repeat for consecutive slices in both directions from the starting slice.

Input Seed points (Step 12) The user inspects one or more slices of a CT scan and visually identifies a potential pleural nodule. By means of a user input device, the user selects two seed points either side of the potential pleural nodule on one of the slices. The seed points need only be located either side of the pleural nodule, and need not be precisely on the boundary. Alternatively, the user may draw a box or other shape around the potential pleural nodule, using the input device. The points of intersection between the shape and the pleura may be taken as the two seed points. As another alternative, the scan image may be pre-processed to identify potential pleural nodules, and the seed points may be input from this pre-processing stage.

Segmentation (Step 14) A sub-image of a plurality (e.g. 30) of slices below and above the slice containing the seed points is selected and coarse segmentation is performed so as to separate the image into the lung space and the pleura (including any nodule attached to the pleura). The segmentation may be performed by applying a predetermined threshold to the image, since the intensity of the lung space is much lower than that of the pleura and nodules. Objects above the threshold but not connected to the pleura are not considered by this algorithm. Alternatively, in order to make the implementation more reproducible, an optimum centre slice may be found and the sub-image may be centred on the optimum centre slice. A window is chosen, for example of 60 pixels x 60 pixels x 10 slices, centred on the mid point between the two seed points. The point with the maximum intensity within the window is found and the corresponding slice containing that maximum intensity point is chosen as the centre slice for the sub-image. The segmentation may be further improved by performing an adaptive segmentation after the initial coarse segmentation. In the adaptive segmentation, the lung space obtained by coarse segmentation is enlarged using a distance transform, to obtain a mask containing the lung space and an envelope around the lung space. The enlargement factor may be 50%. The adaptive segmentation is performed with a threshold which is derived only from the intensity values within the mask. The result of the segmentation is a binary map of the slice, with pixels representing the lung space having one binary value and pixels representing surrounding tissue having another binary value. Pixels on the boundary between the lung space and the surrounding tissue are labelled as boundary pixels. As an example, Figure 3 a shows the scan image of a slice of a lung, while Figure 3b shows the result of the segmentation. Figure 3b is a binary image in which the lung space is white (binary 1) and the surrounding tissue is black (binary 0). Figure 3c shows, in white, the boundary pixels derived from the segmentation.

Determine critical points (Step 16) As a preliminary step for detecting the junction between a nodule and the pleura, critical points on the boundary are determined. Each section of the boundary is identified as concave, convex, or straight. A critical point is either a point of maximum convexity or concavity, or a transitional point between the states of concave, convex, or straight; in other words, between straight and convex or concave, or directly between convex and concave. Critical points are of interest because they can indicate a boundary between the pleura and a nodule. A specific method of determining critical points will now be described. The input is the map of the boundary contour; this could for example be a binar-y array in which the position within the array represents the x and y coordinates of each pixel, and boundary pixels are flagged as binary 1; in other words, a bitmap of the boundary. Next, a boundary pixel is selected as the starting point. This could be any boundary pixel; for example, the first boundary pixel found in a raster scan (left to right, top to bottom) of the bitmap. The algorithm then moves along the boundary, recording the angle between successive neighbouring boundary pixels. As the pixels of each slice are arranged in 2D Cartesian coordinates, there are only 8 possible angles. For each pixel, the neighbours can be represented on a grid:

The angle between each pixel and its neighbours can be encoded as follows:

Where the codes correspond to the following angles:

Therefore, the bitmap is converted to a chain code comprising a one-dimensional vector of codes representing the angle between successive neighbouring boundary pixels in a loop around the boundary, e.g. (1,1,1,2,1,3,3...). Note that these are absolute angles relative to a downwards direction. Next, the chain code is converted to an angle of curvature of the boundary at each boundary pixel. This may be done by taking the chain codes around each boundary pixel and applying a weighted filter so that the chain codes for boundary pixels closer to the current boundary pixel are more heavily weighted. Note that the angle of curvature is relative to the boundary rather than an absolute angle, so that chain codes preceding the current pixel around the loop are subtracted from those following the current pixel around the loop. Ih one example, the angle of curvature for boundary pixel z is calculated as follows:

^{■ m}J∞t+, ;=-« CC₁ = . J Σ m =-» where ni_j is the weighting factor, which may follow a Gaussian distribution; ccj_+j is the chain code for pixel (/+/), and are negative for negative j; n represents the number of pixels taken into account in each direction around the boundary; for example, n=5. The result is a one-dimension vector representing the angle of curvature of the boundary at each boundary pixel, taking into account nearby pixels in either direction around the boundary. The effect of the weighted filter is to smooth the angles from the discrete, granular values of the chain codes. Next, the local maxima and minima of the angles of curvature are identified. For example, a neighbourhood comparison method may be used in which each angle is compared with a predetermined number (e.g. 3) of preceding and following angles in the vector. To identify the transitional points between straight and concave or convex, a range of angles corresponding to 'straight' is defined. For example, all angles of 180°±f may be classified as straight. Figures 4a to 4c show the critical points for/= 5, 15 and 30° respectively. Concave maxima and transitions between straight and concave are labelled with squares, while points of maximum convexity or transitions between straight and convex are labelled with circles. Figure 4a shows that setting /too low results in a large number of critical points representing only slow transitions in the curvature of the boundary. The value of/ may be predefined in the software, and/or adjustable by the user.

Finding the nodule direction Before the nodule is detected, it is first determined in which direction, relative to the two seed points P₁ and p₂, the nodule extends into the lung. In one embodiment, as illustrated in Figure 5, a vector v is formed between the two points pi and p₂, and an orthogonal vector o is constructed from the midpoint of the vector v in both directions. The direction of o which falls into the lung space L indicates the direction of the lung and therefore the direction in which the pleural nodule N extends into the lung from the pleura P. After the initial slice, the nodule direction is determined from the direction of the detected nodule in the previous slice, relative to the seed points pi and p₂.

Move to air (Step 18) For each slice, the seed points pi and p? must first be checked to determine whether they are on the boundary B, and if not, they are moved to a suitable point on the boundary. In the initial slice, the user is not required to position the seed points pi and p₂ precisely on the boundary. After moving the seed points to the boundary, and moving the seed points to the next slice, by changing their z coordinates, the seed points may no longer be on the boundary in the new slice. If either of the seed points pi and ρ₂ is within the pleura P, that point is moved onto the boundary B. The direction and distance of movement may be limited to avoid moving away from a critical point likely to represent an interface between the nodule and the pleura. In particular, the angle of the interface is unlikely to change greatly from one slice to the next. One technique for moving seed points from the tissue to the boundary will now be described. First, it is determined whether there is any lung space L in a window (e.g. 20mm x 20mm) centred on the seed point to be moved. If there is not, then an error condition may be indicated. If there is, then the following steps are performed, as shown in Figure 6: • Draw two lines /;, h from each of the points pi, p₂ which are in the tissue, at 45 and 135 degrees respectively from the vector v, in the direction o of the nodule N, each of 10 units in length. • Get the minimum size rectangle R from the seed point to be moved to the ends of the two lines /_/, h (note that there may only be one pair of such lines if one of the points p_ls p₂ is not in the tissue); • Find the nearest point to the seed point on the boundary B, within the rectangle R. • Move the point to that boundary point provided that the distance moved is less than 10 mm. This technique applies a suitable restriction on the angle by which the vector v can rotate and the distance by which the seed points can move.

Bring two points together (Step 20) If either of the two seed points P₁ and p₂ are in the lung space L, that point is moved towards the other point until it hits the boundary B, as shown in Figure 7. If the moved points are within a predetermined distance (e.g. 2 mm) of each other, the process is terminated as this indicates that there is only air between the points.

Find closest critical points (Step 22) The seed points pi and p₂ should now both be on the boundary B. Next, they are moved to the closest critical points along the boundary B, subject to certain conditions which are designed to avoid a poor choice of critical point, as explained in more detail below. For each seed point, the closest critical point is determined along the boundary B in either direction. If the critical point in one direction along the boundary is significantly closer than the critical point in the other direction, then the seed point is moved to the closer critical point. For example, if the distance between the seed point and the closest critical point in one direction is greater than a threshold, the seed point is moved to the closest critical point in the other direction. The threshold may be a function of the distance between the seed point and the closer of the two closest critical points in either direction, for example twice that distance. If the closest critical points in either direction are approximately equidistant, a critical point is selected based on the amount of lung space in a straight line between the seed point and each critical point. For example, as shown in Figure 9, if the difference between the distances between the seed point P₁ and the two closest critical points C₁, C₂ along the boundary B is less than the threshold, a line I₃ is constructed from the seed point pi to each of the two closest critical points C₁, C₂, and the proportion of the line length which passes through the lung space L is determined. If the proportion is more than a predetermined threshold (e.g. 20%) then the critical point is considered a bad choice and the seed point is moved to the closest critical point C₂ in the other direction, if the proportion for that critical point is less than or equal to the threshold. If both critical points have a proportion of greater than the threshold, then the seed point is not moved to either critical point.

Moving the points if too close hi some cases, one of the seed points might be relatively far from the best critical point and therefore, when finding the closest critical point, both seed points will be moved to the same critical point, or to critical points very close to each other. This may happen when the seed points are mapped from a previous slice and especially when fine segmentation movement is applied, as will be described below. If the distance between the two seed points along the boundary is less than a threshold distance, such as 5 mm, the seed point closer to the critical point is moved to that critical point while the other seed point is moved along the boundary to the nearest critical point away from the other seed point, provided that the distance between the two seed points does not increase beyond a predetermined threshold, such as the original distance between the seed points. In other words, the distance between the seed points after moving one away should not be more than double the distance before moving away.

No critical point nearby In some cases there is no critical point near to one of the seed points, which may cause that seed point to move to the closest critical point to the other seed point. This may be avoided by preventing a seed point from being moved along the boundary by more than a predetermined proportion, such as 50%, of the distance between the seed points before moving; in that case, the seed point far away from any critical point is not moved.

Fine Segmentation (Step 24) After finding the nearest critical points using the coarse segmented boundary, the movement of the seed points may be fine tuned by re-evaluating the boundary B in the vicinity of the seed points using fine segmentation, dependent on the local contrast. For example, the threshold for determining whether each point is foreground or background may be determined by the intensities of points within a mask of local points of predetermined extent surrounding that point. The threshold may be selected so that the mean of the centroids of intensities above and below the threshold is equal to the threshold, or offset from the threshold by a predetermined degree. Next, the critical points are determined on the fine-segmented boundary. The seed points may be moved to the closest critical point on the fine segmented boundary, subject to certain conditions, as set out below by way of example: - the movement of the seed points to the fine-segmented boundary is always away from the direction of nodule. - the distance moved must be less than a predetermined threshold. The threshold is set according to how much the seed point was moved from its initial point within the slice to its final point using coarse segmentation. - the angle by which the line between the seed points rotates when moving to the fine segmented boundary critical points must be less than a predetermined angle, such as 40 degrees. The coarse segmented boundary might not be near the fine-segmented boundary of the nodule where there is a strong partial volume effect, hi this case the nearest critical point might be away from the nodule; only movement away from the nodule is allowed, so the correct critical points might not be found. This may be solved by, if the seed points are in the lung relative to the fine-segmented boundary, moving the seed points towards each other until they hit the boundary, and then finding the closest critical points on the fine- segmented boundary. The effect of fine-segmentation is to remove the effect of material around the nodule, such as infected tissue, which obscures the junction of the nodule to the pleura. Coarse segmentation will normally include the material as part of the boundary, whereas fine segmentation will reclassify the material as background. Multiple Boundaries In some cases, the lung L is split into two or more parts by the coarse segmentation, and the seed points are located in different parts, as illustrated for example in Figure 10. This can occur when a blood vessel extends across the lung in the current slice, and may cause difficulties in determining the extent of the nodule N by including the blood vessel as part of the nodule N. To overcome this problem, it is determined whether the seed points, when moved to the boundary B, form part of different, unconnected boundaries. If so, the variation of the distance between the two boundaries is calculated, proceeding from between the two seed points in the direction o of the nodule, as shown in Figure 11. A sample graph of the variation of the distance is shown in Figure 12. The point g of greatest change in the gradient of the graph is taken and the boundaries B are joined at that point. This allows the nodule N to be separated from the blood vessel.

Contour correction hi some cases the boundary between the detected junction points has overshot edges, as shown for example in Figure 13 a; the critical points might not be at the exact location of nodule attachment to the lung wall, for example because of slow angular changes at the junction. To avoid this problem a contour correction function may used to re-adjust the junction points to allow correct region extraction, as shown in Figures 13b to 13d. First, a line is constructed between the junction points. If the line intersects the boundary, then one junction point is moved towards the other along the boundary until there is no intersection with the boundary on that side. Then the other junction point is moved towards the first junction point until the line no longer intersects the boundary. In some cases, particularly when the nodule is small, the line between the junction points when the edges are overshot does not intersect the boundary, as shown in Figure 14a. In this case, the junction points are moved together along the boundary, as shown in Figure 14b, alternately by a small increment until the line intersects the boundary, as shown in Figure 14c. The contour correction then proceeds as described above, until the line no longer intersects the boundary, as shown in Figure 14d.

Get Nodule Boundary At this stage two junction points have been determined, representing the points along the boundary B at which the nodule N joins the pleura P. These junction points may be used to estimate the boundary of the nodule N within the slice. As a first approximation, a straight line joining the junction points is used to define the junction between the nodule N and the pleura P. Alternatively, a curved line may be used, with a curvature estimated from that of the pleura surrounding the nodule N. Next, the extent of the nodule N is determined. In one embodiment, it is assumed that the nodule N is not attached to or proximate to any features other than the pleura P. The result of the segmentation in step 14 is used to identify all of the foreground region beyond the line joining the junction points as forming part of the nodule N, as shown in Figure 15. In an alternative embodiment, a fuzzy contrast region growing scheme is used to identify the extent of the nodule N. First, the extent of the nodule N is estimated by means of a local threshold-based segmentation process. A local mask is used to derive a threshold intensity value for each point. The threshold is therefore sensitive to the local contrast and can distinguish low contrast nodules from their background. The nodule area N acquired by fine segmentation may include 'holes' i.e. background pixels surrounded by foreground pixels. The holes may result from the sensitivity of the fine segmentation to small local differences in contrast. A suitable hole-filling algorithm is used to fill such holes, in other words to convert them to foreground pixels. Next, the pixel with the highest intensity within the nodule area N is taken as a seed point, and binary region-growing is performed from that seed point to identify a connected foreground region F. This region is enlarged using a distance transform method with an enlargement factor, to obtain a mask M containing both foreground and background. The enlargement factor may be 50%. However, as shown in Figure 16, the mask M does not include any pixels on the other side of the line joining the junction points, including a projection of that line beyond the junction points. Next, the foreground pixels segmented by the fine segmentation process that are not part of the connected foreground region F are removed from the mask M. The background pixels of the mask M are labeled as the background region B . The mean and the standard deviation of the intensity and gradient distributions of the foreground and background regions F and B are determined. The mean of the background intensity μ_B , the mean of the foreground intensity μ_F, the standard deviation of the background intensity σβ and the standard deviation of the foreground intensity σp are calculated. A parameter ε is estimated by counting the number of the foreground standard deviations σ_F that the seed point is away from the background mean intensity μ_B and this is taken as the measure of the contrast, which is subsequently used in constructing a fuzzy map as described below. A fuzzy object extraction technique is used to define a 2D map of fuzzy connectivity of each pixel within the mask M with respect to the seed point. A fuzzy affinity function between adjacent pixels is defined and the fuzzy connectivity between each pixel and the seed point is derived by finding the affinity along a path between the pixel and the seed point. The fuzzy connectivity between two points (not necessarily adjacent) is obtained by considering the path with the strongest affinity between two points. The path with the strongest affinity is chosen as the best path and the strength of each path is equal to that of the weakest affinity of adjacent points along the path. The strength between two adjacent points is the affinity between those points. The affinity between two pixels is a measure of the probability that they belong to the same object. This probability is a function of closeness (i.e. Euclidian distance) and the similarity of the image features (i.e. intensity) between those pixels. The general model for fuzzy affinity is given by: μ_k = (c, d) - h(μ_a(c,d), f(c), f(d), c, d) where h is a scalar value with range [0,1], c and d are image locations of two pixels, and f(i) is the intensity of spel i. μa is an adjacency function based on distance between two pixels which is given by,

[0, otherwise A simplified shift invariant version is defined as μ_k (C, d)

if c≠d, and μ_k (c, c) = \ where the subscripts 'i' represents the calculations related to intensity and 'gk' represents the calculations related to gradient values in relevant direction (which could be x, y, z) respectively, CDJ and ω_g are free parameter weight values whose sum is 1. The value of 0.9 for COj and 0.1 for ω_g has been chosen to allow intensity similarity to have more effect. The fuzzy affinity that was used for the current function is:

where m,-, s_\, m_g and s_g are the Gaussian parameters for the intensity and gradient. These can be predefined, or are estimated from a small region around the seed point as described below. The mean and standard deviation σ of the intensity and gradient are calculated over all points within the mask M. The parameters related to the gradients are computed in three directions (x, y and z) separately. The corresponding σ are calculated based on the difference between maximum and minimum gradient. The calculation of the statistics is now described in more detail. The parameter m_t is taken as the intensity of the seed whereas m_gx , m^. are taken as the means of the gradients in x- and y- direction respectively. The parameters S^ , S₃, are the standard deviation of the gradients in their respective direction. The standard deviation (S _d) appearing in the affinity expression affects the formation of the fuzzy map and hence the determination of the boundary of the nodule. If it is too big, the affinity curve will be relatively flat. Resultantly, the background region will have higher affinity and region growing will result in over-segmentation. On the other hand, if it is too small, the shape of the affinity curve will be narrow and the foreground will have less affinity with the seed and the result would be under-segmented. Ideally, the curve should be spread to such an extent that the background has minimal but finite affinity with the seed. The affinity Gaussian curve is therefore limited or extended by modifying the standard deviation so as to achieve the ideal spread. A fuzzy map is then constructed by finding the fuzzy connectivity value for each pixel relative to the seed point. The fuzzy map may be considered as an enhanced image whose pixels represent how strongly are attached to the seed point. Next, contrast based region growing is applied to the fuzzy map, starting from the seed point. As each pixel is added to the region during region-growing, the peripheral contrast is calculated between the internal and external boundaries of the region. The peripheral contrast of the region may be defined as the difference between the average grey level of the internal boundary and average grey level of the current boundary. At each iteration of the contrast-based region growing, one pixel is selected from the current boundary and added to the current region. The selection priority of pixels in the current boundary is determined on the basis of their intensity and the distance to the centre of the current region. As each pixel is added to the region during region-growing, the peripheral contrast is calculated between the internal and external boundaries of the region. The peripheral contrast of the region may be defined as the difference between the average grey level of the internal boundary and average grey level of the current boundary. The peripheral contrast at each stage is recorded, and the region growing continues until the region fills the mask M. The highest peripheral contrast value obtained during region growing is selected as indicating the optimum region, with a boundary most likely to correspond to that of the nodule N. Map to next slice (Step 28) After the extent of the nodule N in the current slice has been determined, it is determined whether the nodule N in the current slice overlaps the determined extent of the nodule N in the previous, adjacent slice. This may be done in a variety of ways, for example by determining a circular 'core' of a predetermined radius surrounding the centre of the nodule N in each slice, and determining whether the cores overlap in adjacent slices; i.e. if there are any pixels having the same x and y coordinates between the two cores. If there is no overlap, or no nodule has been detected at all in the current slice, this may indicate the limit of the nodule N in the current direction from the starting slice. If the algorithm has proceeded in only one direction from the starting slice, it then proceeds in the other direction. If nodule boundary detection has been performed in both directions, then the algorithm ends, and outputs the detected extent of the nodule in each of the slices in which the nodule N has been detected. If there is overlap with the previous slice, the algorithm proceeds to the next slice, using the junction points J₁, J₂ from the current slice as the two input seed points. P₁ , p₂ for the next slice, as shown in Figure 17. For example, the seed points for the new slice have the same x and y coordinates as the junction points for the previous slice. Results Figure 18 shows the detected extent of the nodule N in a sample scan image. Applicability to the colon The embodiment described above can also be applied to the detection of polyps in a CT scan image of the colon. Polyps are always attached to the colon wall, which has a similar intensity in the CT scan image. Although the shape of polyps is somewhat different from that of lung nodules, the junction between the colon wall and the polyp has similar geometrical properties, and the embodiment may be used to detect that junction. The present invention may be applied to the detection of other abnormal growths attached to walls of internal spaces of the human and/or animal body. Alternative embodiments The embodiments above are described by way of example, and are not intended to limit the scope of the invention. Various alternatives may be envisaged which nevertheless fall within the scope of the claims. As will be apparent from the above discussion, the method can be performed on a 2D image consisting of a single CT slice, or a 3D image consisting of consecutive CT slices.

Claims

Claims 1. A method of identifying the junction between a lesion and a wall adjacent an internal space in a computed tomography scan image, comprising:

a. determining a boundary between the wall and the internal space;

b. determining one or more critical points along the boundary; and

c. selecting first and second junction points on the boundary at either side of the lesion, based on the location of at least one critical point on either side of the lesion, whereby the junction points identify the junction between the lesion and the wall.

2. The method of claim 1, wherein the critical points include at least one point of maximum curvature along the boundary.

3. The method of claim 1 or claim 2, wherein the critical points include at least one point of transition between straight and curved.

4. The method of any preceding claim, including receiving as input first and second seed points at either side of the lesion, wherein the first and second junction points are selected based on the location of the first and second seed points.

5. The method of claim 4, wherein first and second ones of the critical points closest respectively to the first and second seed points are selected respectively as the first and second junction points.

6. The method of claim 5, wherein the distance from the seed points to the critical points is measured along the boundary so as to determine said closest junction points.

7. The method of claim 6, wherein the seed points are moved to the boundary so as to determine their distance along the boundary to the critical points.

8. The method of claim 7, wherein if one of the seed points is located in the internal space, that seed point is moved to the boundary in a direction towards the other one of the seed points.

9. The method of claim 7, wherein if one of the seed points is located within the wall, that seed point is moved to the boundary such that the angle of rotation of a line between the seed points is limited.

10. The method of claim 8 or claim 9, wherein if one of the seed points is located within the wall, it is moved to the boundary if the distance to the boundary is less than a predetermined threshold.

11. The method of any one of claims 5 to 10, wherein for each seed point, the closest critical point in either direction along the boundary is identified and the closer of the closest critical points is selected as the corresponding junction point if the distances along the boundary to the closest critical points in either direction satisfy a predetermined criterion.

12. The method of any preceding claim, wherein if a line between the first and second junction points intersects the boundary, the first and/or second junction points are moved towards one another along the boundary until the line no longer intersects the boundary.

13. The method of any one of claims 1 to 11 , wherein if a line between the first and second junction points lies beyond the lesion, the first and second junction points are moved towards one another along the boundary until the line intersects the lesion but does not intersect the boundary.

14. The method of any one of claims 4 to 11, wherein steps a to c are performed on each of a sequential series of slices of the scan image, wherein the determined first and second junction points of one of the slices are used to generate the first and second seed points for an adjacent one of the slices.

15. The method of claim 14, wherein steps a to c are performed on each of a sequential series of slices of the scan image starting at a predetermined starting slice and proceeding in either direction from the starting slice until the end of the lesion is detected.

16. The method of claim 15, wherein the end of the lesion is detected if no lesion is detected between the seed points in the current slice.

17. The method of claim 15 or claim 16, wherein for each of the sequential series of slices, the extent of the lesion is detected, and the end of the lesion is detected if the extent of the lesion detected in the current slice does not overlap the extent of the lesion detected in an adjacent slice.

18. The method of any preceding claim, wherein the junction between the lesion and the wall is defined by a junction line extending between the first and second junction points, the method further including determining the extent of the lesion such that it is limited by the junction line.

19. The method of claim 18, wherein the extent of the lesion is determined by segmenting the scan image and performing region-growing, limited by the junction line.

20. The method of claim 19, wherein if the first and second junction points are not joined by the boundary, the extent of the lesion is further limited by a boundary connecting line joining separate sections of the boundary on which the junction points are located.

21. The method of claim 20, wherein the boundary connecting line is defined by determining the variation in distance between the separate boundary sections, and identifying the line at which the rate of change of the variation in distance is a maximum.

22. The method of any one of claims 18 to 21 , wherein the extent of the lesion is determined by performing fine segmentation to separate the image around the lesion into foreground and background, using a threshold based on the local contrast; identifying a connected foreground region representing the lesion, limited by the junction line; expanding the foreground region to include background points and to exclude foreground points not included within the foreground region, to create a mask; deriving a fuzzy connectivity map defining the connectivity of points within the mask to a seed point within the lesion; and determining the extent of the lesion from the fuzzy connectivity map.

23. The method of claim 22, wherein the extent of the lesion is determined from the fuzzy connectivity map by growing a connectivity region from the seed point within the fuzzy connectivity map, and outputting the region having the highest boundary contrast as the extent of the lesion.

24. A method of identifying the extent of a lesion attached to a wall in a computed tomography scan image of a lung, comprising:

a. identifying a junction between the lesion and the wall; b. performing fine segmentation to separate the image around the lesion into foreground and background, using a threshold based on the local contrast;

c. growing a foreground region representing the lesion, limited by the junction;

d. expanding the foreground region to include background points while excluding foreground points not included within the foreground region, to create a mask;

e. deriving a fuzzy connectivity map defining the connectivity of points within the mask to a seed point within the lesion; and

f. determining the extent of the lesion from the fuzzy connectivity map.

25. The method of claim 24, wherein step f comprises growing a connectivity region from the seed point within the fuzzy connectivity map, and outputting the region having the highest boundary contrast as the extent of the lesion.

26. The method of any preceding claim, wherein the lesion is a nodule and the internal space is a lung.

27. The method of any one of claims 1 to 25, wherein the lesion is a polyp and the internal space is a colon.

28. A computer program arranged to perform the method of any preceding claim.

29. A computer program product comprising a medium recording the computer program of claim 28.