The invention relates to a method of deriving a dimension of a detail of an object from a data set of data values assigned to a multidimensional space and relating to the object. The invention also relates to a data processor for deriving a dimension of a detail of an object from a data set of data values assigned to a multidimensional space and relating to the object.
A method and a data processor of this kind are known from the international patent application WO 97/13457.
The data set assigns data values to positions in the multidimensional space. The data set notably comprises density values which represent the spatial density distribution of the object. The known method enables the dimensions of a detail, particularly of a blood vessel, to be derived from the data set, notably density values of a patient to be examined. Determination of stenosis of a blood vessel, necessitates accurate determination of the width of the relevant blood vessel. To this end, the known method includes the acquisition of a density profile and a maximum density value of the detail is derived from the density profile. Subsequently, according to the known method there are determined edge points where the density values in the density profile amount to half the maximum density value. The width of the blood vessel is subsequently calculated from the distance between edge points. Even though the known method for the calculation of the width of the blood vessel takes into account the fact that blurring may be involved in the measured data set, it has been found that inaccuracies occur in the measurement of the width of the blood vessel nevertheless.
Inaccuracies in the determination of the width of blood vessels from data acquired by means of computed tomography are also discussed in the article xe2x80x9cEvaluating the potential and problems of three-dimensional computed tomography measurements of arterial stenosisxe2x80x9d by David S. Ebert et al in Journal of Digital Imaging 11 (1998), pp. 151-157.
It is an object of the invention to provide a method of deriving the dimension of a detail of an object from the data set with an accuracy which is superior to that of the known method. It is notably an object of the invention to avoid inaccuracies due to differences in the spatial resolution in different directions during the determination of the dimension.
This object is achieved by a method according to the invention which is characterized in that a preferential direction is chosen in the multidimensional space, the spatial resolution of the data set of data values in the preferential direction being higher than the spatial resolution of the data values in at least one direction other than the preferential direction, the dimension of the detail being derived from data values in the preferential direction.
The data set of data values essentially has the highest spatial resolution in the preferential direction. The resolution represents the smallest dimension of details in the object which can still be faithfully reproduced by the data set. The smaller the smallest faithfully reproduced detail, the higher the spatial resolution will be. Specifically, in comparison with other directions the least blurring has occurred in the preferential direction during the measurement of the data set. It has been found that notably inaccuracies are avoided which, if no further steps were taken, would occur when the spatial resolution of the data set differs in different directions in the object. For example, when the data set is acquired by means of an X-ray computed tomography method, the preferential direction is situated in the scanning plane. This is because it appears that in the case of X-ray computed tomography the highest spatial resolution by far occurs in the scanning plane. In X-ray computed tomography an X-ray source and an X-ray detector are rotated together about the patient in the scanning plane. During the rotation of the X-ray source and the X-ray detector, the patient on the one side and the X-ray source with the X-ray detector on the other side can be displaced relative to one another, notably in the longitudinal direction of the patient, so that the X-ray source and the X-ray detector travel along a helical path relative to the patient. In that case the scanning plane is shifted relative to the patient during the rotation; the scanning plane is notably shifted along the longitudinal axis of the patient. In various orientations of the X-ray source and the X-ray detector, relative to the patient to be examined, the patient is irradiated by means of X-rays and density profiles of the patient to be examined are acquired by measuring the X-ray absorption in different directions in the body of the patient to be examined. Density values in different positions in the body of the patient are reconstructed from the values of the X-ray absorption measured in different directions. When the data values in the preferential direction is utilized to derive the dimension of the detail, the dimension is particularly accurately derived from the data set. The effects of blurring during the determination of the dimension of the detail can thus be avoided to a significant extent. The method according to the invention is particularly suitable for the determination of the dimensions of details whose dimensions hardly differ in various directions. For example, the method according to the invention is particularly suitable for determining the dimensions of the cross-section of arteries of a patient to be examined, because arteries practically always have a substantially round cross-section. The invention notably offers the advantage that an accurate result is obtained for the dimension of the detail of the object, such as the width of the artery, when the resolution of the data set is high in one direction, being the preferential direction, and is low relative to the dimension of the detail in another direction.
While the invention was described herein with an employment of an X-ray computed tomography imaging technique to acquire the data values, the invention may be employed with other techniques to acquire the data values, such as, for example, a magnetic resonance imaging technique or a 3D ultrasound imaging technique.
The data values are preferably acquired in one or more scanning planes. It has been found that the spatial resolution of the data values in such a scanning plane is substantially higher than that in directions outside the scanning plane. This means that the preferential direction is situated in the scanning plane or extends parallel to the scanning plane. The invention is particularly suitable for determining the dimensions of a cross-section of an elongate detail of the object. An artery in the body of the patient to be examined constitutes an example of such an elongate detail. The transverse plane of such an elongate detail is determined substantially perpendicularly to the longitudinal axis. According to the invention the dimension of a detail can be accurately determined by deriving the dimension from data values in the preferential direction. It has been found that the preferential direction is usually situated in the scanning plane in which the data values are acquired. This is the case notably when the data values are acquired by means of an X-ray computed tomography method. In case the transverse plane is situated parallel to the scanning plane, the dimension of the detail can be simply derived from data values in an arbitrary direction in the transverse plane, because the data values are blurred only slightly in essentially all directions in the transverse plane. The invention offers the advantage of accurate results concerning the dimension of the detail also in the case of data sets acquired by means of a magnetic resonance imaging system or a 3D ultrasound method.
Furthermore, the dimension of the detail is preferably derived from data values relating to a perpendicular cross-section of the detail, thus avoiding, the involvement of an oblique projection, relative to the longitudinal axis of the detail, in the determination of the cross-section of the detail. To this end, the (local) longitudinal axis of the detail is derived from the data values in the data set; it is to be noted, however, that the longitudinal axis of the detail can also be derived from anatomical information. The transverse plane is situated substantially perpendicularly to the local longitudinal axis of the detail. The data values which are blurred the least and relate to the cross-section of the detail lie along the line of intersection of the transverse plane and the scanning plane. The width of the detail can thus be accurately determined by deriving the dimension from the data values along the line of intersection. The occurrence of inaccuracies in the determination of the dimension which are due to oblique cross-sections or blurring of the data values is thus avoided as well as possible.
The dimensions of the detail, such as the width of the artery, are accurately determined, for example by determining the locations where strong gradients occur in the data values in the preferential direction. Such gradients represent the edges of the artery; it is notably when the artery is filled with an X-ray-absorbing contrast medium during the acquisition of the data values by means of X-ray computed tomography that the density values in the artery are substantially higher than the data values of the surrounding tissue. Another approach for accurately determining the width of the artery is to determine the locations where the data values amount to a predetermined fraction, for example half, of the maximum density value in the detail, for example the artery. It has been found that the positions in which the density value amounts to half the maximum density value offer an accurate indication of the edges of the artery.
The method according to the invention yields an accurate dimension of the detail, for example the width of the artery. This result is a technical aid that can be used by a physician in making a diagnosis in respect of arterial stenosis in the patient to be examined.
These and other aspects of the invention are apparent from and will be elucidated, by way of example, with reference to the embodiment described hereinafter and the accompanying drawing; therein: