HK1160747B - 3d mammography - Google Patents

Description

3D mammography

Technical Field

The present invention relates to 3D mammography, in which individual images of a breast are taken at different projection angles, typically within an angle of about +/-30 degrees from the vertical, and from which the 3D images are subsequently synthesized by means of suitable image processing software.

Background

Breast cancer is the most common type of cancer in women. According to a survey, about one out of every ten women will be infected with breast cancer at some point in their lifetime. When breast cancer is detected on a symptomatic basis, the disease has often progressed to a stage where the prognosis is rather poor in recovery. Some cases were detected in screening programs scheduled in many countries for women over the age of 40. Screening usually finds the cancer at a very early stage so that its treatment can begin in a timely manner and thus is more likely to recover.

Mammography is a widely used method in breast cancer screening, as a clinical examination method, and in subsequent diagnosis. Mammography is an x-ray imaging method in which a device designed specifically for this purpose is used. In screening studies, mammography has been reported with a sensitivity of 90-93% and specificity of 90-97%. This suggests that screening studies are useful and that early detection of breast cancer by screening can save human life. Mammography has been demonstrated to reduce breast cancer mortality by 35% in women over the age of 50 and 25% -35% in women 40-50 years of age.

Mammography images are examined to detect various abnormalities in the breast, such as calcification, i.e., small calcium deposits in the soft tissue of the breast. Calcifications are generally not detectable by touching the breast, but are visible in x-ray images. Large calcifications are generally not associated with cancer, but smaller clusters of calcium deposits, so-called microcalcifications, are indicative of external breast cell activity, which may be associated with breast cancer. Other features to be detected by mammography include cysts and fibroadenomas, however, these are generally not associated with cancer.

In conventional screening mammography, the breast is typically compressed between two compression plates and exposed to radiation at least twice from above and from an oblique direction. If necessary, a third image is additionally taken at right angles from the side. Since in such imaging the tissue layers are in a top-to-bottom relationship with each other in the direction of the x-ray beam, these illuminations produce a two-dimensional image in which strongly absorbing structures may hinder the detection of structures located underneath them.

The continual improvement of mammography has resulted in new types of mammography methods and apparatus that produce 3D images of a patient's breast. Here, several projections of the breast at different angles are generated and its 3D distribution is created by using an applicable reconstruction algorithm. From the image information, i.e. the individual images, several images representing layers of the breast oriented parallel to the surface of the x-ray detector are typically constructed, thus making it possible to detect tissue structures located in a top-bottom relationship to each other.

A typical digital mammography apparatus comprises a frame part and a C-arm or corresponding structure rotatably connected to the frame part. An x-ray source is arranged at a first end of the C-arm and a radiation detector is arranged at a second end. The term imaging device is often used for these devices. Generally placed in the region between the x-ray source and the detector, typically near the detector, compression plates are arranged, which are designed to position the breast by compression during the duration of the exposure.

In the prior art, in the context of 3D mammography, various approaches to imaging a breast at a plurality of different projection angles have been used or suggested. These include: continuously rotating the x-ray source at a constant or alternating speed along a curved path around the breast, rotating the x-ray source in steps between exposures in which the x-ray source remains stationary, and using a plurality of stationary x-ray sources. With respect to the detector, it may remain stationary, move linearly and/or tilt so that it remains at right angles to the central ray of the x-ray beam at each exposure.

The x-ray source located at the (upper) end of the C-arm is a relatively heavy element. In case of a stepwise movement of the x-ray source, the imaging device should have reached a vibration-free state before each exposure. Thus, the structure of the mammography apparatus should be optimized in view of the number of accelerations, decelerations, and stops (settling times) included in the multi-stage imaging process. The overall time required for an imaging process like this tends to become very long.

On the other hand, in case of continuous motion of the x-ray source, a significantly short exposure time, e.g. less than 50ms, has to be used to avoid the generation of motion artifacts. This in turn requires the use of a sufficiently strong radiation source, which means that even heavier x-ray sources than those typically used in prior art 2D mammography apparatuses are used, so that also other structures of the imaging apparatus have to be designed taking this greater mass into account.

With regard to the arrangement of several x-ray sources in a mammography apparatus, this obviously requires a completely new design of a mammography apparatus in order to make it possible to implement such a specific 3D imaging modality. Based on this mechanical design, it is a challenge to be able to propose a structure that makes the device as practical for use in conventional 2D screening mammography.

Disclosure of Invention

The object of the present invention is to focus on eliminating or reducing at least some of the problems of the imaging systems discussed above. The object of the invention is achieved by the method and the device of the appended independent claims. Some preferred embodiments of the invention are presented in the appended dependent claims.

The present invention makes 3D mammography possible with existing types of mammography apparatuses, i.e. with the same type of x-ray source and C-arm and the associated structures typically used, by enabling a rather long exposure time during the imaging process (even with a continuously moving x-ray source). This is made possible by having the breast follow the movement of the x-ray source during at least one exposure phase of the imaging process. Since the tomographic angle (the angle between the extreme exposure positions of the x-ray source) used in the imaging process may be several tens of degrees, so that a rotation of the breast during multiple exposures is possible in practice, a preferred embodiment of the process cycle of the invention comprises the step of rotating the breast back to its previous/initial position during the (each) non-exposure period of the imaging process.

One of the essential advantages of the invention is that the construction of such a device in a mammography apparatus is much simpler than arranging a corresponding course of motion for the radiation source: the device is capable of repeatedly rotating and stopping the breast during the imaging procedure (i.e., rotating and stopping a breast holding device such as a compression plate). In the present invention, as far as such a radiation source and the structure for moving the radiation source are concerned, there is no need for any special arrangement or basic redesign of the apparatus, but the conventional design used in prior art 2D mammography can be utilized.

Drawings

Some embodiments of the invention and their benefits are described in detail below, also with the aid of the figures, in which

Fig. 1 shows a structure of a typical mammography apparatus.

Fig. 2a and 2b show the movement of an x-ray source of a mammography apparatus according to a prior art method to obtain image information for 3D mammography.

Figures 3a and 3b show the movement of some structures of a mammography apparatus according to the invention, an

Fig. 4 shows a C-arm of a mammography apparatus with an arrangement for pulling tissue into the volume between the compression plates of the apparatus.

Detailed Description

The exemplary mammography apparatus 1 presented in fig. 1 comprises a body part 11 and a C-arm structure 12 connected thereto. Typically, a radiation source 13 and an image data receiving device 15, for example arranged within a so-called lower shelf structure 14, are provided at opposite ends of the C-arm 12. These said means 13, 15, which are located in the cover of the device, are practically invisible in fig. 1.

Furthermore, in the area between said means 13, 15, typically in the vicinity of the image data receiving means 15, means 16, 17 for positioning/locking the object to be imaged within the imaging area are placed. Today, typically such devices are motorized such that the C-arm 12 is arranged to be movable in a vertical direction and rotatable about an axis (typically the physical horizontal axis connecting the C-arm to the body part 11). The positioning/locking means 16, 17 typically comprise an upper compression plate 16 and a lower compression plate 17, which lower compression plate 17 may be arranged integral with the lower shelf structure 14. Within the lower shelf, a grid structure may be located above the image data receiving means 15, which grid structure limits the radiation scattered from the tissue from entering the image data receiving means 15. In the context of the present invention, it is actually necessary that the position of the axis of rotation of the C-arm 12 relative to the compression plates 16, 17 (locking means) is arranged in such a way that the patient can remain in the same position for exposure regardless of the angle of inclination of the C-arm. Such a configuration of a mammography apparatus of this type has been taught in european patent publication 370089.

Fig. 2a and 2b show a prior art system for obtaining image information for 3D mammography. For the sake of clarity, in fig. 2a, 2b, 3a, 3b, the actually cone-shaped x-ray beam originating from the focal spot of the x-ray source 13 is not shown, but only the central ray.

In the prior art system according to fig. 2a, the x-ray source 13 is arranged to move in a continuous manner from a starting position of the imaging process to an ending position of the imaging process, and during this movement the x-ray source 13 is activated for the duration of a plurality of short exposure periods, while the compression plates 16, 17 (and in fig. 2a also the detector 15) remain stationary. The image information detected at the detector 15 is stored and/or transmitted for image processing. In this type of configuration, the use of conventional anti-scatter grids is not possible because, in addition to the exposure angle parallel to the orientation of the grid sheets, the grid will also absorb a portion of the required amount of x-rays at all other exposure angles.

In the prior art system according to fig. 2b, on the other hand, the x-ray source 13 is moved in a stepwise manner, so that for each exposure the x-ray source stops at a predetermined angular position. In fig. 2b, three such stationary exposure positions of the x-ray source 13 are shown.

Fig. 3a and 3b show two basic operating phases of the current mammography imaging invention. Fig. 3a can be seen as showing an exposure phase and fig. 3b can be seen as showing a non-exposure phase of the system, as well as extreme positions of the x-ray source 13. In these figures, these extreme positions of the x-ray source 13 with respect to the vertical direction indicate the width of the tomographic angle of the system, whereas in fig. 3a two positions close to the vertical position of the x-ray source 13, and the corresponding positions of the breast locks 16, 17 and the detector 15, depict the core operational phases of the system according to the invention, and in fig. 3b depicts those phases of a certain preferred embodiment of the invention. In the embodiment of the invention shown as a whole in fig. 3a and 3b, during the exposure phase (fig. 3a) the compression plates 16, 17 are arranged to rotate in synchronism with the movement of the x-ray source 13, whereas during the non-exposure phase (fig. 3b) they rotate in opposite directions. In this embodiment of the invention the detector 15 is arranged to rotate together with the compression plates 16, 17.

The synchronized movement of the x-ray source 13 and the compression plates 16, 17 according to the invention makes it possible to avoid the generation of motion artifacts which are always present when imaging a breast according to the prior art method of fig. 2a, wherein there is a mutual movement between the x-ray source 13 and the breast during exposure. The invention also makes it possible to use longer exposure times than this method, and without having to use a powerful and thus heavy x-ray source.

On the other hand, the time required for the whole imaging process will be significantly shorter than the time required for the prior art process according to fig. 2b, since the x-ray source 13 does not have to be stopped for the duration of the (each) exposure.

Since there are multiple exposures in 3D mammography, the compression plates 16, 17 are repeatedly rotated during each exposure period only in the direction of movement of the x-ray source 13 (and are kept stationary during the non-exposure period), which in total rotates the compression plates 16, 17, for example, by 15 degrees, which can make the imaging process uncomfortable for the patient. To avoid this problem, a preferred embodiment of the invention comprises the following operating phases: during which the x-ray source 15 is not activated (non-irradiation period) and the compression plates 16, 17 (and the detector 15) are rotated in a direction opposite to the direction of movement of the x-ray source 13. According to a preferred embodiment of the invention as shown in fig. 3b, the compression plates 16, 17 and the detector 15 are rotated back to their initial positions at the beginning of the previous illumination period.

The angle by which the compression plates 16, 17 are to be turned can be arranged to be a very small angle and the non-exposure period is longer than the exposure period, so that there will be a lot of time for establishing a stable start-up state for the subsequent exposure period. In other words, according to a preferred embodiment of the present invention, there will be a lot of time for reversing the breast, since the period during which the x-ray source 13 is not activated is arranged to be significantly longer than the period during which it is activated. Considering the example with a typical prior art mammography apparatus and using 12 exposures initiated at 5 degree intervals, the compression plates 16, 17 can be rotated 2 degrees or even less, which easily leaves enough time for the reverse rotation, even considering the time required for acceleration and deceleration of the motion.

In a preferred embodiment of the invention, both the radiation source 13 and the detector 15 are moved around the breast at the same substantially regular angular velocity, as shown in fig. 3a, while the breast is compressed between the compression plates 16, 17, or locked in other means arranged for this purpose.

In practice, with the embodiment as shown in fig. 3a and 3b, it is necessary that the compression plates 16, 17, or said other locking means for the breast, are arranged to be turned a short distance at most from the centre of rotation of the radiation source 13, because during the imaging procedure according to the invention it is not possible to reposition the patient for different projection angles of the radiation.

According to a particularly preferred embodiment of the invention, first, during the exposure of each projection image, the compression plate 16, 17, or said other locking means, is rotated synchronously with the movement of the radiation source 13. Thus, the breast will remain stationary relative to the radiation source 13 during each such irradiation period. Then, secondly, between irradiation periods, the compression plates 16, 17 are rotated back to their position at the beginning of the previous irradiation period. As a result, the total angle over which the locking means 16, 17 will rotate only needs to be as small as the rotation angle needed to create motion synchronization during a single exposure of one projected image. This angle may be arranged to be less than 2 degrees, such as 0.5-2 degrees, for example, which would be tolerable for stretching of the patient. Thus, even in view of the required acceleration and deceleration, as discussed, in the case of images taken at 5 degree intervals within, for example, say, a 50 degree tomographic angle, there will still be a significant amount of time for the locking means (e.g., the upper and lower compression plates 16, 17) to return to their initial positions at the beginning of the exposure phase.

More generally, in the system according to the invention, the breast to be imaged is arranged to be locked in the locking means 16, 17 and during the imaging procedure the x-ray source 13 is moved relative to the position of the breast to be imaged and the breast is irradiated during a plurality of irradiation periods starting at a plurality of angular positions of the x-ray source 13. During the imaging process, the x-ray source 13 is continuously moved and the breast is irradiated during a number of short irradiation periods, and during the period during which the breast is being irradiated, the locking means 16, 17 are moved in synchronism with the movement of the x-ray source 13. With regard to a mammography apparatus according to the invention, which comprises a body part 11 and arranged thereto an x-ray source 13, an image detector 15, and means 16, 17 for locking the breast in the region between the x-ray source 13 and the image detector 15, the x-ray source 13 is arranged movable with respect to the position of said locking means 16, 17. Furthermore, the apparatus comprises a control system arranged to control the operation of the apparatus. The locking means 16, 17 are arranged rotatable and the movement of the locking means 16, 17 and the x-ray source 13 is arranged motorized and the operation of the x-ray source is controlled by the control system such that during the imaging procedure the x-ray source 13 is continuously moved and the breast is irradiated during a plurality of short irradiation periods and during the irradiation periods the locking means 16, 17 are rotated in synchronization with the movement of the x-ray source 13.

The imaging process may comprise a phase before the first irradiation period, wherein the locking means 16, 17 are turned in a direction opposite to the direction when moving synchronously with the movement of the x-ray source 13. Between any number of successive illumination periods there may be periods of backwards movement of the locking means 16, 17. The length of the backward rotational movement of the locking means 16, 17 (and possibly also the length of the backward rotational movement of the image detector 15) may either be exactly the same as during the exposure period, i.e. the locking means 16, 17 may be moved back to its initial position at the beginning of the previous exposure period, or the backward movement may be shorter or longer than the movement that occurred during the previous exposure. The length of the retreat movement need not be any exact multiple of the step of movement during exposure. As an exemplary embodiment of the invention, the imaging process may comprise steps of two exposure periods, between which the locking means 16, 17 are not rotated in any direction, but after the second of these exposure periods, the backward movement will correspond to the total movement of the locking means 16, 17 during these two exposure periods.

During the imaging procedure, the extreme positions of the x-ray source 13 relative to the breast may be arranged to make up a few tens of degrees (e.g. about 50 degrees) of tomographic angle. In a preferred embodiment, the entire movement of the x-ray source 13 is arranged to be symmetrical with respect to the vertical, i.e. the entire tomographic angle is about plus or minus 25 degrees with respect to the vertical. Preferably, the movement of the x-ray source 13 is arranged to follow a curved path, as is the case in typical existing mammography apparatuses, while the principles of the present invention can also be implemented when moving the x-ray source linearly.

Viewed from another point of view, the ratio between the slight individual rotational angle of the locking means 16, 17 relative to the overall displacement of the x-ray source 13 during the exposure period may be arranged to be of the order of 1/10. The imaging process may be arranged to include about 11-15 exposure periods.

Even though varying breast thicknesses and required speed of the x-ray source 13 may affect optimal implementation, preferred embodiments of the present invention include the use of an x-ray source 13 comprising a tungsten anode, and with appropriate arrangements, such as the use of a selenium-based imaging detector and particularly a silver filter of appropriate thickness to absorb low energy x-ray doses that are not capable of penetrating breast tissue, may result in reduced radiation doses when compared to certain other equipment. In the context of a preferred embodiment of the present invention, an exposure time for the projected image of around 50-100ms may be used, and an x-ray tube voltage of around 35-40kV (even up to 45kV) and imaging parameter values of about 5mAs may be used. For kV values of about 30-34, mAs values of about 10-13 may be used.

A preferred embodiment of the invention comprises an arrangement in which, in functional connection with the locking means, means are arranged for pulling tissue into the space between the compression plates 16, 17. Such means may be arranged to include, for example, an arrangement as shown in figure 4 in which the upper and lower tension means 30 are integral with both the compression plates 16, 17. The stretching device 30 may be arranged to include means for engaging and pulling the stretching device, such as a plastic sheet 31, so that, in conjunction with compressing the breast between the compression plates 16, 17, breast tissue will be pulled between the compression plates 16, 17 when positioning the breast for imaging. Such an arrangement enables the use of compression forces in the context of the present invention that are about 10% less than typically used in prior art mammography, which makes the imaging process involving compression and rotation of the breast more convenient.

The invention is applicable both for use in the context of so-called full field size and smaller imaging detectors used in mammography.

Claims (20)

1. A mammography imaging method, wherein a breast to be imaged is arranged locked in a locking device (16, 17) in a region between an x-ray source (13) and an image detector (15) of a mammography apparatus, and wherein during an imaging procedure the x-ray source (13) is moved relative to the position of the breast and the breast is irradiated at a plurality of angular positions of the x-ray source (13) relative to the position of the breast, characterized in that during the imaging procedure the x-ray source (13) is continuously moved and the breast is irradiated during a plurality of short irradiation periods, and during an irradiation period the locking device (16, 17) is rotated synchronously with the movement of the x-ray source (13) to follow the movement of the x-ray source (13).

2. Method according to claim 1, characterized in that before the first irradiation period and/or between any number of subsequent irradiation periods, that is to say during periods when the breast is not being irradiated, the locking means (16, 17) is turned in the opposite direction to that when turned synchronously with the movement of the x-ray source (13).

3. Method according to claim 1 or 2, characterized in that after an irradiation period, during which the locking means (16, 17) is moved in synchronism with the movement of the x-ray source (13), and before at least one subsequent irradiation period, the locking means (16, 17) is rotated at least substantially back to its initial position, i.e. to its position at the beginning of the preceding irradiation period.

4. Method according to claim 1 or 2, characterized in that the imaging process comprises a plurality of irradiation periods during which said locking means (16, 17) is turned synchronously with the movement of the x-ray source (13), and during each non-irradiation period after such an irradiation period the locking means (16, 17) is turned at least substantially back to its position at the beginning of the previous irradiation period.

5. A method according to claim 1 or 2, wherein the movement of the locking means (16, 17) during the irradiation period comprises turning the locking means (16, 17) through an angle of 2 degrees or less.

6. A method according to claim 1 or 2, characterized in that the movement of the locking means (16, 17) during the irradiation period comprises turning the locking means (16, 17) 0.5-2 degrees.

7. The method according to claim 1 or 2, characterized in that the extreme angular position of the x-ray source (13) relative to the breast during the imaging procedure constitutes a tomographic angle of several tens of degrees.

8. Method according to claim 1 or 2, characterized in that the overall movement of the x-ray source (13) is arranged to be symmetrical with respect to the vertical direction.

9. A method according to claim 1 or 2, characterized in that the movement of the X-ray source (13) is arranged to follow a curved path around the breast.

10. The method according to claim 1 or 2,

the locking means (16, 17) comprises compression plates (16, 17), between which plates (16, 17) the breast is compressed during the imaging procedure, and/or

Stretching means (30, 31) are arranged for pulling the breast tissue in between the locking means (16, 17).

11. Method according to claim 1 or 2, characterized in that during an irradiation period, depending on the breast tissue properties, the x-ray source (13) is operated by using imaging parameter values of the x-ray tube comprising a tungsten anode, which are 35-45kV and 5mAs, or 30-34kV and 10-13 mAs.

12. Method according to claim 1 or 2, characterized in that the method comprises 11-15 irradiation periods and/or that the ratio between the angle of rotation of the locking means (16, 17) during an irradiation period and the total tomographic angle of the imaging process is less than 1/10.

13. Mammography-apparatus comprising a body part (11) and an x-ray source (13) arranged thereto, an image detector (15), and a locking device (16, 17) arranged to lock the breast in a region between the x-ray source (13) and the image detector (15), the x-ray source (13) being arranged to be movable relative to the position of said locking device (16, 17), the apparatus further comprising a control system arranged to control the operation of the apparatus, characterized in that said locking device (16, 17) is arranged to be rotatable, and the movement of the locking device (16, 17) and the x-ray source (13) is arranged to be motorized, and the operation of the x-ray source is controlled by said control system such that during the imaging procedure the x-ray source (13) is continuously moved and irradiated during a number of short irradiation periods, and during an irradiation period, the locking device (16, 17) rotates in synchronization with the movement of the x-ray source (13) to follow the movement of the x-ray source (13).

14. Mammography-apparatus according to claim 13, characterized in that the control system is arranged to control the operation of the apparatus such that the locking means (16, 17) is turned in the opposite direction to the direction when turned in synchronism with the movement of the x-ray source (13) before the first irradiation period and/or between any number of subsequent irradiation periods, that is, during periods when the breast is not being irradiated.

15. Mammography-apparatus according to claim 13 or 14, characterized in that the control system is arranged to control the operation of the apparatus such that after an irradiation period, during which the locking means (16, 17) moves in synchronization with the movement of the x-ray source (13), and before at least one subsequent irradiation period, the locking means (16, 17) is turned at least substantially back to its initial position, i.e. at least substantially back to its position at the beginning of the previous said irradiation period.

16. Mammography-apparatus according to claim 13 or 14, characterized in that the control system is arranged to control the operation of the apparatus such that there are a plurality of irradiation periods during which said locking means (16, 17) is turned synchronously with the movement of the x-ray source (13), and during each non-irradiation period after such an irradiation period the locking means (16, 17) is turned at least substantially back to its position at the beginning of such a previous irradiation period.

17. Mammography-apparatus according to claim 13 or 14, characterized in that the control system is arranged to control the operation of the apparatus such that the locking means (16, 17) is turned by an angle of 2 degrees or less during an irradiation period.

18. Mammography-apparatus according to claim 13 or 14, characterized in that the control system is arranged to control the operation of the apparatus such that the locking means (16, 17) is turned 0.5-2 degrees during an irradiation period.

19. Mammography-apparatus according to claim 13 or 14, characterized in that the movement of the x-ray source (13) is arranged to follow a curved path around the breast positioned in the breast-locking means (16, 17).

20. Mammography-apparatus according to claim 13 or 14,

the locking means (16, 17) comprises compression plates (16, 17), between which plates (16, 17) the breast is compressed during the imaging procedure, and/or

Stretching means (30, 31) are arranged to pull the breast tissue in between the locking means (16, 17).