Full field mammography with tissue exposure control, tomosynthesis, and dynamic field of view processing

Field

0001This patent specification is in the field of mammography systems, and is particularly applicable to using large field, flat panel, digital x-ray receptors.

Background

0002X-ray mammography systems typically use an x-ray source mounted at one end of a rotatable c-arm assembly and an image receptor at the other. Between the x-ray source and the image receptor is a device for compressing and immobilizing a breast Until recently, the image receptor was typically a screen-film cassette, which generated an image related to the detected transmission of x-rays through the breast. The device for compressing the breast against the image receptor, or a breast tray covering the receptor, is often called a paddle, and comes in a variety of sizes to match both the cassette size and the breast size. Such matching is desirable because the use of a small size paddle on a large breast can result in uneven and inadequate breast compression and may not allow full-breast imaging, while using a large paddle on a small breast can impede access to the breast, which is important during the compression cycle in order to optimize the amount of breast tissue brought into the field of view of the image receptor.

0003New mammography systems are now being introduced to use digital image receptors in place of screen-film, and have many well recognized advantages. Such a system is currently available from the assignee hereof under the trade name Selenia. The Selenia system uses a digital flat panel detector made by the Digital Radiography Corp. division of the assignee hereof, located in Newark, Delaware. The panel has a 24X29 cm field of view. Various other aspects of a mammography system and method are described in commonly assigned provisional Parent Application Ser. No. <patcit id="pcit0001" dnum="US60350213B"><text>60/350,213 filed October 19, 2001</text></patcit> and in International Application No. <patcit id="pcit0002" dnum="US0233058W"><text>PCT/US02/33058 filed October 17, 2002</text></patcit>.

0004Mammography systems often have provisions for partly or fully automating the selection of appropriate technic factors for an x-ray exposure, such as one or more of kVp (the x-ray tube accelerating potential), mA (x-ray tube current), and exposure time. When a film-screen image receptor is used, this can be done by relying on exposure detectors at the other side of the film from the x-ray source. An imaging exposure of the breast is stopped when these exposure detectors indicate that they have received a sufficient amount of x-radiation. This is not believed practical for use with flat panel image receptors for a number of reasons. Accordingly, one known approach for use with digital flat panel image receptors is to take a short, low x-ray dosage pre-exposure after the breast has been compressed, and then take an imaging exposure while the breast remains immobilized, using technic factors based on measurements taken with the same receptor in the pre-exposure.

0005Another aspect of mammography is proposals for tomographic imaging or tomosynthesis. In principle, a tomographic image of a plane in the breast can be obtained by moving at least one of the x-ray source and the image receptor relative to the breast during the x-ray exposure. If the x-ray source and the image receptor move in opposite directions in parallel planes, with the appropriate geometry, a plane in the breast that is parallel to the image receptor remains in focus during the entire exposure while the images of all other planes in the breast are blurred and become background noise in the final image. One known approach is to keep the image receptor stationary but move the x-ray source in a path suitable for tomosynthesis. One problem with this is that this limits the field of view for the tomosynthesis image. Another is that this makes it difficult to effectively control the effects of scattered radiation as it becomes difficult to maintain the commonly used anti-scatter grids focused on the focal spot of the x-ray source. Yet another problem is that this arrangement allows for only relatively shallow (small) angles relative to a normal to the plane of the receptor.

0006Yet another aspect of mammography using flat panel digital image receptors is the transmission and storage of images. Many health facilities have image storage systems such as PACS, and protocols such as DICOM exist for formatting medical x-ray images for storage and future use. However, in many if not most cases, the breast takes up only a part of the image taken with flat panel digital receptors such that the imaginary rectangle that envelops the image of the breast is smaller than the field of view of the receptor. One proposal has been made for use with a fan beam of x-rays scanning a flat panel digital image receptor, and is believed to involve eliminating from storage image areas that do not contain an image of the breast. However, that proposal is believed to be specific to the use of a scanning fan beam of x-rays.

0007The system disclosed in this patent specification is designed to overcome these and other disadvantages of the known prior proposals.

Summary

0008An object of the disclosed system is to provide a particularly effective and advantageous exposure control for mammography using flat panel, digital x-ray receptors.

0009Aspects of the invention are set out in the accompanying claims.

Brief description of the drawing

0010<ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> illustrates a digital mammography system in which preferred embodiments disclosed herein can be implemented.</li><li><figref idref="f0002">Figure 2</figref> is a flow chart illustrating processes of estimating and using tissue exposure control in a mammography system.</li><li><figref idref="f0001">Figure 3</figref> illustrates a focused anti-scatter grid that can be used in the system of <figref idref="f0003">Figs. 4</figref> and <figref idref="f0001">1</figref>.</li><li><figref idref="f0003">Figure 4</figref> illustrates an aspect oftomosyntesis in mammogrpahy.</li><li><figref idref="f0004">Figure 5</figref> illustrates selection of a decreased size mammography image for storage and transmission.</li><li><figref idref="f0004">Fig. 6</figref> illustrates processes involved in selecting a reduced size image for transmission and storage.</li></ul>

Detailed description of preferred embodiments

0011Referring to <figref idref="f0001">Fig. 1</figref>, a mammography system currently available from the common assignee under the trade name Selenia except for the new features described herein, comprises a stand 10 supporting a C-arm 12 that can move up or down along stand 10, to a selected height, driven by motor(s) controlled by a health professional operating the system. C-arm 12 carries an x-ray tube 12a at an upper end and a breast tray 12b at a lower end. Tray 12b covers a flat panel x-ray image receptor 12c, spaced from the tray by a focused anti-scatter grid 12d (which may be retractable so that it can be removed from the space between tray 12b and receptor 12c). C-arm 12 also carries a compression paddle 12e that is between source 12a and breast tray 12b and is motorized to move away from tray 12b so a patent's breast can fit between tray 12b and paddle 12e, and closer to tray 12b so the patient's breast can be compressed and immobilized. The movement of paddle 12e is motorized and controlled by the health professional. Different size paddles 12e can be fitted to suit different breast sizes for best compression. In addition, the health professional can move paddle 12e along the width of tray 12b to a position in which paddle 12e matches the position of a breast that is not centered on tray 12b, as in the Selenia system currently offered by the common assignee. The system further includes other components, such as a control station 14 comprising interface devices such a keyboard 14a and trackball 14b, a display screen 14c, and control and image processing facilities.

0012In order to carry out tissue exposure control, the currently available Selenia mammography system is modified to incorporate the equipment and process steps illustrated in <figref idref="f0002">Fig. 2</figref>. In particular, a paddle position encoder 20 measures the position of paddle 12e relative to tray 12b as the health professional positions, compresses and immobilizes the patient's breast for imaging. The thickness of the immobilized breast can be measured or estimated in other ways instead. For example, the final position of paddle 12e can be measured in some way, e.g. mechanically or optically or in some other way. The thickness of the immobilized breast may be measured or estimated directly in any one of a number of ways. A calculator 22, which can be implemented by suitably programming the processing unit 14 (<figref idref="f0001">Fig. 1</figref>), calculates the thickness of the compressed and immobilized breast on the basis of the output of encoder 20, or some other means for measuring breast thickness indicators, and provides information about breast thickness for a tissue exposure control calculator 24, which again can be implement through such programming.

0013To estimate technic factors, calculator 24 also relies on information about the x-ray density of the breast. Such information can come from one or more different sources. One example is manual input 26, e.g. keyboard 14a (<figref idref="f0001">Fig. 1</figref>), through which the health professional can input information characterizing the density of the breast. For example, the system can present the health professional with three choices - fatty, normal and dense - and the health professional can make the appropriate choice based on any one or more factors such as physical examination of the breast, information from previous views in the same examination or taken at a much earlier time, or other information about the patient. Another example of a source of information about breast density is previous x-ray images (views) of the patient's breast or breasts. A previous view can be one taken at any time, either in the same visit of the patient or at a previous visit. Information about density can be derived from the previous view(s) by the health professional, or it can be derived automatically - by measuring the overall density of a previous x-ray image and perhaps knowing the technic factors used to take it. If actually measured, the density information from the previous view(s) can be provided to calculator 24 manually or automatically, through a connection from the measuring device to calculator 24. Another source of density information is dual energy arrangement 30 that pulses the immobilized breast with a low dose x-ray energy at each of two different energies, e.g. sequentially, the measurements of x-rays with receptor 12c (<figref idref="f0001">Fig. 1</figref>) at each of the energies are used in a known process, similar to that used in bone densitometry, to estimate breast density and automatically or manually provide the estimate to calculator 24. X-ray tube 12a can be used for such dual energy process, using two different x-ray filters to emit x-rays at the appropriate to different energies or energy bands. Yet another source of information about breast density can be arrangement 32 that measures the force with which paddle 12e compresses the breast and the time such force acts from the start of compression until the breast is immobilized for imaging, and supplies such force/time information to calculator 24, manually or automatically.

0014Calculator 24 can be implemented as look-up table that in effect has an entry for each of a number of combinations of breast thickness and breast density values. The initial values of the entries can be estimated by actual tests, in essence a trial-and-error process, or in some other way. Calculator 24 provides its output to technic factor display 34, which can be display 14c (<figref idref="f0001">Fig. 1</figref>), at which the health professional can see the automatically estimated factors such as one or more of kV, mAs, filter, time, etc. An entry device, which can be keyboard 14a, allows the health professional to confirm or modify the automatically estimated parameters, and control 38 (which can be a part of unit 14 of <figref idref="f0001">Fig. 1</figref>) uses the resulting final tissue exposure control technic factors for an imaging x-ray exposure.

0015The examples disclosed in this patent specification refer to compressing and immobilizing the breast before determining technic factors and imaging. However, alternatives are contemplated in which the breast need not be compressed before imaging; the breast may be simply supported in some manner, such as by a breast tray, or may be suspended in some manner between an x-ray source and an image receptor. In such a case, the breast thickness and density information can come from different sources, such as measurements or estimates of the thickness of the uncompressed breast, or an average of the thickness of the breast portion that will be imaged, of the thickness of the part that is of particular interest for imaging. The density information may come from the health professional, or from prior x-ray images of the breast, or from some other source. The same alternative of imaging the uncompressed breast applies to the other two features discussed below - tomosynthesis and selecting a reduced field of view image for transmission and/or storage - where the alternative dispenses with compression but otherwise conforms to the description below.

0016Another feature of the mammography system disclosed here is tomosynthesis that both allow a large field of view and the use of a focused anti-scatter grid. As illustrated in <figref idref="f0001">Fig. 3</figref>, anti-scatter grid 12d is focused to allow the passage of x-rays along paths 40 that emanate from the focal spot of x-ray source 12a and to suppress (scattered) x-rays that travel along other paths. If such a grid changes its orientation relative to the x-ray source, it would undesirably suppress x-rays that it should be passing. Such change in orientation would result if x-ray tube 12e moves in a direction transverse to the x-ray beam it emits while grid 12d and detector 12c remain stationary. In addition, such motion would reduce the field of view, so a portion of the breast projected on the receptor in one position of the moving source may fall outside the receptor outline at another position of the source.

0017<figref idref="f0003">Fig. 4</figref> illustrates an arrangement that overcomes these deficiencies of a known proposal. In <figref idref="f0003">Fig. 4</figref>, x-ray tube 12a and the combination of anti-scatter grid 12d and receptor 12c rotate as a unit while a compressed and immobilized breast remains between them and in the path of x-rays emitted from tube 12a and impinging on receptor 12c. Anti-scatter grid 12d remains focused on the focal spot of tube 12a, and the effective field of view does not change with angular position of the source-receptor unit. In the currently offered Selenia unit, source 12a and grid 12d and receptor 12c rotate as a unit, also together with compression paddle 12c and breast tray 12b, so a modification is needed to achieve the geometry of <figref idref="f0003">Fig. 4</figref>. This modification involves decoupling a means to compress and immobilize the breast from motion of tube 12a, grid 12d and receptor 12c. For example, this can be done by removing compression paddle 12e and compressing and immobilizing the breast between compression paddles 44 that are appropriately positioned relative to the center of rotation of tube 12a but do not rotate with tube 12a, as illustrated in <figref idref="f0003">Fig. 4</figref>. As an alternative to rotation, one or both of tube 12a and receptor 12c can translate relative to the breast immobilized between paddles 44. In such case, focused grid 12d can be decoupled from receptor 12c and allowed to remain focused at tube 12a, or a different grid can be used that is not focused or is less focused, and/or the motion of tube 12a and /or receptor 12c can be over a more limited path. Discrete x-ray images of the breast are taken at each of a number of different positions of tube 12 relative to the breast. The image data is used for tomosynthesis through the application of known image processing methods.

0018An important advantage of the example of <figref idref="f0003">Fig. 4</figref> is that it allows imaging at relatively large angles between the extreme rotational or translational positions of x-ray tube 12a as compared with known systems.

0019Yet another feature of the mammography system disclosed here is to transmit and store only a portion of the field of view. With a relatively large field-of-view receptor 12c, such as used in the Selenia system (24x29 cm), typically the image of the breast lies within a rectangle that is smaller than the field of view, as illustrated in <figref idref="f0004">Fig. 5</figref>, where the image 46 of a breast is within a notional rectangular outline 48 (reduced field of view) that is much smaller than the field of view 50 of receptor 12c. The area of field of view 50 that is outside the reduced filed of view area 48 may contain little or no information about the breast. To save on transmitting and storing the breast image, only the information within the reduced field of view 48 may be used, and any information outside outline 48 can be discarded. If there is any significant information outside outline 48, only that information can be attached to the information for the image portion inside outline 48.

0020One way to select the position and size of outline 48 is to rely on the selection of the size and position of compression paddle 12e that the health professional has made. As earlier noted, the currently offered Selenia system allows the health professional to select both the size of a paddle and, at least for some paddles, also the position of the paddle relative to receptor 12c, so as to match the size and position on receptor 12c of the breast being x-rayed. The size and position of paddle 12e can be automatically determined, and the result used to in effect crop the resulting breast image before transmitting and/or storing and/or formatting it for transmission or storage, for example according to DICOM standards. Alternatively, the size and position of the breast in the image can be found through image analysis, such as analysis involving edge detection, and the size and position of outline 48 can be found in this manner. Still alternatively, the size and position of outline 48 may be entered by the health professional, e.g., through keyboard 14a, based on viewing the image displayed on monitor 14c. Or, a combination of said methods can be used, e.g., an automatic determination based on one or both of image analysis and paddle selection, followed with a presentation of a recommended outline 48 displayed to the health professional together with the entire image, for confirmation or modification by the health professional.

0021<figref idref="f0004">Fig. 6</figref> illustrates an arrangement for providing a reduced field of view image. A compression paddle size and position encoder, incorporated in C-arm 12 or elsewhere in association with the means for mounting and moving paddle 12e, provides information about the paddle 12e that the health professional has selected, and about the position of the paddle's projection on receptor 12c. A manual input provides information entered by the health professional, which can be similar to that provided by encoder 52 or can be information regarding which of several rectangles within the entire breast image encompasses the breast, or which arbitrary rectangle can encompass the breast on the image. An image analyzer 56 provides information about the area in the overall image occupied by the breast. A calculator 58 uses the information from one or more of units 52, 54 and 57 to calculate the size and position of a reduced field of view that still encompasses the breast, and the calculation is displayed at 60, e.g., as an outline 48 in an image such as illustrated in <figref idref="f0004">Fig. 5</figref>, for the health professional to confirm or modify, e.g. through manual entries. The result is a finalized reduced field of view image at 62 that can be used for further processing, for transmission, and/or storage. While a rectangular outline 48 has been discussed above, in fact outline 48 can have other suitable shapes.