Patent Application
20060170679
This invention relates to the field of medical devices and, in particular, to contour sets describing a volume of interest.
Traditionally, medical imaging was used to represent two-dimensional views of the human anatomy. Modern anatomical imaging modalities such as computed tomography (CT) are able to provide an accurate three-dimensional model of a volume of interest (e.g., skull or tumor bearing portion of the body) generated from a collection of CT slices and, thereby, the volume requiring treatment can be visualized in three dimensions. More particularly, in CT scanning numerous x-ray beams are passed through a volume of interest in a body structure at different angles. Then, sensors measure the amount of radiation absorbed by different tissues. As a patient lies on a couch, an imaging system revolves around the patient emitting and recording x-ray beams from multiple points. A computer program is used to measure the differences in x-ray absorption to form cross-sectional images, or “slices” of the head and brain. These slices are called tomograms, hence the name “computed tomography.”
A volume of interest (VOI) may be defined as a set of planar, closed polygons, as illustrated in
Conventional VOI imaging architectures utilize a three-tier representation structure: VOI-contourslice-contour.
Alternatively, some conventional VOI imaging architectures allow multiple contours to be defined for each contour slice. In this case, VOIs with cavities, branches, and unconnected bodies may be drawn. However, in this case, it is often impossible to perform interpolation without manually labeling on each slice, which contours belong to which part of the structure, thus requiring a large amount of user interaction.
Another problem with conventional architectures is that it may be difficult to achieve conformality when using such architectures for inverse planning in stereotactic radiosurgery. In stereotactic radiosurgery, a collimated radiation source is positioned in a sequence calculated to localize the radiation dose into a VOI that as closely as possible conforms to that requiring treatment, while avoiding exposure of nearby healthy tissue. The degree to which such is achieved is referred to as conformality. Specifically, conformality is a measure of the amount of prescription (Rx) dose (amount of dose applied) within a target volume. Conformality may be measured using a conformality index (CI)=total volume at>=Rx dose/target volume at>=Rx dose. Perfect conformality results in a CI=1. With conventional radiotherapy treatment, using a treatment planning tool, a clinician identifies a contour for a corresponding VOI for application of a treatment dose (e.g., 2000 cGy), as illustrated in
One solution to the above noted problems is to create a pseudo cavity contour by deforming a solid contour into an elongated shape having ends and wrapping its ends around to form a structure illustrated in
The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings.
a-8c illustrate one embodiment of a method of inverse planning.
In the following description, numerous specific details are set forth such as examples of specific systems, components, methods, etc. in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well-known components or methods have not been described in detail in order to avoid unnecessarily obscuring the present invention.
Embodiments of the present invention include various steps, which will be described below. The steps of the present invention may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor programmed with the instructions to perform the steps. Alternatively, the steps may be performed by a combination of hardware and software.
Embodiments of the present invention may be provided as a computer program product, or software, that may include a machine-readable medium having stored thereon instructions, which may be used to program a computer system (or other electronic devices) to perform a process. A machine-readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). The machine-readable medium may include, but is not limited to, magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM); random-access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory; electrical, optical, acoustical, or other form of propagated signal (e.g., carrier waves, infrared signals, digital signals, etc.); or other type of medium suitable for storing electronic instructions.
Embodiments of the present invention may also be practiced in distributed computing environments where the machine-readable medium is stored on and/or executed by more than one computer system. In addition, the information transferred between computer systems may either be pulled or pushed across the communication medium connecting the computer systems, such as in a remote diagnosis or monitoring system. In remote diagnosis or monitoring, a user may utilize embodiments of the present invention to diagnose or monitor a patient despite the existence of a physical separation between the user and the patient.
Some portions of the description that follow are presented in terms of algorithms and symbolic representations of operations on data bits that may be stored within a memory and operated on by a processor. These algorithmic descriptions and representations are the means used by those skilled in the art to effectively convey their work. An algorithm is generally conceived to be a self-consistent sequence of acts leading to a desired result. The acts are those requiring manipulation of quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, parameters, or the like.
A contour based method for representing a VOI is described. In this method, a contour set is used as the basic unit for representing a VOI. A contour set is composed of multiple contours defined on several image slices, with no more than one contour in any single slice. Each contour within a set is defined in the same image plane (axial, sagittal, or coronal). In order to define a VOI, a series of Boolean operators is used to merge the contour sets describing the VOI. For example, where the contour contains a cavity (a.k.a., hole), two contour sets may be developed using Boolean “AND” operators: one contour set for the cavity and one contour set for the surrounding VOI structure. The surrounding VOI as a contour set is merged with the contour set forming the boundary of the cavity using the Boolean “NOT” operator. By using Boolean “AND” and “NOT” operators, a VOI having multiple structures and cavities within each slice can be represented. In one embodiment, the merged contour sets do not all need to be in the same plane as each other. For example, a solid region defined in the axial direction may be merged with a cavity defined in the sagittal direction.
It should be noted that the methods and apparatus are discussed herein in relation to CT imaging only for ease of explanation. The method and apparatus discussed herein may also be used to represent VOIs with other types of medical diagnostic imaging systems, for example, magnetic resonance imaging (MR), ultrasound (US), nuclear medicine (NM) PET/SPECT systems, etc.
The VOI architecture 200 expands the conventional three-tier VOI architecture to a four-tier architecture. VOI architecture 200 includes a contour tier 210, a contour slice tier 220, a VOI tier 230 and a contour set tier 240.
Using architecture 200, a region of interest (ROI) such as VOI image slice 235, can be represented as a Boolean combination of the multiple contour sets 241-244. Each of the contour sets 241-244 is composed of multiple contours defined on multiple image slices, with no more than one contour in any single slice. For example, contour set 241 includes contour slice 221 having a single contour 211. In one embodiment, each contour within a set may be defined in the same image plane (axial, sagittal, or coronal). Each contour within a set may be constructed by identification on a corresponding image slice or through interpolation from other contours on other image slices. It should be noted that interpolation techniques are well known in the art; accordingly, a detailed discussion is not provided herein.
In order to define VOI 231, a series of Boolean operators is used to merge the contour sets 241-244 describing the VOI 231. In one embodiment, contour sets 241-244 may be classified into two different types based on their geometric property: solid and cavity. A solid contour set (e.g., contour sets 241 or 243) represents voxels that exists in an image. While a cavity contour set (e.g., contour sets 242 or 244) represents voxels that need to be removed from a solid contour set. Having multiple contour sets in one VOI 231 may not be sufficient to represent a VOI 231 that contains cavity inside (e.g., cavity 212 or 214). As such, a Boolean operation is performed on the contour sets 241-244 to define such a VOI 231. In this embodiment, VOI 231 contains two bodies B0) and B2 that are defined by two contour sets: contour set 241 (C0) and contour set 243 (C2). In one embodiment, B0 and B2 may be unconnected bodies. The VOI (V) 231 may then be represented by using the Boolean OR operator (∪):
V=C0∪C2 (1)
If a VOI contains one solid body (CB) that has a cavity (CC) inside, then the VOI could be represented suing the Boolean AND operator (∩): V=CB∩{overscore (CC)} . . . In the embodiment illustrated in
The VOI 231 may then be represented by using the Boolean NOT operator:
V=(C0∪C2)∩({overscore (C1∪C3)}) (2)
In general, with a VOI (V) that contains N contour sets, C0 . . . CN-1, if the first K contour sets are of a solid type, and the rest of the contour sets are of a cavity type, the final geometry of the VOI could be represented as:
V=(C0∪C1∪ . . . ∪CK-1)∩{overscore (C)}K∩{overscore (C)}K+1∩ . . . ∩{overscore (C)}N-1 (3)
=(C0∪C1∪ . . . ∪CK-1)∩({overscore (CK∪CK+1520 . . . ∪CN-1)}) (4)
After the determination of the solid and cavity type contour sets, a Boolean OR operation is performed on all solid contour sets, step 330, and a Boolean OR operation is performed on all cavity contour sets, step 340. It should be noted that the Boolean OR operation performed on all cavity contour sets of step 340 may be performed after, prior to, or concurrent with the Boolean OR operation performed on all solid contour sets of step 330. In step 350, the Boolean OR'd solid contour sets are merged with the Boolean OR'd cavity contour sets by taking the OR'd solid contour sets and Boolean AND'ing them with a Boolean NOT of the OR'd cavity contour sets according to equation (4) above.
It should be noted that the merged contour sets do not all need to be in the same plane as each other. Some anatomical locations are much better viewed in one plane than another. As such, it may be desirable to utilize images taken in different planes. Using the method discussed above with respect to
It should be noted that with a conventional 3-tier architecture, VOl editing user interface and contouring tools operate directly on a current selected VOl. With the 4-tier architecture 200 of
In one embodiment, after VOl 231 has be defined using architecture 200, it may be represented as a bit wise mask overlaid on the image, so that each bit is zero or one according to whether the corresponding image voxel is contained within the VOl represented by that bit, as illustrated in
In the 4-tier structure of architecture 200, all contour sets (e.g., contour sets 241-244) of a single VOI (e.g., VOI 231) share the same VOI index, which maps to a single mask bit index in the VOI mask volume 400. Therefore, anything that was based on VOI mask volume in a conventional architecture sees the same single mask plane for each VOI. The internal structure of the VOI is transparent to other algorithms and sub systems. Thus, the addition of the tier 240 in architecture 200 should not affect planning (e.g., dose calculation) and 3D visualization. Inside the VOI, a conventional VOI mask generation algorithm has to be expanded to support the VOI architecture 200 structure.
The method begins at step 510 by clearing the I-th bit of every voxel in volume M. Then, in step 520, a mask is created for all solid contour sets. In particular, for each voxel P that belongs to the VOI mask volume M, if the voxel P belongs to a solid contour set then set the I-th mask bit of voxel P to logic 1.
In step 530, the mask bits for all cavity contour sets are cleared. In particular, for each voxel P that belongs to the VOI mask volume M, if the voxel P belongs to a cavity contour set then set the I-th mask bit of voxel P to logic 0. In step 540, a mask value in the I-th bit of every voxel of the mask volume V is output. It should be noted that in an alternative embodiment, the logic levels corresponding to a solid contour set and a cavity contour set may be switched.
In one embodiment, the above described method of generating a VOI mask volume may be implemented using the following VOI mask volume generation algorithm (with reference to the method steps of
Inputs:
Create mask for all solid contour sets (step 520):
In order to be able to export the VOI architecture 200 structure to a DICOM standard format, the following information is mapped to DICOM standard tags:
The mapping between the DICOM tags and the VOI properties is listed in the table 600 of
The multiple contour set VOI architecture 200 provides an improvement over the conventional architecture. Many applications, which are impossible in the conventional architecture, can be done implemented with architecture 200. One such application involves inverse planning as discussed below with respect to the illustration of
Medical diagnostic imaging system 700 includes an imaging source 710 to generate a beam (e.g., kilo voltage x-rays, mega voltage x-rays, ultrasound, MRI, etc.) and an imager 720 to detect and receive the beam generated by imaging source 710. In an alternative embodiment, system 700 may include two diagnostic X-ray sources and/or two corresponding image detectors. For example, two x-ray sources may be nominally mounted angularly apart (e.g., 90 degrees apart or 45 degree orthogonal angles) and aimed through the patient toward the imager(s). A single large imager, or multiple imagers, can be used that would be illuminated by each x-ray imaging source. Alternatively, other numbers and configurations of imaging sources and imagers may be used.
The imaging source 710 and the imager 720 are coupled to a digital processing system 730 to control the imaging operation. Digital processing system 730 includes a bus or other means 735 for transferring data among components of digital processing system 730. Digital processing system 510 also includes a processing device 740. Processing device 740 may represent one or more general-purpose processors (e.g., a microprocessor), special purpose processor such as a digital signal processor (DSP) or other type of device such as a controller or field programmable gate array (FPGA). Processing device 740 may be configured to execute the instructions for performing the operations and steps discussed herein. In particular, processing device 740 may be configured to execute instructions to perform the Boolean operations on the contour sets 241-244 to define VOI 231 as discussed above with respect to
Digital processing system 730 may also include system memory 750 that may include a random access memory (RAM), or other dynamic storage device, coupled to bus 735 for storing information and instructions to be executed by processing device 740. System memory 750 also may be used for storing temporary variables or other intermediate information during execution of instructions by processing device 740. System memory 750 may also include a read only memory (ROM) and/or other static storage device coupled to bus 735 for storing static information and instructions for processing device 740.
A storage device 760 represents one or more storage devices (e.g., a magnetic disk drive or optical disk drive) coupled to bus 735 for storing information and instructions. Storage device 760 may be used for storing instructions for performing the steps discussed herein.
Digital processing system 730 may also be coupled to a display device 770, such as a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information (e.g., 3D representation of the VOI) to the user. An input device 780, such as a keyboard, may be coupled to digital processing system 730 for communicating information and/or command selections to processing device 740. One or more other user input devices, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processing device 740 and for controlling cursor movement on display 770 may also be used.
It will be appreciated that the digital processing system 730 represents only one example of a system, which may have many different configurations and architectures, and which may be employed with the present invention. For example, some systems often have multiple buses, such as a peripheral bus, a dedicated cache bus, etc.
a-8c illustrate one embodiment of a method of inverse planning. In stereotactic radiosurgery, an accurate three-dimensional model of the skull or other tumor bearing portion of the body is generated from thin-cut CT scans, thus the volume requiring treatment can be visualized in three dimensions. Unlike conventional radiation therapy treatment planning where the beam selection and does is define by the user, in inverse planning, the system user may outline treatment volumes and critical structures on the CT images and prescribe a dose accordingly. The treatment planning system then selects a beam configuration (e.g., direction, distance, number and energy of beams for treatment) and generates a plan. A collimated radiation source is positioned in a sequence calculated by the plan to localize the energy deposition into a VOI that as closely as possible conforms to that requiring treatment, while avoiding exposure of nearby healthy tissue. The dose distribution is an important parameter in stereotactic surgery. If a radiation dose were too low due to unforeseen conditions at a point intended to receive the maximum radiation, then the surgery could be ineffective. If a radiation dose were too high at a particular point in the tissue, the surgery might have negative effects. As such, it is desirable to be able to form constraints on an inverse planning system in such a way that conformality of dose to the treatment target is rewarded such that the treatment target will result in a dose distribution within the prescribed limits and damage to healthy tissue is minimized.
This may be achieved by using the multiple contour set VOI architecture 200, as discussed below in relation to the exemplary brain CT images of
In step 930, the contour set for the target VOI is copied to a new critical structure contour set and the new contour set is dilated in all directions by a certain amount (e.g., 5 mm), or otherwise generated (e.g., by manual tracing), illustrated by contour 812, to create a cavity type contour set. In step 940, the dilated cavity contour set is copied to a new contour set and, itself, dilated in all directions by a certain amount (e.g., 5 mm), or otherwise generated, (illustrated by the contour 813 to created a boundary critical, solid type contour set. The resulting boundary critical structure is defined by the inner contour 812 and the outer contour 813. In step 950, the contour sets are then merged. In particular, a Boolean “NOT” of the cavity contour set (corresponding to contour 812) is Boolean AND'd with the third, solid contour set (corresponding to contour 813) to create a shell VOI of arbitrary thickness around the target body represented in one image slice by contour 811. The inverse planning procedure discussed above can impose a high dose gradient at the edges of the target, thus increasing conformality.
It should be noted that the methods and apparatus described herein are not limited to use only in with medical diagnostic imaging. In alternative embodiments, the methods and apparatus herein may be used outside of the medical technology field, such as non-destructive testing of materials (e.g., motor blocks in the automotive industry and drill cores in the petroleum industry) and seismic surveying.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specifications and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.
