Computer graphics processing and selective visual display system – Computer graphics processing – Three-dimension
Reexamination Certificate
2000-12-22
2004-04-13
Zimmerman, Mark (Department: 2671)
Computer graphics processing and selective visual display system
Computer graphics processing
Three-dimension
C250S363040, C600S436000, C378S021000, C378S062000
Reexamination Certificate
active
06720966
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for reconstructing 3D image data for a volume of interest of an examination subject, wherein radiation emanating from a radiation source is received with a planar detector, of the type wherein a number of 2D central projections are acquired from different projection directions, a volume of interest is marked, and 3D image data of the volume of interest corresponding to the markings are reconstructed from the 2D central projections.
2. Description of the Prior Art
In the current state of computer technology, reconstructive 3D imaging represents has widespread use. This is particularly true for medical-diagnostic imaging, for example computed tomography (CT), magnetic resonance tomography, nuclear medicine, ultrasound and, recently, 3D x-ray technology. These methods are also utilized, outside medical technology in general technology such as, for example, computer tomography for the non-destructive testing of materials (motor blocks in the automotive industry, drill cores in the petroleum industry, etc.).
The term “reconstructive imaging” means that the measured data supplied by the respective detectors are not directly interpreted, but are used as inputs in a procedure that supplies qualitatively new image information, i.e. image data. The mathematical algorithms utilized for this purpose make high demands on the computers employed for the processing of these algorithms with respect to the computing power and data volume.
An important structural feature CT apparatus is a mechanically stable, usually annular gantry that allows substantially vibration-free revolutions of the entire measuring system in the sub-second range. A disadvantage of such a CT apparatus is the limited accessibility to the patient for medical personnel, for example a physician, as a consequence of the gantry. Better accessibility would be desirable for the more frequently utilized minimally invasive and endoscopic surgical techniques (interventions) with additional 3D imaging.
One technique in this direction is the reconstruction of 3D image data from a series of 2D central projections acquired in the form of standard x-ray exposures with a C-arm apparatus that is conventional in terms of its mechanical structure, whereby a planar detector, for example an x-ray image intensifier or, recently, a semiconductor panel is utilized as the radiation receiver.
For example, neuro-radiology is field of employment for such a technique. Vessels filled with contrast agent and their spatial position are imaged with high topical resolution. This is required, for example, in the neuro-surgical treatment of aneurisms. Interventions of this type ensue under constant x-ray monitoring.
The technical realization of the 3D functionality ensues by acquiring the digital data corresponding to the 2D central projections in the course of rotation angiography. For example, a C-arm apparatus distributed by Siemens AG under the name NEUROSTAR® is suitable as a registration device. Typically, 50 2D central projections having 1024×1024 pixels each are registered in five seconds over an angular range of 200°. Due to the mechanical instability of the C-arm, the exact projection geometry must be defined for each of the 2D central projections and must then be taken into consideration in the implementation of the reconstruction algorithm. The reconstruction of the 3D image data ensues according to CT principles.
A C-arm apparatus of this type is described in detail in H. Barfuss, Digitale 3D-Angiographie, VDE-Fachbericht, Vol. 34: Das Digitale Krankenhaus, VDE-Verlag, 1998.
In rotation angiography as a recent 3D imaging method, the preconditions compared to computed tomography are essentially modified by the following points:
A mechanically unstable system with a freely rotatable C-arm is utilized.
The objective is interventional employment, i.e. the image result, must be quickly available during the examination or treatment.
The “field of vision” of the detector, i.e. the aperture angle of the cone-shaped or pyramidal x-ray beam emanating from the x-ray source, is limited compared to computed tomography.
The following facts follow from these points:
1. The entire body is usually not registered, but only a part thereof. This defines a maximum reconstructable volume (MRV).
2. The maximally obtainable spatial resolution of the portrayed volume is limited by the resolution of the 2D central projections; the resolution available to the observer is additionally limited by the selected size of the voxels (voxel=volume element).
3. The number of voxels enters critically into the calculation time. A halving of the size of the voxels with retention of the size of the volume to be reconstructed means, for example, an eight-times increase in the number of voxels and also means an eight-times increase in the size of the dataset. Given limited calculating time (for reconstruction and display), a larger volume with poorer spatial resolution, or a smaller volume with high-spatial resolution (limited by the resolution of the 2D projections) therefore can be reconstructed with a given calculating power.
4. During the implementation of an intervention (for example, placement of platinum coils), the physician is interested in obtaining optimally high resolution, local 3D information with respect to a volume of interest (VOI=volume of interest) within the MRV.
Given employment of rectangular surface detectors, the MRV can be considered approximately as a circular cylinder around the rotational axis of the C-arm in an approximation, as shown in
FIG. 3
herein.
The selection of the volume to be reconstructed based on this approximation is described in detail below.
The definition of the volume within the MRV from which 3D image data are to be reconstructed ensues on the basis of numerical coordinates, usually in a global coordinate system that is preferably oriented with respect to the geometry of the apparatus. For example, the rotational axis of the C-arm corresponds to the z-axis, the rotational plane corresponds to the xy-plane and the x-axis proceeds parallel to the patient support.
The selected volume is geometrically considered as a cuboid, composed of many small cuboids of the same size, i.e., the voxels. The reconstruction allocates a gray scale value to each voxel, this corresponding to the x-ray attenuation coefficient (approximate density) of the subject in the region of the voxel. The reconstructed 3D image data therefore represent a scalar 3D field f (i, j, k), with
i=1, . . . , Nx,
j=1, . . . , Ny,
k=1, . . . , Nz,
where Nx, Ny, Nz reference the number of voxels which are present in the direction of the respective coordinate axis.
The mid-point of each voxel has a geometrical position (xi, yj, zk) allocated to it. When the edge lengths of a voxel are referenced dx, dy, dz, then, for example, the following applies:
xi=x
0
+
i*dx,
yj=y
0
+
j*dy,
zk=z
0
+
k*dz
The reference point (x
0
, y
0
, z
0
) describes the hypothetical voxel that lies outside the cuboid on the spatial diagonal thereof and touches it. Of course, other reference points are possible, for example the mid-point of the cuboid (xM, yM, zM).
N=Nx*Ny*Nz is the overall number of voxels. This number N critically defines the required calculating time. The quantities X=Nx*dx, Y=Ny*dy and Z=Nz*dz describe the edge lengths of the cuboid, i.e. the illustrated overall volume. The quantities dx, dy, dz define the topical resolution of the reconstructed 3D dataset.
Given a constant N, i.e. a given calculating time, one can thus construct either a large volume with poor resolution or a small VOI with good resolution. For evaluating diagnostically or therapeutically relevant structures, for example an aneurism, the latter is preferred. A problem is having to indicate the position of the VOI in the space for a particular examination situation, for which abstract positional coordinates (reference poi
Barth Karl
Brunner Thomas
Mitschke Matthias
Wiesent Karl
Schiff & Hardin LLP
Sealey Lance W.
Siemens Aktiengesellschaft
Zimmerman Mark
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