X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-11-29
2002-06-04
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000, C378S901000
Reexamination Certificate
active
06400790
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for image reconstruction in a computed tomography (CT) apparatus, such as a spiral CT apparatus, as well as to a CT apparatus, such as a spiral CT apparatus, for implementing the method.
2. Description of the Prior Art
A spiral CT (computed tomography) apparatus having a radiation source movable around an examination subject from which a fan-shaped ray beam emanates, and having a detector with a number of lines of detector elements (detector lines) that receives the fan-shaped ray beam, wherein the examination subject and the detector are displaceable relative to one another in the direction of the system axis for the implementation of an examination. It is known to operate such a system to register a number of projections each with several lines of detector elements for a number of projection angles and positions along the system axis, wherein the same lines of detector elements are employed for the registration of all projections. An image is reconstructed from the registered projections.
CT systems of this type are disclosed, for example, in German OS 196 47 435, U.S. Pat. No. 5,682,414 and German OS 42 24 249 as well as U.S. Pat. No. 5,291,402.
For spiral exposures with a CT apparatus that has a detector with a single line of detector elements, an interpolation between the measured values lying in front of and behind the image plane is implemented for generating projections in the desired image plane for each projection angle.
Two interpolation methods are currently most common. In the first, a linear interpolation is undertaken between two measured projections lying closest to the image plane, these having been registered at the same projection angle &agr; but in different revolutions. This type of interpolation is referred to as 360LI interpolation. In the second method, interpolation is carried out between two sets of measured values lying closest to the image plane, one set of these values having been registered at the projection angle &agr;
d′
, the other set at the projection angle &agr;
c′
, complementary thereto. The relation &agr;
c′
=&agr;
d
±Π applies for th e central channel of the detector. This type of interpolation is referred to as 180LI interpolations. It supplies narrower effective slice widths (characterized, for example, by the full width at half-maximum FWHM) than the 360Ll interpolation given the same pitch. As a tradeoff, the image noise is increased compared to 360LI interpolation given the same output power of the X-ray tube (same mA value) and the artifact susceptibility is greater. Both types of interpolation are schematically illustrated in
FIG. 1
, which shows the projection angle &agr; as a function of the detector position in the z-direction during a spiral scan for the pitch p=2 normalized onto the collimated width d of a line of detector elements of the detector, i.e. the collimated slice thickness.
In a CT apparatus having multi-line detectors, the reconstruction of spiral data with exact and approximative methods described in German PS 196 14 223 that in fact take the exact geometry into consideration, but this is very calculation-intensive and therefore is not particularly suited for use in a commercial CT apparatus.
For low line numbers M (for example, M≦5), the angle of inclination—also referred to as the cone angle—of the X-rays (referred to as measuring rays) incident onto the detector relative to a plane perpendicular to the z-axis of the CT apparatus (also referred to as the system axis) can be neglected for reducing the calculating outlay, and the 180LI and 360LI interpolations that are standard for a CT apparatus with a detector having only one line of detector elements can be transferred to multiple detector lines. This is the reconstruction method that is employed in the 2-line CT scanner “Elscint Twin” (see “Dual-slice versus single-slice spiral scanning: Comparison of the physical performances of two computed tomography scanners”, Yun Liang and Robert A. Kruger, Med. Phys. 23(2), Febuary 1996, pp. 205-220).
In a presentation analogous to
FIG. 1
, the principle of the 180LI and 360LI interpolation transferred onto a number of lines is shown in
FIG. 2
for the pitch p=3 with reference to the arbitrarily selected example of a CT apparatus having a detector with four lines of detector elements. The pitch p is the feed in z-direction per revolution of the radiation source with reference to the collimated width d of a line of detector elements of the detector, i.e. the collimated slice thickness. The basic problems in the standard multi-line spiral interpolation become clear from FIG.
2
:
First, in order to generate data for a predetermined projection angle by interpolation, these data corresponding to a corresponding projection in the desired image plane acquired with a detector having only one line of detector elements, the contribution of a number of projections from different revolutions of the spiral scan must be taken into consideration. The interpolation weightings for a specific projection are thus dependent on the z-position of other projections. Given realization on a computer, this makes the processing of the individual projections more difficult. Dependent on the pitch p, moreover, multiple scans occur (in
FIG. 2
, for example, at line 1 and line 4 that scan the same z-positions in successive revolutions), which have to be taken into consideration in the calculation of the interpolation s weightings, making the interpolation more computationally complicated.
Second given pitch values p≧M (M is the number of lines of the detector), a 180LI interpolation must be implemented if the slice sensitivity profile is not to spread unacceptably. For illustration, the full wave at half-maximum FWHM of the slice sensitivity profile occurring given 180LI and 360LI interpolation as function of the pitch value p is shown in
FIG. 3
for the example of the detector having four lines of detector elements.
180LI interpolation given a detector with one line of detector elements means that interpolation is generally carried out between a direct ray and the ray complementary thereto. The situation is more complicated given a detector having a number of lines. In that case, 180LI interpolation means that interpolation is always carried out between the two measured values that lie closest to the image plane. Dependent on the pitch value p and the position of the image plane in the z-direction, interpolation for a specific projection angle &agr; is carried out either between direct measured values, namely when these lie closer to the image plane, or between a direct measured value and a measured value complementary thereto when these lie closer to the measured plane.
When, however, interpolation is carried out between direct and complementary measured values given a projection angle &agr;
d
, the complementary measured value at &bgr;
c
=−&bgr;
d
must be found for every measured value identified by th e direct projection angle &agr;
d
and the corresponding fan angle &bgr;
d
. The projection angles &agr;
d
and &agr;
c′
of direct and complementary projections are offset by exactly 180° only in the rotational center, i.e. for &bgr;
d
=&bgr;
c
=0. The relations
&bgr;
c
=−&bgr;
d
&agr;
c
=&agr;
d
+2&bgr;+Π (1)
apply in the general case, i.e. the complementary measured value at &bgr;
c
for each direct measured value characterized by the projection angle &agr;
d
and the fan angle &bgr;
d
, is from a different projection, that accordingly was registered at a different z-position. Interpolation weightings that are independent of fan angle therefore must be used in the 180LI interpolation, and the contributions of different complementary projections for each direct projection must be considered, this immensely increasing the calculating outlay.
Third, the standard deviation of the pixel noise measured in the image a
Flohr Thomas
Ohnesorge Bernd
Bruce David V.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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