X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-07-13
2001-05-22
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06236707
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method and computed tomography apparatus for reconstructing an image from measured values acquired by conducting a spiral scan of an examination subject.
2. Description of the Prior Art
Methods are known for the reconstruction of images of a slice of an examination subject having a slice thickness with respect to an image plane from measured values acquired with a CT apparatus by conducting a spiral scan of the examination subject with an x-ray source rotating around the examination subject and a detector formed by at least one line of detector elements. In such known methods, measured values are respectively allocated to different projection angles &agr; and to a z-position on the longitudinal axis of the spiral scan, and a constant, dimensionless pitch p is adhered to during the spiral scan. This pitch p is defined as the ratio of the relative longitudinal shift (in mm) between (a) the examination subject, the x-ray source and the detector which occurs per full revolution of the x-ray source around the examination subject, and (b) the longitudinal width (in mm) of a line of the detector. “Longitudinal” means the direction of the longitudinal axis of the spiral scan. CT apparatuses for the implementation of such methods are also known.
Methods and CT apparatuses of this type are disclosed in U.S. Pat. No. 5,559,847, European Application 0 713 678, U.S. Pat. No. 5,539,796 and in Polacin et al., “Evaluation of Section Sensitivity Profiles and Image Noise in Spiral CT”, Radiology, 1992, No. 185, pages 29 through 35.
In the reconstruction of images from measured values acquired by spiral scanning with a CT apparatus having a single-line detector, an interpolation between the measured values lying in front of and behind the image plane is implemented for each projection angle for generating calculated projections in the desired image plane.
Two interpolation methods are currently most standard: In the first, a linear interpolation is undertaken between respectively two measured projections lying closest to the image plane, these having been registered at the same projection angle a but in different revolutions. This type of interpolation is referred as
360
LI interpolation. In the second method, an interpolation also is undertaken between two projections lying closest to the image plane, but one is registered at the projection angle &agr;
d
and the other is registered at the projection angle &agr;
c
complementary thereto (&agr;
c
=&agr;
d
±&pgr; is valid for the middle detector element of the detector). This type of interpolation is referred to as
180
LI interpolation. Given the same pitch, it supplies narrower effective layer widths (characterized, for example, by the full width at half maximum FWHM of the layer sensitivity profile) than the
360
LI interpolation. Given the same output power (mA value) of the x-ray source, for example an x-ray tube, the pixel noise is increased in comparison to the
360
LI interpolation as a tradeoff. The artifact susceptibility is also higher. Both interpolation types are illustrated schematically in
FIG. 2
for the pitch p=2, with the projection angle &agr; being shown as a function of the detector position in the z-direction. The projection angle &agr; is entered on the longitudinal axis of the spiral scan (z-position) relative to the position normalized to the width b of a line of the detector.
All conventional interpolation methods for spiral scanning with a single-line detector have in common the fact that the width of the slice sensitivity profile (characterized, for example, by the full width at half maximum FWHM) increases with increasing pitch p. This is shown in
FIG. 3
for the
180
LI and the
360
LI interpolation, which shows the full width at half maximum FWHM of the slice sensitivity profile referred to the collimated layer thickness d
coll
as a function of the pitch p. The relationship according to
FIG. 3
complicates the procedure, particularly for unfamiliar users and represents a limitation in the selection of the examination parameters.
The situation becomes even less easily predictable when conventional interpolation techniques, for example the
36
OLI or
180
LI interpolation, are employed for spiral scans implemented with a multi-line detector.
FIG. 4
shows the full wave at half maximum FWHM of the slice sensitivity profile deriving in a 360LI and in a 180LI interpolation, again relative to the collimated slice thickness d
coll
, as a function of the pitch p for a CT apparatus having a five-line detector. The full wave at half maximum now changes non-monotonously with the pitch. The relationship is not intuitive and is difficult to understand. Thus, for example, the full wave at half maximum FWHM can decrease given increasing pitch p.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the type initially described wherein a simplified operation of the CT apparatus is possible.
The above object is achieved in accordance with the principles of the present invention in an image reconstruction method, and in a computed tomography apparatus implementing the inventive image reconstruction method, wherein the image is an image of a slice of an examination subject having a slice thickness with respect to an image plane, wherein measured values are acquired in a computed tomography apparatus by conducting a spiral scan of the examination subject with an x-ray source and a radiation detector rotating around the examination subject, the detector having at least one line of detector elements, the measured values being respectively allocated to one of a number of projection angles &agr; and to a z-position along the longitudinal axis of the spiral scan, while adhering to a constant, dimensionless pitch p during the spiral scan, the pitch p being defined as the ratio of the relative shift which occurs between the x-ray source and the detector and the patient during one full revolution of the x-ray source around the subject, with respect to the width of a line of the detector in the direction of the longitudinal axis of the spiral scan, and wherein, for each projection angle, all measured values belonging to this projection angle and lying within a maximum distance from the image plane are employed in the image reconstruction with the measured values being weighted according to a weighting function dependent on their spatial distance in the direction of a longitudinal axis from the image plane, with the weighting function being chosen so that a selectable, functionally defined relationship between the effective slice thickness and the pitch is present.
In the inventive method and apparatus, thus, the dependency of the effective slice thickness on the pitch p can be easily predicted (i.e., identified without resort to complicated calculations) since, differing from the prior art, there is no longer a more or less unpredictable relationship between the two that is defined by the interpolation method which is employed and which cannot be influenced except by the selection of the interpolation method. Instead, a selected functionally defined relationship is present. An inventive CT apparatus thus can be easily operated. This is particularly true when, in a version of the invention, the functional dependency is selected such that the effective slice thickness (the full wave at half maximum of the slice sensitivity profile, for example, can be utilized as criterion for this) is substantially independent of the pitch p.
A further simplification in operation is achieved in an embodiment of the invention, wherein the output power of the x-ray source is set dependent on the pitch p so that the pixel noise is substantially independent of the pitch p. It is then possible to set not only the slice sensitivity profile, but also the pixel noise, independently of the pitch p.
The setting of the desired slice thickness required in the invention ensues, according to one version of the invention, by v
Flohr Thomas
Schaller Stefan
Bruce David V.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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