U.S. patent application number 09/772139 was filed with the patent office on 2001-08-30 for ct apparatus with reduced data transmission rate from the detector system to the image reconstruction computer.
Invention is credited to Doubrava, Clemens, Hahn, Guenter.
Application Number | 20010017909 09/772139 |
Document ID | / |
Family ID | 7628897 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017909 |
Kind Code |
A1 |
Doubrava, Clemens ; et
al. |
August 30, 2001 |
CT apparatus with reduced data transmission rate from the detector
system to the image reconstruction computer
Abstract
A CT (computed tomography) apparatus has a detector system which
produces an x-ray beam that rotates around a system axis, and which
produces data representing attenuation of the x-rays by an
examination subject, and a data transmission path for the
transmission of data from the detector system to an image
reconstruction computer that is stationary relative to the detector
system. Data required for a live image are extracted from the data
supplied by the detector system during the scanning of a subject
under examination, and are transmitted during the scanning to the
image reconstruction computer via the data transmission path and
after completion of the scanning, all data supplied by the detector
system during the scanning are transmitted to the image
reconstruction computer.
Inventors: |
Doubrava, Clemens;
(Nuernberg, DE) ; Hahn, Guenter; (Shanghai,
CN) |
Correspondence
Address: |
SCHIFF HARDIN & WAITE
6600 SEARS TOWER
233 S WACKER DR
CHICAGO
IL
60606-6473
US
|
Family ID: |
7628897 |
Appl. No.: |
09/772139 |
Filed: |
January 29, 2001 |
Current U.S.
Class: |
378/7 ; 378/4;
378/62 |
Current CPC
Class: |
A61B 6/027 20130101;
Y10S 378/901 20130101; A61B 6/032 20130101; G01N 2223/419 20130101;
G01N 23/046 20130101; G06T 15/005 20130101 |
Class at
Publication: |
378/901 ; 378/4;
378/62 |
International
Class: |
A61B 006/03 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 27, 2000 |
DE |
10003518.3 |
Claims
We claim as our invention:
1. A computed tomography apparatus comprising: an x-ray source,
which generates an x-ray beam that rotates around a system axis; a
detector system which rotates around said system axis together with
said x-ray source and which detects radiation attenuated by an
examination subject during scanning of said examination subject and
produces data representing the attenuated radiation; an image
reconstruction computer located remote from said detector system
and not rotating with said detector system; a data transmission
path between said rotating detector system and said image
reconstruction computer for transmitting said data from said
detector system to said image reconstruction computer; a memory
allocated to said detector system for storing said data from said
detector system and rotating with said detector system; a data
preparation unit allocated to said detector system which rotates
with said detector system and which is connected to said memory,
said data preparation unit transmitting said data from said memory
to said image reconstruction computer via said data transmission
path after completion of the scanning, and said data preparation
unit extracting, from the data from the detector system, data
transmitted during the scanning to the image reconstruction
computer via the data transmission path, as extracted data; and a
display connected to said image reconstruction computer, said image
reconstruction computer reconstructing an image of said examination
subject from said extracted data and causing said image to be
displayed on said display.
2. A computed tomography apparatus as claimed in claim 1 wherein,
during said scanning, a continuous rotation of said detector system
occurs around said system axis, and wherein said CT apparatus
comprises means for producing relative displacement between said
detector system and said examination subject during said
scanning.
3. A computed tomography apparatus as claimed in claim 1 wherein
said detector system includes a radiation detector formed by a
two-dimensional array of detector elements.
4. A computed tomography apparatus as claimed in claim 3 wherein
said detector elements are disposed in a plurality of rows and
columns, forming a matrix.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a computed tomography (CT)
apparatus of the type having a detector system that rotates around
a system axis, an image reconstruction computer that is stationary
in relation to the detector system, and a data transmission path
for th transmission of data from the detector system to the image
reconstruction computer, which reconstructs an image from the
transmitted data and present this image on a display.
[0003] 2. Description of the Prior Art
[0004] Users of current CT apparatuses as described, for example in
Japanese Application 102 29 982, Japanese Application 110 76 226,
Japanese Application 112, 35 336 and U.S. Pat. No. 5,867,555,
expect a live reconstruction on the monitor, i.e., an image
reconstructed and presented on the display (live image) immediately
after, or if possible, during, the scanning of a subject under
examination using the CT apparatus.
[0005] In order to satisfy this expectation, it is necessary that
the data supplied by the detector system be transmitted from the
rotating detector system to the stationary image reconstruction
computer at a data rate that is not lower, or is not significantly
lower, than the data rate at which the data are supplied by the
detector system.
[0006] In particular, in a CT apparatus with a detector system
formed by a two-dimensional array of detector elements (known as a
multiple-layer CT apparatus), the satisfaction of this requirement
encounters difficulties, since the data rate his higher by the same
degree as the number of detector elements in relation to a
conventional CT apparatus having a one-dimensional array of
detector elements (detector row).
SUMMARY OF THE INVENTION
[0007] An object of the present invention is to provide a CT
apparatus of the type described above wherein a live image can be
produced even if the data rate at which the data supplied by
rotating detector system are transmitted to the stationary image
reconstruction computer is lower than the data at which the data
are supplied by the detector system.
[0008] This object is achieved in a CT apparatus in accordance with
the invention, wherein in a first phase during the scanning of the
subject under examination (preferably taking place in the form of a
spiral scan), data (in the case of a spiral scan, volume data) are
extracted, using the data preparation unit, from the data supplied
by the detector system and are transmitted to the image
reconstruction computer via the data transmission path during the
scanning, the computer reconstructs an image (live image) from the
extracted data that provides an operator with preliminary
information concerning the results of the examination.
[0009] The extracted data can, for example, be produced such that,
dependent on the algorithm used for the image reconstruction, the
data preparation unit forms the mean value from a number of
(preferably successive) partial rotation data sets or complete
rotation data sets.
[0010] In a second phase, after the completion of the scanning of
the subject under examination, all data supplied by the detector
system during the scanning are transmitted in unmodified form to
the image reconstruction computer via the data transmission
path.
[0011] Since in the first phase only a small part of the data
supplied by the detector system during the scanning of the subject
under examination need to be transmitted, and the transmission
after the scanning of all the data supplied by the detector system
during the scanning of the subject under examination is not
time-critical, there are no particular requirements with respect to
the data transmission rate of the data transmission path. Thus the
requirement of rapid production of a first live image can be
fulfilled with comparatively low outlay and without the necessity
of the data transmission path operating at a high data rate.
Depending on the way in which the extracted data are obtained, for
the transmission of the extracted data a data rate is sufficient
that is on the order of magnitude of one-fourth of the data rate
that would be required for the transmission of all the data during
the scanning.
[0012] The invention is in particular suitable for multiple-layer
CT apparatuses whose detector system is a two-dimensional array of
detector elements that are preferably arranged in a number of rows
and columns. However, the invention is not limited to CT
apparatuses of this sort, but is suitable for CT apparatuses whose
detector system is a one-dimensional array of detector elements,
such as a detector row.
[0013] Moreover, the invention is in particular suitable for CT
apparatuses known as spiral CT apparatuses, in which during a
scanning a continuous rotation of the detector system about the
system axis takes place, with simultaneous displacement of the
subject under examination and the detector system relative to one
another, in the direction of the system axis. In a CT apparatus of
this type, considerable quantities of data can occur during a
scanning, whose transmission during the scanning would be
problematic without the invention.
DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows, in a partially perspective and partially
schematic representation, a CT apparatus suitable for execution of
the inventive method,
[0015] FIG. 2 shows a longitudinal section through the apparatus
according to FIG. 1.
[0016] FIG. 3 shows a schematic diagram of the data preparation
unit of the CT apparatus according to FIGS. 1 and 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] FIGS. 1 and 2 show a third-generation multiple-layer CT
apparatus that is suitable for the execution of the inventive
method. The measurement arrangement, designated generally as 1, has
an x-ray source 2, having a beam diaphragm 3 (FIG. 2) positioned in
front of and close to the source 2. The measurement arrangement 1
also has a detector system 5, fashioned as a planar array of a
number of rows and columns of detector elements (one of these is
designated 4 in FIG. 1), with a beam diaphragm 6 (FIG. 2)
positioned in front of and close to the system 2. The x-ray source
2 with the beam diaphragm 3, and the detector system 5 with the
beam diaphragm 6, are mounted, as shown in FIG. 2, opposite one
another on a rotating frame 7 so that a pyramid-shaped x-ray beam
bundle that emanates from the x-ray source 2 during operation of
the CT apparatus and is gated by the adjustable beam diaphragm 3,
and whose edge beams are designated 8, is incident on the detector
system 5. The beam diaphragm 6 is set to correspond to the
cross-section, using the beam diaphragm 3, of the x-ray beam bundle
so that only that region of the detector system 5 that can be
immediately struck by the x-ray beam bundle is exposed. In the
operating state shown in FIGS. 1 and 2, this corresponds to four
rows of detector elements. The fact that additional rows of
detector elements are present, that are covered or shielded by the
beam diaphragm 6, is indicated by the dots in FIG. 2.
[0018] The rotating frame 7 can be displaced rotationally about a
system axis designated Z using a drive(not shown). The system axis
Z proceeds parallel to the z axis of a rectangular spatial
coordinate system shown in FIG. 1.
[0019] The columns of the detector system 5 likewise proceed in the
direction of the z axis, while the rows, whose width b is measured
in the direction of the z axis, and is for example equal to 1 mm,
proceed transversely to the system axis Z, or the z axis.
[0020] In order to enable a subject under examination, e.g. a
patient, to be brought into the beam path of the x-ray beam bundle,
a positioning arrangement 9 is provided that can be displaced
parallel to the system axis Z, i.e., in the direction of the z
axis.
[0021] For the acquisition of volume data of a subject under
examination located on the positioning arrangement 9, e.g. a
patient, a scanning of the subject under examination takes place by
recording, with motion of the measurement unit 1 around the system
axis Z, a number of projections from various projection directions.
The data supplied by the detector system 5 therefore contain a
number of projections.
[0022] During the continuous rotation of the measurement unit 1
around the system axis Z, the positioning arrangement 9 is
simultaneously continuously displaced relative to the measurement
unit 1 in the direction of the system axis Z, with a
synchronization between the rotational motion of the rotating frame
7 and the translational motion of the positioning arrangement 9, in
the sense that the ratio of translational speed to rotational speed
is constant. This constant ratio can be adjusted by selecting a
value for the advancement h of the positioning arrangement 9 per
rotation of the rotating frame 7 that ensures a complete scanning
of the volume of interest of the subject under examination. Seen
from the subject under examination, the focus F of the x-ray source
2 thus moves around the system axis Z in a spiral path, designated
S in FIG. 1, for which reason scanning using this type of
acquisition of volume data is known as a spiral scan. The volume
data thereby supplied by the detector elements of each row the
detector system 5, relating to projections respectively allocated
to a particular row of the detector system 5 and to a particular
position relative to the system axis Z, are read out in parallel,
are serialized in a pre-processing unit 10, and are transmitted to
an image reconstruction computer 11 via a data transmission path.
The pre-processing unit 10 and a sensor unit 21 a of the data
transmission path are connected to the rotating frame 7 and follow
the motion thereof, which is illustrated by a double connection
line to the rotating frame 7. The receiver unit 21b of the data
transmission path is, like the image construction computer 11 to
which it supplies the received stream of data, stationary relative
to the rotating frame 7.
[0023] After pre-processing the volume data in the pre-processing
unit 12 of the image reconstruction computer 11, the data stream
from the receiver unit 21b reaches a memory 14 in which the volume
data are stored.
[0024] The image reconstruction computer 11 contains a
reconstruction unit 13 that reconstructs image data, e.g. in the
form of tomograms of desired slices of the subject under
examination, from the volume data stored in the memory 14,
according to a method known to those skilled in the art. The image
data reconstructed by the reconstruction unit 13 are likewise
stored in the memory 14 and can be displayed on a display unit 16,
e.g. a video monitor, connected to the image reconstruction
computer 11.
[0025] The x-ray source 2, for example an x-ray tube, is supplied
with the necessary voltage and current by a voltage generator unit
17. In order to enable these to be set to the required values, a
control unit 18 with a keyboard 19 and mouse 20, permitting the
necessary adjustments, are allocated to the generator unit 17.
[0026] The remaining operation and control of the CT apparatus also
takes place by means of the control unit 18 and the keyboard 19 as
well as the mouse 20, which is illustrated by the connection of the
control unit 18 with the image reconstruction computer 11.
[0027] The volume data available at the output of the detector
system 5 are supplied in digital form to the data preparation unit
10, shown in FIG. 3 as a block in broken lines, and these data
reach a memory 22 that stores all the volume data that occur during
a spiral scan.
[0028] During the execution of a spiral scan, a digital signal
processing unit 23 extracts, from the volume data located in the
memory 22, data that permit the reconstruction of a live image, and
transmits the extracted data, via the sensor unit 21 a and the
receiver unit 21b of the data transmission path, to the image
reconstruction computer 11, which reconstructs a live image from
the extracted data, and presents this image on the display unit
16.
[0029] The data rate at which the data transmission path operates
is chosen such that the transmission of the extracted data does not
in any case significantly exceed the duration of the spiral
scan.
[0030] After the completion of the spiral scan, the totality of the
data acquired during the spiral scan and recorded in the memory 22
is transmitted in unmodified form, and at the same data rate at
which the transmission of the extracted data also took place, from
the data preparation unit 10 to the image reconstruction computer
11 via the data transmission path.
[0031] In the inventive CT apparatus, the reconstruction of live
images is possible without the necessity of the data transmission
path operating at a data rate such as would be required for the
transmission during the examination of all the data occurring
during a spiral scan.
[0032] As can be seen in FIG. 3, the cooperation of the memory 22
and the digital signal processing unit 23 of the data preparation
unit 10 is controlled by a control unit 24, for example a
microcontroller.
[0033] The data that reach the memory 22 during a spiral scan from
the individual rows of the detector system 5 contain a number of
partial rotation data sets, or complete rotation data sets, that
are respectively allocated to the individual rows of the detector
system 5. A partial rotation data set contains projections that are
recorded through a projection angle region (i.e., a rotated angle
region of the measurement unit 1) around the system axis Z that is
at least equal to the projection angle region required for the
reconstruction of a tomogram but is smaller than 360.degree.. A
complete rotation data set contains projections that are recorded
through a projection angle of at least 360.degree.. In the
exemplary embodiment, the extracted data are produced by the
digital signal processing unit 23 forming the mean value from a
number of partial rotation data sets or complete rotation data sets
(e.g., four partial rotation or complete rotation data sets) and
transmits the corresponding resulting partial rotation or complete
rotation data set to the image reconstruction computer 11 as
extracted data. The extracted data can be, depending in particular
on the scope and duration of the spiral scan, a single partial
rotation data set or complete rotation data set, or can be a number
of such sets. In the case of the transmission of a number of data
sets, a number of live images can be reconstructed and
displayed.
[0034] The mean value formation explained above is illustrated in
FIG. 3 by a corresponding loop that connects the output and one
input of the digital signal processing unit 23 with one
another.
[0035] As an example, a detector system 5 having 32 rows, having
768 individual detectors, is assumed, and it is further assumed
that 1000 projections are recorded per rotation of the rotating
frame 7 and per row, then if a rotation of the rotating frame lasts
0.4 sec, given a resolution of 17 bits for the complete
transmission of the data supplied by the detector system 5 during a
scanning, a data rate of >1.04448 Gbit/sec is required.
[0036] In contrast, under the precondition that, for obtaining the
extracted data, four partial rotation or complete rotation data
sets are averaged and are used as the basis for the reconstruction
of a live image, a quarter of the data rate required for the
complete transmission is sufficient for the transmission of the
extracted data.
[0037] For the above parameters, and with the additional assumption
of a displacement in the direction of the system axis Z of 200 cm,
given 50 rotations of the measurement unit 1 around the system axis
Z (pitch 4 cm) a capacity of the memory 22 of approximately 21
Gbytes is required for the storage of all data supplied during a
spiral scan by the detector system 5. This capacity can be further
reduced by applying any known loss-free compression technique to
the volume data.
[0038] The production of the extracted data not necessarily take
place using mean value formation. Other procedures are possible,
such as the transmission during the spiral scan of only each nth
partial rotation or complete rotation data set; for example, every
eighth partial rotation or complete rotation data set.
[0039] In the exemplary embodiment, the design of the image
reconstruction computer 11 is described with the pre-processing
unit 12 and the reconstruction unit 13 as hardware components. This
can in fact be the case. As a rule, however, these components are
realized by software modules that operate on a universal computer
provided with the required interfaces, which, differing from FIG.
1, can also take over the function of the control unit 18, which
would then be superfluous.
[0040] In the exemplary embodiment, the CT apparatus has a detector
system 5 having rows whose width, measured in the z direction, are
equal in size, measuring for example 1 mm. However, it is also
within the scope of the invention to employ a detector system whose
rows are of differing widths.
[0041] In the exemplary embodiment, the relative motion between the
measuring unit 1 and the positioning arrangement 9 is produced by
displacing the positioning arrangement 9. However, it is also
within the scope of the invention to leave the positioning
arrangement 9 stationary and to displace the measurement unit 1
instead. Moreover, it is within the scope of the invention to
produce the necessary relative motion by displacing both the
measurement unit 1 and the positioning arrangement 9.
[0042] A third-generation CT apparatus is described used in
connection with the exemplary embodiments, i.e., the x-ray source
and the detector system are displaced in common around the system
axis during the imaging. However, the invention can also be used in
connection with a CT apparatus of the fourth generation, in which
only the x-ray source is displaced around the system axis and
cooperates with a stationary detector ring, as long as the detector
system is a planar array of detector elements.
[0043] The inventive method can also be used in a fifth-generation
CT apparatus, i.e., a CT apparatus in which x-rays emanate not
solely from one focus, but rather from a number of foci of one or
more x-ray sources that are distributed around the system axis, as
long as the detector system is a planar array of detector
elements.
[0044] The CT apparatus used in connection with the exemplary
embodiments has a detector system with detector elements arranged
in the manner of an orthogonal matrix. However, the invention can
also be used in connection with a CT apparatus having a detector
system with detector elements arranged in another manner in a
planar array.
[0045] The exemplary embodiments relate to the medical application
of the inventive method, however, the invention also can be used
outside of medicine, for example, in the examination of luggage or
the examination of other materials.
[0046] Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventors to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of their contribution
to the art.
* * * * *