DE102008058569A1 - Image noise or time resolution optimization method for e.g. two-beam spiral scan computer tomography, involves selecting time resolution for reconstruction angle such that either time resolution or image noise is optimized - Google Patents

Abstract

The method involves providing a computer tomography (CT) apparatus with a storage device for storing an object to be examined. Projection data is produced by a scanning system, where the data represents attenuation of beams when passing through the object to be examined. A pitch with which the system and the device are displaceable, is selected. The object to be examined is spirally scanned with the selected pitch. An image is reconstructed from the data, where time resolution is selected for a reconstruction angle in such a manner that either the time resolution or image noise is optimized.

Description

The The present invention relates to a method for optimizing Image noise or time resolution in dual-beam spiral scan computed tomography as well as in the gated two-emitter spiral scan cardio computed tomography.

at Computed Tomography (CT) devices As is known, an examination subject with X-rays radiates and the attenuation of X-rays along their way from the radiation source (X-ray source) to the detector system (X-ray detector) detected. The weakening is from the irradiated materials along the beam path causing, so that the weakening as line integral over the attenuation coefficients of all volume elements (voxels) along the beam path can be understood. The recorded projection data are not directly interpretable, d. H. they do not give an image of irradiate the layer of the examination object. First over Reconstruction method, it is possible from the projected Attenuation data on the attenuation coefficients μ the to recalculate a single voxel and thus a picture of the To generate distribution of attenuation coefficients. This allows a much more sensitive investigation of the Object of investigation as in pure viewing of projection images.

mit der CT-Zahl C in der Einheit Hounsfield [HU]. Für Wasser ergibt sich ein Wert C_H₂O = 0 HU und für Luft ein Wert C_L = –1000 HU. Da beide Darstellungen ineinander transformierbar bzw. äquivalent sind, bezeichnet der allgemein gewählte Begriff Schwächungswert oder Schwächungskoeffizient sowohl den Schwächungskoeffizienten μ als auch den CT-Wert.To illustrate the attenuation distribution, a value normalized to the attenuation coefficient of water, the so-called CT number, is used instead of the attenuation coefficient μ. This is calculated from a currently determined by measurement attenuation coefficient μ according to the following equation:

with the CT number C in the unit Hounsfield [HU]. For water there is a value C _H₂O = 0 HU and for air a value C _L = -1000 HU. Since both representations are mutually transformable, the commonly chosen term attenuation value or attenuation coefficient designates both the attenuation coefficient μ and the CT value.

For the recording, evaluation and representation of the three-dimensional attenuation distribution become modern X-ray computed tomography devices (CT devices) used. Typically, a CT device includes a source of radiation that is a collimated, pyramidal or fan-shaped Beam from a focus through the object under investigation, For example, a patient, on a built up from a plurality of detector elements Directed detector system. Depending on the design of the CT device are the radiation source and the detector system, for example, on a gantry or a C-arm mounted around a system axis (z-axis) with an angle α are rotatable. Furthermore, a storage device provided for the object of investigation, along the System axis (z-axis) can be moved or moved. While The recording produces each detector element hit by the radiation of the detector system, a signal that is a measure of overall transparency of the examination object for that of the radiation source Outgoing radiation on its way to the detector system or the corresponding Radiation attenuation represents. The set of output signals the detector elements of the detector system, for a certain position of the radiation source is obtained as Projection called. The position from which the ray bundle penetrates the object under investigation, is due to the rotation of the Gantry / C-arm constantly changing. A so-called Scan encompasses a large number of projections that are based on different Positions of the gantry / C-arm and / or various positions the storage device were obtained. One differentiates thereby sequential scanning (axial scan) and spiral scan.

at the sequential scan method is a transversal layer of Object of investigation shown from different angular positions, typically over a range of 360 °. Out The acquired projection values become the corresponding CT image reconstructed. In the spiral scan method, however, no Layer recording, but in principle generates a volume recording. The examination object is typically during the Recording over a variety of 360 ° rotations the scanning system takes place continuously through the measuring field method. The focus of the X-ray tube is running relative to the examination object on a spiral or helical Train around.

On the basis of the data record generated during a scan, as described above, a two-dimensional sectional image of a slice of the examination subject is reconstructed. The quantity and quality of the measurement data acquired during a scan depend on the detector system used. With a detector system comprising an array of multiple rows and columns of detector elements, meh other layers are recorded simultaneously. Today, detector systems with 256 or more lines are known.

The Image quality of the reproduced CT images are typically in the sequential scanning method and in the spiral scanning method only slightly different, because the measuring system and most scanning parameters are in both recording methods identical. So is, for example, the spatial resolution in the image plane same for both methods if same reconstruction parameters to get voted. The radiation quality of the used Spectra is also independent of whether with conventional Single layers or recorded by spiral CT; corresponding are the CT values of arbitrary objects, as far as the layer in Fill the z direction fully in both cases, d. H. the contrasts are unchanged. Similar considerations also apply to other image quality parameters. differences However, dot noise and film sensitivity profiles so also with the spatial resolution in z-direction.

The Pixel noise in spiral CT images is not just the reconstruction algorithm, but also of the selected scan parameters, in particular from the selected table feed or the pitch value p. The pitch value p gives the relation between table feed per gantry rotation and the layer thickness of the detector. there holds that the image noise σ is higher, the higher the table feed or the pitch value p is selected becomes.

Furthermore, for example, from the US 4,991,190 and the DE 29 51 222 A1 CT devices with two scanning systems known, wherein a scanning each comprise a radiation source or a focus and a detector system. The advantage of such CT devices, as disclosed in the references, over a CT scanner with only one scanning system is the ability to inspect a subject at an increased scanning speed or with an increased scanning resolution.

Height Sampling speeds are particularly advantageous when Motion artifacts in the reconstructed image should be minimized. A high scanning speed ensures that all used to reconstruct an image projections from various Projection angles α largely the same state of motion an object, for example the same heart phase of heartbeats, to capture. In the known CT devices are the two Scanning arranged around a common axis of rotation and in the direction of rotation arranged at an angle of 90 degrees to each other, so that the scanning speed when using suitable reconstruction methods doubled.

CT scanners but with multiple scanning systems can also be used to generate used images with a higher resolution become. The Abtastsysteme are for this purpose to the common same Rotary axis arranged so that the projections of the two scanning systems for the same direction of projection an offset to each other which is smaller than a detector element. By evaluation the time of the in the sampling systems from the Projections recorded in each projection direction is a higher resolution image calculable. A higher one Dissolution is advantageous, for example, in the examination of blood vessels, at the small examination volume must be scanned.

Either in the scanning speed increasing mode as well as in the mode for increasing the sampling resolution become the projections produced by the two scanning systems to reconstruct an image for each other. The settlement The data is recorded with knowledge of the system angles, among which the scanning systems in the azimuthal direction about a common axis of rotation are arranged.

task The invention is a method for optimizing image noise or time resolution in dual-beam spiral scan computed tomography specify.

The Task is with the method according to the claims 1 and 2 solved. Advantageous embodiments are the subject the dependent claims or the following description remove.

The inventive method for the optimization of Image noise or time resolution in dual-beam spiral scan computed tomography includes the following process steps.

First, providing a CT device with at least one along a system axis (z-axis) of the CT device arranged storage device for storage of the examination object, two coaxial order Spaced 90 °, coplanar arranged around the system axis (z-axis) rotatable scanning systems, each with a focus from which a fan-shaped beam with a fan angle 2 · β _{max can be} emitted, and an oppositely arranged flat trained detector array with a plurality of distributed detector elements, with the beams of the steel bundle are detectable, wherein by the sampling projection data can be generated, which represent the attenuation of the rays as they pass through the examination subject, and wherein the storage device or the scanning systems along the system axis (z-axis) are movable.

insbesondere p mit:

Θ

maximaler Rekonstuktionswinkel

β_max

halber Fächerwinkel, und

N

Zahl der Detektorzeilen eines Detektors.

Second, selecting a pitch p with which the scanning systems or the storage device are movable, wherein

in particular p with:

Θ

maximum reconstitution angle

β _max

half fan angle, and

N

Number of detector lines of a detector.

As As described above, the pitch value gives p in the spiral scan the relationship between table feed per gantry rotation and the layer thickness of the detector.

Third, Spiral scanning of the examination object with the selected one Pitch p.

für einen Rekonstruktionswinkel Ω derart gewählt wird, dass entweder die Zeitauflösung T_img oder ein Bildrauschen σ_img mit:

optimiert wird, mit:

T_rot

Zeit für eine 2π-Rotation eines Abtastsystems um die Systemachse (z-Achse)

σ(π/2)

Referenzbildrauschen für den Rekonstruktionswinkel Ω = (π/2).

Fourth, reconstruct an image from the projection _data using a time resolution T _img with:

is selected for a reconstruction angle Ω such that either the time resolution T _img or an image noise σ _img with:

optimized, with:

T _red

Time for a 2π rotation of a scanning system around the system axis (z-axis)

σ (π / 2)

Reference image noise for the reconstruction angle Ω = (π / 2).

As is customary in the prior art, the reconstruction angle Ω is understood to mean the projection angle range whose projections are used together to reconstruct a complete planar slice image. For a two-beam system with recording systems offset by 90 ° (= π / 2), the maximum reconstruction angle Θ indicates the projection angle range for which a volume scan of an examination object is gapless. The following relationship applies: π / 2 ≤ Ω ≤ Θ.

When Noise is the worsening of a digital image or electronically recorded image due to interference, which have no relation to the actual image content, the image signal. The disturbing pixels differ in color and brightness those of the actual picture. The signal-to-noise ratio is a measure of the noise component. The reference picture noise is present as the image noise in reconstructed images defined by reconstruction of captured projection data with a reconstruction angle Ω = (π / 2).

wählen. Als Konsequenz ergibt sich das entsprechende Bildrauschen zu

In the present case, it has been recognized that in the case of a spiral feed p <p Θ Max optimize the time resolution or the image noise within the specified limits. For a pitch p <p Θ Max Thus, the time resolution T _img for the reconstruction angle Ω in the range can be

choose. As a consequence, the corresponding image noise results

The staggered scanning generate when scanning a Object to be examined each a set of projection data. in the In general, the two projection data sets overlap with regard to the projection angles detected therein, so that too a projection angle in each projection data set a projection value can be detected. The reconstruction is preferably present here carried out a weighting of the two projection data records, so that each for identical projection angle of the corresponding Projection value of the first and the second projection data set be weighted, with the respective weights to 1 supplement.

• 1. Bereitstellen eines CT-Gerätes mit zumindest einer längs einer Systemachse (z-Achse) des CT-Gerätes angeordneten Lagerungsvorrichtung zur Lagerung eines Untersuchungsobjektes, zwei koaxial um 90° versetzt, koplanar angeordneten, um die Systemachse (z-Achse) rotierbaren Abtastsystemen, mit jeweils einem Fokus, von dem ein fächerförmiges Strahlenbündel Strahlenbündel mit einem Fächerwinkel 2·β_max aussendbar ist, einem gegenüberliegend angeordneten flächig ausgebildeten Detektorarray mit einer Vielzahl von verteilten Detektorelementen, mit dem Strahlen des Stahlenbündels detektierbar sind, wobei durch die Abtastsysteme Projektionsdaten erzeugbar sind, welche die Schwächung der Strahlen beim Durchgang durch das Untersuchungsobjekt repräsentieren, und wobei die Lagerungsvorrichtung oder die Abtastsysteme entlang der Systemachse (z-Achse) verfahrbar sind, und Mitteln zur Erfassung von EKG-Signalen eines schlagenden Herzens, die gegebenenfalls zeitlich korreliert speicherbar sind.
• 2. Ermitteln der Herzrate HR aus den EKG-Signalen.
• 3. Wählen eines Pitch p, mit dem die Abtastsysteme oder die Lagerungsvorrichtung verfahrbar sind, wobei
  
  insbesondere p mit: Ω Rekonstruktionswinkel, Θ maximaler Rekonstruktionswinkel, HR Herzrate, und N Zahl der Detektorzeilen sind.
• 4. Spiral-Abtasten des Untersuchungsobjektes mit dem gewählten Pitch p.
• 5. Rekonstruieren eines Bildes aus den Projektionsdaten, wobei eine Zeitauflösung T_img mit:
  
  für Rekonstruktionswinkel Ω derart gewählt wird, dass entweder die Zeitauflösung T_img oder ein Bildrauschen σ_img mit:
  
  optimiert wird, mit: T_rot Zeit für einen 2π-Rotation eines Abtastsystems um die Systemachse σ(π/2) Referenzbildrauschen für den Rekonstruktionswinkel Ω = (π/2).

Another object of the invention is a method for optimizing image noise or time resolution in gated two-emitter spiral scan cardio computed tomography. This method comprises the following method steps.

• 1. providing a CT apparatus having at least one storage device arranged along a system axis (z-axis) of the CT apparatus for supporting an examination object, two coaxially offset by 90 °, arranged coplanar around the system axis (z-axis) rotatable scanning systems, each having a focus, from which a fan-shaped beam bundle with a fan angle 2 · β _{max can be} emitted, an oppositely arranged areal detector array with a multiplicity of distributed detector elements with which beams of the steel bundle can be detected, wherein projection data can be generated by the scanning systems, which represent the attenuation of the rays as they pass through the examination object, and wherein the storage device or the scanning systems along the system axis (z-axis) are movable, and means for detecting ECG signals of a beating heart, which optionally temporally correlated memory ar are.
• 2. Determine the heart rate HR from the ECG signals.
• 3. Selecting a pitch p with which the scanning systems or the storage device can be moved, wherein
  
  in particular p with: Ω reconstruction angle, Θ maximum reconstruction angle, HR heart rate, and N number of detector rows.
• 4. Spiral scan of the examination object with the selected pitch p.
• 5. Reconstruct an image from the projection _data using a time resolution T _img with:
  
  for reconstruction angle Ω is chosen such that either the time resolution T _img or an image noise σ _img with:
  
  is optimized, with: T _red Time for a 2π rotation of a scanning system about the system axis σ (π / 2) Reference image noise for the reconstruction angle Ω = (π / 2).

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - US 4991190 [0009]
• DE 2951222 A1 [0009]

Claims (4)

Method for optimizing image noise or time resolution in the twin-beam spiral scan computed tomography comprising the following steps: - providing a CT device with at least one storage device arranged along a system axis of the CT device for supporting the object to be examined, two coaxially offset by 90 °, coplanarly arranged to the system axis (z-axis) rotatable scanning systems, each with a focus, from which a fan-shaped beam with a fan angle 2 · β _{max can be} emitted, and an oppositely arranged area detector array with a plurality of distributed detector elements, with the beams of Steel bundles are detectable, wherein projection data can be generated by the scanning, which represent the weakening of the beams as they pass through the examination subject, and wherein the storage device or the scanning systems are moved along the system axis, - Wäh len of a pitch p with which the scanning or the storage device are movable, wherein

with: Θ maximum reconstruction angle, β _max half fan angle, and N number of detector rows, - spiral scan of the examination object with the selected pitch p, - reconstruction of an image from the projection data, with a time resolution T _img with:

is selected for a reconstruction angle Ω such that either the time resolution T _img or an image noise σ _img with:

is optimized, with: T _red Time for a 2π rotation of a scanning system about the system axis σ (π / 2) Reference image noise for the reconstruction angle Ω = (π / 2). Method for optimizing image noise or time resolution in the gated two-beam spiral scan cardio computed tomography comprising the following steps: - Providing a CT device with at least one arranged along a system axis of the CT device storage device for storage of an object to be examined, two offset coaxially by 90 ° , coplanar arranged around the system axis rotatable scanning systems, each with a focus from which a fan-shaped beam bundle beam with a fan angle 2 · β _{max is} emitted, an oppositely arranged area detector array with a plurality of distributed detector elements, with the beams of the steel beam detectable in which projection data can be generated by the scanning systems, which represent the attenuation of the beams as they pass through the examination subject, and wherein the storage device or the scanning systems are along the system axis are arable, and means for detecting ECG signals of a beating heart, the temporally correlated storable, - determining the heart rate HR from the ECG signals, - selecting a pitch p with which the scanning or the storage device are movable, wherein

with: Ω reconstruction angle, Θ maximum reconstruction angle HR heart rate, and N number of detector rows, - spiral scan of the examination object with the selected pitch p, Reconstructing an image from the projection data with a time resolution T _img with:

for reconstruction angle Ω is chosen such that either the time resolution T _img or an image noise σ _img with:

is optimized, with: T _red Time for a 2π rotation of a scanning system about the system axis σ (π / 2) Reference image noise for the reconstruction angle Ω = (π / 2). A method according to claim 1 or 2, characterized in that the pitch p is chosen such that p <p Θ Max is. Method according to one of claims 1, 2 or 3, characterized in that used for the reconstruction Projection data of the two scanning systems a weighting is done so that the weights complement each other in each projection angle.