DE102008058569B4 - Method for optimizing image noise or time resolution in two-beam spiral scan computed tomography - Google Patents

Abstract

A method for optimizing image noise or time resolution in two-emitter spiral scan computed tomography comprising the following steps: Provision of a CT device with at least one storage device arranged along a system axis of the CT device for supporting the examination subject, two coaxially offset by 90 °, arranged coplanar , scanning systems rotatable around the system axis (z-axis), each with a focus from which a fan-shaped beam can be emitted with a fan angle 2 βmax, and an oppositely arranged, flat detector array with a large number of distributed detector elements, with the beam of the beam can be detected, with the scanning systems being able to generate projection data which represent the attenuation of the rays as they pass through the examination object, and the storage device or the scanning systems can be moved along the system axis, selecting a pitch p with which the scanning systems or the storage device can be moved, with: Θ maximum reconstruction angle, βmax half fan angle, and N number of detector lines, - spiral scanning of the examination object with the selected pitch p, - reconstruction of an image from the projection data with a time resolution Timg with: for a reconstruction angle Ω is chosen such that either the time resolution Timg or an image noise σimg is optimized with:, with: Trot time for a 2π rotation of a scanning system around the system axis σ (π / 2) reference image noise for the reconstruction angle Ω = (π / 2).

Description

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 gated two-emitter spiral scan cardio computed tomography.

In computed tomography (CT) devices, it is known to irradiate an examination subject with X-rays and to detect the attenuation of the X-rays along their path from the radiation source (X-ray source) to the detector system (X-ray detector). The attenuation is caused by the irradiated materials along the beam path, so that the attenuation can also be understood as a line integral over the attenuation coefficients of all volume elements (voxels) along the beam path. The captured projection data is not directly interpretable, d. H. they do not give an image of the irradiated layer of the examination subject. Only by means of reconstruction methods is it possible to calculate back from the projected attenuation data to the attenuation coefficients μ of the individual voxels and thus to produce an image of the distribution of the attenuation coefficients. This allows a considerably more sensitive examination of the examination subject than purely viewing projection images.

mit der CT-Zahl C in der Einheit Hounsfield [HU]. Für Wasser ergibt sich ein Wert

= 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

= 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 modern X-ray computed tomography (CT) devices are used. Typically, a CT device includes a radiation source that directs a collimated, pyramidal, or fan-shaped beam from a focus through the subject of the examination, such as a patient, to a detector system constructed of multiple detector elements. Depending on the design of the CT device, the radiation source and the detector system are, for example, mounted on a gantry or a C-arm, which are rotatable about a system axis (z-axis) with an angle α. Furthermore, a storage device for the examination object is provided, which can be moved or moved along the system axis (z-axis). During recording, each detector element of the detector system hit by the radiation produces a signal which represents a measure of the total transparency of the examination object for the radiation emanating from the radiation source on its way to the detector system or the corresponding radiation attenuation. The set of output signals of the detector elements of the detector system, which is obtained for a specific position of the radiation source, is referred to as projection. The position from which the beam penetrates the object of examination is constantly changed as a result of the rotation of the gantry / C-arm. A so-called scan comprises a multiplicity of projections which were obtained at different positions of the gantry / C-arm and / or the various positions of the storage device. A distinction is made between sequential scanning methods (axial scan mode) and spiral scan methods.

In the sequential scanning method, a transversal layer of the examination object is imaged from different angular positions, typically over a range of 360 °. The associated CT image is reconstructed from the acquired projection values. In contrast, the spiral scan method does not produce a layer recording, but in principle generates a volume recording. The object under examination is typically moved continuously through the measuring field during the recording, which takes place over a plurality of 360 ° rotations of the scanning system. The focus of the X-ray tube rotates relative to the examination subject on a spiral or helical path.

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, multiple layers can be recorded simultaneously. Today, detector systems with 256 or more lines are known.

The image quality of the reproduced CT images are typically only slightly different in the sequential scanning method and in the spiral scanning method, since the measuring system and most scanning parameters are identical in both recording methods. Thus, for example, the spatial resolution in the image plane is the same for both methods if identical reconstruction parameters are selected. The beam quality of the spectra used is also independent of whether recorded with conventional single layers or with spiral CT; accordingly, the CT values of arbitrary objects, as far as they fully fill the layer in the z direction, are the same in both cases, ie. H. the contrasts are unchanged. Similar considerations apply to other image quality parameters. Differences arise however with the pixel noise and with the layer sensitivity profiles, thus thus also with the spatial resolution in z-direction.

The pixel noise in spiral CT images is not only dependent on the reconstruction algorithm, but also on the selected scan parameters, in particular on the selected table feed or the pitch value p. The pitch value p indicates the ratio between table feed per gantry rotation and the slice thickness of the detector. The higher the table feed or the pitch value p, the higher the picture noise σ is.

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.

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

CT scanners with multiple scanning systems can also be used to produce higher resolution images. The scanning systems are arranged for this purpose about the common axis of rotation so that the projections of the two scanning systems for the same direction of projection have an offset from each other, which is smaller than a detector element. By evaluating the projections of the scanning systems, which are recorded in temporal succession from the respective projection directions, a higher-resolution image can be calculated. A higher resolution is advantageous, for example, in the examination of blood vessels, in which small examination volumes must be scanned.

In both the scanning speed increasing mode and the scanning resolution increasing mode, projections for reconstructing an image generated by the two scanning systems are offset from each other. The billing of the data takes place with knowledge of the system angle under which the scanning systems are arranged in the azimuthal direction about a common axis of rotation.

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

The object is achieved by the method according to claims 1 and 2. Advantageous embodiments are the subject of the dependent claims or can be found in the following description.

The method according to the invention for optimizing image noise or time resolution in two-beam spiral scan computed tomography comprises the following method 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 is} emitted, and an oppositely arranged areal trained detector array with a plurality of distributed Detector elements, with the beams of the steel beam are detectable, wherein by the scanning 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.

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

With:

Θ

maximum reconstitution angle

β _max

half fan angle, and

N

Number of detector lines of a detector.

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

Third, spiral scan the subject with the selected 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.

Image noise refers to the deterioration of a digital or electronically recorded image due to interference that has no relation to the actual image content, the image signal. The disturbing pixels differ in color and brightness from those of the actual image. The signal-to-noise ratio is a measure of the noise component. The reference image noise is herein defined as the image noise in reconstructed images resulting from reconstructing the acquired projection data at 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 systems each generate a set of projection data when scanning an examination subject. In general, the two projection data sets overlap with respect to the projection angles detected therein, so that a projection value can be recorded at a projection angle in each projection data set. In the present case, a weighting of the two projection data sets is preferably carried out for the reconstruction, so that the corresponding projection value of the first or the second projection data set is weighted in each case for identical projection angles, the respective weights complementing one another.

• 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
  
  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
  
  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).

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, - reconstruction of 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, for projection data of the two scanning systems used for the reconstruction, a weighting is carried out in such a way that the weights complement each other in each projection angle.