DE102005050839B3 - Phantom for magnetic resonance imaging tomography, has set of balls, where phantom is asymmetrically formed such that orientation of phantom during formation of image is determined from image of phantom - Google Patents

Abstract

The phantom (1a) has a set of balls (3, 9, 11), and a three dimensional lattice that is formed by the arrangement of the balls in the phantom. The three dimensional lattice is formed by three principal axis. The phantom is asymmetrically formed such that orientation of the phantom during formation of an image is determined from an image of the phantom, where the balls comprise different diameter.

Description

The The invention relates to a phantom used for calibration of imaging Systems, especially in magnetic resonance imaging systems to calculate the spatial Distortions.

The Magnetic resonance tomography (MR tomography) has been in the last few years Years established as a major imaging method in medicine. MR imaging systems, the sectional images of a to be examined Object, in particular a human body or body part, generate by using nuclear magnetic resonance, are in themselves known. Here, the body to be examined in a strong homogeneous static magnetic field, the so-called main magnetic field, introduced, that in the body an alignment of the nuclear spins of atomic nuclei, in particular of Hydrogen nuclei bound to water (protons) causes. By means of high-frequency excitation pulses then these nuclei become a precession movement stimulated. After the end of a corresponding radio frequency (RF) excitation pulse precess the atomic nuclei with one frequency, the so-called Larmor frequency, those of the strength of the Main magnetic field depends and then oscillate after a predetermined tissue-dependent relaxation time again in the predetermined by the main magnetic field preferred direction one. By computational and / or metrological analysis of the integral, high-frequency nuclear signals can with respect to a body layer from the distribution of spatial Spin density in conjunction with the relaxation times generated an image become. The assignment of the detectable due to the precession movement nuclear magnetic resonance signal to the place of its formation is done by applying linear field gradients. To corresponding gradient fields are superimposed on the main magnetic field and controlled so that only in a layer to be imaged an excitation of the Cores takes place. Both for RF excitation of the nuclear spins as well as for Detection of Kernant word signals an RF coil device is required. Imaging systems based on these physical effects are also known under the names nuclear spin tomography, nuclear magnetic resonance (NMR) tomography or Magnetic Resonance Imaging (MRI).

deviations the magnetic fields, in particular the spatial distribution of their magnetic field strength of their theoretically given and calculated values call in the picture geometric distortions. In particular, methods based on a quantitatively accurate geometric representation of the investigated Areas are based in quality due to such distortions decisively reduced.

• – Die magnetische Suszeptibilität variiert bei unterschiedlichen Materialien und bewirkt so eine geringfügige materialabhängige Änderung des Hauptmagnetfeldes.
• – Statische Magnetfeldinhomogenitäten werden durch die sogenannte Shimmung des Magnetfeldes weitgehend ausgeglichen, geringe Restinhomogenitäten können aber dennoch verbleiben.
• – Nichtlinearitäten der Gradientenspulen verursachen Verzerrungen in alle Richtungen. Üblicherweise sind Gradientenspulen so entworfen, um derartige Verzerrungen im Isozentrum des Gerätes zu minimieren. Daher treten derartige Effekte vornehmlich in Randbereichen von MRT-Bildern auf.
• – Wirbelströme werden in konduktiven Materialien immer dann erzeugt, wenn Änderungen der Magnetfelder durchgeführt werden. Vornehmlich tritt dies beim Schalten der Gradientenfelder auf. Die Stärke und die räumliche Verteilung der Wirbelströme hängen dabei von der angewendeten MRT-Sequenz ab. Ihrerseits verursachen die Wirbelströme wiederum Magnetfelder, die sich anderen Magnetfeldern überlagern und so geometrische Verzeichnungen verursachen.

Primarily, four different causes can be identified that cause geometric distortion:

• - The magnetic susceptibility varies with different materials and thus causes a slight material-dependent change of the main magnetic field.
• Static magnetic field inhomogeneities are largely compensated by the so-called magnetic field shimming, but small residual inhomogeneities can nevertheless remain.
• - Non-linearities of the gradient coils cause distortions in all directions. Typically, gradient coils are designed to minimize such distortion in the isocenter of the device. Therefore, such effects occur mainly in peripheral areas of MRI images.
• - Eddy currents are generated in conductive materials whenever changes in the magnetic fields are made. This occurs primarily when switching the gradient fields. The strength and the spatial distribution of the eddy currents depend on the applied MRI sequence. In turn, the eddy currents cause magnetic fields that overlap other magnetic fields, causing geometric distortions.

A Possibility, detect geometric distortions that occur in a given MRI sequence is the use of a phantom. Such phantoms usually have a three-dimensional grid of structures that fill in the area to be rectified and that are clearly represented in an MRI image. In an image of the Phantoms leaves the image of the three-dimensional grid with the original one Compare lattices. This will create a distortion map the strength and the direction of geometric distortions in different Room points marks. This distortion map will be for correction geometric distortions recorded subsequently with the same MRI sequence Used pictures.

So far There are different ones for this designed phantoms.

That in the US 5,545,995 The disclosed phantom has a plurality of parallel rods whose diameters vary along the longitudinal axis of the rod to form conical structures.

The WO 2004/106962 A1 discloses a phantom whose lattice structure is made Layers is constructed, which form a cuboid grid.

In M. Breeuwer et al., Detection and correction of geometric distortion in 3D MR images ", Proc. SPIE 4322, 1110-1120, 2001 and in M. Holden et al. "Sources and correction of higher order geometrical distortion for serial MR brain imaging ", Proc. SPIE 4322, 69-78 In 2001, phantoms are revealed, their three-dimensional lattice Balls is formed.

Alles The phantoms disclosed have the disadvantage that they have a certain Have symmetry. For example, the above-mentioned phantoms produce a substantially same image when rotated 90 ° or around 180 ° along its longitudinal axis be rotated. Only small production-related asymmetries in the lattice structure of phantoms would be a conclusion allow for the exact location of the phantom. However that is not reliable possible, because deviations from the lattice structure on this scale can also be due to geometric distortions of the imaging system. So far must therefore manually adjust the location and orientation of the phantom the position and orientation of his image. This is especially true in the case of the standard use of the phantom to correct geometric distortions is a possible source of error. Deviations from the grid structure, resulting from manufacturing tolerances can not be automatically separated from deviations resulting from magnetic field inhomogeneities.

In the US 4,888,555 a phantom for measuring image parameters, in particular also the geometric distortion, is disclosed. For this purpose, inter alia, a plane-shaped coordinate system grating is arranged along three mutually orthogonal main planes. In the coordinate system grid, so-called resolution scales ("resolution scales") can be used.

Therefore It is the object of the present invention to provide a phantom in particular for determining geometric distortions in an imaging system is used, so that an automatic adjustment his orientation and the orientation of his image is made possible while at the same time substantially completely covering the imaging volume.

The The object is achieved by a Phantom solved according to claim 1.

advantageous Embodiments of the device are each the subject of further Claims.

The Phantom according to the invention according to claim 1 comprises a plurality of lattice-forming structures, by arranging them in the phantom a three-dimensional lattice, the is characterized by three principal axes, is formed the phantom is asymmetric, so that from an image the phantom's orientation in making the image can be determined in a clear way. The basic structure of the grid is regular. By the lattice-forming structures is a built up of unit cells Room grid shaped. This can be achieved by using at least one the grid-forming structures offset the basic structure of the space grid is arranged asymmetrically and / or by at least one of the lattice-forming Structures at asymmetric position from the other lattice-forming Structures is distinguishable, and / or by at least one further structure arranged at asymmetric position in the phantom is.

by virtue of The asymmetric training may affect the orientation of the phantom closed in the making of the image solely from his image become. This can be done when creating geometric distortion maps capture geometrical distortions more precisely because of production-related Deviations from the ideal lattice structure due to the unique Knowledge of the orientation of the phantom can be reliably taken into account.

In In a variant, this happens because at least in the phantom a part of the lattice-forming structures arranged asymmetrically is. Due to the asymmetrical arrangement of only a part of the lattice-forming structures, the lattice structure remains substantially so that the phantom continues to serve its purpose, namely one To provide grating, the possible to equalize the image volume completely covered. Usually enough it out, a single grid-forming structure in asymmetric Position slightly offset from the grid to arrange. The grid-forming However, structure is shifted so much that it appears in the image of the Phantoms despite the distortions caused by magnetic field inhomogeneities can be identified without doubt.

In another variant is at least one more structure in the phantom arranged at asymmetric position. This further structure serves especially the determination of the orientation of the phantom. This variant has the advantage that the additional structure that is preferably clearly in shape from the lattice-forming structures clearly, despite any geometric distortions can be identified. The orientation can thereby clearly be determined.

In a further variant is at least one of the lattice-forming in the phantom Structures at asymmetric position from the other lattice-forming structures distinguishable trained. Distinguishing feature can, for example the size of the least be a lattice-forming structure that stands out clearly from the others differs grid-forming structures. This variant has the Advantage that the largely regular grid structure itself not changed What will be advantages in implementing an evaluation algorithm Has.

In An embodiment of the phantom is the lattice-forming structures spherical Structures. The symmetry breaking is then in a preferred variant realized by at least one of the spherical structures a from the other spherical ones Structures has distinguishable diameter and in asymmetric Position is arranged.

In a further embodiment In the phantom, the lattice forming structures are a variety of Planes that are arranged so that they intersect and form a three-dimensional grid. The symmetry breaking is then in a preferred embodiment realized by at least one of the levels outside the Grid structure is arranged.

The Invention as well as advantageous embodiments according to the features of the subclaims below with reference to schematically illustrated embodiments explained in the drawing, but not limited thereto to be.

It demonstrate

1 and 2 a longitudinal or a cross section through a phantom whose lattice structure is formed essentially of spheres,

3 and 4 a longitudinal or a cross section through another embodiment of a phantom whose lattice structure is formed from balls,

5 and 6 a longitudinal or a cross section through a further embodiment variant of a phantom whose lattice structure is formed from balls,

7 a three-dimensional schematic representation of a phantom whose lattice structure is formed from intersecting planes.

In 1 and 2 is a longitudinal or a cross section through a phantom shown, which is suitable for determining geometric distortion in an MRI device. The engraved lines II and II-II respectively indicate the location of the longitudinal or the cross section.

The lattice-forming structures are in the phantom 1a from balls 3 educated. In the phantom shown here 1a are the balls 3 formed as a hollow balls, which are filled with water. For easier filling, the balls 3 by hollow rods 5 connected, so that over this water can be filled. The balls 3 are in polymethyl methacrylate 7 embedded, which is distinguishable in a MRI image of water.

The balls 3 are arranged so that they form a three-dimensional, characterized by three main axes and constructed of unit cells grid.

The shape of the lattice-forming structure as a sphere 3 has the advantage that a unique point, namely the center, can be used as a characteristic grid point. Other possible simple shapes which have a similar property and are therefore suitable as lattice-forming structures in the case of a phantom are point-symmetrical structures, such as, for example, cuboids or ellipsoids.

The determination of the geometrical distortions in the imaging volume is made possible by the lattice structure, which substantially completely fills the imaging volume. For this purpose, the geometrical distortions of the individual ball positions in an image of the phantom 1a measured and with the position of the balls 3 in the phantom 1a compared.

A deviation of the image of a sphere 3 from their predetermined nominal position can either result from the geometric distortion of the imaging system or due to small production-related deviations of the phantom 1a arise. Only if the orientation of the phantom 1a can always be clearly determined from his image, production-related deviations in the determination of the geometric distortion can be considered.

In the in 1 and 2 shown phantom 1a this is through two balls 9 . 11 allows, in its size, from the remaining balls 3 differ. Due to the arrangement of the two balls 9 . 11 at different distances to the central sphere 13 can be found in the image of the phantom 1a determine its orientation in the production of the image in a clear way. A rotation about the longitudinal axis of the phantom by 90 °, for example, no longer leads to an identical image.

In 3 and 4 is a longitudinal or a cross section through a further embodiment of the phantom 1b shown. The broken lines III-III and IV-IV indicate the location of the longitudinal or the cross section.

In the embodiment shown here, only one ball differs 15 in their size from the other balls 3 , Because they are outside the two mirror planes 17a . 17b of the phantom 1b is arranged, is the symmetry of the phantom 1b already only by this one ball 15 clearly broken.

In the 1 to 4 presented variants of the phantom 1a . 1b have the advantage that changing the size of some grid-forming structures does not change the grid itself. The imaging volume is also filled in a regular manner by the grid structure.

In 5 and 6 is a longitudinal or a cross section through a further embodiment of the phantom 1c shown. The dashed lines VV and VI-VI respectively denote the location of the longitudinal and the cross section.

In the embodiment shown here, in addition to the lattice-forming balls 3 another structure 19 built into the phantom for symmetry breaking.

7 schematically shows an embodiment in which a cuboid phantom 1d one of levels 21 contains formed lattice structure. For the sake of clarity, the planes in the z-direction are not drawn.

This is a level 23 in the x-direction and another level 25 are arranged in the y-direction outside the lattice structure, is the symmetry of the phantom 1d Broken. The orientation of the phantom 1d can be from one of the phantom 1d produced image can be determined in a clear way.

Claims (5)

Phantom comprising a plurality of lattice-forming structures ( 3 . 9 . 11 . 13 . 15 . 21 . 23 . 25 ), by their arrangement in the phantom ( 1a . 1b . 1c . 1d ) a three-dimensional space grid composed of unit cells, which is characterized by three principal axes, is formed, wherein the phantom ( 1a . 1b . 1c . 1d ) is formed asymmetrically, so that from an image of the phantom ( 1a . 1b . 1c . 1d ) whose orientation can be determined unambiguously during the production of the image, by at least one of the lattice-forming structures ( 23 . 25 ) is arranged asymmetrically to a basic structure of the spatial lattice and / or - by at least one of the lattice-forming structures ( 9 . 11 . 15 ) in asymmetric position from the other lattice-forming structures ( 3 . 13 ) is distinguishable, and / or - by at least one further structure ( 19 ) at asymmetric position in the phantom ( 1a . 1b . 1c . 1d ) is arranged. Phantom according to Claim 1, characterized in that the lattice-forming structures have spherical structures ( 3 ) are. Phantom according to claim 2, characterized in that at least one of the spherical structures ( 9 . 11 . 15 ) one of the other spherical structures ( 3 . 13 ) has a distinguishable diameter and is arranged in an asymmetrical position. Phantom according to Claim 1, characterized in that the lattice-forming structures have a multiplicity of planes ( 21 . 23 . 25 ) arranged to intersect and form a grid. Phantom according to one of claims 1 to 4, characterized in that the phantom ( 1a . 1b . 1c . 1d ) is designed for imaging with a magnetic resonance tomography device.