DE102004033722A1 - Radio frequency volume coil for MRI and MRS has an octahedral shape made up linear elements that are combined to form a control network - Google Patents

Abstract

Radio frequency volume coil for magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) has a coil comprises of linear resonant conducting elements that correspond to the edges of an octahedron, said linear elements being connected to a control network.

Description

The The invention relates to a radio-frequency volume coil, which in the Magnetic Resonance Imaging and Magnetic Resonance Spectroscopy for Transmitting and receiving radio frequency fields at close range, i. in the volume enclosed by the coil. For the application revealed manifold Options. A special field of application of magnetic resonance is medical Range, such as in the study of the human head and of the brain. Another area of application is the investigation of materials on their chemical composition and their physical Properties such as e.g. the diffusion.

In magnetic resonance imaging (MRI) or magnetic resonance spectroscopy (MRS or NMR spectroscopy), the objects to be examined are placed in a static magnetic field (B ₀ ). For atomic nuclei with a non-zero nuclear spin this leads to a splitting of the otherwise degenerate nuclear spin energy levels. By irradiating electromagnetic radiation of suitable frequencies (B ₁ ), the so-called crossover frequencies, these energy levels can be investigated in an NMR spectroscopic experiment. This experiment allows manifold conclusions about the chemical compounds present in the object, their structures and chemical and physical interactions.

By using B ₀ fields superimposed on a gradient field, or by using B ₁ fields having a gradient, the crossover frequency or transition intensity between two nuclear spin levels becomes location dependent. This is used in magnetic resonance imaging and in MR-assisted diffusion experiments.

For the radiation the electromagnetic radiation, which is usually in the radio frequency range must be done, one uses so-called high-frequency (RF) - or radio frequency (RF) coils. The coils can be differentiated according to surface and volume coils. These are among other simple conductor loops, saddle coils, solenoids, Loop gap, birdcage and TEM resonators. Also combinations of Volume coils and surface coils are known.

There are a variety of volume coils known. Volume coils with a homogeneous B ₁ field are in particular TEM coils ( US 5,557,247 ) and Birdcage (J. Link, "The Design of Resonator Probes with Homogeneous Radiofrequency Fields", in "NMR Basic Principles and Progress", Editors: P. Diehl, E. Fluck, H. Gunther, R. Kosfeld, J. Seelig Vol. 26 "In-vivo Magnetic Resonance Spectroscopy I: Probeheads and Radiofrequency Pulse Spectrum Analysis"). From C. Silva, H. Merkle: "Hardware Considerations for Functional Magnetic Resonance Imaging", Concepts in Magnetic Resonance Part A, 16A, 35-49 (2003) discloses volume coils with B ₁ field gradients such as the helix coil, high sensitivity surface coils but inhomogeneous B ₁ field and combinations of volume coil and surface coil (s). The known volume coils have disadvantages in the application. Thus, with a volume coil with homogeneous B ₁ field, although a good illumination of the object can be achieved, but it is to be expected with a lower sensitivity than when using surface coils. A volume coil with a B ₁ field gradient generally has a spatially fixed orientation of the B ₁ field gradient. In general, surface coils have a high sensitivity near the coil, but poor illumination of distant regions. With the combination of volume coil and surface coil (s) a good illumination of the object and a high sensitivity can be achieved. However, the more or less fixed spatial arrangement of volume coil and surface coil allows only one use for special measurements.

• • Beim Einsatz als Volumenspule sollen sich innerhalb des eingeschlossenen Volumens homogene B₁-Felder erzeugen lassen. Dabei soll die Orientierung der erzeugten B₁-Felder in alle 3 Raumrichtungen frei wählbar sein.
• • Beim Einsatz als Volumenspule sollen sich innerhalb des Volumens B₁-Feldgradienten erzeugen lassen, die in alle 3 Raumrichtungen frei wählbar sind.
• • Beim Einsatz als Oberflächenspule soll diese sich in allen 3 Raumrichtungen um das eingeschlossene Volumen orientieren lassen.

The object of the invention is to specify a radio-frequency volume coil for magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS), which behaves both as a surface coil and as a volume coil and thus the advantages of volume coils and surface coils united in itself. The coil should meet the following characteristics:

• • When used as a volume coil, homogeneous B ₁ fields should be generated within the enclosed volume. The orientation of the generated B ₁ fields should be freely selectable in all three spatial directions.
• • When used as a volume coil, B ₁ field gradients should be generated within the volume, which can be freely selected in all three spatial directions.
• • When used as a surface coil, it should be orientated around the enclosed volume in all three spatial directions.

Thus, the abolition of the restriction of the spatial orientation of the B ₁ field should be possible, ie, the orientation of the B ₁ field should be possible in all three spatial directions within the enclosed volume. Furthermore, the design-related separation between homogeneous B ₁ field coil, and B ₁ -Fieldgradientenspule should be repealed the.

According to the invention Task by the claimed in claim 1 radio-frequency volume coil solved, in further claims is designed advantageous.

In the following presentation, the group of points O _h is understood to be the group of points known from FA Cotten, Chemical Applications of Group Theory, 3rd Ed., Wiley-Interscience, 1990.

The octahedron coil according to the invention is defined by the octahedral sequence 12 of resonant line elements. The resonant line elements form the edges of the octahedron coil. The ends of each four line elements form an octahedral corner. The geometry of the coil is described by the point group O _h . It is not required that all 12 line elements are really resonant. Whether a line element must be performed resonantly or can not be performed resonantly depends on the symmetry of the B ₁ field to be generated.

The object is also achieved according to the invention by a distorted octahedron coil. By this is meant a shift of the octahedral corners with respect to the ideal octahedral geometry. In this case, the spatial dimensions of the 12 resonant line elements are not identical. The geometry of the coil can then be described by one of the subgroups of the point group O _h .

The resonant current paths / modes which can be generated in the octahedral coil and the resulting B ₁ fields are given by the irreducible representations of the point group O _h or its subgroups in the case of distorted octahedra. That is, only B ₁ fields can be generated which have the symmetry of one or the linear combination of several irreducible representations.

The resonant frequency of the B ₁ field generated in the octahedron coil corresponds to the resonant frequency of the resonant line elements. By coupling the resonant line elements, the resonance frequency of a vibration mode of the octahedron coil can be shifted in comparison to the resonance frequency of the individual line elements. The frequency shift is proportional to the strength of the coupling of the resonant line elements used. This coupling-related resonance shift can be experimentally determined for any desired mode of oscillation of a constructed octahedral coil. By changing the capacitance or inductance of the individual line elements or by changing the capacitance or inductance of contiguous groups of line elements, this effect can be corrected.

The electronic components (R, C, L, transmission lines, strip conductors), which are used to build the resonant partial coils are freely selectable.

The spatial structure of the resonant line elements determines the spatial B ₁ field intensity within the octahedron coil. By suitable choice of the spatial profile of the resonant line elements, the spatial B ₁ field intensity distribution can be influenced in the desired manner. If the spatial course of the 12 line elements is not identical, this leads to a distortion of the octahedron. Thus, the octahedral coil must be described by a subgroup of the point group O _h .

The arrangement of the resonant line elements is such that the overall arrangement of the 12 line elements results in a regular or distorted octahedron. The 6 connection points of the line elements correspond to the corners of the octahedron. These join points have finite extent and may also be designed as truncated octahedral corners. The connection points usually connect the 4 resonant conductor elements which collide with these. But they can also be used to feed high-frequency currents or to place electronic components that link two or more conductor elements together. The latter is only possible if the symmetry consideration of the desired B ₁ field allows this.

The control of the resonant line elements can be capacitive or inductive. Whether a resonant line element of the octahedron coil must be controlled, and in which phase the control must occur, results from the symmetry consideration of the B ₁ field that is to be generated. Taking into account the symmetry of the B ₁ field to be generated, the driving of the resonant conductor elements can take place via the octahedral corners or along the edges of the octahedra.

The volume coil according to the invention represents a new type. It builds on the knowledge that previously used volume coils can be described analogously to chemical compounds by point groups. For example, the Helmholtz coil developed by Merkte (Silva A. C and Merkle H., Hardware Considerations for Functional Magnetic Resonance Imaging, Concepts in Magnetic Resonance Part A, Vol 16A (1) 35-49 (2003)) has a trigonal pyramidal arrangement of Part coils whose symmetry is described by the point group C _4v leaves. In continuation of this finding, the coil according to the invention allows both the generation of homogeneous B ₁ fields and for the generation of B ₁ field gradients in the enclosed octahedral volume. The orientation of these B ₁ fields or gradients in all three spatial directions is freely selectable.

The coil can also be operated in vibration modes which are subgroups of the O _h group. This can be used to achieve a localization of the B ₁ field within the octahedral volume. This localization results in increased sensitivity of the coil in the selected volume element as compared to volume coil operation.

The Coil can be used both as a parallel transmit, parallel receive as Also, parallel transceiver coil can be used when each coil element or providing each group of coil elements with their own transmit and receive channels becomes.

The single driving and reading out of the 12 resonant line elements makes it possible to use the coil in one and the same experiment to generate homogeneous B ₁ fields, B ₁ field gradients and local B ₁ fields.

The coil according to the invention offers a number of advantages. By suitable driving and resonant tuning of the line elements, homogeneous B ₁ fields and B ₁ field gradients can be generated. By individually controlling the line elements, a targeted localization of the B ₁ field within the octahedral volume can be achieved. All resonant currents or B ₁ fields that can be generated in the coil are given by the octahedral coil geometry, and can be attributed to group or theoretical combinations of single or linear combinations of individual irreducible representations of the octahedral point group O _h or its subgroups. Due to the individual controllability of the octahedron constituent line elements, it can be used for parallel volume excitation or image acquisition. Furthermore, due to its high symmetry, the octahedral coil offers the possibility of changing the spatial orientation of the generated B ₁ field or the B ₁ field gradient in all three spatial directions during an experiment. This can be used to simplify existing experiments as well as lead to the development of novel pulse sequences.

In the following exemplary embodiments, the coil according to the invention is explained with reference to an octahedron coil. It shows how the symmetry to the desired B ₁ fields and the current paths in the octahedron coil are determined. For all examples the in the 1 based on schematically illustrated Oktaederspulenanordnung, which is formed from 12 quarter circles. The twelve red dots indicate the resonant conductor elements.

The 2 shows a plan view of the octahedron coil with the connections 1 to the driving network. It consists of the Plexiglas sphere with a diameter of, for example, about 160 mm, on whose surface quadrants of copper foil 2 are glued on. In the plexiglass ball is a second Plexiglas ball with a diameter of for example about 140 mm, which is the measurement object, such as an agar-agar gel phantom 3 , for which MR measurements are taken. The resonance conditions of the resonant subsegments are achieved by soldering appropriately dimensioned ceramic capacitors 4 set in the Cu film quarter circles. The radio frequency RF is preferably capacitive over semirigid coaxial cable 1 coupled.

example 1

Octahedral coil with O _h symmetry and homogeneous B ₁ field along xy.

In this example, the symmetry for a homogeneous, xy polarized B ₁ field in the point group O _{h is} determined. Subsequently, a current path for this B ₁ field is determined by forming the vector product between the position vector for the center of a resonant conductor element and the B ₁ field vector resulting from the sum of the B ₁ field vectors of the octant enclosing the line element. If the desired B ₁ field and the particular current path belong together then both must have the same representation in the point group O _h .

1.1. B ₁ field symmetry

The reducible representations of a homogeneous B ₁ field polarized in the xy direction can be determined by drawing a B ₁ field vector into each octant of the octahedral coil ( 3 ).

The set of basic functions required for symmetry considerations form the location coordinates and orientations of the eight in 3 drawn B ₁ field vectors. The representation for this basis set is obtained by determining the character for each of the 48 symmetry operations of the point group O _h .

The illustrated homogeneous B ₁ field distribution has the representations Γ = T 1u

1.2. Current path and symmetry

The current distribution given by the vector product to the above homogeneous B ₁ field is in 4 shown. The determination of the symmetry of the current distribution on the resonant line elements is carried out using the location coordinates of the resonant conductor elements and the drawn current vectors.

The represented current distribution has the representation Γ = T 1g

1.3. Result of the symmetry analysis

The representation of the B ₁ field Γ = T _1u and that of the current paths Γ = T _1g correspond to each other. Both contain only the irreducible representation of type T ₁ .

The difference in inversion (subscript "u" and "g" for even and odd) is easy to understand: Recall that a circular current leads to a B ₁ field that is perpendicular to the area enclosed by the circulating current , It follows that the circular current must have a positive sign relative to an inversion in the center of the circle (= subscript g for even), while the B ₁ field has a negative sign (=> subscript u for odd).

The symmetry analysis also shows that there is no current in the line elements lying in the xy plane 5 . 6 . 7 . 8th flows to testify to the desired xy polarized B ₁ field. That is, the line elements 5 . 6 . 7 and 8th can not be performed resonantly.

1.4. Electronic construction a real octahedral spool

The ladder element network of the octahedron coil can be, as in 5 shown, two-dimensional sketch. The numbered lines represent the resonant conductor elements; ie the octahedral edges. The dots represent the nodes; ie the octahedral corners.

The non-resonant conductor elements 5 . 6 . 7 and 8th are designed as R, C, L circuits to avoid Eddi current effects due to switched B ₀ field gradients, as in 6 is shown. It is a series resonant circuit that is not tuned to the resonant frequency of the octahedron coil. However, the resonance frequency is far below that of the resonant conductor elements.

The resonant conductor elements 1 . 2 . 3 . 4 . 9 . 10 . 11 and 12 were constructed as pure inductors ( 7 ). The capacity required for a series resonant circuit, and the drive was placed in the node.

The capacity necessary for the resonance condition was used at the connection points. Capacitive RF coupling was also carried out at these connection points ( 8th ).

1.5. Results

Shown below are three-dimensional polygon plots obtained with the octahedral coil in an MRI experiment ( 9 ). From left to right are shown an axial, a sagittal and a coronal image of a gel sphere in the octahedral coil. It can be seen above all in the axial image that the B ₁ field distribution has the symmetry of the field-generating coil. The illumination in saggital and coronal slice orientation is good.

To measure the homogeneity of the B ₁ field, the "ReferenceGain" for axial layers was determined in 5 mm increments along the z-axis.The ReferenceGain value indicates the attenuation of the RF transmit power required to obtain maximum intensity in That is, the higher the ReferenceGain value, the higher the attenuation of the transmit power and the more sensitive the coil 10 shown plot of the ReferenceGains against the position of the selected axial layer shows a large largely homogeneous B ₁ field area within the Oktaederspulenvolumens.

1.6. Summary

The octahedral coil can generate a homogeneous B ₁ field in the drive derived and described above.

Example 2

Octahedral coil with O _h symmetry but localization of the B ₁ field in the upper hemisphere

In this example, the localization of the B ₁ field in the upper hemisphere of the octahedron coil is achieved.

2.1. B ₁ field symmetry

The 11 shows a B ₁ field located in the upper hemisphere of the octahedral coil, which is represented by the four B ₁ field vectors shown in the figure.

The illustrated localized B ₁ field distribution has the representation in the point group O _h Γ = T 1g + T 2g + 2 · T 1u

However, the illustrated localized B ₁ field distribution has no octahedral symmetry like the coil, but belongs to the point group C _4v , which is a subgroup of the O _h point group.

In the point group C _4v , the localized B ₁ field distribution has the representation Γ = E

2.2. Current path and symmetry

The current distribution given by the vector product to the above local B ₁ field is in the 12 shown.

The illustrated current path has the representation in the point group of the octahedral coil (O _h ) Γ = 2 · T 1g + 2 · T 1u

However, the current path shown has C _4v symmetry and has representation in the associated point group Γ = E

2.3. Result of the symmetry analysis

The representation of the B ₁ field and that of the current paths correspond to each other when the symmetry analysis is based on the reduced symmetry of the localized B ₁ field.

2.4. Electronic construction a real octahedron coil with hemisphere localization

After the above rung would have the line elements 1 . 2 . 3 and 4 resonant, all others could not be resonant. Due to the simple and rapid feasibility, the line elements were maintained as in the construction of Example 1. But in contrast to example 1, in example 2 only one of the two hemispheres was driven with HF radiation.

2.5. Results

The scans obtained for the gel phantom are shown in the polygon plots ( 13 ). A localization of the B ₁ field in the upper hemisphere is achieved. From left to right in the figure, an axial, a sagittal and a coronal image of a gel sphere are shown in the locally controlled octahedral coil. It can be seen in the axial image that the B ₁ field distribution has the symmetry of the field-generating coil and is largely homogeneous. The illumination in saggital and coronal slice orientation shows the expected localization of the B ₁ field in one of the two hemispheres.

The plot of the ReferenceGain (as a measure of the sensitivity of the coil) against the position of the selected axial layer is shown in the graph in 14 applied together with the result for a homogeneous B ₁ field. This plot clearly shows that the localization of the B ₁ field leads to an increased sensitivity of about 3 dB in the respective hemisphere. In this case, 3 dB correspond to a factor of root 2 in relation to the transmission power.

2.6. Summary

In This example shows that the octahedron coil without change their spatial Structure can be used as a local coil.

The local control leads to an increased Sensitivity of the octahedral coil in the selected volume.

Example 3

Octahedral coil with O _h symmetry and B ₁ field gradient along the z axis

This example shows how the octahedral coil can be used to generate B ₁ field gradients.

3.1. B ₁ field symmetry

For a B ₁ field gradient in the z direction, the desired B ₁ field is replaced by that in the 15 shown eight B ₁ field vectors shown.

The illustrated B ₁ field gradient has the representation in the point group O _h Γ = T 1g + T 2g

3.2. Current path and symmetry

The current distribution given by the vector product to the above B ₁ field gradient is in the 16 shown.

The illustrated current path has the representation in the point group O _h Γ = T 1u + T 2u

3.3. Result of the symmetry analysis

The representation of the B ₁ field Γ = T _1g + T _2g and that of the current paths Γ = T _1u + T _2u correspond to each other considering the inverse behavior with respect to the inversion.

The symmetry analysis also shows that there is no current in the line elements lying in the xy plane 5 . 6 . 7 . 8th flows to create the desired B ₁ field. That is, the line elements 5 . 6 . 7 and 8th can not be performed resonantly.

3.4. Electronic construction of a real octahedron coil with B ₁ field gradients in the z-direction

After the above rung would have the line elements 1 . 2 . 3 . 4 . 9 . 10 . 11 and 12 resonant, all others could not be resonant. That is, the structure corresponds to that for the generation of a homogeneous B ₁ field (see 1.4.). However, the current flow direction in the lower hemisphere of the octahedron coil is rotated by 180 °.

3.5. Results

The B ₁ field obtained for the gel phantom is indicated by the polygon plots for axial, sagittal, and coronal slices in FIG 17 documented. One sees the desired B ₁ field gradient in the z-direction. In the equatorial plane of the coil, the zero crossing of the B ₁ field gradient is clearly visible. In the following ReferenceGain plot ( 18 - Comparison of RefGain values: homogeneous vs. Gradient in the B ₁ field) the result is confirmed. It can be seen in this representation of the B ₁ field strength that the gradient is weaker in the region of the upper and lower hemisphere than in the region of the equatorial plane.

3.6. Summary

In this example it is shown that the octahedral coil can be used as a B ₁ field gradient coil without changing its spatial configuration. In this case, the gradient field for the given Oktaederspulenausführung on two regions of different gradient strength. At the equatorial plane, the gradient is very strong, while it is weaker in the lower or upper hemisphere.

Claims (11)

Radio-frequency volume coil for magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS), characterized in that the coil consists of resonant conducting elements corresponding to the edges of an octahedron, the conducting elements being connected to a driving network. Volume spool according to claim 1, characterized the coil consists of 12 resonant conducting elements. Volume coil according to claim 1 and 2, characterized in that the line elements are designed to be resonant depending on the B ₁ field to be configured. Volume spool according to Claims 1 to 3, characterized the octahedron coil is either symmetrical or distorted. Volume spool according to Claims 1 to 4, characterized that the main axis of the octahedron coil is parallel or in a certain, but any angle to the static magnetic field is oriented. Volume spool according to claims 1 to 5, characterized in that that the line elements on the surface of a Plexiglas ball as Quarter circles made of Cu foil are arranged. Volume spool according to Claims 1 to 6, characterized that within the Plexiglas ball with the line elements a arranged further Plexiglas ball for receiving the measurement object is. Volume spool according to Claims 1 to 7, characterized that the resonance conditions of the resonant subsegments means in the cu-foil quarter circle arranged capacitors are designed adjustable. Volume spool according to Claims 1 to 8, characterized that the octahedron coil as a transmit, receive or transceiver coil is operated. Volume spool according to Claims 1 to 9, characterized that each subsegments of the octahedral coil with a receiver and each associated with a transmitter associated therewith. Volume spool according to Claims 1 to 9, characterized that is a linear combination of sub-segments of the octahedron coil with a receiver and in each case a transmitter associated therewith connected is.