DE102020005044A1 - Coil array and its use - Google Patents

Abstract

The invention relates to a coil array which has at least two coil elements and in which at least one coil element is bent and not closed, so that access from at least one supply line to an object to be examined is made possible and its use.

Description

The invention relates to a coil array and its use.

Magnetic resonance imaging (MRI) is one of the most powerful non-invasive medical imaging modalities used in everyday clinical practice. Over the past decade, tremendous technological advances have significantly improved image quality, thereby improving many clinical applications. Every MRI system consists of a number of subsystems that perform dedicated functions that are necessary for the MR image acquisition of an object. As one of the essential magnetic resonance (MR) components, the radio frequency (RF) system plays a key role in generating RF excitation pulses and receiving the MR signal.

For signal reception, it is advantageous to position an RF coil as close as possible to the object, since this achieves the best possible fill factor and thus a higher signal-to-noise ratio (SNR). The SNR increases approximately in proportion to the main magnetic field strength (B ₀ ) [1]. For example, the SNR is twice as high if the B ₀ is increased from 1.5 Tesla to 3 Tesla. The selection of an RF coil is usually based on the desired clinical applications. The main aspects to consider are the field of view (FOV), spatial resolution, sensitivity and parallel imaging capability. RF coils were constructed for each volume of interest (VOI), requiring the purchase of a number of expensive RF coils to cover the imaging of different body parts.

There are several conventional coils that can be used for imaging, particularly of the infant brain. Hughes et al. resized a conventional birdcage coil and fitted it to the infant's head at 3 Tesla [2]. In this way, the MR image quality could be improved compared to the use of commonly available adult coils. Multi-channel array coils are often used to improve the coil sensitivity and to be able to be used at higher B ₀ [3] [4] [5]. A specific type of coil, such as an 8-channel square loop array, is often chosen as a reference coil to evaluate the performance of newly developed arrays [6] [7] that can also be used for infant head studies at 7 Tesla.

Birdcage coils and multi-channel loop arrays are usually mounted on cylindrical housings. This significantly restricts access, especially to lines or hoses for anesthesia, artificial feeding or respiratory aids for infants [8] [9]. In order to avoid this problem, the diameter of the coil or array must inevitably be increased, resulting in a poor fill factor and hence a deterioration in SNR. Pure multi-channel receiving arrays can often be designed to be geometrically more flexible and can therefore be used together with a body coil without the problem of restricted accessibility. Body coils are usually installed in MRI scanners up to 3 Tesla, but not at field strengths of 7 Tesla or higher.

Due to the geometric length of dipoles, which is determined by the wavelength, they are more likely to be used in MRIs with a field strength of 7 Tesla or more, despite the advantage of a greater penetration depth in tissue [10]. Lakshmanan et al. [11] modified the straight dipole antenna by bending it into a "loop shape" and bridging the ends with a capacitor. This enabled them to superimpose both the current distribution of a conventional coil and that of a dipole antenna.

Bo-shimming problems arise in MRI measurements of the brain mainly due to differences in susceptibility between air and biological tissue and are therefore particularly pronounced in the area of air-filled body cavities such as the mouth, nose and paranasal sinuses. In this area, the B ₀ field often cannot be sufficiently homogenized using conventional shim methods.

It is therefore the object of the invention to provide a coil array for magnetic resonance tomography which can be used with VOIs such as subjects and patients, preferably children, in particular infants or small children, but also monkeys and biological tissue. In particular, the coil array should allow good or free access to supply lines, such as lines or hoses, for example for anesthesia, artificial nutrition or ventilation, and other patient connections, such as cables or probes, for example for measurement, without reducing the fill factor . The coil array according to the invention should enable high sensitivity, in particular a high signal-to-noise ratio, in particular in the area of the brain and reduce B ₀ shimming problems, in particular in the area of air-filled cavities such as the nasal cavities, for example in brain MRT.
Furthermore, the smallest possible diameter of the coil or the coil array should be made possible, the fill factor is increased, and a good signal-to-noise ratio is made possible, especially when small objects to be examined, such as the heads of children or infants or monkeys, but also biological tissue, are to be examined by magnetic resonance imaging. The penetration depth of the RF fields should be improved. In particular, the coil array should enable the measurement of volumes to be examined, VOI, which are smaller than the standard adult head. The coil array should also be suitable for magnetic resonance imaging and magnetic resonance spectroscopy.

Based on the preamble of claim 1, the object is achieved according to the invention by the features specified in the characterizing part of claim 1.

With the coil array according to the invention, it is now possible to examine VOI', such as test persons and patients, preferably children, in particular babies or small children, monkeys, in particular their heads, but also biological tissue by magnetic resonance imaging, with good or free access to feeds, such as lines or hoses, e.g. for anesthesia, artificial nutrition or ventilation, as well as other patient connections, such as cables or probes, e.g. for measurement, is made possible without reducing the fill factor or causing the risk of the hoses kinking. The coil array according to the invention has high sensitivity, in particular a high signal-to-noise ratio, especially in the area of air-filled cavities, for example the forehead, and no or reduced B ₀ shimming problems, especially in the area of the nasal cavities, for example in brain MRT. The fill factor of the coil array is increased. The penetration depth of the RF fields into the object to be examined is improved and higher-quality images from deep regions are made possible. In particular, the coil array enables the measurement of volumes to be examined, VOI, which are smaller than the standard adult head. The coil array according to the invention can also be used for magnetic resonance tomography and magnetic resonance spectroscopy.

Advantageous developments of the invention are specified in the dependent claims.

In the following, the invention is described in its general form without this being intended to be restrictive.

According to the invention, the coil array consists of at least two coil elements, of which at least one coil element is curved and open, so that access from leads, such as at least one hose or other patient connection, to an object to be examined is made possible.
This configuration of the coil array has the consequence that the problems with bo-shimming, especially in the vicinity of the nasal cavities, are reduced.
The fact that at least one coil element is curved and open makes it possible for a subject or a patient, in particular a small child or infant, or monkeys, in particular their heads or biological tissue, to be supplied with supplies such as ventilation hoses and/or hoses for anesthesia and /or the artificial nutrition, as well as other connections, such as patient connections, such as cables or probes, for example for measurement, can be supplied well. Especially with small objects to be examined, such as children, small children, infants or monkeys, especially their heads, better images can be generated, which in particular have a good signal-to-noise ratio, since the coil array according to the invention can be made structurally smaller, although a good one Cable access is allowed.
In general, the area of the air-filled cavities, eg mouth, nose and nasal cavities, always causes a B ₀ shimming problem in brain MRI. In the manner according to the invention, therefore, a higher sensitivity is obtained in the region of the forehead and the disturbance associated with Bo-shimming in the vicinity of air-filled cavities is reduced. An essential feature of the bent open dipole is its higher sensitivity in the feed area and the decrease in sensitivity towards the edge of the dipole antenna.

An opening within the meaning of the invention is understood to mean an interruption in the coil or a gap in the coil, which results in the coil not having a closed geometry. This allows access to lines or hoses, for example for anesthesia, artificial nutrition or respiration, as well as other connections or patient connections, such as cables or probes, for example for measurement.

Other coil elements can be closed. These closed coils can be surface coils or loop coils. These coils have no openings and are closed.
This has the advantage that a greater extension of the sensitive area can be achieved along the longitudinal axis of the body or the object to be examined or the longitudinal axis of the space enclosed by the coil array.

A subject or patient, preferably children, in particular infants or monkeys, and in particular their heads, but also biological tissue can be understood as an object to be examined within the meaning of the invention.

Leads within the meaning of the invention can be understood to mean all possible feed lines, such as ventilation hoses and/or hoses for anesthesia and/or artificial nutrition, as well as other connections, in particular patient connections, or other feed lines, such as cables or probes, for example for measurement .

So that the curved, open coil element enables a good supply of at least one supply or the hoses or cables mentioned, it can have an opening in the order of a few millimeters to a few centimeters and is advantageously based on the diameter of the hoses/patient connections provided. For example, the interruption or opening can be 1 mm to 20 cm or 30 cm. However, the size of the interruption can be freely selected and can be adapted to the embodiment of the array according to the invention.

The coil array according to the invention can have a plurality of coil elements. For example, it can have 3, 4, 5, 6 or more, for example up to 16 or 32 individual coil elements. However, shapes with more than 32 elements are also possible.

The coil array according to the invention preferably encloses the object to be examined completely or in a cylindrical arrangement around the object to be examined. In this case, the cylinder can advantageously taper conically into a cone or truncated cone, as a result of which better coverage of the object to be examined, for example the upper head, is achieved. The coil elements can be arranged in a row along the circumference of the cylinder which encloses a Z-axis and/or enclose the circumference or the axis of rotation of the cylinder, cone or truncated cone or be arranged along the circumference in such a way that they at least partially enclose it.

Alternatively, the elements can be arranged freely, e.g. B. in several rows along the circumference of the cylinder or circumference of the truncated cone or in the shape of the hexagons of a soccer ball.

It is important that the open/interrupted or open/interrupted elements are positioned to allow easy access to the tubing/patient connections.

The open elements are preferably also arranged in such a way that they have little sensitivity in the area of the air-filled cavities and thus avoid bo-shimming problems.

The volume enclosed by the coil array can be used in particular for examining small objects to be examined with a circumference of, for example, 35 cm to 45 cm.
A human head typically has a volume of 1260 cm ³ for a male and 1130 cm ³ for a female, and an infant's head typically has a circumference of 35 cm to 45 cm. The coil array according to the invention is therefore suitable for use in the heads of adults, children, small children, infants or monkeys, which typically have the stated dimensions in the stated size range. The publication [12] discloses relevant dimensions of heads that can typically be examined with the coil array according to the invention. The head size measurements given in this publication are part of the disclosure of the application.

At least one bent and open coil element must be present in order to achieve the effect according to the invention. However, it is also conceivable to provide several curved and not open ones, for example 2, 3, 4 or at most all elements of the coil array.
By using several closed coil elements or surface coils, the sensitivity in the area of the air-filled cavities can be adjusted in such a way that the Bo-shimming problem is largely reduced.

In the case where more than one curved, open coil element is present, different hoses, lines or cables can also be routed through different curved, open coil elements to the object to be examined. The use of more than one curved, open coil element results in an improvement in RF field homogeneity in the regions of particular interest and an improvement in RF sensitivity of the region of interest where the coils are located. By way of example, the coil array can have 1 to 4 curved, open coils.

The other coil elements can be closed coils or surface coils such as loop coils.

The number of other surface coils or loop coils can be freely selected.

The surface coils or loop coils each have at least one capacitor in order to produce resonance.

In a simple embodiment, the bent, open coil elements have no capacitor. The physical length of the bent, open coil elements can optionally be increased by the insertion of capacitors, e.g., if the dipole element is too short due to the wavelength.

The coil elements each have means for impedance matching and means for feeding. These means can be implemented by a circuit block. The feed takes place at the point of impedance matching.

The coil elements are decoupled from each other. For this purpose, means known from the prior art for decoupling the individual coil elements can be used. For example, decoupling can be effected by overlapping common legs, capacitive decoupling or inductive decoupling as well as self-decoupling and appropriate means for this. An example is shown in the publication by Yan et al [13]. Combinations of different decoupling means can also be used.

The size of the individual coil elements can be based on the object to be examined.

The small dimensions of the coil array in connection with the configuration of the coil array according to the invention have the result that the image quality, in particular the signal-to-noise ratio, improves and B ₀ shimming problems can be reduced, although supply lines, for example, to supply the living object to be examined, such as a patient, are used with various hoses.

Particularly in the embodiment for small objects to be examined, such as body parts, in particular heads, of children, small children and infants or also monkeys, the coil according to the invention enables a signal-to-noise ratio that is approx. 30% better than the prior art.

so, dass der Füllfaktor für die kleine Spule gegenüber der großen Spule bei diesem Beispiel um den Faktor 4,3 verbessert wird.A good fill factor is achieved by the design of the coil array according to the invention. With a coil array diameter of 26 cm for adult subjects and a smaller coil array with a diameter of 16 cm, the fill factor improves according to Equation 1 $$\text{f}\ddot{\text{and}}\text{factor improvement}={\left(26/16\right)}^3=\mathrm{4.3,}$$

so that the fill factor for the small coil is improved by a factor of 4.3 compared to the large coil in this example.

The coil array according to the invention can be used in magnetic resonance tomography and in magnetic resonance spectroscopy, especially when examining small objects to be examined, especially children, small children, infants and monkeys, especially their heads, but also biological tissue and in MR-compatible versions of newborn incubators.

The figures show coil arrays in schematic form.

• 1: Ein Spulenarray nach dem Stand der Technik
• 2: Ein erfindungsgemäßes Spulenarray
• 3: Ein erfindungsgemäßes Spulenarray
• 4: MR Bilder mit Spulenarrays nach dem Stand der Technik und dem erfindungsgemäßen Spulenarray

It shows:

• 1 : A prior art coil array
• 2 : A coil array according to the invention
• 3 : A coil array according to the invention
• 4 : MR images with coil arrays according to the prior art and the coil array according to the invention

1 shows a prior art coil array. It comprises several coils 1, 1a, 1b, 1c... Each coil has capacitors 2, 2a, 2b, 2c... and a unit for injection and impedance measurement 3. The reference number 4 designates means for decoupling the Wash.

In 2 the same device features have the same reference numbers. In this embodiment, a coil element 5 is bent and interrupted in a partial area, or not closed.

3 shows a further embodiment of the coil array according to the invention, in which the individual coils are arranged in a cylindrical geometry. It is made clear that the coil geometry in the area of the in 2 individual open coil element used as an example has an opening through which hoses and/or patient connections can be routed to the patient or subject ( 3 , to the right). The conventional coil arrangement with closed conductor loops 1 cannot be opened at this point due to the conductor present there ( 3 , Left).

4 shows on the left axial MR images acquired with a conventional 1-channel loop coil and a prior art 1-channel circular dipole antenna. The white dotted box in 4 indicates the area in which the object is positioned and the SNR is calculated.

These clearly show the differences in homogeneity and SNR. The calculated SNRs (=signal mean value/noise standard deviation) of the phantom were 87.3 (=288/3.3) for the conventional design and 115.7 (=344/3.0) for the design according to the invention. The figures show the MR images of a cylindrical, water-filled phantom recorded at a field strength of 7T with a 2D FLASH sequence (TR/TE = 7.6/3.45 ms, 4 averaging, matrix dimensions = 512 x 512, dimensions of the recording field = 250 x 250 mm2 total recording time about 30 s). For recording by means of the dipole element according to the invention on the right side of the 4 a circularly curved dipole with a diameter of 12 cm was used.

State of the art:

• [1] Collins, C.M. and Smith, M.B. (2001), Signal-to-noise ratio and absorbed power as functions of main magnetic field strength, and definition of "90°" RF pulse for the head in the birdcage coil. Magnetic resonance. Med., 45:684-691
• [2] Hughes EJ et al., A dedicated neonatal brain imaging system, Mag. Reson. Med., 2017.
• [3] Corea JR, Flyn AM, Lechene B, et al., Screen-printed flexible MRI receive coils, Nature communications, 2016.
• [4] Roemer PB, et al., The NMR phased array, Mag. Reson. Med., 1990.
• [5] Wiggins GC et al., 32-channel 3 tesla receive-only phased-array head coil with soccer-ball element geometry, Mag. Reson. Med., 2006.
• [6] Hong SM et al., New design concept of monopole antenna array for UHF 7T MRI, Mag. Reson. Med., 2014.
• [7] Yan X et al., 7T transmit/receive arrays using ICE decoupling for human head MR imaging, IEEE Trans. Med. Imaging, 2014.
• [8] Arthurs OJ et al., The challenges of neonatal magnetic resonance imaging, Pediatr Radiol, 2012.
• [9] van Wezel-Meijler et al., Magnetic resonance imaging of the brain in newborn infants: practical aspects, Early Human Development, 2009.
• [10] Raaijmakers AJ et al., Dipole antennas for ultrahigh-field body imaging: a comparison with loop coils, NMR Biomed., 2016.
• [11] Lakshmanan K, Cloos M, Wiggins GC, Circular dipole and surface coil loop structures and methods for using the same, US20150137815A1 , 2015.
• [12] WHO Library Cataloging-in-Publication Data WHO child growth standards : head circumference-for-age, arm circumference-forage, triceps skinfold-for-age and subscapular skinfold-for-age : methods and development. Coordinating team: Mercedes de Onis ... [et al.]. 1. Anthropometry. 2. Anthropometry-methods. 3.Body size standards. 4.Child development. 5.Growth. 6.Reference standards. 7.Nutrition assessment. I. de Onis, Mercedes. II. World Health Organization. III.Title: World Health Organization child growth standards. ISBN 978 92 4 154718 5 (NLM classification: WS 103) © World
• [13] Yan X et al., Self-decoupled RF coil array for MRI, WO2018175530 , 2018.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US20150137815A1 [0044]
• WO 2018175530 [0044]

Claims (15)

Coil array made up of at least two coil elements, characterized in that at least one coil element is bent and has an opening, so that access from at least one supply line to an object to be examined is made possible. coil array after claim 1 , characterized in that the coil elements which are not bent and open are surface coils. coil array after claim 1 or 2 , characterized in that the opening of the coil elements, which are curved and open, is large enough to allow access of at least one of the group consisting of a hose, a ventilation or anesthesia or artificial feeding line, other lines, cables or probes . Coil array according to one of Claims 1 until 3 , characterized in that the openings have a size in a range from 1 mm to 30 cm. Coil array according to one of Claims 1 until 4 , characterized in that it has 2 to 32 coil elements. Coil array according to one of Claims 1 until 5 , characterized in that it has 1 to 4 coil elements with an opening. Coil array according to one of Claims 1 until 6 , characterized in that the coil elements in the array are arranged in the form of a cylinder, a cone or a truncated cone. coil array after claim 7 , characterized in that the coil elements are arranged in a row along the circumference of the cylinder of the cone or of the truncated cone which encloses a z-axis and/or enclose the circumference or are arranged along the circumference in such a way that they at least partially enclose. Coil array according to one of Claims 1 until 8th , characterized in that the curved and open coil elements are arranged in such a way that they have little sensitivity in the area of air-filled cavities of an object to be examined. coil array after a he Claims 1 until 9 , characterized in that the coil elements enclose an object with a circumference of 35 cm to 45 cm or a volume of 1260 cm ³ to 1130 cm ³ . Coil array according to one of claims 2 until 10 , characterized in that the surface coils have at least one capacitor. Coil array according to one of Claims 1 until 11 , characterized in that the bent open coils have at least one capacitor. Coil array according to one of Claims 1 until 11 , characterized in that the coils have means for feeding and impedance matching. Coil array according to one of Claims 1 until 12 , characterized in that it has means for decoupling the individual coils. Use of the coil array according to one of Claims 1 until 13 for magnetic resonance imaging, magnetic resonance spectroscopy, as well as in MR-compatible newborn incubators.

Images