WO2005120340A2

WO2005120340A2 - Radio frequency surface coil designs for magnetic resonance apparatus with improved spatial sensitivity and selectivity

Info

Publication number: WO2005120340A2
Application number: PCT/IB2005/051855
Authority: WO
Inventors: Maria Alfonsetti; Giuseppe Placidi; Antonello Sotgiu; Valeria Clementi; Stefano Iotti; Raffaele Lodi; Bruno Barbiroli; Marcello Alecci
Original assignee: Consiglio Nazionale Delle Ricerche - Infm Istituto Nazionale Per La Fisica Della Materia
Priority date: 2004-06-07
Filing date: 2005-06-07
Publication date: 2005-12-22
Also published as: ITTO20040378A1; WO2005120340A3

Abstract

Surface coil configurations for a magnetic resonance apparatus are described, said configurations comprising a conductive path (10) which extends substantially in a figure of eight forming a pair of adjacent lobes (10a, 10b;) and including a plurality of middle linear sections (20a, 20a', 20a'', 20b, 20b', 20b'';) passed through by current directed in the same direction or in substantially concordant directions and grouped together in a central region of the coil, and sections with an arbitrary shape (22a, 22b) passed through current in the opposite direction in the peripheral region, so that the magnetic radiofrequency field in the central region of the coil is substantially directed in a direction parallel to the local plane of the coil and is spatially non-homogeneous and able to be modulated depending on the number and the geometrical arrangement of said middle linear sections.

Description

Radio frequency surface coil designs for magnetic resonance apparatus with improved spatial sensitivity and selectivity, and a magnetic resonance apparatus including said coil

The present invention relates in general to magnetic resonance apparatus and more particularly to the configuration and use of surface coils for magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) apparatus .

More specifically, the invention relates to radiofrequency surface coil configurations and to a magnetic resonance apparatus comprising such a coil. "Surface coil" is understood as meaning a coil with a two-dimensional configuration, be it of the planar type or formed on a curved or varyingly shaped surface.

Nuclear magnetic resonance is a diagnostic technique used in particular to produce images of the internal organs of the human body by means of investigation of the properties of the molecules, based on an analysis of .the absorption of an electromagnetic radiation (radio waves) due to the transition of the atomic nuclei of the molecules from a low-energy state to a high-energy state following interaction of an external magnetic field with the nuclear spin.

The nuclear spin resonance is detected by means of one or more radiofrequency coils.

The magnetic resonance scanning devices require the use of radiofrequency coils with a particular configuration, which may be both volumetric coils and two-dimensional coils. A single coil for the transmission and reception of the signals or a transmitter coil and a receiver coil may be used. In clinical and research apparatus it is common practice to work with large superconductor magnets which produce a main magnetic field B₀ with high-intensity, for example in the region of 1.5 Tesla or more. Radiofrequency surface coils have proved to be very sensitive, easy to manufacture and compact and provide a means for spatial location of the signal .

The simplest configuration is the planar, circular ring or polygonal configuration, which produces a magnetic field Bi which, in the central region of the coil, is directed substantially in a direction perpendicular to the plane of the coil (so-called axial radiofrequency field) . Figures la- Id show two configurations of axial-field radiofrequency coils.

In magnetic resonance systems the detection sensitivity is proportional to the intensity of the magnetic field B_x per unit of driving current produced by the radiofrequency coil . The spatial distribution of the radiofrequency field B_x is very important both for optimisation of the signal to noise ratio (SNR) in a given volume of interest (VOI) and for spatial discrimination of the signal which may be obtained.

Recently, with the advent of imaging techniques from several reception coils in a parallel configuration, the design and calibration of the spatial distribution of B_x have come to be regarded as an important step for optimisation of the speed of acquisition and quality of the images, for example in clinical cardiology.

The knowledge of the spatial distribution of B_x is moreover very important for multi-nuclei (for example ^Η, ¹³C, ³¹P) clinical magnetic resonance spectroscopy. This technique has been used for several years to assess in a non-invasive manner the physiological and pathological conditions of tissues and organs of the human body.

In order to maximise the sensitivity of the coil in a volume of interest, the radiofrequency field- must be perpendicular to the field B₀ in the same volume.

As shown in Figures 2a-2d, which show various arrangements of a radiofrequency coil C associated with a patient, the use of axial-field coils of the traditional type gives rise to certain constraints in the possible spatial orientation of the coil, limiting the performance and the possible range of applications of the magnetic resonance technique in the clinical and research environment: i) only some orientations of the coil are permitted with respect to the field B₀ (indicated by the arrow) ; ii) the maximum sensitivity of the coil is in the vicinity of the plane of the said coil; iii) the spatial selectivity of the coil may be adjusted only by selecting the diameter thereof (or the linear dimension in the case of a polygonal coil) .

For example, the orientations of the circular ring coil C, shown schematically in Figures 2a and 2b, prove to be adequate, while in the orientation according to Figure 2c, the coil arranged on top of the patient ' s head does not produce any signal in the central region. Moreover, when the coil is oriented at an angle α with respect to the static field Bo, as shown in Figure 2d, there is a significant loss of. signal in its central region.

As shown in Figures 3a-3d, similar problems of significant loss of signal also occur when the circular ring coil C is used in a vertically oriented static field B₀. In this case it is the orientation of Figure 3a which does not produce any signal and that of Figure 3d which results in significant signal losses.

In the case also of coils arranged at an optimum angle of orientation (α=0°) drawbacks arise in terms of spatial selectivity of the radiofrequency field along the axis of the coil, namely in a direction perpendicular to the plane of the coil .

In fact, there exist a wide range of applications where optimisation of the distribution of the radiofrequency field along the axis of the coil is very important .

For example, Figure 4a shows the anatomical details of human calf muscles. It is known that the gastrocnemius muscle (G) , the soleus muscle (S) , the posterior tibial muscle (TP) and the anterior tibial muscle (TA) are arranged along different layers .

Each of the abovementioned calf muscles is involved in a particular movement of the foot and they may be used individually or together by means of- controlled exercises. There exist particular clinical magnetic resonance spectroscopy studies where the response of certain human calf muscles is used to assess the physiological reaction of volunteers and patients affected by muscular disorders.

The image of the calf shown in Figure 4b was obtained by means of magnetic resonance at 1.5 Tesla using a traditional volumetric coil and shows in detail the anatomy of the calf. It should be noted that the magnetic resonance signal received from the various anatomical parts of the calf has a uniform intensity in the various regions . The image of the calf shown in Figure 4c was obtained by means of magnetic resonance at 1.5 Tesla using a traditional square-shaped planar coil, with lateral dimensions of about 15 cm, facing the calf.

With an increase in the distance from the surface of the coil, there is a gradual reduction in the detection sensitivity. The skin, the fat and the gastrocnemius muscle can be observed with the greater sensitivity. Despite the reduction in sensitivity, the soleus and tibial muscles are still clearly visible.

In the particular case of magnetic resonance spectroscopy studies, in order to improve the reliability after a particular controlled exercise it may be desirable to select only the signal of the gastrocnemius muscle. In fact, the response of the other muscles to the exercise has a different metabolic behaviour and would create confusion in the signal. Moreover, the signal received from the skin and from the fatty tissues below the skin could also be a source of disturbance.

With the traditional coil (having a circular, square or rectangular shape) the only possibility available for selecting a limited region of interest in formation of the image of the muscles is that of adjusting the linear dimension of the coil and its position with respect to the calf. For example, a wide coil will allow the detection of the signal received from all the muscles. A more localised response requires a smaller coil size, but this limits also detection of the signal to a smaller portion of the muscles very close to the surface of the coil .

The object of the invention is that of providing an optimum configuration of radiofrequency surface coils such as to produce, in the central region of the coil, a magnetic field B_x substantially directed in a direction parallel to the plane of the coil (so-called transverse radiofrequency field) and such that the spatial distribution of the radiofrequency field is spatially selective and adjustable for any orientation of the coil with respect to the main magnetic field, as well as an improved magnetic resonance apparatus comprising such a coil .

In particular, the object of the invention is to propose a coil configuration for use in magnetic resonance spectroscopy and magnetic resonance imaging, which may be used both in the case of a horizontal main magnetic field B₀ and in the case of a vertical field.

In order to achieve these objects the invention relates to a coil having the characteristic features claimed in Claim 1 and an apparatus having the characteristic features claimed in Claim 10.

In brief, the coil according to the invention is formed with a conductive path substantially in the form of a figure of eight, which overall includes a plurality of middle linear sections passed through by current directed in the same direction, or in any case in substantially concordant directions, and grouped together in a central region of the coil, and sections with an arbitrary, polygonal or semicircular, form passed through by current in the opposite direction in the peripheral region. The geometry of the paths in the central region is essentially of two types: i) parallel linear sections which are arranged at a predetermined distance from each other, and ii) linear sections with an intersecting geometry, at a predetermined angle and arranged at a predetermined distance from each other. The expression "substantially concordant directions" is understood as meaning, in the remainder of the description, directions diverging at an acute angle .

In both configurations the current which flows in the middle linear sections produces a magnetic field Bi which, in the central area of the coil, is a substantially transverse field.

Such a "figure-of-eight" coil has a pronounced spatial non- homogeneity of Bi in a direction perpendicular to the local plane of the coil (along the axis) . Moreover, this configuration may have a greater amplitude of the field B_x in an area located at a predetermined depth along the axis of the coil, than the conventional circular ring coil.

The coil configuration in question has advantageously the following characteristic features : - it allows the coil to be oriented in any direction with respect to the main horizontal or vertical magnetic field B₀ without significant losses of signal in the volume of interest ; - for any orientation of the coil, the distribution of the radiofrequency field in a plane parallel to the surface of the coil and situated at a certain 'distance from the said coil may be adjusted by means of suitable selection of the number and geometric arrangement of the middle linear sections; - for any orientation of the coil, the maximum amplitude of _. the radiofrequency field in a plane parallel to the surface of the coil and situated at a certain distance from the said coil may be adjusted by means of suitable selection of the number and geometric arrangement of the middle linear sections ; - for any orientation of the coil, along the axis of the coil, the distribution of the radiofrequency field is spatially selective and may be adjusted by means of appropriate selection of the number and geometric arrangement of the middle linear sections; in particular configurations an increase in the sensitivity of the radiofrequency field in a selected volume of interest along the axis of the coil may be established; - by means of appropriate selection of the number and geometric arrangement of the middle linear sections and the radiofrequency transmission power it is possible to reduce or even eliminate the unwanted signals received from an area positioned at a predetermined distance from the surface of the coil (for example skin and fatty tissues) ; and - by means of suitable selection of the number and geometrical arrangement of the middle linear sections of the radiofrequency coil it is possible to shape suitably the area in which it is possible to reduce or eliminate the unwanted signal .

Further characteristic features and advantages of the invention will be described more clearly in the following description, provided by way of a non-limiting example, with reference to the accompanying drawings, in which: Figures la-Id are schematic illustrations and practical embodiments of radiofrequency coils of the known circular ring type (la, lc) and square ring type (lb, Id) ; Figures 2a-2d show different spatial orientations of a coil of the known type within a horizontal magnetic field B₀; Figures 3a-3d show various spatial orientations of a coil of the known type within a vertical magnetic field B₀; Figures 4a, 4b and 4c are respectively a schematic illustration of the anatomical details of the human calf muscles, a magnetic resonance image relating to a cross- section obtained using a volumetric coil of the known type, and a magnetic resonance image relating to a cross-section obtained using a planar coil of the known type; Figures 5a-5d are schematic illustrations of variants of the coil according to the invention; Figures 6a-6d are schematic illustrations of variants of a first embodiment of the coil according to the invention; Figures 7a-7c are further illustrations of the first embodiment of the coil according to the invention; Figures 8a and 8b are schematic illustrations of further variants of the first embodiment of the coil according to the invention; Figures 9a and 9b are schematic illustrations of two variants of a second embodiment of the coil according to the invention; Figures 10a and 10b are schematic illustrations of further embodiments of the second embodiment of the coil according to the invention; Figures 11a-lie show, respectively, an example of a clinical application, examples of three magnetic resonance images of the human calf obtained with a coil of the known type in two different orientations of the coil, and a graph showing a comparison between the profiles of the magnetic resonance signal in two different orientations of the coil ; Figures 12a-12c show the radiofrequency field spatial distribution depending on the orientation of a coil of the known type in the configuration of Figure la, in a simulated and measured example respectively, and the corresponding amplitudes in the vicinity of the surface of the coil; Figures 13a-13c show the radiofrequency field spatial distribution depending on the orientation of a coil according to the invention in the configuration of Figure 6a, in a simulated and measured example respectively, and the corresponding amplitudes in the vicinity of the surface of the coil; Figures 14a-14i show, respectively, three possible spatial orientations of any surface coil in different radiological planes and images of a phantom, acquired by means of a coil of the known type and by means of the coil according to the invention in the configuration of Figure 6a; Figures 15a-15b show, respectively, examples of three magnetic resonance images of the human calf obtained with the coil according to the invention in the configuration of Figure 6a in two different orientations of the coil, and a graph showing a comparison between the profiles of the magnetic resonance signal in two different orientations of the coil; Figures 16a-16i show the simulation of radiofrequency field spatial distribution depending on the orientation of the coil according to the invention in the configuration of Figure 6a upon variation in the mutual distance of the two linear sections of the coil; Figures 17-17i show the simulation of the radiofrequency field spatial distribution depending on the orientation of the coil according to the invention in the configuration of Figure 5a in the case of two, four and six middle segments; Figures 18a, 18b and 18c show examples of images of a phantom, obtained in the geometrical arrangement of Figure 14a with a coil of the known type in the configuration of Figure la, with the coil according to the invention in the configuration of Figure 6a, and a graph showing a comparison between the profiles of the magnetic resonance signal along the axis of the coils; Figures 19a, 19b and 19c show examples of images of a human calf, obtained in the geometrical arrangement of Figure 14a, using respectively the known coil in the configuration of Figure la and the coil according to the invention in the configuration of Figure 6a, and a graph showing a comparison between the profiles of the magnetic resonance signal along the axis of the coils; Figure 20 shows a graph comparing the amplitude of the magnetic resonance signal obtained at various intensities of the radiofrequency field in the case of a known coil in the configuration of Figure la and a coil according to the invention in the configuration of Figure 6a; Figure 21 shows a graph comparing the amplitude of the radiofrequency field calculated and measured along the axis of the coil for a known coil in the configuration of Figure lb, a coil according to the invention in the configuration of Figure 6b, and a coil according to the invention in the configuration of Figure 9b; Figures 22a-22f show graphs comparing the amplitude of the radiofrequency field calculated in planes at predefined distances from the plane of the coil, in the case of a known coil in the configuration of Figure lb, of a coil according to the invention in the configuration of Figure 6b and a coil according to the invention in the configuration of Figure 9b; Figures 23a, 23b and 23c show, respectively, examples of magnetic resonance images of the _< human calf obtained depending on the transmission gain with the known coil in the configuration of Figure la, the coil according to the invention in the configuration of Figure 6a, and a graph of the profile of the magnetic resonance signal in both cases; Figures 24a-24d are diagrams which show the contour levels of the theoretical radiofrequency field amplitude (in a flip angle unit) , for the known coi,l in the configuration of Figure lb and the coil according to the invention in the configuration of Figure 5a with two, four and six middle segments, respectively.

With reference to Figure 5a, a generic surface coil according to the invention is formed by two adjacent D-shaped lobes passed through by current and positioned in a mirror arrangement with respect to an axis of symmetry M, each of them including a plurality of middle sections passed through by current directed in the same direction and closed in a peripheral region of the coil by a section of arbitrary shape (preferably linear, polygonal or semicircular) where the current of the middle sections is collected. Each middle section is formed by a plurality of linear current elements.

As shown in Figure 5a, each lobe or current path of the "D- shaped" coil is characterised by the following geometrical parameters: length L, height H, number N of middle sections, length (LI, L2, L3 , ... , LN, greater than or equal to zero) of the N linear current elements parallel to the axis of symmetry, separation (SI, S2, ..., SN, greater than or equal to zero) of each linear element from the line of symmetry, length (Kl, K2, K3, ..., KN, greater than or equal to zero) of the 2N oblique linear current connecting elements, length (TI, T2, ... , TN, greater than or equal to zero) of the 2N linear current elements perpendicular to the axis of symmetry.

By suitably choosing the abovementioned geometrical parameters of the coil according to. the invention it is possible to produce a plurality of transverse field radiofrequency coil configurations, such as for example: i) the coil with two parallel linear middle elements, with square return path (Figure 5b) or semicircular return path (Figures 6a and 6c) ; ii) the coil with two angled middle elements, with square return path (Figure 5c) ; and iii) the coil with two linear middle sections intersecting each other at a predetermined angle and with square return path (Figure 5d) . In this latter case the conductive path has an intersection of two superimposed portions which is achieved by means of an insulating element arranged between the current elements .

In all these configurations and those described in Figures 6- 10, the current which flows in the linear sections produces a magnetic field Bi which, in the central area of the coil, is a substantially transverse field.

As described in detail below, depending on the particular geometrical configuration provided, the coil according to the invention has a transverse field with specific spatial distribution characteristics in planes parallel to the said coil and along the axis of the coil.

For example, as regards the coil according to Figure 5b (also called "figure-of-eight" coil) , it has a pronounced spatial non-homogeneity of B_x in a direction perpendicular to the plane of the coil (along the axis) . Moreover, this configuration may have a greater amplitude of the field B_x in a region located at a predetermined depth along the axis of the coil, than the traditional circular ring coil.

The coil according to Figure 5d (so-called "butterfly" coil) also has a pronounced spatial no -homogeneity of B_x in a direction perpendicular to the plane of the coil (along the axis) . However, this configuration has a progression of the amplitude of the field B_x which decreases in a monotonic manner with a maximum amplitude in the vicinity of the surface of the said coil .

It is worth noting that the traditional coil, with a circular or polygonal ring has only a moderate spatial non-homogeneity of Bi along the axis of the coil, with the maximum value precisely at the plane of the coil (z=0) .

Figures 7a, 7c show three examples of a first embodiment of the coil according to the invention.

Figures 7a, 7b show two examples of embodiment with a series electrical connection, each consisting of a figure-of-eight current path 10, forming two lobes 10a, 10b alongside each other, and formed from a single conductor 12 having an input terminal 14 and an output terminal 16.

The path 10 includes a pair of parallel middle linear sections 20a, 20b (one for each lobe) passed through by current directed in the same direction, and arranged alongside each other in a central region of the coil, and sections of semicircular shape (Figure 7a) or polygonal shape (Figure 7b) 22a, 22b passed through by current in the opposite direction in the peripheral region. The path also has, in the vicinity of the contour of the coil, at the transition point from one lobe to the .other, an intersecting area 24 formed by means of superimposition of two portions, where the superimposed portions are electrically insulated from each other by means of a dielectric spacer 26.

Figure 7c shows an example of embodiment with a parallel electrical connection, where the current path 10 divides at the point 27a, so as to form the two lobes 10a and 10b arranged alongside, and unites again at the point 27b.

Figures 8a and 8b show further variants of the first embodiment of the coil with series electrical connection according to the invention, where the path 10 includes a plurality of parallel linear sections 20a, 20a' , 20a' ' , 20b, 20b¹, 20b¹' (respectively two and three for each lobe) passed through by current directed in the same direction and arranged alongside each other in the central region of the coil .

Figures 9a and 9b show two variants of a second embodiment of the coil according to the invention, again with a series electrical connection.

Said variants consist of a current path 110 which is also in the form of a figure of eight, forming two lobes 110a, 110b arranged alongside each other, and realized by means of a single conductor 112 having an input terminal 114 and an output terminal 116.

In this embodiment, the path 110 includes a pair of linear sections 120a, 120b passed through by current directed in substantially concordant directions, intersecting in a central region of the coil, and sections 122a, 122b of semicircular shape (Figure 9a) or polygonal shape (Figure 9b) passed through by current in the opposite direction in the peripheral region. The path has, at the central point (axis) of the coil at the transition from one lobe to the other, an intersecting area 124 formed by the superimposition of the two linear sections 120a, 120b, the superimposed sections being electrically insulated from each other by means of a dielectric spacer 126.

Figures 10a and 10b show further variants of the second embodiment of the coil with series electrical connection, in which the path 110 includes a plurality of linear sections 120a, 120a', 120a¹', 120b, 120b¹, 120b'' (respectively two and three for each lobe) intersecting in the central region of the coil and passed through by current directed in substantially concordant directions .

In the remainder of the description reference will be made in particular to the configurations of radiofrequency surface coils for magnetic resonance spectroscopy applications, although the novel characteristic features of the invention may be used also for other clinical and research applications by means of magnetic resonance. The embodiments of the invention have been studied from a theoretical point of view by means of a magnetic field simulation program and tested experimentally using laboratory methods and a 1.5 Tesla clinical MRI scanner.

Reference will now be made to Figures 2a-2d already commented on and relating to the known art. In the clinical apparatus traditionally used for diagnostic or research purposes a main magnetic field B₀ is produced, said magnetic field having a horizontal orientation as shown in the figures, substantially along the axis of the patient ' s body lying in the supine position.

In some applications, for example in order to use more easily and more comfortably an ergometer in the study of human calf muscles by means of ³¹P magnetic resonance spectroscopy performed with a clinical scanning detector, as shown in Figure 11a, the coil C must be arranged in a plane at an angle of about α = 45° with respect to the ,main field B₀.

In order to illustrate the limitations due to traditional coils, Figure lib shows examples of 1.5 Tesla magnetic resonance images obtained with a traditional ring coil having a diameter of 10 cm and arranged respectively at angles α of 0° and 45° with respect to the field B₀. In the three sections through the calf shown in Figure lib, the significant loss of signal when the coil is arranged at an angle of 45° is clear.

A graph comparing the profile of the magnetic resonance signal for the section 2 along the axis of the coil in the different orientations of the coil is shown in Figure lie.

In order to quantify the signal loss, a simulation of the three-dimensional radiofrequency field distribution according to the orientation of the coil with respect to the main magnetic field B₀ was performed using Biot-Savart ' s law, based on a predetermined total current value in the coil chosen, by way of example, as 1 ampere in the simulations.

Figure 12a shows examples of radiofrequency field distributions calculated in the axially oriented radiological plane (which interesects the axis of the patient lying in the supine position, as shown for example in Figure 2a) for a cylindrical phantom with a diameter of 12 cm, and obtained when the coil is arranged respectively at angles α of 0°, 45° and 90° with respect to the field B₀. In order to compare the theoretical and experimental results, a 1.5 Tesla clinical MRI scanner was used to acquire, for the same angles, the axial images of a uniform cylindrical phantom (having a diameter of 12 cm and containing water and nickel and manganese salts of suitable composition and concentration) shown in Figure 12b. From the simulated and measured images the corresponding intensity of the signal in a region (of about 5x5 cm) positioned in the vicinity of the surface of the coil were calculated. As shown in the graph of Figure 12c, the results of the simulation (theoretical curve) and the experimental results (discrete points) coincide substantially in showing that, with an increase in the angle α from 0° to 90°, the signal diminishes from a maximum value to zero.

With reference to Figures 3a-3d, already commented on and relating to the known art, said • figures relating to diagnostics and research apparatus generating a main magnetic field B₀ with a vertical orientation (i.e. substantially directed along the axis perpendicular to the body of a patient lying in the supine position) , it should be noted that in this case also there are the limitations in the form of a significant loss of signal when the traditional coil is arranged at an angle α of between 0° to 90° with respect to the main magnetic field B₀ extending vertically.

With the planar coil configuration according to the invention it is possible to overcome the drawback of the signal loss depending on the spatial orientation of the coil, both in the case of a horizontal main magnetic field B₀ and in the case of a vertical field.

Below some applicational examples relating to the embodiment of the coil according to the invention with two parallel linear elements and with a series electrical connection are shown. The same novel characteristics are also applicable for coils with a parallel electrical connection.

An analysis of the differences in terms of quality of the images which can be obtained with some of the variants of the coil according to the invention ("figure of eight" and "butterfly" coils) will be presented further below.

In ■ order to quantify the improvements obtained with use of the coil according to the invention, the distribution of the radiofrequency field depending on the orientation of the coil was calculated by applying Biot-Savart ' s law, based on a predetermined total current value in the coil chosen by way of example as 1 ampere in the simulations. Figures 13a shows examples of radiofrequency field distributions calculated in the axially oriented radiological plane (which intersects the axis of the patient lying in the supine position, as for example shown in Figure 2a) for a cylindrical phantom with a diameter of 12 cm, and obtained when the coil according to the invention is arranged respectively at angles α of 0°, 45° and 90° relative to the field B_o.

In order to compare the theoretical and experimental results, a 1.5 Tesla clinical MRI scanner was used to acquire, for the same angles, the axial images of a uniform cylindrical phantom (with a diameter of 12 cm and containing water and salts of suitable composition and concentration) shown in Figure 13b. From the simulated and measured images the corresponding intensities of the signal in a region (of about 5x5 cm) positioned in the vicinity of the surface of the coil were calculated. As shown in the graph of Figure 13c, the results of the simulation (theoretical curve) and experimental results (discrete points) coincide substantially in showing that, with an increase in the angle α from 0° to 90°, there is only a small reduction of the signal in the central region (less than 20%) . On the other hand, with the traditional ring coil, in the same central region a complete loss of signal was observed (see Figures 12a, 12b, 12c) .

With reference to Figure 14, images were acquired by means of magnetic resonance at 1.5 Tesla of a cylindrical phantom with a 12 cm diameter using the coil according to the invention in the configuration of Figure 7a, arranged in three orientations of the field Bi produced (identified as Figures 14a, 14b and 14c) . The images acquired (Figures 14g-i) are compared with those obtained using a ring coil of the known type (Figures 14d-f) in relation to the same orientations. It is clear that, in the central region, with the coil according to the invention there is no loss of signal in each of the three orientations, while with the ring coil the loss of signal may be observed in the orientation of Figure 14c.

With reference to Figure 15a, 1.5 Tesla images of the human calf were acquired using a coil according to the invention (in the configuration of Figure 6a) with a 10 cm diameter, as above. The first and the second row show the images obtained with the coil arranged at angles of 0° and 45°, respectively. There is no loss of signal in the orientation at 45° although it is possible to note a small variation in the distribution of the radiofrequency field in the central region of the sample. The graph shown in Figure 15b (relating to the section 2) clearly shows that the amplitude of the signal in the axial direction of the coil is conserved when using the coil according to the invention arranged at angles of 0° and 45° .

The results described hitherto were obtained using coils comprising two linear current elements passed through by 1 ampere of current and separated by a distance of 1 cm. The dependency of the radiofrequency field distribution in transverse planes was studied theoretically. Coils with an overall size of 10 cm (diameter in the configuration of Figure 5a, lateral dimensions in the configuration of Figure 6b) were used for the simulation, said coils comprising a pair of parallel middle linear sections separated by a distance of 1 cm, 3 cm and 5 cm. The results of the simulations are shown, respectively, in Figures 16a, d,g,; 16b, e,h; 16c, f,i. As shown in the figures, in the case of the three different spatial orientations (α=0°, 45°, 90°), it was found that, by adjusting the distance between the sections, the distribution of the radiofrequency field and its amplitude in the axial orientation may be adjusted so as to produce a desired spatial profile in the plane xz of the figure (the coil is arranged in the plane xy in accordance with the reference system shown in the figure) .

Simulations were also carried out for the behaviour of coils having dimensions of 10 cm and comprising two, four and six parallel middle linear sections in the initial configuration of Figure 6b and the configurations of Figures 8a and 8b. Figure 17 shows the distributions of the radiofrequency fields for the three different coil configurations, for angles of orientation with respect to B₀ of 0°, 45° and 90°. Figures 17a, d,g relate to a coil with two middle segments, separated by a distance of 2sl=lcm; Figures 17b, e,h relate to a coil with four middle segments separated by distances 2sl=lcm and 2s2=3cm, Figures 17c, f,i relate to a coil with six middle segments separated by distances 2sl=lcm, 2s2=3cm, 2s3=5cm.

The results of the simulation show that at the angles α of 0°, 45° and 90° in the central region of the coil there is no loss of signal and that the distribution of the radiofrequency field may be modulated spatially by selecting the number and the relative position of the middle linear sections of the coil. This is a very useful characteristic of the coil configuration according to the invention, since the distribution of the radiofrequency field, namely the sensitivity of the magnetic resonance, may be optimised depending on the geometrical form of the organ to be examined.

The transverse-field surface coil configuration comprising a plurality of middle linear sections, with a random form of the return current path may also be used advantageously to solve the problem of the spatial selectivity along the axis of the coil.

Magnetic resonance images of a cylindrical phantom with a 12 cm diameter obtained with a traditional coil of the ring type and a novel coil with two linear elements in the configuration of Figure 6a, both with a diameter of 10 cm and in the arrangement of Figure 14a, are shown in Figures 18a and 18b respectively. These images show that it is possible to obtain an improved spatial selectivity along the axis of the coil with the novel coil, compared to the unsatisfactory results of the traditional coil. In order to quantify the spatial selectivity obtained in the experimental example with a cylindrical phantom, the graph in Figure 18c shows the progressions of the magnetic resonance signal along the axis of the coil, for the ring coil of the known type (curve A) and for the "figure-of-eight" coil (curve B) , respectively. With the traditional ring coil the signal reaches a maximum at z=0 cm and gradually diminishes towards the zero value moving away from the surface of the coil . With the novel coil the signal has a zero value at z=0 cm, then rapidly increases until it reaches a maximum at about 1 cm and then rapidly falls towards the zero value. ,

Such a spatial selectivity behaviour along the axis of the coil, for both the coil configurations, may be observed in the real images of a human calf shown in Figures 19a, 19b and a comparison thereof is performed with the aid of the graph of Figure 19c which shows the progressions of the magnetic resonance signal along the axis of the coil, for the ring coil of the known type (curve A) and for the novel coil (curve B) , respectively.

In order to optimise the axial selectivity and the sensitivity of the coils, theoretical and experimental studies were carried out on coils formed with two linear middle sections and a semicircular return path (as shown in Figure 6a) . The components of the field Bi were calculated by means of numerical integration of Biot-Savart ' s law (with a current of 1 ampere) . In order to study the optimum configuration of the coil, estimates were carried out as to the peak values of the field B_x and the sensitivity and axial selectivity of the coil depending on the radius R thereof (ranging between 2 and 15 cm) and the distance 2sl between the two linear sections (ranging between 0.6 and 5 cm). The behaviour of the traditional ring coils was simulated for comparison purposes. Prototypes of the coils according to the invention, with values for the distance between the linear sections of 1, 2 and 3 cm and a radius (for both coils) of 3, 5 and 9 cm, were prepared.

It was noted that the sensitivity of the coils according to the invention depends on the parameters 2si and R and in particular diminishes with an increase in the value of 2sl. The peak value of the field in the case of a coil according to the invention is greater than in the case of the traditional ring coil in the central axial region, depending on 2sl and R. For high radii (R > 8 cm) there exists a wide range of distance values 2si which allows a gain in sensitivity. On the other hand, for smaller radii the gain in sensitivity may be obtained only for values of 2sl < 2 cm.

An example of the improvement in the sensitivity obtained by means of a coil according to the invention compared to a traditional ring coil is shown in the graph of Figure 20, which shows the progressions of the magnetic resonance signal as a function of the field Bi, for the ring coil of the known type (curve A) and for the novel coil (curve B) , respectively. The amplitude of the magnetic resonance signal as a function of the amplitude of the field Bi was measured at a distance z of 5.6 mm from the surface of the coil for a ring coil with a diameter of 10 cm and a novel coil with a diameter of 10 cm and separation distance between the linear sections of 1 cm, by means of imaging methods using magnetic resonance with a magnetic field of 1.5 Tesla at the frequency of 63.9 MHz. As expected, the signal shows a periodic dependency on the amplitude of B_x for both the coils. However, the novel coil has a more rapid variation of the signal compared to the results obtained with the traditional ring coil. This means that the novel coil is more sensitive and the estimated gain in sensitivity between the two coil configurations is equivalent to a factor of about 2.5.

The gain was measured, by means of laboratory methods using a Hall effect probe, also for the traditional coils of Figure lb and the novel coils of Figure 6b and Figure 9b (all with a square configuration having lateral dimensions of 10 cm and a current of 1 ampere) and the results are shown in the graph of Figure 21. The curve A (where the experimental results are indicated by circles and the theoretical results by the continuous curve) relates to the coil of the known type, the curve B (where the experimental results are indicated by squares and the theoretical results by the continuous curve) relates to the novel coil of Figure 6b and the curve C (where the experimental results are indicated by triangles and the theoretical results by the continuous curve) relates to the novel coil of Figure 9b.

The results of Figure 21 show that both the coil in the configuration of Figure 6b and the coil in the configuration of Figure 9b according to the invention have a sensitivity along the central axis which is improved compared to that of the ring coil in a distance range of about 3 cm from the surface of the coil . In the case of ^' greater distances the sensitivity of the coils according to the invention diminishes more rapidly compared to that of the coils in the traditional annular configuration.

The novel coil according to Figure 9b has a sensitivity which diminishes with a monotonic progression and with an amplitude greater than the novel coil of Figure 6b in the vicinity of the surface, especially at distances less than about 15 mm from the surface. However, for this distance the sensitivity of the coil of Figure 5b has a zero value at z=0 cm, then rapidly increases until it reaches a maximum at about 1 cm and then drops off rapidly towards the zero value.

It is shown that, in the case of the novel coil of Figure 6b, the axial spatial selectivity (defined as the distance at which the sensitivity reaches half its maximum value) increases with an increase in the radius R and reaches an asymptotic value depending on the distance 2sl. It was also shown that the axial position of the peak _vvalue of the field Bi increases only by a small amount with the increase in the radius R.

These theoretical and experimental results suggest therefore that it is possible to obtain a significant improvement in the detection sensitivity using coils according to the invention and that, by selecting the values of the radius R and the distance 2sl between the linear sections, it is possible to adjust the selectivity of B_x and the axial position of the peak within the range of a few centimetres .

Where it is required to obtain an improvement in the sensitivity in the vicinity of the surface of the organ being examined, which is independent of the spatial orientation of the said coil with respect to the (horizontal or vertical) field B₀, then it may be more convenient to use the novel coils of Figure 9b.

In order to illustrate more clearly another difference between the innovative coil of Figure 6b and Figure 9b according to the invention, Figure 22 shows the graphs comparing the amplitude of the radiofrequency field calculated in a plane xy positioned at 6 mm (Figures 22a-22c) and at 30 mm (Figures 22d-22f) from the plane of the coil. For comparison purposes the amplitude, of the field also in the case of the known coil is shown. It can be seen that, in the vicinity of the surface, the known coil has in the central zone a uniform spatial distribution and minimum amplitude (Figure 22a) .

On the other hand, the coils according to the invention have, in the same central region, a maximum amplitude, the form and spatial extension of which depends on the particular configuration. For example, the coil of Figure 6b ("figure- of-eight" coil) has a maximum amplitude in a strip oriented in the direction y (parallel to the two linear elements) (Figure 22b), while the coil of Figure 9b ("butterfly" coil) has a maximum amplitude at the intersection of the two linear elements (Figure 22c) .

Finally, the spatial distribution of the field B_x in novel coils according to the configuration of Figure 6a has an interesting characteristic which may be useful for reducing the effects of contamination of the signal which result from fatty tissues and from the epidermis located in a region very close to the surface of the coil . This can be determined from the three calf images shown in Figure 23b, these images having been obtained by means of an innovative coil with a transmission gain of 0, 30 and 60 dB. It can be noted that, with an increase in the gain from 0 dB to 30 dB, a thin dark strip appears in the vicinity of the epidermis and the fatty tissues. With an increase in the transmission gain from 30 to 60 dB, the position of this strip changes, moving away from the surface of the coil. Moreover, the wavy form of the dark strip diminishes and the strip has a flatter appearance. This characteristic may be explained as follows: for a particular transmission gain, the amplitude of the field B_x in the voxel belonging to the strip will produce a reversal angle of 180° , namely a zero magnetic resonance signal, in this particular position. This is shown in the progression of the magnetic resonance signal illustrated in the graph of Figure 23c extracted along the section A-A shown, for a gain of 30 dB, where the curve A relates to the ring coil of the known type (images in Figure 23a) and the curve B relates to the coil according to the invention.

By adjusting the value of the transmission gain the position of the dark strip may be varied within a few centimetres from the surface of the coil. This particular characteristic cannot be observed with traditional ring coils, as can be noted from the magnetic resonance images of Figures 23a and the progression of the signal shown in the graph of Figure 23c.

In order to understand even better this effect the distribution of the radiofrequency field was simulated for a square ring coil with a 10 cm side and for a square novel coil with a 10 cm side, comprising 2, 4 and 6 linear sections, respectively.

The contour levels of the theoretical radiofrequency field in a plane xz perpendicular to the plane of lie of the coil are shown in Figure 24a for the ring coil' of the known type and in Figures 24b-24d for the coils according to the invention.

As shown in Figure 24b, some contour levels correspond very precisely to the form of the dark strip which can be seen in Figure 22b. By selecting the transmission gain, any particular contour level may be set to correspond to a reversal angle of 180°. Moreover, by adjusting the number and the relative position of the central linear sections of the coil, the contour of the 180° reversal angle may be shaped to some extent. This is shown in the profiles of Figures 24b- 24d. The configuration of the ring coil does not show any profile (Figure 24a) which corresponds to the dark strip of Figure 23b .

Conveniently it is possible to envisage a coil according to the invention, of the type described above including a plurality of middle linear sections for each lobe, in which each section has electrical selecting means, such as for example an electrical switch, for insertion or exclusion, respectively, of the corresponding section from the coil configuration and the selection electrically of the configuration of the desired coil. In this way it is possible to adjust the selection of the number of middle sections and, albeit to a limited degree, in the case of a coil including only one pair of middle sections, the 'choice of the distance between the abovementioned sections of the pair.

The abovementioned electrical selecting means, conveniently driven, also allow control of the intensity of the current which flows along each middle section of the coil .

By way of conclusion, the results presented show that the coil configuration according to the invention offers the following advantages compared to the traditional ring coil : - it allows orientation of the coil in any direction with respect to the main, horizontal or vertical, magnetic field Bo without significant signal losses in the volume of interest (VOI) ; for any orientation of the coil, the spatial distribution of the radiofrequency field in a plane parallel to the surface of the coil and situated at a certain distance from the said coil may be adjusted by means of suitable selection of the number and the geometrical arrangement of the middle segments; - for any orientation of the coil, the maximum amplitude of the radiofrequency field in a plane parallel to the surface of the coil and situated at a certain distance from the said coil may be adjusted by means of suitable selection of the number and the geometrical arrangement of the middle segments; - for any orientation of the coil, along the axis of the coil, the distribution of the radiofrequency field is spatially selective and may be adjusted by means of suitable selection of the number and the geometrical arrangement of the middle segments; in particular configurations an increase in the sensitivity of the radiofrequency field in a selected volume of interest along the axis of the coil may also be established; - by means of appropriate selection of the number and geometrical arrangement of the middle segments and the radiofrequency transmission power it is possible to reduce or even eliminate the unwanted signals received from an area positioned at a certain distance from the surface of the coil

(for example epidermis and fatty tissues) ; and - by means of appropriate selection of the number and the geometrical arrangement of the middle segments of the radiofrequency coil it is possible to shape suitably the area in which the unwanted signal is to be reduced or eliminated.

Naturally, without modifying the principle of the invention, the embodiments and the constructional details may be widely varied with respect to that described and illustrated purely by way of a non-limiting example, without thereby departing from the scope of protection of the present invention defined in the accompanying claims.

Claims

1. Surface coil configuration for a magnetic resonance apparatus, characterized in that it comprises a conductive path (10; 110) which extends substantially in a figure of eight forming a pair of adjacent lobes (10a, 10b; 110a, 110b) and includes a plurality of middle linear sections (20a, 20a', 20a'¹, 20b, 20b', 20b'¹; 120a, 120a', 120a'', 120b, 120b', 120b'¹) passed through by current directed in the same direction or in substantially concordant directions and grouped together in a central region of the coil, and sections with an arbitrary shape (22a, 22b; 122a, 122b) passed through by current in the opposite direction in the peripheral region, so that the magnetic radiofrequency field in the central region of the coil is substantially directed in a direction parallel to the local plane of the coil and is spatially non- omogeneous and able to ^'be modulated depending on the number and the geometrical arrangement of said middle linear sections (20a, 20a', 20a'¹, 20b, 20b', 20b'¹; 120a, 120a', 120a¹', 120b, 120b', 120b'¹).

2. Configuration according to Claim 1, in which said adjacent lobes (10a, 10b; 110a, 110b) are positioned in a mirror arrangement with respect to aii axis of symmetry (M) and each middle linear section of the coil includes a linear element parallel to the axis of symmetry (M) , a pair of oblique linear connecting elements and a pair of linear elements perpendicular to the axis of symmetry, the length of each linear element (LI, L2, L3) and its distance (SI, S2, S3) from the axis of symmetry (M) determining the spatial distribution of the magnetic radiofrequency field produced by the coil .

3. Configuration according to Claim 1 or 2, in which said middle linear sections consist of parallel linear elements (20a, 20a', 20a'', 20b, 20b', 20b'') at a predetermined distance from each other.

4. Configuration according to Claim 1 or 2, in which said middle linear sections consist of linear elements (120a, 120a', 120a¹', 120b, 120b', 120b'') intersecting at a predetermined acute angle.

5. Configuration according to Claim 3 or 4 , in which said plurality of middle linear sections (20a, 20a', 20a'¹, 20b, 20b¹, 20b'¹; 120a, 120a', 120a'¹, 120b, 120b', 120b¹') comprises two sets of sections connected electrically in parallel along the conductive path (10, 110) .

6. Configuration according to Claim 3 or 4, in which said plurality of middle linear sections (20a, 20a', 20a¹', 20b, 20b', 20b'¹; 120a', 120a¹', 120b, 120b', 120b¹') comprises two sets of sections connected electrically in series along the conductive path (10, 110) .

7. Configuration according to Claim 5 or 6, in which each set of middle linear sections comprises a single section or a plurality of sections electrically in parallel .

8. Configuration according to Claim 4, in which said intersecting middle linear sections (120a, 120a', 120a¹', 120b, 120b', 120b¹') are superimposed and separated by an electrically insulating means (126) .

9. Configuration according to any one of the preceding claims, in which each middle section (20a, 20a', 20a'', 20b, 20b', 20b'¹; 120a, 120a', 120a' ', 120b, 120b', 120b¹') of the coil has electrical selecting means for insertion or exclusion, respectively, of the corresponding section of the coil configuration or for adjusting the current intensity of each middle section of the coil .

10. Magnetic resonance apparatus comprising at least one transmitter and/or receiver surface coil which has a configuration according to Claims 1 to 9.