Abstract:
The present disclosure provides a method for producing a rigid, shaped, non-planar radiofrequency coil including:
       a) placing a number of coil elements selected from one and a desired plurality of coil elements on to a flexible substrate to make a coil;   b) laying the coil flat and covering it with foam;   c) and subsequently applying the foam covered coil to a first mechanical former capable of holding the coil rigid in a desired configuration.

Description:
BACKGROUND 
       [0001]    This disclosure relates to radiofrequency coils. More particularly it relates to a method for producing rigid, shaped, non-planar radiofrequency coils. 
         [0002]    Radiofrequency coils are used in magnetic resonance imagining (MRI) scanners. These coils can be split into two types. The first are “flat” coils. These coils are typically foam covered and are substantially flat. They may be placed on the target location before a scan begins. These coils would be used for MRI scans on areas of the body such as the abdomen. The coils are lightweight but would not be used on areas where a substantially flat coil is not appropriate. 
         [0003]    The second type of radiofrequency coils currently used are “curved” coils, which take a non-planar form. These are typically encased in hard plastics to hold the coil in the desired configuration. These coils would be used for MRI scans on areas of the body such as the head. The coils and their housing make a unit which is cumbersome and fairly heavy. If the size and shape of “curved” coil required for a particular application is not available it can be expensive and time consuming to make a coil for that application. 
         [0004]    The current disclosure arises from our work seeking to provide non-planar “curved” coils that are lighter than prior such coils while still achieving the required rigidity and both quicker and easier to design and manufacture. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    According to a first aspect of this disclosure, a method for producing a rigid, shaped, non-planar radiofrequency coil comprises the steps of: placing a number of coil elements selected from one and a desired plurality of coil elements on to a flexible substrate to make a coil; laying the said coil flat and covering it with foam; and subsequently applying the said foam covered coil to a first mechanical former capable of holding the said coil rigid in a desired configuration. 
         [0006]    The foam can be applied by a thermoforming method using moulds or by a layered method using foam and adhesives. Using either method we have found that an appropriate foam to use would be a bio-compatible, cross-linked block foam. Applicant has found that suitable foams include bio-compatible PLASTAZOTE® and EVAZOTE® Foams available from Zotefoams Plc. 
         [0007]    The step of applying the foam covered coil to a first mechanical former may be irreversible. 
         [0008]    In an alternative embodiment the step of applying the foam covered coil to a first mechanical former may be reversible. The coil may be removed from the mechanical former using a quick release mechanism. 
         [0009]    The method optionally includes additional steps carried out prior to the step of laying the coil flat and applying foam but subsequent to the step of placing a number of coil elements on to a flexible substrate to make a coil, the optional steps comprising: fitting the coil into a former, which may be the first mechanical former or a different former capable of holding the coil rigid and in the same shape as the first mechanical former; while the coil is held in said former, undertaking a radiofrequency set-up to optimise the radiofrequency characteristics of the coil for the target application. 
         [0010]    This radiofrequency set-up may include obtaining the best coil Q factor; ensuring that the coil has the most suitable unloaded resonance determined by measurement of the circuit&#39;s quality factor; ensuring that the coil is tuned at the nuclear magnetic resonance (NMR) frequency when loaded by the object that is to be the subject of the magnetic resonance (MR) scan; ensuring that in a multi-element design the coil has good radiofrequency isolation from other elements; ensuring that the coil is matched to the output circuitry such that, in the case of a receive coil, the signal detected has optimum signal to noise ratio, and, in the case of a transmit coil, that the coil produces an optimum radiofrequency field for a given radiofrequency power output from an associated amplifier. 
         [0011]    The method may include a preliminary step performed prior to any of the foregoing steps, the preliminary step comprising: designing a former capable of holding a flexible coil in a rigid shape, this rigid shape being the shape required for the final coil. The former may be generally cylindrical in a mathematical sense with a uniform cross-section along its axis, but with that cross-section being circular, oval or irregular. Alternatively the former may be curved but not closed, such that the coil is held in a U- or C-shape. The former may also be designed to extend significantly in two dimensions while having only a low profile in the third dimension. 
         [0012]    The coil may be provided with protrusions capable of reversibly locking into fixing locations on said former such that when the said protrusions are locked into the fixing locations the coil is held rigid in the desired configuration in the said former, the protrusions being designed to extend outside the sections of the coil that are foam covered. The protrusions may be provided on one or more edges of the coil and may be permanently attached to the coil. Alternatively the protrusions may be removed from the coil for the foam covering step and attached once the coil is covered in foam. The protrusions may be slid into mating parts on the coil or be screwed onto the coil through holes in the foam. 
         [0013]    According to a second aspect of this disclosure, there is provided: a rigid, shaped, non-planar radiofrequency coil, the coil comprising a foam covered coil held rigid and in a desired conformation by a mechanical former. 
         [0014]    The coil may be provided on a printed circuit board (PCB). This allows components to be flow soldered on to the PCB. 
         [0015]    The PCB may comprise a plurality of flat sections joined by flexible PCB material or may be a completely flexible, or may comprise a first section comprising a plurality of flat sections joined by flexible PCB material and a second section that is completely flexible. 
         [0016]    The coil may be provided separately from a preamplifier, such that a single set of preamplifiers may be used with several different sets of coils. 
         [0017]    The mechanical former may be designed from a rigid material that is compatible with the coil and an MRI scanner. This rigid material could be formed from rapid prototype plastics material, vacuum cast material, injection moulded material, rim cast material, plastics machine cut parts, stiff foam pieces and/or a stiff plastics foam. 
         [0018]    The mechanical former may be shaped to allow the coil to be used on a person&#39;s head and or neck, the mechanical former being part of a frame which supports the said person&#39;s head and neck and the coil being removable from the frame without moving the person&#39;s head. 
         [0019]    Alternatively the mechanical former may be shaped to allow the coil to cover a focused region such as a person&#39;s forehead, the mechanical former being part of a frame that stabilises the said person&#39;s head and may provide a visual display to the person by means of goggles or minors. 
         [0020]    The coil may be capable of splitting into sections. 
         [0021]    The coil may be made of more than one coil where each coil is for imaging nuclei of different chemical elements. 
         [0022]    The coil may be a transmit coil, whereby the coil transmits the radiofrequency into the target region. Alternatively the coil may be a receive coil, whereby the coil receives the NMR signal. Alternatively the coil may be a transmit/receive coil, whereby the coil both transmits and receives the NMR signal. Alternatively the coil may be made up of more than one coil element where one coil element is for transmitting radiofrequency waves and another coil element is for receiving radiofrequency waves. 
         [0023]    The coils may include at least one fiduciary marker incorporated into the final structure, where a fiduciary marker is an object placed in the field of view of the coil which appears in the final image produced and can be used as a point of reference. 
         [0024]    The coils may include reference samples incorporated into the final structure. 
         [0025]    The foam may be non-conducting. 
         [0026]    Alternatively the foam may include a conducting foam layer which can be used as a conducting radiofrequency screen. 
         [0027]    The foam covering may be designed to act as an enclosure for additional components of the radiofrequency coil. These components may include, but are not limited to, preamplifiers, PIN diode controllers, radiofrequency circuitry for transmit/receive switching (T/R switches), radiofrequency combiners, radiofrequency splitters, quadrature combiners. 
         [0028]    The coil produced may be of any radiofrequency coil type that may be used in MR scanners. Such coil types include, but are not limited to, single element coils, multi-element coils, birdcage coils, transverse electromagnetic (TEM) coils and microstrip coils. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Reference may now be made to the description of preferred embodiments by way of example only with reference to the accompanying drawings, in which: 
           [0030]      FIG. 1  is a perspective view of a printed circuit board with eight elements; 
           [0031]      FIG. 2  is an exploded, perspective view of the printed circuit board of  FIG. 1  with two end rings; 
           [0032]      FIG. 3  is a perspective view of the printed circuit board of  FIG. 1  covered in foam; 
           [0033]      FIG. 4  is a perspective view of the foam covered printed circuit board of  FIG. 3  fitted into two end rings. 
           [0034]      FIG. 5  is a perspective view of a head support; 
           [0035]      FIG. 6  is a perspective view of a coil and former that could be used with the head support of  FIG. 5 ; 
           [0036]      FIG. 7  is a perspective view of a further coil and former that could be used with the head support of  FIG. 5 ; 
           [0037]      FIG. 8  is a perspective view of a further coil and former that could be used with the head support of  FIG. 5 ; 
           [0038]      FIG. 9  is a perspective view of a coil where the end rings are hinged; 
           [0039]      FIG. 10  is a further perspective view of the coil of  FIG. 9 ; 
           [0040]      FIG. 11  shows an exploded, perspective view of a C- or U-shaped coil with a suitable former; 
           [0041]      FIG. 12  shows an exploded, end view of two coils which fit one inside the other; 
           [0042]      FIG. 13  shows an end view of the two coils from  FIG. 12 ; 
           [0043]      FIG. 14  shows a perspective view of a former that may also house reference samples and/or fiducials; 
           [0044]      FIG. 15  shows a perspective view of the former of  FIG. 14  with coils attached; 
           [0045]      FIG. 16  shows a perspective view of a foam enclosure with one open side; 
           [0046]      FIG. 17  shows a perspective view of a printed circuit board and cover to be used in conjunction with the foam enclosure of  FIG. 16 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0047]    A printed circuit board  1  is provided with eight elements  2  which make up a radiofrequency coil  3 . The printed circuit board is fitted with protrusions  4  and then curved such that the protrusions  4  lock into corresponding slots  5  in a mechanical former  6 . Here the mechanical former comprises two end rings  7 . The mechanical former  6  is capable of holding the printed circuit board  1  in a rigid configuration. 
         [0048]    While the printed circuit board  1  is held in the mechanical former  6  a radiofrequency set-up is undertaken to optimise the radiofrequency characteristics of the coil for the target application. This optimisation can involve obtaining the best coil Q factor; ensuring that the coil  3  has the most suitable unloaded resonance, determined by measurement of the circuit&#39;s quality factor; ensuring that the coil  3  is tuned at the nuclear magnetic resonance (NMR) frequency when loaded by the object that is to be the subject of the magnetic resonance (MR) scan; ensuring that in a multi-element design the coil  3  has good radiofrequency isolation from other elements; ensuring that the coil  3  is matched to the output circuitry such that, in the case of a receive coil, the signal detected has optimum signal to noise ratio, and, in the case of a transmit coil, that the coil  3  produces an optimum radiofrequency field for a given radiofrequency power output from an associated amplifier. 
         [0049]    When the radiofrequency set-up is complete, the printed circuit board  1  is removed from the mechanical former, laid flat and covered in foam  8 . The foam  8  can be applied by a thermoforming method using moulds or by a layered method using foam and adhesives. Using either method, we have found that an appropriate foam  8  to use would be a bio-compatible, cross-linked block foam such as bio-compatible PLASTAZOTE® or EVAZOTE® Foam available from Zotefoams Plc 
         [0050]    The protrusions  4  extend out of the foam  8  as do cables  9  associated with each element  2 . 
         [0051]    The foam-covered coil  3  is then fitted into a further mechanical former  11  which comprises two end rings  12  and holds the printed circuit board  1  in a rigid configuration identical to the configuration it was held in by the previous mechanical former  6 . The cables  9  are guided through the mechanical former  11  to an exit point  13 . As shown in  FIG. 4 , the result is effectively a rigid cylindrical RF coil. 
         [0052]    Sometimes it is necessary to obtain an MRI scan of a subject&#39;s head. For that purpose a foam-coated coil  20  can be used with a head support  14 . The head support  14  shown in  FIG. 5  has a head rest  15 , on which a subject may simply lay their head. However, if required, additional fixings could be provided which will hold a subject&#39;s head more securely. The head support  14  is provided with a ring  17  large enough to encircle a subject&#39;s head. The ring  17  provides support for ear defenders  18  and video goggles or mirrors  19  to protect both the subject&#39;s ears and eyes when they are in the support. The ear defenders  17  may be provided with head phones so the subject may listen to music or hear communications from doctors and technicians while using the head support  14 . 
         [0053]    A coil  20  which will be used with the head support  14  could be made in many shapes and sizes. The key requirement is that the coil  20  must be capable of being fitted to the support  14  without the subject having to move their head or the support  14  being adjusted. A suitable arrangement is shown in  FIG. 6  where the coil  20  is formed from two side pieces  21  and  22  and eight top pieces  23 . The left side piece  21  is a mirror image of the right side piece  22 . Both side pieces  21  and  22  have a top edge  24  which is provided with protrusions  4 . The protrusions  4  are capable of locking into corresponding slots  5  which are provided on bottom side  25  of a support ring  26 . The support ring  26  is capable of holding the side pieces  21  and  22  so that together they are held in a cylindrical configuration. The two side pieces  21  and  22  are shaped such that, when used with the head support  14 , the side pieces will stop short of the ear defenders  18  and video goggles or mirrors  19 . Top side  27  of the support ring  26  is provided with slots  5  which correspond to protrusions  4  which are provided on the eight top pieces  23 . When correctly fitted to the support ring  26  and connected to each other by further protrusions  4  and slots  5 , the top pieces  23  are held in a domed configuration which in use will cover the top of a subject&#39;s head. The side pieces  21  and  22  and the top pieces  23  are connected electronically. The coil  20  is capable of being assembled round the head support  14  when it is in use supporting a subject&#39;s head. It will be appreciated that this process involves a coil that can be laid flat for component placement and foam covering, but is subsequently shaped by support ring  26  into the required essentially rigid cylindrical shape, such that it surrounds the subject&#39;s head. 
         [0054]    If coil  20  is required to extend below the ear defenders  18 , it may include a hinged section  28 . This hinged section  28  may be placed on one or both side sections  21 ,  22 . In  FIG. 7  it is shown on both side sections  21  and  22 . The hinged section  28  will comprise a looped piece  29  which is attached to a side section  21  or  22  by a hinge  30 . The looped piece  29  will be electronically connected to the rest of the coil  20 . The hinged section  28  enables the hinge  30  to be in the open position while the coil  20  is being fitted around a head support  14  and then locked into the closed position before imaging commences. 
         [0055]    While not shown, it will readily be appreciated by a person skilled in the art that further hinged sections may be provided which cover the video goggles or mirrors  19 . This will allow for a greater imaging coverage of the brain and the eyes themselves. 
         [0056]    If it was only required that a small section of the subject&#39;s head be scanned then it will be readily appreciated that a much smaller coil  31  would be sufficient.  FIG. 8  shows a coil  31  fitted with protrusions  4  which are capable of clipping into corresponding slots  5  provided on the video goggles or mirrors  19  on the head support  14 . Such a coil would be capable of localised coverage of the forehead or occipital lobe of the subject. 
         [0057]    The assembly can be permanently held in its desired rigid shape. Alternatively the method which fixes the assembly in its desired rigid shape can be reversible. In this reversible approach, the coil  20  is positioned flat while the subject&#39;s head is positioned in the head support  14 , ear defenders  18  and video display  19 . The coil  20  is then shaped around the head and fixed in place using the support ring  26 . A quick release mechanism allows the coil  20  to be laid flat before the subjects head is removed from the support  14 . The quick release mechanism could be made to open the coil  20  quickly, should the subject&#39;s head need to be removed in an emergency. 
         [0058]    Any suitable quick release mechanism can be used. This may include but is not limited to hook and loop fasteners strips or poppers. 
         [0059]    These coils can be made in a variety of shapes and sizes to suit whatever purpose they are intended to be used for. For instance, a cylindrical coil  32  ( FIG. 9 ) could be used for imaging a subject&#39;s arm or leg. The coil  32  is made from three sections. A base section  33  has a semi-circular cross section, a first longitudinal edge  34 , and a second longitudinal edge  35 . A first hinged section  36  has a quarter of a circle cross-section, a first longitudinal edge  37 , and a second longitudinal edge  38 . The first edge  34  of the base section  33  is hinged to the first edge  37  of the first hinged section  36 . The second edge  38  of the first hinged section  36  is provided with protrusions  4 . A second hinged section  39  has the same cross-section as the first hinged section  36  and has a first longitudinal edge  40  and a second longitudinal edge  41 . The second edge  35  of the base section  33  is hinged to the first edge  40  of the section hinged section  39 . The second edge  41  of the second hinged section  39  is provided with slots  5  which correspond to the protrusions  4  located on the second edge  38  of the first hinged section  36 . When the hinged sections  36  and  39  are closed, as illustrated in  FIG. 10 , the coil  32  is cylindrical. 
         [0060]    A coil  42  ( FIG. 11 ) could also be C- or U-shaped if the foam covered printed circuit board  43  is provided with a C- or U-shaped end rings  44 . 
         [0061]    It will be readily appreciated that this technology could be used for two coils where a first coil  45  is positioning inside a second coil  46 . The two separate coils  45  and  46  may be covered in foam  8  together or, as shown in  FIGS. 12 and 13 , separately. When the radiofrequency set-up is undertaken it will include a step of ensuring that there is good isolation between the coils. This approach can be used when the two coils operate at two different frequencies targeting different nuclei or can be used when the coils operate at the same frequency and known techniques are used to isolate the coils from each other during use. For example the first coil  45  may be set-up to transmit and be detuned to receive and the second coil  46  may be set-up to receive and detuned to transmit. 
         [0062]    Sometimes it is desired to include reference samples  47  in a coil  48 . This can be achieved by mounting the reference sample  47  on to a housing  49  ( FIG. 14 ) such that when a foam covered printed circuit board  50  is also mounted to the housing  49 , as shown in  FIG. 15 , the reference sample  47  is in the desired position in the coil  48 . 
         [0063]    As explained above, a required step in creating coil  3  is to cover the printed circuit board  1  in foam  8 . This can be achieved, as shown in  FIGS. 16 and 17 , by making an enclosure  50  out of foam  8  where one side  51  is provided with a keyed opening  52 , and by providing a cover  53  corresponding to the keyed opening  52 . Cover  53  has a first side  54  and a second side  55  where the first side  54  is parallel to the second side  55 . The printed circuit board  1  is attached to the first side  54  of the cover  53 . A plate  56 , suitably formed of plastics material, may be added to the second side  55  for improved security and for mounting connectors. Respective input  57  and output  58  cables will also be mounted to the plastic plate  56 . The printed circuit board  1  is capable of being slid into the enclosure  50  so that the cover  53  slots into the keyed opening  52 , the enclosure  50  thereby covering the printed circuit board  1 . 
         [0064]    While not shown in the drawings, it will be readily understood by a person skilled in the art that the step of covering the printed circuit board  1  in foam  8  may also be achieved using a thermoforming method using moulds or by a layered method using foam and adhesives. 
         [0065]    Different electronic modules may be housed in individual foam housings and then built into single or multilayer structures. These can in turn be housed in a final sealed foam case as desired.