WO2013023186A1

WO2013023186A1 - System and method for the establishment of magnetic field patterns in a coil set with voltage-driven current shunts

Info

Publication number: WO2013023186A1
Application number: PCT/US2012/050462
Authority: WO
Inventors: Hardave S. KHARBANDA
Original assignee: Kharbanda Hardave S
Priority date: 2011-08-10
Filing date: 2012-08-10
Publication date: 2013-02-14

Abstract

The system and method of the present invention employs voltage-driven current shunts to create a non-standard coil set for establishing magnetic fields to be used in such applications as Magnetic Resonance Imaging, Nuclear Magnetic Resonance Spectroscopy, and Electron Paramagnetic Resonance Imaging. By use of the system and method of the present invention, a standard primary coil set, the standard x-gradient, y-gradient, and z-gradient coil sets, and standard shim coil sets may be eliminated and replaced with a single non-standard coil set.

Description

SYSTEM AND METHOD FOR THE ESTABLISHMENT OF MAGNETIC FIELD PATTERNS BY A COIL SET WITH VOLTAGE-DRIVEN CURRENT SHUNTS

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of Provisional U.S. Patent

Application No. 61/574,823 filed on August 10, 2011, entitled Establishment of Magnetic Field Gradients.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH AND DEVELOPMENT

[0002] The invention described in this patent application was not the subject of federally sponsored research or development.

FIELD

[0003] The present invention pertains to the establishment of magnetic field patterns by coil sets through the application of electrical currents; more particularly, the present invention pertains to the establishment of magnetic field patterns by coil sets in the context of Magnetic Resonance Imaging (MRI) scanners and in the context of other systems such as Nuclear Magnetic Resonance Spectroscopy, Electron Paramagnetic Resonance Imaging, and Electron Paramagnetic Resonance Spectroscopy that also require establishment of precise magnetic field patterns by coil sets for the elicitation of information from a subject.

BACKGROUND

[0004] A Magnetic Resonance Imaging (MRI) scanner and other similar devices are systems that establish precise magnetic field patterns by coil sets so as to manipulate the orientations of magnetic moments inherently present within a subject. This manipulation causes the magnetic moments to generate electrical signals within the scanner, and these signals are in turn used to construct detailed images of the internal structure of the subject.

[0005] MRI images have been applied with great success to disease diagnosis.

However, the extension of MRJ to disease screening, including cancer screening, has unfortunately been relatively limited. Two very significant factors limiting the use of MRJ for screening are the relatively high cost generally associated with scanner construction and the relatively small patient space generally existing within the scanner.

[0006] These two limiting factors are ultimately linked to the fact that numerous distinct structures are normally required within an MRI scanner. Each of these structures within an MRI scanner is designed to contribute its own magnetic field pattern within the volume of a scanner specifically designated for imaging. The summed magnetic field patterns result in the magnetic moment orientations desired at a particular instant of time. These structures within an MRI scanner are well-known to those in the art. Specifically, the structures within an MRI scanner include a primary coil set or a permanent magnet for establishment of a B₀ field, a radio frequency coil set for establishment of a B_\ field, an x-gradient coil set, ^-gradient coil set, and z-gradient coil set for respective establishment of an x-gradient field, ^-gradient field, and z-gradient field, and several shim coil sets and permanent magnet shims for adding or subtracting specific magnetic field moments to and from the Bo field to assure its uniformity. For the purposes of this application, the MRI coil sets noted above will be termed "standard coil sets," or groupings of loops and/or coils (i.e., lengths of wire looped around a plurality of times) that carry electrical currents and that are each dedicated to generating one of the specific MRI magnetic field patterns noted above. Each standard coil set is understood to produce its own specific magnetic field pattern, in accordance with the Biot-Savart law, as a result of the exact current values, positions, shapes, and sizes associated with its loops and/or coils.

[0007] There have been endeavors that would allow two or more standard

MRI coil sets to be replaced with a single non-standard coil set, i.e., a grouping of loops and/or coils that carry electrical currents and that is capable of generating more than one of the specific, standard MRI magnetic field patterns noted above. In PCT application WO 98/12964, Cho and Bunney (motivated by the desire to significantly reduce the loud noise produced by the operation of gradient coil sets) replaced the -gradient coil set and ^-gradient coil set with a single, continuously rotating coil set containing a continuous electrical current. Although very quiet, this MRI scanner requires the use of a fairly powerful motor for routine use and would be furthermore greatly restricted with regard to the magnetic field patterns produced.

[0008] U.S. Pat. No. 5,661,401 to Ishikawa and Kida, U.S. Pat. No. 6,067,001 to Xu and Conolly, and U.S. Pat. No. 6,492,817 to Gebhardt et al. each describes the establishment of a desired magnetic field pattern through control of either an array or a mesh of many small current loops. These systems could all in principle be used to produce a magnetic field pattern that is equivalent to the summed magnetic field patterns of two or more of the standard coil sets found in MRI scanners. However, the significant complexity of these systems appears to counter the goal of constructing MRI scanners more economically.

[0009] U.S. Pat. No. 6,933,724 to Watkins et al. has disclosed a coil set within which individual loops have been replaced with separate loop segments or arcs with independent currents. The current pattern in each segmented loop of this coil set is able to represent a sum of current patterns respectively associated with different standard MRI coil sets. Although this system does appear quite able to replace the function of a plurality of standard MRI coil sets, a separate current driver (i.e., voltage source) is noted to be required for each current segment in the coil set. Also, the inventors specifically state that their single coil set can be constructed and run to simultaneously generate a main (i.e., Bo) field, gradient fields, and shim fields.

However, currents that contribute to a Bo field are generally very large, i.e., on the order of tens of thousands of Amps even for low-or mid-field MRI. Therefore, a cluster of like loop segments that contributes to a Bo field in this system will likely need to have a total of several tens of thousands of Amps entering at one end of the cluster and a total of several tens of thousands of Amps leaving at the other end of the cluster. Such very large currents, which do not contribute a magnetic field pattern for imaging, would waste energy and would also potentially affect the precision of the magnetic field patterns within the scanner that are used for imaging.

[00010] Based on the above discussion, it is seen that there is still a need for improvement with respect to the goal of reducing the number of standard coil sets used in an MRI scanner, thereby reducing the cost of an MRI scanner and increasing the space available to patients.

SUMMARY

[00011] The disclosed invention is a non-standard coil set, i.e., a structure that is capable of reducing the number of standard coil sets found in an MRI scanner by replacing two or more of those standard coil sets with a single non-standard coil set. The magnetic field pattern produced by the non-standard coil set of the present invention is understood to be a pattern in the z-component of the magnetic field, where the -direction is by definition taken to be the predominant orientation of the magnetic field vectors associated with the B₀ field of the MRI scanner. Therefore the present invention discloses a way to replace those standard MRI coil sets for which the z-component of the magnetic field is the magnetic field component of interest, i.e., the primary coil set, the x-gradient coil set, the ^-gradient coil set, the z-gradient coil set, and/or shim coil sets, with a single non-standard coil set.

[00012] Once it has been decided which standard MRI coil sets are to be replaced with the disclosed non-standard coil set, the steps for construction and operation of the non-standard coil set of the present invention are as follows:

a) Noting the general current patterns that produce the magnetic field patterns associated with each of those standard coil sets (these general current patterns are well-known in the art);

b) Through techniques well-known in the art (e.g., spherical harmonics, matrix inversion, non-linear optimization, heuristic modification), designating the positions and diameters for a group of loops and/or coils such that each desired standard magnetic field pattern could be produced with sufficient precision if its corresponding general current pattern could somehow be made to appear in one or more specific loops and/or coils from among the full designated group of loops and/or coils;

c) Noting the total current value (i.e., current magnitude and direction) required for each part of each loop and/or coil when the currents demanded by all of the standard magnetic field patterns are considered collectively;

d) Determining if a portion of the current within some or all of the loops and/or coils will arise from a series connection. This would particularly be useful for loops and/or coils that must carry total currents with a relatively large magnitude, because currents used for MRI must generally be kept very stable and it is more economical to use one stable voltage source designed for large currents than a plurality of stable voltage sources designed for large currents. The desired shared current that is to enter and leave each loop via this series connection will here be termed hared and the voltage source driving this shared current will be termed V_Shared,'

e) For each isolated loop or coil, that is, a loop and/or coil not connected to any other loops or coils through a series connection, applying a voltage source across the endpoints of the loop or coil if there is to be a net voltage drop along the loop or coil;

f) For each loop or coil that demands different current values in two or more parts of the loop or coil, attaching voltage-driven current shunts such that each point of the loop or coil where the current value changes is attached to one of the two endpoints of at least one voltage-driven current shunt;

g) For each loop or coil that is connected to other loops and/or coils through a series connection and for which the total voltage drop demanded along the loop or coil will not be equal to the voltage drop along the loop or coil specifically associated with the desired shared current hared, attaching a voltage-driven current shunt with voltage Vi_{oop driver} with one shunt endpoint located at the entry point of I shared and the other endpoint located at the exit point Of Ishared^',

h) Using Kirchhoff s junction rule and loop rule to solve for the currents and voltages required for all the shunts;

i) If a series connection was employed, using Kirchhoff s loop rule along the entire series connection to determine V_sh_are - Because of the inherent symmetries in the current pattern that are generally associated with each standard magnetic field pattern used in MRI, the collective set of net current patterns across all of the loops and/or coils in the MRI scanner as determined by Step c) above will generally exhibit clear symmetries as well. Those skilled in the art will recognize that such symmetries imply that

will most likely not have to be changed at all even as the net magnetic field MRI pattern produced by the present invention is changed as needed.

[00013] The above steps, as written above, apply to the establishment of a net magnetic field pattern associated with a single instant of time. Those skilled in the art will understand that the execution of each step should actually take into account the full range of net magnetic field patterns that those standard coil sets replaced by the non-standard coil set of the present invention would be expected to produce as a subject is scanned.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[00014] A still better understanding of the disclosed system and method for the establishment of magnetic fields by a coil set with voltage-driven current shunts may be had by reference to the drawing figures wherein:

[00015] Figure 1 is a schematic circuit diagram showing four different current values established within the same loop through the attachment of voltage-driven current shunts;

[00016] Figure 2 is a schematic circuit diagram showing how the four different current values of Figure 1 are established within a coil through the attachment of a series of voltage-driven current shunts to a wire that is to be wound n times to form a coil;

[00017] Figure 3 illustrates how the circuit arrangement of Figure 1 would appear within an MRI scanner for the simultaneous establishment of an ^-gradient, a ^-gradient, and a z-gradient without use of the corresponding standard MRI coil sets; [00018] Figure 4 illustrates how the circuit arrangement of Figure 1 would appear within an MRI scanner for the simultaneous establishment of a Bo field, an ^-gradient, a ^-gradient, and a z-gradient without use of the corresponding standard MRI coil sets;

[00019] Figure 5 is an alternative embodiment to that shown in Figure 1 for the establishment of four different current values within the same loop;

[00020] Figure 6A illustrates a cross-sectional view along the middle plane of a loop of a standard primary coil set when a loop of a non-standard gradient coil set has been placed within it;

[00021] Figure 6B is a cross-sectional view at plane VIA - VIA of Figure 6A;

[00022] Figure 6C illustrates a cross-sectional view along the middle plane of a loop of a non-standard gradient coil set when a loop of a standard primary coil set has been placed within it; and

[00023] Figure 6D is a cross-sectional view at plane VIC - VIC of Figure 6C. DESCRIPTION OF THE EMBODIMENTS

[00024] Figures 1 through 5 are schematic circuit diagrams consistent with removal of the standard ^-gradient coil set, the standard s-gradient coil set, and the standard z-gradient coil set from an MRI scanner through use of the present invention 10.

[00025] Figures 1, 2, 4, and 5 are additionally consistent with removal of the standard primary coil set from an MRI scanner.

[00026] All loops described and illustrated below are understood to be centered about the z-axis and to lie in a plane parallel to the x-y plane. The volume of the MRI scanner designated for imaging (i.e., the imaging volume) is understood to be centered at the origin, i.e., at (x,y,z) = (0,0,0). [00027] Figure 1 may be understood by first noting, as is well-known in the art, that an x-gradient magnetic field (i.e., a magnetic field for which the magnitude has a slope or an approximately linear change in the x-direction) can be established at the origin through the application of two currents of magnitude β that are anti-symmetric with respect to x = 0; a ^-gradient magnetic field can be established at the origin through the application of two currents of magnitude / that are anti-symmetric with respect to = 0; a z-gradient magnetic field can be established at the origin through the application of two loop currents of magnitude δ that are anti-symmetric with respect to z = 0; and a Bo magnetic field can be established at the origin through loop currents that are symmetric with respect to z - 0.

[00028] Figure 1 represents a circular loop 100 that contributes to all four of these magnetic field patterns because its current values are seen to be the sum of two currents of magnitude β that are anti-symmetric with respect to x = 0, two currents of magnitude /that are anti-symmetric with respect to y = 0, a current of magnitude δ throughout the loop, and a current of magnitude I_Shared throughout the loop, where in this context the I_s ared value contributes to the Bo field (and where it is understood that there should be a partner loop on the other side of the z = 0 plane, and the same distance from the z = 0 plane, with the same current values but with current δ substituted with -<5).

[00029] The current values in the circular loop 100 of Figure 1 are seen to change at the bottommost, rightmost, uppermost, and leftmost points of the circle. In accordance with Step f) described in the Summary above, voltage-driven current Shunt A 20 and voltage-driven current Shunt B 40 have been attached to the loop 100 such that each of those points of current change 22, 24, 42, and 44 is attached to one shunt endpoint. Additionally, because of the entry and exit of the desired shared current I shared into the loop 100 and the fact that the voltage drop along the loop 100 is not simply equal to the product of I_Sh_ared and the total loop resistance, in accordance with Step g) described in the Summary above a voltage-driven current shunt with voltage Vioop driver has been attached to the loop at the entry and exit points of Ishared 48, 46.

[00030] Kirchhoffs junction rule and loop rule were used to solve for the three shunt currents and voltages, which are as follows.

I A = 2β

h = 2y

hoop driver ^~ δ— β—

V_A = (2fi)R_A + 2{I_shared + δ + β)11_α

V_B = {2_Y)R_B + 2(I_shared+ δ + f)R_q

Vloop driver ~ {5 ~ β ~ f)Rloop driver +

⁺ fyRq,

where R_q is the resistance of each quarter of the loop, RA is the total resistance associated with shunt A, Rg is the total resistance associated with shunt B, and Rioop drive,- is the total resistance associated with the leads and battery of the loop driver.

[00031] As reflected by the above solutions for I_A, and , it can be seen that voltage-driven current Shunt A 20 simply transfers current 2β from the topmost point of the loop 100 to the bottommost point of the loop 100 and that voltage-driven current Shunt B 40 simply transfers current 2f from the leftmost point of the loop 100 to the rightmost point of the loop 100.

[00032] For clarification, if the I_shared segments had been missing from Figure 1 then the value Ishare would not have appeared as part of the current values in the loop 100. Furthermore, the points where Ishared had entered and exited the loop 100 would have been seen to be the endpoints of the loop 100, and a voltage source would have been placed across the endpoints of the loop 100 in accordance with Step e) described in the Summary above.

[00033] Several practical notes regarding the loop 100 of Figure 1 must be made here. First, because /,^ would be on the order of tens of thousands of Amps to be able to contribute to a B field, it is understood that the physical loop corresponding to the schematic loop 100 shown in Figure 1 has a thickness on the order of that associated with a coil in a standard primary coil set to prevent excessive heating. Second, the physical loop corresponding to the schematic loop 100 shown in Figure 1 may have to contain slots that prevent the formation of eddy currents within the loop 100. These slots should be designed so as to not affect the overall precision of the magnetic field pattern of the loop. Third, it can be seen that the voltage sources that drive the shunts 20, 40 can be used to overcome the inductance of the loop 100, thus allowing the magnetic field established by the loop 100 to be changed as quickly as is typically required for MRI scanning (i.e., in about one half of a millisecond). Fourth, the voltage source V_sh_ared generating the series current I_Shared that allows the loop 100 to contribute to the B₀ field will clearly have to be one that is equipped to produce a very large, stable current within a low-resistance series connection. This may be achieved, for example, through use of a stack of rectifier-controller units wired together in parallel and employing insulated gate bipolar transistors (IGBTs), thyristors, or other semiconductor technologies.

[00034] Figure 2 illustrates how shunts would be used to establish the same current patterns that were seen in Figure 1 within a coil instead of within a loop. The wire 200 in Figure 2 is to be wound n times to form a coil. Consequently, the current values in the wire 200 are the same four current values seen within Figure 1 , reduced by a factor of n and understood to be repeated n times in a row. This way, when the coil is wound, the n parts of the wire 200 with current value (I shared + S - β - γ)Ιη can sum to produce current I_{s ar}ed + S- β - γ, the n parts of the wire with current value (Shared + δ+ β - γ)Ι^η can sum to produce current I shared + δ+ β - γ, etc. A pair of voltage-driven current shunts 60, 80 is used to establish each set of four current values in a completely analogous way to Figure 1.

[00035] In Figure 2, only one voltage-driven current shunt with voltage source V/oop driver is necessary, as opposed to n such voltage-driven current shunts, since the desired shared current (I_shareJri) used in this context only has one entrance point 98 and one exit point 96 in the wire. The configuration associated with the voltage source Vi_oop driver ^ d the current (I_Sharec/n) in Figure 2 is understood to be the same as the configuration of the voltage sourt Vi_{oop c}i_ri_Ver

the current I shared in Figure 1 , with the shared current entrance point 98 and exit point 96 in figure 2 simply substituting for the shared current entrance point 48 and exit point 46 in Figure 1.

[00036] In Figure 2, Ii_{oop driver} equals

(ίδ -β - yyri)

and V/oop dri_Ver equals

((δ- β - γ)Ιή)Ι{,_οορ driver + 4(I_shared + S)Rg,

where R_q corresponds to the resistance of the length of wire associated with a quarter loop when the wire is wound.

[00037] Figure 3 illustrates how the present invention 10 would manifest itself in an actual MRI scanner when the present invention 10 is used to replace standard x-gradient, y-gradient, and z-gradient coil sets. In this scenario, the inner loops are involved with the establishment of an ^-gradient and ^-gradient and all four loops are involved with the establishment of a z-gradient (via loop currents of magnitude δ_\ and loop currents of magnitude <% that are each anti-symmetric with respect to z = 0). The circuit structure shown in Figure 1 is understood to apply to the loops of Figure 3, but with no voltage-driven current shunts used for the outer loops, no series current I shared running to and from the loops, the endpoints of each loop and the voltage sources across the endpoints of each loop left understood, and the voltage sources for the inner loop shunts left understood.

[00038] Those skilled in the art will readily appreciate that the course of the voltage-driven current shunts associated with the present invention can be set so as to avoid or minimize any negative impact of the voltage-driven current shunts' magnetic fields on the net magnetic field pattern established by the loops of the present invention. This is manifested in two different ways for the voltage-driven current shunts shown in Figure 3.

[00039] Firstly, the currents in the voltage-driven current shunts arising from the inner loops are seen to be directed away from the center of the scanner (i.e., away from the imaging volume) along the z-axis, to cross over in the left-to-right or top-to- bottom directions, and then to return to the inner loops along the z-axis. This is a favorable arrangement because currents that are parallel to the z-axis and small compared to 2¾-level currents have negligible impact on magnetic field patterns in the imaging volume, and currents not parallel to the z-axis but relatively removed from the imaging volume have limited impact on the magnetic field patterns in the imaging volume.

[00040] Secondly, the current in each voltage-driven current shunt in Figure 3 is seen to be intentionally split into two branches before it crosses in the left-to-right or top-to-bottom directions, with the split shunt currents traveling along arcs of the same diameter as the loop from which the currents arose. With this approach, the two voltage-driven current shunts, each carrying a total current of 2β produce an ^-gradient with a small slope at the scanner center and the two voltage-driven current shunts carrying total current of 2/produce a^-gradient with a small slope at the scanner center. Thus, rather than the currents in the voltage-driven current shunts distorting the net magnetic field pattern formed by the loops of the invention, the currents in the voltage-driven current shunts just slightly modify the slopes of the x-gradient and ^-gradient formed by the loops of the invention. The x-gradient and the ^-gradient slopes can be shown to be increased in magnitude by these currents in the voltage-driven current shunts if the currents in the voltage-driven current shunts are sufficiently removed from the center of the scanner.

[00041] Figure 4 demonstrates how the present invention would manifest itself in an actual MRI scanner if it is used to replace the standard primary coil set in addition to the standard x-gradient, >-gradient, and z-gradient coil sets. Figure 4 is the same as Figure 3 except for the entry and exit of a large I_Shared current into and from each loop, as well as the thickness of the loops being more in line with the thickness of coils seen in standard MRI primary coil sets. The circuit structure shown in Figure 1 is understood to apply to the loops shown in Figure 4, but with no shunts used for the outer loops, the voltage-driven current shunts attached to the loops at the entry and exit points of hared and their accompanying voltage sources Vi_oop driver left understood, and the voltage sources for the inner loop shunts left understood.

[00042] Figure 5 is an alternative embodiment to the preferred embodiment shown in Figure 1. The circuit illustrated in Figure 5 produces the exact same current values within the loop as the circuit of Figure 1, but does so with five voltage-driven current shunts instead of three. Although the use of relatively fewer voltage-driven current shunts would generally be preferred in the present invention, the circuit of Figure 5 has an infinite number of solutions for its shunt currents and voltages. It therefore may be of use in situations in which significant flexibility is needed in the current and voltage values associated with the voltage-driven current shunts.

[00043] The shunt currents and voltages are indicated below:

IA is arbitrary

= IA— 2_y

Ic = lA ~ 2(fi + y)

I_D = I_A - 2fi

hoop driver ~ δ ~ β ~

V_A = I_ARA + (Ishared + δ + β + y)R_Q [I A arbitrary]

V_B = (IA ^~ 2y)R_B + (I_shared + δ + β - y)R_Q

Vc = (I A -2(β + y))Rc + Shared + 0- fi - _R)R_Q - 4(I_shared + S)R_Q

V_D = (IA - 2(J)RB + (Ishared + δ- β + r)R_Q

Vloop driver ~ (δ^~ β ^~ )Rfoop driver +

+ S)R_q,

where R_q is the resistance of each quarter of the loop, RA-RD are the total resistances respectively associated with shunts A, B, C, and D, and Ri_oop driver is the total resistance associated with the leads and battery of the loop driver.

[00044] Figures 6A, 6B, 6C, and 6D may be better understood by first noting, as is well-known in the art, that a structure exposed to a strong magnetic field (such as the magnetic fields surrounding the conductors of a standard primary coil set) and also containing a current that is changed over time will generally vibrate from Lorentz forces and thereby produce acoustic noise. The coil set of Figure 3, which can here be termed a "non-standard gradient coil set" of the present invention because it is a nonstandard coil set that replaces one or more of the standard ^-gradient, ^-gradient, and z-gradient coil sets (and is additionally understood to be used in conjunction with a standard primary coil set), would consequently be expected to vibrate and produce acoustic noise as standard ^-gradient, ^-gradient, and z-gradient coil sets do. While in the Advantages section below it will be explained why a non-standard coil set associated with the present invention would probably be easier to place within evacuated tube structure than a standard MRI coil set would be, Figures 6A, 6B, 6C, and 6D represent a way to reduce the acoustic noise of a non-standard gradient coil set of the present invention that does not involve evacuated tubes. Specifically, as part of Step b) described in the Summary above, one or more loops and/or coils of the non-standard gradient coil set are designed to be square-shaped and to have the same positions and general dimensions as one or more square-shaped loops and/or coils of the standard primary coil set. Those primary coil set loops and/or coils are furthermore designed to each have a continuous hollow or tunnel within them, and the loops of the non-standard gradient coil set are placed within the tunnels of the corresponding loops and/or coils of the primary coil set. The loops of the nonstandard gradient coil set will be expected to experience reduced vibration because the magnetic field within a circular tunnel established in the middle of a long, straight, circularly symmetric structure with uniform longitudinal currents should be significantly reduced in magnitude in comparison to the magnetic field outside of the long, straight, circularly symmetric structure.

[00045] Figure 6A shows a cross sectional view along the middle plane of a loop 400 of a standard primary coil set when a loop of a non-standard gradient coil set 12 has been placed within the continuous tunnel 402 established within the loop of the primary coil set.

[00046] Figure 6B shows a cross section along the thickness of the composite structure of Figure 6A, and assumes that a shunt 30 is projecting outwards from the loop of the non-standard gradient coil set 12 and is piercing the loop 400 of the primary coil set at this particular cross section.

[00047] If, as was described above, one or more loops and/or coils of the nonstandard gradient coil set are designed to be square-shaped and to have the same positions and general dimensions as one or more square-shaped loops and/or coils of the standard primary coil set, a continuous hollow tunnel could be made within each of the loops and/or coils of the non-standard gradient coil set instead of within each of the loops and/or coils of the standard primary coil set. In this scenario the loops and/or coils of the standard primary coil set are placed within the tunnels of the corresponding loops and/or coils of the non-standard gradient coil set. In this case, the loops of the non-standard gradient coil set will experience reduced vibration because a long, straight, circularly symmetric structure with longitudinal, changing currents enveloping a long, straight, circularly symmetric structure with longitudinal, strong currents should experience Lorentz forces due to those longitudinal, strong currents that are significantly balanced in comparison to the Lorentz forces that would be experienced if the structure with longitudinal, changing currents were located outside of the structure with longitudinal, strong currents.

[00048] Figure 6C shows a cross section along the middle plane of a loop 14 of a non-standard gradient coil set when a loop of a standard primary coil set 500 has been placed within the continuous tunnel 15 established within the loop 14 of the nonstandard gradient coil set.

[00049] Figure 6D shows a cross section along the thickness of the composite structure of Figure 6C, and assumes that a voltage-driven current shunt 50 is projecting outwards from the loop of the non-standard gradient coil set at this particular cross section: [00050] Square-shaped loops and/or coils were employed to pursue acoustic noise reduction in the scenarios described just above because square-shaped or rectangular-shaped loops and/or coils, assuming that they have a circular cross section, clearly contain sections that can be considered long, straight, and circularly symmetric. An analogous approach to noise reduction with loops and/or coils representing non-square shapes, including the use of circular loops and/or coils, could be taken if the diameters of those loops and/or coils are large enough for sections of the loops and/or coils to be approximated as long, straight, and circularly symmetric.

[00051] Based on the foregoing disclosure of the structure of the present invention, the following method steps are used for its implementation:

a) determining the general current patterns that produce magnetic field patterns associated with the standard coil sets used in a device to establish a plurality of magnetic field patterns with the same magnetic component of interest;

b) designating the positions and diameters for a group of loops and/or coils to enable production of the magnetic field patterns associated with the standard coil sets used in a device to establish magnetic field patterns for the elicitation of information from a subject;

c) determining the current value for each part of each loop and/or coil when the currents needed by all of the standard magnetic field patterns are considered collectively;

d) determining if a portion of the current within some or all of the loops and/or coils will arise from a series connection;

e) applying a voltage source across the endpoints of end loop or coil if there is to be a net voltage drop along the loop or coil; f) attaching one or more voltage-driven current shunts to each loop or coil that needs different current values in two or more part of the parts of the loop or coil;

g) attaching one or more voltage-driven current shunts to each loop or coil that is connected to other loops and/or coils through a series connection and for which the total voltage drop along the loop or coil will not be equal to the voltage drop along the loop or coil specifically associated with the desired shared current of the series conneciton;

h) determining the currents and voltages needed for the voltage-driven current shunt; and

i) determining the voltage needed for a desired shared current.

[00052] Those of ordinary skill in the art will understand that there are many possible variations to the disclosed invention including those described in the following paragraphs.

[00053] The loops and/or coils of the present invention do not have to be planar, i.e., they do not have to completely lie within planes parallel to the x-y plane. The loops and/or coils do not necessarily have to possess the same overall geometric shape.

[00054] The two endpoints of each voltage-driven current shunt used to help achieve different current values in two or more parts of individual loops and/or coils of the present invention do not necessarily have to be attached to the same loop and/or coil. Also, one or both endpoints of each voltage-driven current shunt used to help achieve different current values in two or more parts of individual loops and/or coils of the present invention could be attached to some region within another voltage- driven current shunt. Additionally, each voltage-driven current shunt used to help achieve different current values in two or more parts of individual loops and/or coils of the present invention can branch out and rejoin an arbitrary number of times between its endpoints.

[00055] Each voltage-driven current shunt could possess some variable resistance that could be used in addition to its voltage source to help achieve the required current values in the loops and/or coils of the present invention. In the embodiment represented in Figure 5, certain values of 1 and I_shwed in fact make it possible for one or more of the shunts to only possess variable resistance (i.e., to not be voltage-driven to begin with).

[00056] The present invention can clearly be used to replace electrical coil sets in systems other than MRI scanners that produce magnetic field patterns. In those cases, when a plurality of coil sets is to be replaced by the present invention it is understood that the coil sets to be replaced establish magnetic field patterns associated with the same magnetic field component of interest (e.g., the z-component of the magnetic field for systems that, like MRI, have a 2¾ field). Nuclear Magnetic Resonance Spectroscopy, Electron Paramagnetic Resonance Spectroscopy, and Electron Paramagnetic Resonance Imaging are three examples of non-MRI methods to which the present invention can be applied.

[00057] Having now disclosed the system and method of the present invention, those of ordinary skill in the art will understand that the advantages described in the following paragraphs are now enabled.

[00058] The present invention provides a practical way to replace any or all of the standard MRI coil sets for which the magnetic field component of interest is along the z-dimension with a single new coil set. This means that the present invention can in principle be a practical substitute for all of the non-radiofrequency coil sets used in an MRI scanner, including the primary coil set. Unlike the segmented coil set disclosed in U.S. Patent No. 6,933,724 to Watkins et al, that was discussed in the Background above, establishment of the Bo field with the present invention will not require a plurality of entry and exit points of very large current into each loop that is contributing to the Bo field which, as noted above, implies a significant waste of energyas well as potential field distortion. Accordingly to the present invention, each loop contributing to the Bo field need to have only one entry and one exit point for a very large current, and this very large current can furthermore be potentially generated by a single stack of voltage sources that supplies current through a series connection to all loops contributing to the Bo field.

[00059] A system that replaces the standard primary coil set, standard gradient coil sets, and standard shim coil sets with a single coil set of the present invention, and that consists of relatively thick loops, such as those illustrated in Figure 4, and attached voltage-driven current shunts, will have numerous advantageous features. As these advantageous features are enumerated below, a scanner that uses the coil set just described will be referred as a "thick-loop scanner."

[00060] The first advantage of the thick-loop scanner can be seen from the fact that, given that the precision of the B field magnetic field pattern is particularly important in MRI, the loops of the thick-loop coil scanner will likely be designed to have positions, diameters, and thicknesses equal or approximately equal to positions, diameters, and thicknesses of coils associated with a standard primary coil set. This means that, assuming that the paths of the shunts are set to be outside of the volume enclosed by the thick loops, as shown in Figure 4, from a spaciousness perspective the thick-loop scanner will be equivalent to an MRI scanner that contains only a primary coil set and a radiofrequency coil set. The size of the radiofrequency coil set may be able to be made larger than is usual due to the space freed up within the thick-loop scanner. The greatly increased sense of spaciousness would be likely to make disease screening more palatable to the general population, and would also increase opportunities for the imaging of obese individuals, the imaging of individuals with claustrophobia, veterinary imaging, and imaging during interventional or surgical procedures.

[00061] The second advantageous feature of the thick-loop scanner is the relatively low manufacturing cost expected. Only one coil set other than the radiofrequency coil set would have to be manufactured for the scanner. Furthermore, the thick loops of that single coil set would presumably be molded and consequently be more cost-effective to make in comparison to structures formed from the careful, repeated winding of wires into thick coils. Molded structures may also be less susceptible to errors arising from the mechanical stresses of transport than wound structures are, and for that reason it might be more economical to disassemble a thick- loop scanner and reassemble it elsewhere, for example, for donation to a developing nation, than would be the case for a scanner with a large number of windings. It is true that voltage-driven current shunts will have to be manufactured along with the loops of this thick-loop scanner, and attached to those loops; however, like the molded loops themselves, the voltage-driven current shunts are relatively simple structures. Additionally, if the loops had a shared current through a series connection, a thick connector would have to be constructed and attached to the loops; although the currents of the connector would have to be arranged to assure that it produces a net magnetic field of approximately zero, it would still be a relatively basic structure to build. [00062] A third advantageous feature of the thick-loop scanner is its capability to provide relatively quiet operation. In standard MRI, the different coil sets are often placed within one another in the form of tightly-fitting concentric cylinders; however, the non-standard coil sets of the present invention can generally be developed to have comparatively more free space at their interior and/or exterior volumes because of the accompanying reduction in the number of standard MRI coil sets within the scanner. Part of this increased space could be devoted to the construction of slender evacuated tubes around the individual loops and voltage-driven current shunts of the nonstandard coil set, which would significantly reduce the noise transmission resulting from Lorentz forces acting on the loops and voltage-driven current shunts when their currents change in value. For the case of the thick-loop scanner, evacuated tubes would only have to be placed around the voltage-driven current shunts because the loops would each probably weigh on the order of 1000 kg and would therefore be unlikely to significantly vibrate as their currents change. If the shunts to be used to help achieve different current values in the loops of the thick-loop scanner happen to be identical to the voltage-driven current shunts depicted in Figure 4, then the evacuated tubes used to enclose the shunts could simply consist of eight straight evacuated tubes and two circular evacuated rings.

[00063] While the thick-loop scanner would especially exhibit the qualities of relative spaciousness, cost-effectiveness in manufacturing, and quietness, other nonstandard coil sets of the present invention would be generally expected to share all three of those qualities as well. Therefore, for an MRI scanner that uses permanent magnets to generate the Bo field and shim fields, it would probably be beneficial to apply the present invention to build a single coil set (such as the coil set shown in Figure 3) to establish the scanner's three gradient fields. It may be noted that, assuming loops with no series connection in this scenario, 25% fewer voltage sources would be required to establish those gradient fields than if the gradient fields were obtained with the segmented coil set disclosed in U.S. Patent 6,933,724 to Watkins et al, as Figure 1 in U.S. Patent No. 6,933,724 suggests that the invention would require four voltage sources per loop while Figure 1 of the present invention suggests that the present invention would require only three voltage sources per loop.

[00064] Having now read and understood the disclosed system and method for the establishment of magnetic field patterns by a coil set with voltage-driven current shunts, those of ordinary skill in the art will recognize other advantages, variations, and embodiments that have been enabled by the foregoing disclosure. Such advantages, variations, and embodiments shall be considered to be part of the scope and meaning of the appended claims.

Claims

CLAIMS I claim:

1. An electrical coil set for establishing a magnetic field pattern, said coil set comprising:

one or more loops and/or coils;

one or more voltage-driven current shunts;

each of said one or more loops and/or coils potentially requiring distinct current values within two or more of its parts, to help establish said magnetic field pattern;

each point at which a current value changes within each said one or more loops and/or coils connected to an endpoint of at least one of said voltage-driven current shunts;

a series connection between a plurality of said one or more loops and/or coils, with the shared current of said series connection driven by its own voltage source; each of said one or more loops and/or coils that is a member of said series connection potentially requiring a voltage drop other than that associated with the desired shared current of said series connection, to help establish said magnetic field pattern;

the two endpoints of one of said voltage-driven current shunts respectively attached to the entry and exit points of said shared current for each of said one or more loops and/or coils that is a member of said series connection and that requires a voltage drop other than that associated with the desired shared current of said series connection; and whereby the use of this coil set will remove the need for two or more standard coil sets in systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest.

2. The electrical coil set as defined in Claim 1 wherein the magnetic field pattern established is equivalent to the magnetic field pattern established by a single standard coil set in systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest.

3. The electrical coil set as defined in Claim 1 wherein the magnetic field pattern established is equivalent to the magnetic field pattern established by one or more standard gradient coil sets in systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest, and wherein said one or more loops and/or coils of said electrical coil set are placed within hollows formed within loops and/or coils of a standard primary coil set used in systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest.

4. The electrical coil set as defined in Claim 1 wherein the magnetic field pattern established is equivalent to the magnetic field pattern established by one or more standard gradient coil sets in systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest, and wherein loops and/or coils of a standard primary coil set used in systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest are placed within hollows established within said one or more loops and/or coils of said electrical coil set.

5. The electrical coil set as defined in Claim 1 wherein the electrical power for said shared current or for individual loops and/or coils is obtained from a stack of rectifier-controller units wired together in parallel and employing one or more insulated gate bipolar transistors, thyristors or other semiconductors.

6. A method for the establishment of magnetic fields in coil sets used systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest, said method comprising the steps of:

e) applying a voltage source across the endpoints of each loop or coil if there is to be a net voltage drop along the loop or coil;

f) attaching one or more voltage-driven current shunts to each loop or coil that needs different current values in two or more parts of the loop or coil;

g) attaching one or more voltage-driven current shunts to each loop or coil that is connected to other loops and/or coils through a series connection and for which the total voltage drop along the loop or coil will not be equal to the voltage drop along the loop or coil specifically associated with the desired shared current of the series connection;

h) determining the currents and voltages needed for the voltage-driven current shunts; and

i) determining the voltage needed for a desired shared current.

7. A device that establishes a plurality of magnetic field patterns with the same magnetic field component of interest which requires the establishment of a predetermined magnetic field pattern, said device that establishes a plurality of magnetic field patterns with the same magnetic field component of interest comprising:

an electrical coil set, said electrical coil set including:

one or more loops and/or coils;

one or more voltage-driven current shunts;

each of said one or more loops and/or coils potentially requiring distinct current values within two or more of its parts, to help establish the predetermined magnetic field pattern;

each point at which a current value changes with each of said one or more loops and/or coils connection to an endpoint of at least one of said voltage- driven current shunts;

a series connection between a plurality of said one or more loops and/or coils, with shared current of said series connection being driven by its own voltage source;

each of said one or more loops and/or coils that is a member of said series connection potentially requiring a voltage drop other than that associated with the desired shard current of said series connection, to help establish said magnetic field pattern;

the two endpoints of one of said voltage-driven current shunts respectively attached to the entry and exit points of said shared current for each of said one or more loops and/or coils that is a member of said series connection and that requires a voltage drop other than that associated with the desired shared current of said series connection; and

whereby the use of this coil set will remove the need for two or more standard coil sets in systems that establish a plurality of magnetic field patterns with the same magnetic field component of interest.

8. The device that establishes a plurality of magnetic field patterns with the same magnetic field component of interest as defined in Claim 7 wherein the electrical power for said shared current or for individual loops and/or coils is obtained from a stack of rectifier-controller units wired together in parallel and employing one or more insulated gate bipolar transistors, thyristors or other semiconductors

9. The device that establishes a plurality of magnetic field patterns with the same magnetic field component of interest as defined in Claim 7 wherein said electrical coil set is a non-standard gradient coil set and is placed within a tunnel in a primary coil set.

10. The device that establishes a plurality of magnetic field patterns with the same magnetic field component of interest as defined in Claim 7 wherein said electrical coil set is a non-standard gradient coil set and a primary set is placed within a tunnel formed in said non-standard gradient coil set.