Arrangement of coil windings for MR systems

Although magnet coil windings are distributed discontinuously, the distribution of magnetic fields generated actually approaches a desired continuous distribution. The desired continuous distribution is defined analytically, as closely as possible. A system for acquiring magnetic resonance data has a coil segment formed by winding a conductor on a bobbin representing an axial direction. A magnetic field is generated by supplying current into the conductor. Winding positions at which the conductor is wound turn-by-turn on the bobbin are determined in agreement with a specified current step value and sequentially from a winding positioned at an outermost end of the coil segment in the axial direction. Additionally, a shunt element for shunting the current carried turn-by-turn is arranged in the coil segment in relation to a pattern of turns of the coil segment by winding the conductor into a plurality of current flows through a plurality of shunt paths.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a coil unit for generating magnetic fields 
exhibiting desired spatial distribution. More particularly, this invention 
is concerned with an epoch-making way of winding a coil to be included in 
a magnetic-field generation coil unit preferably of a magnetic resonance 
imaging (MRI) system or magnetic resonance spectroscopy (MRS) system that 
utilizes a resonance phenomenon exhibited by nuclear spins of a subject. 
2. Description of the Related Art 
A coil for generating a magnetic field is an indispensable element for many 
electric circuits or electric equipment. The gantry of a medical-purpose 
magnetic resonance imaging (MRI) system or magnetic resonance spectroscopy 
(MRS) system is one such system. A static coil for generating a static 
magnetic field, shim coils used to compensate for inhomogeneities in the 
static magnetic field, gradient coils for generating a magnetic field 
gradient to be superposed on the static magnetic field, and a 
radio-frequency coil used to transmit or receive radio-frequency signals 
are used as magnetic-field generation coils. 
These coils employed in an MRI or MRS system are, unlike an inductive 
element in an ordinary electric circuit, requested to meet another 
requirement that they must generate magnetic fields exhibiting a spatially 
desired distribution (also having a desired magnetic field strength). In 
particular, the gradient coils to which a pulsating current is fed are 
supposed to meet requirements defining switching characteristics such as 
the rise time required until a maximum magnetic field gradient strength is 
attained. 
A space in which a subject and the radio-frequency coil are inserted must 
be preserved inside the gantry of the MRI or MRS system. Various coils are 
arranged around the space. The gantry itself therefore tends to get large 
in size. Currently, it is sought to improve the ability to generate 
magnetic fields while avoiding an increase in size. A coil layer 
containing the static coil, shim coils, and gradient coils must be wound 
as thinly as possible. Especially, in the case of gradient coils to be 
stored in an already-defined-size bore of the static coil (for example, a 
superconducting magnet), the coil layer must be wound in a layer. 
With respect to the gradient coils, shielded coils capable of preventing 
magnetic leakage have been widely adopted in recent years. One of the 
shielded gradient coils is an actively (self-) shielded gradient coil 
(ASGC). This coil assembly has a dual coil structure having a main coil 
enclosed with a shield coil. It is therefore required that the main coil 
and shield coil are each wound in a layer in order to realize a thin coil 
assembly. 
As far as a coil assembly employed in an MRI or MRS system is concerned, 
the positions of windings forming a coil must be determined so that a 
spatially desired distribution of magnetic fields can be attained. As a 
known method of designing a coil, a technique using a continuous 
distribution or function to design a coil that exhibits a desired 
distribution of magnetic field is well-known. Also known is a technique 
described in "Gradient Coil Design: A Review of Methods" written by R. 
Turner (Magnetic Resonance Imaging, Vol. 11, pp.903-920, 1993). According 
to Turner's proposal, "integrated currents (amp-turns)" are calculated by 
integrating a distribution of current densities (See FIG. 7(A) in p. 911 
in the same thesis). Spatial positions associated with the integrated 
values of current densities are defined as coil positions by increasing 
the same value on a curve indicating the integrated values. This technique 
is called "target field approach in Turner's proposal. 
Still, as a conventional coil arrangement design technique, a distribution 
of current densities is used to determine the positions of windings, which 
is described, for example, in "Designing an NMR Actively Shielded Gradient 
Coil" written by Kiyoshi Yoda (T. IEE Japan, Vol. 110-A, No. 4, p.275-281, 
1990), is known. This technique is such that a desired distribution of 
current densities is calculated for an axial distance on a cylindrical 
bobbin of a coil, the distribution is integrated sequentially from the 
axial center of the bobbin toward each axial end thereof, and the 
positions of windings (turns) are determined from the axial center toward 
each axial end by examining axial points where the integrated values 
become I/2, I, . . . , I,I/2 (I: coil drive current value). 
Thus, in the known method proposed by Yoda, an ideal continuous 
distribution of currents obtained analytically is replaced with a 
discontinuous distribution of currents externalized as windings (changed 
into a discrete distribution) in order to create a coil assembly having a 
wire wound in a layer. 
However, in the foregoing method of designing a coil, since an ideal 
continuous distribution obtained analytically is replaced with 
discontinuous coil positions, an error occurring in an actual distribution 
of currents is basically unavoidable. For this reason, a desired ideal 
distribution of magnetic fields cannot be attained in many cases. Taking a 
gradient coil for instance, the linearities of magnetic field gradients 
realized with the continuous distribution deteriorate. Even the static 
coil and shim coils are designed according to the foregoing technique. For 
the same reason, there arises a problem that a distribution of magnetic 
fields deviates from a desired ideal state, and the homogeneities in a 
static magnetic field cannot be attained as expected. When an actual 
distribution of magnetic fields deviates from an ideal state, an adverse 
effect imposed on the qualities of MR images becomes serious, and the 
reliabilities of the images are impaired. From this viewpoint, there is a 
need for attaining a desired ideal distribution of magnetic fields. 
In addition to basic problems concerning the change from continuity to 
discontinuity for attaining a discrete distribution, there are problems in 
the arrangement of coil windings proposed by A Yoda, which are described 
below. 
A Yoda-proposed arrangement design needs a prerequisite that the design is 
carried out under strict restrictions including a condition where the peak 
value of a streamline function curve calculated based on a distribution of 
current densities supplied to a shield coil of Z channel is exactly an 
integer-times the coil drive current I. In the actual design, however, it 
will hardly happen that a solution to meet such condition will have found, 
in most cases, a remaining current which cannot covered by the windings 
being left. The remaining current thus appears at and have influence on 
the axial ends of a shield coil, because the positions of windings are 
determined from the axial center of the shield coil to the axial ends 
thereof. In consequence, at the axial end portions of a shield coil is 
provided a vacant gap which is relatively large and has no turns. Magnetic 
fluxes will leak because of the gap, causing eddy currents to flow on and 
in surrounding metal frames. In particular, the eddy currents thus-caused 
at the axial end portions have unfavorable deteriorating effects on the 
quality of MR images. On one hand, analytically designing the positions of 
windings of a coil based on Yoda's proposal should sacrifice its 
performance such as linearity. Additionally, the inductance and resistance 
values of the coil becomes large in Yoda's proposal, resulting in a 
larger-sized (i.e. enhanced power output) gradient coil. The coil design 
by Yoda's proposal is thus faced with various difficulties for practical 
use. 
Still, in addition to the basic problem in changing into discrete winding 
positions described above, the conventional coil design techniques 
including the foregoing Yoda's proposal have problems as below. 
First, there is a problem of physical restrictions to be imposed on an 
arrangement of windings forming a coil. Assuming that a coil having a wire 
is created by winding a wire, which has a certain width, about a 
cylindrical bobbin, the actual width of the coil is determined with the 
width over windings of the area most crowded with windings (turns) (area 
in which windings are most dense). In other words, there is the 
restriction that a wire wider than the width over windings of an area most 
crowded with windings cannot be used. Because of this restriction, when an 
ideal continuous distribution of currents is replaced with a discontinuous 
distribution of currents, a wide gap in which no wire exists is created 
between windings of a coil. 
The gap between windings poses a serious problem on, especially, an 
actively shielded gradient coil. Whether a gradient coil is of a saddle 
type or solenoid type, the size of the gap varies depending on the 
position in a coil unit. A wide gap between windings allows magnetic 
fluxes to leak out. As a result, eddy currents are induced in an external 
conductor. Despite the actively shielded gradient coil, the magnetic 
fields affected by the eddy currents invite deterioration of qualities of 
MR images. This has become a serious problem in recent years. 
The above situation will be described further. With the advancements of 
various electronic technologies and superconducting technologies, echo 
planar imaging (EPI) is one fast imaging technique enabling fast imaging 
that is faster than known spin echo (SE) imaging and fast spin echo (FAST 
SE) imaging and it has come to be a mainstream imaging technique in recent 
years. Spin echo imaging requires certain performance in relation to 
magnetic field gradients, for example, a maximum magnetic field gradient 
strength of 10 mT/m and a rise time of 1 msec required until the maximum 
magnetic field gradient strength is attained. By contrast, echo planar 
imaging requires certain performance in relation to magnetic field 
gradients, for example, a maximum magnetic field gradient strength of 30 
mT/m and a rise time of 0.1 msec required until the maximum magnetic field 
gradient strength is attained. 
With such increases in maximum magnetic field gradient strength and 
decreases in rise time, magnetic leakage increases. The increase in 
magnetic leakage brings about various deteriorations in image quality. 
This problem has become especially significant in recent years. 
Even when the structure of an ASGC is adopted, the problem that eddy 
currents are induced is pointed out even in "Design and Evaluation of 
Shielded Gradient Coils" written by J. W. Carlson et al. (Magnetic 
Resonance Imaging, Vol. 26, pp.191-206, 1992). As for the problem that 
eddy currents are induced, various countermeasures have been proposed. The 
problem of eddy currents is solved by improving a pulse sequence used to 
acquire an MR signal or by optimizing the phase of an radio-frequency 
pulse to be applied. 
However, when any of such proposed countermeasures is adopted, sequence 
control becomes complex. Besides, the practical efficacy is very low. The 
reasons are as follows: if magnetic fields affected by eddy currents 
exhibit the same spatial distribution (are the same magnetic field 
components) as magnetic field gradients, correction through pulse sequence 
control can be achieved. However, in reality, almost all magnetic fields 
affected by eddy currents contain magnetic components different from 
magnetic field gradients. It is, in principle, impossible to correct such 
magnetic fields affected by eddy currents on an ex post facto basis by 
controlling magnetic field gradients, a radio-frequency pulse, and a pulse 
sequence. In short, there is no better measure other than suppressing 
induction of eddy currents themselves. At present, eddy currents resultant 
from a gap between shielding windings included in a self-shielded gradient 
coil are thought to be unavoidable. This problem has remained unsolved. 
For smoothing a distribution of actually generated magnetic fields and 
approximating it to a desired distribution of magnetic fields, it is 
thought that a markedly thin wire is used in order to increase the number 
of turns. However, such a coil has a markedly high resistance and 
inductance. Unless the current-carrying capacity of a power supply 
increases enormously, a current cannot be supplied to the coil. It is 
actually very hard to manufacture such a large-capacity power supply. At 
present, it is rather unfeasible to manufacture such a large-capacity 
power supply. 
SUMMARY OF THE INVENTION 
The present invention attempts to break through the aforesaid various 
difficulties underlying the prior art. 
An object of the present invention is to provide, in particular, a system 
for acquiring magnetic resonance data comprising an ASGC having a coil 
segment generating a desired magnetic distribution, wherein at the axial 
end portions of a bobbin on which the coil segment is wound, the 
generation of eddy currents due to flux leakage can be minimized, avoiding 
excess deterioration in MR image quality. 
Another object of the present invention is to provide a system for 
acquiring MR data, which is capable of approaching a distribution of 
magnetic fields to be generated actually to a desired continuous 
distribution obtained analytically while maintaining a structure in which 
windings (wire) are distributed discontinuously. 
Another object of the present invention is to provide a system for 
acquiring MR data, which makes it possible to employ a power supply having 
a current-carrying capacity of a level accepted at present without the 
necessity of increasing the capacity of the power supply for feeding a 
current to a coil unit, and capable of approaching a distribution of 
magnetic fields to be generated actually to a desired continuous 
distribution obtained analytically while maintaining a structure, in which 
windings (wire) are distributed discontinuously, under the conditions for 
the power supply. 
Yet another object of the present invention is to provide a system for 
acquiring MR data, which is preferably adaptable to a static coil, shim 
coils, and gradient coils. 
Yet another object of the present invention is to provide a system for 
acquiring MR data, which when provided with a shielding ability, can 
fulfill the shielding ability by suppressing magnetic leakage or 
magnetostriction, and suppress unwanted eddy currents induced in 
surrounding metals. 
Still another object of the present invention is to provide a system for 
acquiring MR data, in which ex post facto data correction may be used as a 
countermeasure, which is intended to eliminate the adverse effect of eddy 
currents, while approaching a distribution of magnetic fields to be 
generated actually to a desired spatial magnetic distribution and 
suppressing induction of the unwanted eddy currents in surrounding metals. 
The present invention has been devised when the present inventor is 
motivated in pursuit of the structure of an MRI or MRA system which can 
provide a desired distribution of magnetic fields (that is, desired 
performance of a coil) and which can be realized using power supply and 
manufacturing facilities whose current-carrying capacity and scale are of 
levels accepted at present. 
First, the first aspect of the present invention concerning coil winding 
techniques will be explained in comparison with the conventional technique 
proposed by Yoda. 
An ASGC, the total current amount supplied through a main coil generating a 
gradient field can be expressed as follows; 
EQU Ic.times.Nc 
where Ic is a value of current flowing through the main coil and Nc is the 
total number of windings of the main coil. A shield coil, which is placed 
in a radially outer space of the main coil and generates a shielding 
magnetic field for the gradient field generated by the main coil, should 
have the total number of windings which is an integer, like the main coil. 
A value of current supplied into the shield coil can be obtained by 
dividing the analytically-requested total amount of current by a given 
total number of windings. This current value obtained by the division 
becomes a current step value for determining the winding positions (turn 
positions) of the shield coil. This is the basic principle of conventional 
techniques including Yoda's proposal. 
Like the main coil, the total amount of current through the shield coil can 
be expressed by 
EQU Is.times.Ns 
where Is is a current value through the shield coil and Ns is the total 
number of windings thereof. 
When driven by the same power source, both the main and shield coils are 
electrically connected in series, and it is desirable that the currents Ic 
equal Is. Yoda's proposal is based on a specific restriction that Ic=Is 
and there is no remaining current which cannot be covered by the turns. 
This restriction offers a severe obstacle to designers. Thus, a more 
generalized coil arrangement design technique, which provides well-defined 
positions of windings of a coil without such specific restriction, has 
long been desired. 
The present inventor paid attention to the direction to determine the 
positions of windings of a coil, since in the case of a Z channel, for 
example, it is a fact that the influence of eddy currents (caused by flux 
leakage) on MR image quality is less at the axial center portion of the 
coil than at the axial end portions thereof. Therefore, the present 
invention provides a technique, which gives priority to the axial end 
portions, by which the positions of windings are sequentially and turn by 
turn determined from each axially outermost winding position toward the 
axial center with the current step value for the main coil (or, a current 
step value for the shield coil, determined on the determination of the 
number of windings of the shield coil). 
When representing the analytically-requested total current amount for 
shield coil by the current Is and total winding number Ns determined in 
the foregoing conventional manner, representing the total winding number 
Ns40 of the shield coil determined on the present invention, and 
representing a remaining current amount by dNI, 
EQU Is.times.Ns=Ic.times.Ns'+dNI 
is established. 
Determining the positions of windings from each axial end position toward 
the axial center will cause the remaining current dNI to be collected to 
and concentrate in the axial center portion. In the Z channel, flows of 
the remaining current collected from each axial side are opposite to each 
other at the axial center portion, resulting in less magnetic field 
components in charge of the remaining current. 
When a specific condition is realized which the remaining current dNI=O[A], 
the conventional Yoda's proposal is identical to the present invention in 
finally-determined winding positions. However, as described above, the 
remaining current amount dNI does not become zero in most cases. 
Therefore, using the generalized manner of the present invention makes it 
possible to determine the positions of windings for the shield coil 
connected to the main coil in series, regardless of a situation the 
remaining current amount dNI is equal to zero or not. Coil designers thus 
gain increased degrees of freedom in design. 
On the above first aspect, there are provided features of the present 
invention as follows. 
There is provided a system for acquiring magnetic resonance data having a 
coil segment formed by winding a conductor on a bobbin representing an 
axial direction, a magnetic field being generated by supplying current 
into the conductor. In the system, winding positions at which the 
conductor is wound turn by turn on the bobbin is determined in agreement 
with a specified current step value and sequentially from a winding 
positioned at an outermost end of the coil segment in the axial direction. 
Preferably, the system further comprises an actively shielded gradient coil 
(ASGC) incorporating therein a Z coil having a main and shield coils and 
generating a gradient in a Z-direction defined by an XYZ-coordinate set 
for the system, and the coil segment being installed in the shield coil. 
It is also preferred that the shield coil having a coil segment group 
consisting of two of the coil segments wound around the bobbin in series 
electrical connection and series spatial arrangement states, the winding 
positions are sequentially determined from each winding positioned at each 
of outermost both ends of the coil segment in the axial direction toward a 
center of the coil segment in the axial direction. As an example, the 
specified current step value equals a value of current being supplied into 
the main coil. 
Concerning coil winding, the second aspect of the present invention will be 
described below, which can compensate the foregoing first aspect or can 
solely be practiced. 
For designing and manufacturing an actual coil unit, the coil unit must 
have a size enabling the coil unit to lie in a limited space. Moreover, 
there are manufacturing-related restrictions. Although a single-layer 
winding is not always necessary, it is therefore preferable that each 
channel of the coil unit is formed by winding a wire in a layer. (A return 
wire of each channel is routed through a space between channels, and the 
one-layered wound state is ensured for even one channel.) 
In a magnetic-field generation coil, assuming that a wire having a certain 
width is used as a coil element, when a continuous desired function of 
current densities is changed to discontinuous or discrete positions of 
windings, a distribution of magnetic fields is, as mentioned above, 
deviated from an ideal distribution of magnetic fields. Moreover, the gap 
between windings gets larger. This is because a turn at each of discrete 
windings conducts a desired current I. The present inventor has noted this 
fact. 
A major structure of a solving means proposed by the present inventor is 
such that one turn (winding) for conducting a desired current I is divided 
into a plurality, n, of turns. The winding or turn for shunting the 
current I into n windings shall be referred to as a "fractional turn." The 
fractional turn is used in the following form. In one use form, one turn 
(winding) that has a wider gap relative to an adjoining one than the other 
turns and that conducts a desired current I is branched into n windings 
each conducting a current I/n (n is an integer equal to or larger than 2), 
whereby a distribution of magnetic fields is approached to a desired 
distribution, and the gap of the winding relative to an adjoining one is 
narrowed. Otherwise, the fractional turn may be added to a gap between 
windings that is wider than the other gaps. Incidentally, a current 
flowing into a plurality of windings constituting a fractional turn may, 
for example, be a current 2I/3 or I/3, or in other words, may be 
differentiated according to a desired distribution of magnetic fields. 
Thus, a current need not always be distributed into the plurality of turns 
by an equal amount. 
An exemplary structure satisfying the foregoing basic purport in accordance 
with the present invention will be described below. 
According to the second aspect of the present invention, there are provided 
various features as follows. 
A shunt element for shunting the current carried turn by turn by winding 
the conductor into a plurality of current flows though a plurality of 
shunt paths is arranged in a coil segment in relation to a pattern of 
turns of the coil segment. 
As an example, the plurality of shunt paths are two in number and the two 
shunt paths are branched at a position in the middle of the conductor and 
joined together at another position in the middle of the conductor. 
Preferably, the shunt element is inserted into the conductor as an 
alternative to part of the conductor wound in agreement with the pattern 
of turns. For example, the conductor of the coil segment is wound on the 
same plane of one layer. The coil segment is one of a plurality of coil 
segments constituting a saddle type coil. 
As another example, the shunt element is additionally attached to the coil 
segment as part of the conductor and wound in agreement with the pattern 
of turns. In this case, for example, the coil segment is formed into a 
solenoid type coil. 
It is preferred that the system comprises a gradient coil unit 
incorporating the coil segment therein. Preferably, the gradient coil unit 
comprises X, Y, and Z coil assemblies generating gradients in X-, Y-, and 
Z-directions respectively, and at least one of the coil assemblies 
includes the coil segment. For example, the gradient coil unit is an 
actively shielded gradient coil (ASGC) and the at least one coil assembly 
comprises a main coil generating a gradient and a shield coil generating a 
magnetic field for shielding the gradient. Preferably, the at least one 
coil assembly is the Z coil assembly having the main and shield coils and 
at least one of the main and shield coils has the coil segment. 
In this case, it is preferred that the shield coil has the coil segment. 
Also, preferably, each of the main and shield coils are formed into a 
solenoid type coil made up of two solenoid-type coil segments arranged in 
series on the bobbin and the shunt element is attached to the shield coil 
at an axial position of at least one of axial end portions and an axial 
center portion of the shield coil. Preferably, the shunt element consists 
of two shunt paths branched at a position in the middle of the conductor 
and joined together at another position in the middle of the conductor. 
As another feature, there is provided a configuration that, for example, 
the two shunt paths are wound so as to produce two shunt current flows 
therethrough in the same turn direction in accord with the pattern of 
turns. In contrast, there is provided another configuration that the two 
shunt paths are wound so as to produce two shunt current flows 
therethrough in mutually-opposite directions in accord with the pattern of 
turns.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Magnetic-field generation coils in accordance with the embodiments of the 
present invention will be described below. 
First Embodiment 
The first embodiment of the present invention will now be described with 
reference to FIGS. 1 to 16. 
In the first embodiment, a coil unit forming Z channel (Z coil) of an 
actively shielded gradient coil (ASGC) unit for an MRl system will be 
exemplified as a magnetic-field generation coil unit. The present 
invention can be adapted to an X channel (X coil) and Y channel (Y coil) 
of the ASGC unit. Moreover, the present invention can preferably be 
adapted to a static coil and shim coils. 
A magnetic-field generation coil unit of the present invention can be 
implemented irrespective of the shape of a coil and the method of 
generating magnetic fields; that is, can be implemented in a cylindrical 
coil, an opposed type coil, a surface type coil, or an open type coil. 
FIG. 1 schematically shows a longitudinal section of a gantry 1 of an MRl 
system. The gantry 1 is shaped like a cylinder as a whole. A central bore 
of the gantry 1 serves as a diagnostic space. For diagnosis, a subject P 
can be inserted in the bore. 
The gantry 1 comprises a substantially-cylindrical static coil unit 11, a 
substantially-cylindrical gradient coil unit 12 located on the bore side 
of the coil unit 11, a shim coil unit 13 mounted on, for example, the 
outer circumference of the unit 12, and an radio-frequency coil 14 located 
in the bore inside the gradient coil unit 12. The subject P is asked to 
lie down on the couchtop of a patient couch that is not shown, and 
inserted into the bore (diagnostic space) defined by the radio-frequency 
coil 14. 
The static coil unit 11 is made of a superconducting magnet. That is to 
say, a plurality of heat radiation-shielded containers and a single liquid 
helium container are stowed in an outer vacuum container. A 
superconducting coil is wound and placed in the liquid helium container. 
The gradient coil unit 12 is actively shielded. The coil unit 12 has a coil 
assembly for forming each of the X, Y, and Z channels so as to generate 
gradient pulses in each of the X-axis, Y-axis, and Z-axis directions. The 
coil assemblies have a shielded structure allowing only very little 
magnetic field gradients to leak out along each channel. 
The MRl system, as shown in FIG. 1, comprises a static power supply 51 
supplying power to the static coil unit 11, a gradient power supply 52 
supplying current to the gradient coil unit 12, a transmitter/receiver 53 
not only transmitting an RF signal to the RF coil 14 but also receiving an 
MR signal from the RF coil 14, and a sequencer 54 controlling operation of 
the gradient power supply and transmitter/receiver on the basis of a given 
sequence. The system further comprises a controller 55 controlling the 
whole system and a unit 56 for reconstructing MR images from the MR 
signal, in addition to a monitor 57, memory 58, and input device 59. 
The actively shielded gradient coil (ASGC) unit 12 has, as shown in FIG. 2, 
an X coil assembly 12X, Y coil assembly 12Y, and Z coil assembly 12Z, 
which form the X, Y, and Z channels respectively, isolated layer by layer 
and laminated, and is shaped substantially like a cylinder as a whole. The 
X coil assembly 12X, Y coil assembly 12Y, and Z coil assembly 12Z each 
include a main coil and a shield coil. Each of the main and shield coils 
is provided with a plurality of coil segments. This enables each coil 
assembly to not only generate magnetic fields changing in each axial 
direction but also achieve a shielding structure that does not allow most 
magnetic field gradients to leak out. 
To begin with, the Z coil assembly 12Z forming the Z channel will be 
described in conjunction with FIGS. 3 and 4. The Z coil assembly 12Z 
includes two bobbins B1 and B2 arranged as coaxial cylinders and having 
different inner diameters, and a main coil 12ZM having main coil segments 
12Z-1 and 12Z-2, and a shield coil 12ZS having shield coil segments 12Z-3 
and 12Z-4 which are paired and created by winding a wire in a layer. The 
assembly of the shield coil 12ZS is placed on the bobbin whose diameter is 
larger than the bobbin on which the main coil 12ZM is placed. The shield 
coil is covering the outer circumferences of the main coil. The paired 
main coil segments 12Z-1 and 12Z-2 are created by winding a wire so that 
pulsating currents flow in mutually opposite directions. Another pair or 
the paired shield coil segments 12Z-3 and 12Z-4 are created by winding a 
wire so that pulsating currents flow in mutually opposite directions. As 
for a main coil and shield coil which are mutually opposed in a radial 
direction, currents flow in mutually opposite directions between one pair 
of the main coil segment 12Z-1 and shield coil segment 12Z-3, and currents 
flow in mutually opposite directions between another pair of the main coil 
segment 12Z-2 and shield coil segment 12Z-4. 
FIG. 5 shows an electrically equivalent circuit of the whole Z coil 
assembly 12Z. Two main coil segments 12Z-1 and 12Z-2 and two shield coil 
segments 12Z-3 and 12Z-4 are connected in series respectively. A desired 
current I is supplied from a single gradient power supply 17 used to 
generate magnetic field gradients at the same time. A waveform shaper 18 
is connected to the power supply 17. The waveform shaper 18 receives 
waveform data concerning magnetic field gradients to be generated by the Z 
channel, which reflects a command issued from a sequencer that is not 
shown, and outputs a waveform control signal proportional to the data to 
the power supply 17. With the waveform control signal, the power supply 17 
outputs a pulsating current I, which is used by the Z channel for 
generating magnetic field gradients intended by the sequencer, to the 
group of coil segments connected in series. 
The positions of windings forming the main coil 12ZM in the Z-axis 
direction on the bobbin B1 are determined according to a known technique. 
A wire is wound exactly at the positions in the form of a solenoid 
(however, the winding method of the present invention that will be 
described later can be adopted for the main coil segments 12Z-1 and 
12Z-2). 
By contrast, the positions of the windings forming the shield coil 12ZS are 
determined in this embodiment as mentioned below. 
To begin with, the spatially linear characteristic Bz relevant to magnetic 
field gradients that change in the Z-axis direction is determined 
analytically as shown in FIG. 6. The relationship of an ideal function of 
current densities [A/m] to positions in the Z-axis direction, which 
satisfies the characteristic and applies to the shield coil segments 12Z-3 
and 12Z-4, is obtained by carrying out known computation. An example of 
the ideal function of current densities is shown in FIG. 7. 
Next, the ideal function of current densities is integrated relative to a 
specific range of the positions in the Z-axis direction, whereby an ideal 
streamline function is obtained. An example of an obtained ideal 
streamline function is shown in FIG. 8(a). 
Next, the obtained ideal streamlined function is used to determine 
positions of windings forming the shield coil segments 12Z-3 and 12Z-4. In 
other words, on this stage, an analytically-obtained continuous 
distribution of currents is replaced with a discontinuous distribution of 
currents externalized by physical entities that are windings (changed into 
a discrete distribution). 
The positions of the windings of the shield coil segments 12Z-3 and 12Z-4 
is determined in sequence from each outermost position in the axial 
direction of the bobbin B2 to its axial center with a current step value I 
(=Ic=Is), where Ic is a value of current passing the main coil segments 
12Z-1 and 12Z-2 (their total number of windings is Nc) and Is is a value 
of current passing the shield coil segments 12Z-3 and 12Z-4 (with total 
number of windings is Ns). 
First, at both the axial end portions, specific position -Z1 and Z1 of the 
outermost two windings are given on the bobbin B2. Each of the outermost 
windings is wound, as shown in FIG. 8(b), by one turn at each of the 
outermost positions -Z1 and Z1, thus raising stepwise the actual 
(discrete) streamline function by the current step value I, as shown in 
FIG. 8(b). Any positions may be selected as the specific positions -Z1 and 
Z1 in consideration with the curves of a given ideal streamline function 
and a stepwise actual streamline function to be formed by the positions 
-Z1 and Z1 and the current step value I; for example, the positions -Z1 
and Z1 are selected so that both the curves of the actual and ideal 
streamline functions crosses with each other at a point of I/2. 
In response to this initial determination, for example, the winding 
positions -Z2, -Z3, and -Z4 and Z2, Z3 and Z4 corresponding to 3I/2, 5I/2 
and 7I/2 on the longitudinal axis expressing NI are sequentially 
determined toward the axial center of the bobbin. 
This winding method by which determination of the winding positions first 
starts at the axial outermost ones and then goes sequentially to the axial 
center makes it possible to give priority to winding positions in the 
axial end portions, where ideal winding positions can be set according to 
curvature in the foot range of an ideal streamline function curve. Thus, 
leakage of magnetic flux from the axial end portions of the shied coil is 
reduced, contributing to prevention MR image quality from being 
deteriorated. Remaining currents which cannot be covered by all the 
windings are brought to the axial center range and flow in mutually 
opposite ways. As a result, magnetic fields in charge of the remaining 
currents are reduced and leakage of magnetic flux in the axial center are 
also lowered. Compared with the conventional coil arrangement design 
(Ic=Is), the leakage of magnetic field will be improved by 1/2.6 in this 
embodiment. 
Still, the winding method according to this embodiment excludes the 
necessity of adjustment of current of the shield coil by connecting a 
correction resister to the shield coil in parallel. When such a correction 
resister is used, the shield coil has changed resistance due to increase 
in the temperature of the shield coil, causing deviation from a desired 
shunt state. This will also lead to changes in shield states, resulting in 
a rise in eddy currents due to magnetic flux leakage. Therefore, it was 
required to use such correction method employing correction resisters. By 
contrast, the foregoing winding method of the embodiment enable one to 
exclude such temperature-affected unstable factors and gain a stable 
shielding state. 
Now, the results of simulation to which the coil winding method of the 
present embodiment has been applied will be described. 
FIGS. 9 and 10 show curves of ideal streamline functions (integrated 
functions of current densities) exhibited by a Z channel of an ASGC unit 
which result from simulation performed by the present inventor. The curve 
shown in FIG. 9 is the curve of a streamline function exhibited by a main 
coil, and the curve shown in FIG. 10 is the curve of a streamline function 
exhibited by a shield coil. Both the curves are obtained theoretically 
using the computation devised by J. W. Carlson et al. Specifically, when 
streamline functions are plotted as smooth curves assumed ideal values, 
the generation of magnetic field gradients by the Z channel and the 
self-shielding performance of the Z channel become ideal (come to desired 
states). 
Next, a desired current I (=Ic=Is) is set to 99.345 A, and the positions of 
windings forming the main coil and shield coil constituting a Z channel 
are determined according to the technique proposed by Yoda. The positions 
are shown in columns (a) and (b) of FIG. 11. The positions of windings in 
the drawings are positions on the positive side in the Z-axis direction. 
Actually, a wire is wound at symmetric positions on the negative side. The 
plus and minus signs in the figure show the directions of windings 
(current flows) opposite to each other on the circumferential surface of 
the bobbin. 
The less than preferable result is shown in FIG. 12. FIG. 12 shows the 
results of simulation performed using coils designed according to the 
positions of windings listed in (a) and (b) of FIG. 11, and shows a 
streamline function of eddy currents induced by the Z coil assembly. The 
simulation can be carried out using a finite element method or boundary 
element method. Eddy currents induced by magnetic leakage occurring at 
positions within a radius of 0.5 m are calculated. As apparent from FIG. 
12, the streamline function of eddy currents rises sharply and broadly at 
both edges in the Z-axis direction and in the center therein. This is 
attributable to the fact that a magnetic flux passes through a wide gap 
between windings of the shield coil. The inner radius of the bore of the 
static coil unit is usually about 0.5 m. The eddy currents induced at both 
edges in the Z-axis direction and in the center therein according to the 
graph shown in FIG. 12 are induced at positions opposed to the conductor 
of the static coil unit and at surrounding positions. The eddy currents 
invite, as mentioned above, deterioration of the qualities of MR images, 
though the coil unit is an ASGC unit. 
By contrast, the columns (a) and (b) of FIG. 13 show the simulated 
positions of windings of a main and shield coils to which the winding 
method of the embodiment is applied, by which determination of their 
positions is carried out first at each axial outermost position and then 
goes to its axial center. By using the positions and the same drive 
conditions as the above comparative example shown in FIGS. 11 and 12, the 
actual streamline functions of the main and shield coils are shown by 
FIGS. 14 and 15 which closely approximate to the ideal streamline 
functions shown in FIGS. 9 and 10. 
In simulation based on this improved winding method, a streamline function 
of eddy currents caused by magnetic leakage has been resulted in FIG. 16, 
which should be comparative to FIG. 12. As understood from comparing both 
the curves, FIG. 16 shows noticeably decreased levels of eddy currents 
over the entire range 
Second Embodiment 
A magnetic-field generation coil unit in accordance with the second 
embodiment of the present invention will be described in conjunction with 
FIGS. 17 to 22. In the magnetic-field generation coil unit in this 
embodiment, like that in the first embodiment, the present invention is 
implemented in a shield coil 12Z having shield coil segments 12Z-3 and 
12Z-4 included in a cylindrical z coil assembly that is one of coil 
assemblies constituting an ASGC unit. Herein, components identical or 
similar to those in the first embodiment will be assigned the same 
reference numerals. The description of the components will be omitted or 
briefed. 
The second embodiment is to employ the same winding method as one described 
by the first embodiment and to make sure that the magnetic leakage is more 
deeply suppressed. In order to achieve this, a winding technique called 
"fractional turn" is introduced. 
For an ideal streamline function shown in (a) of FIG. 17, the positions of 
windings are sequentially determined, like the first embodiment, from each 
outermost position in the axial direction of the bobbin B2 to the center 
therein. Additionally, in this embodiment, for the windings forming the 
shield coil segments 12Z-3 and 12Z-4 which are located in the center and 
both edges in the Z-axis direction of the bobbin B2, a way of winding 
referred to as "fractional turn" whose concept has been introduced by the 
present inventor is adopted. 
The "fractional turn" is a way of winding based on an unprecedented novel 
concept, wherein a given current I [A] carried by one normal turn is 
shunted into I/N passing n local windings (where n is an integer equal to 
or larger than 2). The introduction of the "fractional turn" is intended 
to smoothen an actual streamline relationship and make it more closely 
approach an ideal streamline function. A fractional turn employed herein 
is "I/2 turn" formed in each end in the bobbin axial direction and in the 
center therein. 
To be more specific, as shown in (a) of FIG. 17, when a desired current is 
I (for example, 100 A), positions -Z1, -Z2, Z1 and Z2 of windings in the 
axial direction, at which the ideal streamline function curve and current 
values I/4 and 3I/4 on the longitudinal axis expressing NI (N is the total 
number of windings and I is coil current) in FIG. 17, are determined as 
the positions of the "fractional turn I/2" arranged in each axial end 
range. Then, winding positions -Z3, -Z4, -Z5, Z3, Z4, Z5 which correspond 
to 3I/2, 5I/2, 7I/2 on the longitudinal axis, which are increased stepwise 
by the current step value 1, are determined in turn. Then winding 
positions -Z6, -Z7 (about zero), Z6 and Z7 (about zero) corresponding to 
17I/4 and 19I/4 are determined for the fractional turns arranged in the 
axial center. 
First, fractional turns FTa and FTb at both edges will be described. 
Positions of windings -Z1, -Z2, Z2, and Z1, which are associated with 
currents I/4 and 3I/4 on the axis of ordinates and into which a current 
I/2 that is a half of the current I flows, (half current turns, that is, 
n=2) are determined. The fractional turn FTa at the left-hand edge in FIG. 
17 is created by bifurcating a lead extending from a power supply at the 
positions of windings -Z1 and -Z2, and winding the resultant windings F1 
and F2 by one turn. The windings F1 and F2 are then merged into one 
winding, and linked to a winding at the next position -Z3. The fractional 
turn FTb at the right-hand edge in the drawing is created by bifurcating a 
winding extending from a position Z3 at the positions of windings Z2 and 
Z1, and winding the resultant windings F3 and F4 by one turn. The windings 
F3 and F4 are then merged into one winding and linked to a lead extending 
from the power supply. 
Fractional turns FTc and FTd for conducting a current I/2 are created as 
illustrated in the center in the Z-axis direction of the bobbin. 
Specifically, positions -Z6 and Z6 associated with a current 17I/4 by the 
streamline functional curve are defined as the positions of ones of 
windings constituting the fractional turns FTc and FTd which conduct the 
current I/2. At the same time, the positions of the other windings 
constituting the fractional turns FTc and FTd which conduct the current 
I/2 (remaining turns) are determined at positions -Z7 and Z7 of nearly 
zero which are associated with a current 19I/4 by the streamline 
functional curve. 
The fractional turn FTc that is one of the fractional turns in the center 
of the bobbin are created by bifurcating a winding extending from a 
position -Z5 at a position -Z6 and a position -Z7 of a nearly zero on the 
negative side in the Z-axis direction, and winding the resultant windings 
F5 and F6 by one turn. The two turned shunt windings F5 and F6 are passed 
from the negative side in the Z-axis direction to the positive side 
therein in such a way that the windings will not cross each other. The two 
crossovers are linked to the other fractional turn FTd. The two crossovers 
are routed in a direction opposite to the direction in which the windings 
F5 and F6 are routed, and linked to the windings F8 and F7 at the position 
Z7 of nearly zero on the positive side in the Z-axis direction and the 
position Z6 on the positive side therein respectively. The two windings F8 
and F7 are then wound by one turn and then merged into one winding. The 
resultant winding is then linked to a winding at the position Z5. 
As a result, in the center in the Z-axis direction on the bobbin, the two 
adjoining windings F6 and F7 at the positions -Z7 and Z7 are regarded as 
substantially the same winding. Currents flowing into the two turns or 
windings are oriented in mutually opposite directions. Magnetic fields 
generated by the two windings F6 and F7 are canceled out. At both the 
center positions -Z6 and Z6, it can therefore be thought that a half 
current turn is realized substantially without the necessity of shunt. 
FIG. 18 shows a simplified and conceptual circuit of the whole Z coil 
assembly 12Z. Two main coil segments 12Z-1 and 12Z-2 and two shield coil 
segments 12Z-3 and 12Z-4 are connected in series respectively. A desired 
current I is supplied from a single power supply 17 used to generate 
magnetic field gradients at the same time. In the shield coil segments 
12Z-3 and 12Z-4, windings F1 to F8 are equivalent to the aforesaid 
windings constituting fractional turns (half current turns). A waveform 
shaper 18 is connected to the power supply 17. The waveform shaper 18 
receives waveform data concerning magnetic field gradients to be generated 
by the Z channel, which reflects a command issued from a sequencer that is 
not shown, and outputs a waveform control signal proportional to the data 
to the power supply 17. With the waveform control signal, the power supply 
outputs a pulsating current I, which is used by the Z channel for 
generating magnetic field gradients and intended by the sequencer, to the 
group of coil segments connected in series. The power supply 17 and the 
waveform shaper 18 are included in the gradient power supply 52. 
When the fractional turns FTa to FTd are thus added to the shield coil 
segment 12Z-3 and 12Z-4, although the shield coil segments are formed by 
winding a wire in a layer, a streamline function which is desired to be 
actually exhibited by the shield coil segments 12Z-3 and 12Z-4 can be 
caused to more closely approach an ideal streamline function. Moreover, 
the concentration of windings in the center in the Z-axis direction and at 
both edges therein can be raised. 
This will be detailed below. Conventionally, there is a problem in that 
gaps between windings are formed widely in the ends in the axial direction 
of the bobbin and in the center therein, since a wire member having a 
specified width is used. Assuming that, as shown in FIG. 19, a coil is 
created by winding a wire 100, which has a certain width, about a 
cylindrical bobbin 101, the actual width of the coil is determined with 
the width over windings of the area most crowded with windings (turns) 
(area in which windings are most dense). In other words, there is the 
restriction that a wire wider than the width over windings of an area A 
most crowded with windings cannot be used. Because of this restriction, 
when an ideal continuous distribution of currents is replaced with a 
discontinuous distribution of currents, a wide gap in which no wire exists 
is created between windings of a coil. 
By contrast, the addition of fractional turns makes it possible to more 
closely approach the curve plotting a streamline function to an ideal 
state. Consequently, the state of generating shielding magnetic fluxes 
better approaches an ideal state. As a result, the performance of 
shielding magnetic fields improves. 
In other words, a streamline functional curve that is smoothly streamlined 
relative to positions in the Z-axis direction can be obtained. The 
streamline function is closer to a continuous function. Therefore, in 
cooperation with the winding method described in the first embodiment, the 
qualities of MR images produced by the MRI system employing this ASGC unit 
can be improved outstandingly. The same is applied to MRS system. 
Also, an ASGC unit can be designed with a one-layered wound state retained, 
if desired. It will not take place that the ASGC unit gets thicker in a 
radial direction thereof and causes a whole gantry to become larger. The 
introduction of the new concept of the fractional turns makes it possible 
to still use, as one auxiliary countermeasure, ex post facto correction of 
magnetic fields affected by eddy currents, such as, optimization of a 
pulse sequence which has been attempted in the past, or control of the 
phase of a radio-frequency pulse. This is accompanied by the secondary 
merit that one can prevent control of a pulse sequence from becoming too 
complex. The new concept will prove effective for spatial high-order 
components of magnetic fields affected by eddy currents, which cannot be 
treated by such ex post facto correction. In other words, the additional 
use of fractional turns of the present invention makes it possible to 
suppress eddy currents themselves to a low level. Such high-order magnetic 
field components themselves can therefore be minimized remarkably. This 
obviates the need for subsequent correction. Moreover, the resistances and 
inductances of coils themselves hardly vary. Therefore, even when 
fractional turns are added, the current-carrying capacity of a power 
supply used to generate magnetic field gradients need not be increased. A 
conventional power supply can be used. 
Now, the qualitatively described effect of the present invention will be 
described more quantitatively by showing an example of the results of 
simulation. 
Columns (a) and (b) of FIG. 20 individually show simulated winding 
positions of a main coil and a shield coil of a Z coil assembly including 
the fractional turns described above. A desired current I is set to 
99.345[A]. The positions of windings in the drawing are positions on the 
positive side in the Z-axis direction. Actually, a wire is wound at 
symmetric positions on the negative side. Based on the winding positions, 
there is provided a streamline function curve of the shield coil shown in 
FIG. 21. Fractional turns for conducting a current 1/2 (where I equals to 
99.345 A) are added to two positions, that is, in the center on the 
positive side in the Z-axis direction (positions in the Z-axis direction 
of about zero meter and of 0.0400 m) and at edges (positions of 0.97500 m 
and 1.03500 m). As apparent from FIG. 21, the streamline function curve is 
more smoothened and closer to its ideal curve, compared with that in the 
first embodiment, in the center in the Z-axis direction, that is, a range 
of Z=-0.3 [m] to 0.3 [m] or thereabout, and at both edges in the Z-axis 
direction, that is, ranges Z=-0.7 [m] to -1 [m] and Z=0.7 [m] to 1 [m] or 
thereabout. As a result, a streamline function is plotted in FIG. 22 for 
eddy currents caused by magnetic leakage. Compared with the curve shown in 
the first embodiment, induction of eddy currents in the center in the 
Z-axis direction and at both edges therein within the radius of 0.5 [m] is 
suppressed markedly and evened over all the positions in the Z-axis 
direction. 
This means that the fractional turns work as expected. Thus, including such 
ASGC unit into an MRI system or MRS system enables one to increase the 
qualities of acquired MR data. 
In the Z coil assembly in the second embodiment, as described above, 
fractional turns are implemented in the center in the Z-axis direction and 
at both edges therein. Alternatively, the fractional turns may be 
implemented either in the center or at edges. The fractional turns may be 
added to the shield coils alone of the Z coil assembly. Alternatively, the 
fractional turns may be added to both the main coils and shield coils. 
Otherwise, the fractional turns may be added to the main coils alone. Even 
when the fractional turns are added to the main coils, a streamline 
function of currents flowing into the main coils can be smoothed. The 
characteristic relevant to magnetic field gradients can be improved. This 
eventually contributes to improvement of qualities of MR images or MR 
data. 
In the second embodiment, the technique of calculating a streamline 
function and determining positions of windings in relation to stepped 
currents using the function is adopted for determining the positions of 
windings forming coils. Alternatively, like in the prior art, a function 
of each current density may be integrated, and the position of a winding 
may be determined on the basis of the obtained integral of a function of 
current density. 
Third Embodiment 
A magnetic-field generation coil unit in accordance with the third 
embodiment of the present invention will be described in conjunction with 
FIGS. 23 to 26. In the magnetic-field generation coil unit in this 
embodiment, like that in the second embodiment, the present invention is 
implemented in a shield coil 12ZS having shield coil segments 12Z-3 and 
12Z-4 included in a cylindrical Z coil assembly that is one of coil 
assemblies constituting an ASGC unit. Herein, components identical or 
similar to those in the first embodiment will be assigned the same 
reference numerals. The description of the components will be omitted or 
briefed. 
The fractional turns described in the second embodiment have been 
configured such that shunt currents flow through a plurality shunt paths 
in the same circumferential direction. Hereinafter, such fractional turns 
are referred to as "non-reversed fractional turns". In contrast, 
fractional turns introduced herein are referred to as "reversed fractional 
turns", where shunt currents flow in mutually-opposite directions through 
each shunt path. The winding method may also be additionally applied to 
the shield coil in this embodiment. 
As shown in (b) and (c) of FIG. 23, reversed fractional turns FTi and FTj 
are formed at the axial outermost ends of shield coil segments 12Z-3 and 
12Z-4, respectively, and non-reversed fractional turns FTc and FTd are 
formed at the center thereof, respectively, in the same way as the second 
embodiment. In one shield coil segment 12Z-3, a lead extending from a 
power supply is branched into opposite-directionally wound windings F21 
and F22 at specific winding positions -Z1 and -Z2 on one end portion of 
the bobbin B2, and the windings F21 and F22 are wound around the bobbin by 
one turn, respectively. After the one turn, the windings F21 and F22 are 
merged into the next turn wound at a position -Z3. The windings F21 and 
F22 are set to have the same resistance value. Therefore, currents flowing 
through the windings F21 and F22 are the same value of I/2, but +I/2 
passes one winding and -I/2 passes through the remaining winding, 
producing two currents flowing in mutually-opposite circumferential 
directions. In the other shield coil segment, windings F23 and F24 make 
another reversed fractional turn FTj configured identically to the turn 
FTi. 
With the reversed fractional turns FTi and FTj, an actual streamline 
function is provided as represented in (a) of FIG. 23. Ampere turn NI is 
stepwise raised at the position of the winding F21 (F24) by I/2 and then 
stepwise lowered at the adjoining position of the winding F22 (F23). This 
ups and downs in an actual streamline function are advantageous to, in 
particular, a gentle foot curve of -an ideal streamline function, 
approximating the actual curve closer to the ideal one. Thus, there are 
obtained the identical operation and advantages to the second embodiment. 
The results of simulation carried out the inventor for a shield coil 
including non-reversed fractional turns (bobbin edges) and reversed 
fractional turns (bobbin center) which are described in FIG. 23 are shown. 
Specifically, columns (a) and (b) of FIG. 24 individually show simulated 
winding positions of a main coil and a shield coil of a Z coil assembly 
including the fractional turns described above. For the shield coil, as 
shown in (b) of FIG. 24 a non-reversed fractional turn is added at the 
center in the Z-axis plus direction (at Z=appr. zero and 0.0400 [m]), 
while a reversed fractional turn is added at the edge therein (at 
Z=0.97500 [m] and 1.03500 [m]). In the Z-axis minus direction the 
identical turns are symmetrically added, which are not shown. These 
fractional turns are made up of I/2 turns (I =99.345 [A]). The remaining 
turns shown in (b) in FIG. 24 are normally-wound non-fractional one turns. 
Column (a) of FIG. 24, which is the same as (a) of FIG. 20, is listed 
again for each comparison. A desired drive current I is 99.345 [A]. 
A streamline function curve of eddy currents is simulated based on the 
foregoing coil winding arrangement and is shown in FIG. 25. As apparent 
from the figure, the streamline function curve is more smooth and closer 
to its ideal curve, compared with that in the first embodiment, in the 
center in the Z-axis direction, that is, a range of Z=-0.3 [m] to 0.3 [m] 
or thereabout, and at both edges in the Z-axis direction, that is, ranges 
Z=-0.7 [m] to -1.5 [m] and Z=0.7 [m] to 1.5 [m] or thereabout. As a 
result, a streamline function is plotted in FIG. 26 for eddy currents 
caused by magnetic leakage. Compared with the curve shown in the first 
embodiment, induction of eddy currents in the center in the Z-axis 
direction and at both edges therein within the radius of 0.5 [m] is 
suppressed markedly and evened over all the positions in the Z-axis 
direction. 
This means that, like the second embodiment, the non-reversed and reversed 
fractional turns work as expected. Thus, including such ASGC unit into an 
MRI system or MRA system enables one to increase the qualities of acquired 
MR data. 
Fourth Embodiment 
A magnetic-field generation coil unit in accordance with fourth embodiment 
of the present invention will be described in conjunction with FIG. 27. In 
the magnetic-field generation coil unit in this embodiment, like that in 
the other exemplary embodiments, the present invention is implemented in a 
shield coil 12Z having shield coil segments 12Z-3 and 12Z-4 included in a 
cylindrical Z coil assembly that is one of coil assemblies constituting an 
ASGC unit. 
The shield coil in this embodiment employs only one reversed fractional 
turn FTk arranged in the axial center range of the bobbin B2, in addition 
to non-reversed fractional turns arranged in both the axial end ranges 
thereof. The winding method according to the first embodiment, by which 
the positions of windings are sequentially determined from each of the 
axial outermost positions of the bobbin toward the axial center thereof, 
is also applied to the shield coil of this embodiment (The same will be 
applied to the following embodiments). 
The reversed fractional turn FTk is made up of two windings F25 and F26 
wound around the bobbin B2 at axially-centered winding positions -Z6 and 
Z6 thereof. A lead extending from a non-fractional winding around at a 
position -Z5 branches at the position -Z6 into the winding F25 (wound at 
the position -Z6) and the winding F26 (wound at the position Z6) of which 
circumferential winding directions are opposite to each other on the 
bobbin. Each of the two windings wound by one turn is merged at the 
position Z6 into a non-fractional adjoining winding wound at a position 
Z5. This winding structure makes it possible for shunt currents flowing 
through the two winding F25 and F26 to achieve I/2 current, respectively, 
and mutually-opposite flowing directions, when both the windings have the 
same value of impedance. 
By the present embodiment, in addition to identical or similar operation 
and advantages to those described by the above embodiments, there is 
provided an advantage that the number of fractional turns is reduced to 
one, for the reversed fractional turn is placed in the axial center range 
of the bobbin. This is equivalent to a situation that in the foregoing 
embodiments, shunt currents passing the windings F6 and F7 wound in the 
axial center of the bobbin are canceled out, while shunts currents passing 
windings F5 and F8 solely make a contribution to an actual streamline 
function curve. In this way, arranging one reversed fractional turn in the 
center range of a bobbin permits a winding structure to be more simplified 
and winding work to be easier. 
In the winding structure shown in FIG. 27, a crossed portion D, at which 
the two windings F25 and F26 cross each other, is constructed such that 
two half-thickness plate-like lead wires are laid on each other in an 
insulated state. This maintains a one-layered configuration of the shield 
coil. 
Additionally, when a reversed fractional turn is placed in the axial center 
of a bobbin, there is provided an alternative shown in FIG. 28 in which a 
reversed fractional turn FTk' is formed. In this turn, two windings 
forming the turn are bridged over between two winding positions -Z6 and Z6 
at a half-turn circumferential position on the bobbin. This also achieves 
an effective reversed fraction turn in a simplified winding configuration 
and the like. 
Fifth Embodiment 
A magnetic-field generation coil unit in accordance with the fifth 
embodiment of the present invention will be described in conjunction with 
FIGS. 29 to 31. In the magnetic-field generation coil unit in this 
embodiment, like that in the foregoing exemplary embodiments, the present 
invention is implemented in a shield coil having shield coil segments 
12Z-3 and 12Z-4 included in a cylindrical Z coil assembly that is one of 
coil assemblies constituting an ASGC unit. Herein, components identical or 
similar to those in the first embodiment will be assigned the same 
reference numerals. The description of the components will be omitted or 
more brief. 
The fifth embodiment attempts to exert the same operation and effect as the 
foregoing embodiments, and to set the resistances and/or inductances of 
turns (shunt paths) including fractional turns to the same values and 
achieve shunt of a current to an intended number of paths reliably. The 
points described below should be noted in creating fractional turns, 
though they are not referred to in the description of the magnetic-field 
generation coil unit of the foregoing embodiments. 
(1) For shunting a current equally to turns, the resistances of turns 
(shunt paths) forming a fractional turn must be equalized. In designing a 
static coil or shim coils into which a steady-state current flows, this 
condition (1) should preferably be satisfied as a top priority. 
(2) In designing a coil into which a pulsating current flows such as a coil 
for generating magnetic field gradients, the condition (1) must be 
satisfied, and the inductances of turns (shunt paths) constituting a 
fractional turn should preferably be set to an equal value. 
For satisfying the conditions (1) and (2), the arrangement of windings and 
the way of winding shown in (b) and (c) of FIG. 29 are adopted for the 
shield coil segments 12Z-3 and 12Z-4 of the Z coil assembly of this 
embodiment. As illustrated, the technique described in conjunction with 
FIG. 17 in relation to the second embodiment is adopted for the shield 
coil segments 12Z-3 and 12Z-4, and non-fractional, fractional turns are 
created in the center in the Z-axis direction of the bobbin and at both 
edges therein. (b) and (c) of FIGS. 29 shows the same-direction flanks A 
and B of the shield coil segments 12Z-3 and 12Z-4. 
To begin with, the fractional turns FTg and FTh in the center will be 
described. One of the fractional turns that is the fractional turn FTg is 
composed of windings F5 and F6, which are half current turns, located at 
positions -Z6 and -Z7 near the center on the negative side of the Z axis. 
The other fractional turn FTh is composed of windings F7 and F8, which are 
half current turns, located at positions Z7 and Z6 near the center on the 
positive side of the Z axis. 
The positions of windings constituting the fractional turns FTg and FTh are 
identical to those in the second embodiment. When the windings F5 and F6 
of the fractional turn FTg are linked to the other fractional turn FTh, 
ones of the adjoining windings to be folded back, that is, the windings F5 
and F6 (F7 and F8) are crossed with each other (See area B indicated with 
a dot-dash line). The equivalent circuit is shown in FIG. 30. The lengths 
of the shunt paths (that is, the windings F5 and F7, or the windings F6 
and F8) are set to an equal value. The resistances of the shunt paths are 
therefore set to the same value. 
The orientations of the currents I/2 flowing into the fractional turns FTg 
and FTh are held to be the same as those in the second embodiment. The 
ability to cancel out magnetic fluxes generated by the windings F6 and F7 
is guaranteed. Since the windings are crossed with each other, the 
orientations of currents (See arrows in FIG. 30) induced by interlinked 
magnetic fluxes and flowing into the two closed shunt loops S1 and S2 are 
the same. The magnetic fluxes generated by the closed loops S1 and S2 are 
therefore canceled out. The inductances of the shunt paths of the windings 
F6 and F8 or of the windings F5 and F7 become equal. In other words, the 
inductances of the windings F6 and F5 are equal to each other, and the 
inductances of the windings F7 and F8 are equal to each other. 
Among the windings forming the shield coil segments 12Z-3 and 12Z-4, a 
normal winding that is not shunted (into which a current I flows), which 
is, for example, marked with circle A indicated with a dot-dash line in 
(b) of FIG. 29, is formed with a wire whose vertical and lateral lengths 
is t and whose sectional area is t.sup.2 as shown in (a) of FIG. 31. Two 
shunt paths of the windings F5 and F7 or windings F6 and F8 constituting a 
fractional turn are, as shown in (b) or (c) of FIG. 31, formed with a wire 
having a vertical length or thickness of t/2 over the whole paths. The 
thickness of an intersection between the windings F5 (F7) and F6 (F8) is 
therefore confined to t. The whole shield coil segments 12Z-3 and 12Z-4 
may therefore be created by winding a wire in a layer. 
When the cross-sectional shape shown in (b) of FIG. 31 is adopted for the 
two shunt paths, although the thickness is t/2, since the width is 2t or 
twice as large as t, the cross-sectional area of each path is the same of 
the ordinary winding. The resistance per unit length across a fractional 
turn is a half that of the normal winding (an ampere turn, that is, a turn 
for conducting a current I). When the cross-sectional shape shown in (b) 
of FIG. 31 is adopted, the thickness of each of the shunt paths is t/2 and 
the width thereof is t as shown in FIG. 31c. That is a wire whose 
thickness is half of that of the normal winding is employed. The 
resistance per unit length across a fractional turn never the less equal 
to that of the normal winding (I ampere turn) because two wires are 
connected in parallel. 
The fractional turns FTe and FTf at both edges of the bobbin are half 
current turns, that is, turns for conducting the current I/2 located at 
the same positions as the second embodiment. In consideration of the 
conditions (1) and (2), the resistances and inductances of the shunt paths 
constituting the fractional turn FTe or FTf are equalized. For equalizing 
the resistances, the lengths of the shunt paths (windings) are made equal 
with each other. For equalizing the inductances, as shown in circle C 
indicated with a dot-dash line in (c) of FIG. 29, the windings F1 and F2 
(or F3 and F4) are crossed with each other. Thus, the ability to cancel 
out interlocked magnetic fluxes that induce currents, which is the same as 
the ability of the fractional turns in the center, is present. 
As mentioned above, in the fifth embodiment, not only the operation and 
effect of the second embodiment can be ensured but also the resistances 
and inductances of the windings (shunt paths) of a fractional turn are set 
to the same values. Thus, when a pulsating current (magnetic field) is 
handled in order to, for example, generate magnetic field gradients, the 
ability to shunt a current reliably can be ensured owing to the fractional 
turns. 
When the temporal variation of a pulsating current is moderate, the 
inductances of the windings of a fractional turn need not always be the 
same. In this case, for example, only the resistances of the windings may 
be made equal with one another. 
The fractional turns in this embodiment can be adapted not only to shield 
coils but also to main coils. The fractional turns can be adapted not only 
to the Z channel of a gradient coil unit but also the X or Y channel 
thereof. 
Only the resistances or inductances of the shunt paths constituting a 
fractional turn may be made equal with one another. Such fractional turns 
may be adapted to a static coil or shim coils. 
In the foregoing second to fourth embodiments, intersecting the windings 
forming a fractional turn has not been explained. It is, of course, 
preferable that the fractional turns in those embodiments adopt such 
winding crossing configuration, if needed. 
Still, the number of intersections formed in one fractional turn is not 
necessarily limited to one. The number can be determined with reference to 
a wide range of factors including magnetic flux generated from coils in 
other channels and can be developed into various modes. As an example, a 
mode is provided in which the number of intersections should be one for 
larger magnetic influence from other channels, while the number for larger 
magnetic influence from other channels and its complicated magnetic 
spatial distribution (for example, the number of intersections are three 
like shown by a fractional turn FTw in FIG. 45). On one hand, when such 
magnetic influence is comparatively fewer, an intersection is not always 
need. 
The structure of the fractional turns FTe and FTf implemented at the edges 
in the Z-axis direction of a bobbin in the fifth embodiment may be adapted 
to the fractional turns in the center thereof. An example is shown in (a) 
and (b) of FIG. 32. In this example, fractional turns FTe and FTf and 
fractional turns FTe' and FTf', which have the same structure, are adopted 
for the edges and center of the bobbin respectively. The resistances and 
inductances of the windings that are each a pair of shunt paths 
constituting each fractional turn are made equal with one another, whereby 
the ability to cancel out interlocked magnetic fluxes inducing currents 
can be better achieved. In particular, since all the fractional turns have 
the same structure, designing can be simplified. 
The aforesaid fractional turns are not limited to specific windings 
positioned in the center in the Z-axis direction of a bobbin and at both 
edges therein. If desired, any middle winding (turn) may be replaced with 
a fractional turn. Alternatively, a fractional turn in accordance with the 
present invention may be additionally interposed between windings. Even 
when a different shape of a coil unit or a different way of winding for 
creating a coil is adopted, parts fractional turn can be carried out at 
any position. 
Sixth Embodiment 
The sixth embodiment of the present invention will be described in 
conjunction with FIG. 33. A magnetic-field generation coil unit of this 
embodiment includes the same shield coil segments 12Z-3 and 12Z-4 as that 
of the fifth embodiment. In this embodiment, a non-reversed fractional 
turn is developed into three shunt paths. 
The parts (b) and (c) of FIG. 33 show models of the same-direction flanks A 
and B of a cylindrical bobbin B2, illustrating the positions of windings. 
The shield coil segments 12Z-3 and 12Z-4 are paired to constitute a 
shielding Z channel. At both edges in the Z-axis direction of the shield 
coil segments 12Z-3 and 12Z-4, and in the center thereof, "one-third 
current turns" FTm, FTn, FTo, and FTp, that is, turns for conducting a 
current I/3 are formed as non-reversed fractional turns. The positions of 
windings constituting the one-third current turns FTm to FTp are 
determined according to the method employed in the aforesaid embodiment. 
The lengths of the shunt paths (windings) constituting each of the 
one-third current turns FTm to FTp are made equal with one another and 
their resistances are set to the same value. Moreover, since the windings 
intersect one another, the inductances thereof are also made equal with 
one another. Owing to the one-third current turns, an actual distribution 
of magnetic fields can be smoothed more finely and caused to better 
approach an ideal distribution of magnetic fields. Moreover, the same 
opera ion and effect as the fifth embodiment can be realized for a 
pulsating current. 
Alternatively, when a coil unit has fractional turns at both the axial 
edges of its cylindrical bobbin and the axial center thereof, it is not 
always necessary that their fractional turns be the same in shunt paths. 
For example, a modification is provided that the number of shunt paths is 
two for each of the fractional turns wound at the axial edges and three 
for the fractional turn wound in the axial center. Various modifications 
can be achieved in relation to conditions in designing coils. 
Seventh Embodiment 
A magnetic-field generation coil unit of the seventh exemplary embodiment 
of the present invention will be described in conjunction with FIGS. 34 to 
37. A Y coil assembly 12Y forming the Y channel of the actively shielded 
gradient coil unit 12 shown in FIG. 2 will be described as the 
magnetic-field generation coil unit. 
The Y coil assembly 12Y including bobbins is, as shown in FIG. 34, shaped 
substantially like a cylinder as a whole. The coil unit includes a main 
coil 12YM and shield coil 12YS. The main coil 12YM are arranged 
symmetrically with respect to a ZX plane with an origin (X, Y, Z)=(0, 0, 
0) in the space as a center. The main coil includes two pairs of (four) 
semicylindrical bobbins B and two pairs of main coil segments 12YM-1 and 
12YM-2 placed on the bobbins B. Each pair of main coil segments 12YM-1 (or 
12YM-2) are formed with two coil segments CS that are patterned in the 
form of a saddle on the bobbins B and that are opposed to each other with 
the XZ plane between them. 
The shield coil 12YS is located around the outer circumferences of the main 
coil 12YM while separated from the main coil by a given length in a radial 
direction, and shaped like a cylinder covering the main coil 12YM 
entirely. The shield coil 12YS has the same structure as the main coil 
12YM, though they have a different diameter. Specifically, the shield coil 
12YS includes two pairs of semicylindrical bobbins B, which are juxtaposed 
in the Z-axis direction with the original (X, Y, Z)=(0, 0, 0) in the space 
between them, and two pairs of shield coil segments 12YS-1 and 12YS-2 
placed on the bobbins B. Each pair of shield coil segments 12YS-1 (or 
12YS-2) are formed with two coil segments CS that are patterned in the 
form of a saddle on the bobbins and that are opposed to each other with 
the XZ plane between them. 
The coil patterns on the coil segments CS of the main coil 12YM and shield 
coil 12YS are spiral patterns each of which looks like it contains 
multiple spiral turns in development. The coil patterns may be produced 
using known techniques. 
In this embodiment, fractional patterning of the present invention is 
performed on the coil segments CS included in the Y channel. The 
patterning is illustrated in the form of a model in FIG. 35. In FIG. 35, 
for a better understanding, one coil segment CS is shown as a saddle coil 
having only three spiral turns. Still, the positions of windings of the 
shield coil of Y channel in this embodiment are determined based on the 
method described by the first embodiment. In other words, for each coil 
segment CS, the winding positions are determined in the direction starting 
at each edge in the Z-axis direction and advancing toward the center 
portion of the coil segment, as shown by an arrows K1 or K2 in FIG. 34. 
In FIG. 35, the three turns of current paths shall be referred to as paths 
Cout, Cmid, and Cin orderly from the outer path. The outermost path Cout 
has a fractional turn FTout inserted partly in the middle thereof. The 
fractional turn FTout is composed of shunt paths F11 and F12 bifurcating 
from the current path Cout at a given position on the current path Cout. 
The shunt paths F11 and F12 are crossed at a given middle position 
(however, mutually isolated), and merged into the current path Cout at a 
given position on the current path Cout. 
The remaining part of the outer current path Cout other than the fractional 
turn FTout is formed with one wire having a thickness t (width t). The 
aspect ratio of the wire may be determined arbitrarily. By contrast, the 
shunt paths F11 and F12 of the fractional turn FTout are each formed with 
a wire having a thickness t/2 (width 2t or t). The lengths of the shunt 
paths F11 and F12 along the current path are set to the same value. The 
resistances of the shunt paths F11 and F12 are therefore equal to each 
other. Since the shunt paths F11 and F12 are crossed with each other, 
electromotive forces of the closed loops causing eddy currents can be 
canceled out. Consequently, the inductances of the shunt paths F11 and F12 
are equal to each other. The total thickness of the wires at the 
intersection is t (=t/2+t/2). 
The middle current path Cmid is formed with one wire (having a thickness t 
and width t) over the whole circumference thereof. 
The innermost current path Cin is formed with a fractional turn FTin over 
the whole circumference thereof. Specifically, the inner current path Cin 
(=FTin) is bifurcated into shunt paths F13 and F14 at the start point. The 
shunt paths F13 and F14 are crossed each other at a given position in the 
middle of the current path, and merged into the current path at the end 
point after making one turn. In the case of the fractional turn FTin 
formed over the whole circumference of the current path, the shunt paths 
F13 and F14 are formed by turning a wire having a thickness t/2 (width 2t 
or t) according to a given pattern from beginning to end. Even in this 
case, the resistances and inductances of the shunt paths F13 and F14 are 
equal to each other. The total thickness of the wires at the intersection 
is retained at t. 
A pulsating current I fed to a feed terminal of the outer current path Cout 
is bisected exactly into currents I/2 by the shunt paths F11 and F12, and 
merged into the current I. The current I flows along the middle current 
path Cmin and then enters the innermost current path Cin. In the current 
path Cin, the exactly bisected currents I/2 flow into the shunt paths F13 
and F14 from the beginning, makes a turn, and then flow into a subsequent 
coil segment through another feed terminal. FIG. 36 shows an equivalent 
circuit of the Y channel in which the coil segments CS of the main coil 
and those of the shield coil are connected in series respectively, and 
into which the pulsating current I flows. Fractional turns are not shown 
to be included in the coils of the shield coil 12YS in FIG. 36. However, 
the coil segments CS each include fractional turns FTout and FTin. 
The coil segments CS are placed on the bobbins shown in FIG. 34. When 
fractional turns are not employed, as shown in FIG. 37, a gap is, as 
mentioned previously, created between wires in the center in the Z-axis 
direction of a spiral coil segment and at edges thereof. Using the 
fractional turns of this embodiment, the gaps can be filled up. 
Consequently, the curve of a streamlined function exhibited by shield 
coils can be smoothed and approaches more closely to an ideal curve. 
Magnetic fields leaking out from the gaps can be shielded properly. This 
enables, as described previously, suppression of eddy currents. The 
original object of an ASGC can be achieved and image quality can be 
improved. Moreover, the compactness of a coil unit in a radial direction 
will not be impaired while a one-layered wound state is retained. 
Furthermore, the resistances and inductances of shunt paths constituting a 
fractional turn are equal to each other. Even a pulsating current can be 
shunted exactly. A high-precision shielding ability can be provided 
reliably. 
In the foregoing embodiment, both the resistances and inductances of paths 
constituting a fractional turn are made equal with each other. For a 
structural reason or the like, either of the resistances and inductances 
may be made equal with each other. Fractional turns may also be performed 
on the main coil of the Y channel or may be performed only on the main 
coil. 
The number of fractional turns FTout and FTin and the positions thereof (to 
which turn the fractional turns are inserted) can be determined 
arbitrarily. As far as an actual streamline function can be smoothed to 
approach to an ideal streamline function as closely as possible, the 
number of fractional turns and the positions thereof may be determined in 
consideration of actual physical conditions. In the above description, the 
outer fractional turn FTout is inserted partly in the middle of the 
current path Cout. Alternatively, like the inner fractional turn FTin, the 
outer fractional turn FTout may be formed along the whole length thereof. 
Furthermore, the fractional turns may be implemented in the X coil assembly 
12X for producing magnetic field gradients that change in the X-axis 
direction. The X coil assembly 12X is equivalent to a coil assembly made 
by turning the Y coil assembly 12Y by 90.degree. about the Z axis. The 
fractional turns can be adapted in appropriate forms to the main coil of 
the X coil assembly or/and the shield coil thereof. 
Still, in the winding structure exemplified by FIG. 35, the position of the 
crossed point of windings formed in each of the fractional turns FTout and 
FTin is changeable in the whole length of each turn, not necessarily fixed 
at the center position therein. In the case that, for example, magnetic 
flux generated from another channel intersects the loop of shunt paths in 
a way that magnetic imbalance occurs therein, the position of the crossed 
point of windings in a fractional turn can be adjusted to exclude effects 
caused by the magnetic imbalance. This adjustment is particularly 
preferable to a coil unit used for generating an asymmetric gradient. 
Still further, although the foregoing inner fractional turn FTin employs a 
non-reversed fractional turn, this fractional turn may be replaced by a 
reversed fractional turn, which is exemplified in FIG. 38. 
Eighth Embodiment 
The eighth embodiment of the present invention will be described in 
conjunction with FIG. 39. In this embodiment, like the seventh embodiment, 
the present invention is implemented in the shield coil segment pairs 
12YS-1 and 12YS-2 of the Y coil assembly 12Y of the actively shielded 
gradient coil unit 12. Each non-reversed fractional turn is developed into 
a one-third current turn, that is, a turn for conducting a current I/3. 
Each shielding coil segment CS (shield coil) of the Y coil assembly 12Y 
has, as shown in FIG. 39, three turns of current paths Cout, Cmid, and 
Cin. Among the current paths, the current path Cout turning on the outside 
has a fractional turn FTout, which is a one-third current turn for 
branching the current path into three paths and conducting the current 
I/3, formed partly in the middle thereof. The inner current path Cin has a 
fractional turn FTin, which is a one-third current turn for conducting the 
current I/3, formed along the whole length thereof. 
Three branch wires (shunt paths) of the outer fractional turn FTout each 
have the same width t as one unbranched wire but have a thickness t/3 (See 
sectional explanatory insets (a) and (b) of FIG. 39). The cross-sectional 
area of one branch wire is one-third of that of the unbranched wire. Since 
the three wires are connected in parallel, the three wires have the same 
resistance as an ampere turn that is a turn for conducting the current I. 
The lengths of the three branch wires of the outer fractional turn FTout 
are equal to one another. The three branch wires are intersected at a 
given middle position. The resistances and inductances of the shunt paths 
are therefore equal to one another. Not only a steady-state current but 
also a pulsating current can be shunted exactly by the one-third current 
turn. At an intersection, since the thickness of each wire is set to t/3, 
a one-layered wound state can be maintained for the whole of a shield coil 
included in the Y coil assembly 12Y even at the intersection of the wires. 
The compactness of the Y coil assembly in the radial direction can be 
maintained. Incidentally, the thickness of each of the three branch wires 
is not limited to t/3. When the thickness of one layer of a shield coil 
can be increased, the branch wires may be made thicker (the thicknesses 
may be set to a larger absolute value). 
By the way, the three branch wires (shunt paths) of the inner fractional 
turn FTin are intersected at a given middle position. The inner fractional 
turn FTin thus serves as a one-third current turn. However, as shown in 
circle C drawn with a dashed line in FIG. 39, the branch wires are 
intersected one another at three positions by crossing twos of the branch 
wires. Even when the thickness of each branch wire is set to t/2, the 
one-layered wound state can be retained. The resistances and inductances 
of the shunt paths constituting the inner fractional turn FTin are set to 
equal values. Consequently, a shunt ability can be provided reliably. 
Ninth Embodiment 
The ninth embodiment of the present invention will be described in 
conjunction with FIG. 40. This embodiment results from further 
exploitation of the eighth embodiment, wherein a one-n-th current turn, 
that is, a turn for conducting a current I/n (where n is an integer larger 
than 3) is implemented in the aforesaid shield coil 12YS. 
Each shielding coil segment CS (shield coil) of the Y coil assembly 12Y is, 
as shown in FIG. 40, composed of three current paths Cout, Cmid, and Cin, 
for example. Among the current paths, the current path Cout turning on the 
outside has a fractional turn FTout, which is a one-n-th current turn for 
branching the current path into n paths, formed partly in the middle 
thereof. The inner current path Cin has a fractional turn FTin, which is a 
one-n-th current turn, formed along the whole length thereof. 
Each of the fractional turns FTout and FTin is composed of n branch wires 
(shunt paths) and has the structural characteristics of the aforesaid 
one-third current turn. Since the thickness of one branch wire is set to, 
for example, t/n, the one-layered wound state is retained for the whole of 
the shield coil even at the intersection of the n branch wires. 
Consequently, the operation and effect equivalent to the aforesaid ones can 
be provided. Moreover, since the number of branches is increased, a 
stepwise streamlined function exhibited by the coil structure can be 
smoothed more precisely and approach more closely to a continuous ideal 
streamlined function. 
Tenth Embodiment 
A magnetic-field generation coil unit of the tenth embodiment will be 
described in conjunction with FIGS. 41 and 42. As the magnetic-field 
generation coil unit, a shim coil 8 (for correcting a static magnetic 
field) for producing magnetic fields directed in the Z-axis direction (Z 
channel) will be exemplified. 
FIG. 41 shows a model indicating the positions of windings forming a shim 
coil 13Z for producing magnetic fields directed in the Z-axis direction. 
As illustrated, fractional turns FT.sub.D-1 and FT.sub.D-2 in accordance 
with the present invention are formed in the center in the Z-axis 
direction. Specifically, the fractional turn FT.sub.D-1 on the left side 
in the Z-axis direction in (b) of FIG. 41 is composed of shunt windings 
F12-1 and F13-1 located at positions -Za and -Zb in the Z-axis direction 
which are associated with currents 13I/4 and 15I/4 by a streamline 
function. The fractional turn FT.sub.D-2 on the right side in the Z-axis 
direction is composed of shunt windings F13-2 and F12-2 located at 
positions Zb and Za in the Z-axis direction which are associated with 
currents 17I/4 and 19I/4 by the streamline function. A left-hand winding 
F11-1 associated with a current 5I/2 associated with a current 5I/2 by the 
streamline function is linked to one fractional turn FT.sub.D-1. The 
fractional turn FT.sub.D-1 is linked to the other fractional turn 
FT.sub.D-2 by way of a crossover running in the Z-axis direction. The 
fractional turn FT.sub.D-2 reaches a right-hand winding F11-2 associated 
with a current 11I/2 by the streamline function. The lengths of the shunt 
windings F12 and F13 of the fractional turn FT.sub.D-1 or FT.sub.D-2 
should preferably be set to the same value, and the resistances thereof 
should preferably be set to the same value. 
By thus replacing part of turns with fractional turns FT.sub.D, as shown in 
(a) of FIG. 41, the transition of the streamline function NI from a point 
13I/4[A.multidot.T] to a point 19I/4[A.multidot.T] becomes smoother than 
that of a function exhibited by coils having known windings. The 
streamline function approaches more closely to an ideal (desired) 
streamline function. When a state of windings shown in (b) of FIG. 42 
(identical to the state shown in (b) of FIG. 41 except that normal turns 
for conducting the current I are substituted for the fractional turns 
FT.sub.D-1 and FT.sub.D-2) is expressed using a streamline function as 
shown in (a) of FIG. 42 for comparison, it will be apparent that the 
streamline function is improved with addition of the fractional turns. 
Consequently, the performance of the shim coil 13Z, which is an integrant 
part of the Z channel, for homogenizing a static magnetic field can be 
improved. 
A given number of fractional turns may be formed at any positions in the 
Z-axis direction. Moreover, the fractional turns can be added irrespective 
of the order of a shim coil unit, that is, the number of inhomogeneities 
to be shimmed by the shim coil unit, and can be adapted to any of channels 
XY, X.sup.2 -Y.sup.2, Z.sup.3, Z, X, Y, ZXY, Z(X.sup.2 -Y.sup.2), and so 
on. 
Various embodiments have been described by exemplifying various coils 
employed in a horizontal magnetic field system in which a static magnetic 
field is generated in a horizontal direction. The present invention is not 
limited by the system of generating the static magnetic field. The present 
invention can be implemented in various coils (static coil, shim coils, 
gradient coils, and radio-frequency coil) employed in, for example, a 
vertical magnetic field system in which the static magnetic field is, as 
shown in FIGS. 43 and 44, generated in a vertical direction. 
FIG. 43 shows the outline structure of a Z coil assembly 30Z of an ASGC 
unit employed in the vertical magnetic field system, and FIG. 44 shows the 
outline structure of a Y coil assembly 30Y thereof. In FIG. 43, there are 
shown a main coil 30ZM and a shield coil 30ZS. In FIG. 44, there are shown 
a main coil 30YM and a shield coil 30YS. Fractional turns FT.sub.v similar 
to the aforesaid fractional turns are implemente in at least part of the 
coil segments of the main coils and/or shield coils, whereby the same 
operation and effect as the aforesaid ones can be exerted. 
In the case of a method of creating a coil by corroding a copper plate, 
such as, by performing etching, a portion of a conductor having a larger 
width is created. In general, therefore, as far as an MRI system of an EPI 
type which requires fast switching is concerned, eddy currents induced in 
a conductor cannot be ignored. Such a coil created by performing etching 
is therefore unacceptable. Even in this case, a distribution of currents 
can approach an ideal distribution by adopting the aforesaid method (for 
example, when a litz wire made by twisting thin wires is used as a wire, 
eddy currents induced in the wire itself due to fast switching or the like 
will pose no problem). In other words, even when a coil is created by 
performing etching, part of turns forming the coil can be regarded as a 
straight angle wire having a large width. A turn can therefore be divided 
into portions in the same manner as mentioned above. Specifically, for 
example, the width of a turn located at an edge can be narrowed and the 
number of turns forming a spiral can be reduced. For realizing crossings 
of windings according to the present invention in the course of creating a 
coil through etching, the copper in, for example, a circular portion drawn 
with a dot-dash line in FIG. 35 is corroded in advance, and the conductors 
are linked by two wires having a thickness t/2 in the same manner as shown 
in FIG. 35. Even when such a coil creating method as etching or milling is 
adopted, a coil pattern closer to an ideal pattern can be created by 
adopting fractional turns of the present invention. 
In the aforesaid embodiments and their variants, fractional turns have been 
described as turns for conducting a current I/n (n is equal to or larger 
than 2). However, the present invention is not limited to this form. 
Assuming, for example, that the current is divided into two portions, the 
width of one turn is made twice as large as that of the other turn (the 
thickness is the same), and the ratio of the sectional areas S of the 
wires (or conductors) of the windings are set to 2:1 (the ratio of 
resistances is 1:2). Thus, although the number of turn divisions, n, is 2, 
the currents flowing through the turns are 2I/3 and I/3. Namely, 
fractional turns one of which carries a larger amount of current can be 
created. This leads to higher freedom in designing a coil using fractional 
turns. A desired distribution of magnetic fields can be attained readily. 
Still, in the foregoing embodiments and their variations, the gradient coil 
unit is formed into an integrated type of unit, as shown in FIGS. 1 to 3, 
in which the main and shield coils are integrated as one unit. Application 
of the present invention, however, is not limited to such integrated type 
of gradient coil unit. For example, the present invention can suitably 
applied to a gradient coil unit formed such that a main and shield coils 
are physically separated to be installed at spatially separated positions 
set in a gantry. 
Still further, the coil units having the fractional turn, which have been 
described in the second to tenth embodiments and their variations, may 
form based on conventional winding methods by which the winding positions 
are sequentially determined from the center in the Z-axis direction of a 
bobbin to each edge therein for a Z-channel gradient coil assembly, for 
example, instead of employing the winding method described in the first 
embodiment. 
As described so far, for winding a conductor around a bobbin to coil 
segment, the winding positions on the bobbin may be sequentially 
determined from the outermost end position in the axial direction of the 
bobbin toward the axial center therein with a given current step value 
such as main coil drive current. Further, a magnetic-field generation coil 
unit may include coil segments on which a conductor makes turns, and 
generates magnetic fields when a current is fed to the conductor. Shunt 
conductors for shunting a current, which is carried by the conductor, into 
a plurality of paths as a non-reversed or reversed fractional turn are 
included in the coil segments in relation to the pattern of turns of the 
conductor. A structure in which windings (wire) are distributed 
discontinuously can be maintained. Moreover, a distribution of magnetic 
fields to be generated actually can approach a desired continuous 
distribution, which is defined analytically, as closely as possible. 
Consequently, magnetic fields that are very precisely distributed 
spatially can be generated. 
A power supply having a current-carrying capacity of a level accepted at 
present can be employed without the necessity of increasing the 
current-carrying capacity of the power supply for feeding a current to a 
coil unit. Under the conditions of the power supply, a structure in which 
windings (wire) are arranged in a layer and distributed discontinuously 
can be maintained, and a distribution of magnetic fields to be generated 
actually approach a desired continuous distribution, which is defined 
analytically, as closely as possible. 
Furthermore, when a coil unit is provided with a shielding ability, the 
shielding ability can be fulfilled by suppressing magnetic leakage and 
magnetostriction, and unwanted eddy currents induced in surrounding metals 
can be minimized. A distribution of magnetic fields to be generated 
actually approaches to a desired distribution of magnetic fields, and 
induction of unwanted eddy currents in surrounding metals is suppressed. 
Thereby, ex post facto data correction for eliminating adverse effects of 
the eddy currents may still be carried out as an auxiliary countermeasure. 
When the coil unit is adapted, for example, to a gradient coil unit, static 
coil unit, shim coil unit, or radio-frequency coil unit for, for example, 
an MRI system or MRS system, high-precision and high-quality MR images or 
MR data can be produced owing to such direct effects as nullified 
irregular sensitivity, improved linearity, and improved homogeneities.