Gradient magnetic field generator for MRI system

Disclosed is a gradient magnetic field generator for an MRI system including a bobbin, formed of a non-magnetic body, being shaped into an appoximate hollow cylinder, and gradient coils embedded in the bobbin for generating gradient magnetic fields with supplied currents. The bobbin is formed such that thickness of the axial central portion of the bobbin is thicker than that of the axial end portions for enhancing the rigidity on the axial central portion.

BACKGROUND OF THE INVENTION 
The present invention relates to a gradient magnetic field generator in an 
MRI (Magnetic Resonance Imaging) system using an NMR (Nuclear Magnetic 
Resonance) phenomenon, and more specifically, to the improvement in 
reducing noise caused by pulsed electromagnetic forces exerted on gradient 
coils. 
It is well known that a medical MRI system includes a bed device for 
putting a patient thereon, a magnet device for forming a static magnetic 
field, a transmitter-receiver for transmitting a high frequency wave for 
generating magnetic resonance to the patient and for receiving an MR 
signal from the patient to detect and amplify the MR signal, a gradient 
magnetic field generator for discriminating the position of the MR signal 
and a processor-controller for controlling the entire system and 
reconstructing the image. 
The gradient magnetic field generator has a gradient coil unit and a 
gradient magnetic field power supply. The gradient coil unit comprises 
three groups of coils commonly called as x-coils, y-coils and z-coils for 
forming gradient magnetic fields linearly changed in their intensities in 
respective coordinate directions, with supplied pulse currents from the 
gradient magnetic field power supply. 
One type of the gradient coil unit is formed by mechanically fixing a 
plurality of x-coils, y-coils and z-coils to be paired with respect to the 
Z-direction (i.e. the longitudinal direction of the unit), respectively, 
on the outer peripheral side of a bobbin made of a fiber reinforced 
composite material (hereinafter referred to as FRP) impregnated with 
non-magnetic resin. Also, another type of the gradient coil unit is formed 
by winding x-coils, y-coils and z-coils on the outer peripheral surface 
side of an internal cylinder made of FRP, surrounding the outside thereof 
by another cylinder made of FRP, and impregnating the gap between the two 
cylinders with resin into an integrated structure, thereby winding the 
x-coils, y-coils and z-coils in the state of embedding them within a 
bobbin. 
The gradient magnetic field power supply is to be a pulse current source 
for supplying a pulse current with a pulse of a several millseconds in its 
first and last transitions to x-coils, y-coils and z-coils through 
operation of a processor-controller. 
Such MRI system as mentioned above has been disadvantageous in that, as a 
pulse current is applied from the gradient magnetic field power supply to 
the x-coils, y-coils and z-coils in a static magnetic field, the coils 
(such as flat lead wires) receive pulsed eletromagnetic forces in various 
directions, which causes the deflection of the bobbin resulting in the 
generated noise. 
In order to reduce the noise, there has been proposed such a technique that 
a sound absorption material is wound on the outer peripheral side of the 
gradient coil unit, or is inserted between a superconductive magnet as a 
magnet device and a frame cover. However, in the coil attachment structure 
of the above mentioned gradient coil unit, the coil attachment is 
restricted so as not to increase the rigidity, and also the gradient coil 
unit itself is low in its rigiditiy. Therefore, the noise generating 
energy is left as being high, and also if being absorbed by a sound 
absorption material, sound components not to be absorbed and to be leaked 
are left. They are transferred to patients in a diagnostic dome as 
non-continuation sound of about 70 to 80 phons, thus still bringing 
inhospitable feeling on the patients. 
Another technique for reducing the noise is that prefabricated 
saddle-shaped x-coils and y-coils and ring-shaped z-coils are rigidly 
combined by using support elements to form a coil basket and the coil 
basket is supported via insulating elastic support elements on a hollow 
cylindrical body. However, forming the coil basket and attaching the coil 
basket to the hollow cylindrical body via the elastic support elements 
leads to increased size of the gradient coil unit in the radial direction 
and increasd assembly time. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
gradient magnetic field generator for an MRI system by which generation of 
noise caused by electromagnetic forces exerted on gradient coils can be 
reduced to hence relieve the inhospitable feeling of patients in a 
diagnostic dome. 
It is another object of the present invention to provide a gradient 
magnetic field generator for an MRI system in which only the form of a 
bobbin is changed to reduce noise, without the need of complicating the 
whole structure of a magnetic unit. 
These and other objects can be achieved according to the present invention, 
in one aspect by providing a gradient magnetic field generator for a 
magnetic resonance imaging system comprising a bobbin formed of a 
non-magnetic body and formed into a hollow cylindrical shape having inner 
and outer peripheral surfaces extending in an axial direction of the 
bobbin and having a certain thickness in a radial direction of the bobbin 
and gradient coils embedded in the bobbin for generating gradient magnetic 
fields with supplied currents, wherein the thickness of the bobbin is 
formed to be larger on an axial central portion than on axial end 
portions. 
Preferably, at least one of the inner and outer peripheral surfaces has a 
protruding portion protruded on the axial central portion. The protruding 
portion has the thickness which is continuously or steppedly increased as 
nearing from the axial end portions to the axial central portion. The 
gradient coils comprise x-coils, y-coils and z-coils corresponding to a 
coordinate system of X-, Y- and Z-axes when the axial direction of the 
bobbin is set to be the Z-direction. Further, the bobbin is impregnated 
with epoxy resin and the x-coils, y-coils and z-coils are placed at 
predetermined positions in the epoxy resin. 
In the bobbin thus constructed, three groups of gradient coils (a plurality 
of x-coils, y-coils and z-coils) form gradient magnetic fields in X, Y and 
Z-directions, respectively. When pulse currents are applied to the 
x-coils, y-coils and z-coils, respective lead wires of the coils are 
exerted with electromagnetic forces, to thereby cause the total moment in 
such a direction as bending the central portion of the bobbin. However, 
the central portion of the bobbin is thicker in its wall thickness and 
hence is high in rigidity, so that the deflection of the bobbin caused by 
the bending moment can be suppressed. The mechanical deflection of the 
bobbin is therefore certainly suppressed, resulting in the reduced noise. 
Furtheremore, it can be achieved only by changing the axial thickness of 
the bobbin, and consequently needs the only simple arrangement without 
complicating the whole construction of the magnet. Also, since the bobbin 
is thinner on the end portions less liable to suffer from defflection than 
on the central portion, the total thickness is minimized as required, 
thereby preventing the weight thereof from being increased uselessly. 
Preferably, the protruding portion is integratedly formed on the outer 
peripheral surface and is constituted of a plurality of continuous 
protruding stripes having heights being gradually increased as nearing 
from the axial end portions to the axial central portion. A plurality of 
the protruding stripes are shaped into a waveform in cross section taken 
along the radial direction of the bobbin. Further, the gradient coils 
comprise x-coils, y-coils, and z-coils. Further, the protruding direction 
of the protruding stripes is specified to be the Y-direction. 
As a result, it can be achieved to reduce noise as mentioned above, to 
enhance heat radiating effect of the bobbin, and to save the material 
thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the exemplary embodiments will be described. 
FIRST EMBODIMENT 
The first embodiment will be described with respect to FIGS. 1 to 6. 
Referring now to FIG. 1, a magnet device 1 for an MRI system includes a 
cylindrical dome 10 made of a non-magnetic body. A superconductive magnet 
12 for forming a static magnetic field is disposed on the outer peripheral 
surface side of the dome 10 through metal fittings 11. Also, a gradient 
coil unit 14 being approximately cylindrical in its whole shape is mounted 
on the inner peripheral surface of the dome 10 through metal fittings 13. 
Moreover, on the inner peripheral surface of the dome 10, there is rigidly 
formed a whole body (WB) coil 15 for detecting a magnetic resonance signal 
of the object to be detected with fittings (not shown) in such a way as to 
surround the gradient coil unit 14. In this case, the dome 10 may be 
eliminated by directly attaching the gradient coil unit 14 on a frame 
located on the inner peripheral side of the superconductive magnet 12, and 
also attaching the WB coil 15 on the gradient coil unit 14 or on a frame 
of the superconductive magnet 12. 
As shown in FIGS. 2 and 3, the above gradient coil unit 14 made of a 
non-magnetic body includes a bobbin 20 as a spool being approximately 
cylindrical in its whole shape, and gradient coils wound in the state of 
being embedded within the bobbin 20, that is, x-coils 21x . . . 21x, 
y-coils 21y . . . 21y, and z-coils 21z and 21z. 
The bobbin 20 is formed into an approximate hollow cylinder shape having a 
wall whose thickness changes along its axial direction. The inner 
peripheral surface straightly extends and holds the same radius along the 
axial (z-axis) direction. Both of the axial end portions of the bobbin 20 
have a specified wall thickness. The outer peripheral surface of the 
bobbin 20 forms a protruding portion, which is integrated with a main body 
of the bobbin 20, by gradually and continuously increasing the wall 
thickness as nearing from both axial end portions to the axial central 
portion. The bobbin 20 is formed disposing the coils 21x . . . 21z at the 
specified positions within a preset mold, respectively, and allowing them 
to be impregnated with non-magnetic impregnating agent such as epoxy 
resin. 
Incidentally, the inclination of the outer peripheral surface of the bobbin 
20 is determined based on the result of the analysis for the magnitudes 
and directions of the electromagnetic forces exerted on the coils 21x . . 
. 21z located at respective position within the bobbin 20 when the coils 
21x . . . 21z are applied with pulse currents thereby forming gradient 
magnetic fields. 
The bobbin 20 in this embodiment is sticked with reinforced sheets made of 
a non-magnetic body such as CFRP (Carbon Fiber Reinforced Plastic) 22 and 
22 on the inner and outer peripheral surfaces, respectively. 
The gradient coils 21x . . . 21x, 21y . . . 21y, and 21z and 21z form three 
groups being paired by a plurality of coils in X, Y, and Z-directions, 
respectively. One group of coils 21x and 21x, 21y and 21y, and 21z are 
insulatedly embedded in an axial one-half side of the bobbin 20, while the 
other group of coils 21x and 21x, 21y and 21y, and 21z are insulatedly 
embedded in the axially opposed half side of the bobbin 20, as shown in 
FIGS. 2 and 3. These gradient coils 21x . . . 21z are connected with a 
gradient magnetic field power supply (not shown) constituting a part of a 
gradient magnetic field generator together with the gradient coils, 
respectively, and are supplied with pulse currents according to a 
controlled command from a central control unit (not shown). 
The function of the first embodiment will be described below. 
As pulse currents are supplied with specified sequences from the gradient 
magnetic field power supply to the gradient coils 21x . . . 21z, the 
adjacent coils in the axial (Z-axis) direction generate the magnetic fluxs 
reversed in phase to each other, and the adjacent coils in the X and 
Y-directions generate the magnetic fluxs identical in phase to each other, 
thereby forming gradient magnetic fields in X, Y and Z-directions. The 
gradient magnetic fields are superposed to a static magnetic field formed 
by the superconductive magnet 12, to form the total magnetic field for 
diagnosis. 
Referring to FIGS. 4 to 6, there will be described electromagnetic forces 
exerted to the coils 21x . . . 21z in formation of the gradient magnetic 
fields. In these figures, a reference numeral B shows the static magnetic 
field formed by the superconductive magnet 12. As a pulse current I is 
applied to lead wires of the coils 21x . . . 21z in the static magnetic 
field B, each lead wire receives electromagnetic force F (proportional to 
the magnitudes of the static magnetic field B and the current I) in the 
direction determined by the directions of the static magnetic field B and 
the current I. 
For example, in the case of the z-coils 21z and 21z, the electromagnetic 
force F is generated in such a direction as shown in FIG. 4, so that the 
z-coils 21z and 21z receive the forces deflected in the inner and outer 
peripheral directions, respectively- The x-coils 21x . . . 21x receive 
electromagnetic forces generated in such directions as shown in FIG. 5A. 
Namely, the lead wire pieces in the central portion receive the 
electromagnetic forces in the minus X-direction, respectively, while the 
lead wire pieces in the end portions receive the electromagnetic forces in 
the plus X-direction, respectively. 
Thus, as shown in FIG. 5B, the bending moment of the electromagnetic forces 
F . . . F is exerted to the x-coils 21x . . . 21x, as a result of which, 
assuming that both of the axial end portions are fixedly supported, the 
central portion of the bobbin 20 integratedly receives such a force as to 
deflect the bobbin 20 upwardly(in the minus X-directon), as shown by a 
two-dotted line in FIG. 5B. Furthermore, in the case of the y-coils 21y . 
. . 21y, as shown in FIG. 6, the electromagnetic forces are exerted 
thereto in the identical direction to that obtained by rotating the 
x-coils 21x . . . 21x by 90 degrees with respect to the z axis. 
Incidentally, it is confirmed that the electromagnetic forces (namely, 
noise) caused by the x-coils 21x . . . 21x are larger than those caused by 
the y-coils 21y . . . 21y and z-coils 21z and 21z. 
As mentioned above, the elctromagnetic forces F . . . F exerted to the all 
of coils 21x . . . 21z are complicatedly synthesized or independently 
effected, and are liable to cause the maximum deflection at the central 
portion of the bobbin 20. However, in this embodiment, the wall thickness 
of the bobbin 20 is gradually increased as nearing the central portion so 
as to enhance the rigidity of the central portion compared with the 
conventional one, so that the whole deflection of the bobbin 20 caused by 
the electromagnetic forces F . . . F can be certainly suppressed, thereby 
certainly reducing the intensity in noise. 
Therefore, differently from the conventional passive noise reducing measure 
wherein the generated noise is absorbed by means of a sound absorption 
material, the noise transferred to patients is remarkably suppressed 
thereby extremely reducing inhospitable feeling. Meanwhile, this 
embodiment is characterized by changing the wall thickness near the 
central portion of the bobbin 20 according to the condition of generating 
electromagnetic forces, and consequently the arrangement can be carried 
out without complicating the construction thereby unaffecting the change 
in design of the whole magnet device 1. In addition, it is unnecessary to 
use support elements used in a conventional technique, thereby decreasing 
the radial size of the gradient coil unit and decreasing assembly time 
compared with the conventional one. 
Furthermore, in consideration of the fact that both the end portions of the 
bobbin 20 receive a small deflection, the end portions are kept to be 
thinner. Thus, only the portions requiring larger rigidity are thickened, 
which excludes the disadvantage of incurring increase in weight and 
material cost. In addition, according to the form of bobbin 20 of this 
embodiment, there can be secured a space for a patient on the inner 
peripheral side. 
The primary object of this embodiment is to reduce noise; however, in 
another viewpoint, by enhancing the rigidity of the bobbin 20, 
flactuations in position of gradient coils 21x . . . 21z can be 
suppressed. Thus, there can be obtained the secondary effect of improving 
the resolution of an MRI diagnostic image. 
The above-mentioned bobbin 20 is so constructed that the outer peripheral 
surfaces is changed in inclination according to its axial position. 
However, as a result of structual and mechanical analysis, and also in 
consideration of manufacturing cost, there may be adopted various kind of 
the modifications as shown in FIGS. 7 to 12. 
In changing the wall thickness of the outer peripheral surface of the 
bobbin according to the axial position thereof, the following 
modifications may be proposed: a bobbin 35 provided with a recess 35a on 
the outer inclined peripheral surface at the axial central portion as 
shown in FIG. 7; a bobbin 36 having an axially level portion of the outer 
inclined peripheral surface at the axial central portion as shown in FIG. 
8; and a bobbin 37 provided with a steppedly protruding portion 37a at the 
axial central portion as shown in FIG. 9. 
Furthermore, a bobbin 38 shown in FIGS. 10A and 10B has such an object as 
to enhance the rigidity for electromagnetic forces caused by the x-coils 
21x . . . 21x. The bobbin 38 has protruding stripes 38a . . . 38a, each 
having a waveform radial cross section, which are integratedly formed 
partly on both sides in the Y-direction respectively. The wall thickness 
of the main body of the bobbin 38 is kept constant in the axial direction, 
in contrast, the radial thicknesses of the top portions of the protruding 
stripes 38a . . . 38a are increased as nearing the X-direction central 
portion of the bobbin 38. Accordingly, in the viewpoint of the whole 
bobbin, the rigidity of the central portion in the Z-direction is larger 
than that of axial end portions with respect to the X-direction. 
Therefore, the deflection of the bobbin 38 caused by the electromagnetic 
forces of x-coils 21x . . . 21x, which are usually the largest magnitude, 
can be certainly suppressed, thus remarkably reducing the total noise. 
Moreover, the bobbin 38 shown in FIGS. 10A and 10B has the outside surface 
area increased more than that of the simple cylindrical type by adding the 
protruding stripes 38a . . . 38a, and consequently it is excellent in heat 
radiation effect, and can be saved in its resin material compared with the 
cylindrical bobbin having the thickened central portion. 
A bobbin 39 shown in FIGS. 11A and 11B has also such a construction as to 
further correspond to the electromagnetic forces in the Y-direction. The 
bobbin 39 is formed to be increased in its wall thickness as nearing the 
central portion in the Z-direction and to be provided with slits 39a . . . 
39a for forming protruding stripes 39b . . . 39b with a specified wall 
thickness left axially formed on the outer peripheral surface every 
specified distance in the circumferential direction. With such protruding 
stripes 39b . . . 39b, it can be achieved to enhance the rigidity for the 
deflection caused by the electromagnetic forces in the X and Y-directions, 
and to obtain the same effect as in the above mentioned stripes with 
respect to the heat radiation and saving of the material. 
Against the above mentioned forms, the inner peripheral surface may be 
changed in inclination along its axial direction. For such modification, 
there may be adopted a bobbin 40 shown in FIG. 12, wherein the inclination 
of the inner peripheral surface is increased as nearing the axial central 
portion while the outer peripheral surface is not inclined along its axial 
direction. 
SECOND EMBODIMENT 
The second embodiment will be described with respect to FIGS. 13 and 14, 
wherein elements similar to those in the first embodiment are denoted by 
the same reference numerals, and the explanation thereof is simplified or 
omitted. 
A gradient coil unit 50 shown in FIGS. 13 and 14 includes an approximately 
cylindrical bobbin 51, and gradient coils 21x . . . 21z wound in the state 
of being embedded within the bobbin 51. The bobbin 51 has an outer 
peripheral surface formed in a continuously-thickened protruding portion 
having the top at the axial central portion similarly to the first 
embodiment, and also has an inner peripheral surface formed in a 
cone-shape (i.e. protruding portions) symmetric with the outer peripheral 
surface. Reinforced sheets 52 and 52 are sticked on the inner and outer 
peripheral surfaces, respectively. The other construction is similar to 
that in the first embodiment. 
According to the second embodiment, it is possible not only to obtain the 
same functional effect as in the first embodiment, but also to further 
enhance the rigidity near the central portion of the bobbin 51 which more 
certainly reduces the intensity in noise caused by the electromagnetic 
forces, thereby enhancing the noise reducing effect. 
Incidentally, in the above embodiments, the reinforced sheets respectively 
sticked on the inner and outer peripheral surfaces may be eliminated.