MRI system with open access to patient image volume

An MRI system using a pair of opposed magnetic poles to create the requisite static magnetic field H.sub.o is configured so as to provide open and unobstructed patient access areas communicating directly with an MRI imaging volume along directions which are perpendicular to the patient transport axis. In this manner, operator/doctor access to the patient is maximized while potential claustrophobic reactions from the patient are minimized.

This invention relates generally to magnetic resonance imaging (MRI) 
systems. In particular, it relates to the spatial arrangement of the 
(static and gradient) magnetic field generating components of an MRI 
system so as to maximize open and unobstructed patient access areas 
communicating directly with an MRI imaging volume along directions 
perpendicular to the patient transport axis. 
The art of magnetic resonance imaging (MRI) is now well developed and 
several different types of MRI systems are commercially available. In all 
of them, some means is provided to produce a very strong static magnetic 
field H.sub.o and controlled spatial gradients therein (e.g., along three 
mutually orthogonal coordinate axes). The static magnetic field is 
typically of an approximately homogenous nature within a predefined 
imaging volume and the controlled gradients are typically approximately 
linear with respect to spatial displacements there-within. 
A programmed sequence of radio frequency pulses is transmitted into body 
portions located within the imaging volume at predetermined frequency 
distributions so as to selectively nutate the magnetic moment of certain 
atoms by predetermined amounts in accordance with well-known nuclear 
magnetic resonance (NMR) principles. After cessation of such transmitted 
RF pulses, the NMR nutated atoms tend to relax back toward alignment with 
the static magnetic field H.sub.o and, in the process, produce 
characteristic NMR RF signals. Such RF signals are received, detected and 
processed to thereafter produce a desired MRI image of the body portion 
located within the imaging area in accordance with any one of many known 
MRI techniques as will be appreciated by those in the art. The transmitted 
RF pulses typically are synchronized with a special sequence of current 
pulses passed through various magnetic gradient coils during the imaging 
process so as to effect spatial information encoding processes and/or to 
provide known types of NMR phasing control. 
In some MRI apparatus, the static magnetic field H.sub.o and/or the 
magnetic gradient coils are realized in the form of large solenoidal coils 
or, in the case of gradient coils, saddle-shaped coils conformed to a 
generally tubular configuration. In such cases, it is naturally necessary 
for patient access to the imaging volume to be provided only along a 
narrow tunnel through the tubular shaped apparatus. With some patients, 
this may give rise to claustrophobic reactions. It also makes it extremely 
cumbersome to access the image volume (e.g., so as to adjust the relative 
positioning of RF transmit and/or receive coils or to attend to patient 
needs). 
Other types of MRI systems utilize a pair of magnetic poles (e.g., 
permanent magnets or electromagnetic magnets with ferromagnetic or air 
cores) disposed on opposite sides of the image volume to create the 
requisite static magnetic field H.sub.o. In the past, either necessary 
magnetic circuits for return flux (i.e., outside the image volume) between 
the magnetic poles and/or the magnetic gradient coils (e.g., in a tubular 
form rather than flat) or decorative cover systems have been constructed 
so as to also limit access to the image volume except along a generally 
tunnel-shaped area through which the patient is transported into the image 
volume. Thus, as with the solenoidal field generating devices, access to 
the image volume has been essentially limited to only one or two open and 
unobstructed patient access ports or areas--i.e., the ends of the patient 
transport tunnel aligned with the patient transport axis. 
Some typical examples of the latter type of prior art MRI system structures 
are depicted in FIGS. 1-3. In each case, access to the image volume 10 is 
limited to either a single port (FIG. 2) or a pair of aligned ports (FIGS. 
1 and 3) along the patient transport axis 12. In every case, there is no 
open and unobstructed patient access path communicating directly with the 
image volume 10 in a direction perpendicular to the patient transport axis 
12. Instead, any such potential access is blocked either by return 
magnetic flux circuit structure 14 and/or by a gradient coil, (FIG. 1) 
and/or by housing structure 16 (FIGS. 2 and 3). As those in the art will 
appreciate, behind the housing structures 16 are typically further 
obstructions to access in directions perpendicular to the patient 
transport axis 12 (e.g., magnetic flux return circuits and/or magnetic 
gradient coil structures or the like). 
I have now discovered an improved magnetic resonance imaging apparatus 
wherein the static field magnet and gradient coil and decorative cover 
structures are configured so as to leave an open and obstructed patient 
access area communicating directly with the image volume along a direction 
perpendicular to the patient transport axis. In the preferred exemplary 
embodiment, such transverse access to the imaging volume may be had from 
two opposite sides of the patient transport mechanism while in yet another 
exemplary embodiment, such transverse access to the imaging volume passes 
virtually through the top of the MRI system. In such exemplary 
embodiments, magnetic flux return circuits are preferably in the form of 
cylindrical columns (e.g., four of them) disposed radially outwardly of 
the magnetic poles. In this manner, transverse unobstructed access to the 
imaging volume is provided not only along the patient transport axis but 
also through at least one additional transverse port provided between such 
columnar return flux circuit structures. Although the four column static 
magnet construction is a standard available vendor design, I have taken 
unique advantage of such an open static magnet structure by coordinating 
gradient coil and housing structures so as to maintain such "openness" in 
the final completed MRI structure. That is, no obstructing housings or 
other structures are used to obstruct such transverse access paths. 
Accordingly, in the preferred exemplary embodiment of this invention, a 
magnetic resonance imaging apparatus includes a main static magnetic field 
structure for producing the requisite static magnetic field H.sub.o within 
a predetermined patient imaging volume through which a patient transport 
is arranged along a predetermined Z axis. Magnetic gradient coils 
associated with the main static field structure are provided for effecting 
controlled gradients in the static magnetic field H.sub.o along mutually 
orthogonal x,y,z axes within the patient imaging volume. However, the main 
static field structure and the gradient coils are configured so as to 
leave an open and unobstructed patient access area/path communicating 
directly with the imaging volume along a direction that is perpendicular 
to the patient transport z-axis. 
Indeed, in the preferred exemplary embodiment, permanent magnet poles are 
disposed with horizontal poles planes below and above the imaging volume 
so as to produce a vertically oriented H.sub.o field--while magnetic flux 
return paths are provided through four vertical columns which also support 
the upper permanent magnet structure above the imaging volume. In this 
manner, patient transport access is provided along the z-axis into and out 
of the imaging volume while a pair of opposing, open and unobstructed 
patient access areas/paths also communicate directly and transversely with 
the imaging volume along directions perpendicular to the patient transport 
z-axis. 
Stated somewhat differently, in the exemplary embodiment, there are at 
least three (and preferably at least four) open and unobstructed patient 
access areas communicating directly with the imaging volume along 
respective directions which are all perpendicular to the static H.sub.o 
field--and at least one (preferably two) of which is(are) also 
perpendicular to the patient transport axis z.

As will be appreciated throughout the following discussion, the 
denomination of specific axes as being x,y, or z axes is purely a matter 
of convention adopted to facilitate description of relative directions and 
dimensions in one exemplary embodiment. Other definitions may 
alternatively be used for descriptive purposes. 
As depicted in FIG. 4, the imaging volume 50 is sandwiched between upper 
and lower magnetic field producing assemblies 100. Within the imaging 
volume 50 (e.g., a 30 cm diameter spherical volume), cooperating permanent 
magnets 102 create a substantially homogenous static magnetic field 
H.sub.o (e.g., 650 Gauss.+-.100 ppm with a gap width between magnet poles 
of approximately 60 cm and a magnet pole diameter of approximately 1300 
mm). Conventional "Rose" shims 104 may be used to help insure sufficient 
field uniformity within the imaging volume 50. 
The permanent magnets 102 and Rose shims 104 are of conventional design 
using conventional ferrite materials with iron or steel pole tips. For 
example, suitable such permanent magnets (with shims) are available from 
Sumitomo Special Metals Company Ltd., Osaka, Japan. 
An exploded view of the assembly 100 comprising permanent magnet 102, Rose 
shim 104 and gradient coils 106 is shown in FIG. 5. The gradient coil 
structures may also be of conventional design. In general, the y-gradient 
coil comprises circular loops while the x and z-gradient coils comprise 
respectively orthogonal sets of back-to-back D-shaped coils arranged so 
that current in the parallel straight conductor segments within each set 
passes in the same direction. In the exemplary embodiment, the individual 
coil windings are formed from approximately 0.1 inch square copper wire 
with the active straight portion of the turns being spaced apart from one 
another by approximately one conductor width. 
In the exemplary embodiment, the gradient coils 106 are sandwiched together 
as closely as possible (with suitable allowances for insulating 
fiberglass/epoxy tape and potting materials) and fitted within the annular 
Rose shim 104. Thus, the gradient coils 106 are substantially parallel to 
the face of permanent magnet 102 and the composite magnetic field 
producing structure 100 is an essentially "flat" or "pancake" type of 
structure. 
As those in the art will appreciate, the gradient coils 106 as well as 
suitable RF coils are connected to suitable electrical driving, RF 
transmitting and RF receiving circuits (not shown) so as to complete an 
NMR imaging system. 
As those in the art will also appreciate, suitable magnetic circuits 200 
must be provided for return flux between the permanent magnets 102 located 
outside the imaging volume 50. In the preferred exemplary embodiment, such 
return paths are located so as to leave open access along opposing patient 
transport ports 300 and 302 while simultaneously also leaving transverse 
open patient access ports 400 and 402 communicating directly and without 
obstruction to the imaging volume 50 along directions perpendicular to the 
patient transport axis-z. 
In the exemplary embodiment of FIGS. 6-7, the magnetic flux return circuit 
200 comprises flux conductive (e.g., iron or steel) members 200a, 200b, 
200c, 200d, 200e, and 200f). As will be appreciated, the vertical 
cylindrical columns 200a-200d suffice to concentrate most of the return 
flux between upper and lower members 200e and 200f outside the imaging 
volume 50 (which is disposed between the magnetic field producing 
structures 100 near the center of the apparatus). A suitable patient 
transport structure 500 may be utilized for transporting a patient into 
and out of the imaging volume through open access ports 300 and 302 
parallel to the z-axis (as in the conventional systems of FIGS. 1-3). 
However, in addition, the embodiment of FIGS. 6-7 provides transverse open 
access ports 400 and 402 directly into the imaging volume. 
Accordingly, as a patient passes into the imaging volume, an essentially 
open and substantially unobstructed feeling is encountered so as to 
suppress or minimize possible claustrophobic reactions. Furthermore, 
doctors, technicians, nurses and/or MRI system operating personnel have 
ready access to the imaging volume for the purpose of attending to patient 
needs and/or adjusting RF coils 600, 601 with respect to the patient 
and/or imaging volume. 
Although some sort of RF coil structures 600, 601 will also have to be 
present in the vicinity of the patient imaging volume, such structures may 
take the form of surface coils or may include suitable access ports 602, 
604 (or maybe made so as to be at least partially transparent in selected 
areas) or may be of very narrow dimensions in the z-axis direction. 
Although the basic structure of the magnets 102 and return flux structures 
200a-200f may be conventional in and of themselves (e.g., as available 
from Sumitomo Special Metals Company, Ltd.), use heretofore with MRI 
systems has failed to utilize the potential for transverse open access 
areas as in the embodiment of FIGS. 6-7. Instead, prior approaches have 
used additional magnetic flux return circuit components and/or other types 
of magnetic gradient coil structures and/or other types of external 
housings so as to effectively obstruct access to the imaging volume 50 
except along a narrow patient transport tunnel. 
Another exemplary embodiment is depicted in FIGS. 8-9. It is essentially 
similar to that of FIGS. 6-7 except that the structure is rotated by 
90.degree. so that the static magnetic field H.sub.o is oriented in a 
horizontal direction rather than vertically. And, as a further consequence 
of such rotation, the transverse open access port 400 is also now disposed 
vertically with respect to the patient transport axis-z. In some 
circumstances, this orientation of an open access port may be preferred 
since the patient will typically view an unobstructed area as he/she 
passes face up into the imaging volume along the patient transport axis-z. 
Although body coil structures 600 and 601 are depicted for illustration 
purposes in FIGS. 6-9, other types of RF coil structures (e.g., head 
coils, surface coils, etc.) may be used in addition or alternatively 
depending upon the MRI imaging procedure to be employed. Furthermore, such 
RF coil structures may be constructed so as to have narrow z-axis 
dimensions and/or to be partially transparent and/or to have limited 
access ports therethrough if desired. Even if no such access ports are 
provided, a substantially more open presentation is made to the patient 
(thus minimizing possible claustrophobic reactions) and substantially 
greater access is provided (e.g., so as to adjust the RF coils if for no 
other purpose) to operating personnel. 
Although only a few exemplary embodiments of this invention have been 
described in detail, those skilled in the art will recognize that many 
variations and modifications may be made in these exemplary embodiments 
while yet maintaining many of the novel features and advantages of this 
invention. Accordingly, all such variations and modifications are intended 
to be included within the scope of the appended claims.