U.S. patent application number 09/681494 was filed with the patent office on 2002-11-07 for switchable field of view apparatus and method for magnetic resonance imaging.
Invention is credited to Boskamp, Ed B., Weyers, Daniel J..
Application Number | 20020165447 09/681494 |
Document ID | / |
Family ID | 24735505 |
Filed Date | 2002-11-07 |
United States Patent
Application |
20020165447 |
Kind Code |
A1 |
Boskamp, Ed B. ; et
al. |
November 7, 2002 |
Switchable field of view apparatus and method for magnetic
resonance imaging
Abstract
An MRI apparatus and method for reducing wrap around artifacts
during image reconstruction is provided. An RF coil and control
configuration includes a center coil and at least one pair of end
RF coils configured to encode and excite spins over an adjustable
field-of-view (FOV). The adjustable FOV has at least two size
designations in response to an FOV size request input. The control
connected to the RF coil assembly is capable of switching between
the FOV size designations by switching power to activate a center
coil alone or the center coil in conjunction with the end coils in
unison.
Inventors: |
Boskamp, Ed B.; (Menomonee
Falls, WI) ; Weyers, Daniel J.; (Wauwatosa,
WI) |
Correspondence
Address: |
COOK & FRANKE S.C. (GEMS)
660 EAST MASON STREET
MILWAUKEE
WI
53202
US
|
Family ID: |
24735505 |
Appl. No.: |
09/681494 |
Filed: |
April 17, 2001 |
Current U.S.
Class: |
600/422 |
Current CPC
Class: |
G01R 33/34046 20130101;
G01R 33/56545 20130101 |
Class at
Publication: |
600/422 |
International
Class: |
A61B 005/05 |
Claims
1. An RF coil and control configuration comprising: an RF coil
assembly having a center coil and a pair of end coils, each end
coil comprised of at least one RF coil, the RF coil assembly
configured to encode and excite spins over an adjustable FOV; and a
control connected to the RF coil assembly to switch the RF coil
assembly between at least two general FOV size designations based
on an FOV size input, the at least two general FOV size
designations determined by the control switching power to activate
one of: the center coil only; and the center coil and the pair of
end coils in unison.
2. The RF coil and control configuration of claim 1 wherein the at
least two general FOV size designations include a small FOV size in
which the center coil only is activated and a large FOV size in
which the center coil and the pair of end coils are activated.
3. The RF coil and control configuration of claim 1 wherein the RF
coil assembly transmits and receives signals to acquire a set of
data for the FOV size input.
4. The RF coil and control configuration of claim 1 wherein
acquisition of data in an axial imaging plane can be accomplished
with the center coil only.
5. The RF coil and control configuration of claim 1 further
comprising: a computer configured to generate control signals; an
unequal power splitter connected to the center coil and the
computer; an equal power splitter connected to the pair of end
coils and the unequal power splitter; and wherein the control
switching power to the center coil and the pair of end coils in
unison generates a magnetic field amplitude in the RF coil assembly
substantially equivalent to that of a single birdcage.
6. The RF coil and control configuration of claim 5 wherein the
center coil has a longitudinal length less than a standard
whole-body RF coil.
7. The RF coil and control configuration of claim 1 further
comprising: a computer configured to generate control signals; a
first amplifier connected to the center coil and the computer; a
second amplifier connected to the computer; an equal power splitter
connected to the pair of end coils and the second amplifier; and
wherein the control switching power to the center coil and the pair
of end coils in unison generates a uniform amplitude magnetic field
in the RF coil assembly.
8. The RF coil and control configuration of claim 1 wherein the
center coil and the pair of end coils partially overlap so as to
minimize a mutual inductance between the center coil and the pair
of end coils.
9. The RF coil and control configuration of claim 1 wherein one of
the center coil only and the pair of end coils only comprises a
birdcage resonator.
10. The RF coil and control configuration of claim 1 wherein the
center coil and the pair of end coils are each a birdcage
resonator.
11. An MRI apparatus to acquire images comprising: a magnetic
resonance imaging (MRI) system having a plurality of gradient coils
positioned about a bore of a magnet to impress a polarizing
magnetic field and an RF transceiver system and an RF switch
controlled by a pulse module to transmit RF signals to an RF
resonator assembly having a center resonator enclosed by a set of
end resonators to acquire MR images; and a computer programmed to:
receive an FOV input; and based on the FOV input, cause
transmission of a signal from one of: the center resonator only;
and the center resonator and the set of end resonators
simultaneously.
12. The MRI apparatus of claim 11 further comprising: an unequal
power splitter connected to the center resonator and the computer
such that the computer causes an output from only the center
resonator when the received FOV input is selected to cause
transmission from the center resonance only; and an equal power
splitter connected to the set of end resonators and the unequal
power splitter, such that the computer causes equal outputs from
the center resonator and the set of end resonators when the
received FOV input is selected to cause transmission from the
center resonator and the set of end resonators simultaneously.
13. The MRI apparatus of claim 11 further comprising: a first
amplifier connected to the center resonator and the computer; a
second amplifier connected to the computer; an equal power splitter
connected to the set of end resonators and the second amplifier;
and wherein the control switching power to the center resonator and
the set of end resonators simultaneously generates the equal output
from the center resonator and the set of end resonators.
14. The MRI apparatus of claim 11 wherein the center resonator and
the set of end resonators are each birdcage resonators.
15. The MRI apparatus of claim 11 wherein the center resonator
comprises a birdcage resonator having simultaneous electrical
sinusoidal and cosinusoidal waveform inputs upon energization of
the center resonator.
16. The MRI apparatus of claim 11 wherein the computer is further
programmed to acquire a set of data for image reconstruction
limited to an imaging volume defined by the FOV input.
17. The MRI apparatus of claim 11 wherein the center resonator and
the set of end resonators partially overlap so as to minimize
mutual inductance therebetween.
18. The MRI apparatus of claim 11 wherein an FOV input designating
a large FOV in a non-axial image acquisition causes activation of
the center resonator and the set of end resonators.
19. The MRI apparatus of claim 11 wherein an FOV input designating
a small FOV cause activation of the center resonator only.
20. A method of controlling an MRI device having an RF coil
assembly comprised of a central RF coil and at least one pair of
end RF coils comprising: creating a predetermined parameter based
on a length of the central RF coil; and comparing a desired FOV to
the predetermined parameter; and if the desired FOV is not greater
than the predetermined parameter, selecting the central RF coil for
energization so as to perform an image scan of the desired FOV; and
if the desired FOV is greater than the selected parameter,
selecting the central RF coil and the at least one pair of end RF
coils for energization so as to perform an imaging scan of the
desired FOV.
21. The method of claim 20 wherein the length of the central RF
coil is less than a standard MRI full-body RF coil.
22. The method of claim 20 wherein the central RF coil and the at
least one pair of end RF coils have approximately a 1 centimeter
overlap so as to minimize mutual inductance therebetween.
23. The method of claim 20 wherein the central RF coil and the at
least one pair of end RF coils transmit and receive signals if the
desired FOV has a longitudinal length greater than the length of
the central RF coil.
24. The method of claim 20 further comprising the steps of:
creating a linear uniform magnetic field in the desired FOV;
polarizing and exciting a set of spins in the desired FOV; and
encoding the set of spins.
25. The method of claim 20 wherein the central RF coil is a
birdcage coil.
26. The method of claim 20 wherein the desired FOV is one of a
whole-body FOV designation and a FOV designation less than the
predetermined parameter.
Description
BACKGROUND OF INVENTION
[0001] The present invention relates generally to magnetic
resonance imaging (MRI), and more particularly, to a switchable
field-of-view (FOV) apparatus and method to reduce the occurrences
of wrap around artifacts during image reconstruction.
[0002] When a substance such as human tissue is subjected to a
uniform magnetic field (polarizing field B.sub.0), the individual
magnetic moments of the spins in the tissue attempt to align with
this polarizing field along a Z-axis, but precess about it in
random order at their characteristic Larmor frequency. If the
substance, or tissue, is subjected to a magnetic field (excitation
field B.sub.1) which is in an x-y plane and which is near the
Larmor frequency, the net aligned moment, or "longitudinal
magnetization", M.sub.Z may be rotated, or "tipped", into the x-y
plane to produce a net transverse magnetic moment M.sub.t. A signal
is emitted by the excited spins after the excitation signal B.sub.1
is terminated and this signal may be received and processed to form
an image.
[0003] When utilizing these signals to produce images, magnetic
field gradients (G.sub.x G .sub.y and G.sub.z) are employed using
gradient coils. Typically, the region to be imaged is scanned by a
sequence of measurement cycles in which these gradients vary
according to the particular localization method being used. The
resulting set of received MRI signals are digitized and processed
to reconstruct the image using one of many well-known
reconstruction techniques.
[0004] The transmission of a radio frequency (RF) excitation pulse
through a subject and the reception of the resulting resonant
signal is known in the art of nuclear magnetic resonance imaging.
Examples of structures capable of transmitting and receiving RF
pulses include a helical coil, saddle coil, resonant cavity, and a
birdcage resonator or coil. While the use of these structures for
transmission and reception of image signals has greatly improved
reconstruction of an image, there are some existing drawbacks to
the current designs. For example, the occurrence of wrap around
artifacts can create distortion during the image reconstruction
process. Due to the non-linearity of the gradient coils and the
in-homogeneity of the uniform background magnetic field B.sub.0,
there are areas outside the FOV that resonate at the same frequency
as portions inside the FOV, such that aliasing wrap around
artifacts are displayed upon image reconstruction. These
undesirable wrap around artifacts cause a portion of the imaged
subject, which is located outside the FOV, to appear inside of the
FOV as part of the volume imaged.
[0005] The birdcage resonator and other structures cause
limitations as to the strengths of magnetic fields introduced in
the system because of their whole-body imaging methodology.
Whole-body coils cause more irradiation of patient volume than
equivalent shorter length coils. Irradiation levels are regulated
according to an average specific power absorption rate (SAR) per
unit mass for patients under examination. As a result, there is a
need for an apparatus capable of operating with increased magnetic
field strength without exceeding regulated absorption rates that
also reduces the occurrence of wrap around artifacts during image
reconstruction.
[0006] It would therefore be desirable to have an RF coil and
control apparatus to restrict sensitivity in areas outside a
selected FOV, thereby reducing the occurrence of wrap-around
artifacts caused by extended areas of a whole-body coil while
maintaining good sensitivity and homogeneity within the FOV.
SUMMARY OF INVENTION
[0007] The present invention provides a switchable FOV magnetic
resonance imaging apparatus and method solving the aforementioned
drawbacks.
[0008] The invention includes the use of a magnet to produce a
magnetic field for MRI imaging of a subject. After the subject is
placed within a bore of the magnet having a uniform linear magnetic
field so that nuclei within the subject are excited and encoded
using a set of magnetic field gradient coils, a desired FOV size is
selected by an operator. This selection causes an automatic
determination and activation of a center coil only or the center
coil and a pair of end coils in combination for transmission of a
signal desired to perform imaging of a subject. During activation
of the center coil only, the effective longitudinal length of the
RF coil is less causing a reduction in wrap around artifacts as
compared to a standard whole-body coil. A shorter RF coil causes
limited excitation of nuclei in areas outside the desired FOV.
[0009] In accordance with one aspect of the invention, an RF coil
and control configuration includes an RF coil assembly having a
center coil and a pair of end coils. Each end coil is comprised of
at least one RF coil, but can include more. The RF coil assembly is
configured to encode and excite spins over an adjustable FOV. With
activation of both the center coil and the pair of end coils, an
imaging scan can be acquired that is comparable to a whole-body
coil imaging scan. A control is connected to the RF coil assembly
to switch the RF coil assembly between at least two general FOV
sizes based on an FOV size input from an operator. The at least two
general FOV size designations are determined by the control
switching power to activate either a center coil alone or the
center coil and the pair of end coils in unison.
[0010] In accordance with another aspect of the invention an MRI
apparatus to acquire images is disclosed having an MRI system
having a plurality of gradient coils positioned about a bore of a
magnet to impress a polarizing magnetic field B.sub.0 through a
subject under examination. An RF transceiver system and an RF
switch controlled by a pulse module are included to transmit RF
signals to an RF resonator assembly having a center resonator
enclosed by a set of end resonators to acquire magnetic resonance
(MR) images of the subject. A computer is further included and
programmed to receive an FOV input and based on that input, cause
transmission of a signal from either the center resonator only or
the center resonator and the end resonators simultaneously.
[0011] In accordance with yet another aspect of the present
invention, a technique of controlling an MRI device having a
central RF coil assembly comprised of a central RF coil and at
least one pair of end RF coils includes the steps of creating a
predetermined parameter based on a length of the central RF coil
and then comparing a desired FOV to the predetermined parameter. If
the desired FOV is not greater than the predetermined parameter,
then the central RF coil is selected for energization so as to
perform an image scan of the desired FOV. If the FOV is greater
than the selected parameter, then the central RF coil and the at
least one pair of end RF coils are selected for energization
thereby performing an imaging scan of the desired FOV.
[0012] Various other features, objects and advantages of the
present invention will be made apparent from the following detailed
description and the drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0013] The drawings illustrate one preferred embodiment presently
contemplated for carrying out the invention. In the drawings:
[0014] FIG. 1 is a block diagram of a system in accordance with the
present invention.
[0015] FIG. 2 is a schematic of an RF coil assembly connected to a
block diagram of a control system in accordance with the present
invention.
[0016] FIG. 3 is a schematic of another embodiment of an RF coil
assembly connected to a block diagram of a control system in
accordance with the present invention.
[0017] FIG. 4 is a flow chart showing a process to control the RF
coil assembly of FIG. 2 and implemented in the system of FIG.
1.
DETAILED DESCRIPTION
[0018] The embodiments of the present invention will now be
described in more detail with reference to the accompanying
drawings. Referring to FIG. 1, the major components of a preferred
magnetic resonance imaging (MRI) system 10 incorporating the
present invention are shown. The operation of the system is
controlled from an operator console 12 which includes a keyboard or
other input device 13, a control panel 14, and a display 16. The
console 12 communicates through a link 18 with a separate computer
system 20 that enables an operator to control the production and
display of images on the screen 16. The computer system 20 includes
a number of modules which communicate with each other through a
backplane 20a. These include an image processor module 22, a CPU
module 24 and a memory module 26, known in the art as a frame
buffer for storing image data arrays. The computer system 20 is
linked to disk storage 28 and tape drive 30 for storage of image
data and programs, and communicates with a separate system control
32 through a high speed serial link 34. The input device 13 can
include a mouse, joystick, keyboard, track ball, touch activated
screen, light wand, voice control, or any similar or equivalent
input device, and may be used for interactive geometry
prescription.
[0019] The system control 32 includes a set of modules connected
together by a backplane 32a. These include a CPU module 36 and a
pulse generator module 38 which connects to the operator console 12
through a serial link 40. It is through link 40 that the system
control 32 receives commands from the operator to indicate the scan
sequence that is to be performed. The pulse generator module 38
operates the system components to carry out the desired scan
sequence and produces data which indicates the timing, strength and
shape of the RF pulses produced, and the timing and length of the
data acquisition window. The pulse generator module 38 connects to
a set of gradient amplifiers 42, to indicate the timing and shape
of the gradient pulses that are produced during the scan. The pulse
generator module 38 can also receive patient data from a
physiological acquisition controller 44 that receives signals from
a number of different sensors connected to the patient or subject,
such as ECG signals from electrodes attached to the patient. And
finally, the pulse generator module 38 connects to a scan room
interface circuit 46 which receives signals from various sensors
associated with the condition of the patient and the magnet system.
It is also through the scan room interface circuit 46 that a
patient positioning system 48 receives commands to move the patient
to the desired position for the scan.
[0020] The gradient waveforms produced by the pulse generator
module 38 are applied to the gradient amplifier system 42 having
G.sub.x, G.sub.y, and G.sub.z amplifiers. Each gradient amplifier
excites a corresponding physical gradient coil in a gradient coil
assembly generally designated 50 to produce the magnetic field
gradients used for spatially encoding acquired signals. The
gradient coil assembly 50 includes an RF shield to form part of a
magnet assembly 52 which includes a polarizing magnet 54 and an RF
coil assembly 56. A transceiver module 58 in the system control 32
produces pulses which are amplified by an RF amplifier 60 and
coupled to the RF coil assembly 56 by a transmit/receive switch 62.
The resulting signals emitted by the excited nuclei in the patient
may be sensed by the same RF coil assembly 56 or a portion thereof
and coupled through the transmit/receive switch 62 to a
preamplifier 64. The amplified MR signals are demodulated,
filtered, and digitized in the receiver section of the transceiver
58. The transmit/receive switch 62 is controlled by a signal from
the pulse generator module 38 to electrically connect the RF
amplifier 60 to the coil assembly 56 during the transmit mode and
to connect the preamplifier 64 to the coil assembly 56 during the
receive mode. The transmit/receive switch 62 can also enable a
separate RF coil (for example, a surface coil) to be used in either
the transmit or receive mode.
[0021] The MR signals picked up by the selected RF coil are
digitized by the transceiver module 58 and transferred to a memory
module 66 in the system control 32. A scan is complete when an
array of raw k-space data has been acquired in the memory module
66. This raw k-space data is rearranged into separate k-space data
arrays for each image to be reconstructed, and each of these is
input to an array processor 68 which operates to Fourier transform
the data into an array of image data. This image data is conveyed
through the serial link 34 to the computer system 20 where it is
stored in memory, such as disk storage 28. In response to commands
received from the operator console 12, this image data may be
archived in long term storage, such as on the tape drive 30, or it
may be further processed by the image processor 22 and conveyed to
the operator console 12 and presented on the display 16.
[0022] FIG. 2 shows one embodiment of the magnet assembly 52 of
FIG. 1 in accordance with the present invention. The configuration
of the magnetic assembly 52 includes a uniform cylindrical
polarizing magnet 54 for generating the static magnetic field
B.sub.0 in its internal space 55. Disposed within the polarizing
magnet 54 is a gradient coil assembly 50 comprised of one or more
gradient coils, an RF coil assembly 56, and an RF shield (not
shown) that is used to shield the RF energy from penetrating or
coupling to the gradients or magnet. The RF coil assembly 56 has a
generally cylindrical center coil 70 and at least one pair of end
coils 72. Each end coil 72 has one or more RF coils generally
aligned along a common longitudinal axis. The pair of end coils 72
partially overlap the center coil 70 so as to minimize any mutual
inductance between the center coil 70 and the end coils 72.
Preferably, the end coils 72 overlap the center coil 70 by
approximately 1 centimeter (cm) and are longitudinally shorter than
the center coil 70 having a longitudinal length along a Z-axis 74
of approximately 12 cm. The diameters of the center coil 70 and end
coils 72 as measured in the X-Y plane are substantially identical.
The center coil 70, in a preferred embodiment, has a longitudinal
length of 42 centimeters. Longitudinal lengths less than a standard
whole-body coil length of 64 cm are also contemplated and within
the scope of the invention.
[0023] The center and end coils 70, 72 are configured to encode and
excite nuclear spins situated within the uniform magnetic field
B.sub.0 in an adjustable FOV or FOV size designation based on an
FOV size input. The polarizing magnet 54, gradient coils and RF
shield 52, center coil 70, and the pair of end coils 72 are aligned
along a common longitudinal center Z-axis 74 that is parallel to
the static magnetic field B.sub.0. X-axis 76 and Y-axis 78 define
vertical and horizontal axes, respectively, for defining spatial
positions of nuclei within the RF coil assembly 56 during the
encoding process. A patient 80, shown in phantom, is positioned
within the RF coil assembly 56 such that the volume to be imaged
within the field-of-view (FOV) is centered at the origin of the
axes 74-78.
[0024] To control the RF coil assembly 56, in one embodiment, an
equal power splitter 82 is connected to the pair of end coils 72 so
as to provide equal power and alternating electrical current to
drive both end coils 72. An unequal power splitter 84 is connected
to the center coil 70, generally having a length less than a
standard whole-body RF coil, and the equal power splitter 82 to
supply power and drive the center coil 70 in unison with the end
coils 72 during larger FOV imaging scans. The unequal power
splitter 84 is also connected to a computer 86 that includes
operator console 12, computer system 20, disk storage 28, tape 30,
control 32, physiological acquisition controller 44, scan room
interface 46, patient positioning system 48, transmit/receive
switch 62, and amplifiers 60, 64, as shown in FIG. 1 . The computer
86 is designed to generate control signals, for larger FOV size
designations, that cause control 32 to switch power to the center
coil 70 and the pair of end coils 72 in unison thereby generating a
uniform amplitude magnetic field in the RF coil assembly 56. The
computer 86 also causes outputs from the center coil 70 and/or the
end coils 72 when the received FOV input is selected. The output
from the center coil 70 and the end coils 72 activated together is
equal.
[0025] FIG. 3 shows another embodiment to control the RF coil
assembly 56 shown in FIG. 1. The magnet assembly 52 and coil
configuration 70, 72 is similar to that shown in FIG. 2. The set of
end coils 72 are again connected to the equal power splitter 82
providing equal power to the end coils 72. The equal power splitter
is also connected to a first amplifier 88 that is coupled to the
computer 86. The computer 86 provides signals to the end coils 72
to generate a magnetic field in the internal space 55 of the end
coils 72 of a particular amplitude. The computer 86 also provides
signals to a second amplifier connected to the center coil 70. The
signals from the second amplifier are amplified in such a manner
that the current amplitude in the center coil 70, upon activation
of the center coil 70 and end coils 72 simultaneously, matches the
current amplitude in the end coils 72 such that the summation of
currents produce an equivalent magnetic field amplitude similar to
that produced in a single length birdcage coil having a total
length equal to the combination of the center and end coils.
Generally, since the end coils 72 are smaller than the center coil
70, less power is required to generate an equivalent current
amplitude in the end coils 72 than that of the center coil 70.
[0026] The center coil 70 can be activated by computer 86 either
alone or in unison with end coils 72 to transmit and/or receive RF
signals. The transmitted signals are designed to obtain spatial
spin data which is then processed by the computer 86 to reconstruct
a subject 80. The computer 86 can be programmed to acquire a set of
data for image reconstruction that is limited to an imaging volume
defined by the FOV input. Thus, during reception of signals, the
transverse magnetization of precessed nuclei are recorded and
acquired by the selected receive coil, such as coils 70, 72 or a
separate surface coil, as they precess in an X-Y plane defined by
the X-Y axes 76, 78. In this manner, different imaging techniques
can be used for image reconstruction.
[0027] In one embodiment, the center coil 70 and/or pair of end
coils 72 can be birdcage resonators. Birdcage resonators are
particularly beneficial in whole-body MR imaging. Such a resonator
is constructed as a ladder circuit that closes upon itself. The
current flow around the resonator is distributed sinusoidally.
Further, the birdcage is designed such that a phase shift is
discreetly distributed around the circumference of the resonator
from 0 to 2 .pi. (or 2 .pi. k where k is an integer). Preferably,
the center birdcage 70 has simultaneous electrical sinusoidal and
cosinusoidal waveform inputs upon energization of the coil 70. In
addition to birdcage resonators, other coil arrays, for example
saddle and helical coils, are contemplated for use with the present
invention.
[0028] In operation, a subject or patient 80 is placed inside the
coil assembly 56 by the patient positioning system 48 such that the
region desired to be scanned is within a designated FOV centered at
the origin of the coordinate system axes 74-78. The RF coil
assembly 56 is configured to encode and excite spins within the
adjustable FOV having at least two general FOV size designations.
An FOV size designation is based on an FOV size input is determined
by control 32 switching power to activate either the center coil 70
only or center coil 70 in conjunction with the pair of end coils
72. Upon activation of the selected coils or resonators 70, 72, one
or more RF signals or pulses are transmitted from the coils 70, 72
and then the coils can be switched to receive data, or a separate
RF surface coil can be employed for data reception. The signals are
then amplified 64 and processed by the control 32 to reconstruct an
image of the subject 80 within the selected FOV, which can be
displayed at 16. Generally, whole-body imaging used for large FOV
images of a patient 80 in a sagittal or coronal plane, or oblique
planes, is conducted by activating both the center coil 70 and the
pair of end coils 72. The equal power splitter 82, connected to the
pair of end coils 72, and the unequal power splitter 84 connected
to the equal power splitter 82 and computer 86 causes signals from
the computer 86 to create equal amplitude currents in the center
coil 70 and the pair of end coils 72 matching a field equivalent to
the equal length birdcage coil. Smaller or less than large FOV"s,
such as certain axial imaging scans and small FOV sagittal,
coronal, and oblique imaging scans, can be conducted by activating
the central coil 70 only.
[0029] The activation of the center coil 70 only or in unison with
the pair of end coils 72 provides a method for switching the FOV
between a shorter or longer FOV. Decreasing the center coil 70 size
relative to a standard whole-body coil results in less wrap around
artifacts during image reconstruction and less irradiation of
patient volume. Switching between the FOV"s is controlled by system
control 32 which passes commands for the desired coil activation
via scan room interface 46 and/or the transmit/receive switch
62.
[0030] Referring to FIG. 4, a technique of controlling an MRI
device having a central RF coil and a pair of end RF coils is
initiated at 100, and then a determination of the desired FOV size
is made 102. If the size requested is large 104 (i.e., to cover the
equivalent of a standard whole-body coil), the central RF coil and
all end RF coils are activated 106 causing the transmission of a
signal which passes through the patient and is received by the
receiving coils, whether they are the same coils or a separate
receiving coil. If the FOV size requested is not large 102, 108
then only the center coil is activated to transmit 110.
[0031] Both FOV designations 104, 108 independently decide whether
to utilize their respective transmission coils 106, 110 as
receiving coils 112, 114. Use of the transmitting coils as
receiving coils for the large FOV 112, 116 results in signal
reception at 118. The system then determines if data acquisition is
complete 120, and if so 122, the process ends 124. If data
acquisition is not complete 120, 126 then the technique loops back
to step 102 and signal transmission occurs again. Likewise, if the
transmitting coils are chosen to transmit and receive for an FOV
designation that is something less than large 114, 128, then the
center coil only can receive 130. Again the system determines if
data acquisition is complete 120, and the technique either ends 124
or returns to step 102 causing a new transmission.
[0032] If the transmission coils are not selected as the receiving
coils 132, 134, then the transmitting coils are deactivated 136.
For the large FOV designation 104, both the central coil and the
end coils are inactive. For the less than large FOV designation
108, only the center coil is deactivated since any end coils are
already inactive. The transmitted signal is received with surface
coils at 138 and then a determination is made on whether data
acquisition is complete 120 similar to the previous determinations
that used the transmitting coils as receiving coils.
[0033] In instances where a FOV has a longitudinal length greater
than the length of the central RF coil, the method can be
predetermined so as to automatically transmit and receive signals
using both the central RF coil and the pair of end coils.
[0034] Accordingly, an RF coil and control configuration includes
an RF coil assembly having a center coil and a pair of generally
shorter end coils, wherein each end coil has at least one RF coil.
The RF coil assembly is configured to encode and excite spins over
an adjustable FOV, such as a large FOV designation for imaging in
the coronal, sagittal, and/or oblique planes of a patient, and some
large FOV axial planes, or a small FOV designation for partial body
or certain axial imaging. A control connected to the RF coil
assembly switches the RF coil assembly between at least two general
FOV sizes by switching power to activate one of a center coil only
or the center coil and the pair of end coils in unison.
[0035] In accordance with another aspect of the invention an MRI
apparatus to acquire images is disclosed comprising a magnetic
resonance imaging system having a plurality of gradient coils
positioned about a bore of a magnet to impress a polarizing B.sub.0
magnetic field through a patient under examination causing an
alignment of the patient's nuclei spins. An RF transceiver system
and an RF switch controlled by a pulse module transmit RF signals
to an RF resonator assembly having a center resonator enclosed by a
set of end resonators to acquire magnetic resonance images of the
patient. A computer is also included and programmed to receive a
FOV input and based on that input, cause transmission of a signal
from either the center resonator only or the center resonator and
the set of end resonators simultaneously.
[0036] A method of controlling an MRI device having an RF coil
assembly comprised of a central RF coil and at least one pair of
end RF coils is also disclosed. The technique includes the steps of
creating a predetermined parameter based on a length of a central
RF coil and then comparing a desired FOV to the predetermined
parameter. If the desired FOV is not greater than the predetermined
parameter, then the central RF coil is selected for energization so
as to perform an image scan of the desired FOV, such as a partial
body imaging scan. If the desired FOV is greater than the selected
parameter, then the central RF coil and at least one pair of end RF
coils are selected for energization thereby performing an imaging
scan of the desired FOV.
[0037] The present invention has been described in terms of the
preferred embodiment, and it is recognized that equivalents,
alternatives, and modifications, aside from those expressly stated,
are possible and within the scope of the appending claims.
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