U.S. patent application number 09/682913 was filed with the patent office on 2003-05-01 for method and apparatus of mr data acquisition using ensemble sampling.
Invention is credited to Collick, Bruce D., Frigo, Frederick J., Frigo, Louis M., Hartley, Michael R., Pettersson, Bo.
Application Number | 20030083568 09/682913 |
Document ID | / |
Family ID | 24741737 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030083568 |
Kind Code |
A1 |
Frigo, Frederick J. ; et
al. |
May 1, 2003 |
METHOD AND APPARATUS OF MR DATA ACQUISITION USING ENSEMBLE
SAMPLING
Abstract
The present invention provides a system and method of increasing
the sampling rate for MR data acquisition overcoming the
aforementioned drawbacks. By implementing ensemble sampling
techniques, the present invention provides higher data sampling
rates that are useful for several MR data acquisition applications
including Echo Planar Imaging, Functional Magnetic Resonance
Imaging, and Sensitivity Encoding Imaging (SENSE) techniques. By
multiplying an MR signal by a series of pure sinusoids having the
same frequency but shifted by an incremental phase, the MR signal
may be separated into a number of channels which can be sampled at
lower rates by analog-to-digital converters. The output from the
converters may then be reconstructed using one of a number of
interpolation techniques to create a single digital channel with
increased bandwidth. The single channel with increased bandwidth
may then be used to acquire MR data with an improved sampling
rate.
Inventors: |
Frigo, Frederick J.;
(Waukesha, WI) ; Collick, Bruce D.; (Madison,
WI) ; Frigo, Louis M.; (Brookfield, WI) ;
Hartley, Michael R.; (Pewaukee, WI) ; Pettersson,
Bo; (Waukesha, WI) |
Correspondence
Address: |
ZIOLKOWSKI PATENT SOLUTIONS GROUP, LLC (GEMS)
14135 NORTH CEDARBURG ROAD
MEQUON
WI
53097
US
|
Family ID: |
24741737 |
Appl. No.: |
09/682913 |
Filed: |
October 31, 2001 |
Current U.S.
Class: |
600/410 |
Current CPC
Class: |
G01R 33/3621
20130101 |
Class at
Publication: |
600/410 |
International
Class: |
A61B 005/055 |
Claims
1. A method of increasing the sampling rate used for MR data
acquisition, the method comprising: acquiring MR data; combining
the MR data with an ensemble function; separating the MR data and
ensemble function into a number of channels; sampling and
converting data from the number of channels to digitize the data;
and reconstructing the digitized data to create a single channel of
data with increased bandwidth.
2. The method of claim 1 wherein combining includes multiplying the
MR data with a series of pure sinusoids having a common frequency
and shifted by an incremental phase to create a number of channels
of data.
3. The method of claim 1 wherein acquiring includes: generating a
polarizing magnetic field across an FOV; applying an RF signal to
produce transverse magnetization in the FOV; and detecting an MR
signal resulting from the transverse magnetization.
4. The method of claim 3 further comprising demodulating and
filtering the MR signal.
5. The method of claim 1 further comprising adjusting each of the
number of channels to account for at least one of gain and phase
incoherence.
6. The method of claim 1 wherein reconstructing includes
interpolating the digitized data using a sinc function.
7. A computer readable storage medium having stored thereon a
computer program representing a set of instructions that when
executed by a computer causes the computer to: detect an MR data
signal; demodulate the MR data signal to generate a band-limited
analog MR data signal, S; combine the band-limited analog MR data
signal with at least two ensemble functions; input the band-limited
analog MR data signal and the at least two ensemble functions to a
number of analog-to-digital converters; detect A/D converter
output; and generate a single digital channel, X, with increased
bandwidth from the converter output.
8. The computer readable storage of claim 7 wherein the set of
instructions further causes the computer to input the band-limited
analog MR data signal into a single A/D converter without ensemble
sampling for imaging applications requiring lower bandwidth.
9. The computer readable storage of claim 7 wherein the single
digital channel includes a single real-valued or single
complex-valued MR data set.
10. The computer readable storage of claim 7 wherein the set of
instructions further causes the computer to input S into an
anti-aliasing filter.
11. The computer readable storage of claim 10 wherein the set of
instructions further causes the computer to multiply S by a number
of ensemble functions, .psi..sub.n, and generate a set of ensemble
signals, E.sub.n.
12. The computer readable storage of claim 11 wherein the set of
instructions further causes the computer to input E.sub.n to the
number of A/D converters configured to convert E.sub.n to a number
of discrete ensemble digital data channels, .phi..sub.n.
13. The computer readable storage of claim 12 wherein the set of
instructions further causes the computer to combine .phi..sub.n
into a single real-valued MR data set or a single complex-valued MR
data set, X, representative of S with an increased sampling
rate.
14. The computer readable storage of claim 13 wherein the set of
instructions further causes the computer to interpolate .phi..sub.n
to combine .phi..sub.n into a single digital channel, X, with
increased bandwidth.
15. The computer readable storage of claim 14 wherein the set of
instructions further causes the computer to utilize the MR imaging
data contained in X for image reconstruction with an increased
sampling rate.
16. An MRI apparatus 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 coil Assembly to acquire MR images;
and a computer programmed to: receive an MR data signal and
demodulate the MR data signal into a band-limited MR signal S;
generate a set of ensemble signal channels, E.sub.n, by combining S
with a plurality of ensemble functions, .psi..sub.n andconvert
E.sub.n to a number of discrete ensemble digital channels,
.phi..sub.n; and form .phi..sub.n into a single discrete digital
channel, X, having increased bandwidth compared to that of S.
17. The system of claim 16 wherein the computer is further
programmed to combine .phi..sub.n into the single discrete digital
channel, X, by interpolating .phi..sub.n with a sinc function.
18. The system of claim 16 wherein E.sub.n=S*.psi..sub.n and
wherein .psi..sub.n=cos((.omega.+.theta..sub.n)t), where
.theta..sub.n=(.pi./M)*n- , and M equals the number discrete
ensemble digital channels for n=0, . . . , (M-1).
19. The system of claim 16 wherein the computer is further
programmed to input S into an anti-aliasing filter and multiply an
anti-aliasing filter output by .psi..sub.n.
20. The system of claim 16 wherein X includes one of a single
real-valued MR data set and a single complex-valued MR data
set.
21. The system of claim 16 wherein the computer is programmed to
employ at least one of a gain calibration and a phase calibration
to compensate for differences in gain of each A/D converter and to
compensate for non-uniform phase differences of each discrete
ensemble digital channel, respectively.
22. The system of claim 16 wherein the computer is programmed to
acquire MR imaging data with an increased sampling rate.
23. A method of increasing the sampling rate used for MR data
acquisition, the method comprising: generating a polarizing
magnetic field across an FOV; applying an RF signal to produce
transverse magnetization in the FOV; detecting and demodulating an
MR signal resulting from the transverse magnetization; generating a
band-limited MR signal S from the demodulated MR signal;
multiplying the band-limited MR signal Sby an ensemble function
.psi..sub.n; separating the multiplied band-limited MR signal
E.sub.n into a number of channels; digitizing the number of
channels to generate a number of digital MR data channels
.phi..sub.n; and combining .phi..sub.n into a digital data channel
X.
24. The method of claim 23 wherein .psi..sub.n includes a plurality
of pure sinusoids, each sinusoid having a constant frequency,
.omega., shifted by a constant phase, .theta..sub.n; wherein
.theta..sub.n=(.pi./M)*n, where M is the number of channels for
n=0, . . . , (M-1); and .psi..sub.n=cos((.omega.+.theta..sub.n)t)
for n=0, . . . , (M-1).
25. The method of claim 24 wherein E.sub.n=S*.psi..sub.n and
further comprising interpolating .phi..sub.n to generate X.
26. The method of claim 23 further comprising adjusting each of the
number of channels, .phi..sub.n, to account for at least one of
gain and phase incoherence.
27. The method of claim 23 wherein X includes a single real-valued
or complex-valued digital MR data set.
28. An apparatus to produce an MR data set from an MR signal, the
apparatus comprising: a magnet configured to produce a polarizing
magnetic field; an RF coil configured to produce transverse
magnetization in a region of interest of the polarizing field; a
receiver configured to receive an MR signal resulting from the
transverse magnetization; a demodulator configured to generate a
band-limited MR signal S; a data acquisition system configured to
sample the band-limited MR signal and generate an MR data set
having a sampling rate more than twice that of the band-limited MR
signal; a gradient coil assembly configured to produce a readout
gradient during sampling of the receive MR signal; a pulse
generation system for controlling the synchronized operation of the
RF coil, gradient coil assembly, and data acquisition system; and a
processing system for generating results from the MR data set.
29. The apparatus of claim 28 further comprising an anti-aliasing
filter configured to filter the band-limited MR signals.
30. The apparatus of claim 29 further comprising an ensemble
function generator configured to generate a series of pure
sinusoids having a common frequency and shifted by an incremental
phase.
31. The apparatus of claim 30 further comprising a multiplier
connected to the anti-aliasing filter and the ensemble function
generator and configured to multiply the band-limited MR signal by
the series of pure sinusoids.
32. The apparatus of claim 31 further comprising a plurality of A/D
converters configured to digitize the number of data channels.
33. The apparatus of claim 32 further comprising an interpolator
configured to combine A/D converter output into a single digital
channel of MR data having increased bandwidth.
34. The apparatus of claim 28 further comprising a bypass
processing system configured to input the band-limited MR signal
into a single A/D converter to generate a set of MR imaging data
without ensemble sampling for low bandwidth applications.
Description
BACKGROUND OF INVENTION
[0001] The present invention relates generally to data acquisition
and, more particularly, to a method and apparatus of increasing the
sampling rate used for data MR acquisition using ensemble sampling
techniques.
[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, 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 the x-y plane and which is near the Larmor frequency, the net
aligned moment, or "longitudinal magnetization", Mz, 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.xG.sub.y and G.sub.z) are employed.
Typically, the region to be imaged is scanned by a sequence z of
measurement cycles in which these gradients vary according to the
particular localization method being used. The resulting set of
received NMR signals are digitized and processed to reconstruct the
image using one of many well known reconstruction techniques.
[0004] Generally, the MR signal resulting from the net transverse
magnetic moment is demodulated to generate a band-limited MR
signal. During sampling of the band-limited MR signal by a data
acquisition system, a gradient coil assembly produces a readout
gradient. The data acquisition system then generates an MR data set
which is ultimately used to reconstruct an image using one of many
well known reconstruction techniques.
[0005] A number of factors influence sampling rate which can be
classified into four distinct categories: (1) analog front end; (2)
input signal characteristics; (3) hardware; and (4)
analog-to-digital parameters. Factors associated with the analog
front end include the sensitivities of the demodulator and the
anti-aliasing filter as well as the sample-hold circuitry. Input
signal characteristics include the frequency of the input signal,
its amplitude and bandwidth, and the noise of the signal. Hardware
implemented with the system also may affect the sampling rate,
i.e., number of bits, the maximum sampling rate, and the
sensitivity of the analog-to-digital converter. Special
characteristics of the A/D converter may also affect the sampling
rate. The clock frequency, linearity, and operating temperature of
the A/D converter can each affect the sampling rate of a
signal.
[0006] Several techniques have been developed to increase the
sampling rate used for MR data acquisition. One known technique
uses "quadrature sampling" to receive "input" signals from which
images may be created. Quadrature sampling involves separating an
input signal into two channels by multiplying the original input
signal by cos(.omega.) to form an in-phase (I-channel) and by
sin(.omega.) to form the quadrature-phase (Q-channel). In
accordance with this technique, a local oscillator produces an
in-phase signal and a quadrature signal. The phase of the end-phase
signal is then shifted by 90.degree.. The shifted end-phase signal
and the quadrature signal are then mixed and further processed to
generate an output signal having a desired sampling rate and a
desired bandwidth that may be used by an MRI system control.
Sampling the I and Q channels in parallel effectively doubles the
effective sampling rate of the original input signal. This
technique and other known techniques, however, fail to adequately
increase the sampling rate and bandwidth that are needed for
several MR data acquisition applications.
[0007] It would therefore be desirable to have a system and method
capable of creating a single digital channel with increased
bandwidth and subsequent improved sampling rate for MR data
acquisition.
BRIEF DESCRIPTION OF INVENTION
[0008] The present invention provides a system and method of
increasing the sampling rate for MR data acquisition overcoming the
aforementioned drawbacks. By implementing ensemble sampling
techniques, the present invention provides higher data sampling
rates that are useful for several MR data acquisition applications
including Echo Planar Imaging, Functional Magnetic Resonance
Imaging, and Sensitivity Encoding Imaging (SENSE) techniques. By
multiplying an MR signal by a series of pure sinusoids having the
same frequency but shifted by an incremental phase, the MR signal
may be separated into a number of channels which can be sampled at
lower rates by analog-to-digital converters. The output from the
converters may then be reconstructed using one of a number of
interpolation techniques to create a single digital channel with
increased bandwidth. The single channel with increased bandwidth
may then be used to acquire MR data with an improved sampling
rate.
[0009] Therefore, in accordance with one aspect of the present
invention, a method of increasing the sampling rate used for MR
data acquisition is provided and includes acquiring MR data and
combining the MR data with an ensemble function. The method further
includes separating the MR data and ensemble function into a number
of channels and sampling and converting data from the number of
channels to digitize the data. The method also includes
reconstructing the digitized data to create a single channel of
data with increased bandwidth.
[0010] In accordance with a further aspect of the present
invention, a computer readable storage medium is provided having
stored thereon a computer program representing a set of
instructions that when executed by a computer causes a transceiver
to detect an MR data signal and demodulate the MR data signal to
generate a band-limited analog MR data signal. The computer is
further programmed to combine the band-limited analog MR data
signal with a number of ensemble functions and input the signal
resulting therefrom into a number of analog-to-digital converters.
The computer program then causes the computer to detect
analog-to-digital converter output and generate a single digital
channel with increased bandwidth from the output.
[0011] In accordance with another aspect of the present invention,
an MRI apparatus to acquire MR data with increased sampling is
disclosed. The apparatus includes an MRI system having a number of
gradient coils positioned about a bore of a magnet to impress a
polarizing magnetic field and an RF transceiver system and an RF
modulator controlled by a pulse control module to transmit RF
signals to an RF coil assembly to acquire MR images. The MRI
apparatus also includes a computer programmed to input an MR data
signal to a demodulator configured to demodulate the MR data signal
into a band-limited MR signal. The computer is further programmed
to generate a set of ensemble signal channels by combining the
band-limited MR signal by a plurality of ensemble functions and
convert the ensemble signal channels to a number of discrete
ensemble digital channels. The number of discrete ensemble digital
channels is then formed into a single discrete digital channel
having an increased bandwidth.
[0012] In accordance with yet another aspect of the present
invention, a method of increasing the sampling rate used for MR
data acquisition is provided and includes generating a polarizing
magnetic field across a field of view. An RF signal is then applied
to produce transverse magnetization in a region of interest of the
magnetic field. The method further includes detecting an MR signal
resulting from the transverse magnetization and demodulating the
signal to generate a band-limited MR signal. The band-limited MR
signal is then multiplied by a plurality of ensemble functions.
Thereafter, the multiplied band-limited MR signal is then separated
into a number of channels whereupon each channel is sampled to
generate a single digital channel with increased bandwidth thereby
providing an increased sampling rate for data acquisition.
[0013] In accordance with yet a further aspect of the present
invention, an apparatus for producing an MR data set from an MR
signal is provided and comprises a magnet for producing a
polarizing magnetic field and an RF coil for producing transverse
magnetization in a region of interest of the polarizing field. The
apparatus further includes a receiver configured to receive an MR
signal resulting from the transverse magnetization as well as a
demodulator configured to demodulate the signal to generate a
band-limited MR signal. A gradient coil assembly is provided for
producing a readout gradient during sampling of the received MR
signal as well as a data acquisition system for sampling the
band-limited MR signal and generating an MR data set having a
sampling rate more than twice that of the band-limited MR signal.
The apparatus further includes a pulse generation system configured
to control the synchronized operation of the RF coil, gradient coil
assembly, and the data acquisition system. A processing system is
also provided for reconstructing an image from the data set.
[0014] 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
[0015] The drawings illustrate one preferred embodiment presently
contemplated for carrying out the invention.
[0016] In the drawings:
[0017] FIG. 1 is a schematic block diagram of an MR imaging system
for use with the present invention.
[0018] FIG. 2 is a flow chart showing an example for increasing
sampling rate for MR data acquisition in accordance with the
present invention.
[0019] FIG. 3 is a graphical representation of a band-limited MR
signal.
[0020] FIG. 4 is a graphical representation of a set of ensemble
functions in accordance with the present invention.
[0021] FIG. 5 is a graphical representation of a combination of the
signals shown in FIGS. 3 and 4.
[0022] FIG. 6 is a graphical representation of an output signal
similar to that shown in FIG. 3 but having increased sampling
rate.
[0023] FIG. 7 is a schematic block diagram of a signal processing
system in accordance with the present invention.
DETAILED DESCRIPTION
[0024] 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 or a removable digital
media 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.
[0025] 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, 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.
[0026] 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 forms part of a magnet assembly 52 which
includes a polarizing magnet 54 and a whole-body RF coil 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 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 56 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 56 during the transmit mode and to connect
the preamplifier 64 to the coil 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.
[0027] The MR signals picked up by the RF coil 56 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 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.
[0028] The present invention is directed to acquiring MR imaging
data with an increased and/or improved sampling rate. The steps of
a process to generate a digital data acquisition channel with
increased bandwidth are set forth in FIG. 2. The present invention
also contemplates a computer program to automatically carry out the
steps necessary to acquire MR imaging data with a faster sampling
rate. The commands of the computer program will likewise be
referenced with the description below of FIG. 2.
[0029] A process 100 for increasing the sampling rate used for MR
data acquisition begins at 110 with generation of a polarizing
magnetic field 120 across the subject to be scanned, i.e. a medical
patient. After the polarizing magnetic field is generated, an RF
coil assembly 56, FIG. 1, is energized to produce transverse
magnetization in a volume-of-interest in the polarizing field 130.
Signals emitted by the excited nuclei resulting from the transverse
magnetization are detected at 140 and demodulated at 150.
Demodulation of the detected MR signals results in generation of a
band-limited MR signal, S. The band-limited MR signal is then
filtered at 160 by an anti-aliasing filter, as is well known.
[0030] At 170, the filtered band-limited MR signal is multiplied by
a set of ensemble functions. In a preferred embodiment, the set of
ensemble functions is a series of pure sinusoids of the same
frequency shifted by an incremental phase. Multiplying the
band-limited MR signal by the pure sinusoids results in a set of
ensemble signals, E.sub.n, which may be separated into multiple
channels that may be sampled at lower rates by analog-to-digital
converters, as will be discussed shortly. The set of ensemble
signals, E.sub.b, may be defined by:
E.sub.n=S*.psi., (Eqn.1).
[0031] where S is the band-limited MR signal and .psi..sub.n is the
set of ensemble sampling functions at a constant frequency,
.omega., shifted by a constant phase, .theta..sub.n. .theta..sub.n
may be defined by:
.theta..sub.n=(.pi./M)*n, (Eqn. 2).
[0032] where M is the number of channels for n=0 . . . , (M-1).
[0033] The set of ensemble sampling functions, .psi..sub.n, may be
defined by:
.psi..sub.n=cos((.omega.+.theta..sub.n)t) for n=0, . . . , (M-1)
(Eqn. 3).
[0034] After the band-limited MR signal is multiplied by the
ensemble functions at 170 to generate a set of ensemble signals,
the ensemble signals are separated into a number of channels at
180. That is, the demodulated MR signal is multiplied by four
sinusoids out of phase with respect to one another at 170 and then
the multiplied signals are separated into four channels at 180.
There is a one-to-one relationship between the number of channels
into which the signals are separated and the number of sinusoids
multiplied to the demodulated signal at 170. Once the signals are
separated into a number of channels at 180, each channel is
digitized by an analog-to-digital converter configured to convert
an ensemble signal to a discrete ensemble digital data channel 190.
After each channel is digitized at 190 by the analog-to-digital
converter, a number of discrete ensemble digital channels,
.phi..sub.n, results. As indicated previously, n, since the MR
signal is separated into multiple channels, the MR signal may be
sampled at lower rates by the analog-to-digital converters.
[0035] Following digitization of each channel at 190, each channel
is adjusted at 200 for a gain and phase incoherence. A number gain
calibration and phase calibration techniques may be used to
compensate for differences in the gain of the individual
analog-to-digital converter stages and for non-uniform phase
differences that may exist in individual channels, respectively.
Since each separate channel is sampled with a different A/D
converter and each separate channel has its own sample-hold
circuitry, each channel should, in a preferred embodiment, undergo
a gain calibration. Further, each channel is created by multiplying
an input signal by a sinusoidal input, therefore, each channel
should, in a preferred embodiment, undergo a phase calibration to
assure phase coherence among the signals.
[0036] After each channel is adjusted to compensate for gain and
phase incoherencies, the multitude of discrete ensemble digital
channels are combined 210 to generate a single digital channel with
increased bandwidth. The set of digital data channels is combined
into a single real-valued or complex-valued MR data set, X, which
represents a sampled version of the band-limited MR signal, S, set
forth above. A number of interpolation schemes may be utilized to
combine the data channels into a single channel, such as a sinc
interpolation. Because the single digital data channel has an
increased bandwidth, the channel may be processed with a greater
sampling rate to generate MR imaging data for image reconstruction
at 220. Acquiring data for image reconstruction with an improved
sampling rate allows for reconstruction of an image with greater
sensitivity to the volume of interest, improved signal to noise,
improved image quality, and improved overall diagnostic value.
After the single digital data channel is processed at 220, an image
is reconstructed using known imaging techniques at 230 whereupon
process 100 ends at 240.
[0037] Now referring to FIGS. 3-6, representations of signals
generated in accordance with the ensemble sampling techniques of
the present invention are shown. FIGS. 3-6 will be discussed with
reference to a four channel ensemble sampling technique, but the
present invention is applicable with less than four channels as
well as more than four channels. Shown in FIG. 3 is a
representation of a typical band-limited MR signal 250 detected
from excited nuclei following application of an RF signal to
produce transverse magnetization and a volume of interest of a
polarized magnetic field, having been demodulated and filtered
using known processing techniques. In accordance with the present
invention, signal 250 is multiplied by a number of ensemble
functions, shown in FIG. 4. In a four channel embodiment, four
ensemble functions, generally referenced 260 include four pure
sinusoids having a constant frequency but shifted by incremental
phase. As indicated previously, the ensemble when multiplied to the
MR signal 250 will subsequently allow for separation of the
band-limited MR signal into a number of data channels that may be
digitized at lower sampling rates.
[0038] FIG. 5 is a representation of ensemble signals 270 that
result in the multiplication of signal 250 and functions 260. The
ensemble signals 270 are then input to a number of
analog-to-digital converters to convert the signals into discrete
ensemble digital channels. The discrete ensemble digital data
channels are then combined into a single real-valued or
complex-valued MR data set, X, which represents a sampled version
of the band-limited MR signal 250 of FIG. 3. FIG. 6 is a
representation of the output signal 280 resulting from the
combination of the set of digital data channels into a single
real-valued or complex-valued MR data set. As indicated previously,
output signal 280 represents a sampled version of the band-limited
MR signal 250 shown in FIG. 3. A number of interpolation schemes
may be employed such as sinc interpolation to combine the discrete
ensemble digital data channels into the single channel 280 shown in
FIG. 6. When combined, output signal 280 has a sampling rate
greater than the sampling rate of each A/D converter. For example,
using four A/D converters in parallel at 1 MHz will result in a
single digital output signal with a 4 MHz sampling rate. In another
embodiment, an eight channel ensemble sampling mechanism may be
implemented to effectively increase the sampling rate of an
original MR input signal by eight fold.
[0039] FIG. 7 is a schematic block diagram of a data acquisition
apparatus 300 for use with the present invention. Apparatus 300
includes a signal detector 310 configured to detect an MR signal
and input the MR signal into a demodulator 320. The demodulator 320
is connected to an anti-aliasing filter 330 and configured to input
into filter 330 a demodulated MR signal. Connected to the
anti-aliasing filter 330 are in the preferred embodiment, a number
of signal multipliers 340. Each signal multiplier 340 is configured
to combine the filtered MR signal output from filter 330 with an
ensemble function created output by an ensemble function generator
350. In a preferred embodiment, the ensemble functions are a series
of pure sinusoids having a common frequency and shifted by an
incremental phase. The outputs of each signal multiplier create a
separate data channel as shown in FIG. 5. The number of data
channels are then input to a corresponding number of
analog-to-digital converters 370 which are configured to digitize
the analog signal and generate a set of digital data channels
therefrom. Output from the analog-to-digital converters 370 is then
input into an interpolator 380 which is connected to the A/D
converters 370 and configured to combine the number of digital data
channels into a single digital data channel having increased
bandwidth. In a preferred embodiment, interpolator 380 combines the
multiple data channels into a single digital data channel using an
interpolation technique. Connected to the interpolator 380 is an
image reconstructor 390 that is configured to take as input the
interpolator output and reconstruct an MR image therefrom. In an
alternative embodiment, apparatus 300 further includes a bypass
processing system generally referenced 394 wherein output from the
anti-aliasing filter 330 is input to a single analog-to-digital
converter 396 having an output that is then input to the image
reconstructor 390. With this alternate embodiment, the demodulated
and filtered MR signal does not undergo ensemble sampling and is
therefore applicable for those imaging applications requiring lower
bandwidth.
[0040] The present invention provides a process for increasing a
sampling rate used for MR data acquisition by employing ensemble
sampling techniques. Higher data sampling rates are useful for many
MR data acquisition applications such as Echo Planar Imaging,
Functional Magnetic Resonance Imaging, and Sensitivity Encoding
(SENSE) Imaging techniques. Improving the sampling rate used for
data acquisition yields a more sensitive reconstructed image having
improved signal to noise ratio and overall image quality.
[0041] Therefore, in accordance with one embodiment of the present
invention, a method of increasing the sampling rate used for MR
data acquisition is provided and includes acquiring MR data and
combining the MR data with an ensemble function. The method further
includes separating the MR data and ensemble function into a number
of channels and sampling and converting data from the number of
channels to digitize the data. The method also includes
reconstructing the digitized data to create a single channel of
data with increased bandwidth.
[0042] In accordance with another embodiment of the present
invention, a method of increasing the sampling rate used for MR
data acquisition is provided and includes generating a polarizing
magnetic field across a field of view. An RF signal is then applied
to produce transverse magnetization in a region of interest of the
magnetic field. The method further includes detecting an MR signal
resulting from the transverse magnetization and demodulating the
signal to generate a band-limited MR signal. The band-limited MR
signal is then multiplied by a plurality of ensemble functions.
Thereafter, the multiplied band-limited MR signal is then separated
into a number of channels whereupon each channel is sampled to
generate a single digital channel with increased bandwidth thereby
providing an increased sampling rate for data acquisition.
[0043] In accordance with a further embodiment of the present
invention, a computer readable storage medium is provided having
stored thereon, a computer program representing a set of
instructions that when executed by a computer causes the
transceiver MR data signal and demodulate the MR data signal to
generate a band-limited analog MR data signal. The computer is
further programmed to combine the band-limited analog MR data
signal with a number of ensemble functions and input the signal
resulting therefrom into a number of analog-to-digital converters.
The computer program then causes the computer to detect
analog-to-digital converter output and generate a single digital
channel with increased bandwidth from the output.
[0044] In accordance with another embodiment of the present
invention, an MRI apparatus to acquire MR data with increased
sampling is disclosed. The apparatus includes an MRI system having
a number of gradient coils positioned about a bore of a magnet to
impress a polarizing magnetic field and an RF transceiver system
and an RF modulator controlled by a pulse control module to
transmit RF signals to an RF coil assembly to acquire MR images.
The MRI apparatus also includes a computer programmed to input an
MR data signal to a demodulator configured to demodulate the MR
data signal into a band-limited MR signal. The computer is further
programmed to generate a set of ensemble signals by multiplying the
band-limited MR signal by a plurality of ensemble functions. The
set of ensemble signals is the input into a number of
analog-to-digital converters configured to convert the ensemble
signals to a number of discrete ensemble digital channels. The
number of discrete ensemble digital channels is then combined into
a single discrete digital channel having an increased
bandwidth.
[0045] In accordance with yet another embodiment of the present
invention, an apparatus for producing an MR data set from an MR
signal is provided and comprises a magnet for producing a
polarizing magnetic field and an RF coil for producing transverse
magnetization in a region of interest of the polarizing field. The
apparatus further includes a receiver configured to receive an MR
signal resulting from the transverse magnetization as well as a
demodulator configured to demodulate the signal to generate a
band-limited MR signal. A gradient coil assembly is provided for
producing a readout gradient during sampling of the received MR
signal as well as a data acquisition system for sampling the
band-limited MR signal and generating an MR data set having a
sampling rate more than twice that of the band-limited MR signal.
The apparatus further includes a pulse generation system configured
to control the synchronized operation of the RF coil, gradient coil
assembly, and the data acquisition system. A processing system is
also provided for reconstructing an image from the data set.
[0046] In an alternate embodiment of the present invention, a
single analog-to-digital converter is used as a bypass processing
system without ensemble sampling for applications requiring lower
bandwidth. In this embodiment, the band-limited MR signal is input
to a single analog-to-digital converter. The analog-to-digital
converter processes the band-limited MR signal to output a single
real-valued or complex-valued MR data set that may be sampled to
acquire data for image reconstruction.
[0047] 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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