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BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging methods
and systems. More particularly, the invention relates to a method for
reducing the time required to perform an NMR scan.
Any nucleus which possesses a magnetic moment attempts to align itself with
the direction of the magnetic field in which it is located. In doing so,
however, the nucleus precesses around this direction at a characteristic
angular frequency (Larmor frequency) which is dependent on the strength of
the magnetic field and on the properties of the specific nuclear species
(the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit
this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic
field (polarizing field B.sub.z), 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.
A net magnetic moment M.sub.z is produced in the direction of the
polarizing field, but the randomly oriented magnetic components in the
perpendicular, or transverse, plane (x-y plane) cancel one another. If,
however, 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, M.sub.z, may be rotated, or
"tipped", into the z-y plane to produce a net transverse magnetic moment
M.sub.1, which is rotating, or spinning, in the x-y plane at the Larmor
frequency. The degree to which the net magnetic moment M.sub.z is tipped,
and hence the magnitude of the net transverse magnetic moment M.sub.1
depends primarily on the length of time and the magnitude of the applied
excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is
emitted by the excited spins after the excitation signal B.sub.1 is
terminated. In simple systems the excited spins induce an oscillating sine
wave signal in a receiving coil. The frequency of this signal is the
Larmor frequency, and its initial amplitude, A.sub.0, is determined by the
magnitude of the transverse magnetic moment M.sub.1. The amplitude, A, of
the emission signal decays in an exponential fashion with time, t:
A=A.sub.0 e.sup.-t /T*.sub.2
The NMR measurements of particular relevance to the present invention are
called "pulsed NMR measurements". Such NMR measurements are divided into a
period of excitation and a period of signal emission. Such measurements
are performed in a cyclic manner in which the NMR measurement is repeated
many times to accumulate different data during each cycle or to make the
same measurement at different locations in the subject. A wide variety of
preparative excitation techniques are known which involve the application
of one or more excitation pulses (B.sub.1) of varying magnitude and
duration. Such excitation pulses may have a narrow frequency spectrum
(selective excitation pulse), or they may have a broad frequency spectrum
(nonselective excitation pulse) which produces transverse magnetization
M.sub.1 over a range of resonant frequencies. The prior art is replete
with excitation techniques that are designed to take advantage of
particular NMR phenomena and which overcome particular problems in the NMR
measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR
signals from specific locations in the subject. Typically, the region
which is to be imaged (region of interest) is scanned by a sequence of NMR
measurement cycles which 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. To perform such a scan, it is, of course,
necessary to elicit NMR signals from specific locations in the subject.
This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and
G.sub.z) which have the same direction as the polarizing field B.sub.0,
but which have a gradient along the respective x, y and z axes. By
controlling the strength of these gradients during each NMR cycle, the
spatial distribution of spin excitation can be controlled and the location
of the resulting NMR signals can be identified.
NMR data for constructing images can be collected using one of many
available techniques, such as multiple angle projection reconstruction and
Fourier transform (FT). Typically, such techniques comprise a scan made up
of a plurality of sequentially implemented views. Each view may include
one or more NMR experiments, each of which comprises at least an RF
excitation pulse and a magnetic field gradient pulse to encode spatial
information into the resulting NMR signal. As is well known, the NMR
signal may be a free induction decay (FID) or, preferably, a spin-echo
signal.
The preferred embodiments of the invention will be described in detail with
reference to a variant of the well known FT technique, which is frequently
referred to as "spin-warp". The spin-warp technique is discussed in an
article entitled "Spin Warp NMR Imaging and Applications to Human
Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and
Biology, Vol. 25, pp. 751-756 (1980).
Spin-echo imaging using relatively long echo times (TE) is a very effective
diagnostic tool. Typically, such sequences have a fixed echo time (TE) of
80 to 100 milliseconds and a scan is comprised of 256 views with one or
two averages. The main drawback is the long scanning time, as long as 17
minutes. To reduce total scan time, clinicians often reduce the number of
views and accept the consequent reduction in resolution. For example, the
scan time can be reduced 25% if the number of views is reduced from 256 to
192, however, the resulting image resolution is also reduced from 256 by
256 to 192 by 256.
The relatively long echo times (TE) are required to provide the desired
T.sub.1 and T.sub.2 weighting in the image. For example, echo times of 80
to 100 milliseconds are preferred for revealing brain pathology, while
much shorter echo times of from 15 to 30 milliseconds generally provide
T.sub.1 weighting and are preferred, for example, when forming images in
conjunction with contrast agents. Whatever echo time is selected, the
pulse sequence remains fixed during the entire scan to provide a set of
data from which an image having the desired T.sub.1 and T.sub.2 contrast
weighting is produced.
SUMMARY OF THE INVENTION
The present invention relates to a method for performing an NMR scan to
acquire image data in which the echo time and/or the repetition time are
varied during the scan. More specifically, a scan is performed by
conducting a series of pulse sequences in which each pulse sequence has a
repetition time TR and includes an excitation pulse followed by a phase
encoding gradient pulse and followed at a time TE by an NMR signal, and in
which the phase encoding gradient pulse is stepped through a set of values
to acquire an NMR image data set, and the time TE or the time TR, or both
the times TE and TR are varied during the scan to acquire NMR signals in
the NMR image data set at a plurality of TE times.
A general object of the invention is to reduce the scan time necessary to
acquire an NMR image data set. It has been discovered that if the central
views are acquired with a desired TR time, the peripheral views (i.e. high
spatial frequency) can be acquired with shorter TR times without
significantly changing the diagnostic character of the image. With a
shorter TR time for many of the views, the total scan time can be reduced.
It has also been discovered that the time TE can be similarly shortened
during the acquisition of the peripheral views without significantly
changing the diagnostic value of the reconstructed image. This enables a
further shortening of the repetition time TR, and hence, the total scan
time.
A more specific object of the invention is to weight the contrast
differently for different sizes of objects in the reconstructed image. By
using longer TE times for the central views and shorter TE times for the
peripheral views, the reconstructed image will have T.sub.1 and T.sub.2
weighted characteristics which are different for large objects than for
small objects. This is due to the fact that the central views contribute
to the definition of large objects in the reconstructed image, whereas the
peripheral views contribute to the definition of small objects and
features.
Yet another specific object of the invention is to increase the
signal-to-noise ratio of the acquired NMR data. By shortening the echo
time TE for peripheral views, the SNR for those views is increased and the
SNR for the entire data set is thus increased.
The foregoing and other objects and advantages of the invention will appear
from the following description. In the description, reference is made to
the accompanying drawings which form a part hereof, and in which there is
shown by way of illustration a preferred embodiment of the invention. Such
embodiment does not necessarily represent the full scope of the invention,
however, and reference is made therefore to the claims herein for
interpreting the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an NMR system which employs the present
invention;
FIG. 2 is an electrical block diagram of the transceiver which forms part
of the NMR system of FIG. 1;
FIG. 3 is a graphic representation of a conventional NMR pulse sequence
used to acquire data to produce an image;
FIG. 4 is a pictorial representation of how an image is reconstructed from
the NMR data acquired using the pulse sequence of FIG. 3;
FIGS. 5A and 5B are graphic representations of the manner in which echo
time TE and repetition time TR may be varied throughout a scan; and
FIGS. 6A-6D are graphic representations showing how the pulse sequence of
FIG. 3 can be changed by shortening TE, TR or both.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring FIG. 1, there is shown in block diagram form the major components
of a preferred NMR system which incorporates the present invention and
which is sold by the General Electric Company under the trademark "SIGNA".
The overall operation of the system is under the control of a host
computer system generally designated 100 which includes a main computer
101 (a Data General MV4000). The computer 100 includes an interface 102
through which a plurality of computer peripheral devices and other NMR
system components are coupled to the main computer 101. Among the computer
peripheral devices is a magnetic tape drive 104 which may be utilized
under the direction of the main computer 101 for archiving patient data
and image data to tape. Processed patient data may also be stored in an
image disc storage device designated 110. An array processor 106 is
utilized for preprocessing acquired NMR data and for image reconstruction.
The function of image processor 108 is to provide interactive image
display manipulation such as magnification, image comparison, grayscale
adjustment and real time data display. The computer system 100 also
includes a means to store raw NMR data (i.e. before image construction)
which employs a disc data storage system designated 112. An operator
console 116 is also coupled to the main computer 101 by means of interface
102, and it provides the operator with the means to input data pertinent
to a patient study as well as additional data necessary for proper NMR
system operation, such as calibrating, initiating and terminating scans.
The operator console is also used to display images stored on disc or
magnetic tape.
The computer system 100 exercises control over the NMR system by means of a
system control 118 and a gradient amplifier system 128. Under the
direction of a stored program, the computer 100 communicates with system
control 118 by means of a serial communication network 103 (such as the
Ethernet network) in a manner well known to those skilled in the art. The
system control 118 includes several subsystems such as a pulse control
module (PCM) 120, a radio frequency transceiver 122, a status control
module (SCM) 124, and power supplies generally designated 126. The PCM 120
utilizes control signals generated under program control by main computer
101 to generate digital waveforms which control gradient coil excitation,
as well as RF envelope waveforms utilized in the transceiver 122 for
modulating the RF excitation pulses. The gradient waveforms are applied to
the gradient amplifier system 128 which is comprised of G.sub.x, G.sub.y
and G.sub.z amplifiers 130, 132 and 134, respectively. Each amplifier 130,
132 and 134 is utilized to excite a corresponding gradient coil in an
assembly designated 136 which is part of a magnet assembly 146. When
energized, the gradient coils generate magnetic field gradients G.sub.x,
G.sub.y and G.sub.z.
The gradient magnetic fields are utilized in combination with radio
frequency pulses generated by transceiver 122, RF amp 123 and RF coil 138
to encode spatial information into the NMR signals emanating from the
region of the patient being studied. Waveforms and control signals
provided by the pulse control module 120 are utilized by the transceiver
subsystem 122 for RF carrier modulation and mode control. In the transmit
mode, the transmitter provides a radio frequency signal to an RF power
amplifier 123 which then energizes RF coils 138 which are situated within
main magnet assembly 146. The NMR signals radiated by the excited spins in
the patient are sensed by the same or a different RF coil than is used for
transmitting. The signals are detected, amplified, demodulated, filtered,
and digitized in the receiver section of the transceiver 122. The
processed signals are transmitted to the main computer 101 by means of a
dedicated, unidirectional, high-speed digital link 105 which links
interface 102 and transceiver 122.
The PCM 120 and SCM 124 are independent subsystems both of which
communicate with main computer 101, peripheral systems, such as patient
positioning system 152, as well as to one another by means of serial
communications link 103. The PCM 120 and SCM 124 are each comprised of a
16-bit microprocessor (such as Intel 8086) for processing commands from
the main computer 101. The SCM 124 includes means for acquiring
information regarding patient cradle position, and the position of the
moveable patient alignment light fan beam (not shown). This information is
used by main computer 101 to modify image display and reconstruction
parameters. The SCM 124 also initiates functions such as actuation of the
patient transport and alignment systems.
The gradient coil assembly 136 and the RF transmit and receiver coils 138
are mounted within the bore of the magnet utilized to produce the
polarizing magnetic field. The magnet forms a part of the main magnet
assembly which includes the patient alignment system 148, a shim coil
power supply 140, and a main magnet power supply 142. The main power
supply 412 is utilized to bring the polarizing field produced by the
magnet to the proper operating strength of 1.5 Tesla and is then
disconnected.
To minimize interference from external sources, the NMR system components
comprised of the main magnet assembly, the gradient coil assembly, and the
RF transmit and receiver coils, as well as the patient-handling devices,
are enclosed in an RF shielded room generally designated 144. The
shielding is generally provided by a copper or aluminum screen network
which encloses the entire room. The screen network serves to contain the
RF signals generated by the system, while shielding the system from RF
signals generated outside the room.
Referring particularly to FIGS. 1 and 2, the transceiver 122 includes
components which produce the RF excitation field B.sub.1 through power
amplifier 123 at a coil 138A and components which receive the resulting
NMR signal induced in a coil 138B. The base, or carrier, frequency of the
RF excitation field is produced by a frequency synthesizer 200 which
receives a set of digital signals through the communications link 103 from
the main computer 101. These digital signals indicate the frequency which
is to be produced at an output 201 at a resolution of one Hertz. This
commanded RF carrier is applied to a modulator 202 where it is frequency
and amplitude modulated in response to signals received through line 203,
and the resulting RF excitation signal is turned on and off in response to
a control signal which is received from the PCM 120 through line 204. The
magnitude of the RF excitation pulse output through line 205 is attenuated
by a transmit attenuator circuit 206 which receives a digital signal from
the main computer 101 through communications link 103. The attenuated RF
excitation pulses are applied to the power amplifier 123 that drives the
RF transmitter coil 138A.
Referring still to FIGS. 1 and 2, the NMR signal produced by the excited
spins in the subject is picked up by the receiver coil 138B and applied to
the input of a receiver 207. The receiver 207 amplifies the NMR signal and
this is attenuated by an amount determined by a digital attenuation signal
received from the main computer 101 through link 103. The receiver 207 is
also turned on and off by a signal through line 208 from the PCM 120 such
that the NMR signal is acquired only over the time intervals required by
the particular acquisition being performed.
The received NMR signal is demodulated by a quadrature detector 209 to
produce two signals I and Q that are coupled through anti-aliasing filters
216 and 217 to a pair of analog to digital converters indicated
collectively at 218. The quadrature detector 209 also receives an RF
reference signal from a second frequency synthesizer 210 and this is
employed by the quadrature detector 209 to sense the amplitude of that
component of the NMR signal which is in phase with the transmitter RF
carrier (I signal) and the amplitude of that component of the NMR signal
which is in quadrature therewith (Q signal).
The I and Q components of the received NMR signal are continuously sampled
and digitized by the A/D converter 218 at a sample rate of 64 kHz
throughout the acquisition period. A set of 256 digital numbers are
acquired for each I and Q component of the NMR signal, and these digital
numbers are conveyed to the main computer 101 through the serial link 105.
The NMR system of FIG. 1 performs a series of pulse sequences to collect
sufficient NMR data to reconstruct an image. One such pulse sequence is
shown in FIG. 3. This sequence performs a slice selection by applying a
90.degree. selective RF excitation pulse 300 in the presence of a z axis
gradient pulse 301 and its associated rephasing pulse 302. After an
interval TE/2, a 180.degree. selective RF excitation pulse 303 is applied
in the presence of another z axis gradient pulse 304 to refocus the
transverse magnetization at the echo time TE and produce an echo NMR
signal 305.
To position encode the echo NMR signal 305, an x axis read-out gradient
pulse 306 is applied during the acquisition of the NMR signal 305. The
read-out gradient frequency encodes the NMR signal 305 in the well known
manner. In addition, the echo NMR signal 305 is position encoded along the
y axis by a phase encoding gradient pulse 307. The phase encoding gradient
pulse 307 has one strength during each echo pulse sequence and associated
NMR echo signal 305, and it is typically incremented in steps through 256
discrete strengths (-128 to +128) during the entire scan. As a result,
each of the 256 NMR echo signals 305 acquired during the scan is uniquely
phase encoded.
It is, of course, usual practice to repeat the pulse sequence for each
phase encoding gradient value one or more times and to combine the
acquired NMR signals in some manner to improve signal-to-noise and to
offset irregularities in the magnetic fields. In the following discussion,
it is assumed that such techniques may be used. Also, while an RF
referenced spin echo signal is produced in the pulse sequence of FIG. 3,
it is also possible to practice the present invention using a pulse
sequence that produces a gradient-recalled echo signal.
Referring particularly to FIG. 4, the acquired NMR data is stored in the
data disk 112 (FIG. 1) in the form of two 256X256 element arrays 310 and
311. The array 310 contains the in-phase magnitude values I and the array
311 contains the quadrature values Q. Together these arrays 310 and 311
form an NMR image data set which defines the acquired image in what is
referred to in the art as "k-space".
To convert this k-space NMR data set into data which defines the image in
real space (i.e. Cartesian coordinates), a two step Fourier transformation
is performed on the I and Q arrays 310 and 311. The transformation is
performed first in the read-out direction which is the horizontal rows of
the arrays 310 and 311 to produce two 256X256 element arrays 312 and 313.
The array 312 contains the in-phase data and is labeled I', while the
array 313 contains the quadrature data and is labeled Q'. The I' and Q'
arrays 312 and 313 define the acquired image in what is referred to in the
art as "hybrid-space". This first transformation of the acquired NMR data
set is expressed mathematically as follows:
##EQU1##
The second transformation is performed in the phase encoding direction
which is the vertical columns of the arrays 312 and 313 to produce two
256X256 element arrays 314 and 315. The array 314 contains the transformed
in-phase values and is labeled I", while the array 315 contains the
quadrature values and is labeled Q". This second transformation may be
expressed mathematically as follows:
##EQU2##
The arrays 314 and 315 are a data set which defines the acquired image in
real space and the elements thereof are used to calculate the intensity
values in a 256X256 element image array 316 in accordance with the
following expression:
##EQU3##
The 256X256 elements of the image array 316 are mapped to the main
operator console 116 (FIG. 1) for display on a CRT screen.
The above described NMR system and pulse sequence produces an image in
which the contrast may be determined by spin density alone or it may be
enhanced by T.sub.1 or T.sub.2 effects. For example, cerebrospinal fluid
(CSF) exhibits a considerably longer T.sub.1 and T.sub.2 than brain
tissue, and these differences can be exploited to produce different
contrast effects in the reconstructed image. If TE is short, brain tissue
appears brighter than CSF, and if TE is set to a larger value, CSF appears
brighter than brain tissue. A range of contrast effects can be achieved by
selecting an echo time TE intermediate these two extremes.
In prior NMR systems the desired echo time is fixed at the beginning of the
scan to achieve the desired contrast enhancement and the entire scan is
conducted with this fixed echo time. This is illustrated graphically in
FIG. 5 where the echo time TE is plotted as a function of view number. As
shown by line 325, for all views, or phase encoding values (k.sub.y),
ranging from -N/2, through 0 to +N/2, the echo time TE remains fixed at a
constant value TE.sub.L. In contrast, it is the teaching of the present
invention that the echo time TE can be varied as a function of view
number.
One such variation in echo time TE as a function of view number is shown in
FIG. 5A by dotted line 326. In this preferred embodiment of the invention
the echo time TE is maintained at the longer time period TE.sub.L for the
central views of the scan (-n.sub.1 through +n.sub.1). Over a range of
peripheral views (-n.sub.2 through -N/2 and +n.sub.2 through +N/2) the
short echo time TE.sub.S is maintained at a constant value.
Referring particularly to FIG. 3, the echo time TE is varied by controlling
the timing between the 90.degree. RF excitation pulse 300 and the
180.degree. RF excitation pulse 303. This timing is determined by the
pulse control module 120 (FIG. 1) which generates command signals over the
link 103 to the transceiver 122 as explained above. Of course, when the
time TE/2 is changed, the timing of the slice select pulse 304 must also
be changed and the timing of the readout gradient pulse 306 must be
changed to maintain its symmetry about the center of the echo NMR signal
305.
In the preferred embodiment 256 views are acquired (N/2=128). The central
views have a long echo time TE.sub.L of 80 milliseconds and a short echo
time TE.sub.S of 20 milliseconds. The central region where the echo time
TE.sub.L is employed ranges from -n.sub.1 =-20 to +n.sub.1 =20. The
peripheral regions where the short echo time TE.sub.S is employed begins
at n.sub.2 =60. The echo time TE diminishes linearly in the transition
regions (n.sub.1 through n.sub.2).
The image which is reconstructed from NMR data acquired with the variable
echo time TE of FIG. 5A is not reduced in diagnostic value. The image
tends to have an increase in edge definition compared to that of an image
acquired with a constant echo time TE.sub.L. Likewise, small objects tend
to look somewhat more intense because the higher spatial frequencies are
emphasized. This emphasis is caused by the shorter TE which produces an
echo signal that has undergone less T.sub.2 decay. This same increase in
echo signal strength also results in an improved signal-to-noise ratio
which improves image quality.
Another aspect of the invention is to employ a variable repetition time TR
during the scan. This is illustrated in FIG. 5B where solid line 325'
indicates a normal scan with constant repetition time TR.sub.L for all
views, and dashed line 326' illustrates a preferred embodiment of the
invention in which repetition time is shortened to TR.sub.S for the
peripheral views. The break points .+-.n.sub.1 and .+-.n.sub.z in this
scan are the same as those described above for FIG. 5A.
When the variable repetition time is employed in an NMR scan, the total
scan time is directly reduced. The scan time is given by
T.sub.S =TR.N.NEX
where:
TR=Pulse repetition time which is longer than TE and is usually much longer
to reduce T.sub.1 effects;
N=number of views; and
NEX=number of repetitions of each view.
In the preferred embodiment, a reduction of 30% is achieved in total scan
time.
It should be apparent to those skilled in the art that performance may be
improved by employing either variable echo times TE, or variable
repetition times TR, or both. This is illustrated in FIGS. 6A through 6D,
where FIG. 6A illustrates a pulse sequence in which both times are at
their maximum values TE.sub.L and TR.sub.L. FIG. 6B shows a pulse sequence
in which the repetition time is shortened to TR.sub.S to reduce total scan
time, and FIG. 6C shows a pulse sequence in which echo time is shortened
to TE.sub.S to improve signal-to-noise. FIG. 6D shows a pulse sequence in
which both times have been shortened to TE.sub.S and TR.sub.S.
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