U.S. patent number 4,458,203 [Application Number 06/326,361] was granted by the patent office on 1984-07-03 for nuclear magnetic resonance imaging.
This patent grant is currently assigned to Picker International Limited. Invention is credited to Ian R. Young.
United States Patent |
4,458,203 |
Young |
July 3, 1984 |
Nuclear magnetic resonance imaging
Abstract
A method of imaging a body by nuclear magnetic resonance wherein
volume scanning of a region of the body is achieved by scanning a
first planar slice of the region and at least one further slice of
the region in the relaxation time for the scan of the first
slice.
Inventors: |
Young; Ian R. (Middlesex,
GB2) |
Assignee: |
Picker International Limited
(Middlesex, GB2)
|
Family
ID: |
10517908 |
Appl.
No.: |
06/326,361 |
Filed: |
December 1, 1981 |
Foreign Application Priority Data
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Dec 11, 1980 [GB] |
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8039702 |
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Current U.S.
Class: |
324/309;
324/307 |
Current CPC
Class: |
G01R
33/4835 (20130101) |
Current International
Class: |
G01R
33/54 (20060101); G01R 033/08 () |
Field of
Search: |
;324/300,307,309 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1525564 |
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Sep 1978 |
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GB |
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2037996 |
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Jul 1980 |
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GB |
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Primary Examiner: Tokar; Michael J.
Attorney, Agent or Firm: Watts, Hoffmann, Fisher &
Heinke Co.
Claims
I claim:
1. A method of imaging a body by nuclear magnetic resonance using
data obtained by T.sub.1 experiments wherein volume scanning of a
region of the body is achieved by scanning for T.sub.1 data in each
of a plurality of planar parallel slices of said region by applying
in turn a series of .pi./2 radio frequency pulses, each with
magnetic field gradients for selecting a different one of said
slices, between successive ones of a sequence of .pi. radio
frequency pulses, the scan of at least one slice occurring in the
relaxation time for the scan of at least one other slice.
2. A method according to claim 1 wherein slice selection is
effected by controlling the frequency of the (.pi./2) pulses.
3. A method according to claim 1 wherein the slice selection field
gradients include a phase correction magnetic field gradient pulse
whose size is varied so as to resolve a scanned slice into a
plurality of subslices.
4. A method of imaging a body by nuclear magnetic resonance using
data obtained by T.sub.1 experiments, said method comprising the
steps of:
(a) producing a sequence of .pi. radio frequency pulses;
(b) applying in turn a series of 90 /2 radio frequency pulses, each
with a magnetic field gradient for selecting a different slice of
the body;
(c) scanning for T.sub.1 data in each of a plurality of planar
parallel slices of said region to achieve volume scanning of a
region, said .pi./2 frequency pulses taking place between
successive ones of said sequence of radio frequency pulses, and
(d) wherein said scanning of at least one slice occurs during the
relaxation time for the scan of another slice.
5. A system for imaging a body by nuclear magnetic resonance using
data obtained by T.sub.1 experiments, said system comprising:
(a) apparatus and circuitry for producing a series of .pi. radio
frequency pulses;
(b) second apparatus and circuitry for applying in turn a series of
.pi./2 radio frequency pulses, each with a magnetic field gradient
for selecting a different slice of the body, said .pi./2 frequency
pulses being timed to take place between successive ones of said
sequence of .pi. radio frequency pulses, said second apparatus and
circuitry comprising means for scanning for T.sub.1 data in each of
a plurality of planar parallel slices of said region to achieve
volume scanning of a region, and
(c) timing means for effecting said scanning of at least one slice
during the relaxation time for the scan of another slice.
Description
This invention relates to imaging a body by nuclear magnetic
resonance (NMR) and is particularly related to imaging a volume of
a body rather than a substantially planar slice. NMR imaging is
especially suitable for examination of the body of a patient for
medical purposes.
Proposals have been made for scanning volumes of bodies by NMR
using various specialised procedures. These are lengthy procedures
because, inter alia, on the assumption of three dimensional
scanning, n.sup.2 experiments are needed, with n lines from each
experiment, for an n.sup.3 volume. Thus, if the time needed for
relaxation, in each experiment is T.sub.R, the minimum scanning
time is n.sup.2 T.sub.R. It is an object of this invention to
provide a more rapid volume scanning procedure.
According to the invention in a method of imaging a body by nuclear
magnetic resonance volume scanning of a region of the body is
achieved by scanning in a first planar slice of the region and
scanning in at least one further planar slice of the region in the
relaxation time for the scan of the first slice.
In order that the invention may be clearly understood and readily
carried into effect an example thereof will now be described with
reference to the accompanying drawings in which:
FIG. 1 shows a pulse sequence of a conventional NMR imaging
method;
FIG. 2 illustrates a pulse sequence of a method according to the
present invention;
FIG. 3 is a diagram of an NMR apparatus suitable for carrying out
the method according to the invention;
FIG. 4 shows a distribution of field sensing probes used in the
apparatus of FIG. 3; and
FIG. 5 illustrates the circuits used to control the application of
pulses to the field coils of the apparatus of FIG. 2.
The basic principles of NMR imaging are now well documented, for
example, in U.S. Pat. No. 4,284,948 which is hereby incorporated in
the present specification by reference, and will not therefore be
described here.
The method to be described by way of example relates to imaging of
a body using so-called T.sub.1 experiments yielding data relating
to the spin-lattice relaxation time (T.sub.1) of protons in the
body under examination.
Referring to FIG. 1, in a normal T.sub.1 experiment a .pi. radio
frequency (RF) pulse is applied without magnetic field gradients
for slice selection, followed by a .pi./2 RF pulse, with field
gradients for slice selection after a relaxation period .tau.,
immediately following which the free induction decay (FID) is
measured. Thus the sequence used is, .pi. (unselected), .tau.,
.pi./2 (selected), FID measured.
The signal is then of the form M(1-2T.tau.) when M is the number of
available protons and T.tau.=e (-.tau./T.sub.1) (.tau..noteq.0.69
T.sub.1).
For volume scanning there is established a regular sequence of .pi.
pulses as shown in FIG. 1, at intervals of time t.sub.B. At some
time after one of these a slice is selected and imaged. If this is
at a time t.sub.A before the following pulse, and the experiment is
repeated with one slice selection for each pair of .pi. pulses,
then the signal is of the form M(1-T.sub.B)(1-2T.sub.C), where
##EQU1##
Now t.sub.B -t.sub.A for any slice is known, and can be written
kt.sub.B so the signal is of the form M(1-T.sub.B)(1-yT.sub.B),
that is of the form
where z and y are constants for a particular slice.
Similar relations exist for systems with more .pi. flips between
slice selections.
This indicates that most of each interval between .pi. pulses can
be used for scanning slices, as long as the slices scanned between
successive .pi. pulses are all different.
Thus, in accordance with the invention a series of .pi./2 RF pulses
each with field gradients for selecting a different slice are
applied in turn between successive .pi. pulses, as illustrated in
FIG. 2. If x is the number of slices scanned between successive
.pi. pulses then the minimum scanning time for an n.sup.3 volume is
reduced from the value of n.sup.2 T.sub.R stated above to
##EQU2##
As well as scanning different slices, the same slice may be scanned
for different relaxation times t.sub.B -t.sub.A.
The time taken to scan a single slice is typically about 50
milliseconds. Hence, with a typical value for t.sub.B of between
200 and 1,000 milliseconds, four to twenty different slices may be
scanned between successive .pi. pulses. Due to slice edge effects
contiguous slices cannot be scanned between successive .pi. pulses
so that even if it were possible to scan enough slices between
successive .pi. pulses to achieve adequate resolution through a
volume being examined, it would not be practical to do so. A
minimum scan time is thus obtained when x=0.5 n.
In practice it is easier using conventional apparatus to scan fewer
slices, and to achieve adequate resolution by varying the size of
the usual phase correction magnetic field gradient pulse applied
following the slice selection field gradient pulse. Thus, if the
correct phase correction gradient pulse is a fraction k of the
slice selection one, fractions (k-.delta.) and (k+.delta.) result
in gradients across the slice, and are analoguous to the gradient
steps in the modified echo planar imaging system (see United
Kingdom Patent Specification No. 2056078A). A slice can thus be
resolved into three subslices. Such a configuration can be used to
produce a volume scan in six times the time for a single slice,
with acceptable duty cycle for the slice selection system.
As an alternative it is possible to rephase z inexactly to give
small z resolution for up to eight slices.
The easiest way of effecting slice selection is to use a pulse of
frequency (f+.delta.f) for the .pi./2 RF pulses where .delta.f
ranges over a set of increments .+-.m.delta.f.sub.o (including
m.sub.o =0), and then demodulate with f (or f+offset). This
requires the RF coil system to be of relatively wide bandwidth, but
the receiver maintains a normal passband.
This invention may be implemented on a suitable NMR examining
apparatus such as that disclosed in U.S. Pat. No. 4,284,948 which
is shown in simplified form in FIG. 3. Illustrated schematically
only are: coils 6, which provide a steady B.sub.o field; 7, which
provide a G.sub.x field gradient and 8 which provide a G.sub.y
field gradient together forming a field in the z direction with a
gradient in a variable direction R orthogonal to the z direction;
9, which provide the RF fields; and 10, which provide a G.sub.z
field gradient. The coils are driven by B.sub.o G.sub.x, G.sub.y,
RF and G.sub.z drive amplifiers 11, 12, 13, 14, and 15
respectively, controlled by B.sub.o, G.sub.xy, RF and G.sub.z
control circuits 16, 17, 18 and 19 respectively. These circuits can
take suitable forms which will be well known to those with
experience of NMR equipment and other apparatus using coil induced
magnetic fields. The circuits are controlled by a central
processing and control unit 20 to achieve a desired pulse sequence
such as that of this invention.
The NMR signals subsequently sensed are received in this example by
the RF coils 9 and are amplified by an RF amplifier 21 before being
applied to signal handling circuits 22. In certain circumstances it
may be preferable to provide separate coils specifically designed
for the purpose, to sense the signal. The circuits 22 are arranged
to make any appropriate calibrations and corrections but
essentially transmit the signals to the processing circuits to
provide the required representation of the examined slice. These
circuits can conveniently be combined with the circuits which
control the pulse sequence and thus are included in the circuits
indicated at 20. The information thus obtained can be displayed on
a display 23, such as a television monitor, and this may include
inputs and other peripherals 24 for the provision of commands and
instructions to the machine, or other forms of output.
The apparatus also includes field measurements and error signal
circuits 25 which receive signals via amplifiers 26 from field
probes X.sub.1, X.sub.2, Y.sub.1, and Y.sub.2, shown. The positions
of the probes, in relation to the examined slice of the body 27 of
the patient, are further shown in FIG. 4. X.sub.1, X.sub.2, Y.sub.1
and Y.sub.2 are in this example simply miniature cells of pure, or
slightly doped water (such as a closed test tube) surrounded by a
small coil. Preferably the water is doped to have a suitable value
of T.sub.1, relaxation time-constant. The probes give a reliable
resonance of 4.26 kH.sub.z /Oe. Other types of probe may be used as
desired.
FIG. 5 shows, in schematic view, components which may typically
form part of the processing and control circuits 20 of FIG. 3. Four
profile stores PS1-4 store the required pulse shapes in terms of a
sequence of current amplitudes and the required duration (number of
clock pulses) at each amplitude. The specified current at any
instant is then supplied by the corresponding drive circuit (12,
13, 19 or 14) to the corresponding coil (7, 8, 10 or 9). The
operation of the four profile stores is controlled by a sequence
control store SCS which stores the sequence of activation of the
profile stores and the duration (number of clock pulses) of
operation of each stage of the sequence (including gaps in the
pulse sequence). A clock CL controls the overall timing operation
of the circuits.
Clearly this invention may be implemented by appropriately storing
in the profile and sequence control stores information about the
various field pulses required to be applied. Other implementations
are, of course, possible within the scope of the invention.
* * * * *