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BACKGROUND OF THE INVENTION
This invention relates to test devices, generally referred to as phantoms.
More specifically, this invention relates to a phantom useful with nuclear
magnetic resonance (NMR) scanner apparatus to carry out performance and
calibration measurements in three dimensions without repositioning the
phantom.
A phantom generally comprises a test object constructed to simulate
structures and conditions encountered in actual use. In the case of
medical diagnostic equipment, the phantom can be made to simulate various
types of tissue and can be used as a substitute test object in operator
training, as well as a calibration device to ascertain the level of
equipment performance. In some cases, it is desirable to ascertain the
degree of equipment operability by daily calibration procedures. The
phantom, therefore, must be constructed to allow evaluation of multiple
image quality parameters with relative ease and a minimum expenditure of
operator time and effort. Accordingly, factors such as scan time to
acquire the test data, phantom set-up time, phantom weight, and cost must
be minimized. Conversely, factors such as realiability, repeatability and
simplicity must be maximized.
Phantoms have been utilized in the past in such diagnostic modalities as
transmission computed tomography (CT) and digital radiography. Phantoms
for use with NMR apparatus, however, must meet different performance
requirements than those of other modalities. This is due, at least in
part, to the fact that NMR scanner operation is different from other
modalities in that it is capable of detecting tissue parameters which are
not measureable by any other means. Additionally, NMR has significantly
longer imaging times, of the order of five minutes, than the
afore-mentioned modalities, so that the need to optimize phantom
performance is apparent. To better appreciate the unique requirements
associated with NMR phantoms, it is beneficial to consider some
fundamental NMR scanning principles.
By way of background, the nuclear magnetic resonance phenomenon occurs in
atomic nuclei having an odd number of protons or neutrons. Due to the spin
of the protons and the neutrons, each such nucleus exhibits a magnetic
moment, such that, when a sample including such nuclei is placed in a
static, homogeneous magnetic field, B.sub.o, a greater number of nuclear
magnetic moments align with the field to produce a net macroscopic
magnetization M in the direction of the field. Under the influence of the
B.sub.o magnetic field, the magnetic moments precess about the axis of the
field at a frequency which is dependent on the strength of the applied
magnetic field and on the characteristics of the nuclei. The angular
precession frequency .omega., also referred to as the Larmor frequency, is
given by the Lamor equation .omega.=.gamma.B, in which .gamma. is the
gyromagnetic ratio which is constant for each NMR isotope and wherein B is
the magnetic field acting upon the nuclear spins. It will be thus apparent
that the precession frequency is dependent on the strength of the magnetic
field in which the sample is positioned.
In order to observe an NMR signal, the orientation of magnetization M,
normally directed along the magnetic field B.sub.o, must be perturbed by
the application of a magnetic field oscillating at the Larmor frequency so
as to create a transverse magnetization component in a plane orthogonal to
the field B.sub.o. This is accomplished by applying a magnetic field,
designated B.sub.1, in a plane orthogonal to the direction of the static
field B.sub.o by means of radio frequency (RF) excitation pulses through
coils connected to RF transmitting apparatus. The effect of field B.sub.1
is to rotate magnetization M in the volume of the object being studied
which lies in the field of the RF coil. When the RF excitation is removed,
magnetization M returns to its equilibrium position by a variety of
processes and in the course of doing so, generates a detectable NMR
signal.
While it is adequate for some purposes to simply detect the NMR signal
originating from the entire volume lying within the field of the coil, it
is frequently necessary to identify spatially where in the volume the NMR
signal originates. One such application is in NMR imaging. Spatial
localization is achieved by the application of the G.sub.x, G.sub.y and
G.sub.z magnetic-field gradients directed along the x, y, and z axes of
the conventional Cartesian coordinate system. The gradients are generally
of the form
G.sub.x (t)=.differential.B.sub.o /.differential.x
G.sub.y (t)=.differential.B.sub.o /.differential.y
G.sub.z (t)=.differential.B.sub.o /.differential.z
The G.sub.x, G.sub.y, and G.sub.z gradients are constant throughout the
imaging volume, but their magnitudes are typically time dependent. The
gradients are utilized with radio frequency excitation pulses in various
imaging techniques, such as those conventionally referred to as
multiple-angle-projection reconstruction, and spin warp to acquire
spatially resolvable NMR information.
A refinement of the technique to localize the NMR signal to a particular
volume of interest (such as a slice, for example), rather than the sample
volume lying within the field of the RF coil, is to utilize RF excitation
pulses which are modulated to have a predetermined frequency content. Such
RF pulses applied in the presence of magnetic field gradients are
effective in exciting nuclear spins situated in preselected regions of the
sample having resonant frequencies as predicted by the afore-described
larmor equation. Radio frequency pulses modulated in this manner are
referred to as being "selective." These should be contrasted to
non-selective RF pulses which are applied in the absence of magnetic field
gradients, as disclosed hereinbefore, and which affect all of the nuclear
spins in the field of the coil.
It will be therefore recognized that judicious choice of RF and gradient
pulses permits NMR information to be acquired from any preselected plane
within the object. Typically, it is possible to collect NMR data to permit
image reconstruction in any of three orthogonal planes of the object. The
primary planes are generally referred to as the coronal, axial, and
sagittal planes. The NMR data acquisition process is, however, not limited
to these planes but is capable of acquiring data from oblique planes as
well. With such multiplanar data-acquisition capability, it is desirable
to test system operation in at least some representative orientations
(e.g., coronal, sagittal, and axial planes). If performance is
satisfactory for these, it can then be assumed that the system will
operate satisfactorily in other orientations. This should be contrasted to
CT where imaging information is acquired by measuring x-ray attenuation
through the object slice of interest in a single transverse plane
coincident with the plane of the x-ray beam. In CT, only if several
contiguous planes are scanned, by advancing the object through the x-ray
beam, can images corresponding to other orientations be calculated
indirectly. In CT, therefore, there is no need for multiplanar system test
capability.
Thus, it is apparent that a need exists in NMR for a phantom having the
capability to provide multiplanar test data regarding NMR system operation
without requiring the phantom to be repositioned, and with ease and low
cost as described hereinbefore. The phantom should also provide a
multi-parameter testing capability. It is, therefore, a principal object
of the present invention to provide such a phantom.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided a phantom for testing
the performance of an NMR scanner having the capability of acquiring NMR
data from nuclei situated in a plurality of planes within an object. The
phantom includes at least first and second test plates each including
means useful in testing at least one performance parameter of the NMR
scanner. The test plates are arranged relative to one another so as to be
simultaneously positionable in use at the isocenter of the NMR scanner to
enable testing NMR scanner performance in each of the planes containing
the test plate without the need to reposition the phantom.
In the preferred embodiment, the phantom is made up of three mutually
orthogonal test plates which enable NMR scanner performance to be tested
in each of the axial, sagittal and coronal planes without requiring the
phantom to be repositioned.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth with
particularly in the appended claims. The invention itself, however, both
as to its organization and method of operation, together with further
objects and advantages thereof, may best be understood by reference to the
following description taken in conjunction with the accompanying drawings
in which:
FIG. 1 is a perspective view of the inventive phantom;
FIG. 2 is a front view of a single element of the inventive phantom
illustrating details of construction;
FIG. 2A is a sectional view taken along line 2A--2A shown in FIG. 2.
FIG. 3 is an exploded view of the new phantom illustrating the manner in
which the constituent elements are configured for assembly in one
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, there is shown a perspective view of the inventive
three-dimensional NMR phantom, generally designated by reference numeral
10. The phantom comprises three mutually orthogonal test plate elements
12, 14, and 16 useful in testing NMR system parameters in the sagittal,
axial, and coronal planes, respectively. The test plates are fabricated
from materials compatible with NMR applications. Such materials are, for
example, non-metallic, non-magnetic, and non-hydroscopic. Suitable
materials include various types of polycarbonate, acrylic, and polystyrene
plastics. Acrylic materials are preferred due to their desirable bonding
and machining properties. In the embodiment depicted in FIG. 1, test
plates 12, 14, and 16 are shown as having a circular configuration so that
the assembled phantom is spherical. It will be recognized, of course, that
the circular/spherical geometry is merely exemplary and, in fact, other
suitable geometries may be advantageously employed. For example, in the
embodiment depicted in FIG. 3 (which will be described in greater detail
hereinafter) sagittal plate 12 and coronal plate 16 both have a
rectangular configuration.
Regardless of the geometry selected, the test plates are enclosed in a
protective shell having the appropriate configuration (e.g., spherical or
rectangular). The shell, which may be fabricated from any of the suitable
materials described hereinabove, is not shown to preserve figure clarity.
In a completed phantom, the shell is filled with an NMR-active substance,
preferably a liquid such as one of the glycerine, copper sulfate, or
magnesium chloride solution. The fluid simulates in-vivo tissue, while the
areas without liquid simulate substantially non-NMR-active regions, such
as bone. For example, the fluid acts to absorb radio-frequency energy, and
emits NMR signals in a manner which approximates a human head. Other areas
in the test plate (such as those designated 18 in FIG. 1) may be filled
with fluids or gels which differ in their NMR properties providing
additional flexibility and utility in the use of the phantom.
Continuing with reference to FIG. 1, a position marker 20, comprised of two
orthogonal alignment lines, is provided, preferably on the phantom shell,
so as to locate the center of intersection between the axial and sagittal
test plates 14 and 12, respectively. Another position marker 21 is
provided laterally on coronal plate 16. A similar marker (not visible in
FIG. 1) is provided on the side of the coronal plate opposite that on
which marker 21 is located. Overhead and lateral laser patient alignment
lights (not shown), which typically form part of the NMR scanner, can then
be used in conjunction with position markers 20 and 21, respectively, to
align center 22 of the phantom with the center of the homogeneous region
of magnetic field B.sub.o located within the magnet bore and commonly
referred to as the system isocenter. The isocenter also coincides with the
zero gradient point corresponding to the intersection of the G.sub.x,
G.sub.y, and G.sub.z gradient magnetic fields discussed previously. When
aligned with the isocenter, the phantom is positioned such that the
coronal, sagittal, and axial plates can be imaged by the NMR scanner
without repositioning the phantom. The position markers enable the phantom
to be identically repositioned within the NMR system as daily calibration
studies are performed, thereby enabling meaningful comparison of
day-to-day calibration tests. Such tests are useful in determining whether
system performance has departed from pre-established levels.
The detailed construction of one embodiment of a test plate will be
disclosed next by way of example with reference to axial test plate 14
(FIG. 1), a front view of which is depicted in FIG. 2. It will be, of
course, understood that functionally test plates 12 and 16 are
substantially identical to plate 14. Referring now to FIG. 2, test plate
14 may comprise a circular member having a diameter of, for example, 8.75
inches and being 0.75 inch thick. An array of square cells, such as those
designated 24, is disposed in a generally square area located on the face
of test plate 14. The cells are created by sets of narrow, parallel
dividers 26 which are arranged orthogonal to a second set of parallel
dividers 28. The cells are adapted for holding an NMR-active liquid used
to fill the protective shell in a completed phantom. The array comprises
an image distortion grid and is used to visually (or through computer
image analysis) inspect the image for warping or distortion in the square
apertures. The distortion grid is composed of straight lines due to the
fact that the human eye is particularly sensitive to irregularities in
straight-line patterns. Such irregularities would be indicative of
non-uniformities in the main magnetic field B.sub.o or the G.sub.x,
G.sub.y, and G.sub.z gradient fields. It will be, of course, recognized
that the exact configuration need not be as described. All that is
required is a substantially rectilinear pattern in which warped or
distorted lines are easily recognizable.
Referring again to FIG. 2, the image distortion grid described above is
divided into four substantially triangularly-shaped areas by a pair of
diagonal channels 23 and 25, the extremes of which are designated A, A',
and B, B', respectively. These channels are used to perform an image
uniformity test. The test is accomplished by locating a line N-pixels wide
along each of the diagonal areas between points A-A' and B-B'. The
standard deviation of the pixel values is then calculated and compared to
values previously set for the system. Departures from the preset values
would be indicative of non-uniformity in the B.sub.o, B.sub.1, G.sub.x,
G.sub.y, and G.sub.z magnetic fields.
Test plate 14 also includes two high contrast resolution grids, generally
designated 42 and 44, each made up of a series of regularly shaped cells
30-34 and 36-40, respectively, which are filled with an NMR-active fluid.
In the preferred embodiment, these cells (as well as those of the image
distortion grid) are in liquid communication with the remainder of the
phantom so that the fluid held within the protective shell fills the
cells. In pattern 42, the cells are more closely spaced than those of
pattern 44 and are designed to test for fine resolution using, for
example, a head radio-frequency coil which is capable of higher resolution
than the body radio-frequency coils. Test pattern 44, having wider
separations than pattern 42, is used to test for resolution in body
images.
The manner in which the resolution test is performed is identical for both
patterns and will be described by way of example with reference to pattern
42. To perform the test, pixel values in a line, such as the one
designated by reference numeral 46, N pixels wide, are sampled. The
percent modulation of the pixel values along line 46 is then calculated.
The percent modulation is the total range of pixel values divided by the
modulation range of the values. The higher the percent of modulation, the
higher the resolution. Again, comparison the test value is compared to a
previously recorded file value to determine whether a change in resolution
has occurred which may be indicative of sub-standard system performance.
Test plate 14 is also provided with special compartments 48 and 50 which
are isolated from the fluid used to fill the protective shell.
Compartments 48 and 50 are used in performing a contrast ratio test.
Compartments 48 and 50 are filled through openings 52 and 54,
respectively, with a specific NMR-active gel or liquid material having
T.sub.1, T.sub.2, and nuclear spin-density constants comparable to those
of human tissue over a range of B.sub.o magnetic field strengths of
between about 0.35 tesla and 2.0 tesla. Moreover, the materials are
selected to have different NMR constants relative to one another.
Additionally, the constant values for each material are selected to fall
into different parts of the range of T.sub.1, T.sub.2, and spin-density
values for human tissue. The materials must also be selected so as to be
non-toxic, inert with respect to the material from which the phantom is
constructed and must also be temporally stable. Reagent grade materials
are preferred to ensure quality uniformity. It has been found that
solutions of reagent grade glycerine with water to achieve 85% and 15%
glycerine concentrations work satisfactorily.
The contrast ratio is determined by defining areas 56 and 58 in
compartments 48 and 50, respectively, and calculating the means pixel
values in each area. The contrast ratio is the quotient of the sum of mean
pixel values in areas 56 and 58 and the difference therebetween.
A third compartment 58, similar to compartments 48 and 50, is provided in
test plate 14 and is used in calculating the signal-to-noise ratio (S/N).
This is accomplished by introducing an NMR-active material (such as 100%
glycerine) into compartment 58 through a fill opening 60. The
signal-to-noise ratio is then calculating by locating an area, such as
that designated 62, centered in compartment 58 and an area 64 comprising
material which forms part of test plate 14 and, therefore, is
substantially non-NMR active. The signal-to-noise ratio is determined by
calculating the mean pixel values in area 62 and dividing by the standard
deviation of area 64.
The thickness of the selected imaging slice can be tested by a means
generally designated 66 in FIG. 2, a sectional view of which, taken along
line 2A--2A, is shown in FIG. 2A. Referring now to FIGS. 2 and 2A, means
66 is provided with a slot 68 which is adapted for holding an NMR-active
fluid. As best seen in FIG. 2A, slice thickness, designated "T" is related
to the height "h" of slot 68. The slice thickness is determined by
considering that the NMR signal originates from the shaded portion of slot
68 as shown in FIG. 2A. The NMR signal is small in regions 70 and 72 and
increases toward the center of the shaded region as would be expected due
to increased volume of NMR-active fluid in that region. The slice
thickness is determined by taking the full width half maximum (FWHM) of
the resulting signal profile waveform.
The phantom can be constructed by any convenient method following any
desired assembly procedure. FIG. 3 depicts an exploded view of one
possible assembly technique. For example, axial test plate 14 may be
fabricated as a single circular piece, while coronal plate 16 can be
fabricated as two pieces 16a and 16b and sagittal plate 12 is fabricated
from four pieces 12a, 12b, 12c, and 12d. A set four grooves 80-83 is
provided on the face of plate 14 visible in FIG. 3. Test plate 14 is
provided on its other surface with a similar set of grooves (of which only
those designated 84 and 85 are visible) in general alignment with slots
80-83. Test plate halves 16a and 16b, which form part of the coronal test
plate 16, are each provided with a set of tongues, visible ones of which
are designated 88-90, which are sized to fit into the grooves machined in
axial plate 14. Thus, for example, groove 82 and tongue 90 form one joint
when bonded, while groove 85 and tongue 89 form another.
Similarly, elements 12a-12d, comprising sagittal plate 12 in the assembled
phantom, are provided with tongues which are sized to fit grooves formed
in the coronal plate 16 and in axial plate 14. Thus, by way of example,
element 12a is provided with tongues 92 and 93 which fit into grooves 81
and 98, respectively, formed in the axial and coronal plates,
respectively. Elements 12b-12d are provided with similar tongues which fit
into grooves provided in the axial and coronal plates and are assembled in
a manner identical to that described with reference to element 12a.
While this invention has been described with reference to particular
embodiments and examples, other modifications and variations will occur to
those skilled in the art in view of the above teachings. Accordingly, it
should be understood that within the scope of the appended claims the
invention may be practiced otherwise than is specifically described.
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