 |
Description  |
|
|
This invention relates to phantoms or test bodies generally, and in
particular to phantoms for use with nuclear magnetic resonance machines to
test the operational characteristics of the machine.
Nuclear magnetic resonance (NMR) techniques are being developed for use in
many fields for chemical analysis, one of the most important of which is
as a diagnostic tool in the medical field. For example, it has been
suggested that the NMR technique can be used to detect cancer in human and
animal tissue and to indicate the degree of malignancy (See U.S. Pat. No.
3,789,832 which issued Feb. 5, 1974 and is entitled "Apparatus and Method
for Detecting Cancer in Tissue"). Therefore, large NMR machines are being
developed and used to make measurements along planes passing through
portions of a human body.
An NMR machine measures three characteristics of the structure of an object
or substance: spin density, spin-lattice relaxation time, T1, and
spin-spin relaxation time, T2.
The person analyzing the results of an NMR scan must have a reasonable
knowledge of the operating characteristics of the particular machine that
he is using. For example, he must know generally the ability of the
machine to distinguish small objects, in other words, the resolution of
the machine, which is a measure of its ability to delineate detail or
distinguish objects or substances having nearly equal values of a quanity.
He should also know how accurate the machine is in measuring spin density,
T1, and T2.
It is therefore an object of this invention to provide a test body or
"phantom" that can be used by the operator of an NMR machine to measure
the performance characteristics of the machine by using substances having
known characteristics.
It is a further object of this invention to provide such a phantom that
indicates the resolution of an NMR machine.
These and other objects, advantages and features of this invention will be
apparent to those skilled in the art from a consideration of this
specification including the attached drawings and appended claims.
In the drawings:
FIG. 1 is an isometric view, partly in elevation and partly in section, of
a NMR machine of the type used for making whole body measurements showing
a patient positioned in the magnet section of the machine;
FIG. 2 is an isometric view of three phantom inserts of this invention with
a portion of the base member that supports the phantom inserts broken away
to show the various positions that the phantom inserts can be located on
the base member to obtain scans along different planes; actually thin
cross sections, through the sensitive volume areas of the NMR machine.
FIG. 3 is a sectional view taken along Line 3--3 of FIG. 2;
FIG. 4 is typical of the image produced by a scan through one of the
phantom inserts of FIG. 2 perpendicular to the longitudinal axis of the
tubular elements of the phantom;
FIG. 5 is another embodiment of the phantom of this invention; and
FIG. 6 is a cross sectional view taken along line 6--6 of FIG. 5.
FIG. 7 is a typical output signal produced by a scan through the leg FIG.
6.
In FIG. 1, NMR machine 10 is one designed for use in medical diagnosis by
measuring the spin density, T1, and T2 of the human body along selected
planes or slices through the body. Section 11 has cylindrical cavity 12 in
which the patent is positioned. The magnetic field and frequency that is
used to measure the NMR characteristics are provided by electronics
located in section 11. Only a portion of the cavity has the right mix of
magnetic field and RF frequency to provide the designed results. This is
called the "sensitive volume". The part of the patent to be examined is
located in the sensitive volume. The output from section 12 is an analog
electrical signal. Usually, it is fed to a computer having an
analog/digital converter, to a CRT, where the shape of the signal is
displayed, or to a tomographic instrument for displaying an image of the
section of the body along the plane that the scan was made or any
combination thereof.
In accordance with this invention, a phantom is provided for testing the
operational characteristics of an NMR machine that includes a plurality of
containers of non-magnetic material such as glass, each of which contain a
material having a known spin density, T1, or T2 characteristic that
differs a preselected amount from the spin density, T1, or T2
characteristic of the material in the other containers. It may be possible
to use solids in these phantoms, but at the present time T1 and T2 of any
appropriate solids are of such values that it is unpractical to use solid
materials at this time.
In FIG. 2, three phantoms are shown A, B, and C. Each phantom is identical
so only one, phantom A, will be described in detail.
In this embodiment, the containers are tubular members. They must be made
of a non-magnetic material. Glass and certain plastic materials are
suitable and convenient to use for this purpose. In this embodiment, glass
tubing serves as the containers.
In the phatom shown, a plurality of glass tubes are used. Each tube is
filled with a liquid having a known spin density, T1, or T2 characteristic
that differs a preselected amount from that same characteristic of the
liquid in the other tubes. For example, if this phantom is designed for
measuring spin density, then several tubes 20, five in this embodiment,
are arranged in a row with liquids that have, for example, spin densities
that are 100%, 90%, 80%, 70%, and 60% of spin density of distilled water.
The liquid used will have a T1 and T2 characteristic but it is difficult
enough to get the right progressive relationship of one characteristic
between five liquids without having to worry about getting that same
progressive relationship for T1 and T2. Therefore, each phantom will be
designed to measure one characteristic only. The use of a simply parametic
test object is encouraged to provide a less demanding and error
susceptible evaluation of machine performance. Complex compounds are
available, but the present state of the art of NMR imaging systems could
be unequally penalized in the evolution process by a complex solution of
T1, T2, and spin density materials. The same materials are placed in
intermediate tubes 22 and the smaller diameter tubes 24, although more
complex arrays could be chosen. The different diameter tubes provides a
test for resolution as well as accuracy at the same time that the operator
is measuring, for example, spin density. Preferably, the phantoms should
include tubes of a diameter small enough that the machine is not expected
to resolve them so the lower limit of performance will be confirmed and
the operator will know what this limit is.
Means are provided to support the containers of the phantoms in a
predetermined reproducible arrangement in the NMR machine to allow the
machine to measure the known characteristics of the liquids to test the
operational characteristics of the machine. The reproducible geometry for
testing is essential if valid comparisons are to be made of the same or
differential machines. In the embodiment shown in FIG. 2, tubes 20, 22,
and 24 are mounted on mounting plate 26. The mounting plate is supported
by base member 28, which is a rectangularly shaped, four-sided, box having
square or round openings 30 with coded indents located in the side walls
and bottom thereof. Mounting pin 32 as shown in FIG. 3, can extend into
any one of square openings 30 and support the containers of the phantom
inside base member 28 in the desired position. All components of the
phantom support means are made of a non-magnetic material, such as glass,
lucite, or lexan.
In FIG. 2, phantom A is mounted on the left hand side of the box, phantom B
at the far end, and phantom C on the bottom. With this arrangement, the
machine can be tested along three mutually perpendicular planes. For
example a scan can be taken through phantom A perpendicular to the
longitudinal axis of the tubes and parallel to the side wall 28a. The same
can be done through phantom B along a plane parallel to end wall 28b of
the base member and through phantom C along a plane parallel to bottom
28c. The phantoms then can be moved to various other locations in the box
as desired to obtain as an extensive test pattern over the sensitive
volume of the machine. The inserts can be mounted internally or externally
to the base frame in the same holes in the side and end walls.
FIG. 4 is typical of the image obtained by a scan along a plane that cuts
through all of the tubes. The bottom line is just noise indicating that
the lower set of tubes 24 could not be resolved.
An alternate embodiment of the phantom of this invention is shown in FIG.
5. The purpose of this phantom indicated generally by the number 40, is to
provide the same tests as would be obtained with the phantom shown in FIG.
2, but to also provide some additional information. Here, the lattice or
framework of a polyhedron is formed by a plurality of concentric glass
tubes with each tube forming one leg of the polyhedron. Each tube is
filled with a material having a known characteristic that differs a
preselected amount from that same characteristic of the liquid in the
other tubes. For example, as shown in FIG. 5, each leg 42 of phantom 40
consists of four concentric glass tubes 44, 46, 48, and 50. Assume this
phantom is for measuring T1. The liquid in central tube 44 can have a T1
of 300 milliseconds. The liquid in the annular space between tube 44 and
46, 400 milliseconds, the next 600 milliseconds, with the liquid in the
outer annulus having a T1 of 800 milliseconds. The arrangement could be
reversed, of course, with the T1 of the liquids increasing inwardly.
Preferably, the diameters of the tubes vary in some preselected rates. For
example, the diameter of each tube could increase by a factor of three.
D.sub.2 would equal 3D.sub.1, D.sub.3 would equal 3D.sub.2, etc.
Also, the tubes themselves could be used to provide a test of resolution.
For example, two concentric tubes having outside diameters equal to
D.sub.2 and D.sub.4 and inside diameters equal to D.sub.1 and D.sub.2
would be used where the material of the tubes have a preselected
characteristic that could be the same or different.
FIG. 7 shows a typical output signal for a scan through the leg of FIG. 6,
where inside tube 44 cannot be resolved.
In use, the polyhedron is positioned in the sensitive volume of a NMR
machine. It is self-supporting on any one of its faces. Slices through the
polyhedron can be taken along any desired plane. The angle that the slice
is taken can be confirmed from the resulting pattern of the cross-sections
of the legs. The ability of the NMR machine to measure the selected
characteristic plus its resolution ability can also be determined. The
angle that the cut is taken can be determined because unless the plane is
horizontal and exactly midway between the top and bottom of the
polyhedron, it will cut several of the concentric glass tubing legs at an
angle. Knowing the angle of the legs relative to each other and to the
horizontal, the computer can determine the angle of the cut by the
distance between the outer edges of the outer glass tube along the major
axis of the eliptical shape of the cut.
The phantoms described herein can be used to measure the following
performance characteristics of an NMR machine: precision, accuracy,
spatial, contrast, noise, slice thickness, linearity, reproducibility,
repositioning and positioning accuracy in the sensitive volume, indicated
plane versus actual plane, evaluate accuracy of image processing, as
summing, weighting, and subtracting, and pulse sequence precision and
accuracy for NMR parameter determination.
From the foregoing, it will be seen that this invention is one well adapted
to attain all of the ends and objects hereinabove set forth, together with
other advantages that are obvious and that are inherent to the apparatus.
It will be understood that certain features and subcombinations are of
utility and may be employed without reference to other features and
subcombinations. This is contemplated by and is within the scope of the
claims.
Since the phantom of this invention may take shape other than those
described, without departing from the scope of this invention, it is to be
understood that all matter herein set forth or shown in the accompanying
drawings is to be interpreted as illustrative and not in a limiting sense.
* * * * *
|
|
 |
|
 |
Description |
|
