Radiant energy – Calibration or standardization methods
Reexamination Certificate
2002-04-18
2004-06-01
Lee, John R. (Department: 2881)
Radiant energy
Calibration or standardization methods
Reexamination Certificate
active
06744039
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to phantoms for nuclear imaging, and more specifically to a phantom fillable with a solution of a radioisotope for providing radioactive regions or “hot spots” within a less radioactive or “warm” background.
Medical physicists and researchers in nuclear imaging commonly use fillable phantoms for characterizing the imaging capabilities of both single photon emission computed tomography (SPECT) and positron emission tomography (PET) systems. Most common phantoms have simple designs and often are designed to measure specific imaging parameters, such as line sources for spatial resolution and open fillable chambers for uniformity. However, in recent years, there has been growing interest in more complex phantom designs to assess imaging performance in realistic imaging situations.
With the more widespread use of radiopharmaceuticals such as
18
F-fluorodeoxyglucose (FDG),
99m
Tc-Sestamibi, and
111
In-labeled and
131
I-labeled monoclonal antibodies for oncology imaging, the ability of SPECT and PET scanners to detect lesions of higher activity concentration with respect to the surrounding tissue is of high interest. To simulate the imaging of tumors in patients, phantoms with fillable spheres are typically used. Before imaging the phantom, the physicist or technologist fills the spheres and background chamber with solutions with the desired concentration ratio of activity. Although the design of this phantom is relatively simple, there are several disadvantages to this approach. The accuracy and reproducibility of the spheres-to-background concentration ratio is not guaranteed, since the steps of measuring the activities and volumes and of filling the chambers have the possibility of error. In addition, such phantoms with multiple fillable chambers are inconvenient to use because of the number of steps involved in preparation. The time required to fill these phantoms (typically 30 minutes) is costly, and it also prevents lesion detectability studies with isotopes with very short half-lives. A more convenient and more reproducible phantom design that simulates active lesions in a patient is therefore highly desirable.
Attenuation correction in nuclear medicine also is rising in significance. Attenuation correction provides a more quantitative uptake distribution in images, and many believe that more accurate diagnosis can be obtained. The commercial implementations of attenuation correction in nuclear imaging systems are many and often fundamentally different. Commercial attenuation correction approaches range from stationary line sources, to scanning point sources, to multi-modality x-ray computed tomography (CT) systems: CT/SPECT and CT/PET. Comparing the imaging capabilities between scanners and ensuring the daily quality of attenuation-corrected images is vital. Other than “cold-spot” phantoms using inserts of different materials, there are no phantoms specifically designed to test the attenuation correction capability of nuclear medicine systems, even though the need for such a phantom is growing.
Anthropomorphic phantom designs are also of interest. Phantoms with inserts to simulate cardiac uptake are commercially available, and anthropomorphic phantoms with chambers for lungs, heart, and liver are also available. Such phantoms are useful for better simulating patient imaging and for observing the effect of attenuation correction and scatter correction. While these complex phantoms provide more imaging detail, they are correspondingly more inconvenient to use because of the increased number of chambers to fill.
Phantoms are routinely used in nuclear medicine for several purposes. Phantoms for single photon emission computed tomography (SPECT) and positron emission tomography (PET) are fixtures that contain a radioisotope source of a specific geometry. Often, phantoms are used for characterizing the performance of SPECT and PET systems. There are guidelines from organizations such as the National Electrical Manufacturers Association (NEMA) and the American Association of Physicists in Medicine (AAPM) which recommend several phantoms, such as point sources, line sources, and fillable cylindrical chambers. These phantoms are specially designed to measure specific performance characteristics of the scanner, including spatial resolution and sensitivity. Manufacturers also use these performance measures in marketing their SPECT and PET systems.
An important use for phantoms is for quality assurance (QA) of SPECT and PET scanners. Hospital physicists follow specific daily, weekly, and monthly protocols to ensure proper operation of their imaging systems. Regular QA is critical for ensuring proper image quality and proper diagnostic accuracy of patient images. While the methodology for performance testing from NEMA gives valuable information about system performance, many of the tests require specialized equipment and sophisticated software. As a result, more convenient QA phantoms are used for routine testing. The most desirable characteristics of phantoms for QA are ease of use and reproducibility. Therefore, QA phantoms are designed to be simple and convenient to fill.
For example, as shown in
FIG. 1A
, the Jaszczak phantom
10
comprises a cylindrical chamber
12
containing arrays of solid plastic rods
14
and several solid plastic spheres
16
of various diameters. The Jaszczak phantom is what is commonly referred to as a “cold spot” phantom having a “warm background,” in that the spheres show up on a scan as radio-neutral regions in a radioactive background. The Jaszczak phantom
10
is pillable using a single injection of radioisotope, and the resulting images yield information regarding scanner contrast resolution and performance, but not lesion conspicuity.
Another field of interest for SPECT and PET phantoms is research. Physicists utilize phantoms of various geometries, and often simulate the human body in order to test novel image reconstruction algorithms and data correction capabilities. Examples of such phantoms are the anthropomorphic torso phantom
20
available from Data Spectrum, shown in
FIG. 1B
, and the Hoffman brain phantom
30
, shown in FIG.
1
C.
The torso phantom
20
comprises a plurality of individual chambers
22
, representing human organs, which can be filled with a radioactive solution. A main chamber
24
can be filled with a different radioactive solution. Thus, the torso phantom
20
can be used to provide “hot” spots in a “warm” background. However, the steps of filing each of the individual chambers
22
of the torso phantom
20
is time consuming. Further, the requirement of the preparation of different radioactive solutions for producing contrast among the chambers
22
and/or between the chambers
22
and the main chamber
24
leads to poor repeatability, since human error will naturally produce variations in the concentrations of the solutions each time they are prepared.
The Hoffman brain phantom
30
comprises a single fillable chamber defined by nineteen individual plates
32
that stack within a cylindrical container. The plates
32
each include open chamber portions
34
which hold the radioactive solution when the phantom
30
is filled. When the plates
32
are stacked together, varying thicknesses of the chamber portions
34
and surrounding solid portions cooperate to simulate the gray and white matter of the human brain.
Nuclear medicine research using phantoms has been steadily increasing in recent years because of the availability of advanced imaging hardware and software. The efficacy of methods for quantitative SPECT and PET, such as attenuation correction, scatter correction, and collimator deblurring, continues to be investigated by researchers. Of particular interest is the effect of these corrections on specific clinical applications, such as for oncology and cardiac imaging.
Another approach to phantoms for nuclear imaging was recently developed in which an ink-jet printer loaded with radioisotope solution is used to print pages with the desired planar isotop
Biomec Inc.
Kalivoda Christopher M.
Lee John R.
Pearne & Gordon LLP
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