Chemistry: analytical and immunological testing – Composition for standardization – calibration – simulation,...
Reexamination Certificate
2001-07-27
2003-10-21
Wallenhorst, Maureen M. (Department: 1743)
Chemistry: analytical and immunological testing
Composition for standardization, calibration, simulation,...
C073S001860, C073S866400, C324S308000
Reexamination Certificate
active
06635486
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the field of phantoms for use with medical imaging such as ultrasound scanning, magnetic resonance imaging and computed tomography.
BACKGROUND OF THE INVENTION
There has been a tremendous surge in the number of ultrasound guided transperineal prostate implants performed in recent years. Effective implants require involved treatment planning based on three-dimensional multi-modality (magnetic resonance imaging, ultrasound, computed tomography) images used in combination with one another. A multi-modality prostate imaging phantom could have applications in quality assurance, image registration and treatment planning. Three human soft tissues relevant to a prostate phantom that should be mimicked for magnetic resonance imaging (MRI), ultrasound, and computed tomography (CT) are prostate parenchyma, skeletal muscle and adipose (fat) tissue.
Tissue-mimicking (TM) materials must exhibit the same properties relevant to a particular imaging modality as actual human soft tissues. Tissue-mimicking materials for use in magnetic resonance imaging phantoms should have values of characteristic relaxation times, T
1
and T
2
, which correspond to those of the tissue represented at the Larmor frequency of concern. Soft tissues exhibit T
1
values ranging from about 200-1200 ms and T
2
values from about 40-200 ms. For a given soft tissue, T
1
in particular can exhibit a significant dependence on frequency as well as on temperature. However, for multi-modality imaging phantoms, in general, measurements may be performed near the available clinical Larmor frequency of an MRI system (typically 64 MHz or 85 MHz) and at room temperature. Phantoms must be assumed to be useful at room temperature even though their properties must mimic those of soft tissues at the normal body temperature of 37° C.
The ideal tissue-mimicking material for use in ultrasound should have the same ranges of speeds of sound, attenuation coefficients, and backscatter coefficients as soft tissue. These parameters should be controllable in the manufacturing process of the phantom material, and their variation within the range of room temperatures should be small. Speeds of sound in human soft tissues vary over a fairly small range with an average value of about 1540 m/s. The speed of sound in fat is thought to be about 1470 m/s. The amplitude attenuation coefficients appear to vary over the range from 0.4 dB/cm to about 2 dB/cm at a frequency of 1 MHz in these tissues. The frequency dependencies of the attenuation coefficient of some soft tissues have been studied and, typically, it has been reported that the attenuation coefficient is approximately proportional to the ultrasonic frequency in the diagnostic frequency range of 1 to 10 MHz. An exception is breast fat, in which the attenuation coefficient is proportional to the frequency to the 1.7 power. F. T. D'Astous and F. S. Foster, “Frequency Dependence of Attenuation and Backscatter in Breast Tissue,” Ultrasound in Med. & Biol., Vol. 12, pp. 795-808 (1986).
For use in computed tomography (CT), the tissue-mimicking materials must exhibit the same CT number as that of the tissue being mimicked. The CT numbers for most soft tissues lie in the range of about 20-70 at the typical effective x-ray energy of a clinical CT scanner except for fat where the CT number is about −100.
In addition to the individual imaging modality parameters listed above, tissue-mimicking materials must also exhibit long term stability and ease of storage without which they are rendered useless in an imaging phantom.
An ultrasound phantom containing tissue-mimicking material is disclosed in U.S. Pat. No. 4,277,367, to Madsen, et al., entitled Phantom Material and Methods, in which both the speed of sound and the ultrasonic attenuation properties could be simultaneously controlled in a mimicking material based on water based gels, such as those derived from animal hides. In one embodiment, ultrasound phantoms embodying the desired features for mimicking soft tissue were prepared from a mixture of gelatin, water, n-propanol and graphite powder, with a preservative. In another embodiment, an oil and gelatin mixture formed the basis of the tissue-mimicking material.
Tissue-mimicking material is typically used to form the body of an ultrasound scanner phantom. This is accomplished by enclosing the material in a container which is closed by an ultrasound transmitting window cover. The tissue-mimicking material is admitted to the container in such a way as to exclude air bubbles from forming in the container. Tissue-mimicking materials may contain scattering particles, spaced sufficiently close to each other that an ultrasound scanner is incapable of resolving individual scattering particles. Testing spheres of tissue-mimicking material, or other targets, may be located within the phantom container, suspended in the tissue-mimicking material body. The objective is for the ultrasound scanner to resolve the testing spheres or other targets from the background material and scattering particles. This type of ultrasound phantom is described in U.S. Pat. No. 4,843,866, to Madsen, et al., entitled Ultrasound Phantom.
U.S. Pat. No. 5,625,137 to Madsen, et al. discloses a tissue-mimicking material for ultrasound phantoms with very low acoustic backscatter coefficient that may be in liquid or solid form. A component in both the liquid and solid forms is a filtered aqueous mixture of large organic water soluble molecules and an emulsion of fatty acid esters, which may be based on a combination of milk and water. Hydroxy compounds, such as n-propanol, can be used to control the ultrasonic speed of propagation through the material and a preservative from bacterial invasion can also be included. The use of scattering particles allows a very broad range of relative backscatter levels to be achieved.
Hydrogen magnetic resonance imaging (MRI) (also known as nuclear magnetic resonance, or NMR, imaging) is generally a more complicated imaging procedure than X-ray or ultrasound since it does not measure just one dominant property, such as electron density in the case of X-ray computed tomography, but is affected by the hydrogen atom density, flow, and two relaxation phenomena. The contrast, or differences in image brightness, in an MRI image is primarily due to differences in the relaxation times of tissues. It has been found that there are relaxation time differences between normal tissue and certain tumors, which makes MRI imaging potentially very valuable in early detection of such tumors.
A satisfactory MRI phantom must satisfy several requirements. First, the material of which the phantom is made should mimic the hydrogen density and relaxation times of several types of tissues. Second, the relaxation times of the material should not change over time, such as over several months or years, so that the phantom can be used in tests of imager reproducibility. Third, if the phantom includes inclusions of materials within the surrounding matrix which have different NMR characteristics than the surrounding matrix, these inclusions must be stable over time in both shape and in NMR relaxation times, T
1
and T
2
.
Soft tissues exhibit T
2
's from about 40 ms to 200 ms. Typical values for the ratio T
1
/T
2
lie between 4 and 10 for soft tissues. For a given soft tissue parenchyma, T
1
in particular can exhibit a significant dependence on frequency as well as temperature.
Materials which have been proposed for use in phantoms to mimic soft tissues with respect to one or more NMR properties include aqueous solutions of paramagnetic salts and water based gels of various forms. Such gels may also contain additives such as a paramagnetic salt for control of T
1
. Aqueous solutions of paramagnetic salts can be used in phantoms to produce a desired value of either T
1
or T
2
. The ratio of T
1
/T
2
in the salt solutions is almost always less than 2, however, rendering such solutions inadequate for the close mimicking of soft tissue, with the possible exception
D'Souza Warren D.
Frank Gary R.
Madsen Ernest L.
Foley & Lardner
Wallenhorst Maureen M.
Wisconsin Alumni Research Foundation
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