News Column

Patent Issued for Method for Absorption Correction of PET Data and MR-PET System

July 15, 2014



By a News Reporter-Staff News Editor at Life Science Weekly -- According to news reporting originating from Alexandria, Virginia, by NewsRx journalists, a patent by the inventor Ladebeck, Ralf (Erlangen, DE), filed on June 24, 2010, was published online on July 1, 2014 (see also Siemens Aktiengesellschaft).

The assignee for this patent, patent number 8768432, is Siemens Aktiengesellschaft (Munich, DE).

Reporters obtained the following quote from the background information supplied by the inventors: "Positron Emission Tomography (PET) is a widely-used method of functional imaging. During an examination a weak radioactive substance, of which the distribution in the organism is made visible by way of PET, is administered to a person being examined. This enables biochemical and physiological functions of the organism to be mapped. In such cases molecules which are marked with a radio nuclide which emits positrons are used as radiopharmaceuticals. The high-energy photons, which are emitted at an angle of 180.degree. to each other, produced in the body of the person under examination during the annihilation of the positron with an electron are detected by a plurality of detectors arranged in a ring around the person under examination. Only coincident events which have been detected with two opposite detectors are evaluated in each case.

"From the registered coincident decay events deductions are made about the spatial distribution of the radio pharmaceutical within the body and a series of image slices are computed. The image can be reconstructed in such cases with a filtered back projection or an iteration method, with the spatial resolution usually lagging behind the resolution of conventional computer tomography (CT) or magnetic resonance tomography (MRT).

"On their passage through material the photons produced during the annihilation can be absorbed, with the absorption probability depending on the path length through the material and the corresponding absorption coefficient of the material. Accordingly in PET a correction of the signals in relation to the attenuation by components which are located in the beam path is necessary. In particular such a correction has to be undertaken if a quantitative analysis of the data is to be carried out for example for quantifying accumulations of the marked substance (i.e. the radiopharmaceutical) in areas of the person under examination. In image reconstruction too not taking account of the absorption of the radiation leads to the occurrence of artifacts, since the measured activity distribution without absorption correction does not match the actual distribution. The correction of the attenuation of the radiation requires the knowledge of the location of the attenuating structures which are taken into account during reconstruction of PET image data by means of an attenuation correction map (.mu.-map).

"An attenuation correction map can be determined with a combined PET/CT system. The correction maps can be calculated in such cases from the Hounsfield values of the CT data. This method is made possible by the x-ray radiation of the CT undergoing a similar attenuation on its passage through the person under examination to the high-energy photons during the recording of the PET signals. Furthermore such systems enable the high local resolution of CT to be combined with the functional imaging of PET.

"CT devices have the disadvantage however that damaging x-rays are used and that only a low soft tissue contrast can be achieved without contrast media. However a high soft tissue contrast is desirable, especially in functional imaging of the brain.

"A high local resolution with simultaneous high soft tissue contrast as well as a functional imaging can be achieved with a combination of PET and magnetic resonance tomography (MRT). Such a system simultaneously enables high resolution images of the brain structure to be delivered and functional activities in the brain to the mapped. MRT allows different types of tissue to be differentiated, while PET makes physiological and biochemical activities visible. However it is problematic to derive coefficients of attenuation for the high-energy photons of the PET imaging from the MRT image data, i.e. to determine the attenuation correction map. Furthermore the recording of MRT image data demands a significantly longer acquisition time than the creation of computer tomographies.

"To take into account the attenuation of the emitted photons through the body any deviations of the MRT imaging from the true geometry are also disruptive. In this case areas of the body which, although they lie in the PET beam path, are not mapped or not mapped at the correct position pose a particular problem. In particular because of the high ratio of the attenuation coefficient (.mu. value) of human tissue to air it is desirable for a correct attenuation correction to determine the spatial transition from air to tissue as exactly as possible."

In addition to obtaining background information on this patent, NewsRx editors also obtained the inventor's summary information for this patent: "In at least one embodiment of the present invention, an improved absorption correction of PET data is provided in an MR-PET system.

"In accordance with the first aspect of at least one embodiment of the present invention an apparatus for combined magnetic resonance tomography and positron emission tomography (PET) imaging is provided which is equipped for recording PET image data of a person under examination from an area under examination. The apparatus includes an additional scanning unit which is equipped to scan a prespecified area of the person under examination and, based on the scanning, to determine a contour of the person under examination for the prespecified area, as well as a processing unit which is equipped, on the basis of the contour determined, to carry out an absorption correction of PET data which has been recorded from the prespecified area of the person under examination.

"By way of the additional scanning unit the contour or surface of the person under examination can be detected so that the position of the body parts of the person under examination is known and can be used for absorption correction of PET signals or PET image data. By way of the scanning unit the contour of the person under examination can be determined significantly more quickly and with a larger field of view than with an imaging MRT measurement and an improved absorption correction is made possible.

"In one embodiment, the additional scanning unit can be arranged in the direction in which the person under examination is pushed into the examination area, with the scanning unit having a scanning area which is different from the examination area of the apparatus. In the examination the person under examination is thus for example initially moved through the scanning area of the scanning unit and subsequently through the examination area of the apparatus, i.e. of the MR-PET system. The scanning unit can thus be integrated more easily into the MR-PET system and existing MR-PET systems can be upgraded in a simple manner. In particular it is advantageous for the scanning area to have a larger field of view than the examination area of the MRT imaging.

"The scanning unit is equipped for example for scanning the surface of a slice of the person under examination. The apparatus can have a patient table for moving the person under examination through the examination area, with the scanning unit scanning the prespecified area of the person under examination in slices during a typically step-by step method of movement of the patient table. Consequently the surface of the person under examination in the prespecified area can be determined in three dimensions by placing the scanned slices next to each other, with the prespecified area also able to comprise the whole person under examination.

"The apparatus can be embodied for simultaneously executing a PET measurement and scanning the person under examination by way of the scanning unit. The recording of the PET data or signals and the determination of the contour of the person under examination by means of scanning can therefore take place during one measurement pass. It is also possible to simultaneously carry out an MRT measurement during the scanning of the person under examination with the scanning unit. It is likewise possible to record both PET and MRT data in one measurement run and also to carry out the scanning of the person under examination. This enables locally high-resolution image data with functional information from the PET image data to be obtained, which has been absorption-corrected based on the scanning carried out simultaneously.

"The processing unit can be embodied, based on the contour determined, to create an attenuation correction map (.mu.-map) of the scanned area. The absorption correction of the PET data can be undertaken on the basis of the attenuation correction map. A rapid determination of the attenuation correction map is thus possible with the aid of the scanning unit which approximately specifies the spatial distribution of the attenuation coefficient .mu. for the attenuation of the high-energy protons occurring during the positron annihilation.

"The processing unit can for example be designed so that areas which are lying within the contour determined will be assigned a constant prespecified attenuation coefficient .mu.. The contour determined is for example the surface of the person under examination or an area of the person under examination, so that the constant attenuation coefficient, such as a previously determined attenuation coefficient for water or tissue for example, can be assigned to the inside of the person under examination. Since the variation of the attenuation coefficient from the surrounding air to the inside of the body is significantly greater than the variation within the body of the person under examination, an approximated attenuation correction map can thus be obtained in a simple manner.

"The processing unit can also be designed to create the attenuation correction map taking into account information from MRT image data which was recorded by way of the apparatus during MRT imaging of the prespecified area of person under examination. Information obtained from the MRT image data can for example include the position of bones and/or of the lungs of the person under examination with these are then able to be taken into account during creation of the attenuation correction map for example in the form of prespecified attenuation coefficients for the different structures. The accuracy of the attenuation correction map can thus be improved and an improved absorption correction of the PET data can be undertaken.

"In accordance with one embodiment the additional scanning unit comprises a terahertz (THz) scanner which detects terahertz radiation emitted from the body of the person under examination or reflected or scattered on the latter. Terahertz radiation typically lies within a frequency range of 100 GHz to 30 THz or in a wavelength range from 3000 .mu.m to 10 .mu.m. By scanning the person under examination by way of the terahertz scanner the surface of the person under examination can be determined quickly and without harmful radiation. It is also advantageous that the terahertz radiation can penetrate many materials, such as plastics or the clothing of the person under examination for example, so that a precise determination of the contours or of the surface of the person under examination is made possible. The patient table with which the person under examination is moved through the examination area of the apparatus can also be embodied so that terahertz radiation penetrates through it.

"The terahertz scanner can especially be embodied so that, by emitting electromagnetic terahertz radiation and detection of terahertz radiation reflected at the person under examination, it detects the contour of the person under examination in a scanning area. The terahertz scanner can thus not only detect terahertz radiation emitted passively from the body of the person under examination, but can also actively scan the body of the person under examination by means of a terahertz radiation source. This can for example be done using a fine microwave beam, with the scanning unit also able to feature a number of radiation sources.

"In accordance with a further embodiment, the additional scanning unit comprises an x-ray scanner which is designed to detect the contour of the person under examination in a scanning area by irradiating x-rays and detection of the x-rays scattered back at the person under examination. An x-ray scanner based on backscattering has the particular advantage that a weak high-energy x-ray source can be used so that the person under examination is subjected to a lower radiation dose than with conventional CT systems. The x-rays likewise penetrate the clothes of the person under examination so that the surface of the person under examination can be precisely mapped and determined by way of the scanning unit.

"With the apparatus previously described a precise determination of the surface of the person under examination is thus made possible with a short measurement duration so that movement artifacts from movements of the person under examination are reduced and the surface determination is thereby improved. The scanning unit can also have a significantly larger field of view than the recording unit for MRT imaging so that an attenuation correction map can also be created for the areas of the person under examination which, although they lie within the path of the PET beam, will not be mapped however by the MRT imaging. This makes an improved absorption correction of recorded PET data possible and thereby a spatially more accurate functional mapping which is suitable for quantification.

"In accordance with a further aspect of at least one embodiment of the present invention a method for absorption correction of PET data is provided, with the method being carried out with an apparatus for combined magnetic resonance tomography (MRT) and positron emission tomography (PET) imaging which is embodied for recording PET data of a person under examination from an examination area and which has an additional scanning unit. The method includes the scanning of a prespecified area of the person under examination with the scanning unit, the determination of the contour of the person under examination in the scanned area based on the scanning and the execution of an absorption correction of PET data which has been recorded from the prespecified area of the person under examination on the basis of the determined contour.

"Similar advantages to those described in relation to the apparatus are obtained with at least one embodiment of the inventive method. The method makes possible an improved absorption correction of recorded PET data, i.e. through a correction of PET signals or PET image data or through an absorption correction during the reconstruction of PET image data.

"In accordance with an embodiment of the inventive method the scanning can be undertaken slice-by slice. For example the scanning of the person under examination can be undertaken by moving a patient table on which the person under examination is arranged through the scanning area of the scanning unit and by slice-by-slice scanning of the prespecified area by means of the scanning unit. During the scanning of the person under examination by means of the scanning unit a PET measurement can be carried out simultaneously. It is also possible to carry out an MRT measurement simultaneously. In the method, once again based on the contour determined, an attenuation correction map (.mu.-map) of the scanned area can be created on the basis of which the absorption correction of the PET data is undertaken. The attenuation correction map can especially be used for the reconstruction of PET image data.

"Furthermore at least one embodiment of the method can comprise one or more method steps which have been described in advance with regard to the apparatus. Naturally the features of the previously described aspects and embodiments of the invention can be combined."

For more information, see this patent: Ladebeck, Ralf. Method for Absorption Correction of PET Data and MR-PET System. U.S. Patent Number 8768432, filed June 24, 2010, and published online on July 1, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8768432.PN.&OS=PN/8768432RS=PN/8768432

Keywords for this news article include: Biochemical, Biochemistry, Siemens Aktiengesellschaft.

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Source: Life Science Weekly


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