X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2002-02-21
2003-06-03
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S005000, C378S018000, C378S053000, C378S056000, C378S098110, C378S207000
Reexamination Certificate
active
06574302
ABSTRACT:
The invention relates to a method of determining a density of a volume in an image data set of an object to be examined, which volume can be distinguished from its environment in that the volume contains a material having a characteristic parameter (&mgr;) which can be dedicated, which method includes the step of exposing the volume and the environment to X-rays in different exposure positions, in each exposure position there being performed an exposure with a first energy and with a second energy of the X-rays so as to obtain on an X-ray detector a first set of transmission images of the object which corresponds to these energies.
A method of this kind can be used in the field of bone densitometry in which a mineral density of a trabecular bone is to be determined. In this case the volume to be imaged contains the trabecular bone that can be distinguished from the environment (the soft tissue, the cortical bone) on the basis of its characteristic linear absorption coefficient (&mgr;). The value of the mineral density in the trabecular bone can be used for further diagnosis of a patient, for example, for diagnosing osteoporosis. Therefore, it is important that the value of the mineral density can be quantitatively be determined without inaccuracies which are due to other tissues present in a region of interest to be imaged. Tissues of this kind are, for example, cortical bone and possible calcifications of the blood vessels present in the region of interest to be imaged.
A method of performing densitometry on bones is known from U.S. Pat. No. 5,778,045. In conformity with the known method transmission images of a patient to be examined are formed by means of an X-ray apparatus that includes an X-ray source for generating X-rays. The X-ray source is rotatable about the object and the generated X-rays are collimated so as to form a thin beam. The known apparatus is provided with means for generating the X-rays with two different energies, that is, to carry out the so-termed dual-energy method. In this case an exposure is performed with two energies in each exposure position. In accordance with the known method, subsequently an operation is performed so as to obtain a quantity that is related to the degree of absorption of the total bone in the exposed volume for both energies. In order to limit a contribution of a soft tissue to the ultimate image, this calculated quantity is graphically represented as a function of the distance in an imaging plane within the region of interest. The soft tissue is then graphically eliminated by applying a threshold value to the calculated quantity.
It is a drawback of the known method that a volume corresponding to the soft tissue is not accurately eliminated. Granted, the calculated quantity exhibits a characteristic behavior, that is, a low, comparatively flat line, corresponding to the absorption of the soft tissue, and a slightly higher region that corresponds to the absorption of the bone. For a vertebra, for example, the resultant graphic representation of the behavior of the calculated quantity will exhibit approximately a high region which corresponds to the vertebra, and to the left and the right thereof two regions of lower value which correspond to the soft tissue. It is unlikely that the absolute value of the magnitude of the calculated quantity is of the same value in the regions that correspond to the soft tissue on both sides of the vertebra. When only one value is chosen for the threshold, it is inevitable that either the soft tissue is not completely eliminated from the images or that a part of the bone volume is lost. It is a further drawback of the known method that an exposure is performed only in a cross-section of a patient. For the acquisition of volume information it is necessary to repeat the movement of the X-ray source, making this procedure a time-consuming operation. A further drawback of the known method consists in the fact that the data concerning the bone density is presented only two-dimensionally.
It is an object of the invention to provide a method in which the contribution of the soft tissue is accurately and substantially completely eliminated in the images to be analyzed. It is a further object of the invention to provide an image of the volume which corresponds to the trabecular bone, the absolute value of the mineral density of the trabecular bone being represented in said image. To this end, the invention is characterized in that the method also includes:
the use of a two-dimensional detector as the X-ray detector,
performing a first calibration in conformity with a dual-energy method so as to obtain a first set of parameters (x
p1
) and a second set of parameters (x
a1
);
performing a second calibration in conformity with a dual-energy method so as to obtain a third parameter (&phgr;) and a fourth parameter (f), the third parameter being characteristic of the environment and the fourth parameter being characteristic of the material;
performing a first operation on the first set of transmission images while using the first set of parameters (x
p1
) and the second set of parameters (x
a1
) so as to obtain a set of density images;
applying the third parameter (&phgr;) to the set of density images so as to eliminate the environment from the set of density images;
carrying out a 3D reconstruction algorithm with information of the set of density images so as to obtain an image of the volume;
applying the fourth parameter (f) to the image of the volume so as to calculate the density of the material in the volume.
Because use is made of a two-dimensional X-ray detector, it suffices in principle to perform only one motion of the X-ray source around the object, during which motion the exposures take place in a number of positions along an arc while using the first and the second energy of the X-rays. The known method, however, utilizes a thin beam geometry and a corresponding array detector. Generating such tomographic images is known per se and will be evident to those skilled in the art. It is also possible to perform first a movement of the X-ray source around the patient during which exposure takes place with the first energy only. In that case a supplementary second tomographic exposure must take place with the second X-ray energy. In this case it is also possible to expose the patient continuously, or to perform exposures with the first and the second energy in corresponding positions on the arc. A further advantage of the use of a two-dimensional X-ray detector resides in the resultant isotropic distribution of the cubic voxels in the volume image data to be reconstructed. This results in an enhanced accuracy of the 3D reconstruction and also reduces the minimum dose that has to be applied to an object to be studied. A characteristic format of a suitable detector is approximately 15×15 cm
2
. The method in accordance with the invention also includes the execution of calibrations in conformity with the dual-energy method. A calibration of this kind is known per se from H. Neale Cardinal and A. Fenster “An accurate method for direct dual-energy calibration and decomposition”, Med. Phys. 17 (3), 1990, p. 327. This method is based on the insight that X-ray absorption of a material can be represented by a linear combination of two basic materials. In accordance with the invention the acquisition of the first set of calibration parameters (x
p1
) and the second set (x
a1
) of calibration parameters by means of the dual-energy method takes place while utilizing a calibration phantom that is especially designed for this purpose and consists of two basic materials. The second calibration in accordance with the method of the invention is performed by means of a second calibration phantom that substantially represents the composition of the environment of the material to be examined. After the formation of the transmission images of the second calibration phantom in conformity with the dual-energy method, they are further processed so as to achieve the decomposition of the environment to a linear combination o
Dunn Drew A.
Ho Allen C.
Vodopia John
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