X-ray CT image reconstruction device, X-ray CT image reconstruction method, and X-ray CT apparatus
View Patent ↗Base substance thickness projection estimation values ( 124 ) and base substance thickness projection estimation error values ( 125 ) of M (M≤N) base substances are estimated based on measured count projection values ( 121 ) using N energy windows for a subject. The base substance thickness projection estimated values ( 124 ) are updated such that a likelihood in accordance with a joint probability density of all of X-ray projection paths based on the base substance thickness projection estimated error values ( 125 ) increases. The obtained second base substance thickness projection assumed value ( 127 ) is subjected to back projection, thereby obtaining a base substance concentration image estimation value ( 128 ).
1. An X-ray CT image reconstruction device that is connected to an X-ray detector that measures a detection count value of X-ray transmitted through a subject, in response to each of N different energy spectra (N is an integer satisfying a relationship of N≥2), the device comprising:
storing means for storing the detection count value of X-ray which is measured by the X-ray detector, in association with each of X-ray transmission paths and each of the energy spectra when the X-ray is transmitted through the subject;
base substance thickness estimating means for estimating, for each of the X-ray transmission paths, a thickness of each of M base substances (M is a positive integer satisfying a relationship of M≤N) constituting the subject, based on the detection count value of X-ray stored in the storing means;
error estimating means for estimating an error value regarding the thickness of each of the base substances, based on the detection count value of X-ray;
likelihood calculating means for calculating a likelihood which is an index of a probability of an assumed value of a thickness of each of the base substances for each of the X-ray transmission paths with the thickness of each of the base substances for each of the X-ray transmission paths, which has been estimated by the base substance thickness estimating means, as an initial value, based on the error value regarding the thickness of each of the base substances for each of the X-ray transmission paths, which has been estimated by the error estimating means;
base substance thickness updating means for updating the assumed value of the thickness of each of the base substances for each of the X-ray transmission paths such that a likelihood that is to be calculated by the likelihood calculating means increases; and
base substance concentration image reconstructing means for reconstructing a base substance concentration image regarding the M base substances of the subject by performing back projection of the assumed value of the thickness of each of the base substances for each of the X-ray transmission paths, which has been updated by the base substance thickness updating means.
2. The X-ray CT image reconstruction device according to claim 1 ,
wherein the likelihood calculating means
estimates probability density regarding the thickness of each of the base substances for each of the X-ray transmission paths, based on the error value regarding the thickness of each of the base substances for each of the X-ray transmission paths, which has been estimated by the error estimating means,
calculates joint probability density regarding the thickness of each of the base substances for all of the X-ray transmission paths, based on the estimated probability density regarding the thickness of each of the base substances for each of the X-ray transmission paths, and
calculates the likelihood in accordance with the calculated joint probability density.
3. The X-ray CT image reconstruction device according to claim 2 ,
wherein the base substance thickness updating means updates the thickness of each of the base substances for each of the X-ray transmission paths, based on a likelihood difference between an average value likelihood and an assumed value likelihood, the average value likelihood being a likelihood obtained by applying, to the joint probability density, an average value of thicknesses of the base substances on all of the X-ray transmission paths, which have been estimated by the base substance thickness estimating means, the assumed value likelihood being a likelihood obtained by applying, to the joint probability density, the assumed value of the thickness of each of the base substances, which is different from the average value of the thicknesses of the base substances.
4. An X-ray CT image reconstruction method that causes a computer connected to an X-ray detector that measures a detection count value of X-ray transmitted through a subject, in response to each of N different energy spectra (N is an integer satisfying a relationship of N≥2) to execute:
a first step of storing, in a storage device, the detection count value of X-ray which is measured by the X-ray detector, in association with each of X-ray transmission paths and each of the energy spectra when the X-ray is transmitted through the subject;
a second step of estimating, for each of the X-ray transmission paths, a thickness of each of M base substances (M is a positive integer satisfying a relationship of M≤N) constituting the subject, based on the detection count value of X-ray stored in the storage device;
a third step of estimating an error value regarding the thickness of each of the base substances, based on the detection count value of X-ray;
a fourth step of calculating a likelihood which is an index of a probability of an assumed value of a thickness of each of the base substances for each of the X-ray transmission paths with the thickness of each of the base substances for each of the X-ray transmission paths, which has been estimated in the second step, as an initial value, based on the error value regarding the thickness of each of the base substances for each of the X-ray transmission paths, which has been estimated in the third step;
a fifth step of updating the assumed value of the thickness of each of the base substances for each of the X-ray transmission paths such that a likelihood that is to be calculated in the fourth step increases; and
a sixth step of reconstructing a base substance concentration image regarding the M base substances of the subject by performing back projection of the assumed value of the thickness of each of the base substances for each of the X-ray transmission paths, which has been updated in the fifth step.
5. The X-ray CT image reconstruction method according to claim 4 ,
wherein the computer, in the fourth step,
estimates probability density regarding thicknesses of the base substances for each of the X-ray transmission paths, based on the error value regarding the thickness of each of the base substances for each of the X-ray transmission paths, which has been estimated in the third step;
calculates joint probability density regarding the thicknesses of the base substances for all of the X-ray transmission paths, based on the estimated probability density regarding the thicknesses of the base substances for each of the X-ray transmission paths, and
calculates the likelihood in accordance with the calculated joint probability density.
6. The X-ray CT image reconstruction method according to claim 5 ,
wherein the computer, in the fifth step,
updates the thickness of each of the base substances for each of the X-ray transmission paths, based on a likelihood difference between an average value likelihood and an assumed value likelihood, the average value likelihood being a likelihood obtained by applying, to the joint probability density, an average value of thicknesses of the base substances on all of the X-ray transmission paths, which have been estimated in the second step, the assumed value likelihood being a likelihood obtained by applying, to the joint probability density, the assumed value of the thickness of each of the base substances, which is different from the average value of the thicknesses of the base substances.
7. An X-ray CT apparatus comprising:
an X-ray tube configured to irradiate a subject with an X-ray;
an X-ray detector configured to measure a detection count value of X-ray with which irradiation from the X-ray tube is performed and which is transmitted through the subject, in response to a plurality of different energy spectra; and
the X-ray CT image reconstruction device according to claim 1 .