![]() So, the signal transmitted by the detector is processed by the PC in the form of digital information, the CT image reconstruction. CT numbers based on measurements with the EMI scanner invented by Sir Godfrey Hounsfield 6, a Nobel prize winner for his work in 1979, related the linear attenuation coefficient of a localized region with the attenuation coefficient of water, the multiplication factor of 1000 is used for CT number integers. However, several features distinguish it from conventional radiology: the image is reconstructed from a large number of measurements of attenuation coefficient.īefore the data are presented on the screen, the conventional rescaling was made into CT numbers, expressed in dimensionless Hounsfield Units (HU), as mentioned above. The CT deals with the attenuation of the x-rays during the passage through the body segment. The action of the filter function is such that the negative value created is the filtered projection, when projected backwards, is removed, and an image is produced, which is the accurate representation of the original object. To avoid this, the method of filtered back projection is used. In the process of the image, the value of the attenuated coefficient for each voxel corresponding to these pixels needs to be calculated.Įach image point is surrounded by a halo-shaped star that degrades the contrast and blurs the boundary of the object. The typical CT image is composed of 512 rows, each of 512 pixels, i.e., a square matrix of 512 x 512 = 262,144 pixels (one for each voxel). In conclusion, a measurement made by a detector CT is proportional to the sum of the attenuation coefficients. The CT image is a digital image and consists of a square matrix of elements ( pixel), each of which represents a voxel (volume element) of the tissue of the patient. Using the computer, it presents the elementary surfaces of the reconstructed image from a projection of the data matrix reconstruction, the tone depending on the attenuation coefficients. It gathers together all the data coming from the elementary volumes of material through the detectors. The image of the section of the object irradiated by the x-ray is reconstructed from a large number of measurements of attenuation coefficient. Sir Godfrey Hounsfield, the co-developer of CT, chose a scale that reflects the four basic x-ray densities in the human body, with the following values: I(x) is smaller where the body is more radiopaque. The outgoing intensity I(x) of the beam of photons measured will depend on the location. If the x-ray at the exit of the tube is made monochromatic or quasimonochromatic with the proper filter, one can calculate the attenuation coefficient corresponding to the volume of irradiated tissue by the application of the general formula of absorption of the x-rays in the field (see Figure 1). To obtain tomographic images of the patient from the "preprocessed" CT dataset, the computer uses complex mathematical algorithms for image reconstruction. The image of that section is taken from different angles, and this allows to retrieve the information on the depth (in the third dimension). They measure the transmission of a thin beam (1-10 mm) of x-rays through a full CT of the body. Unlike x-ray radiography, the detectors of the CT scanner do not produce an image. In the particular case of CT, the emitter of x-rays rotates around the patient and the detector, placed on diametrically opposite sides, picks up the image of a body section (beam and detector move in synchrony). This phenomenon is represented by a single coefficient, mju. There are two processes of absorption: the photoelectric effect and the Compton effect. Attenuation values of the x-ray beam are recorded and the data is used to build a 3D representation of the scanned object/tissue. A detector placed at the exit of the sample, measures N + ΔN photons, ΔN smaller than 0. The emitted x-rays form a beam that passes through the layer of biological material of thickness Δx. The CT x -ray tube (typically with energy levels between 20 and 150 keV), emits N photons (monochromatic) per unit of time. Using this principle, CT allows the reconstruction of the density of the body, by a two-dimensional section perpendicular to the axis of the acquisition system. Cross-sections are reconstructed from measurements of attenuation coefficients of x-ray beams passing through the volume of the object studied.ĬT is based on the fundamental principle that the density of the tissue passed by the x-ray beam can be measured from the calculation of the attenuation coefficient. Computed tomography (CT), also known as, especially in the older literature and textbooks, computerized axial tomography (CAT), is an imaging modality that uses x -rays to build cross-sectional images ("slices") of the body.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |