![]() Axial (a) and coronal (b) reformatted CT images (soft-tissue window) of a 71-year-old woman who had undergone spinal fusion surgery for scoliosis show that the bright and dark streaks from the pedicle screws (made of titanium) and rods (made of cobalt-chrome) obscure the depiction of adjacent tissues. Such data result in fine bright and dark streaks that appear preferentially along the direction of greatest attenuation, resulting in severe streaks on the reconstruction image ( Fig 2).įigure 2a. The result is that projection data with large statistical error are obtained ( 6, 7). This phenomenon is known as photon starvation. The x-ray beam becomes markedly attenuated after passing through metallic hardware, and insufficient numbers of photons reach the detectors. Because the probability of the photoelectric effect is proportional to the cube of the atomic number for a given x-ray energy, the absorption in metallic hardware is amplified relative to the absorption in the surrounding soft tissue. Metals have much larger atomic numbers than the atomic numbers for soft tissues. The probability of Compton scatter is usually constant for different energies, although it slowly decreases at higher energies ( 5). On the other hand, the probability of Compton scatter is essentially independent of Z. At the energy levels used for diagnostic imaging, the general relationship is that the probability of the photoelectric effect is proportional to ( Z/ E) 3, where Z is the atomic number, and E is the energy of the photons. The total rate at which photons interact with a material (attenuation coefficient value) depends on the individual rates associated with the photoelectric effect and Compton scatter. As a practical guide, the optimal MAR methods are discussed for frequently encountered clinical situations. Special attention is paid to the strengths and limitations of each method. Then, the theoretical basics of state-of-the-art artifact reduction techniques are described, emphasizing the important differences between the projection-based metal artifact reduction (MAR) technique and the dual-energy CT technique. In this article, the two major mechanisms of metal artifact generation, photon starvation and beam hardening, are thoroughly reviewed. Metal artifacts are caused by a combination of multiple mechanisms, including photon starvation, beam hardening, scattering, partial volume effects, undersampling, and patient motion ( 4). Relevant anatomic structures are often completely obscured by the artifacts, which increase the risk of missing relevant findings. This problem often leads to impaired image quality of the adjacent tissue as well as of the metallic implant itself. ![]() Artifacts caused by metallic implants, such as dental fillings, surgical clips, coils, wires, and orthopedic hardware, appear as bright and dark streaks across the reconstruction image ( 3). Metal-related artifacts have limited the diagnostic value of computed tomographic (CT) images since the early days ( 1, 2). Radiologists should be more familiar with the clinical and technical features of each method and should be able to choose the optimal method according to the clinical situation. In clinical practice, although MAR algorithms can be applied after image acquisition, the decision whether to apply dual-energy CT for the patient usually needs to be made before image acquisition. Dual-energy CT provides synthesized virtual monochromatic images at different photon energy (kiloelectron volt) levels, and virtual monochromatic images obtained at high kiloelectron volt levels are known to reduce the effects of beam hardening. The dual-energy CT technique is characterized by data acquisition at two different energy spectra. MAR algorithms primarily suppress artifacts that are due to photon starvation. Projection-based metal artifact reduction (MAR) algorithms act in projection space and replace corrupted projections caused by metal with interpolation from neighboring uncorrupted projections. To improve image quality and recover information about underlying structures, several artifact reduction methods have been introduced in modern CT systems. When x-rays pass through a metal object, depending on its size and composition, different physical effects negatively affect the measurements in the detector, most notably the effects of photon starvation and beam hardening. Artifacts caused by metallic implants appear as dark and bright streaks at computed tomography (CT), which severely degrade the image quality and decrease the diagnostic value of the examination.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |