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Nuclear Medicine is a medical speciality that employs tomography procedures for the diagnosis, treatment and prevention of diseases [1]. One of the most commonly used apparatus is the Single Photon Emission Computed Tomography (SPECT). To perform exams, a very small amount of a radiopharmaceutical must be given to the patient. Then, a gamma camera is placed in convenient positions to perform the photon counting, which is used to reconstruct a full 3 dimensional distribution of the radionuclide inside the body or organ. This reconstruction provides a 3-dimensional image in spatial coordinates, of the body or organ under study, allowing the physician to give the diagnostic. Image reconstruction is usually worked in the frequency domain, due to a great simplification introduced by the Fourier decomposition of image spectra. After the reconstruction, an inverse Fourier transform must be applied to trace back the image into spatial coordinates. To optimize this reconstruction procedure, digital filters are used to remove undesirable components of frequency, which can "shadow" relevant physical signatures of diseases. Unfortunately, the efficiency of the applied filter are strongly dependent on its own mathematical parameters. Particularly in this work we demonstrate how filters interfere on image quality in cardiology examinations with SPECT, concerning perfusion and myocardial viability and the importance of the medical physicist in the choice of the right filters avoiding some serious problems that could occur in the inadequate processing of an image damaging the medical diagnosis.

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By appreciating the technical basis of Fourier representations and filtering, a better interpretation of SPECT images can be gained. The ramp filter has no selectable parameters and is the required filter used in tomography. The resulting noisy reconstructed image is smoothed by using a filter where the cut-off and order can be selected. By relating frequencies to sizes of structures in the image, reasonable values for the cut-off frequency for the smoothing filter can be selected. Most important, the realization that the ramp-filtered image (without the smoothing filter applied) is the most correct image means that subsequent smoothing can slightly degrade the numeric correctness of the reconstructed image. Smoothing filters make the image easier for clinical interpretation because noisy structures are difficult for the human eye to perceive. Selection of the smoothing filter to maximize noise reduction and image structure preservation is accomplished by matching the cut-off frequency to the image noise or camera resolution. This understanding should reduce the amount of time spent searching for a smoothing filter, which is used routinely in clinical imaging studies.
Influence of Digital Filters used to obtain Cerebral Image in Nuclear Medicine
  • A L S Lima
  • Jesus M De
  • J Santos
  • B Megueriam
Lima, A. L. S.; de Jesus M. C; dos Santos, J. A; Megueriam, B. A, "Influence of Digital Filters used to obtain Cerebral Image in Nuclear Medicine", to appear in this conference.