Variation of the count‐dependent Metz filter with imaging system modulation transfer function

Abstract

A systematic investigation was conducted of how a number of parameters which alter the system modulation transfer function (MTF) influence the count-dependent Metz filter. Since restoration filters are most effective at those frequencies where the object power spectrum dominates that of the noise, it was observed that parameters which significantly degrade the MTF at low spatial frequencies strongly influence the formation of the Metz filter. Thus the radionuclide imaged and the depth of the source in a scattering medium had the most influence. This is because they alter the relative amount of scattered radiation being imaged. For low-energy photon emitters, the collimator employed and the distance from the collimator were found to have less of an influence but still to be significant. These cause alterations in the MTF which are more gradual, and hence are most pronounced at mid to high spatial frequencies. As long as adequate spatial sampling is employed, the Metz filter was determined to be independent of the exact size of the sampling bin width, to a first approximation. For planar and single photon emission computed tomographic (SPECT) imaging, it is shown that two-dimensional filtering with the Metz filter optimized for the imaging conditions is able to deconvolve scatter and other causes of spatial resolution loss while diminishing noise, all in a balanced manner.

Optimum tomographic reconstruction parameters for HMPAO brain SPET imaging: a practical approach based on subjective and objective indexes

“…is constant, the Gilbert index is equivalent to the normalized mean square error [11]. The lower the Gilbert index, the better is the reconstruction.…”

“…In other studies [5,9], observers were asked to rank images by their degree of quality. Another approach is to use discrepancy indexes such as the Gilbert index [10] or the normalized mean square error (NMSE) [11]. Since they require an ideal reconstructed image, discrepancy indexes are difficult to reproduce in clinical SPET.…”

Quantitative analysis in single photon emission tomography (SPET)

“…Well-known examples of these filters are the Wiener and Metz filters. Several investigators have analyzed these filters in scintigraphic imaging (King et al 1984(King et al , 1986(King et al , 1987Honda et al 1987;Gilland et al 1988;Yanch et al 1988Yanch et al , 1990Links et al 1990) and compared their relative performance (King et al 1984) and with other filters like the Hanning and Butterworth filters (Gilland et al 1988). …”

“…Instead of using the PSRF for deconvolution of the images, often a generalized exponential form of the PSRF is applied. King et al (1986) investigated the different parameters of this generalized Metz filter. Their results showed an improved image contrast and a reduced noise when the Metz filter was optimized with respect to the total image counts.…”

“…Notwithstanding the low signal-to-noise ratio of the planar scintigrams obtained with a gamma-camera, a skilled and experienced human observer could visually interpret the images. Several methods have been published to enhance the quality of scintigrams using computers and image processing tools like filtering (Sebern et al 1976; King et al 1986) and background activity subtraction Offprint requests to: J.A.K. Blokland in 2°iT1 myocardial scintigrams (Goris 1979;Watson et al 1981).…”