The aims of this study were to investigate if digital filtering can increase the sensitivity and specificity of bone scans, and to find the type of filter most suitable for bone images at various count levels. We also wished to examine if filtering allows the administered activity or the examination time to be reduced, and if it is easier to detect low-contrast uptake using a digital filter. Images containing a total of 100, 350 and 1000 kcounts were acquired in a 256 x 256 matrix using a transmission phantom simulating the thoracolumbar region. Receiver operating characteristic (ROC) analysis demonstrated that digital filtering increases the sensitivity and specificity of bone scintigraphy. A low-pass filter for images with low statistics (100 kcounts), which contain 2-5 counts per pixel in the ribs and vertebrae respectively, and a Metz filter for images with normal (5-19 counts per pixel) to good (20-54 counts per pixel) statistics, increase the area under the ROC curve significantly (99% confidence level) compared to unfiltered images. Filtering also increases the detectability of low-contrast objects compared with unfiltered images. Digital filtering might be an alternative to raising the number of counts in the image. An alternative is to reduce the administered activity and hence the effective dose to the patient, or to reduce the examination time-which is an advantage when treating elderly patients or patients with pain.

Absolute measurement of activity implies a determination of effective depths and effective attenuation coefficients. In order to define restoration filters, it is necessary to measure the transfer function, i.e. position a line source at an effective depth for the specific measurement situation. A phantom was designed which can simulate an organ with a certain thickness at a certain depth. The phantom was used to measure transfer functions and a comparison was made with transfer functions from a line source to determine effective depths. Effective attenuation coefficients were calculated for 99mTc, 111In and 201Tl for different organ thicknesses and depths of simulated organs. The effective attenuation coefficient for 99mTc was found to be 0.124 +/- 0.006 cm-1, in good agreement with previously published values. For 111In, the attenuation coefficient decreased with the depth of an organ due to the use of two energy windows in the measurements and a corresponding change in mean photon energy by depth. For 201Tl, the attenuation coefficient decreased with increasing organ thickness due to the increasing fraction of scattered radiation in the 40% energy window used. Using attenuation coefficients of 0.124, 0.184 and 0.11 cm-1 for 99mTc, 201Tl and 111In respectively, the derived equations can be used to calculate the position of a conventional line source for measurements of transfer functions for a specific organ with a certain thickness at a certain depth for definition of different types of restoration filter.

Digital image processing in terms of image restoration and noise reduction assumes a thorough knowledge of the gamma camera transfer function in any clinical measurement. A phantom is designed which can simulate a slice of activity through an organ and hence can be used to determine the corresponding transfer function. The advantage of this phantom compared to an ordinary line source is that the organ thickness can easily be simulated and calculations of an effective depth of an organ and the corresponding attenuation coefficient are not necessary. On the other hand the phantom can be used to determine the depth at which a line source should be positioned for measurement of a certain clinical situation.

A selection of commonly used reconstruction and filter techniques in the processing of 99mTc oxidronate (i.e., 99mTc hydroxymethane diphosphonate) single photon emission computed tomography (SPECT) of the spine was compared. The possible additional value of scatter correction on image contrast was also evaluated. Twenty-eight bone SPECT examinations of consecutive patients were studied retrospectively. The reconstruction techniques used were filtered back-projection and iterative reconstruction with the use of ordered subsets estimation maximization. Three-dimensional post-filtering with a Metz filter and a Butterworth filter was used. Each combination was evaluated with or without scatter correction. Each study was also processed with the department's standard technique of two-dimensional pre-filtering with a Metz filter followed by filtered back-projection (without scatter correction). Five observers evaluated the image quality of reconstructed coronal and sagittal slices, with special reference to the resolution of vertebrae, vertebral processes, the spinal canal and suspected abnormal uptakes. A grading scale from -2 to +2 was used with the standard technique as the reference. The best image quality was found with iterative reconstruction in combination with a contrast enhancing Metz filter or a noise reducing Butterworth filter. Scatter correction did not improve image quality.