On Artifact-Free Projection Overlaps in Multi-Pinhole Tomographic Imaging

Abstract

In multi-pinhole SPECT, overlapping the projections from different pinholes has been used to increase sensitivity. However, the prevailing view is that the overall quality of the reconstructed image is not improved by the overlaps in the projections. It is often stated in literatures that overlaps introduce ambiguous information, which can lead to nonuniqueness of solution for the inverse problem, and thus artifacts are introduced in the reconstructed image. On the other hand, contrary to the prevailing view, a recent study on slit-slat collimators shows that artifacts can be removed with the "help" of an extra complete nonoverlapped projection data set. In this paper, two types of artifact-free projection overlaps are defined in general, and the criteria for designing artifact-free multi-pinhole systems with overlaps are proposed. It is shown that once the criteria are satisfied, the solution of the inverse problem is unique, and thus no artifact is expected in the reconstructed image. Via extensive simulation study, various artifact-free overlapping multi-pinhole systems are designed and validated. It is shown that overlaps in the artifact-free systems can improve contrast-to-noise ratio (CNR). With a proper design, the CNR for an artifact-free overlapping system can be significantly higher than that for the corresponding nonoverlapping system. The improved image quality is also confirmed with noisy reconstructions.

Reducing Multiplexing Artifacts in Multi-Pinhole SPECT With a Stacked Silicon-Germanium System: A Simulation Study

“…Other studies using mixed-data projections have focused on more traditional MLEM and OSEM, such as Mahmood et al [15], where OSEM was utilized with both multiplexed and non-multiplexed projections in each subset, which is similar to the OSEM one-phase reconstruction tested in this work. Lin [9] also used a typical OSEM algorithm, as projections in that study are not easily separable into low or highly-multiplexed groups. In Havelin et al [13], where limited-angle tomography was performed with 6 different geometric configurations, the quality of their reconstructed images might improve if, instead of the published MLEM reconstruction, an alternative reconstruction scheme were used that altered how projections were combined.…”

“…This allows for novel system configurations to be developed that have increased sensitivity due to smaller radii of rotation. An extension of this idea to multiplexing was investigated by Lin that determined that if a subset of non-multiplexed projections exist that is independently sufficient to reconstruct an object, and they are consistent with the multiplexed projections, then reconstructions with both the multiplexed and non-multiplexed data will yield an artifact-free image [9]. This study also concluded that given the above conditions, in some cases contrast-to-noise ratio can be improved even after the surface area of the detector is fully utilized.…”

The Evaluation of Data Completeness and Image Quality in Multiplexing Multi-Pinhole SPECT

“…Recently, an article has been published by Lin [17] in which a method was developed to predict the presence of artifacts in multiplexing systems based on data completeness. This method was developed independently from ours but it shows many similarities.…”

“…NPW-SNR is then defined as (17) with the observer response averaged over all lesions and all noise realizations and with the standard deviation of the observer response.…”

“…This is not a necessary condition, certainly not for systems like 2 MPH A (which corresponds to multiplexing Type I in [17]). Most systems discussed in literature are of Type I.…”