Reduction of collimator correction artefacts with bayesian reconstruction in spect

Abstract

Poor resolution of single photon emission computed tomography (SPECT) has degraded its use in clinical practice. Collimator correction has been shown to improve the reconstructed resolution, but the correction can generate ringing artefacts, which lower image quality. This paper investigates whether Bayesian reconstruction methods could reduce these artefacts. We have applied and tested three Bayesian reconstruction methods: smoothing prior, median root prior, and anatomical prior. To demonstrate the efficacy of these methods, we compared their physical and visual performance both in phantom and patient studies. All the three Bayesian reconstruction methods reduced the collimator correction artefacts. Images reconstructed using the smoothing prior and the median root prior had slightly lower contrast than the standard reconstruction with collimator correction, whereas the anatomical prior produced images with good resolution and contrast.

Figure 1

An example of the Gibbs-like ringing artefacts. Image (a) represents a reconstructed transverse slice of a phantom with active spheres with different diameters without collimator response correction, and image (b) shows a transverse slice with collimator response correction. While the collimator response correction improves the image resolution, it generates an artefact that can be seen as a hole in the middle of the two biggest circles, indicated by black arrows.

Figure 2

Block diagram of OSEM and OSL reconstruction algorithms. OSEM iteration (inside the dashed rectangle) consists of the following steps: forward-projection of the current reconstructed image, division of measured and calculated projections, back-projection of the quotient, and multiplication of the current reconstructed image and the back-projected correction factors. OSL iteration differs only by the multiplication with the penalty factors that have been calculated by using the current reconstructed image.

Figure 3

PTW-Freiburg's phantom with the hot-sphere insert attached (a) and the hot-sphere insert (b) on its own. The insert includes hollow spheres with different diameters, which can be filled via thin capillaries.

Figure 4

Veenstra Instruments' SPECT-phantom's inserts. Images (a)–(c) show the cold lesion insert, the grid insert, and the hot lesion insert, respectively.

Figure 5

A representative slice taken from the PTW-Freiburg's PET/SPECT phantom with the hot-sphere insert with the five different reconstruction methods used and also the equivalent CT slice. Below, the images profiles for the largest sphere (black line) and the corresponding theoretical profile (grey line) scaled to the reconstructed image's maximum value are shown. From left to right: OSEM without collimator correction (OSEM NORR), OSEM reconstruction with collimator correction (OSEM RR), Median root prior (MRP), Quadratic smoothing prior (SMOOTH), Bowsher prior (AMAP), and low-dose CT slice, which has been resampled to SPECT image size. The black arrow marks the location of the missing sphere.

Figure 6

A representative slice taken from the three inserts of the Veenstra Instruments' SPECT-phantom with the different reconstruction methods used and also the equivalent CT slice. From left to right: OSEM without collimator correction (OSEM NORR), OSEM with collimator correction (OSEM RR), Median root prior (MRP), Quadratic smoothing prior (SMOOTH), Bowsher prior (AMAP), and low-dose CT slice, which has been resampled to SPECT image size. Arrows show faint ringing artefacts on the largest rods.

Figure 7

Clinical bone SPECT reconstructed with the five different algorithms. From left to right: OSEM without collimator correction (OSEM NORR), OSEM with collimator correction (OSEM RR), Median root prior (MRP), Quadratic smoothing prior (SMOOTH), Bowsher prior (AMAP), and low-dose CT slice, which has been resampled to SPECT image size.