Calculation of attenuation factors from combined singles and coincidence emission projections

Attenuation correction in emission tomography using the emission data--A review

The problem of attenuation correction (AC) for quantitative positron emission tomography (PET) had been considered solved to a large extent after the commercial availability of devices combining PET with computed tomography (CT) in 2001; single photon emission computed tomography (SPECT) has seen a similar development. However, stimulated in particular by technical advances toward clinical systems combining PET and magnetic resonance imaging (MRI), research interest in alternative approaches for PET AC has grown substantially in the last years. In this comprehensive literature review, the authors first present theoretical results with relevance to simultaneous reconstruction of attenuation and activity. The authors then look back at the early history of this research area especially in PET; since this history is closely interwoven with that of similar approaches in SPECT, these will also be covered. We then review algorithmic advances in PET, including analytic and iterative algorithms. The analytic approaches are either based on the Helgason-Ludwig data consistency conditions of the Radon transform, or generalizations of John's partial differential equation; with respect to iterative methods, we discuss maximum likelihood reconstruction of attenuation and activity (MLAA), the maximum likelihood attenuation correction factors (MLACF) algorithm, and their offspring. The description of methods is followed by a structured account of applications for simultaneous reconstruction techniques: this discussion covers organ-specific applications, applications specific to PET/MRI, applications using supplemental transmission information, and motion-aware applications. After briefly summarizing SPECT applications, we consider recent developments using emission data other than unscattered photons. In summary, developments using time-of-flight (TOF) PET emission data for AC have shown promising advances and open a wide range of applications. These techniques may both remedy deficiencies of purely MRI-based AC approaches in PET/MRI and improve standalone PET imaging.

Joint Estimation of Activity and Attenuation in Whole-Body TOF PET/MRI Using Constrained Gaussian Mixture Models

It has recently been shown that the attenuation map can be estimated from time-of-flight (TOF) PET emission data using joint maximum likelihood reconstruction of attenuation and activity (MLAA). In this work, we propose a novel MRI-guided MLAA algorithm for emission-based attenuation correction in whole-body PET/MR imaging. The algorithm imposes MR spatial and CT statistical constraints on the MLAA estimation of attenuation maps using a constrained Gaussian mixture model (GMM) and a Markov random field smoothness prior. Dixon water and fat MR images were segmented into outside air, lung, fat and soft-tissue classes and an MR low-intensity (unknown) class corresponding to air cavities, cortical bone and susceptibility artifacts. The attenuation coefficients over the unknown class were estimated using a mixture of four Gaussians, and those over the known tissue classes using unimodal Gaussians, parameterized over a patient population. To eliminate misclassification of spongy bones with surrounding tissues, and thus include them in the unknown class, we heuristically suppressed fat in water images and also used a co-registered bone probability map. The proposed MLAA-GMM algorithm was compared with the MLAA algorithms proposed by Rezaei and Salomon using simulation and clinical studies with two different tracer distributions. The results showed that our proposed algorithm outperforms its counterparts in suppressing the cross-talk and scaling problems of activity and attenuation and thus produces PET images of improved quantitative accuracy. It can be concluded that the proposed algorithm effectively exploits the MR information and can pave the way toward accurate emission-based attenuation correction in TOF PET/MRI.

Anomaly Detection and Artifact Recovery in PET Attenuation-Correction Images Using the Likelihood Function

In dual modality PET/CT, CT data are used to generate the attenuation correction applied in the reconstruction of the PET emission image. This requires converting the CT image into a 511-keV attenuation map. Algorithms for making this transformation require assumptions about the makeup of material within the patient. Anomalous material such as contrast agent administered to enhance the CT scan confounds conversion algorithms and has been observed to result in inaccuracies, i.e., inconsistencies with the true 511-keV attenuation present at the time of the PET emission scan. These attenuation artifacts carry through to the final attenuation-corrected PET emission image and can resemble diseased tissue. We propose an approach to correcting this problem that employs the attenuation information carried by the PET emission data. A likelihood-based algorithm for identifying and correcting of contrast is presented and tested. The algorithm exploits the fact that contrast artifacts manifest as too-high attenuation values in an otherwise high quality attenuation image. In a separate study, the performance of the loglikelihood as an objective-function component of a detection/correction algorithm, independent of any particular algorithm was mapped out for several imaging scenarios as a function of statistical noise. Both the full algorithm and the loglikelihood performed well in studies with simulated data. Additional studies including those with patient data are required to fully understand their capabilities.

Characterization of septal penetration in 511 keV SPECT

BACKGROUND

Single photon emission computed tomography (SPECT) with 511 keV photons is a challenging modality and collimators for this purpose require trade-offs among resolution, sensitivity and septal penetration. While PET is the modality of choice for imaging at 511 keV, there are some procedures, e.g., dual-isotope imaging, in which 511 keV SPECT has a role.

AIM

To measure the imaging properties of a VPC-93 SPECT collimator designed for imaging at 511 keV and to isolate the effects of septal penetration.

METHODS

NaI gamma camera projection images of (18)F (511 keV) and (99m)Tc (140 keV) point sources were measured and the corresponding modulation transfer functions calculated. The projection images were reconstructed via filtered back-projection to obtain the tomographic three-dimensional (3-D) point spread function. Differences between the 511 and 140 keV results were attributed mainly to septal penetration. Contrast measurements were made separately using (18)F and (99m)Tc of a 20 cm phantom containing hot spheres and a warm background. Both isotopes were also used in imaging studies of a 3-D Hoffman brain phantom.

RESULTS

Reconstructed 511 keV point source images were spatially extended with more than half of the total reconstructed counts appearing away from the point source region. The number of false counts contained in the image as a function of distance from the true source location remains approximately constant for large distances out to at least 14 cm. Septal penetration results in a rapid roll-off with spatial frequency of collimator response. The response of the collimator to 511 keV photons falls to half of its 0-frequency response at 0.03 cm(-1). For 140 keV photons this value is 0.20 cm(-1). A result is reduced image contrast as measured in the phantom sphere studies. Septal penetration causes image degradation through large-scale blurring. Image noise characteristics are modified and correlations are extended into many transaxial planes.

CONCLUSIONS

Both 2-D and 3-D point spread functions for 511 and 140 keV photons using the VPC-93 collimator have been measured. Septal penetration unfavourably affects image resolution and changes image noise characteristics. Without compensation, the effects of septal penetration are readily apparent in images of real objects.