To create a realistic PET-MR simulation platform for use in neuro-PET-MR imaging research. The recent introduction of integrated PET-MR scanners has opened up a plethora of new areas of research utilizing multimodal patient data. However, due to practical, logistical, and monetary limitations, the acquisition of real-world tri-modality (PET/MR/CT) data can be problematic. The developed E-phantom platform solves the PET data insufficiency problem for researchers and streamlines the research process without compromising anatomical diversity or PET realism.

Phantom. The first step of the simulation process is the creation of patient-based PET E-phantoms from co-registered MR and CT images of the same subject. The most fundamental component of each E-phantom is an activity map of the desired tracer distribution. Activity maps are built from tissue-segmented MR (brain) and CT (skull/soft tissue) images with tracer-appropriate relative activity levels assigned by tissue class and literature-based tracer distributions. Activity maps may also be drawn from acquired PET data when available. Additional anatomical structures, such as hot or cold lesions, can also be added to the activity phantom. Lesions may be targeted to brain regions using a registered label atlas or limited to certain tissue classes. Each phantom also requires a true attenuation map created by conversion of CT-derived Hounsfield units into linear attenuation coefficients and, optionally, an attenuation correction map (μ-map).

Simulation. PET data acquisition is simulated by projection of the activity and true attenuation map into sinogram space using the geometry of any PET-MR system. The simulated PET data are then iteratively reconstructed using the industry-standard OSEM method accelerated by GPU. The reconstruction platform is capable of performing attenuation, randoms, and system uniformity corrections on the simulated data but does not support scatter simulation or correction. Reconstructed PET images can be matched to manufacturer reconstruction space, allowing straightforward comparison to acquired PET images.

Validation and capabilities of the simulation platform were demonstrated using tri-modality (¹⁸F-Florbetapir PET/MR/CT) data acquired from 28 subjects. The process was validated by comparing simulated to acquired PET images. Acquired PET data were reconstructed using manufacturer software with gold-standard CT-based attenuation correction. These reconstructions were then used as the activity map input to the E-phantom platform and reconstructed using the same gold-standard CT-based attenuation correction. Bulk regional and whole-brain relative error was calculated from the difference of the manufacturer and simulation platform reconstructions.

Mean whole-brain error (±standard deviation) between the reconstruction platforms was 1.54% (±0.46%). Error analysis of MR-based μ-map performance revealed strong linear correlation (slope = 0.90, r² = 0.93) between errors determined using manufacturer processing software and phantom data reconstructed on the E-phantom platform. The total time required to simulate acquisition of 3.5x10⁸ lines of response (LORs), collapse to 7.3x10⁷ LORs, apply correction factors, and iteratively reconstruct one phantom was 15 minutes on a computer with Intel i7-860 CPU, Nvidia GTX 780 GPU, and 8 GB RAM.The E-phantom simulation platform provides numerous advantages over real-world acquired PET data. Since only bi-modal (MR/CT) data are required to utilize the E-phantom platform, larger research datasets can be constructed than typically available to PET-MR researchers. These large datasets can improve anatomical diversity and statistical power over acquired PET data alone. Additional capabilities of the E-phantom platform include the ability to selectively add region targeted hot and cold lesions and to model different PET tracer distributions, such as PIB, FDG, or Tau. The platform has been demonstrated using MR-based μ-map evaluation; other applications include study of PET dose reduction and MR-driven reconstruction methods.