Attenuation correction for pelvis PET imaging

Attenuation correction for pelvis PET-MR imaging

The pelvis is another area where MR-based approaches that do not properly account for bone attenuation could lead to substantial bias in PET data quantification and deep learning approaches have been proposed to minimize this bias starting from the MR data acquired using specialized MR sequences. Instead, we implemented a deep learning-based approach to generate pseudo-CT maps exclusively from the Dixon-VIBE MR images routinely acquired for attenuation correction on the Biograph mMR scanner. Using our 2D network that takes four contrasts as inputs (water, fat, in-phase, and out-of-phase Dixon-VIBE images), the mean absolute relative change in PET values in the pelvis area was 2.36% ± 3.15% (Torrado-Carvajal et al 2019).

One remaining challenge in using convolutional neuronal networks to synthesize CT from MR images of the pelvis is the presence of air pockets (i.e., digestive tract gas) in this area. As CT and MR images are acquired on separate scanners at different times, the locations and sizes of these air pockets can change between the two scans, which can lead to errors in both the MR-CT co-registration and image synthesis tasks. We trained and evaluated CNNs to automatically segment air pockets from MR CAIPIRINHA-accelerated Dixon images and assessed the quantitative impact on the reconstructed PET images (Sari et al, submitted to JNM).

Siemens Biograph mMR

The Biograph mMR scanner (Siemens Healthineers, Erlangen, Germany) consists of a 3T whole-body superconductive magnet with active shielding and external interference shielding and a whole-body PET scanner. It is equipped with a gradient system with a maximum gradient amplitude of 45 mT/m and a maximal slew rate of 200 T/m/s.  Separate cooling channels that simultaneously cool primary and secondary coils allow the application of extremely gradient intensive techniques. 

This scanner is equipped with the “TIM” RF coils that were custom designed to minimize the 511 keV photons attenuation. The fully-integrated PET detectors use avalanche photodiode (APD) technology and LSO scintillator crystals (eight rings with 56 detectors blocks per ring, each consisting of 8×8 arrays of 4×4×20 mm3crystals read out by a 3×3 array of APDs).  The PET scanner’s transaxial and axial fields of view are 594 mm and 25.8 cm, respectively.

The Biograph mMR was installed at the Martinos Center in June 2011.