Interpolated Average CT for Attenuation Correction in PET—A Simulation Study

Abstract

Previously, we proposed using an interpolated average CT (IACT) method for attenuation correction (AC) in positron emission tomography (PET), which is a good, low-dose approximation of cine average CT (CACT) to reduce misalignments and improve quantification in PET/CT. This study aims to evaluate the performance of IACT for different motion amplitudes. We used the digital four-dimensional (4-D) extended cardiac-torso phantom (XCAT) to simulate maximum of 2, 3, and 4 cm respiratory motions. The respiratory cycle was divided into 13 phases, with average activity and attenuation maps to represent 18F-fluorodeoxyglucose (18F-FDG) distributions with average respiratory motions and CACT, respectively. The end-inspiration, end-expiration, and midrespiratory phases of the XCAT attenuation maps represented three different helical CTs (i.e., HCT-1, HCT-5, and HCT-8). The IACTs were generated using: 1) 2 extreme+11 interpolated phases (IACT 2o ); 2) 2 phases right after the extreme phases+11 interpolated phases (IACT 2s); 3) 4 original+9 interpolated phases (IACT 4o). A spherical lesion with a target-to-background ratio (TBR) of 4:1 and a diameter of 25 mm was placed in the base of right lung. The noise-free and noisy sinograms with attenuation modeling were generated and reconstructed with different noise-free and noisy AC maps (CACT, HCTs, and IACTs) by Software for Tomographic Image Reconstruction, respectively, using ordered subset expectation maximization(OS-EM) with up to 300 updates. Normalized mean-square error, mutual information (MI), TBR, image profile, and noise-contrast tradeoff were analyzed. The PET reconstructed images with AC using CACT showed least difference as compared to the original phantom, followed by IACT 4o, IACT 2o , IACT 2s, HCT-5, HCT-8, and HCT-1. Significant artifacts were observed in the reconstructed images using HCTs for AC. The MI differences between IACT 2o and IACT 4o /CACT were<0.41% and <2.17%, respectively. With a slight misplacement of the two extreme phases, IACT 2s was still comparable to IACT 2o with MI difference of <2.23%. The IACT is a robust and accurate low-dose alternate to CACT.

Motion Freeze for Respiration Motion Correction in PET/CT: A Preliminary Investigation With Lung Cancer Patient Data

“…Motion freeze technology combining the interpolated CT technology has high potential to overcome these challenges by using the entire acquired data to create a single 3D motion corrected image and to provide quantitative accuracy equivalent to 4D PET/CT with shorter acquisition times and less dose reduction [13,14]. Unlike conventional 4D PET imaging, motion freeze technology combines 100% of the PET counts into a 3D motion corrected image that has a comparable acquisition time and the equivalent image noise of a static acquisition.…”

“…These methods include breathing instruction, respiratory-gate (4D) PET/CT, motion-corrected PET reconstruction, and postprocessing methods [9][10][11][12][13][14][15]. Breathing instruction methods like deepinspiration breath-hold (DIBH) during the CT scan reduce the occurrence and the severity of respiratory curvilinear artifacts on coregistered PET/CT images.…”

“…The main problem of 4D technology is relatively high radiation dose compared to conventional HCT. With combining gated CT and imaging processing, the interpolated average CT used for PET/CT AC corrected the PET/CT misregistration and enhanced lesion quantitation accompanied with radiation dose reduction was proposed by Huang et al [13,14]. However averaging caused the lack of total effective counts in PET data.…”

Thoracic Tumor Volume Delineation in 4D-PET/CT by Low Dose Interpolated CT for Attenuation Correction

“…The optical flow method (OFM), a deformable image registration algorithm based on the image intensity gradient, was applied to calculate the motion displacement in three dimensions for each voxel on two original extreme CT phases in the respiratory cycle, end-inspiration (phase T5%) and end-expiration (phase T55%) [16,18]. The interpolated phases were generated using the deformation matrix from the OFM registration.…”

“…Its clinical merits in six oncologic patients and its potential application in cardiology have also been reported [17]. ICT has been reported to be a robust, accurate, low-dose alternative to 4D-CT and it works well for a large range of respiratory motion amplitudes [18]. This study aims to assess the interpretation of 4D-PET using ICT for AC on 4D-PET/CT in the spatial matching of PET and CT, localization of tumors, quantification of tumor volume and quantitation of SUV for clinical thoracic cancer patients.…”