Amplitude Binning Research Articles

Retrospective data-driven respiratory gating for PET/CT

Respiratory motion can adversely affect both PET and CT acquisitions. Respiratory gating allows an acquisition to be divided into a series of motion-reduced bins according to the respiratory signal, which is typically hardware acquired. In order that the effects of motion can potentially be corrected for, we have developed a novel, automatic, data-driven gating method which retrospectively derives the respiratory signal from the acquired PET and CT data. PET data are acquired in listmode and analysed in sinogram space, and CT data are acquired in cine mode and analysed in image space. Spectral analysis is used to identify regions within the CT and PET data which are subject to respiratory motion, and the variation of counts within these regions is used to estimate the respiratory signal. Amplitude binning is then used to create motion-reduced PET and CT frames. The method was demonstrated with four patient datasets acquired on a 4-slice PET/CT system. To assess the accuracy of the data-derived respiratory signal, a hardware-based signal was acquired for comparison. Data-driven gating was successfully performed on PET and CT datasets for all four patients. Gated images demonstrated respiratory motion throughout the bin sequences for all PET and CT series, and image analysis and direct comparison of the traces derived from the data-driven method with the hardware-acquired traces indicated accurate recovery of the respiratory signal.

4D-CT Reconstruction Based on Accurate Vector Field Inter/Extrapolation

Four-dimensional computed tomography (4D-CT) generated using amplitude binning performs better and contains fewer artifacts, compared to those based on phase binning. However, missing amplitude values may appear during image acquisition using the external respiratory signal provided by a Real-time Position Management system (RPM) Here, we propose a method to overcome possible missing amplitude values based on accurate motion inter/extrapolation.

TH‐C‐332‐04: Comparison of Residual Motion Artifacts in Phase‐ and Amplitude‐Binned Onboard 4D‐CBCT Imaging

Purpose: Linac‐mounted 4D‐Cone Beam CT (CBCT) imaging is an important tool for extending 3D IGRT techniques to respiration‐motion synchronized treatments. Our purpose is to evaluate the effects of two different projection binning strategies, phase and amplitude binning, on the image quality of our recently implemented 4D‐CBCT method using the Varian On Board Imager and RPM breathing motion surrogate. Method and Materials: We have implemented a 4D‐CBCT imaging procedure that adjusts the acquisition parameters to the breathing period. The Varian Real‐time Position Management system (RPM) was used to establish temporal coincidence between the respiratory motion and the CBCT projections. Projections were sorted into 10 bins. For the projections sorted into each bin, the variances about the RPM amplitude and phase means was calculated and compared to the image quality achieved in CBCT images. 4D‐CBCTs were studied with a phantom with irregular motion cycle. The phantom consists of two acrylic spheres and one cylinder on a motion platform to mimic the superior‐inferior tumor motion mostly seen in patients. Results: With breathing cycle period variation but not amplitude variation, we found that amplitude binning produced fewer blurring artifacts which was consistent with RPM analysis which showed significantly smaller amplitude variances for amplitude binning. For the patient subjects, the RPM amplitude but not phase variances were smaller for amplitude binning. However, image quality was significantly better when phase binning was used. We hypothesize that the under‐sampling artifacts play a major role over the motion artifacts which is the case for amplitude binning, while the phase binning has a good control of aliasing artifacts. Residual motion artifacts persist in both phase‐ and amplitude‐ binning 4D‐CBCT images. Conclusion: Optimizing the binning strategies requires understanding complex interplay between residual motion in each bin and other factors such as uniformity and adequacy of angular sampling.Supported by PPG‐NIH‐P01116602.

SU‐GG‐J‐131: Motion Assessment Using Phase‐ and Amplitude‐Binned 4D‐CT: Comparison with Cine CT

Purpose: To determine the accuracy of tumor motion extent measurements derived from phase‐ and amplitude‐binned 4D‐CT maximum intensity projections (MIPs). Method and Materials: Twelve stage I patients (13 lesions) received cine CT scans for 4D‐CT radiotherapy simulation. All lesions were in the middle of the lung parenchyma and not adjacent to surrounding tissue. Two 4D‐CT image sets were constructed: One sorted by phase, the other sorted by amplitude. MIPs were formed from each 4D‐CT image set and from unsorted cine CT data. Lesions were auto‐segmented on each image set using a threshold of −225 HU. Using the auto‐segmented contours as a guide, three‐dimensional motion extent was assessed visually on both 4D‐CT MIPs and compared with motion extent assessed on MIP from cine CT. The cine MIP was considered the reference image set because it includes all images at each couch position and therefore more accurately samples the respiratory pattern. Results: For phase‐binned 4D‐CT, motion extent was accurately identified on 12 of 13 lesions. For amplitude‐binned 4D‐CT, motion extent was accurately identified on 11 of 13 lesions. The two patients whose motion extent was missed on amplitude‐binned 4D‐CT demonstrated a substantial discrepancy near the superior edge of the tumor. Upon further investigation, this discrepancy was found to be caused by lag between the respiratory surrogate and internal anatomy. Conclusion: Both phase binning and amplitude binning were unable to accurately identify the motion extent for all 13 lesions. Though generally regarded as more artifact‐free, amplitude‐binned 4D‐CT demonstrated increased sensitivity to the correlation between respiratory surrogate and internal anatomy which caused inaccurate assessments of motion extent. MIP processed directly from cine CT should be used in place of MIP processed from 4D‐CT (either phase‐ or amplitude‐binned) wherever possible.

Evaluation of amplitude-based sorting algorithm to reduce lung tumor blurring in PET images using 4D NCAT phantom

Purpose: develop and validate a PET sorting algorithm based on the respiratory amplitude to correct for abnormal respiratory cycles. Method and materials: using the 4D NCAT phantom model, 3D PET images were simulated in lung and other structures at different times within a respiratory cycle and noise was added. To validate the amplitude binning algorithm, NCAT phantom was used to simulate one case of five different respiratory periods and another case of five respiratory periods alone with five respiratory amplitudes. Comparison was performed for gated and un-gated images and for the new amplitude binning algorithm with the time binning algorithm by calculating the mean number of counts in the ROI (region of interest). Results: an average of 8.87 ± 5.10% improvement was reported for total 16 tumors with different tumor sizes and different T/B (tumor to background) ratios using the new sorting algorithm. As both the T/B ratio and tumor size decreases, image degradation due to respiration increases. The greater benefit for smaller diameter tumor and lower T/B ratio indicates a potential improvement in detecting more problematic tumors.

Phase and amplitude binning for 4D-CT imaging

We compare the consistency and accuracy of two image binning approaches used in 4D-CT imaging. One approach, phase binning (PB), assigns each breathing cycle 2π rad, within which the images are grouped. In amplitude binning (AB), the images are assigned bins according to the breathing signal's full amplitude. To quantitate both approaches we used a NEMA NU2-2001 IEC phantom oscillating in the axial direction and at random frequencies and amplitudes, approximately simulating a patient's breathing. 4D-CT images were obtained using a four-slice GE Lightspeed CT scanner operating in cine mode. We define consistency error as a measure of ability to correctly bin over repeated cycles in the same field of view. Average consistency error μe ± σe in PB ranged from 18% ± 20% to 30% ± 35%, while in AB the error ranged from 11% ± 14% to 20% ± 24%. In PB nearly all bins contained sphere slices. AB was more accurate, revealing empty bins where no sphere slices existed. As a proof of principle, we present examples of two non-small cell lung carcinoma patients' 4D-CT lung images binned by both approaches. While AB can lead to gaps in the coronal images, depending on the patient's breathing pattern, PB exhibits no gaps but suffers visible artifacts due to misbinning, yielding images that cover a relatively large amplitude range. AB was more consistent, though often resulted in gaps when no data existed due to patients' breathing pattern. We conclude AB is more accurate than PB. This has important consequences to treatment planning and diagnosis.

SU‐EE‐A4‐04: Phase and Amplitude Binning for 4D‐CT Imaging

We compare the consistency and accuracy of two image binning approaches used in 4D‐CT imaging. In 4D‐CT the images and respiratory motion are correlated via RPM Respiratory Gating system (Varian, Palo Alto, CA). In phase binning (PB), RPM assigns each breathing cycle 2π radians, within which the images are grouped. Alternately, the images are assigned bins according to the signal's amplitude (AB). To quantitate both approaches, we used NEMA NU2‐2001 IEC phantom oscillating at random frequencies and amplitudes, simulating patients breathing. 4D‐CT images were obtained using 4‐slice GE Lightspeed CT operating in cine mode. We define consistency error as ability to correctly bin over breathing cycles in the same FOV. Average consistency error in PB ranged from 18%±20% to 30%±35%, while in AB the error ranged from 11%±14% to 20%±24%. For 28mm sphere, PB images were hardly consistent, with error of 43%±47%, while AB images for the same sphere resulted in error of only 18%±22%. In PB, while not all breathing cycles covered all phases, nearly all bins contained sphere slices. AB was more accurate, revealing empty bins where no sphere slices existed. As a proof of principle, we present examples of two NSCLC patients' 4D‐CT lung images binned by both approaches. While AB can lead to gaps in the images, depending on patients' breathing pattern, PB exhibits no gaps but suffers visible artifacts due to misbinning, yielding images that covered a relatively large amplitude range. AB was more consistent, though often resulting in gaps corresponding to CT slices where no data existed due to patients' breathing. We conclude AB is more accurate than PB, which should be factored into treatment planning and diagnosis.Work supported by GE Healthcare Technologies and Varian Medical Systems.