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Background: Data‐driven gated (DDG) PET has gained clinical acceptance and has been shown to match or outperform external‐device gated (EDG) PET. However, in most clinical applications, DDG PET is matched with helical CT acquired in free breathing (FB) at a random respiratory phase, leaving registration, and optimal attenuation correction (AC) to chance. Furthermore, DDG PET requires additional scan time to reduce image noise as it only preserves 35%–50% of the PET data at or near the end‐expiratory phase of the breathing cycle. Purpose: A new full‐counts, phase‐matched (FCPM) DDG PET/CT was developed based on a low‐dose cine CT to improve registration between DDG PET and DDG CT, to reduce image noise, and to avoid increasing acquisition times in DDG PET. Methods: A new DDG CT was developed for three respiratory phases of CT images from a low dose cine CT acquisition of 1.35 mSv for a coverage of about 15.4 cm: end‐inspiration (EI), average (AVG), and end‐expiration (EE) to match with the three corresponding phases of DDG PET data: −10% to 15%; 15% to 30%, and 80% to 90%; and 30% to 80%, respectively. The EI and EE phases of DDG CT were selected based on the physiological changes in lung density and body outlines reflected in the dynamic cine CT images. The AVG phase was derived from averaging of all phases of the cine CT images. The cine CT was acquired over the lower lungs and/or upper abdomen for correction of misregistration between PET and FB CT as well as DDG PET and FB CT. The three phases of DDG CT were used for AC of the corresponding phases of PET. After phase‐matched AC of each PET dataset, the EI and AVG PET data were registered to the EE PET data with deformable image registration. The final result was FCPM DDG PET/CT which accounts for all PET data registered at the EE phase. We applied this approach to 14 18F‐FDG lung cancer patient studies acquired at 2 min/bed position on the GE Discovery MI (25‐cm axial FOV) and evaluated its efficacy in improved quantification and noise reduction. Results: Relative to static PET/CT, the SUVmax increases for the EI, AVG, EE, and FCPM DDG PET/CT were 1.67 ± 0.40, 1.50 ± 0.28, 1.64 ± 0.36, and 1.49 ± 0.28, respectively. There were 10.8% and 9.1% average decreases in SUVmax from EI and EE to FCPM DDG PET/CT, respectively. EI, AVG, and EE DDG PET/CT all maintained increased image noise relative to static PET/CT. However, the noise levels of FCPM and static PET were statistically equivalent, suggesting the inclusion of all counts was able to decrease the image noise relative to EI and EE DDG PET/CT. Conclusions: A new FCPM DDG PET/CT has been developed to account for 100% of collected PET data in DDG PET applications. Image noise in FCPM is comparable to static PET, while small decreases in SUVmax were also observed in FCPM when compared to either EI or EE DDG PET/CT. [ABSTRACT FROM AUTHOR]

Background: Misregistration between CT and PET data can result in mis‐localization and inaccurate quantification of functional uptake in whole body PET/CT imaging. This problem is exacerbated when an abnormal inspiration occurs during the free‐breathing helical CT (FB CT) used for attenuation correction of PET data. In data‐driven gated (DDG) PET, the data selected for reconstruction is typically derived from the end‐expiration (EE) phase of the breathing cycle, making this potential issue worse. Purpose: The objective of this study is to develop a deformable image registration (DIR)‐based respiratory motion model to improve the registration and quantification between misregistered FB CT and PET. Methods: Twenty‐two whole‐body 18F‐FDG PET/CT scans encompassing 48 lesions in misregistered regions were analyzed in this study. End‐inspiration (EI) and EE PET data were derived from −10% to 15% and 30% to 80% of the breathing cycle, respectively. DIR was used to estimate a motion model from the EE to EI phase of the PET data. The model was then used to generate PET images at any phase of up to four times the amplitude of motion between EE and EI for correlation with the misregistered FB CT. Once a matched phase of the FB CT was determined, FB CT was deformed to a pseudo CT at the EE phase (DIR CT). DIR CT was compared with the ground truth DDG CT for AC and localization of the DDG PET. Results: Between DDG PET/FB CT and DDG PET/DIR CT, a significant increase in ∆%SUV was observed (p < 0.01), with median values elevating from 26.7% to 42.4%. This new method was most effective for lesions ≤3 cm proximal to the diaphragm (p < 0.001) but showed decreasing efficacy as the distance increased. When FB CT was severely misregistered with DDG PET (>3 cm), DDG PET/DIR CT outperformed DDG PET/FB CT alone (p < 0.05). Even when patients showed varied breathing patterns during the PET/CT scan, DDG PET/DIR CT still surpassed the efficiency of DDG PET/FB CT (p < 0.01). Though DDG PET/DIR CT couldn't match the performance of the DDG PET/CT ground truth (42.4% vs. 53.6%, p < 0.01), it reached 84% of its quantification, demonstrating good agreement and a strong overall correlation (regression coefficient of 0.94, p < 0.0001). In some cases, anatomical distortion and blurring, and misregistration error were observed in DIR CT, rendering it still unable to correct inaccurate localization near the boundaries of two organs. Conclusions: Based on the motion model derived from gated PET data, DIR CT can significantly improve the quantification and localization of DDG PET. This approach can achieve a performance level of about 84% of the ground truth established by DDG PET/CT. These results show that self‐gated PET and DIR CT may offer an alternative clinical solution to DDG PET and FB CT for quantification without the need for additional cine‐CT imaging. DIR CT was at times inferior to DDG CT due to some distortion and blurring of anatomy and misregistration error. [ABSTRACT FROM AUTHOR]

It was hypothesized that the use of attenuation correction could compensate for degradation in the uniformity of apparent localization of imaging agents seen in cardiac walls when patients are imaged with arms at their sides. Noise-free simulations of the digital MCAT phantom were employed to investigate this hypothesis. Four variations in camera size and collimation scheme were investigated. We observed that: 1) without attenuation correction, the arms had little additional influences on the uniformity of the heart for 180 [degrees] reconstructions and caused a small increase in nonuniformity for 360 [degrees] reconstructions, where the impact of both arms was included; 2) change in patient size had more of an impact on count uniformity than the presence of the arms, either with or without attenuation correction; 3) for a low number of iterations and large patient size, slightly better uniformity was obtained from parallel emission data than from fan-beam emission data, independent of whether parallel or fan-beam transmission data was used to reconstruct the attenuation maps; and 4) for all camera configurations, uniformity was improved with attenuation correction and, given sufficient number of iterations, it was compatible among different imaging geometry combinations. Thus, iterative algorithms can compensate for the additional attenuation imposed by larger patients or having the arms on the sides. When the arms are at the sides of the patient, however, a larger radius of rotation may be required, resulting in decreased spatial resolution.

Purpose: We use central frequency ratio and noise magnitude ratio from noise power spectrum (NPS) to evaluate the noise reduction techniques of ASiR and ASiR‐V of GE, SAFIRE and ADMIRE of Siemens, and PixelShine of AlgoMedica. ASiR, ASiR‐V, SAFIRE and ADMIRE use a combination of image and projection data whereas PixelShine uses artificial intelligence neural network for noise reduction. Methods and materials: The homogeneous module of the ACR computed tomography (CT) phantom was scanned on a GE Revolution HD 64‐slice CT for ASiR and ASiR‐V, a Siemens Somatom Force for ADMIRE, and a Siemens Definition AS 64‐slice for SAFIRE for NPS calculation. The baseline filtered back‐projection (FBP) reconstructions were derived from the standard kernel on Revolution HD, Hr44f on Force and D40s on Definition AS. The central frequency ratio (CFR) indicates the degree of shift in the central frequency of NPS after noise reduction. A smaller CFR means a larger shift of the NPS curve, or a larger degree of image blurring. The noise magnitude ratio (NMR) indicates the amount of noise removed. A smaller NMR means a larger degree of noise reduction. An ideal noise reduction shall maintain a CFR close to 1 and a NMR close to 0. Results: The ideal noise reduction by increasing radiation exposure did not shift the central frequency when the image noise was reduced. PixelShine was the closest to the ideal noise reduction in CFR, and was followed by SAFIRE, ASiR‐V, ADMIRE and ASiR, in sequence. Similarly, PixelShine had the smallest NMR, and was followed by SAFIRE, ASiR‐V, ADMIRE and ASiR in sequence. Overall, PixelShine had the least central frequency shift for the same amount of noise reduction or the most noise reduction for the same amount of central frequency shift. For the same CFR, ASiR‐V reduced more noise than ASiR; and SAFIRE reduced more noise than ADMIRE. Conclusions: We introduced two new parameters of CFR and NMR from NPS to compare the reconstructions from different manufacturers. PixelShine had the least central frequency shift for the same amount of noise reduction or the most noise reduction for the same amount of central frequency shift. For the same central frequency shift, ASiR‐V reduced more noise than ASiR, and SAFIRE reduced more noise than ADMIRE. [ABSTRACT FROM AUTHOR]

Purpose: To perform dosimetric validation of the plan adaptation and high-definition motion management (HDMM) system of Gamma Knife® IconTM in various clinical scenarios. Methods and materials: We built an assembly for a pitch-adjustable anthropomorphic head phantom. We then used films to measure dosimetric and positional accuracy in 13 clinical scenarios, including movement near HDMM thresholds, multiple plan adaptations, frequent coughing, and initial setup error. Results: The dose for the superiorly located 4-mm shot was decreased up to 7-13% near 2- to 3-mm HDMM thresholds in the chin-down position. Dosimetric deviation was within ±3.5% for initial pitch angles of up to 20°. Multiple treatment interruption and frequent coughing did not cause substantial dosimetric deviation (<2%). Conclusion: Our results indicated that dosimetric accuracy of the Gamma Knife® IconTM system is reliable even in extreme treatment conditions. However, the user should exercise caution for superiorly located small lesions with an HDMM threshold ≥2 mm or in the scenario of large initial setup error. [ABSTRACT FROM AUTHOR]

Purpose: We present our institutional experience in treating brain metastases with GK-SRS and a headframe fixed to the skull with only 3 pins to avoid collisions between the headframe and the Gamma Knife (GK) machine. Methods and materials: Among 3500 consecutive patients who received GK-SRS in 2011-2017, 50 had 1 of the 2 anterior pins removed immediately before treatment of ≥1 brain lesion. Endpoints were local control, dosimetric parameters, and toxicity. Results: Median follow-up time for the 49 patients with follow-up was 7.0 months (range 0.2- 57.0). Median number of lesions treated per session was 6 (range 1-18); a median 1 lesion was treated with 3-pin fixation (range 1-2) and a median 5 lesions treated with 4-pin fixation (range 0-17) during the same session. Lesions treated with 3-pin fixation were in the occipital lobe (n=41), cerebellum (n=9), or temporal lobe (n=1). No local failures were noted. The sole grade 2 toxicity (partial seizure) was attributed to treatment of a 4-pin-fixed lesion. Except for gradient index, dosimetry did not vary for lesions treated with 3-pin versus 4-pin fixation. Conclusions: Treating brain metastases with 3-pin fixation did not compromise treatment outcome and is a good option for posterior brain metastases that cannot otherwise be treated with 4-pin GK-SRS. [ABSTRACT FROM AUTHOR]

Purpose: To assess the dosimetric effects of respiratory motion on the target and spinal cord in spinal stereotactic body radiation therapy (SBRT). Methods and materials: Thirty patients with 33 lesions were enrolled on a prospective clinical protocol and simulated with both free-breathing and four-dimensional (4-D) computed tomography (CT). We studied the target motion using 4-D data (10 phases) by registering a secondary image dataset (phase 1 to 9) to a primary image dataset (phase 0) and analyzing the displacement in both translational and rotational directions. The study of dosimetric impacts from respiration includes both the effect of potential target and spinal cord motion and anatomic changes in the beam path. A clinical step and shoot IMRT plan generated on the free-breathing CT was copied to the 4-D datasets to evaluate the difference in the dose-volume histogram of target and normal tissues in each phase of a breathing cycle. Results: Twenty three lesions had no motion in a breathing cycle; four lesions had anterior-posterior motion ⩽ 0.2 mm; two lesions had lateral motion ⩽ 0.2 mm; and eight lesions had superior-inferior motion, most ⩽ 0.2 mm with the worst at 0.6 mm. The difference of maximum dose to 0.01 cm3 of spinal cord in different phases of a breathing cycle was within 20 cGy in worst case. Target volumes that received the prescription dose (V100) varied little, with deviations of V100 of each phase from the average CT < 1% in most cases. Only when lesions were close to the diaphragm (e.g., at T11) did the V100 deviate by about 7% in the worst case scenario. However, this was caused by a small dose difference of 20 cGy to part of the target volume.Conclusions: Breathing induced target and spinal cord motion is negligible compared with other setup uncertainties. Dose calculation using averaged or free-breathing CT is reliable when posterior beams are used. [ABSTRACT FROM AUTHOR]