Dose Optimization in 18F-FDG PET Based on Noise-Equivalent Count Rate Measurement and Image Quality Assessment

The present study aimed to optimize the injected dose of ¹⁸F-FDG in whole-body PET/CT scans and assess its effect on noise-equivalent count rate (NECR) and visual image quality (IQ). Methods: Patients scheduled to undergo ¹⁸F-FDG PET/CT were prospectively recruited in the study from January to December 2019, regardless of the indication or underlying disease. Patients were divided into 4 groups and injected with different amounts of ¹⁸F-FDG radioactivity per kilogram of body weight (1.85, 3.7, 5.5, and 7.4 MBq/kg). All patients underwent ¹⁸F-FDG PET/CT studies, and NECR_local was calculated by noting the trues rate, total prompts, and randoms rate for each bed position. Whole-body NECR_global was calculated as the average NECR for all bed positions. IQ was qualitatively assessed for each bed position (IQ_local) and for whole-body PET (IQ_global) by 2 readers using 5-point scores based on prevalence of noise, contrast, and lesion detectability. NECR and IQ were compared among all 4 activity groups. Patients were also subdivided into 4 body-mass-index groups (group I, 15-20 kg/m²; group II, 20.1-25 kg/m²; group III, 25.1-30 kg/m²; and group IV, 30.1-35 kg/m²) for comparison. A P value of less than 0.05 was considered significant. Results: In total, 109 patients underwent ¹⁸F-FDG PET/CT studies after injection of different amounts of ¹⁸F-FDG radioactivity and a mean uptake time of 62.32 min. The mean NECR_global and IQ_global for each group were significantly different from other groups (P < 0.05), with NECR and IQ being higher in high-activity groups than in low-activity groups. The overall IQ was acceptable in all patients, even in the lowest-activity group (1.84 MBq/kg). The mean NECR_global and IQ_global were significantly different in all 4 body-mass-index groups (P < 0.05), except between groups II and III (P > 0.05). NECR_local and IQ_local correlated moderately (r = 0.64). Conclusion: Optimization of injected ¹⁸F-FDG radioactivity from 7.4 MBq/kg (200 μCi/kg) to 1.85 MBq/kg (50 μCi/kg) resulted in acceptable IQ, despite a reduction in NECR.

Dosage optimization in positron emission tomography: state-of-the-art methods and future prospects

Positron emission tomography (PET) is widely used nowadays for tumor staging and therapy response in the clinic. However, average PET radiation exposure has increased due to higher PET utilization. This study aims to review state-of-the-art PET tracer dosage optimization methods after accounting for the effects of human body attenuation and scan protocol parameters on the counting rate. In particular, the relationship between the noise equivalent count rate (NECR) and the dosage (NECR-dosage curve) for a range of clinical PET systems and body attenuation sizes will be systematically studied to prospectively estimate the minimum dosage required for sufficiently high NECR. The optimization criterion can be determined either as a function of the peak of the NECR-dosage curve or as a fixed NECR score when NECR uniformity across a patient population is important. In addition, the systematic NECR assessments within a controllable environment of realistic simulations and phantom experiments can lead to a NECR-dosage response model, capable of predicting the optimal dosage for every individual PET scan. Unlike conventional guidelines suggesting considerably large dosage levels for obese patients, NECR-based optimization recommends: i) moderate dosage to achieve 90% of peak NECR for obese patients, ii) considerable dosage reduction for slimmer patients such that uniform NECR is attained across the patient population, and iii) prolongation of scans for PET/MR protocols, where longer PET acquisitions are affordable due to lengthy MR sequences, with motion compensation becoming important then. Finally, the need for continuous adaptation of dosage optimization to emerging technologies will be discussed.

Clinical image quality perception and its relation to NECR measurements in PET

Background

The purpose of this study is to describe a clinical relation of noise equivalent count rate (NECR) - an objective measurement of positron emission tomography (PET) systems - measured in a large number of patients, to clinical image quality of PET and their relation to 18F-fluoro-2-deoxyglucose (FDG) activity and patient's weight.

Methods

A total of 71 consecutive patients were evaluated in this retrospective study. All data was automatically analysed using Matlab to estimate the noise equivalent count rate. Then, image quality was evaluated according to two subjective scores: the IQ local score was a 3-point scale assigned to each bed position in all patients and the IQ global score was a 10-point scale assigned after evaluating the coronal whole-body PET. Patient data was also analysed concerning weight, body mass index, FDG dose at the start of acquisition (D_Acq), presence of bowel uptake and presence of FDG-positive pathologic lesions. Two additional parameters were defined for each patient: the ratio between D_Acq and patient weight (R_DW) and the ratio between D_Acq and patient BMI (R_DBMI).

Results

Clinically perceived image quality in PET has a significant positive correlation with NECR measured in patients, R_DW, R_DBMI and presence of pathologic lesions. Clinical image quality furthermore has significant negative correlation with weight, body mass index (BMI) and presence of bowel uptake. Thresholds of R_DW and R_DBMI in which clinical IQ is good to excellent in more than 90% of the patients were 2.6 and 8.0, respectively.

Conclusions

Clinically perceived image quality in PET systems is positively and significantly related to NECR measured in patients. An optimal threshold for the R_DW and R_DBMI was defined in which clinical IQ is good to excellent in more than 90% of patients. With this data, it is possible to extrapolate technical as well as clinical image quality to other PET system and to predict clinical image perception.

Optimization of the Energy Window for PETbox4, a Preclinical PET Tomograph With a Small Inner Diameter

Small animal positron emission tomography (PET) systems are often designed by employing close geometry configurations. Due to the different characteristics caused by geometrical factors, these tomographs require data acquisition protocols that differ from those optimized for conventional large diameter ring systems. In this work we optimized the energy window for data acquisitions with PETbox4, a 50 mm detector separation (box-like geometry) pre-clinical PET scanner, using the Geant4 Application for Tomographic Emission (GATE). The fractions of different types of events were estimated using a voxelized phantom including a mouse as well as its supporting chamber, mimicking a realistic mouse imaging environment. Separate code was developed to extract additional information about the gamma interactions for more accurate event type classification. Three types of detector backscatter events were identified in addition to the trues, phantom scatters and randoms. The energy window was optimized based on the noise equivalent count rate (NECR) and scatter fraction (SF) with lower-level discriminators (LLD) corresponding to energies from 150 keV to 450 keV. The results were validated based on the calculated image uniformity, spillover ratio (SOR) and recovery coefficient (RC) from physical measurements using the National Electrical Manufacturers Association (NEMA) NU-4 image quality phantom. These results indicate that when PETbox4 is operated with a more narrow energy window (350-650 keV), detector backscatter rejection is unnecessary. For the NEMA NU-4 image quality phantom, the SOR for the water chamber decreases by about 45% from 15.1% to 8.3%, and the SOR for the air chamber decreases by 31% from 12.0% to 8.3% at the LLDs of 150 and 350 keV, without obvious change in uniformity, further supporting the simulation based optimization. The optimization described in this work is not limited to PETbox4, but also applicable or helpful to other small inner diameter geometry scanners.