Scanner Protocol Design

Purpose

Kinetic parameters from dynamic ¹⁸F-fluorodeoxyglucose (FDG) imaging offer complementary insights to the study of disease compared to static clinical imaging. However, dynamic imaging protocols are cumbersome due to the long acquisition time. Long axial field-of-view (LAFOV) PET scanners (> 70 cm) have two advantages for dynamic imaging over clinical PET scanners with a standard axial field-of-view (SAFOV; 16–30 cm). The large axial coverage enables multi-organ dynamic imaging in a single bed position, and the high sensitivity may enable clinically routine abbreviated dynamic imaging protocols.

Methods

In this work, we studied two abbreviated protocols using data from a 65-min dynamic ¹⁸F-FDG scan: (A) dynamic imaging immediately post-injection (p.i.) for variable durations, and (B) dynamic imaging immediately p.i. for variable durations plus a 1-h p.i. (5-min-long) datapoint. Nine cancer patients were imaged on the Biograph Vision Quadra (Siemens Healthineers). Time-activity curves over the lesions (N = 39) were fitted using the Patlak graphical analysis and a 2-tissue-compartment (2C, k₄ = 0) model for variable scan durations (5–60 min). Kinetic parameters from the complete dataset served as the reference. Lesions from all cancers were grouped into low, medium, and high flux groups, and bias and precision of K_i (Patlak) and K_i, K₁, k₂, and k₃ (2C) were calculated for each group.

Results

Using only early dynamic data with the 2C (or Patlak) model, accurate quantification of K_i required at least 50 (or 55) min of dynamic data for low flux lesions, at least 30 (or 40) min for medium flux lesions, and at least 15 (or 20) min for high flux lesions to achieve both 10% bias and precision. The addition of the final (5-min) datapoint allowed for accurate quantification of K_i with a bias and precision of 10% using only 10–15 min of early dynamic data for either model.

Conclusion

Dynamic imaging for 10–15 min immediately p.i. followed by a 5-min scan at 1-h p.i can accurately and precisely quantify ¹⁸F-FDG on a long axial FOV scanner, potentially allowing for more widespread use of dynamic ¹⁸F-FDG imaging.

CT guidance is widely used for many interventional spine procedures. These procedures are less invasive than open surgery, and the use of CT guidance can shorten procedure time, lessen postprocedural care, and minimize cost. CT is generally used to gain important information for locating and tracking a safe path to the target and evaluating efficacy in reaching the target [1]. However, the medical community and general public have raised concerns pertaining to the possible harmful consequences of medical radiation exposure [2].

A technique that we refer to as the “scout no scan” technique eliminates the step of performing traditional planning CT for target localization. The scout no scan technique includes three main steps: using prior imaging to determine target location, mapping target localization directly from the scout image, and performing low-dose targeted CT fluoroscopy [3]. Our study reports how the scout no scan technique can be used for CT-guided spinal biopsies to reduce radiation exposure while maintaining good image quality with high diagnostic yields.

Background: Recent technological advances in myocardial perfusion imaging may warrant the use of lower injected activity. We evaluated whether quantitative measures of stress myocardial perfusion defects using Tc-99m sestamibi and low-energy high-resolution (LEHR) collimators are equivalent to lower dose SPECT-CT with cardiac multifocal collimators and software (IQ·SPECT).

Methods: 93 patients underwent one-day rest-stress gated SPECT-CT. Following conventional rest imaging, 925-1100 MBq (25-30 mCi) of Tc-99m sestamibi was injected during stress testing. Stress SPECT-CT images were acquired two ways: with LEHR (13 minutes) and IQ·SPECT (7 minutes). Low-dose IQ·SPECT stress was simulated by subsampling the full-dose data to half-, quarter-, and eighth-count levels. Abnormalities were quantified using the total perfusion deficit (TPD) score and dose-specific databases.

Results: The mean ± SD of the differences between LEHR and IQ·SPECT TPD scores were −1.01 ± 5.36%, −0.10 ± 5.81%, 1.78 ± 4.81%, and 1.75 ± 6.05% at full, half, quarter, and eighth doses, respectively. Differences were statistically significant for quarter and eighth doses. Correlation between LEHR and IQ·SPECT was excellent at all doses (R ≥ 0.93). Bland-Altman plots demonstrated minimal bias.

Conclusions: With IQ·SPECT, quantitative stress SPECT-CT imaging is possible with half of the standard injected activity in half the time.

Rationale and Objectives: Positron emission tomography (PET) is used to evaluate response to therapy with increasing interest in having PET provide endpoints for clinical trials. Here we demonstrate impacts of PET measurement error and choice of quantification method on clinical trial design.

Materials and Methods: Sample size was calculated for two-arm randomized trials with percent change in ¹⁸F-fluorodeoxyglucose (FDG) PET uptake as an efficacy endpoint. Two methods of uptake quantification were considered: standardized uptake values (SUVs) and kinetic measures from dynamic imaging. Calculations assumed a 20 percentage point difference in treatment groups’ average percent change, and yielded 80% power at α = 0.05. The range of precision (10%–40%) in PET uptake measures was based on review of the literature. The range of SUV sensitivities (50%–100%) relative to kinetic analyses was based on a study of 75 locally advanced breast cancerpatients.

Results: Sample sizes increased from 8 to 126 as PET precision worsened from 10% to 40% at full measurement sensitivity to true change. In a subgroup with low initial FDG uptake, a sample size of 126 was required under 20% standard deviation using clinical SUVs. More sophisticated imaging quantification could reduce this sample size to 32.

Conclusions: The dependence of sample size on measurement precision and the sensitivity of imaging measures to true change should be considered in single site and multicenter PET trials to avoid underpowered studies with inconclusive results. Sophisticated PET imaging methods that are more sensitive to changes in uptake may be advantageous in early studies with limited patient numbers.

PET image quality depends strongly on patient weight and habitus, decreasing for increasing weight and body mass index. Common adult injection rules prescribe either a dose proportional to weight or a fixed dose. In light patients, image quality may improve for decreasing weight more than by inverse proportion. If better quality than in average-adult studies does not justify the associated dose burden, attractive options are to reduce scan time, reduce dose, or any combination of the 2. The objective of this study was to determine quantitative injection rules for pediatric PET allowing clinical implementation of these trade-offs. Methods: Literature methods combining phantom with clinical data were followed to derive patient-specific noise-equivalent count rate density (NECRD) curves as a function of injected dose. From these, it was possible to estimate retrospectively for each patient the scan time that would have been sufficient for the same NECRD as in a 70-kg reference adult; the reduced dose sufficient for constant NECRD and scan time; and a general relationship among scan time, dose, and NECRD. Correlation to the patient statistic giving highest correlation, which was found to be weight, provided rules applicable prospectively. Data from 73 patients (weight, 11.5–91.4 kg; mean, 45.4 kg) were acquired and analyzed. Results: Following the clinical injection rule, which was proportional to weight, the NECRD increased linearly with decreasing weight. The expression exp[0.019 × (weight [kg] − 70)] for the time reduction possible with the current dose at constant NECRD correlated well with data (R² = 0.86). The dose (in MBq) necessary for constant NECRD that should be injected 60 min before imaging is predicted well by 14.8 × exp[0.046 × weight (kg)] (R²= 0.88) with the current scan time. A more complex expression to convert NECRD in whole or part to both dose and time savings was also derived. Comparison to common pediatric injection rules showed reasonable agreement with Clark's rule, albeit not at all weights. Conclusion: Results suggest that pediatric PET of constant image quality (in an NECRD sense) can be performed with time or dose savings, up to 50% for the lightest patients (10–20 kg).