Original Research
Nuclear Medicine
June 22, 2022

Simulated Reduced-Count Whole-Body FDG PET: Evaluation in Children and Young Adults Imaged on a Digital PET Scanner

Abstract

BACKGROUND. Digital PET scanners with increased sensitivity may allow shorter scan acquisition times or reductions in administered radiopharmaceutical activities.
OBJECTIVE. The purpose of this study was to evaluate in children and young adults the impact of shorter simulated acquisition times on the quality of whole-body FDG PET images obtained using a digital PET/CT system.
METHODS. This retrospective study included 27 children and young adults (nine male and 18 female patients) who underwent clinically indicated whole-body FDG PET/CT examinations performed using a 25-cm axial FOV PET/CT system at 90 s per bed position (expressed hereafter as seconds per bed). Raw list-mode data were reprocessed to simulate acquisition times of 60, 55, 50, 45, 40, and 30 s/bed. Three radiologists independently reviewed reconstructed images and assigned Likert scores for lesion conspicuity, normal structure conspicuity, image quality, and image noise. A separate observer recorded the SUVmax, SUVmean, and SD of the SUV (SUVSD) for liver, thigh, and the most FDG-avid lesion. The SUVSD/SUVmean (the SUVSD divided by the SUVmean) was calculated as a surrogate of image noise. ANOVA, the Friedman test, and the Dunn test were used to compare qualitative measures (combining reader scores) and SUV measurements.
RESULTS. The mean patient age was 10.8 ± 8.3 (SD) years, mean BMI was 18.7 ± 2.9, and mean administered FDG activity was 4.44 ± 0.37 MBq/kg (0.12 ± 0.01 mCi/kg). No qualitative measure showed a significant difference versus 90 s/bed for the simulated acquisition at 60 s/bed (all p > .05). Significant differences (all p < .05) versus 90 s/bed were observed for lesion conspicuity at at most 40 s/bed, conspicuity of normal structures and overall image quality at at most 45 s/bed, and image noise at at most 55 s/bed. SUVmean was not significantly different from 90 s/bed for any site for any reduced-count simulation (all p > .05). SUVSD/SUVmean and SUVmax showed gradual increases with decreasing acquisition times and were significantly different from 90 s/bed only for liver at 60 s/bed (for SUVmax: 1.00 ± 0.00 vs 1.05 ± 0.03, p = .02; for SUVSD/SUVmean: 0.09 ± 0.02 vs 0.11 ± 0.02, p = .04).
CONCLUSION. Favorable findings for the simulated acquisition at 60 s/bed suggest that, in children and young adults who undergo imaging performed using a 25-cm FOV digital PET scanner, acquisition time or administered FDG activity may be decreased by approximately 33% from the clinical standard without significantly impacting image quality.
CLINICAL IMPACT. A 25-cm axial FOV digital scanner may allow FDG PET/CT examinations to be performed with reduced radiation exposure or faster scan acquisition times.

HIGHLIGHTS

Key Finding
The clinical standard FDG PET/CT acquisition of 90 s/bed and the simulated acquisition of 60 s/bed were not significantly different for any qualitative measure and for only a single SUV-based measurement, suggesting that acquisition time or administered FDG activity can be reduced by approximately 33% without significantly impacting image quality.
Importance
A 25-cm FOV digital scanner may allow faster scans or reduced administered activity (thus lowering radiation exposure) for children and young adults undergoing FDG PET/CT.
FDG PET/CT is an important tool for making clinical decisions regarding many cancers in children and young adults. Both the PET and CT localization components of the examination impart radiation exposure to the patient. In contemporary practice that incorporates optimized technique for CT localization and attenuation correction as well as optimized radiopharmaceutical dosing, the PET component contributes the greater proportion of the patient's radiation dose [1]. In addition to patient radiation exposure, PET/CT examinations typically are long examinations that require sedation or anesthesia in very young children [2, 3]. Most of the duration of PET/CT examinations involves acquisition of the PET portion.
PET acquisition times (i.e., the duration of PET data acquisition) vary according to the administered activity and the sensitivity of the scanner. Professional nuclear medicine societies report typical acquisition times of approximately 1–3 minutes per bed position for pediatric oncologic imaging [4]. In general, increased PET acquisition times provide higher counts and lower image noise. However, increased PET acquisition times contribute to patient discomfort, motion artifacts, increased personnel costs, and increased costs for adjunct services such as sedation or anesthesia [5, 6].
New PET/CT scanners that have increased z-axis (head-to-foot) FOVs and improved detector technology provide greater sensitivity and thus capture more counts for image generation, potentially allowing reduced administered radiopharmaceutical activity or reduced acquisition time [6, 7]. Studies in adults have explored the use of these scanners to achieve reductions in administered activity or scan acquisition time [613]. In broad terms, these studies have shown that reductions of at least 20% in administered FDG activity are possible while maintaining PET image quality. Leveraging scanner improvements to reduce administered radiopharmaceutical activity or reduce the scan acquisition time would be particularly relevant to the clinical care of children and young adults. Additionally, these gains may help expand the application of PET/CT to disease states other than cancer.
The aim of the present study was to evaluate in children and young adults the impact of shorter simulated acquisition times on the quality of whole-body FDG PET images obtained using a state-of-the-art digital PET/CT system. The results of this evaluation may provide an indication of the anticipated clinical impact of lower administered radiopharmaceutical activities.

Methods

This retrospective HIPAA-compliant study was performed at Cincinnati Children's Hospital Medical Center, a tertiary pediatric academic center. The study was approved by the institutional review board, and a waiver of documentation of written informed consent was obtained.
A research fellow (V.P.V.A.) used an imaging report search engine (Illuminate InSight, version 4.3, Softek Illuminate) to identify unique patients who underwent a clinically indicated whole-body FDG PET/CT examination between December 2020 (the month of installation of a digital PET/CT scanner) and July 2021, yielding a total of 333 patients. Patients were then excluded for the following reasons: uptake time (i.e., the interval between FDG administration and initiation of acquisition) of either less than 50 minutes or more than 70 minutes (n = 61), Langerhans cell histiocytosis due to lower administered FDG activities for this indication (n = 16), abnormal renal function based on the serum creatinine level or measured glomerular filtration rate (n = 93), FDG PET/CT performed at an institution other than the study institution (n = 33), and weight greater than 70 kg (n = 35). These exclusions resulted in 95 eligible patients. Seven groups based on patient weight were designated (0–9, 10–19, 20–29, 30–39, 40–49, 50–59, and 60–69 kg), and from among the 95 eligible patients, we selected the four patients in each weight group who had most recently undergone FDG PET/CT examinations. This process resulted in a final study sample of 27 pediatric and young adult patients who underwent a total of 27 clinically indicated FDG PET/CT examinations, with only three patients available for inclusion in the group weighing 0–9 kg but with four patients included in each of the six other groups weighing from 10–19 kg to 60–69 kg. Figure 1 summarizes the patient selection process.
Fig. 1 —Flow diagram shows study sample selection. Boxes on right denote excluded patients. s/bed = Seconds per bed position.

Scanner Details

All FDG PET/CT examinations were acquired using a PET/CT scanner (Discovery MI Gen 2, GE Healthcare). The scanner was equipped with detectors consisting of lutetium crystals and silicon photomultipliers with 25 cm of z-axis coverage (a five-ring configuration). Sensitivity, which was based on in-house National Electrical Manufacturers Association (NEMA) testing, was 26 counts per second (cps) per kilobecquerel at the center of the PET gantry. In comparison, an analog PET/CT system (Ingenuity TF, Philips Healthcare) that had previously been used at the institution had sensitivity of 7.3 cps/kBq.

Clinical FDG PET/CT Protocol

At the study institution, the standard instructions for patient preparation required patients to fast for a minimum of 4 hours before FDG administration and necessitated that dextrose-containing IV fluids and medications be stopped during this same period. Patients were instructed to abstain from vigorous physical activity for 24 hours before the examination.
On arrival in the radiology department, patients were ushered into a warm room (21°C). Warm blankets were placed on the patient for 40–60 minutes before FDG injection, to minimize FDG uptake in brown adipose tissue. Vascular access was established by placing an IV line or by accessing an indwelling central venous catheter. The blood glucose level was assessed at the time of vascular access by use of a point-of-care glucometer, to confirm that the blood glucose level was less than 150 mg/dL. To further inhibit FDG uptake in brown adipose tissue, IV fentanyl was administered at the discretion of the nuclear medicine physician according to a sliding scale 10 minutes before FDG administration [14].
FDG was administered IV at an activity of 3.7 MBq/kg (0.1 mCi/kg) for patients who weighed less than 20 kg and 4.44 MBq/kg (0.12 mCi/kg) for patients who weighed 20 kg or more, with a maximum administered activity of 370 MBq (10 mCi). After FDG was injected, patients were instructed to remain still and quiet during the uptake period before the start of imaging. Patients were instructed to void immediately before undergoing imaging.

FDG PET/CT Acquisition

PET/CT acquisition was conducted approximately 60 minutes after FDG administration, and patients were imaged from the vertex to the toes. CT acquisition was performed first according to institutional parameters for attenuation correction and localization; no examinations included a diagnostic CT acquisition or used oral or IV contrast material for the CT component. PET acquisition in 3D list mode began after completion of the CT acquisition and routinely acquired data for 90 s per bed position (hereafter, reported as seconds per bed) with an overlap between bed positions of 21%. The raw PET count data were stored.

PET Image Processing

For the purposes of this investigation, the stored clinically acquired raw list-mode PET data were used for retrospective reconstruction of multiple image sets per patient. Time-of-flight reconstructions were performed without application of commercially available regularized reconstruction methods (e.g., Q.Clear [GE Healthcare]). Parameters included: VUE Point Fx (GE Healthcare) as the reconstruction method, point-spread function reconstruction algorithm Sharp IR (GE Healthcare) as the quantitation method, a standard z-axis filter, a 6.4-mm filter cutoff, 16 subsets, three iterations, a 256 × 256 matrix, and attenuation correction.
Raw list-mode data were reprocessed to simulate six reduced acquisition times (60, 55, 50, 45, 40, and 30 s/bed) in addition to the clinical standard of 90 s/bed, yielding a total of seven acquisition times that were evaluated (Fig. 2 and Fig. S1 [available in the online supplement]). The simulations were achieved by trimming counts backward from the end of the original full-count list-mode acquisition and thereby decreasing the total number of counts used for the reconstructed image sets, as described in prior studies in adults and children [2, 3, 5, 1517]. This truncation process may be used to simulate either shorter acquisitions times with no change in administered activities or reduced administered activities with no change in acquisition times. Under the assumption that the reductions in acquisition time and administered activity were directly proportional, the simulated acquisition durations of 60, 55, 50, 45, 40, and 30 s/bed correspond to reductions in administered activities of approximately 33%, 40%, 45%, 50%, 55%, and 67%, respectively, in comparison with the previously described clinical standard.
Fig. 2A —Likert scores for simulated acquisition times.
A, Graphs show mean subjective Likert scores for simulated reduced acquisition times for lesion conspicuity (A), conspicuity of normal structures (B), overall image quality (C), and image noise (D). Circles denote mean subjective Likert scores and whiskers indicate 95% CIs. s/bed = Seconds per bed position.
Fig. 2B —Likert scores for simulated acquisition times.
B, Graphs show mean subjective Likert scores for simulated reduced acquisition times for lesion conspicuity (A), conspicuity of normal structures (B), overall image quality (C), and image noise (D). Circles denote mean subjective Likert scores and whiskers indicate 95% CIs. s/bed = Seconds per bed position.
Fig. 2C —Likert scores for simulated acquisition times.
C, Graphs show mean subjective Likert scores for simulated reduced acquisition times for lesion conspicuity (A), conspicuity of normal structures (B), overall image quality (C), and image noise (D). Circles denote mean subjective Likert scores and whiskers indicate 95% CIs. s/bed = Seconds per bed position.
Fig. 2D —Likert scores for simulated acquisition times.
D, Graphs show mean subjective Likert scores for simulated reduced acquisition times for lesion conspicuity (A), conspicuity of normal structures (B), overall image quality (C), and image noise (D). Circles denote mean subjective Likert scores and whiskers indicate 95% CIs. s/bed = Seconds per bed position.
Respiratory gating was turned off during reconstruction to prevent any additional counts in gated bed positions from impacting image analysis. A total of 189 image sets (seven simulated acquisition times for each of 27 patients) were reconstructed, anonymized, and loaded onto clinical viewing software (MIM, version 7.1.4, MIM Software) for evaluation.

Qualitative Image Assessment

Three radiologists who regularly interpret pediatric FDG PET/CT examinations independently rated the reconstructed image sets, blinded to patient history and simulated acquisition time. Two of the readers were board certified in pediatric radiology (S.E.S. and Y.L., with 14 and 3 years of postfellowship experience, respectively), and one reader (N.A.A.) had completed 2 years of pediatric radiology fellowship and was in an accredited nuclear radiology fellowship at the time of image review. Each reader completed seven review sessions, with a minimum of 7 days between sessions. Twenty-seven randomly selected reconstructed PET image sets were reviewed in each session. The readers evaluated PET images only; CT or fused PET/CT images were not viewed. Readers assessed each reconstructed PET image set in terms of lesion conspicuity, conspicuity of normal structures, and overall image quality, by use of a 5-point Likert scale (with a score of 1 denoting nondiagnostic image quality; 2, barely diagnostic; 3, equal to current clinical practice; 4, superior to current clinical practice; and 5, state-of-the-art image quality). Perceived image noise was also scored by use of a 4-point Likert scale (with a score of 1 denoting poor [marked noise]; 2, fair [some diagnostically relevant noise]; 3, good [some diagnostically irrelevant noise]; and 4, excellent [optimally low noise]). The reviewers were instructed to base the rating of lesion conspicuity on what was, in their judgment, the most FDG-avid lesion(s) and to not rate lesion conspicuity if they judged no FDG-avid lesions to be present.

Quantitative Image Assessment

Quantitative assessment was performed by a research fellow (V.P.V.A.) who placed ROIs that were stored and subsequently reviewed by a board-certified radiologist (A.T.T.) with additional certification in nuclear radiology and 7 years of postfellowship experience. Using the full-count (90 s/bed) images, 3D spheric ROIs with a diameter of 3 cm were drawn in a homogeneous area of liver parenchyma, with large vessels and lesions avoided, as well as in anterior or anterolateral muscle of the right mid thigh, with bone avoided. In addition, a gradient-based tool (PET Edge, MIM Software) was used to contour the most FDG-avid lesion in each patient, as selected by the two reviewers (V.P.V.A. and A.T.T.) in consensus. The lesions selected for this purpose did not necessarily correspond to the lesions judged to be the most FDG avid for the purposes of the previously described qualitative lesion conspicuity evaluation. The gradient-based tool detected the steepest drop-off in SUV to automatically create a contour boundary. ROIs were then propagated to the six reduced-count reconstructions. The SUVmax, SUVmean, and SD of the SUV (SUVSD) were recorded, and the SUVSD/SUVmean (the SUVSD divided by the mean of the SUV [SUVmean]) was calculated as a surrogate of perceived image noise.

Statistical Analysis

Descriptive analysis was performed to summarize patient demographics using means, medians, SDs, and IQRs. Likert scores were converted to a binary classification of superior or equal to clinical practice versus worse than clinical practice, and the percentage of agreement among the three readers was determined (with agreement defined as all three readers providing the same binary classification). For qualitative measures, for each reconstruction the mean score across patients was computed from the median scores calculated for each patient across three readers. For quantitative measures, for each reconstruction the mean value across patients was computed and expressed as a ratio relative to the values at 90 s/bed. ANOVA, the Friedman test, and the Dunn test were used to compare qualitative and quantitative measures between simulated reduced acquisitions times and the clinical standard of 90 s/bed. For qualitative measures, these comparisons were performed using data pooled across the three readers, and they were summarized as a mean for the study sample. For each patient, graphs were generated that depicted the variation across simulated acquisition times of SUVmean and SUVmax of liver, thigh, and FDG-avid lesions computed as ratios relative to the values at 90 s/bed position. A statistically significant difference was defined as p < .05. Statistical analyses were performed using GraphPad Prism software (version 8.0.0 for Windows, GraphPad Software).

Results

The final sample of 27 patients included nine male and 18 female patients. The mean patient age was 10.8 ± 8.3 years (range, 0–34 years); four patients (15%) were older than 18 years old. The mean BMI was 18.7 ± 2.9. Table S1 (available in the online supplement) summarizes the clinical indications for the FDG PET/CT examinations. The two most common indications were sarcoma (11/27 [40.7%]) and neuroblastoma (4/27 [14.8%]). At least one FDG-avid lesion was identified in 25 patients (92.6%); two patients (7.4%) had no FDG-avid lesion. For the 25 patients with at least one FDG-avid lesion, characteristics of the most FDG-avid lesion that was selected for quantitative evaluation in each patient are found in Table S2 (available in the online supplement). Additional demographic data and data on FDG administration are found in Table 1.
TABLE 1: Demographic Characteristics of Study Sample and Characteristics of FDG Administration
CharacteristicMean ± SDRange
Age (y)10.8 ± 8.30–34
Weight (kg)36 ± 208–67
Height (cm)134.6 ± 32.870.9–184.0
Administered FDG activity (MBq)a170.2 ± 92.540.7–303.4
Administered FDG activity per patient weight (MBq/kg)b4.44 ± 0.373.33–5.18
Uptake time (min)58.6 ± 5.750–69
Fasting serum glucose level (mg/dL)89.3 ± 8.562–103
a
Mean, 4.6 ± 2.5 mCi; range, 1.1–8.2 mCi.
b
Mean, 0.1 ± 0.01 mCi/kg; range, 0.09–0.14 mCi/kg.

Qualitative Analysis

Interobserver agreement for the qualitative scores is summarized in Table 2. Agreement for the four measures ranged from 92.6% to 100.0% for 90 s/bed, 70.4–100.0% for 60 s/bed, 70.4–96.3% for 55 s/bed, 74.1–100.0% for 50 s/bed, 59.3–100.0% for 45 s/bed, 48.1–92.6% for 40 s/bed, and 25.9–81.5% for 30 s/bed.
TABLE 2: Interobserver Agreement for Qualitative Image Scores by Simulated Acquisition Time
CharacteristicOverall90 s/bed60 s/bed55 s/bed50 s/bed45 s/bed40 s/bed30 s/bed
Lesion conspicuity92.6 (175/189)100.0 (27/27)100.0 (27/27)96.3 (26/27)96.3 (26/27)100.0 (27/27)85.2 (23/27)70.4 (19/27)
Conspicuity of normal structures94.7 (179/189)100.0 (27/27)100.0 (27/27)92.6 (25/27)100.0 (27/27)100.0 (27/27)88.9 (24/27)81.5 (22/27)
Overall image quality91.0 (172/189)100.0 (27/27)100.0 (27/27)92.6 (25/27)96.3 (26/27)92.6 (25/27)92.6 (25/27)66.7 (18/27)
Image noise63.0 (119/189)92.6 (25/27)70.4 (19/27)70.4 (19/27)74.1 (20/27)59.3 (16/27)48.1 (13/27)25.9 (7/27)

Note—Data are expressed as the percentage, with the numerator and denominator in parentheses. Scores are stratified in a binary fashion as superior or equal to clinical practice versus worse than clinical practice. Agreement required the same binary assessment by all three reviewers. s/bed = Seconds per bed position.

Table 3 and Figure 3 provide the qualitative image scores for the clinical standard of 90 s/bed and for the simulated reduced acquisition times. Each qualitative measure showed an overall pattern of Likert scores decreasing with shorter simulated acquisition times. However, no qualitative measure showed a significant difference versus the clinical standard of 90 s/bed for the simulated acquisition at 60 s/bed (all p > .05). Mean lesion conspicuity showed significant differences versus the clinical standard of 90 s/bed (3.9 ± 0.9) for 40 s/bed (3.4 ± 0.7; p = .007) and 30 s/bed (3.1 ± 0.6; p < .001). Mean conspicuity of normal structures showed significant differences versus the clinical standard of 90 s/bed (3.9 ± 0.9) for 45 s/bed (3.5 ± 0.7, p = .04), 40 s/bed (3.4 ± 0.7, p = .005), and 30 s/bed (3.1 ± 0.6, p < .001). Mean overall image quality showed significant differences versus the clinical standard of 90 s/bed (3.9 ± 0.9) for 45 s/bed (3.5 ± 0.7, p = .007), 40 s/bed (3.4 ± 0.7, p = .002), and 30 s/bed (3.1 ± 0.6, p < .001). Mean image noise showed significant differences versus the clinical standard of 90 s/bed (3.3 ± 0.5) for 55 s/bed (3.0 ± 0.5, p = .03), 50 s/bed (3.0 ± 0.5, p < .01), 45 s/bed (2.8 ± 0.5, p < .001), 40 s/bed (2.8 ± 0.6, p < .001), and 30 s/bed (2.4 ± 0.6, p < .001). The qualitative scores of the individual readers are summarized in Table S3 (available in the online supplement).
TABLE 3: Summary of Qualitative Image Scores by Simulated Acquisition Time
Qualitative ParameterAcquisition at 90 s/bedaSimulated Reduced Acquisition Time (s/bed)
605550454030
Lesion conspicuityb3.9 ± 0.93.7 ± 0.8 (p > .99)3.6 ± 0.8 (p = .45)3.6 ± 0.8 (p = .56)3.5 ± 0.7 (p > .10)3.4 ± 0.7 (p = .007)3.1 ± 0.6 (p < .001)
Conspicuity of normal structuresb3.9 ± 0.93.7 ± 0.7 (p > .99)3.6 ± 0.8 (p = .26)3.6 ± 0.7 (p = .35)3.5 ± 0.4 (p = .04)3.4 ± 0.7 (p = .005)3.1 ± 0.5 (p < .001)
Overall image qualityb3.9 ± 0.93.7 ± 0.8 (p > .99)3.6 ± 0.8 (p = .21)3.6 ± 0.7 (p = .07)3.5 ± 0.7 (p = .007)3.4 ± 0.7 (p = .002)3.1 ± 0.6 (p < .001)
Image noisec3.2 ± 0.53.1 ± 0.5 (p = .24)3.0 ± 0.5 (p = .03)3.0 ± 0.5 (p = .01)2.8 ± 0.5 (p < .001)2.8 ± 0.6 (p < .001)2.4 ± 0.6 (p < .001)

Note—Likert scores are the mean ± SD computed from the median scores for each patient across the three reviewers, with values in parentheses denoting p values for comparison with the clinical standard of acquisition at 90 s/bed. s/bed = Seconds per bed position.

a
Clinical standard full-count acquisition.
b
Scored using a 5-rank Likert scale.
c
Scored using a 4-rank Likert scale.
Fig. 3A —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
A, Axial PET images show examples of clinical standard full-count acquisition at 90 s/bed (A) (lesion SUVmax = 5.69) and simulated acquisitions at 60 s/bed (approximately 67% administered activity) (B) (lesion SUVmax = 5.84), 55 s/bed (approximately 60% administered activity) (C) (lesion SUVmax = 5.86), 50 s/bed (approximately 55% administered activity) (D) (lesion SUVmax = 5.69), (E) 45 s/bed (approximately 50% administered activity) (lesion SUVmax = 5.86), 40 s/bed (approximately 45% administered activity) (F) (lesion SUVmax = 5.69), and 30 s/bed (approximately 33% administered activity) (G) (lesion SUVmax = 5.96).
Fig. 3B —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
B, Axial PET images show examples of clinical standard full-count acquisition at 90 s/bed (A) (lesion SUVmax = 5.69) and simulated acquisitions at 60 s/bed (approximately 67% administered activity) (B) (lesion SUVmax = 5.84), 55 s/bed (approximately 60% administered activity) (C) (lesion SUVmax = 5.86), 50 s/bed (approximately 55% administered activity) (D) (lesion SUVmax = 5.69), (E) 45 s/bed (approximately 50% administered activity) (lesion SUVmax = 5.86), 40 s/bed (approximately 45% administered activity) (F) (lesion SUVmax = 5.69), and 30 s/bed (approximately 33% administered activity) (G) (lesion SUVmax = 5.96).
Fig. 3C —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
C, Axial PET images show examples of clinical standard full-count acquisition at 90 s/bed (A) (lesion SUVmax = 5.69) and simulated acquisitions at 60 s/bed (approximately 67% administered activity) (B) (lesion SUVmax = 5.84), 55 s/bed (approximately 60% administered activity) (C) (lesion SUVmax = 5.86), 50 s/bed (approximately 55% administered activity) (D) (lesion SUVmax = 5.69), (E) 45 s/bed (approximately 50% administered activity) (lesion SUVmax = 5.86), 40 s/bed (approximately 45% administered activity) (F) (lesion SUVmax = 5.69), and 30 s/bed (approximately 33% administered activity) (G) (lesion SUVmax = 5.96).
Fig. 3D —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
D, Axial PET images show examples of clinical standard full-count acquisition at 90 s/bed (A) (lesion SUVmax = 5.69) and simulated acquisitions at 60 s/bed (approximately 67% administered activity) (B) (lesion SUVmax = 5.84), 55 s/bed (approximately 60% administered activity) (C) (lesion SUVmax = 5.86), 50 s/bed (approximately 55% administered activity) (D) (lesion SUVmax = 5.69), (E) 45 s/bed (approximately 50% administered activity) (lesion SUVmax = 5.86), 40 s/bed (approximately 45% administered activity) (F) (lesion SUVmax = 5.69), and 30 s/bed (approximately 33% administered activity) (G) (lesion SUVmax = 5.96).
Fig. 3E —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
E, Axial PET images show examples of clinical standard full-count acquisition at 90 s/bed (A) (lesion SUVmax = 5.69) and simulated acquisitions at 60 s/bed (approximately 67% administered activity) (B) (lesion SUVmax = 5.84), 55 s/bed (approximately 60% administered activity) (C) (lesion SUVmax = 5.86), 50 s/bed (approximately 55% administered activity) (D) (lesion SUVmax = 5.69), (E) 45 s/bed (approximately 50% administered activity) (lesion SUVmax = 5.86), 40 s/bed (approximately 45% administered activity) (F) (lesion SUVmax = 5.69), and 30 s/bed (approximately 33% administered activity) (G) (lesion SUVmax = 5.96).
Fig. 3F —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
F, Axial PET images show examples of clinical standard full-count acquisition at 90 s/bed (A) (lesion SUVmax = 5.69) and simulated acquisitions at 60 s/bed (approximately 67% administered activity) (B) (lesion SUVmax = 5.84), 55 s/bed (approximately 60% administered activity) (C) (lesion SUVmax = 5.86), 50 s/bed (approximately 55% administered activity) (D) (lesion SUVmax = 5.69), (E) 45 s/bed (approximately 50% administered activity) (lesion SUVmax = 5.86), 40 s/bed (approximately 45% administered activity) (F) (lesion SUVmax = 5.69), and 30 s/bed (approximately 33% administered activity) (G) (lesion SUVmax = 5.96).
Fig. 3G —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
G, Axial PET images show examples of clinical standard full-count acquisition at 90 s/bed (A) (lesion SUVmax = 5.69) and simulated acquisitions at 60 s/bed (approximately 67% administered activity) (B) (lesion SUVmax = 5.84), 55 s/bed (approximately 60% administered activity) (C) (lesion SUVmax = 5.86), 50 s/bed (approximately 55% administered activity) (D) (lesion SUVmax = 5.69), (E) 45 s/bed (approximately 50% administered activity) (lesion SUVmax = 5.86), 40 s/bed (approximately 45% administered activity) (F) (lesion SUVmax = 5.69), and 30 s/bed (approximately 33% administered activity) (G) (lesion SUVmax = 5.96).
Fig. 3H —3-year-old patient weighing 17.4 kg who underwent FDG PET/CT as evaluation for neuroblastoma. Patient received 95.5 MBq (5.18 MBq/kg or 0.14 mCi/kg) FDG, and PET acquisition was started 65 minutes after radiopharmaceutical administration.
H, Axial fused PET/CT image using full-count (90 s/bed) reconstruction shows large right upper quadrant lesion (arrow), which was selected as most FDG-avid lesion for purposes of quantitative evaluation; axial PET reconstructions shown in A–G are at level of this lesion. PT = PET image number, U = upper limit of SUV color scale, L = lower limit of SUV color scale, SUVbw = SUV by body weight.

Quantitative Analysis

Table 4 provides ratios of mean SUVs for the clinical standard of 90 s/bed and for the simulated reduced acquisition times. SUVmean did not show significant differences compared with the clinical standard of 90 s/bed for any simulated reduced acquisition time for the thigh, liver, or selected FDG-avid lesions (all p > .05). However, SUVSD/SUVmean and the ratio of mean SUVmax showed gradual increases with decreasing simulated acquisition times, and multiple simulated reduced acquisition times for liver, thigh, and FDG-avid lesions showed significant differences with respect to the clinical standard of 90 s/bed. For example, for FDG-avid lesions, the ratio of mean SUVmax showed significant differences versus the clinical standard of 90 s/bed (1.00 ± 0.00) for 45 s/bed (1.03 ± 0.04, p = .002), 40 s/bed (1.03 ± 0.05, p < .001), and 30 s/bed (1.05 ± 0.06, p < .001). For the 60 s/bed simulated acquisition, significant differences from the clinical standard of 90 s/bed among the quantitative measures were seen only for liver SUVmax (1.00 ± 0.00 at 90 s/bed vs 1.05 ± 0.03 for 60 s/bed, p = .02) and liver SUVSD/SUVmean (0.09 ± 0.02 at 90 s/bed vs 0.11 ± 0.02 for 60 s/bed, p = .04); other comparisons of quantitative measures between these two acquisition times were not significantly different (all p > .05). Figure S2 (available in the online supplement) shows the increasing deviation in the ratios of mean SUVmax and mean SUVmean from the values obtained at 90 s/bed with decreasing simulated acquisition times.
TABLE 4: Summary of Quantitative Measures Across Simulated Acquisition Times
Quantitative ParameterAcquisition at 90 s/bedaSimulated Reduced Acquisition Time (s/bed)
605550454030
SUVmean       
 Liver1.00 ± 0.001.00 ± 0.01 (p > .99)1.00 ± 0.01 (p > .99)1.00 ± 0.01 (p > .99)1.00 ± 0.02 (p > .99)1.00 ± 0.02 (p > .99)0.99 ± 0.02 (p > .99)
 Thigh1.00 ± 0.000.99 ± 0.02 (p > .99)0.99 ± 0.02 (p > .99)0.99 ± 0.02 (p > .99)0.99 ± 0.02 (p > .99)1.00 ± 0.02 (p > .99)1.00 ± 0.03 (p > .99)
 Lesion1.00 ± 0.000.99 ± 0.01 (p > .99)0.99 ± 0.01 (p > .99)0.99 ± 0.02 (p > .99)1.00 ± 0.02 (p > .99)0.99 ± 0.02 (p > .99)0.99 ± 0.02 (p = .57)
SUVSD/SUVmean       
 Liver0.09 ± 0.020.11 ± 0.02 (p = .04)0.12 ± 0.02 (p = .001)0.12 ± 0.02 (p < .001)0.13 ± 0.02 (p < .001)0.14 ± 0.02 (p < .001)0.16 ± 0.03 (p < .001)
 Thigh0.14 ± 0.050.16 ± 0.05 (p = .35)0.17 ± 0.05 (p = .02)0.18 ± 0.05 (p < .001)0.18 ± 0.05 (p < .001)0.19 ± 0.05 (p < .001)0.21 ± 0.06 (p < .001)
 Lesion0.29 ± 0.140.30 ± 0.13 (p = .53)0.30 ± 0.13 (p = .49)0.30 ± 0.13 (p = .02)0.30 ± 0.13 (p = .006)0.31 ± 0.13 (p < .001)0.32 ± 0.13 (p < .001)
SUVmax       
 Liver1.00 ± 0.001.05 ± 0.03 (p = .02)1.07 ± 0.04 (p < .001)1.08 ± 0.05 (p < .001)1.10 ± 0.05 (p < .001)1.12 ± 0.07 (p < .001)1.18 ± 0.08 (p < .001)
 Thigh1.00 ± 0.001.03 ± 0.05 (p > .99)1.05 ± 0.08 (p = .40)1.03 ± 0.09 (p = .03)1.08 ± 0.11 (p = .02)1.12 ± 0.10 (p < .001)1.19 ± 0.18 (p < .001)
 Lesion1.00 ± 0.001.01 ± 0.02 (p = .90)1.01 ± 0.03 (p = .96)1.03 ± 0.03 (p = .07)1.03 ± 0.04 (p = .002)1.03 ± 0.05 (p < .001)1.05 ± 0.06 (p < .001)

Note—Data are the mean ± SD for 27 patients (25 lesions), expressed as a ratio relative to values at 90 s/bed with values in parentheses denoting p values for comparison with the clinical standard of acquisition at 90 s/bed. s/bed = Seconds per bed position, SUVSD/SUVmean = SUVSD divided by the SUVmean.

a
Clinical standard full-count acquisition.

Discussion

The present study shows the potential for reduced image acquisition times or reduced administered FDG activities for children and young adults undergoing whole-body FDG PET on a state-of-the-art, digital 25-cm axial FOV scanner. With shorter simulated acquisition times, subjective image quality measures declined and subjective image noise increased, as scored by three independent observers. However, the scores were significantly different from the reference full-count acquisition only at simulated acquisition times of at most 40 s/bed (approximately 45% administered activity) for lesion conspicuity, at most 45 s/bed (approximately 50% administered activity) for conspicuity of normal structures and overall image quality, and at most 55 s/bed (approximately 60% administered activity) for image noise; no qualitative measure showed a significant difference between 60 s/bed (approximately 33% administered activity) and the reference full-count acquisition. SUVmean did not show a significant difference from the reference full-count acquisition for any of the simulated reduced acquisition times. Liver SUVmax and SUVSD/SUVmean showed a statistically significant difference between the 90 s/bed and 60 s/bed reconstructions that was small and unlikely to be clinically relevant. Otherwise, SUVmax and SUVSD/SUVmean (a surrogate of perceived image noise) across all assessed sites showed significant differences from the reference full-count acquisition only at simulated reduced acquisition times of at most 55 s/bed (approximately 60% administered activity). In total, the findings support an approximately 33% reduction in image acquisition time or administered activity from the clinical standard of 90 s/bed in combination with an administered FDG activity of 4.44 MBq/kg (0.12 mCi/kg; 3.7 MBq/kg [0.1 mCi/kg] for patients who weighed less than 20 kg; maximum, 370 MBq [10 mCi]) when children and young adults underwent imaging performed using this digital scanner and time-of-flight reconstruction technique. Greater reductions might be possible when using commercially available advanced reconstruction techniques such as regularized reconstruction (e.g., Q.Clear) and when using future artificial intelligence and deep-learning reconstruction approaches [18].
The new generation of digital PET/CT systems uses silicon photomultipliers in place of the photomultiplier tube detectors coupled to multiple scintillation crystals that are generally used in analog PET/CT systems. These improvements allow improved localization of annihilation events, higher sensitivities, higher system coincidence timing resolution, and higher spatial resolution [7, 1921]. Indeed, in-house testing confirmed the higher sensitivity of the digital PET system used in this study in comparison with a prior analog system.
Several prior studies in children have shown the potential to decrease acquisition times or administered radiopharmaceutical activities while maintaining PET image quality for patients who undergo imaging performed using digital PET scanners (Table 5). These studies varied in terms of the scanners and baseline techniques used before reductions were applied. Nonetheless, the present findings are overall concordant with those of prior studies. These studies all propose minimum acquisition times and activities for digital PET scanners that are 50% or less than those described by Alessio et al. [3] for an earlier-generation nondigital scanner. The earlier study that is most comparable to the present study is the one by Schmall et al. [17], which used the Signa PET/MRI scanner (GE Healthcare), which has the same detector configuration and z-axis FOV view as the scanner used in the present investigation. In their study, Schmall et al. showed no degradation of image quality for simulated administered activities as low as 1.2 MBq/kg with images acquired for 180 s/bed, but they ultimately recommended an administered activity of 2.46 MBq/kg. Differences in minimum acceptable activities or acquisition times listed in Table 5 likely relate to a combination of differences between scanner platforms as well as differences in the reviewers' opinion of acceptable image quality.
TABLE 5: Details of Studies That Compared Image Quality Across Decreased Acquisition Times or Administered Activities for Whole-Body Digital FDG PET/CT in Children and Young Adults
Authors [Reference]Study Sample DetailsAdministered FDG Activity (MBq/kg)bPET Scanner (Manufacturer)Reconstruction Parameter(s)Scanner z-Axis FOV (cm)Original PET Acquisition Time (s/bed)Simulated Activity or Simulated Acquisition TimeRecommended Minimum Administered Activity or Acquisition TimeApproximate Recommended Time × Activityc
No. of PatientsAge (y)aWeight Range (kg)
Alves et al. [present study]270–347.6–67.14.44 ± 0.37GE Discovery Ml Gen 2 (GE Healthcare)TOF and Sharp IR259060, 55, 50, 45, 40, and 30 s/bed60 s/bed266 MBq/s
Gatidis et al. [15]241–183.1 ± 0.65Biograph mMR (Siemens Healthcare)3D OP OSEM222402.5, 2, 1.5, 1, 0.5, and 0.25 MBq/kg1.5 MBq/kg360 MBq/s
Lasnon et al. [16]28d7.5 ± 4.45.8 ± 0.4Vereos Digital (Philips Healthcare)16.49090, 60, 30, 20, and 10 s/bed30 s/bed174 MBq/s
Schmall et al. [17]112–1914–493.7eSigna (GE Healthcare)251803.7, 1.9, 1.2, 0.6, and 0.4 MBq/kg2.46 MBq/kgf443 MBq/s
Zhao et al. [5]330.8–17.68.5–58.53.7 ± 0.37uEXPLORER (United Imaging)OSEM and TOF1946003.7, 1.85, 0.37, 0.25, and 0.12 MBq/kg0.37 MBq/kg222 MBq/s
Zucchetta et al. [2]177–1727–853.00 ± 0.2Biograph mMR (Siemens Healthcare)3D OSEM223003.0, 2.4, 1.8, 1.5, 1.2 and 0.6 MBq/kg1.5 MBq/kg450 MBq/s
Alessio et al. [3]g131–2313–1095.3eDiscovery VCT (GE Healthcare)h15.7300 (> 22 kg)
180 (≤ 22 kg)
300, 240, 180, 120, and 60 s/bed180 s/bed954 MBq/s

Note—Dash (—) denotes data not reported. s/bed = Seconds per bed position, TOF = time of flight, Sharp IR = point-spread function reconstruction algorithm (GE Healthcare), OP OSEM = ordinary Poisson ordered-subset-expectation-maximization.

a
Data are range or mean ± SD.
b
Except where otherwise indicated, data are mean ± SD.
c
To facilitate comparisons of recommendations between studies, data were calculated by multiplying recommended time by actual activity or recommended activity by actual time.
d
Nine pediatric patients.
e
Reported target activity rather than administered activity.
f
Showed no degradation of image quality based on quantitative and qualitative analysis for simulated activities as low as 1.2 MBq/kg but recommends a higher administered activity.
g
This study was included to provide a comparable evaluation for a nondigital PET scanner.
h
Analog PET/CT scanner with bismuth germanium oxide detector.
We evaluated image quality in terms of both subjective reviewer assessments and quantitative analysis of SUVs, with the latter representing the most commonly used semiquantitative parameter for analysis of radiotracer uptake [2, 10, 22]. SUVmean did not significantly change across simulated reduced acquisition times, regardless of anatomic location. However, depending on the site, SUVmax was significantly different at simulated acquisition times of at most 60 s/bed. These findings are not surprising, as SUVmean represents a mean value across voxels and is expected to be relatively stable, whereas SUVmax, a measure of the single voxel with the highest uptake, is dependent on image noise and statistical fluctuations. These results are concordant with those in the prior literature. For example, Zucchetta et al. [2] showed that SUVmax was the quantitative value most affected by tracer activity reduction. Likewise, Gatidis et al. [15] observed only small differences in the SUV mean in reduced-activity PET datasets as compared with full-activity datasets, whereas SUVmax was more sensitive to tracer activity reduction. In that study, what was called SUVvar (which corresponds to SUVSD in the present study) showed a steadily increasing deviation with decreasing tracer activity. That finding is concordant with the present observation of a gradual increase in SUVSD/SUVmean with decreasing simulated acquisition times.
Although some studies have focused on investigating reductions in administered activities, others have simulated decreased acquisition times. Each approach has advantages. Reducing administered activity implies decreasing radiation exposure, which is a particular concern in children given that their risk of radiation-induced malignancies is higher than that in adults [5]. Radiation exposure reduction is also critical for clinical staff around the PET environment [10]. In addition, the reduced activity leads to reduced radiopharmaceutical utilization, which facilitates access and decreases resource needs, which is especially important for underresourced centers and for tracers of limited availability [11]. Decreased acquisition times, on the other hand, provide scan acquisitions that are faster, more accessible, and more comfortable and have a lower risk of motion artifacts. Likewise, a faster scan acquisition time can allow a decrease in sedation or anesthesia times as well as a possible reduction in associated complications and reduced equipment costs [6, 16]. As an example, in a prospective study in adults, Hornnes et al. [6] showed that reducing the whole-body FDG PET/CT acquisition time to 60 s/bed would lead to an estimated savings of €16,000 per scanner per year. Overall, the balance of reducing administered activity and, thus, patient radiation exposure versus reducing time should be carefully weighed, with patient population-specific preferences and needs considered.
The present study has limitations. First, the results represent a retrospective simulation of reduced-count acquisitions based on list-mode reconstructions. The results need to be verified prospectively through real reductions in administered radiopharmaceutical activity or image acquisition time. Second, the study sample was small and included patients with only a limited range of different types of tumors, so the findings therefore may not be uniformly generalizable. Third, the results were based on images acquired on a single digital PET/CT scanner; results may not apply to different PET systems that have distinct sensitivity performances. Fourth, qualitative assessment was evaluated by three readers from a single institution; other readers may have unique interpretation preferences. Fifth, reconstruction parameters were uniform across reconstructions and were not optimized for the different acquisition durations. Optimizing reconstruction parameters for each acquisition duration could potentially have improved image quality for the reconstructions with relatively short acquisition times [3]. Sixth, we did not analyze the impact of reduced-count reconstructions on every FDG-avid lesion that was present. The impact of reconstruction differences may vary among lesions, particularly lesions of different sizes. Finally, the simulation method entailed removing counts from the ends of the acquisitions, which may have altered the impact of patient motion during the acquisition on the reconstructed images. Other approaches for reducing counts—for example, removing evenly interspersed or randomly selected counts across the course of the acquisition—could have simulated reduced activities with the same acquisition duration while eliminating any potential variation in patient motion across reconstructions.
In conclusion, the findings of the present study suggest that in children and young adults who undergo FDG PET/CT performed using an extended-FOV digital whole-body PET/CT scanner, acquisition time or injected radiopharmaceutical activity may be decreased by approximately 33% from the clinical standard without significantly impacting image quality. Such reduction provides an opportunity to reduce radiation exposure and/or provide faster scan acquisition.

Supplemental Content

File (22_27894_suppl.pdf)

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 952 - 961
PubMed: 35731102

History

Submitted: April 25, 2022
Revision requested: May 18, 2022
Revision received: May 27, 2022
Accepted: June 9, 2022
Version of record online: June 22, 2022

Keywords

  1. children
  2. decreased acquisition time
  3. decreased dose
  4. digital PET
  5. young adults

Authors

Affiliations

Vinicius P. V. Alves, MD
Department of Radiology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Kasota Bldg MLC 5031, Cincinnati, OH 45226.
Samuel Brady, PhD
Department of Radiology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Kasota Bldg MLC 5031, Cincinnati, OH 45226.
Department of Radiology, University of Cincinnati College of Medicine, Cincinnati, OH.
Nadeen Abu Ata, MD
Department of Radiology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Kasota Bldg MLC 5031, Cincinnati, OH 45226.
Yinan Li, MD
Department of Radiology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Kasota Bldg MLC 5031, Cincinnati, OH 45226.
Department of Radiology, University of Cincinnati College of Medicine, Cincinnati, OH.
Joseph MacLean, MHA, CNMT
Department of Radiology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Kasota Bldg MLC 5031, Cincinnati, OH 45226.
Bin Zhang, PhD
Division of Biostatistics and Epidemiology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH.
Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, OH.
Susan E. Sharp, MD
Department of Radiology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Kasota Bldg MLC 5031, Cincinnati, OH 45226.
Department of Radiology, University of Cincinnati College of Medicine, Cincinnati, OH.
Andrew T. Trout, MD [email protected]
Department of Radiology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Kasota Bldg MLC 5031, Cincinnati, OH 45226.
Department of Radiology, University of Cincinnati College of Medicine, Cincinnati, OH.
Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, OH.

Notes

Address correspondence to A. T. Trout ([email protected]).
The authors declare that there are no disclosures relevant to the subject matter of this article.
Based on presentations at the Society for Pediatric Radiology 2022 annual meeting (Denver, CO) and the Society for Nuclear Medicine and Molecular Imaging Annual Meeting 2022 annual meeting (Vancouver, BC, Canada).

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