(18)F-FDG PET-CT respiratory gating in characterization of pulmonary lesions: approximation towards clinical indications

Comparison of the variability and diagnostic efficacy of respiratory-gated PET/CT based radiomics features with ungated PET/CT in lung lesions

OBJECTIVES

To investigate the variability and diagnostic efficacy of respiratory-gated (RG) PET/CT based radiomics features compared to ungated (UG) PET/CT in the differentiation of non-small cell lung cancer (NSCLC) and benign lesions.

METHODS

117 patients with suspected lung lesions from March 2020 to May 2021 and consent to undergo UG PET/CT and chest RG PET/CT (including phase-based quiescent period gating, pQPG and phase-matched 4D PET/CT, 4DRG) were prospectively included. 377 radiomics features were extracted from PET images of each scan. Paired t test was used to compare UG and RG features for inter-scan variability analysis. We developed three radiomics models with UG and RG features (i.e. UGModel, pQPGModel and 4DRGModel). ROC curves were used to compare diagnostic efficiencies, and the model-level comparison of diagnostic value was performed by five-fold cross-validation. A P value < 0.05 was considered as statistically significant.

RESULTS

A total of 111 patients (average age ± standard deviation was 59.1 ± 11.6 y, range, 29 - 88 y, and 63 were males) with 209 lung lesions were analyzed for features variability and the subgroup of 126 non-metastasis lesions in 91 patients without treatment before PET/CT were included for diagnosis analysis. 101/377 (26.8 %) 4DRG features and 82/377 (21.8 %) pQPG features showed significant difference compared to UG features (both P<0.05). 61/377 (16.2 %) and 59/377 (15.6 %) of them showed significantly better discriminant ability (ΔAUC% (i.e. (AUCRG - AUCUG) / AUCUG×100 %) > 0 and P<0.05) in malignant recognition, respectively. For the model-level comparison, 4DRGModel achieved the highest diagnostic efficacy (sen 73.2 %, spe 87.3 %) compared with UGModel (sen 57.7 %, spe 76.4 %) and pQPGModel (sen 63.4 %, spe 81.8 %).

CONCLUSION

RG PET/CT performs better in the quantitative assessment of metabolic heterogeneity for lung lesions and the subsequent diagnosis in patients with NSCLC compared with UG PET/CT.

Evaluation of Data-Driven Respiration Gating in Continuous Bed Motion in Lung Lesions

Respiration gating is used in PET to prevent image quality degradation due to respiratory effects. In this study, we evaluated a type of data-driven respiration gating for continuous bed motion, OncoFreeze AI, which was implemented to improve image quality and the accuracy of semiquantitative uptake values affected by respiratory motion. Methods: ¹⁸F-FDG PET/CT was performed on 32 patients with lung lesions. Two types of respiration-gated images (OncoFreeze AI with data-driven respiration gating, device-based amplitude-based OncoFreeze with elastic motion compensation) and ungated images (static) were reconstructed. For each image, we calculated SUV and metabolic tumor volume (MTV). The improvement rate (IR) from respiration gating and the contrast-to-noise ratio (CNR), which indicates the improvement in image noise, were also calculated for these indices. IR was also calculated for the upper and lower lobes of the lung. As OncoFreeze AI assumes the presence of respiratory motion, we examined quantitative accuracy in regions where respiratory motion was not present using a ⁶⁸Ge cylinder phantom with known quantitative accuracy. Results: OncoFreeze and OncoFreeze AI showed similar values, with a significant increase in SUV and decrease in MTV compared with static reconstruction. OncoFreeze and OncoFreeze AI also showed similar values for IR and CNR. OncoFreeze AI increased SUV_max by an average of 18% and decreased MTV by an average of 25% compared with static reconstruction. From the IR results, both OncoFreeze and OncoFreeze AI showed a greater IR from static reconstruction in the lower lobe than in the upper lobe. OncoFreeze and OncoFreeze AI increased CNR by 17.9% and 18.0%, respectively, compared with static reconstruction. The quantitative accuracy of the ⁶⁸Ge phantom, assuming a region of no respiratory motion, was almost equal for the static reconstruction and OncoFreeze AI. Conclusion: OncoFreeze AI improved the influence of respiratory motion in the assessment of lung lesion uptake to a level comparable to that of the previously launched OncoFreeze. OncoFreeze AI provides more accurate imaging with significantly larger SUVs and smaller MTVs than static reconstruction.

Respiratory-gated 18F Fluorodeoxyglucose Positron Emission Tomography/Magnetic Resonance Imaging in Evaluation of Primary Gastric Lesions and Gastric Lymph Nodes in Patients with Gastric Cancer

AIMS

To evaluate the added value of respiratory-gated positron emission tomography (PET) in ¹⁸F fluorodeoxyglucose (FDG) PET/magnetic resonance imaging (MRI) in the visual and semi-quantitative assessment of primary gastric lesions and gastric lymph nodes for patients with gastric cancer.

MATERIALS AND METHODS

In total, 102 upper abdominal respiratory-gated and whole-body ¹⁸F FDG PET/MRI of 88 patients with gastric cancer were evaluated visually and semi-quantitatively. For 41 patients who underwent surgery, histopathological and PET findings were compared. Three PET images were obtained from upper abdominal PET data: non-Q static (non-QS) PET from all counts, respiratory-gated Q static (QS) PET from counts in the end-expiration phase of breathing, shortened 4 min (S4min) PET that was reconstructed to obtain similar counts to QS PET. The semi-quantitative parameters (standardised uptake values, metabolic tumour volume, total lesion glycolysis) of primary lesions for each PET image, the sizes of primary lesions and the patient's body mass index were recorded. According to lymph node stations, the presence and numbers of positive lymph nodes and visual scores of lymph nodes for each PET image were recorded.

RESULTS

The patients with smaller gastric lesions (≤30 mm) or higher body mass index (>30) had significantly higher standardised uptake value percentage changes in QS PET compared with non-QS PET (all P < 0.05). The third (lesser curvature), fourth (greater curvature) and sixth (infra-pyloric) lymph node stations had significantly higher visual scores in the QS PET than in the others. The fourth lymph node station had a significantly higher number of FDG-positive lymph node in the QS PET than in the non-QS and the whole-body PET images. In the fourth station, sensitivity, positive predictive value, negative predictive value and accuracy increased in the QS PET compared with the others.

CONCLUSION

Respiratory-gated PET/MRI was found to be significantly superior in the evaluation of especially the fourth lymph node station, smaller gastric lesions and in the patients with a higher BMI compared with the non-respiratory-gated PET images.

Respiratory Gating and the Performance of PET/CT in Pulmonary Lesions

Background

Motion artifacts related to the patient’s breathing can be the cause of underestimation of the lesion uptake and can lead to missing of small lung lesions. The respiratory gating (RG) technology has demonstrated a significant increase in image quality.

Objective

The aim of this paper was to evaluate the advantages of RG technique on PET/CT performance in lung lesions. The impact of 4D-PET/CT on diagnosis (metabolic characterization), staging and re-staging lung cancer was also assessed, including its application for radiotherapy planning. Finally, new technologies for respiratory motion management were also discussed.

Methods

A comprehensive electronic search of the literature was performed by using Medline database (PubMed) searching “PET/CT”, “gated” and “lung”. Original articles, review articles, and editorials published in the last 10 years were selected, included and critically reviewed in order to select relevant articles.

Results

Many papers compared Standardized Uptake Value (SUV) in gated and ungated PET studies showing an increase in SUV of gated images, particularly for the small lesions located in medium and lower lung. In addition, other features as Metabolic Tumor Volume (MTV), Total Lesion Glycolysis (TLG) and textural-features presented differences when obtained from gated and ungated PET acquisitions. Besides the increase in quantification, gating techniques can determine an increase in the diagnostic accuracy of PET/CT. Gated PET/CT was evaluated for lung cancer staging, therapy response assessment and for radiation therapy planning.

Conclusion

New technologies able to track the motion of organs lesion directly from raw PET data, can reduce or definitively solve problems (i.e.: extended acquisition time, radiation exposure) currently limiting the use of gated PET/CT in clinical routine.

Improvement of image quality using amplitude-based respiratory gating in PET-computed tomography scanning

OBJECTIVES

This study sought to provide data supporting the expanded clinical use of respiratory gating by assessing the diagnostic accuracy of breathing motion correction using amplitude-based respiratory gating.

METHODS

A respiratory movement tracking device was attached to a PET-computed tomography scanner, and images were obtained in respiratory gating mode using a motion phantom that was capable of sensing vertical motion. Specifically, after setting amplitude changes and intervals according to the movement cycle using a total of nine combinations of three waveforms and three amplitude ranges, respiratory motion-corrected images were reconstructed using the filtered back projection method. After defining areas of interest in the acquired images in the same image planes, statistical analyses were performed to compare differences in standardized uptake value (SUV), lesion volume, full width at half maximum (FWHM), signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR).

RESULTS

SUVmax increased by 89.9%, and lesion volume decreased by 27.9%. Full width at half maximum decreased by 53.9%, signal-to-noise ratio increased by 11% and contrast-to-noise ratio increased by 16.3%. Optimal results were obtained when using a rest waveform and 35% duty cycle, in which the change in amplitude in the respiratory phase signal was low, and a constant level of long breaths was maintained.

CONCLUSIONS

These results demonstrate that respiratory-gated PET-CT imaging can be used to accurately correct for SUV changes and image distortion caused by respiratory motion, thereby providing excellent imaging information and quality.

Prone position [18F]FDG PET/CT to reduce respiratory motion artefacts in the evaluation of lung nodules

OBJECTIVES

2-Deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) positron-emission tomography/computed tomography (PET/CT) is widely used to evaluate lung nodules, although respiratory motion artefacts may occur. We investigated the value of prone position PET/CT (pPET/CT) in lung nodule evaluation compared with standard supine position PET/CT (sPET/CT).

METHODS

We retrospectively reviewed 28 consecutive patients (20 men; age, 65.6 ± 12.1 years) with a lung nodule (size, 16.8 ± 5.5 mm) located below the sub-carinal level who underwent [¹⁸F]FDG PET/CT in a standard supine position and additional prone position. The maximum standardised uptake value (SUV_max), metabolic tumour volume (MTV), difference of diaphragm position between PET and CT (DDP), Dice's similarity coefficient (DSC) and occurrence of mis-registration were analysed. The [¹⁸F]FDG uptake of 20 biopsy-confirmed (15 malignant) nodules was evaluated visually.

RESULTS

pPET/CT yielded a significantly higher SUV_max, lower MTV and shorter DDP than with sPET/CT (p = 0.043, 0.007 and 0.021, respectively). Mis-registration occurred in 53.6% of cases in sPET/CT and in 28.6% of cases in pPET/CT (p = 0.092). Among the 15 patients with mis-registration in sPET/CT, 10 patients (66.7%) did not show mis-registration in pPET/CT. DSC was higher in pPET/CT than in sPET/CT in 18 out of 28 patients (64.3%). In visual analysis, malignant nodules exhibited a higher [¹⁸F]FDG uptake positivity than benign nodules in pPET/CT (93.3% vs. 40.0%, p = 0.032) but not in sPET/CT (80.0% vs. 40.0%, p = 0.131).

CONCLUSIONS

pPET/CT reduces respiratory motion artefact and enables more-precise measurements of PET parameters.

KEY POINTS

• In prone position PET/CT, the decrease in the blurring effect caused by reduced respiratory motion resulted in a higher SUV_max and lower MTV in lung nodules than that with supine position PET/CT. • Prone position PET/CT was useful to interpret correctly malignant lung nodules as being positive in individual cases that had a negative result in supine position PET/CT.

Functional image in soft tissue sarcomas: An update of the indications of 18F-FDG-PET/CT

Soft tissue sarcomas (STS) are a rare and heterogeneous group of tumors. They account for 1% of solid malignant tumors in adults and 7% in children and are responsible for 2% of cancer mortality. They require a multidisciplinary approach in centers with experience. This collaboration aims to update the scientific evidence to strengthen, together with clinical experience, the bases for the use and limitations of ¹⁸F-FDG-PET/CT in STSs. The general recommendations for the use of PET/CT in STS at present are summarized as the initial evaluation of soft tissue tumours when conventional image does not establish benignity with certainty and this determines the approach; in biopsy guiding in selected cases; in the initial staging, as additional tool, for rhabdomyosarcoma and STS of extremities or superficial trunk and head and neck tumours; in the suspicion of local recurrence when the CT or MRI are inconclusive and in the presence of osteosynthesis or prosthetic material and in assessment of therapy response to local/systemic therapy in stages ii/iii. In addition, PET/CT has the added value of being a surrogate marker of the histopathological response and it provides prognostic information, both in the baseline study and after treatment.

Gated 18F-FDG PET/CT of the Lung Using a Respiratory Spirometric Gating Device: A Feasibility Study

Spirometric gating devices (SGDs) can measure the respiratory signal with high temporal resolution and accuracy. The primary objective of this study was to assess the feasibility and tolerance of a gated lung PET/CT acquisition using an SGD. The secondary objective was to compare the technical quality, accuracy, and interoperability of the SGD with that of a standard respiratory gating device, Real-Time Position Management (RPM), based on measurement of vertical thoracoabdominal displacement. Methods: A prospective phase I monocentric clinical study was performed on patients undergoing ¹⁸F-FDG PET/CT for assessment of a solitary lung nodule, staging of lung malignancy, or planning of radiotherapy. After whole-body PET/CT, a centered gated acquisition of both PET and CT was simultaneously obtained with the SGD and RPM during normal breathing. Results: Of the 46 patients who were included, 6 were prematurely excluded (1 because of hyperglycemia and 5 because of distant metastases revealed by whole-body PET/CT, leading to an unjustified extra gated acquisition). No serious adverse events were observed. Of the 40 remaining patients, the gated acquisition was prematurely stopped in 1 patient because of mask discomfort (2.5%; confidence interval [CI], 0.1%-13.2%). This event was considered patient tolerance failure. The SGD generated accurately gated PET/CT images, with more than 95% of the breathing cycle detected and high temporal resolution, in 34 of the 39 patients (87.2%; 95% CI, 60.0%-100.0%) and failed to generate a biologic tumor volume in 1 of 21 patients with increased ¹⁸F-FDG uptake (4.8%; 95% CI, 0.1%-26.5%). The quality and accuracy of respiratory signal detection and synchronization were significantly better than those obtained with RPM (P < 0.05). Conclusion: This trial supports the use of an SGD for gated lung PET/CT because of its high patient tolerance and accuracy. Although this technique seems to technically outperform RPM for gated PET/CT, further assessment of its superiority and the clinical benefit is warranted. We believe that this technique could be used as a gold standard to develop innovative approaches to eliminate respiration-induced blurring artifacts.

Respiratory-gated PET/CT for pulmonary lesion characterisation-promises and problems

2-deoxy-2-(¹⁸Fluorine)-fluoro-D-glucose (FDG) PET/CT is an integral part of lung carcinoma staging and frequently used in the assessment of solitary pulmonary nodules. However, a limitation of conventional three-dimensional PET/CT when imaging the thorax is its susceptibility to motion artefact, which blurs the signal from the lesion resulting in inaccurate representation of size and metabolic activity. Respiratory gated (four-dimensional) PET/CT aims to negate the effects of motion artefact and provide a more accurate interpretation of pulmonary nodules and lymphadenopathy. There have been recent advances in technology and a shift from traditional hardware to more streamlined software methods for respiratory gating which should allow more widespread use of respiratory-gating in the future. The purpose of this article is to review the evidence surrounding four-dimensional PET/CT in pulmonary lesion characterisation.

Added diagnostic value of respiratory-gated 4D 18F-FDG PET/CT in the detection of liver lesions: a multicenter study

PURPOSE

The aim of the present study was to evaluate the added diagnostic value of respiratory-gated 4D18F-FDG PET/CT in liver lesion detection and characterization in a European multicenter retrospective study.

METHODS

Fifty-six oncological patients (29 males and 27 females, mean age, 61.2 ± 11.2 years) from five European centers, submitted to standard 3D-PET/CT and liver 4D-PET/CT were retrospectively evaluated. Based on visual analysis, liver PET/CT findings were scored as positive, negative, or equivocal both in 3D and 4D PET/CT. The impact of 4D-PET/CT on the confidence in classifying liver lesions was assessed. PET/CT findings were compared to histology and clinical follow-up as standard reference and diagnostic accuracy was calculated for both techniques. At semi-quantitative analysis, SUVmax was calculated for each detected lesion in 3D and 4D-PET/CT.

RESULTS

Overall, 72 liver lesions were considered for the analysis. Based on visual analysis in 3D-PET/CT, 32/72 (44.4%) lesions were considered positive, 21/72 (29.2%) negative, and 19/72 (26.4%) equivocal, while in 4D-PET/CT 48/72 (66.7%) lesions were defined positive, 23/72 (31.9%) negative, and 1/72 (1.4%) equivocal. 4D-PET/CT findings increased the confidence in lesion definition in 37/72 lesions (51.4%). Considering 3D equivocal lesions as positive, sensitivity, specificity, and accuracy were 88.9, 70.0, and 83.1%, respectively, while the same figures were 67.7, 90.0, and 73.8% if 3D equivocal findings were included as negative. 4D-PET/CT sensitivity, specificity, and accuracy were 97.8, 90.0, and 95.4%, respectively, considering equivocal lesions as positive and 95.6, 90.0, and 93.8% considering equivocal lesions as negative. The SUVmax of the liver lesions in 4D-PET (mean ± SD, 6.9 ± 3.2) was significantly higher (p < 0.001) than SUVmax in 3D-PET (mean ± SD, 5.2 ± 2.3).

CONCLUSIONS

Respiratory-gated PET/CT technique is a valuable clinical tool in diagnosing liver lesions, reducing 3D undetermined findings, improving diagnostic accuracy, and confidence in reporting. 4D-PET/CT also improved the quantification of SUVmax of liver lesions.

Respiratory-gated time-of-flight PET/CT during whole-body scan for lung lesions: feasibility in a routine clinical setting and quantitative analysis

PURPOSE

To demonstrate the feasibility of respiratory gating during whole-body scan for lung lesions in routine ¹⁸F-FDG PET/CT examinations using a time-of-flight (TOF)-capable scanner to determine the effect of respiratory gating on reduction of both misregistration (between CT and PET) and image blurring, and on improvement of the maximum standardized uptake value (SUVmax).

MATERIALS AND METHODS

Patients with lung lesions who received FDG PET/CT were prospectively studied. Misregistration, volume of PET (Vp), and SUVmax were compared between ungated and gated images. The difference in respiratory gating effects was compared between lesions located in the upper or middle lobes (UML) and the lower lobe (LL). The correlation between three parameters (% change in misregistration, % change in Vp, and lesion size) and % change in SUVmax was analyzed.

RESULTS

The study population consisted of 60 patients (37 males, 23 females; age 68 ± 12 years) with lung lesions (2.5 ± 1.7 cm). Fifty-eight out of sixty respiratory gating studies were successfully completed with a total scan time of 20.9 ± 1.9 min. Eight patients' data were not suitable for analysis, while the remaining 50 patients' data were analyzed. Respiratory gating reduced both misregistration by 21.4 % (p < 0.001) and Vp by 14.2 % (p < 0.001). The SUVmax of gated images improved by 14.8 % (p < 0.001). The % change in misregistration, Vp, and SUVmax by respiratory gating tended to be larger in LL lesions than in UML lesions. The correlation with % change in SUVmax was stronger in % change in Vp (r = 0.57) than % change in misregistration (r = 0.35). There was no statistically significant correlation between lesion size and % change in SUVmax (r = -0.20).

CONCLUSIONS

Respiratory gating during whole-body scan in routine TOF PET/CT examinations is feasible and can reduce both misregistration and PET image blurring, and improve the SUVmax of lung lesions located primarily in the LL.

Applications and limitations of radiomics

Radiomics is an emerging field in quantitative imaging that uses advanced imaging features to objectively and quantitatively describe tumour phenotypes. Radiomic features have recently drawn considerable interest due to its potential predictive power for treatment outcomes and cancer genetics, which may have important applications in personalized medicine. In this technical review, we describe applications and challenges of the radiomic field. We will review radiomic application areas and technical issues, as well as proper practices for the designs of radiomic studies.

Reduction of Respiratory Motion During PET/CT by Pulsatile-Flow Ventilation: A First Clinical Evaluation

Respiratory motion negatively affects PET/CT image quality and quantitation. A novel Pulsatile-Flow Ventilation (PFV) system reducing respiratory motion was applied in spontaneously breathing patients to induce sustained apnea during PET/CT.

METHODS

Four patients (aged 65 ± 14 y) underwent PET/CT for pulmonary nodule staging (mean, 11 ± 7 mm; range, 5-18 mm) at 63 ± 3 min after (18)F-FDG injection and then at 47 ± 7 min afterward, during PFV-induced apnea (with imaging lasting ≥8.5 min). Anterior-posterior thoracic amplitude, SUVmax, and SUVpeak (SUVmean in a 1-cm-diameter sphere) were compared.

RESULTS

PFV PET/CT reduced thoracic amplitude (80%), increased mean lesion SUVmax (29%) and SUVpeak (11%), decreased lung background SUVpeak (25%), improved lesion detectability, and increased SUVpeak lesion-to-background ratio (54%). On linear regressions, SUVmax and SUVpeak significantly improved (by 35% and 23%, respectively; P ≤ 0.02).

CONCLUSION

PFV-induced apnea reduces thoracic organ motion and increases lesion SUV, detectability, and delineation, thus potentially affecting patient management by improving diagnosis, prognostication, monitoring, and external-radiation therapy planning.

(18)F-FDG-PET/CT in the assessment of pulmonary solitary nodules: comparison of different analysis methods and risk variables in the prediction of malignancy

OBJECTIVE

To compare the diagnostic performance of different metabolical, morphological and clinical criteria for correct presurgical classification of the solitary pulmonary nodule (SPN).

METHODS

Fifty-five patients, with SPN were retrospectively analyzed. All patients underwent preoperative (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT). Maximum diameter in CT, maximum standard uptake value (SUVmax), histopathologic result, age, smoking history and gender were obtained. Different criteria were established to classify a SPN as malignant: (I) visually detectable metabolism, (II) SUVmax >2.5 regardless of SPN diameter, (III) SUVmax threshold depending of SPN diameter, and (IV) ratio SUVmax/diameter greater than 1. For each criterion, statistical diagnostic parameters were obtained. Receiver operating characteristic (ROC) analysis was performed to select the best diagnostic SUVmax and SUVmax/diameter cutoff. Additionally, a predictive model of malignancy of the SPN was derived by multivariate logistic regression.

RESULTS

Fifteen SPN (27.3%) were benign and 40 (72.7%) malignant. The mean values ± standard deviation (SD) of SPN diameter and SUVmax were 1.93±0.57 cm and 3.93±2.67 respectively. Sensitivity (Se) and specificity (Sp) of the different diagnostic criteria were (I): 97.5% and 13.1%; (II) 67.5% and 53.3%; (III) 70% and 53.3%; and (IV) 85% and 33.3%, respectively. The SUVmax cut-off value with the best diagnostic performance was 1.95 (Se: 80%; Sp: 53.3%). The predictive model had a Se of 87.5% and Sp of 46.7%. The SUVmax was independent variables to predict malignancy.

CONCLUSIONS

The assessment by semiquantitative methods did not improve the Se of visual analysis. The limited Sp was independent on the method used. However, the predictive model combining SUVmax and age was the best diagnostic approach.

Utility of respiratory-gated small-animal PET/CT in the chronologic evaluation of an orthotopic lung cancer transplantation mouse model

Our aim in this study was to clarify the effects of respiratory-gated PET in the evaluation of lung cancer according to the (18)F-FDG uptake in an orthotopic transplantation mouse model. We created such a model, and we performed PET/CT. The mice were divided into two groups according to tumor volume: a small-tumor group (<20 mm(3)) and a large-tumor group (>20 mm(3)). We reconstructed the following conditions based on list-mode data: non-gated (3D) images and gated (4D) images, divided based on the respiratory cycle (expiration phase, stable phase, and inspiration phase). We calculated the maximum standardized uptake values (SUVmax) in each phase. We used the % difference [= (4D SUVmax - 3D SUVmax)/3D PET SUVmax × 100 (%)] to evaluate the differences in the 4D SUVmax and 3D SUVmax. The 4D SUVmax values were significantly higher than the 3D SUVmax, regardless of the tumor size. The % difference for the small tumors was greater than that for the large tumors, and it was highest in the stable phase. We conclude that the SUVmax in the stable phase under respiratory-gated PET are the most reliable. The SUVmax observed under non-gated PET are considered to be more frequently underestimated in cases involving small tumors than in those involving large tumors. In the chronologic study evaluating the time course of tumor development, the size of the tumor is small in early stage, and respiratory-gated PET is effective in reducing the underestimation of such tumors caused by respiratory motion.

Respiratory-gated (4D) FDG-PET detects tumour and normal lung response after stereotactic radiotherapy for pulmonary metastases

BACKGROUND

Response assessment after stereotactic ablative body radiotherapy (SABR) in lung can be confounded by radiation-induced inflammation, fibrosis and subsequent alteration of tumour motion. The purpose of this prospective pilot study was to evaluate the utility of four-dimensional (4D) FDG-PET/CT for post-SABR tumour and normal lung response assessment in pulmonary oligometastases.

MATERIAL AND METHODS

Patients enrolled from February 2010 to December 2011 in this prospective ethics approved study had 1-2 pulmonary metastases on staging FDG-PET. Serial contemporaneous 3D and 4D FDG-PET/CT scans were performed at baseline, 14 days and 70 days after a single fraction of 26 Gy SABR. Tumour response was evaluated in 3D and 4D using SUVmax, RECIST and PERCIST criteria. Normal lung radiotoxicity was evaluated using SUVmean within 0-2 Gy, 2-5 Gy, 5-10 Gy, 10-20 Gy and 20 + Gy isodose volumes.

RESULTS

In total, 17 patients were enrolled of which seven were ineligible due to interval progression from staging PET to baseline 4D-PET. The mean time between scans was 62 days. At a median follow-up of 16 months, 10 patients with 13 metastases received SABR, with no patient having local progression. The vector of tumour motion was larger in patients with discordant 3D and 4D PET PERCIST response (p < 0.01), with a mean (± SEM) motion of 10.5 mm (± 0.96 mm) versus 6.14 mm (± 0.81 mm) in those patients with concordant 3D and 4D response. Surrounding normal lung FDG uptake at 70 days was strongly correlated to delivered radiation dose (r(2) = 0.99, p < 0.01), with significant elevations across all dose levels (p ≤ 0.05), except the < 2 Gy volume (p = 0.30).

CONCLUSIONS

We demonstrate high rates of interval progression between staging PET scans in patients with oligometastases. We found that tumour response on conventional 3D PET is not concordant with 4D PET for tumours with large motion. Normal lung metabolic uptake is strongly dose dependent after SABR, a novel finding that should be further validated.

Heterogeneity in [18F]fluorodeoxyglucose positron emission tomography/computed tomography of non-small cell lung carcinoma and its relationship to metabolic parameters and pathologic staging

To investigate the relationships between tumor heterogeneity, assessed by texture analysis of [18F]fluorodeoxyglucose-positron emission tomography (FDG-PET) images, metabolic parameters, and pathologic staging in patients with non-small cell lung carcinoma (NSCLC). A retrospective analysis of 38 patients with histologically confirmed NSCLC who underwent staging FDG-PET/computed tomography was performed. Tumor images were segmented using a standardized uptake value (SUV) cutoff of 2.5. Five textural features, related to the heterogeneity of gray-level distribution, were computed (energy, entropy, contrast, homogeneity, and correlation). Additionally, metabolic parameters such as SUVmax, SUVmean, metabolic tumor volume (MTV), and total lesion glycolysis (TLG), as well as pathologic staging, histologic subtype, and tumor diameter, were obtained. Finally, a correlation analysis was carried out. Of 38 tumors, 63.2% were epidermoid and 36.8% were adenocarcinomas. The mean ± standard deviation values of MTV and TLG were 30.47 ± 25.17 mL and 197.81 ± 251.11 g, respectively. There was a positive relationship of all metabolic parameters (SUVmax, SUVmean, MTV, and TLG) with entropy, correlation, and homogeneity and a negative relationship with energy and contrast. The T component of the pathologic TNM staging (pT) was similarly correlated with these textural parameters. Textural features associated with tumor heterogeneity were shown to be related to global metabolic parameters and pathologic staging.

Comparison of texture features derived from static and respiratory-gated PET images in non-small cell lung cancer

BACKGROUND

PET-based texture features have been used to quantify tumor heterogeneity due to their predictive power in treatment outcome. We investigated the sensitivity of texture features to tumor motion by comparing static (3D) and respiratory-gated (4D) PET imaging.

METHODS

Twenty-six patients (34 lesions) received 3D and 4D [18F]FDG-PET scans before the chemo-radiotherapy. The acquired 4D data were retrospectively binned into five breathing phases to create the 4D image sequence. Texture features, including Maximal correlation coefficient (MCC), Long run low gray (LRLG), Coarseness, Contrast, and Busyness, were computed within the physician-defined tumor volume. The relative difference (δ3D-4D) in each texture between the 3D- and 4D-PET imaging was calculated. Coefficient of variation (CV) was used to determine the variability in the textures between all 4D-PET phases. Correlations between tumor volume, motion amplitude, and δ3D-4D were also assessed.

RESULTS

4D-PET increased LRLG ( = 1%-2%, p < 0.02), Busyness ( = 7%-19%, p < 0.01), and decreased MCC ( = 1%-2%, p < 7.5 × 10(-3)), Coarseness ( = 5%-10%, p < 0.05) and Contrast ( = 4%-6%, p > 0.08) compared to 3D-PET. Nearly negligible variability was found between the 4D phase bins with CV < 5% for MCC, LRLG, and Coarseness. For Contrast and Busyness, moderate variability was found with CV = 9% and 10%, respectively. No strong correlation was found between the tumor volume and δ3D-4D for the texture features. Motion amplitude had moderate impact on δ for MCC and Busyness and no impact for LRLG, Coarseness, and Contrast.

CONCLUSIONS

Significant differences were found in MCC, LRLG, Coarseness, and Busyness between 3D and 4D PET imaging. The variability between phase bins for MCC, LRLG, and Coarseness was negligible, suggesting that similar quantification can be obtained from all phases. Texture features, blurred out by respiratory motion during 3D-PET acquisition, can be better resolved by 4D-PET imaging. 4D-PET textures may have better prognostic value as they are less susceptible to tumor motion.

[Improvement of quantitative accuracy using phase-based respiratory-gated PET/CT in phantom and clinical studies]

OBJECTIVE

The present study aimed at determining the quantitative accuracy of phase-based respiratory-gated PET/CT imaging using phantom and clinical studies.

METHODS

The effects of target size, target-to-background ratio (TBR), and respiratory motion on PET images were estimated using a NEMA body phantom comprising six spheres (diameter 10-37mm) in a solution of F-18 of three different TBRs (4, 6, 8). The phantom was moved in a superior-inferior direction at motion displacements of 0, 10, 20 and 30 mm. Stationary images of the phantom as well as non-gated (3D) and gated (4D) images of the phantom while moving were reconstructed and the recovery coefficient (RC) of individual spheres was calculated from each image. We then determined the RC improvement rate to evaluate improvements conferred by 4D-PET/CT. We retrospectively analyzed data from 14 patients with lung cancer who were examined by 3D- and 4D-PET/CT. Each lesion on the 3D-PET/CT and each of the five phases of the 4D-PET/CT were analyzed.

RESULTS

Larger motion displacement and TBR resulted in increased RC degradation for small spheres. The RC improvement rate showed that 4D acquisition improved the RC of spheres with larger motion displacement exceeding 13 mm in diameter. 4D-PET/CT alone can reduce the effects of motion blurring, but partial volume effects may still be the dominant source of quantitative inaccuracy for small lesions. The trends of phantom and clinical studies for evaluating the improvement rate were similar.

CONCLUSIONS

4D-PET/CT significantly improved the quantitative accuracy of PET images particularly when larger motion displacement exceeded 17mm in diameter such as in lung cancer.

Management of respiratory motion in PET/computed tomography: the state of the art

Combined PET/computed tomography (CT) is of value in cancer diagnosis, follow-up, and treatment planning. For cancers located in the thorax or abdomen, the patient’s breathing causes artifacts and errors in PET and CT images. Many different approaches for artifact avoidance or correction have been developed; most are based on gated acquisition and synchronization between the respiratory signal and PET acquisition. The respiratory signal is usually produced by an external sensor that tracks a physiological characteristic related to the patient’s breathing. Respiratory gating is a compensation technique in which time or amplitude binning is used to exclude the motion in reconstructed PET images. Although this technique is performed in routine clinical practice, it fails to adequately correct for respiratory motion because each gate can mix several tissue positions. Researchers have suggested either selecting PET events from gated acquisitions or performing several PET acquisitions (corresponding to a breath-hold CT position). However, the PET acquisition time must be increased if adequate counting statistics are to be obtained in the different gates after binning. Hence, other researchers have assessed correction techniques that take account of all the counting statistics (without increasing the acquisition duration) and integrate motion information before, during, or after the reconstruction process. Here, we provide an overview of how motion is managed to overcome respiratory motion in PET/CT images.

Comparing respiratory gated with delayed scans in the detection of colorectal carcinoma hepatic and pulmonary metastases with 18F-FDG PET-CT

PURPOSE

In patients experiencing colorectal carcinoma, exhaustive analysis of indicates extent of hepatic and pulmonary surgery that prolongs survival of patients.

PATIENTS AND METHODS

To localize metastasis, we compared 2 F-FDG PET-CT imaging protocols, early respiratory gated scan, and delayed scan, to standard PET imaging procedure. SUVmax and lesion-to-healthy tissue ratio were measured in 60 pulmonary and 21 hepatic lesions by each of the 2 imaging protocols.

RESULTS

In the liver, metastatic lesion-to-healthy tissue ratios significantly increased on delayed scans as compared with early scans (P < 0.001). Better ratios could not be obtained when using respiratory gated scans, whereas more lesions were detected on delayed scans. In the lungs, metastatic lesion- to-healthy tissue ratio increased significantly on delayed scans for largest lesions (P < 0.001). Ratios were not better when exploring smallest metastatic lesions or when using respiratory gated scans. Factors interfering with F-FDG PET-CT sensitivity are discussed, such as respiratory motion and high FDG physiological uptake in healthy liver parenchyma.

CONCLUSIONS

Our studies indicate that routine acquisitions should use delayed hepatic scans for all patients referred for evaluation, recurrence check, or monitoring of colorectal adenocarcinoma. Delayed pulmonary scans are useful for the largest metastatic lesions and should be used in addition to early scan. In our experience, respiratory gated scans seem to be less convenient because of a low sensitivity in detection, as compared with the delayed technique; in addition, this technique is currently complicated by some technical issues, although these might be overcome with new gated protocols.

4D PET/CT as a Strategy to Reduce Respiratory Motion Artifacts in FDG-PET/CT

The improved accuracy in tumor identification with FDG-PET has led to its increased utilization in target volume delineation for radiotherapy treatment planning in the treatment of lung cancer. However, PET/CT has constantly been influenced by respiratory motion-related image degradation, which is especially prominent for small lung tumors in the peri-diaphragmatic regions of the thorax. Here, we describe the current findings on respiratory motion-related image degradation in PET/CT, which may bring uncertainties to target volume delineation for image guided radiotherapy (IGRT) for lung cancer. Furthermore, we describe the evidence suggesting 4D PET/CT to be one strategy to minimize the impact of respiratory motion-related image degradation on tumor target delineation for thoracic IGRT. This, in our opinion, warrants further investigation in future IGRT-based lung cancer trials.

Imaging techniques for tumour delineation and heterogeneity quantification of lung cancer: overview of current possibilities

Imaging techniques for the characterization and delineation of primary lung tumours and lymph nodes are a prerequisite for adequate radiotherapy. Numerous imaging modalities have been proposed for this purpose, but only computed tomography (CT) and FDG-PET have been implemented in clinical routine. Hypoxia PET, dynamic contrast-enhanced CT (DCE-CT), dual energy CT (DECT) and (functional) magnetic resonance imaging (MRI) hold promise for the future. Besides information on the primary tumour, these techniques can be used for quantification of tissue heterogeneity and response. In the future, treatment strategies may be designed which are based on imaging techniques to optimize individual treatment.

A prospective investigation into the clinical impact of 4D-PET/CT in the characterisation of solitary pulmonary nodules

Background

While the effects of respiratory motion on measuring metabolic signal in PET/CT scanning are well known, it is still standard practice in most centres to scan patients while breathing freely with no correction for the effects of respiratory motion. The aim of this study was to investigate the impact of 4D-PET/CT in classifying lesions in patients with a radiologically-indeterminate solitary pulmonary nodule.

Methods

Twenty consecutive patients with a solitary pulmonary nodule for investigation were prospectively recruited and completed a whole-body (WB)-PET/CT and 4D-PET/CT in the same session. The reporting physician initially classified the nodule using a 5-point scale (Definitely Malignant, Probably Malignant, Indeterminate, Probably benign, Definitely Benign) on the WB-PET/CT. The physician was then shown the 4D-PET/CT and asked if they would re-classify the lesion. Frequency, sensitivity, specificity and accuracy values were calculated for WB-PET/CT alone and then with the addition of the 4D-PET/CT.

Results

There were no changes in the classification for nodules initially classed as either benign or malignant with the addition of a 4D-PET/CT. However changes were observed between WB and 4D-PET/CT scans in lesions initially classified as indeterminate. When indeterminate lesions were defined as malignant there was a minor increase in sensitivity (from 73% to 75%), in specificity (56%-63%) and in accuracy (65%-70%) but these results do not reach statistical significance. When the Indeterminate lesions were defined as benign there was an increase in sensitivity (from 55% to 67%) but there was a reduction in the specificity (100%-75%) and accuracy (75%-70%) with the addition of the 4D-PET/CT but again the results did not reach statistical significance.

Conclusion

The addition of 4D-PET/CT is most likely to have an impact on those nodules initially classified as indeterminate on standard WB-PET/CT. In lesions classified as benign or malignant on standard WB-PET/CT the addition of a 4D-PET/CT is less likely to impact lesion classification. While 4D-PET/CT does improve the measurement of the metabolic signal, it does not overcome inherent limitations of FDG in differentiating a malignant lesion from inflammatory processes, correct for partial volume effects or compensate for the low intrinsic FDG-avidity of some malignancies.

Respiratory-gated PET/CT versus delayed images for the quantitative evaluation of lower pulmonary and hepatic lesions

PURPOSE

Respiratory motion degrades fluorodeoxyglucose positron emission tomography (FDG PET) images of the lower chest and upper abdomen, as the blur introduced by breathing motion increases the apparent size of the moving tumour lesions and decreases their apparent uptake, reducing the sensitivity of PET in detection of small lesions. We assessed the role of delayed and respiratory-gated PET acquisition in the quantitative evaluation of lung and liver lesions.

METHODS

A retrospective analysis of 64 lesions was performed. After initial non-gated whole-body PET/CT, respiratory gating was performed with 15 min in list mode. Non-gated delayed images were obtained by summing all list mode data. SUV(max) adjusted for lean body mass (SUL(max)) was measured in the initial whole-body scan, the delayed non-gated scans and the individual gated bins for each lesion. The axial z-position of SUL(max) for each lesion in five respiratory-gated bins was determined. The mean SUL of the non-pathological liver parenchyma was also recorded for each patient.

RESULTS

Tumour lesion SUL(max) increased by an average of 34% in the delayed non-gated scan as compared with the whole-body initial scan and further by an additional 17.2% in respiratory-gated images. The maximum lesion displacement was 6.2 ± 5.0 mm.

CONCLUSION

Delayed imaging alone substantially increases the magnitude of the SUL of liver and lung lesions as compared with standard whole-body images and may allow for a more accurate definition of the lesion's volume and localisation and improve tracer quantitation in malignant lesions in the lungs or upper abdomen. While respiratory gating provides more optimal imaging with greatest increase in SUL(max), the benefit is small, and delayed imaging appears sufficient in most cases.

External radioactive markers for PET data-driven respiratory gating in positron emission tomography

PURPOSE

Respiratory gating is an established approach to overcoming respiration-induced image artefacts in PET. Of special interest in this respect are raw PET data-driven gating methods which do not require additional hardware to acquire respiratory signals during the scan. However, these methods rely heavily on the quality of the acquired PET data (statistical properties, data contrast, etc.). We therefore combined external radioactive markers with data-driven respiratory gating in PET/CT. The feasibility and accuracy of this approach was studied for [(18)F]FDG PET/CT imaging in patients with malignant liver and lung lesions.

METHODS

PET data from 30 patients with abdominal or thoracic [(18)F]FDG-positive lesions (primary tumours or metastases) were included in this prospective study. The patients underwent a 10-min list-mode PET scan with a single bed position following a standard clinical whole-body [(18)F]FDG PET/CT scan. During this scan, one to three radioactive point sources (either (22)Na or (18)F, 50-100 kBq) in a dedicated holder were attached the patient's abdomen. The list mode data acquired were retrospectively analysed for respiratory signals using established data-driven gating approaches and additionally by tracking the motion of the point sources in sinogram space. Gated reconstructions were examined qualitatively, in terms of the amount of respiratory displacement and in respect of changes in local image intensity in the gated images.

RESULTS

The presence of the external markers did not affect whole-body PET/CT image quality. Tracking of the markers led to characteristic respiratory curves in all patients. Applying these curves for gated reconstructions resulted in images in which motion was well resolved. Quantitatively, the performance of the external marker-based approach was similar to that of the best intrinsic data-driven methods. Overall, the gain in measured tumour uptake from the nongated to the gated images indicating successful removal of respiratory motion was correlated with the magnitude of the respiratory displacement of the respective tumour lesion, but not with lesion size.

CONCLUSION

Respiratory information can be assessed from list-mode PET/CT through PET data-derived tracking of external radioactive markers. This information can be successfully applied to respiratory gating to reduce motion-related image blurring. In contrast to other previously described PET data-driven approaches, the external marker approach is independent of tumour uptake and thereby applicable even in patients with poor uptake and small tumours.

The retrospective binning method improves the consistency of phase binning in respiratory-gated PET/CT

This study assesses the accuracy of prospective phase-gated PET/CT data binning and presents a retrospective data binning method that improves image quality and consistency. Respiratory signals from 17 patients who underwent 4D PET/CT were analysed to evaluate the reproducibility of temporal triggers used for the standard phase-based gating method. Breathing signals were reprocessed to implement retrospective PET data binning. The mean and standard deviation of time lags between automatic triggers provided by the Real-time Position Management (RPM, Varian) gating device and inhalation peaks derived from respiratory curves were computed for each patient. The total number of respiratory cycles available for 4D PET/CT according to the binning mode (prospective versus retrospective) was compared. The maximum standardized uptake value (SUV(max)), biological tumour volume (BTV) and tumour trajectory measures were determined from the PET/CT images of five patients. Compared to retrospective binning (RB), prospective gating approach led to (i) a significant loss in breathing cycles (15%) and (ii) the inconsistency of data binning due to temporal dispersion of triggers (average 396 ms). Consequently, tumour characterization could be impacted. In retrospective mode, SUV(max) was up to 27% higher, where no significant difference appeared in BTV. In addition, prospective mode gave an inconsistent spatial location of the tumour throughout the bins. Improved consistency with breathing patterns and greater motion amplitude of the tumour centroid were observed with retrospective mode. The detection of the tumour motion and trajectory was improved also for small temporal dispersion of triggers. This study shows that the binning mode could have a significant impact on 4D PET images. The consistency of triggers with breathing signals should be checked before clinical use of gated PET/CT images, and our RB method improves 4D PET/CT image quantification.

Role of respiratory-gated PET/CT for pancreatic tumors: a preliminary result

PURPOSE

The aim of this study is to ascertain role of respiratory-gated PET/CT for accurate diagnosis of pancreatic tumors.

MATERIALS AND METHODS

Prior to clinical study, the phantom study was performed to evaluate the impact of respiratory motion on lesion quantification. Twenty-two patients (mean age 65 years) with pancreatic tumors were enrolled. Pathological diagnoses by surgical specimens consisted of pancreatic cancer (n=15) and benign intraductal papillary mucinous neoplasm (IPMN, n=7). Whole-body scan of non-respiratory-gated PET/CT was performed at first, and subsequent respiratory-gated PET/CT for one bed position was performed. All PET/CT studies were performed prior to surgery. The SUV max obtained by non-respiratory-gated PET/CT and respiratory-gated PET/CT, and percent difference in SUVmax (%SUVmax) were compared.

RESULTS

The profile curve of 5 respiratory bin image was most similar to that of static image. The third bin of 5 respiratory bin image showed highest FWHM (24.0mm) and FWTM (32.7 mm). The mean SUVmax of pancreatic cancer was similar to that of benign IPMN on non-respiratory-gated PET/CT (p=0.05), whereas significant difference was found between two groups on respiratory-gated PET/CT (p=0.016). The mean %SUV of pancreatic cancer was greater than that of benign IPMN (p<0.0001). Identification of the primary tumor in pancreatic head (n=13, 59%) was improved by using respiratory-gated PET/CT because of minimal affection of physiological accumulation in duodenum.

CONCLUSION

Respiratory-gated PET/CT is a feasible technique for evaluation of pancreatic tumors and allows more accurate identification of pancreatic tumors compared with non-respiratory-gated PET/CT.

The clinical application of 4D 18F-FDG PET/CT on gross tumor volume delineation for radiotherapy planning in esophageal squamous cell cancer

A combination of four-dimensional computed tomography with (18)F-fluorodeoxyglucose positron emission tomography (4D CT-FDG PET) was used to delineate gross tumor volume (GTV) in esophageal cancer (EC). Eighteen patients with EC were prospectively enrolled. Using 4D images taken during the respiratory cycle, the average CT image phase was fused with the average FDG PET phase in order to analyze the optimal standardized uptake values (SUV) or threshold. PET-based GTV (GTV(PET)) was determined with eight different threshold methods using the auto-contouring function on the PET workstation. The difference in volume ratio (VR) and conformality index (CI) between GTV(PET) and CT-based GTV (GTV(CT)) was investigated. The image sets via automatic co-registrations of 4D CT-FDG PET were available for 12 patients with 13 GTV(CT) values. The decision coefficient (R(2)) of tumor length difference at the threshold levels of SUV 2.5, SUV 20% and SUV 25% were 0.79, 0.65 and 0.54, respectively. The mean volume of GTV(CT) was 29.41 ± 19.14 ml. The mean VR ranged from 0.30 to 1.48. The optimal VR of 0.98, close to 1, was at SUV 20% or SUV 2.5. The mean CI ranged from 0.28 to 0.58. The best CI was at SUV 20% (0.58) or SUV 2.5 (0.57). The auto-contouring function of the SUV threshold has the potential to assist in contouring the GTV. The SUV threshold setting of SUV 20% or SUV 2.5 achieves the optimal correlation of tumor length, VR, and CI using 4D-PET/CT images.

Respiratory gated PET/CT in a European multicentre retrospective study: added diagnostic value in detection and characterization of lung lesions

PURPOSE

The aim of our work is to evaluate the added diagnostic value of respiratory gated (4-D) positron emission tomography/computed tomography (PET/CT) in lung lesion detection/characterization in a large patient population of a multicentre retrospective study.

METHODS

The data of 155 patients (89 men, 66 women, mean age 63.9 ± 11.1 years) from 5 European centres and submitted to standard (3-D) and 4-D PET/CT were retrospectively analysed. Overall, 206 lung lesions were considered for the analysis (mean ± SD lesions dimension 14.7 ± 11.8 mm). Maximum standardized uptake values (SUV(max)) and lesion detectability were assessed for both 3-D and 4-D PET/CT studies; 3-D and 4-D PET/CT findings were compared to clinical follow-up as standard reference.

RESULTS

Mean ± SD 3-D and 4-D SUV(max) values were 5.2 ± 5.1 and 6.8 ± 6.1 (p < 0.0001), respectively, with an average percentage increase of 30.8 %. In 3-D PET/CT, 86 of 206 (41.7 %) lesions were considered positive, 70 of 206 (34 %) negative and 50 of 206 (24.3 %) equivocal, while in 4-D PET/CT 117 of 206 (56.8 %) lesions were defined as positive, 80 of 206 (38.8 %) negative and 9 of 206 (4.4 %) equivocal. In 34 of 50 (68 %) 3-D equivocal lesions follow-up data were available and the presence of malignancy was confirmed in 21 of 34 (61.8 %) lesions, while in 13 of 34 (38.2 %) was excluded. In 31 of these 34 controlled lesions, 20 of 34 (58.8 %) and 11 of 34 (32.4 %) were correctly classified by 4-D PET/CT as positive and negative, respectively; 3 of 34 (8.8 %) remained equivocal. With equivocal lesions classified as positive, the overall accuracy of 3-D and 4-D was 85.7 and 92.8 %, respectively, while the same figures were 80.5 and 94.2 % when equivocal lesions were classified as negative.

CONCLUSION

The respiratory gated PET/CT technique is a valuable clinical tool in diagnosing lung lesions, improving quantification and confidence in reporting, reducing 3-D undetermined findings and increasing the overall accuracy in lung lesion detection and characterization.

Evaluating FDG uptake changes between pre and post therapy respiratory gated PET scans

PURPOSE

Whole body (3D) and respiratory gated (4D) FDG-PET/CT scans performed pre-radiotherapy (pre-RT) and post-radiotherapy (post-RT) were analyzed to investigate the impact of 4D PET in evaluating 18F-fluorodeoxyglucose (FDG) uptake changes due to therapy, relative to traditional 3D PET.

METHODS AND MATERIALS

3D and 4D sequential FDG-PET/CT scans were acquired pre-RT and approximately one month post-RT for patients with non-small cell lung cancer (NSCLC). The lesions of high uptake targeted with radiotherapy were identified on the pre-RT scan of each patient. Each lesion on the 3D and each of the five phases of the 4D scan were analyzed using a region of interest (ROI). For each patient the ROIs of the pre-RT scans were used to locate the areas of initial FDG uptake on the post-RT scans following rigid registration. Post-RT ROIs were drawn and the FDG uptake was compared with that of the pre-RT scans.

RESULTS

Sixteen distinct lesions from 12 patients were identified and analyzed. Standardized uptake value (SUV) maxima were significantly higher (p-value <0.005) for the lesions as measured on the 4D compared to 3D PET. Comparison of serial pre and post-RT scans showed a mean 62% decrease in SUV with the 3D PET scan (range 36-89%), and a 67% decrease with the 4D PET scan (range 30-89%). The mean absolute difference in SUV change on 3D versus 4D scans was 4.9%, with a range 0-15% (p-value = 0.07).

CONCLUSIONS

Signal recovery with 4D PET results in higher SUVs when compared to standard 3D PET. Consequently, differences in the evaluation of SUV changes between pre and post-RT plans were observed. Such difference can have a significant impact in PET-based response assessment.

The clinical significance and management of lesion motion due to respiration during PET/CT scanning

Lesion movement during positron emission tomography (PET) scan acquisition due to normal respiration is a common source of artefact. A PET scan is acquired in multiple couch positions of between 2 and 5 min duration with the patient breathing freely. A PET-avid lesion will become blurred if affected by respiratory motion, an effect similar to that created when a person moves in a photograph. This motion also frequently causes misregistration between the PET and computed tomography (CT) scan acquired for attenuation correction and anatomical correlation on hybrid scanners. The compounding effects of blurring and misregistration in whole-body PET/CT imaging make accurate characterization of PET-avid disease in areas of high respiratory motion challenging. There is also increasing interest in using PET quantitatively to assess disease response in both clinical reporting and trials. However, at this stage, no response criteria take the effect of respiratory motion into account when calculating the standardized uptake value on a PET scan. A number of different approaches have been described in the literature to address the issue of respiratory motion in PET/CT scanning. This review details the clinical significance of lesion movement due to respiration and discusses various imaging techniques that have been investigated to manage the effects of respiratory motion in PET/CT scanning.

Small lesions detectability with the Biograph 16 Hi-Rez PET/CT scanner and fast imaging protocols: performance evaluation using an anthropomorphic thoracic phantom and ROC analyses

OBJECTIVES

The purpose of this study was to evaluate the impact on lesion detectability of fast imaging protocols using 18F-FDG and a 3-dimensional LSO-based PET/CT scanner.

METHODS

An anthropomorphic thoracic phantom was used simulating the anatomical structures of radioactivity distribution for the upper torso of an underweight patient. Irregularly shaped targets of small dimensions, the zeolites, were located inside the phantom in an unpredictable position for the observers. Target-to background ratios and target dimensions were selected in order to sample the range of detectability. Repeated imaging was performed to acquire PET images with varying emission scan duration (ESD) of 1, 2, 3 and 4 min/bed and background activity concentrations of 10, 5 and 3 kBq/mL in the torso cavity. Three observers ranked the targets and a receiver operating characteristic analysis was performed for each acquisition protocol.

RESULTS

Detection performances improved when passing from a short (ESD = 1 min) protocol to longer (ESD C 2 min) protocols. This improvement was established with adequate statistical significance.

CONCLUSIONS

Short image acquisition times of 1 min/bed using 18F-FDG and the specific scanner model considered in the study lead to reduced lesion detectability and should be avoided also in underweight patients.

Respiratory-gated 18F-FDG PET imaging in lung cancer: effects on sensitivity and specificity

BACKGROUND

Respiratory motion is known to deteriorate positron emission tomography (PET) images and may lead to potential diagnostic errors when a standardized uptake value (SUV) cut-off threshold is used to discriminate between benign and malignant lesions.

PURPOSE

To evaluate and compare ungated and respiratory-gated 18F-fluorodeoxyglucose PET/computed tomography (CT) methods for the characterization of pulmonary nodules.

MATERIAL AND METHODS

The list-mode acquisition during respiratory-gated PET was combined with a short breath-hold CT scan to form the CT-based images. We studied 48 lesions in 43 patients. PET images were analyzed in terms of the maximum SUV (SUV(max)) and the lesion location.

RESULTS

Using receiver-operating characteristic (ROC) curves, the optimal SUV cut-off thresholds for the ungated and CT-based methods were calculated to be 2.0 and 2.2, respectively. The corresponding sensitivity values were 83% and 92%, respectively, with a specificity of 67% for both methods. The two methods gave equivalent performance levels for the upper and middle lobes (sensitivity 93%, specificity 62%). They differed for the lower lobes, where the CT-based method outperformed the ungated method (sensitivity values of 90% and 70%, respectively, and a specificity of 73% with both methods) - especially for lesions smaller than 15 mm.

CONCLUSION

The CT-based method increased sensitivity and did not diminish specificity, compared with the ungated method. It was more efficient than the ungated method for imaging the lower lobes and smallest lesions, which are most affected by respiratory motion.