Standardization of Preclinical PET/CT Imaging to Improve Quantitative Accuracy, Precision, and Reproducibility: A Multicenter Study.

Preclinical PET/CT is a well-established noninvasive imaging tool for studying disease development/progression and the development of novel radiotracers and pharmaceuticals for clinical applications. Despite this pivotal role, standardization of preclinical PET/CT protocols, including CT absorbed dose guidelines, is essentially nonexistent. This study (1) quantitatively assesses the variability of current preclinical PET/CT acquisition and reconstruction protocols routinely used across multiple centers and scanners; and (2) proposes acquisition and reconstruction PET/CT protocols for standardization of multicenter data, optimized for routine scanning in the preclinical PET/CT laboratory. Methods: Five different commercial preclinical PET/CT scanners in Europe and the United States were enrolled. Seven different PET/CT phantoms were used for evaluating biases on default/general scanner protocols, followed by developing standardized protocols. PET, CT, and absorbed dose biases were assessed. Results: Site default CT protocols were the following: greatest extracted Hounsfield units (HU) were 133 HU for water and -967 HU for air; significant differences in all tissue equivalent material (TEM) groups were measured. The average CT absorbed doses for mouse and rat were 72 mGy and 40 mGy, respectively. Standardized CT protocol were the following: greatest extracted HU were -77 HU for water and -990 HU for air; TEM precision improved with a reduction in variability for each tissue group. The average CT absorbed dose for mouse and rat decreased to 37 mGy and 24 mGy, respectively. Site default PET protocols were the following: uniformity was substandard in one scanner, recovery coefficients (RCs) were either over- or underestimated (maximum of 43%), standard uptake values (SUVs) were biased by a maximum of 44%. Standardized PET protocols were the following: scanner with substandard uniformity improved by 36%, RC variability decreased by 13% points, and SUV accuracy improved to 10%. Conclusion: Data revealed important quantitative biases in preclinical PET/CT and absorbed doses with default protocols. Standardized protocols showed improvements in measured PET/CT accuracy and precision with reduced CT absorbed dose across sites. Adhering to standardized protocols generates reproducible and consistent preclinical imaging datasets, thus augmenting translation of research findings to the clinic.

Magnetically targeted viral envelopes: a PET investigation of initial biodistribution.

Gene and drug therapy for organ-specific diseases in part depends on the efficient delivery to a particular region of the body. We examined the biodistribution of a viral envelope commonly used as a nanoscale gene delivery vehicle using positron emission tomography (PET) and investigated the magnetic alteration of its biodistribution. Iron oxide nanoparticles and (18)F-fluoride were encapsulated by hemagglutinating virus of Japan envelopes (HVJ-Es). HVJ-Es were then injected intravenously in the rat and imaged dynamically using high-resolution PET. Control subjects received injections of encapsulated materials alone. For magnetic targeting, permanent magnets were fixed on the head during the scan. Based on the quantitative analysis of PET images, HVJ-Es accumulated in the liver and spleen and activity remained higher than control subjects for 2 h. Histological sections of the liver confirmed imaging findings. Pixel-wise activity patterns on coregistered PET images of the head showed a significantly different pattern for the subjects receiving magnetic targeting as compared to all control groups. Imaging demonstrated the initial biodistribution of a viral envelope within the rodent by providing quantitative behavior over time and in specific anatomical regions. Magnetic force altered the biodistribution of the viral envelope to a target structure, and could enable region-specific delivery of therapeutic vehicles noninvasively.

Evaluation of 18F-annexin V as a PET imaging agent in an animal model of apoptosis.

UNLABELLED: Annexin V is a 36-kDa protein that binds with high affinity to phosphatidylserine lipids in the cell membrane. Because one of the earliest measurable events in apoptosis is the eversion of phosphatidylserine from the inner membrane leaflet to the outer cell surface, annexin V has proven useful for detecting the earliest stages of apoptosis. METHODS: Annexin V was radiolabeled with 18F using N-succinimidyl-4-18F-fluorobenzoic acid chemistry, to a specific activity of 555-925 kBq/mug of protein. 18F-Annexin V (14.8-51.8 MBq) was administered intravenously to rats after pretreatment with cycloheximide (5 mg/kg) to induce liver apoptosis, and the injected rats were imaged by PET over 2 h. After imaging, rats were dissected and individual organs were weighed and counted. RESULTS: Pretreatment of rats with cycloheximide resulted in a 3- to 9-fold increase in uptake of 18F-annexin V in the liver of treated animals at 2 h, compared with controls. By morphologic analysis, treated livers showed a 3- to 6-fold higher level of apoptosis than controls, with higher levels also seen with longer exposure to cycloheximide. Terminal deoxynucleotide end-labeling (TUNEL) assays performed on liver slices showed that cycloheximide induced a 5- to 8-fold increase in the number of TUNEL-positive nuclei. These TUNEL results correlated with the uptake of 18F-annexin V in dissected liver tissue, with an r2 value of 0.89. Biodistribution analysis of normal rats showed highest uptake of 18F-annexin V in the kidneys and urinary bladder, indicating rapid renal clearance of 18F-annexin V metabolites. CONCLUSION: The PET data, the organ-specific uptake data from dissection, and the morphologic and TUNEL measures of apoptosis together indicate that 18F-annexin V binds specifically to apoptotic tissues in this model of chemically induced apoptosis in rat liver. The short physical half-life of 18F-annexin V and the rapid clearance of its metabolites to the urinary system suggest that 18F-annexin V will be useful in early assessment of the clinical response to cancer therapy in individual patients.

FDG-PET/CT-guided intensity modulated head and neck radiotherapy: a pilot investigation.

BACKGROUND: 2-deoxy-2[(18)F]fluoro-D-glucose-positron emission tomography (FDG-PET) imaging can be registered with CT images and can potentially improve neck staging sensitivity and specificity in patients with head and neck squamous cell cancer. The intent of this study was to examine the use of registered FDG-PET/CT imaging to guide head and neck intensity modulated radiotherapy (IMRT) planning. METHODS: Twenty patients with squamous cell carcinoma of the oral cavity, oropharynx, larynx, or hypopharynx underwent FDG-PET and contrast-enhanced CT imaging of the head and neck before neck dissection surgery. Combined FDG-PET/CT images were created by use of a nonrigid image registration algorithm. All IMRT plans were theoretical and were not used for treatment. We prescribed 66 Gy in 30 fractions to FDG-avid CT abnormalities and nodal zones directly involved with disease, without prophylactic coverage of uninvolved neck levels. Matched CT-guided IMRT plans designed according to the specifications of Radiation Therapy Oncology Group (RTOG) H-0022 were available for comparison. We investigated the feasibility of FDG-PET/CT-directed IMRT dose escalation in five patients with FDG-avid disease located away from critical normal structures. After 66 Gy, FDG-avid disease with 0.5-cm margins was boosted in 220 cGy increments until dose-limiting criteria were reached. RESULTS: Elimination of prophylactic coverage to FDG-PET/CT-negative neck levels markedly reduced mean dose (Dmean) to the contralateral parotid gland (p < .001) and Dmean to the laryngeal cartilage (p = .001). No FDG-PET/CT-directed plan missed pathologically verified nodal disease. During the dose escalation exercise, we successfully increased the dose to 95% of the planning target volume (PTV95%) to a mean of 7490 cGy (range, 7153-8098 cGy). CONCLUSIONS: We demonstrate early proof of the principle that FDG-PET/CT-guided IMRT planning can selectively target and intensify treatment of head and neck disease while reducing critical normal tissue doses. Routine clinical use of such planning should not be engaged until the accuracy of FDG-PET/CT is fully validated. Future directions, including refinement of treatment to gross disease and radiologically uninvolved neck nodal levels, are discussed.

18F-FDG PET of gliomas at delayed intervals: improved distinction between tumor and normal gray matter.

UNLABELLED: We hypothesized that delineation of gliomas from gray matter with 18F-FDG PET could be improved by extending the interval between 18F-FDG administration and PET data acquisition. The purposes of this study were, first, to analyze standard and delayed 18F-FDG PET images visually and quantitatively to determine whether definition of tumor improved at later imaging times and, second, to investigate the dynamics of model-derived kinetic rate constants, particularly k4. METHODS: Nineteen adult patients with supratentorial gliomas were imaged from 0 to 90 min and once or twice later at 180-480 min after injection. In 15 patients, arterial sampling provided the early input function. Venous sampling provided the remaining curve to the end of the imaging sequence. Standardized uptake value (SUV) was calculated as tissue concentration of tracer per injected tracer dose per body weight. Ratios of tumor SUV relative to the SUV of gray matter, brain (including gray and white matter), or white matter were calculated at each imaging time point. Dynamic image data from tumor, gray matter, brain, or white matter were analyzed using a 2-compartment, 4-parameter model applied for the entire duration of imaging, in which delay, K1, distribution volume, k3, and k4 were optimized using a nonlinear optimization method. Parameter estimation for each region included both an early subset of data from a conventional dynamic imaging period (0-60 min) and the full, extended dataset for each region. RESULTS: In 12 of the 19 patients, visual analysis showed that the delayed images better distinguished the high uptake in tumors relative to uptake in gray matter. SUV comparisons also showed greater uptake in the tumors than in gray matter, brain, or white matter at the delayed times. The estimated k4 values for tumors were not significantly different from those for gray matter in early imaging analysis but were lower (P < 0.01) using the extended-time data. CONCLUSION: The kinetic parameter results confirm the visual and SUV interpretation that tumor enhancement is greater than enhancement of surrounding brain regions at later imaging times, consistent with a greater effect of FDG-6-phosphate degradation on normal brain relative to glioma.

Production of [F-18]fluoroannexin for imaging apoptosis with PET.

Recombinant human-annexin-V was conjugated with 4-[F-18]fluorobenzoic acid (FBA) via its reaction with the N-hydroxysuccinimidyl ester (FBA-OSu) at pH 8.5. A series of reactions using varying amounts of annexin-V, unlabeled FBA-OSu, and time produced products with different conjugation levels. Products were characterized by mass spectrometry and a cell-binding assay to assess the effect of conjugation. In each case, the conjugated protein was a mixture of proteins with a range of conjugation. Annexin-V could be conjugated with an average of two FBA mole equivalents without decreasing its affinity for red blood cells (K(d) 6-10 nM) with exposed phosphatidylserine. An average conjugation of 7.7 (range 3-13) diminished the binding 3-fold. Large-scale production and purification of [F-18]FBA-OSu from [F-18]fluoride was accomplished within 90 min and in 77% radiochemical yield (decay-corrected to the end of cyclotron bombardment). The conjugation reaction of annexin with [F-18]FBA-OSu was studied with respect to activity level, protein mass, and concentration. Under the most favorable conditions, >25 mCi [F-18]fluoroannexin (FAN) was isolated in 64% yield (decay-corrected for a 22 min conjugation process) from labeling 1.1 mg of annexin-V. A pilot PET imaging study of [F-18]fluoroannexin in normal rats showed high uptake in the renal excretory system and demonstrated sufficient clearance from most other internal organs within 1 h. [F-18]Fluoroannexin should prove useful in imaging targeted apoptosis.

SUV varies with time after injection in (18)F-FDG PET of breast cancer: characterization and method to adjust for time differences.

UNLABELLED: The purpose of this study was to measure how (18)F-FDG PET standardized uptake values (SUVs) change over time in breast cancer and to examine the feasibility of a method to adjust for modest variations in the time of uptake measurement experienced in clinical practice. METHODS: (18)F-FDG PET was performed as 60-min dynamic imaging with an additional image acquired at approximately 75 min after injection. For 20 newly diagnosed, untreated, locally advanced breast cancer patients, both the maximum SUV and the average SUV within the lesion were calculated with and without correction for blood glucose concentration. A linear regression analysis of the portion of the time-activity curves starting at 27 min after injection was used to estimate the rate of SUV change per minute during the interval from 27 to 75 min. The rate of SUV change with time was compared with the instantaneous SUV obtained at different times from 27 to 75 min. RESULTS: In untreated breast cancer, (18)F-FDG SUV values changed approximately linearly after 27 min at a rate ranging from -0.02 to 0.15 per minute. In addition, the rate of SUV change was linearly correlated with the instantaneous SUV measured at different times after injection (r(2) ranged from 0.82 to 0.94; P < 0.001). Using this information, an empirical linear model of SUV variation with time from injection to uptake measurement was formulated. The comparison method was then applied prospectively to a second set of 20 locally advanced breast cancer lesions not included in the initial analysis. The average percent error using the method to adjust for time differences was 8% and 5% for maximum SUVs and average SUVs ranging from 2 to 12. CONCLUSION: In untreated breast cancer, the SUV at any time point approximately predicts the rate of change of SUV over time. A comparison method based on this finding appears feasible and may improve the usefulness of the SUV by providing a means of comparing SUV acquired at different times after injection.

2-[(18)F]Fluoro-2-deoxyglucose and glucose uptake in malignant gliomas before and after radiotherapy: correlation with outcome.

PURPOSE: To examine whether quantitative 1-[(11)C]glucose- or 2-[(18)F]fluoro-2-deoxyglucose (FDG)-positron emission tomography performed before and/or after radiotherapy (RT) of malignant gliomas correlates with treatment outcome. Changes in metabolism between the start and finish of RT, and immediate post-RT studies have received little attention. EXPERIMENTAL DESIGN: Adults with malignant gliomas were imaged within 2 weeks before and/or 2 weeks after RT. Four patients were imaged only before RT, 12 only after RT, and 14 both before and after RT. Each 1-[(11)C]glucose and FDG study included arterial plasma sampling. Kinetic parameters, glucose metabolic rate (MRGlc), and FDG metabolic rate (MRFDG) were estimated by an optimization program based on a three compartment, four rate constant model. Changes in MRGlc or MRFDG from pre-RT to post-RT were calculated for the 14 patients studied at both times. Overall survival was examined, and survival was computed relative to historical controls in matched prognostic classes. RESULTS: Low pre-RT MRGlc (P < 0.02) or MRFDG (P < 0.03), or an increase from pre- to post-RT in MRGlc (P < 0.004) or MRFDG (P < 0.006) are correlating with longer survival (4 patients still alive). Strikingly, the post-RT studies (n = 26) showed no correlation between MRGlc or MRFDG and survival (P = 0.73 and P = 0.46 respectively). CONCLUSIONS: Low MRGlc or MRFDG before RT probably indicates less aggressive disease. An increase in MRGlc or MRFDG from pre- to post-RT in the tumors of patients with longer survival could be because of one or more of the following or other reasons: (a) apoptosis of tumor cells in response to RT requires energy; (b) decreased tumor cell density by the RT leaving normal cells with higher metabolism; or (c) inflammatory cells infiltrate and take up glucose or FDG where tumor cells are dying. Quantitative 1-[(11)C]glucose or FDG uptake in the early weeks post-RT correlates poorly with survival.