Is the physical decay correction of the (18)F-FDG input function in dynamic PET imaging justified?

UNLABELLED: In this theoretic note, the rationale for the physical decay correction of the (18)F-FDG input function in dynamic PET is investigated, using the Patlak equation as an example. METHODS: The Patlak equation conventionally obtained when correcting the (18)F-FDG input function and correcting the tissue activity measurement for (18)F physical decay can also be derived from a 2-compartment analysis that does not conceptually involve any physical decay correction of the (18)F-FDG input function but accounts only for the physical decay of the trapped tracer. RESULTS: We demonstrate that exactly the same equation can be derived from the 2 conceptual approaches, and hence each approach yields the correct uptake rate of the tracer. CONCLUSION: No advantage in (18)F-FDG dynamic PET can be expected from using the concept of uncorrected data rather than that of decay-corrected data. Nevertheless, conceptually, we show that correcting the (18)F-FDG input function for radioactive decay cannot be justified and that this correction is not compatible with the calculation of patient radiation dose.

Assessment of dual-time-point 18F-FDG-PET imaging for pulmonary lesions.

OBJECTIVE: The objective of this study was to assess suitability of dual-time-point 18F-FDG [(18F)-fluoro-2-deoxyglucose]-PET imaging for differentiating between malignant and benign pulmonary lesions, whose size and maximal standardized uptake values (SUVs) are greater than 10 mm and 2.5, respectively. METHODS: A total of 38 patients, 27 with malignant lesions (n = 30), and 11 with benign lesions (n = 22), were investigated by performing two static acquisitions started at mean times t = 79 and t = 158 min after the tracer injection. A model analysis involving tissue 18F-FDG uptake and release has been developed and applied. RESULTS: Malignant lesions showed a SUV increase between the two acquisitions for 27 of 30 lesions, and a SUV decrease or constancy for the other three. Benign lesions showed a SUV increase in 19 of 22 lesions, and a SUV decrease in three (both increase and decrease were observed for multiple benign lesions in two patients). CONCLUSION: It is recommended that dual-time-point 18F-FDG-PET imaging is not indicated to differentiate between malignant and benign pulmonary lesions, whose size and maximal SUV are greater than 10 mm and 2.5, respectively. Furthermore, a model analysis suggests that the variation in SUV observed between early and delayed scans may be explained by different values of the 18F-FDG release/uptake ratio.

The effect of renal failure on 18F-FDG uptake: a theoretic assessment.

UNLABELLED: This work addresses the issue of using (18)F-FDG PET in patients with renal failure. METHODS: A model analysis has been developed to compare tissue (18)F-FDG uptake in a patient who has normal renal function with uptake in a theoretic limiting case that assumes tracer plasma decay is tracer physical decay and is trapped irreversibly. RESULTS: This comparison has allowed us to propose, in the limiting case, that the usually injected activity be lowered by a factor of 3. We also proposed that the PET static acquisition be obtained at about 160 min after tracer injection. These 2 proposals were aimed at obtaining a similar patient radiation dose and similar tissue (18)F-FDG uptake. CONCLUSION: In patients with arbitrary renal failure (i.e., between the 2 extremes of normal function and the theoretic limiting case), we propose that the injected activity be lowered (without exceeding a factor of 3) and that the acquisition be started between 45 and 160 min after tracer injection, depending on the severity of renal failure. Furthermore, the model also shows that the more severe the renal failure is, the more overestimated is the standardized uptake value, unless the renal failure indirectly impairs tissue sensitivity to insulin and hence glucose metabolism.

Systemic tuberculosis presenting as an epiglottic mass detected on F-18 FDG PET/CT.

A 32-year-old man presented with asthenia, weight loss, cough, and dysphagia following a recent stay in Morocco. Endoscopy showed a bulky mass of the epiglottis suspected of being a malignant tumor. The patient underwent jointly an F-18 FDG PET/CT and a biopsy of the tumor. Against all expectations, biopsy revealed granulomatous inflammation with epitheloid giant cells and caeseating necrosis. These findings associated with the presence of acid-fast bacilli in the sputum smears were highly suggestive of laryngeal tuberculosis, which was confirmed later after cultivation of mycobacteria. F-18 FDG PET showed diffuse pharyngolaryngeal and lung uptake with bilateral cervical and abdominal nodes, but also one thoracic vertebral uptake. Lung CT could have revealed carcinomatous dissemination, but cavitary lesions in some pulmonary segments were more evocative of tuberculosis. Moreover, cerebral MRI showed brain tuberculomas not visualized on F-18 FDG PET/CT. The patient was treated with a 5-antituberculosis drug regimen, which improved clinical symptoms with epiglottis mass regression, and lung CT image reduction, clinching the systemic tuberculosis diagnosis. A control F-18 FDG PET/CT performed 5 months later showed disappearance of the pharyngolaryngeal and node uptake, with an improvement of lung uptake without normalization, arguing for persistent disease. Unexpected pathologic findings may be present in more than 3% of neck dissections. Although this is usually indolent, with the underlying SCC remaining the main prognostic determinate, it may significantly complicate postoperative management.

Feasibility of an SUV normalization to 1 hour after the 18F-FDG injection.

The aim of this work was to reduce the SUV variability related to the time delay between 18F-FDG injection and the static PET acquisition, by means of a normalization to a 1-h time delay. Two static PET acquisitions separated by approximately 1 h were performed on each of 14 cancer patients, with SUVs on 22 hypermetabolic lesions calculated for both scans. The pairs of SUVs were normalized to each other using the parameterized input function with one free parameter (alpha3). This optimized parameter was found by computing the value which yielded equal normalized SUV pairs, on average, over the whole series. Without normalization, SUVs measured at later scans were found to be significantly greater than the earlier ones: mean (+/- SD) ratio of 0.84 (+/-0.08; range 0.69-0.97). After normalization, with an alpha3 value of 0.0257 min(-1), as expected, the mean (+/- SD) ratio was 1.00 (+/-0.07; range 0.88-1.10).

Normalization in quantitative [18F]FDG PET imaging: the 'body surface area' may be a volume.

Non-invasive methods for quantifying [(18)F]FDG uptake in tumours often require normalization to either body weight or body surface area (BSA), as a surrogate for [(18)F]FDG distribution volume (DV). Whereas three dimensions are involved in DV and weight (assuming that weight is proportional to volume), only two dimensions are obviously involved in BSA. However, a fractal geometry interpretation, related to an allometric scaling, suggests that the so-called 'body surface area' may stand for DV.

Quantitative MRI comparison of pulmonary hemodynamics in mustard/senning-repaired patients suffering from transposition of the great arteries and healthy volunteers at rest.

In Mustard/Senning-repaired (MSR) patients, the right and left ventricles (RV, LV) act as the systemic and pulmonary ventricle, respectively. The purpose of the study was to compare non-invasively, at rest, pulmonary ventricle systolic function and hemodynamics in MSR patients with those of healthy volunteers. Velocity-encoded MR imaging was performed at the level of the main pulmonary artery (MPA) in ten male patients late after a Mustard/Senning correction performed early in infancy and in ten male volunteers. Both blood flow and MPA cross-sectional area variations were recorded over a complete cardiac cycle. MPA distensibility, body surface area (BSA)-normalized pulmonary ventricle systolic power and work were significantly lower in the MSR patients compared to volunteers. In particular, BSA-normalized LV systolic power and work in MSR patients were equal to 82 and 77% on average of those of the RV in volunteers (0.32 vs. 0.39 W/m2 and 0.10 vs. 0.13 J/m2), respectively. We conclude that in MSR patients at rest two unrelated findings were observed: (1) a reduced MPA distensibility and (2) a significantly lower systolic mechanical performance of the pulmonary LV compared to that of the RV in healthy volunteers. The latter quantification indirectly confirms the lowest systemic RV systolic mechanical performance previously published.

131I in blood samples: a danger for professionals? A problem for immunoassays?

OBJECTIVE: Our objective was to investigate the safety of radioactive blood samples from patients receiving 131I and whether the radioactivity affects the validity of assays. METHODS: First, the activity of samples from patients given 131I was measured by 3 methods and compared with the upper threshold. Then, pilot sera were spiked with 131I, and possible interference was investigated using 2 immunoradiometric assays. RESULTS: The activity of 13 of the 15 samples was below the European limit; the other 2 samples were from patients with reduced renal clearance rates. No differences in thyroglobulin level or thyroid-stimulating hormone level were found between sera that were spiked with 131I and sera that were not. CONCLUSION: These blood samples are safe because they contain negligible activity, and the use of radioimmunoassays or immunoradiometric assays on them produces reliable results.

A method to quantify at late imaging a release rate of 18F-FDG in tissues.

This theoretical work shows that the rate constant for the (18)F-FDG release in tissues can be assessed without needing any arterial blood sampling. The method requires that the clearance of (18)F-FDG from plasma has occurred, whereas (18)F-FDG is still present in the tissue. This condition can be met dating from 3 h after (18)F-FDG injection, when hydration and/or phlorizin injection are applied after the routine static acquisition. The release rate constant can be obtained from a graphical analysis performed at the later decreasing phase of the tissue tracer activity. A two-compartment and a three-compartment model are developed, both in accordance with one another. To cite this article: E. Laffon et al., C. R. Biologies 328 (2005).

Feasibility of aortic pulse pressure and pressure wave velocity MRI measurement in young adults.

PURPOSE: To investigate the feasibility of assessing, noninvasively, aortic pulse pressure (APP) and pulse wave velocity (PWV) in the ascending aorta of young adults by means of velocity-encoded magnetic resonance (MR) imaging. MATERIALS AND METHODS: In a series of 11 healthy volunteers, velocity-encoded MR imaging provided pairs of magnitude and phase-contrast images. Blood flow velocity and aortic cross-sectional area (CSA) were determined with a 30-msec temporal resolution. A model analysis revealed that variation in aortic CSA and in maximal blood flow velocity throughout systole could be used to estimate APP and, hence, to derive PWV by means of two different methods. RESULTS: Mean +/- SD values of the APP for the series were 54.2 +/- 16.4 mmHg (range 32.2-84.1 mmHg). The ascending aortic PWV mean +/- SD values were 5.03 +/- 1.10 m/second and 5.37 +/- 1.23 m/second according to the two methods, and both estimates were not significantly different (95% confidence level). CONCLUSION: These results are in agreement with previously published data, suggesting that APP and PWV can be determined, noninvasively, in young adults using MRI.

A method to quantify the uptake rate of 2-[18F]fluoro-2-deoxy-D-glucose in tissues.

A two-compartment model is used to derive the tissue time-activity curve of 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG). At the later phase of the 18F-FDG activity in the tissue, the model leads to a method which allows the assessment of the 18F-FDG uptake rate constant. The proposed method is both compared with the standard uptake value (SUV) method and with Patlak's graphical analysis.

Quantitative MRI comparison of systemic hemodynamics in Mustard/Senning repaired patients and healthy volunteers at rest.

The purpose of the present study was to non-invasively compare ascending aortic blood flow and cross-sectional area (CSA) variations vs time in Mustard or Senning repaired (MSR) patients against those of healthy volunteers at rest. Velocity-encoded MR imaging was performed in 10 male patients (age range 18-24 years, median age 20.5 years) late after a Mustard or Senning correction, and in 10 male healthy volunteers (age range 21-25 years, median age 22.5 years), at the upper part of the ascending aorta. Both aortic cross-sectional area (CSA) and blood-flow variations were recorded over a complete cardiac cycle, with a 30-ms time of resolution. The body-surface area (BSA), the mean CSA over the systolic phase, and the BSA-normalized systemic ventricle power and work were significantly lower in the patient series compared with those of the volunteer series. The BSA-normalized right ventricle (RV) power and work of MSR patients were equal to 87 and 83% on average of those of the left ventricle (LV) of healthy volunteers. We conclude that, at rest, the mechanical performance of the systemic RV in MSR patients is significantly lower than that of the LV in healthy volunteers. Furthermore, the significantly lower aortic CSA found in MSR patients than in healthy volunteers may reveal an increase in the vasomotor tone.

A computed method for noninvasive MRI assessment of pulmonary arterial hypertension.

The present method enables the noninvasive assessment of mean pulmonary arterial pressure from magnetic resonance phase mapping by computing both physical and biophysical parameters. The physical parameters include the mean blood flow velocity over the cross-sectional area of the main pulmonary artery (MPA) at the systolic peak and the maximal systolic MPA cross-sectional area value, whereas the biophysical parameters are related to each patient, such as height, weight, and heart rate. These parameters have been measured in a series of 31 patients undergoing right-side heart catheterization, and the computed mean pulmonary arterial pressure value (Ppa(Comp)) has been compared with the mean pressure value obtained from catheterization (Ppa(Cat)) in each patient. A significant correlation was found that did not differ from the identity line Ppa(Comp) = Ppa(Cat) (r = 0.92). The mean and maximal absolute differences between Ppa(Comp) and Ppa(Cat) were 5.4 and 11.9 mmHg, respectively. The method was also applied to compute the MPA systolic and diastolic pressures in the same patient series. We conclude that this computed method, which combines physical (whoever the patient) and biophysical parameters (related to each patient), improves the accuracy of MRI to noninvasively estimate pulmonary arterial pressures.

MRI quantification of the role of the reflected pressure wave on coronary and ascending aortic blood flow.

The respective roles of the windkessel effect, which is related to the vessel compliance, and of the returning pressure wave, which is reflected off peripheral resistances, in generating the total diastolic blood flow in the ascending aorta (VT: coronary + flow through the upper part of the ascending aorta) were assessed under physiological conditions. In ten healthy young men (21-30 years), magnetic resonance phase mapping was used to assess non-invasively both blood flow and vessel cross-sectional area (CSA) at the upper part of the ascending aorta. Measurements of blood flow velocities and CSAs were carried out over complete cardiac cycles, with a 30 ms resolution time. The total diastolic blood flow volume (VT) was significantly greater than the diastolic blood flow volume related to the windkessel effect (Vw). Consequently, since the windkessel effect could not account for the whole diastolic blood flow volume, the difference was attributed to the diastolic blood flow volume related to the reflected pressure wave (Vpw). The mean and SD of Vw and Vpw were estimated to be 4.34 +/- 1.94 ml/cycle and 6.80 +/- 2.40 ml/cycle (n = 10), respectively, for a measurement slice placed at 5.12 cm (on average) from the aortic valve and a mean coronary diastolic blood flow volume assumed to be equal to 4 ml/cardiac cycle in all subjects. The present non-invasive MR phase mapping study indicates that the windkessel effect and the reflected pressure wave might play a similar role in generating the total diastolic blood flow in the ascending aorta, under physiological conditions. Furthermore, pathophysiologic implications are discussed.