[Recovery coefficients for the quantification of the arterial input functions from dynamic PET measurements: experimental and theoretical determination]

AIM

For kinetic modelling of dynamic PET data, the arterial input function can be determined directly from the PET scans if a large artery is visualized on the images. It was the purpose of this study to experimentally and theoretically determine recovery coefficients for cylinders as a function of the diameter and level of background activity.

METHODS

The measurements were performed using a phantom with seven cylinder inserts (Ø = 5-46 mm). The cylinders were filled with an aqueous 68Ga solution while the main chamber was filled with a 18F solution in order to obtain a varying concentration ratio between the cylinders and the background due to the different isotope half lives. After iterative image reconstruction, the activity concentrations were measured in the center of the cylinders and the recovery coefficients were calculated as a function of the diameter and the background activity. Based on the imaging properties of the PET system, we also developed a model for the quantitative assessment of recovery coefficients.

RESULTS

The functional dependence of the measured recovery data from the cylinder diameter and the concentration ratio is well described by our model. For dynamic PET measurements, the recovery correction must take into account the decreasing concentration ratio between the blood vessel and the surrounding tissue. Under the realized measurement and data analysis conditions, a recovery correction is required for vessels with a diameter of up to 25 mm.

CONCLUSIONS

Based on the experimentally verified model, the activity concentration in large arteries can be calculated from the measured activity concentration in the blood vessel and the background activity. The presented approach offers the possibility to determine the arterial input function for pharmacokinetic PET studies non-invasively from large arteries (especially the aorta).

Comparison of pharmacokinetic MRI and [18F] fluorodeoxyglucose PET in the diagnosis of breast cancer: initial experience

It was the aim of this methodology-oriented clinical pilot study to compare the potential of dynamic MRI and 2-[18F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) for the detection and characterization of breast cancer. Fourteen women with suspicious breast lesions were examined. The MRI data were acquired with a turbo fast low-angle shot sequence and analyzed using a pharmacokinetic model. Emission data were detected in the sensitive 3D modus, iteratively reconstructed, and superimposed onto corresponding transmission images. In the 14 patients, 13 breast masses with a suspicious contrast enhancement and FDG uptake were detected. For these lesions, no statistically significant correlation between evaluated MR and PET parameters was found. Of the 9 histologically confirmed carcinomas, 8 were correctly characterized with MRI and PET. Two inflammatory lesions were concordantly classified as cancer. Moreover, dynamic MRI yielded another false-positive finding. In 6 patients, PET detected occult lymph node and/or distant metastases. Although both functional imaging techniques provide independent tissue information, the results concerning the diagnosis of primary breast lesions were almost identical. An advantage of PET, however, is its ability to localize lymph node involvement and distant metastases as an integral part of the examination.

Chances and shortcomins of adenovirus-mediated ATP7B gene transfer in Wilson disease: proof of principle demonstrated in a pilot study with LEC rats

BACKGROUND & AIMS

Wilson disease (WD) is an autosomal recessively inherited copper storage disease due to mutations in the ATP7B gene. It results in impaired biliary copper excretion followed by liver injury leading to cirrhosis. In parallel, copper accumulates in other tissues e. g. basal ganglia of the brain inducing motoric disorders. Phenotypical cure of Wilson disease by liver transplantation raised the question whether gene therapy may represent a successful alternative treatment procedure. To examine the principle feasibility of this approach we investigated the effects of gene transfer using an adenoviral vector construct expressing the human ATP7B cDNA in an established rodent model for WD, the Long-Evans Cinnemon rat (LEC).

METHODS

Transduction efficiency was assessed by RT-PCR, Western blot and immunofluorescence analysis. The therapeutic effect was estimated by analyzing holoceruloplasmin and its ferroxidase activity in serum, and the copper content of excrements. Changes in copper homeostasis were determined by positron emission tomography (PET).

RESULTS

Successful, but temporary gene transfer was clearly detectable on RNA and protein levels. In parallel the temporary therapeutic effect was documented by restoration of serum holoceruloplasmin and of its ferroxidase activity. Additionally the Ad-ATP7B treated LEC rat revealed a higher (64)Cu content in stool. PET was able to visualize differences in (64)Cu distribution between wild type and LEC rats, indicating its principle usefulness as analytical tool.

CONCLUSION

The data demonstrate proof of principle of successful gene therapy in an experimental model of WD. As a consequence of successful but only transient therapeutic effect of adenoviral gene transfer we can now focus more efficient and permanent gene transfer strategies.

Quantification of [(18)F]FDG uptake in the normal liver using dynamic PET: impact and modeling of the dual hepatic blood supply

For quantification of hepatic [(18)F]FDG uptake, the dual blood supply to the liver must be considered. In contrast to the arterial input, however, the portal venous blood supply to the liver cannot be monitored directly by PET because of the inaccessibility of the portal vein on PET scans. In this study, we investigated whether the dual hepatic input can be predicted from the measurable arterial input. Moreover, we assessed the effect of different input models on the rate constants of the standard 3-compartment model describing regional uptake of FDG.

METHODS

Dynamic FDG PET scanning was performed on 5 foxhounds. Activity concentrations in blood from the aorta and the portal vein were measured simultaneously using external circuits. After image reconstruction, time--activity courses were determined from the aorta and the liver. The venous input was approximated by convolving the arterial input with a notional system function describing the dispersion of the arterial input on its way through the gastrointestinal tract. On the basis of these data, 5 different hepatic input models, which pertain to a single-input as well as a dual-input scenario, were statistically compared with regard to the adequacy of the model fits to liver data and to differences in the estimated rate constants.

RESULTS

Portal venous input to the liver could be approximated by convolving the arterial input function with a system function. From this function, a mean transit time of 25 s was computed for FDG to pass through the gastrointestinal tract. According to the statistical analysis, dual-input models were superior to their single-input counterparts. However, differences in the rate constants estimated for the 5 input models were in the same order as interindividual variations within the different model groups. For the dephosphorylation rate constant, a consistent value of 0.05 +/- 0.01 min(-1) was found.

CONCLUSION

Dual-input models proved to be superior to single-input models with respect to the adequacy of FDG model fits to normal liver data. However, the hepatic blood supply may be approximated by the arterial input function as well, especially for the evaluation of liver lesions mainly fed by the hepatic artery.

PET imaging of somatostatin receptors using [68GA]DOTA-D-Phe1-Tyr3-octreotide: first results in patients with meningiomas

Imaging of somatostatin receptors (SSTRs) using [111In]diethylenetriaminepentaacetic-acid-octreotide (DTPAOC) has proven to be helpful in the differentiation of meningiomas, neurinomas or neurofibromas, and metastases as well as in the follow-up of meningiomas. A drawback of the SPECT method is its limited sensitivity in detecting small meningiomas. Because of PET's increased spatial resolution and its ability to absolutely quantify biodistribution, a PET tracer for SSTR imaging would be desirable.

METHODS

1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic-acid-D-Phe1-Tyr3-octreotide (DOTATOC) was labeled using the positron-emitting generator nuclide 68Ga. We acquired dynamic PET images over 120 min after intravenous injection of 175 MBq [68Ga]DOTATOC in 3 patients suffering from 8 meningiomas (WHO I degrees; 7- to 25-mm diameter). Patients' heads had been fixed using individually shaped fiber masks equipped with an external stereotactic localizer system to match PET, CT, and MRI datasets.

RESULTS

[68Ga]DOTATOC was rapidly cleared from the blood (half-life alpha, 3.5 min; half-life beta, 63 min). Standardized uptake values (SUVs) of meningiomas increased immediately after injection and reached a plateau 60-120 min after injection (mean SUV, 10.6). No tracer could be found in the surrounding healthy brain tissue. All meningiomas (even the 3 smallest [7- to 8-mm diameter]) showed high tracer uptake and could be visualized clearly. Tracer boundaries showed a good correspondence with the matched CT and MRI images. PET provided valuable additional information regarding the extent of meningiomas located beneath osseous structures, especially at the base of the skull.

CONCLUSION

According to our initial experiences, [68Ga]DOTATOC seems to be a very promising new PET tracer for imaging SSTRs even in small meningiomas, offering excellent imaging properties and a very high tumor-to-background ratio.

Immunoscintigraphy with positron emission tomography: gallium-68 chelate imaging of breast cancer pretargeted with bispecific anti-MUC1/anti-Ga chelate antibodies

Pretargeting techniques that are based on the sequential administrations of bispecific antitumor/antimetal chelate antibodies (BS-MAbs), a blocker to saturate the anti-chelate binding sites of the BS-MAb still present in the circulation, and the radiolabeled chelate are suitable to increase tumor-to-normal tissue contrasts and enable positron emission tomography (PET) as an imaging method. As demonstrated in the nude mouse model, a combination of pretargeted immunoscintigraphy and PET markedly improved the detection of tumor xenografts. With the presented preliminary clinical trial, we attempted to assess the efficacy of pretargeting and PET for breast cancer localization in patients. The BS-MAb used for pretargeting was synthesized from the F(ab')(2) fragments of the anti-MUC1 MAb 12H12, which reacts with the vast majority of breast tumors, and the F(ab') fragments of an anti-gallium (Ga) chelate MAb via a mixed functional chemical linker. For labeling of the Ga-chelate, we used the short-lived positron emitter Ga-68 (t(1/2), 68 min; beta(+), 88%). The dose and time schedule of pretargeting was deduced from previous animal experiments. Ten patients with biopsy-proven, primary breast carcinoma were infused with 10 mg of the BS-MAB: Eighteen h later, they received i.v. injections of 10.7 mg of a blocker and, 15 min later, 9.6 microg of the Ga chelate labeled with 230-300 MBq of (68)GA: PET imaging was started 60-90 min after injection of the (68)Ga chelate. Average tumor-to-blood and tumor:normal breast tissue ratios were 0.9 and 3.0 at 1 h postinjection. Tumor uptake amounted to approximately 0.003% iD/g corresponding to a standard uptake value of approximately 2. Blood clearance of the (68)Ga chelate showed a t(1/2) beta of approximately 100 min. Fourteen of 17 known lesions, averaging 25 +/- 16 mm in size, were clearly visualized as foci of increased activity with PET. No false-positive but three false-negative readings were obtained. An enhanced, bilateral activity uptake in the whole breast parenchyma, found in 4 of the 10 patients, compromised the recognition of these tumor sites. Although the shedding of the MUC1 antigen and the comparatively low tumor affinity of the BS-MAb, common to all anti-mucin MAbs, proved not to be optimal for increasing tumor:tissue ratios with a pretargeting technique, PET imaging offered better sensitivity for the detection of breast cancer at low tumor contrasts than conventional immunoscintigraphy. This could be demonstrated by the clear visualization of tumor sites 10 mm in size, which contrasted only by a factor of 2 from surrounding normal breast tissue.

[Hepatitis C virus contamination of endoscopes and biopsy forceps]

BACKGROUND

Procedures such as digestive endoscopy may explain some unclear contaminations by HCV.

AIMS

The aims of this study were to detect HCV genome on endoscopes and biopsy-forceps used in patients with known chronic HCV infection and to determine its presence in their gastric juice and saliva.

METHODS

A gastroscopy with antral biopsies was performed in 48 patients with non-treated replicative chronic hepatitis C. Samples were obtained after pushing 10 mL of sterile water through the biopsy-suction channel and after immersing the brush used to clean this channel. The biopsy-forceps were also immersed and their tips brushed in 10 mL of sterile water. This sampling technique was repeated three times: immediately after the endoscopic procedure (T0), after washing with a detergent (T1) and after immersion for 20 minutes in a 2% glutaraldehyde solution (T2). The HCV genome was detected by polymerase chain reaction (PCR, Amplicor - Roche Diagnostics Systems). For the last 15 patients, samples of gastric juice and saliva were obtained before antral biopsies and used to detect HCV genome.

RESULTS

HCV genome was detected in the biopsy-suction channel in 13 cases (27%) at T0 and in one case (2%) at T1. It was undetectable after completion of the disinfection procedure (T2). Three biopsy-forceps (6%) were PCR positive immediately after the endoscopy but none at T1 and T2. HCV genome was found in the gastric juice in three cases. In all of them, it was also found at T0 in the biopsy-suction channel but not on the biopsy-forceps. When saliva contained HCV genome (4 cases), it was present in the biopsy-suction channel in only one case. In this case, the gastric juice was also PCR positive.

CONCLUSIONS

HCV genome is detected in 27% of cases in the biopsy-suction channel after an endoscopic procedure performed on patients with chronic HCV infection. The biopsy-forceps are PCR positive in 6% of cases. The infected gastric juice may play a role in the contamination of the endoscopes. The complete disinfection procedure seems effective to eliminate HCV.

[Diagnostic evaluation of the breast using PET: optimization of data acquisition and postprocessing]

PURPOSE

Development and evaluation of an optimized protocol for PET examinations of the female breast with 2-F-18-fluoro-2-deoxyglucose (F-18-FDG).

METHODS

All PET measurements were performed with a whole-body PET system (ECAT EXACT HR+). In order to examine the women with the breasts freely pendant, a special extension for the patient table made of carbon layer composite was designed. After data acquisition in the 3D modus, emission data were sorted into 2D sinograms using the Fourier rebinning algorithm and reconstructed by means of an ultra-fast iterative 2D algorithm (HOSP). The reconstructed emission scans were superimposed onto the corresponding transmission images. The protocol presented was evaluated in examinations on 6 women with breast lesions after the administration of 150-220 MBq F-18-FDG. From two adjacent bed positions, emission and transmission data were acquired over periods of 20 min and 10 min, respectively. For comparison, dynamic magnetic resonance (MR) image series were acquired with a whole-body MR system (MAGNETOM SP 4000) using a double-breast coil.

RESULTS AND CONCLUSION

Using the designed extension of the patient table, it was possible to examine corpulent women despite the limited patient port of the PET system in the prone position with the breasts freely pendant. Alongside a reduction in motion artifacts, this positioning also offers the possibility of making a direct comparison between PET and MR images. Despite the fact that the amount of F-18-FDG applied to the patient was markedly reduced, the combination of 3D data acquisition and iterative image reconstruction resulted in excellent quality of the emission scans. By super-positioning of iteratively reconstructed emission and transmission scans, anatomical localization of breast lesions visualized on the emission scans could be improved. The postprocessing of the PET data described was completed in 60 min, this meaning that the presented concept can readily be employed in clinical practice.

[Reduction of radiation exposure in PET examinations by data acquisition in the 3D mode]

AIM

Modern volume PET systems offer the possibility to measure without the shadowing effect of interplane septa (2D mode) and thus to detect coincident events between detectors on distant rings (3D mode). It was the aim of the present paper to characterize the count rate behaviour of a latest-generation whole-body PET system in the 2D and 3D mode as well as to discuss the consequences for the radiation hygiene of PET examinations with 2-[F-18]-fluoro-2-deoxyglucose (18-F-FDG).

METHODS

All experiments were performed with the PET system ECAT EXACT HR+. For 2D data acquisition, a collimator of thin tungsten septa was positioned in the field-of-view. The count rate behaviour of the scanner was evaluated in the 2D and 3D mode over a wide range of F-18 activity concentrations following the NEMA protocol. Moreover, PET images of the EEC whole-body phantom with different inserts were acquired in the 2D and 3D mode over a period of 15 min each. For the 3D measurement, the activity concentrations of the F-18 solution were only half of those used for the 2D measurement.

RESULTS

For the circular NEMA phantom (phi = 19.4 cm, length = 19.0 cm), we observed an increase of the system sensitivity in the 3D mode by a factor of about 5 with respect to the 2D mode (27.7 vs. 5.7 cps/Bq/ml). The evaluation of the activity distributions of the EEC phantom reconstructed from the 3D data set revealed a superior image quality compared to the corresponding 2D images despite the fact that the activity concentrations were only half as high.

CONCLUSIONS

By using the 3D data acquisition mode, it is possible to markedly reduce the amount of activity to be applied to the patient and nevertheless to improve image quality. In our experience, it is sufficient to administer an activity of 150-200 MBq for whole-body examinations with F-18-FDG, which results in an effective equivalent dose of 3 or 4 mSv, respectively.

Reduktion der Strahlenexposition bei PET-Untersuchungen durch Datenakquisition im 3D-Modus

Ziel: Moderne Volumen-PET-Systeme bieten die Möglichkeit, auf die abschirmende Wirkung von Septen zwischen den einzelnen Detektorringen (2D-Modus) zu verzichten, so daß auch koinzidente Ereignisse zwischen Detektoren auf weiter entfernt liegenden Ringen erfaßt werden können (3D-Modus). Ziel der vorliegenden Arbeit war es, das Zählratenverhalten eines PET-Scanners der neuesten Generation im 2D- und im 3D-Modus anhand von Phantommessungen zu untersuchen und die sich daraus ergebenden Konsequenzen für die Strahlenhygiene bei PETUntersuchungen mit 2-[F-18]-Fluor-2-Desoxyglukose (F-18-FDG) zu diskutieren. Methoden: Alle Messungen wurden am Ganzkörper-PET-System ECAT EXACT HR+ durchgeführt. Für die 2D-Messungen wurde ein Kollimator aus dünnen Wolframlamellen in das Gesichtsfeld eingebracht. Das Zählratenverhalten des Scanners wurde dem NEMA-Protokoll folgend über einen weiten Bereich von F-18-Aktivitätskonzentrationen untersucht. Darüber hinaus wurden PET-Aufnahmen des EEC-Ganzkörperphantoms mit verschiedenen Einsätzen im 2D- und im 3D-Modus jeweils über 15 min akquiriert, wobei die F-18-Aktivitätskonzentrationen bei der 3D-Messung halb so hoch waren wie bei der 2D-Messung. Ergebnisse: Für das zylinderförmige NEMA-Phantom (Ø = 19,4 cm, Länge = 19,0 cm) ergab sich im 3D-Modus eine im Vergleich zur 2D-Akquisition etwa fünffach höhere Systemsensitivität (27,7 statt 5,7 cps/Bq/ml). Die Auswertung der rekonstruierten Aktivitätsverteilungen des EECPhantoms ergab, daß die Qualität der aus dem 3D-Projektionsdatensatz berechneten PET-Aufnahmen besser war als die der korrespondierenden 2D-Aufnahmen, obwohl die Aktivitätskonzentrationen nur halb so hoch waren. Schlußfolgerungen: Durch die Datenakquisition im 3DModus kann die zu applizierende Aktivitätsmenge bei gleichzeitiger Verbesserung der Bildqualität erheblich reduziert werden. Für Patientenuntersuchungen mit F-18-FDG im Ganzkörperbereich reicht es unserer Erfahrung nach aus, eine Aktivität zwischen 150 und 200 MBq zu applizieren. Dies entspricht einer effektiven Äquivalentdosis von 3 beziehungsweise 4 mSv.

Uncoupling of 2-fluoro-2-deoxyglucose transport and phosphorylation in rat hepatoma during gene therapy with HSV thymidine kinase

This animal study investigates the application of positron emission tomography (PET) with tracers of tumour metabolism for monitoring suicide gene therapy with herpes simplex virus thymidine kinase (HSVtk). After transplantation of HSVtk-expressing Morris hepatoma cells into ACI rats, dynamic PET measurements of 18F-labeled 2-fluoro-2-deoxyglucose (FDG) uptake were performed in animals 2 days (n = 7) and 4 days (n = 5) after the onset of therapy with 100 mg ganciclovir (GCV)/kg body weight as well as after administration of sodium chloride (n = 8). The arterial FDG plasma concentration was measured dynamically in an extracorporeal loop and the rate constants for FDG transport (K1, k2) and FDG phosphorylation (k3) were calculated using a three-compartment model modified for heterogeneous tissues. Also, quantification using the metabolic rate of FDG turnover and the standardized uptake value (SUV) was done. Furthermore, the thymidine incorporation into the tumour DNA was determined after i.v. administration of 3H-thymidine. An uncoupling of FDG transport and phosphorylation was found with enhanced K1 and k2 values and a normal k3 after 2 days of GCV treatment. The increase in FDG transport normalized after 4 days whereas the phosphorylation rate k3 increased. Quantification using the metabolic rate or the SUV showed congruent but less sensitive results compared with the modeling approach. The thymidine incorporation into the DNA of the tumours declined to 10.5% of the controls after 4 days of GCV treatment. The data indicate that PET with 18FDG and 11C-thymidine may be applied for monitoring of gene therapy with the HSVtk/GCV suicide system. Increased transport rates are evidence of stress reactions early after therapy. The measurement of thymidine incorporation into the tumour DNA can be used as an indicator of therapy efficacy.

[Optimization of PET image quality by means of 3D data acquisition and iterative image reconstruction]

PURPOSE

In the recent past, several algorithms have been developed in order to transform 3D sinograms acquired at volume PET systems into 2D data sets. These methods offer the possibility to combine the high sensitivity of the 3D measurement with the advantages of iterative 2D image reconstruction. The purpose of our study was the assessment of this approach by using phantom measurements and patient examinations.

METHODS

The experiments were performed at the latest-generation whole-body PET system ECAT EXACT HR+. For 2D data acquisition, a collimator of thin tungsten septa was positioned in the field-of-view. Prior to image reconstruction, the measured 3D data were sorted into 2D sinograms by suing the Fourier rebinning (FORE) algorithm developed by M. Defrise. The standard filtered backprojection (FBP) method and an optimized ML/EM algorithm with overrelaxation for accelerated convergence were employed for image reconstruction. The spatial resolution of both methods as well as the convergence and noise properties of the ML/EM algorithm were studied in phantom measurements. Furthermore, patient data were acquired in the 2D mode as well as in the 3D mode and reconstructed with both techniques.

RESULTS

At the same spatial resolution, the ML/EM-reconstructed images showed fewer and less prominent artefacts than the FBP-reconstructed images. The resulting improved detail conspicuously was achieved for the data acquired in the 2D mode as well as in the 3D mode. The best image quality was obtained by iterative 2D reconstruction of 3D data sets which were previously rebinned into 2D sinograms with help of the FORE algorithm. The phantom measurements revealed that 50 iteration steps with the optimized ML/EM algorithm were sufficient to keep the relative quantitation error below 5%.

CONCLUSION

Our measurements show that the image quality in 3D PET can be improved by using iterative reconstruction techniques. The concept of 3D data acquisition and combining the FORE algorithm with 2D ML/EM reconstruction can readily be employed in clinical practice since the computation time is not considerably longer than that in iterative reconstruction of true 2D data.

Intravenous and intra-arterial oxygen-15-labeled water and fluorine-18-labeled fluorouracil in patients with liver metastases from colorectal carcinoma

Intra-arterial chemotherapy can potentially increase drug delivery at the tumor sites and has therefore been used for the therapy of metastatic colorectal cancer.

METHODS

Dynamic PET and [18F]fluorouracil (18F-FU) were used in patients with liver metastases from colorectal cancer to examine the pharmacokinetics of the drug up to 120 min after intravenous and intra-arterial injection of the same dose of fluorouracil (FU). All patients included in the study (n = 15) had surgically implanted catheters in the gastroduodenal artery. Dynamic PET studies (up to 5 min) with 15O-labeled water were performed for the evaluation of the access to the lesions immediately before the 18F-FU study using both administration routes. The final evaluation included 24 metastases, obtained from 15 patients.

RESULTS

Of 24 lesions, 21 (87.5%) showed an improved access using the intra-arterial approach, and 20 (83.3%) demonstrated a better FU influx after intra-arterial 18F-FU infusion. Metastases reached the highest 18F-FU concentrations after intra-arterial administration, with a maximum standardized uptake values of 18.75 for the FU influx and of 5.03 for FU trapping. Of 24 metastases, eight (33.3%) demonstrated enhanced FU trapping after the intra-arterial administration. Cluster analysis revealed a group of metastases (n = 6) with a nonperfusion-dependent FU transport using the intravenous application. Of these six lesions, five (83.3%) did not show any enhancement of the 18F-FU trapping after intra-arterial application. The data gave evidence for at least one different, energy-dependent transport system, which can be saturated even after intravenous administration of the drug.

CONCLUSION

The data show that the main limiting factor for a therapy response is the very high and rapid elimination of the cytostatic agent out of the tumor cells. Furthermore, it was not possible to predict the pharmacokinetics of FU after intra-arterial application using an intravenous PET study. It may be possible, using intravenous PET double-tracer studies, to identify metastases having a nonperfusion-dependent transport system and exclude them from an intra-arterial treatment protocol.

Performance evaluation of a whole-body PET scanner using the NEMA protocol. National Electrical Manufacturers Association

This study evaluates the performance of the newly developed high-resolution whole-body PET scanner ECAT EXACT HR+.

METHODS

The scanner consists of four rings of 72 bismuth germanate block detectors each, covering an axial field of view of 15.5 cm with a patient port of 56.2 cm. A single block detector is divided into an 8 x 8 matrix, giving a total of 32 rings with 576 detectors each. The dimensions of a single detector element are 4.39 x 4.05 x 30 mm3. The scanner is equipped with extendable tungsten septa for two-dimensional two-dimensional measurements, as well as with three 68Ge line sources for transmission scans and daily quality control. The spatial resolution, scatter fraction, count rate, sensitivity, uniformity and accuracy of the implemented correction algorithms were evaluated after the National Electrical Manufacturers Association protocol using the standard acquisition parameters.

RESULTS

The transaxial resolution in the two-dimensional mode is 4.3 mm (4.4 mm) in the center and increases to 4.7 mm (4.8 mm) tangential and to 8.3 mm (8.0 mm) radial at a distance of r = 20 cm from the center. The axial slice width measured in the two-dimensional mode varies between 4.2 and 6.6 mm FWHM over the transaxial field of view. In the three-dimensional mode the average axial resolution varies between 4.1 mm FWHM in the center and 7.8 mm at r = 20 cm. The scatter fraction is 17.1% (32.5%) for a lower energy discriminator level of 350 keV. The maximum true event count rate of 263 (345) kcps was measured at an activity concentration of 142 (26.9) kBq/ml. The total system sensitivity for true events is 5.7 (27.7) cps/Bq/ml. From the uniformity measurements, we obtained a volume variance of 3.9% (5.0%) and a system variance of 1.6% (1.7%). The implemented three-dimensional scatter correction algorithm reveals very favorable properties, whereas the three-dimensional attenuation correction yields slightly inaccurate results in low- and high-density regions.

CONCLUSION

The ECAT EXACT HR+ has an excellent, nearly isotropic spatial resolution, which is advantageous for brain and small animal studies. While the relatively low slice sensitivity may hamper the capability for performing fast dynamic two-dimensional studies, the scanner offers a sufficient sensitivity and count rate capacity for fully three-dimensional whole-body imaging.

PET 2-fluoro-2-deoxyglucose uptake in rat prostate adenocarcinoma during chemotherapy with gemcitabine

This study was performed to investigate the effect of the new chemotherapeutic agent gemcitabine on glucose transport and metabolism in prostate carcinoma in vitro and in vivo.

METHODS

After transplantation of rat prostate adenocarcinoma cells, dynamic PET measurements with fluorine-18-labeled 2-fluoro-2-deoxy-D-glucose (18FDG) were performed in 15 animals before and 1 day after therapy with 90 mg/kg of body weight (n = 8) and 180 mg/kg of body weight (n = 7) gemcitabine. In the second examination, the animals received a simultaneous injection of 18FDG and [3H]thymidine. Quantitative evaluation of the PET data was done using the standardized uptake value (SUV) as well as a three-compartment pharmacokinetic model. Furthermore, the incorporation of [3H]thymidine into the DNA was determined. In vitro measurements of the FDG, 3-O-methylglucose and thymidine uptake were performed immediately and 4 hr after a 24-hr incubation period with different doses of gemcitabine.

RESULTS

FDG-SUV and the metabolic rate of FD 3 utilization did not change significantly after therapy. However, the values for the transport rate constants K1 and K2 increased significantly. The incorporation of thymidine into the DNA of treated tumors showed an 80% decline as compared with a control group. In the cell culture experiments, a dose-dependent increase of FDG (up to 178%) and 3-O-methylglucose uptake (up to 305%) was demonstrated. The thymidine uptake showed a 96% decline in the nucleic acid fraction and an increase of up to 337% in the cytoplasmic fraction.

CONCLUSION

The more global measures of FDG metabolism as SUV and metabolic rate of FDG utilization were unchanged after therapy, while DNA synthesis and cell viability declined. However, in vitro and in vivo evidence of an enhancement of glucose transport is presented, indicating that quantification by modelling may be superior for the evaluation of metabolic effects during chemotherapy.

Use of scanner characteristics in iterative image reconstruction for high-resolution positron emission tomography studies of small animals

The purpose of this work was to improve of the spatial resolution of a whole-body positron emission tomography (PET) system for experimental studies of small animals by incorporation of scanner characteristics into the process of iterative image reconstruction. The image-forming characteristics of the PET camera were characterized by a spatially variant line-spread function (LSF), which was determined from 49 activated copper-64 line sources positioned over a field of view (FOV) of 21.0 cm. This information was used to model the image degradation process. During the course of iterative image reconstruction, the forward projection of the estimated image was blurred with the LSF at each iteration step before the estimated projections were compared with the measured projections. The imaging characteristics of the high-resolution algorithm were investigated in phantom experiments. Moreover, imaging studies of a rat and two nude mice were performed to evaluate the imaging properties of our approach in vivo. The spatial resolution of the scanner perpendicular to the direction of projection could be approximated by a one-dimensional Gaussian-shaped LSF with a full-width at half-maximum increasing from 6.5 mm at the centre to 6.7 mm at a radial distance of 10.5 cm. The incorporation of this blurring kernel into the iteration formula resulted in a significantly improved spatial resolution of about 3.9 mm over the examined FOV. As demonstrated by the phantom and the animal experiments, the high-resolution algorithm not only led to a better contrast resolution in the reconstructed emission scans but also improved the accuracy for quantitating activity concentrations in small tissue structures without leading to an amplification of image noise or image mottle. The presented data-handling strategy incorporates the image restoration step directly into the process of algebraic image reconstruction and obviates the need for ill-conditioned "deconvolution" procedures to be performed on the projections or on the reconstructed image. In our experience, the proposed algorithm is of special interest in experimental studies of small animals.

Microbial communities of continuously cropped, irrigated rice fields

In continuously cropped, irrigated rice fields, soil microbial biomass as measured by total phospholipid fatty acid concentrations declined during the second half of the crop cycle. This decline was also observed in other components of the microbial community assessed by viable counts, including denitrifiers and sporeformers. Simultaneous with total biomass decline was the increase in potential indicators of nutrient stress--such as ratios of cyclopropanol ((Sigma)[cy/(omega)7c]) and trans ((Sigma)[(omega)7t/(omega)7c]) phospholipid fatty acids--in plain crop soil but not in the rhizosphere. Polyhydroxyalkanoate levels were enhanced in the root environment of mature rice. Polyunsaturated eukaryotic biomarkers accounted for only 13 to 16 mol% of the total phospholipids, including 2 mol% of 18:2(omega)6, which is considered a fungal biomarker. Single biomarkers for defined physiological groups of bacteria did not follow the declining trend of total microbial biomass. Signature compounds for gram-positive and gram-negative fermenters (plasmalogen phospholipids), methanogenic bacteria (diether lipids), and methanotrophs (18:1(omega)8c) increased as the crop approached maturity. Methanotrophs were not particularly enriched in the rhizosphere. Methanogenic biomarkers were, however, most abundant in root extracts from mature rice plants. Assuming that soil microbial biomass plays a significant role as a passive nutrient pool, its reduction during the second half of the cropping season suggests a mechanism that may ultimately contribute to declining productivity in irrigated, continuous rice cropping systems.

[PET studies with C-11 ethanol in intratumoral therapy of hepatocellular carcinomas]

Positron emission tomography (PET) is a noninvasive functional method for the study of solid tumor perfusion, metabolism and interaction with different therapeutic agents. The aim of the study was the investigation of the metabolism of hepatocellular carcinomas (HCC) and the kinetics during a treatment with intratumoral ethanol by PET. The ongoing study includes seven patients with child. A cirrhosis and HCC (UICC stage III-IVA; tumor size 3-6 cm). Dynamic PET studies (60 min) with 18F-fluordeoxyglucose (FDG) were performed prior to therapy to assess tumor viability. The evaluation of the FDG data demonstrated a liver-equivalent uptake in six of the tumors (well and moderately differentiated HCC), which were poorly delineated against the normal liver parenchyma. One moderately differentiated HCC showed an increased FDG metabolism, indicating no correlation between histology and metabolism. A dose of 37-80 MBq 11C-ethanol was applied together with a nonlabelled therapeutic dose of the drug via a puncture needle positioned under sonography. Five out of seven tumors demonstrated a high 11C uptake shortly after the end of the ethanol injection followed by constant 11C-ethanol concentration during the whole study period of 45 min. The PET data demonstrated no significant elimination of the 11C-ethanol from the tumor and no accumulation in the surrounding liver tissue. One case showed a decrease of the intra-tumoral 11C-ethanol concentration due to a punkture of a tumor vein, and in another case the surrounding liver parenchyma demonstrated significant 11C uptake in the early phase following paratumoral injection of the drug. In conclusion, PET is a useful tool for the study of the mechanism and the kinetics of percutaneous intratumoral ethanol injection of HCC.

[Predictive value of a rapid negativity of serum virus C viremia during treatment with interferon alpha in patients with chronic hepatitis C]

Interferon alpha (INF) used for chronic hepatitis C treatment induces a response in less than 25% of patients. The aim of this study was to appreciate the predictive value of the delay of clearance of hepatitis C virus (HCV) viremia after onset therapy and to compare it with other virological markers such as viral load before treatment and viral type. Thirty one patients with chronic hepatitis C, treated with 3 MU Interferon, 3 times a week for 6 months, were followed until 6 months post-treatment. Response was defined according to the normalisation of transaminases levels and loss of HCV viremia. Five patients were long term responder, eleven patients were complete to relapse responder and fifteen patients were non responder. Serum HCV RNA level, HCV type and serial detection of serum HCV RNA were determined and correlated with the long term response to INF. Patients with long term response had lower pre-INF viral load compared to the complete to relapse responder or to the non responder (p < 0.001). A rapid clearance of serum HCV RNA (1 month) is observed for all the long term responder (p < 0.001). The lowest viral load is also observed in these patients (p < 0 05). By contrast, although the number of patient is low, we were not able to observe a relation between the viral type and the response to treatment. In conclusion this data indicate that the delay of clearance of HCV RNA is also a good predictor of response to INF therapy. Furthermore a rapid clearance of HCV RNA in patients with a very weak pre-INF viral load is strongly associated with long term response (positive predictive value: 100%).

Multistep tumor targeting in nude mice using bispecific antibodies and a gallium chelate suitable for immunoscintigraphy with positron emission tomography

To improve tumor:tissue ratios in immunoscintigraphy, a three-step targeting method has been developed. The reagents used were (a) a radioactive, low molecular weight chelate prepared from ionic gallium and a phenolic polyaminocarboxylic acid, which can be labeled either with the single-photon emitter 67Ga or with the short-lived positron emitter 68Ga (t1/2 = 68 min); (b) a bispecific monoclonal antibody (bs-mAb) synthesized from the F(ab)2 fragment of the 1.1ASML antibody specific for the glycoprotein CD44v associated with a rat pancreas carcinoma cell line and the F(ab') fragment of an antibody specific for the gallium chelate; and (c) the nonradioactive gallium chelate covalently coupled to transferrin, which served as a high molecular weight blocker to prevent binding of the radioactive gallium chelate to bs-mAbs in the circulation. Targeting experiments in tumor-bearing nude mice with different doses of bs-mAbs, blocker, and 67Ga chelate were adjusted to maximize tumor to tissue contrasts and tumor uptake. Compared with the biodistribution of the 131I-labeled, native 1.1ASML antibody 24 h postinjection, a schedule using 100 pmol bs-mab 24 h later 100 pmol blocker, 15 min later 16 pmol 67Ga chelate, 1 h later examination, increased tumor:blood and tumor: liver ratios by a factor of 5 while keeping the localization of radioactivity in the tumor constant (10.1% injected dose/g). High-contrast images using either 67Ga or 68Ga were obtained within 1 h. The targeting method described enables the use of the short-lived positron emitter 68Ga and thus allows the combination of an improved immunoscintigraphy and positron emission tomography.