Hermann Ostertag

Performance evaluation of the whole-body PET scanner ECAT EXACT HR/sup +/ following the IEC standard

The performance parameters of the whole-body PET scanner ECAT EXACT HR/sup +/ (CTI/Siemens, Knoxville, TN) were determined following the standard proposed by the International Electrotechnical Commission (IEC). The tests were expanded by some measurements concerning the accuracy of the correction algorithms and the geometric fidelity of the reconstructed images. The scanner consists of 32 rings, each with 576 BGO detectors (4.05/spl times/4.39/spl times/30 mm/sup 3/), covering an axial field-of-view of 15.5 cm and a patient port of 56.2 cm. The transaxial FWHM determined by a Gaussian fit in the 2D (3D) mode is 4.5 (4.3) mm at the center. It increases to 8.9 (8.3) mm radially and to 5.8 (5.2) mm tangentially at a radial distance of r=20 cm. The average axial resolution varies between 4.9 (4.1) mm FWHM at the center and 8.8 (8.1) mm at r=20 cm. The system sensitivity for unscattered true events is 5.85 (26.4) cps/Bq/ml (measured with a 20 cm cylinder). The 50% dead-time losses were reached for a true event count rate (including scatter) of 286 (500) kcps at an activity concentration of 74 (25) kBq/ml. The system scatter fraction is 0.24 (0.35). With the exception of the 3D attenuation correction algorithm, all correction algorithms work reliably. The results reveal that the ECAT EXACT HR/sup +/ has a good and nearly isotropic spatial resolution. Due to the small detector elements, however, it has a low slice sensitivity which is a limiting factor for image quality.

Measured attenuation correction methods.

Accurate attenuation correction is a prerequisite for the determination of exact local radioactivity concentrations in positron emission tomography. Attenuation correction factors range from 4–5 in brain studies to 50–100 in whole body measurements. This report gives an overview of the different methods of determining the attenuation correction factors by transmission measurements using an external positron emitting source. The long-lived generator nuclide68Ge/68Ga is commonly used for this purpose. The additional patient dose from the transmission source is usually a small fraction of the dose due to the subsequent emission measurement. Ring-shaped transmission sources as well as rotating point or line sources are employed in modern positron tomographs. By masking a rotating line or point source, random and scattered events in the transmission scans can be effectively suppressed. The problems of measured attenuation correction are discussed: transmission/emission mismatch, random and scattered event contamination, counting statistics, transmission/emission scatter compensation, transmission scan after administration of activity to the patient. By using a double masking technique simultaneous emission and transmission scans become feasible.

Uptake and turnover of L-(13N)-glutamate in the normal human heart and in patients with coronary artery disease

L-(13N)-glutamate (4–8 mCi) was administered IV to 27 patients with coronary artery disease and to 12 control subjects.Quantitative whole body imaging of the 13N label was performed in 31 individuals at different time intervals following the injection. Initial uptake of the total myocardium was estimated to be 5.0±0.88% of the dose. Standardized areas of reduced size on the projection plane contained 2.38±0.41% of the total dose in control subjects and 2.67±0.49% in coronary patients. Subsequent imaging exhibited significant differences in the dynamic behavior of both groups: 13N activity loss within 10 min was 3.2±4.2% of the initial value in control subjects and 16.0±9.8% in coronary patients. In individual cases a high myocardial accumulation of the 13N label was observed in regions of reduced 201Tl uptake. The findings are explained by an augmented extraction efficiency in cases of flow reduction.Glutamate utilization may be involved in metabolic adaptations of the myocardium to chronic or repetitive ischemia and may be worthy of further investigation by positron emission tomography.

Scatter suppression by using a rotating pin source in PET transmission measurements

Transmission scans used essentially for attenuation correction in whole-body studies with positron-emission tomography (PET) contain different amounts of scatter contamination, depending on the source used. It is shown by phantom studies that scatter causes spatial distortions and nonuniformities in the attenuation images, leading to errors in the attenuation correction factors. For these measurements a ring-shaped transmission source is simulated by a 6-cm-long rotating pin source containing 5 mCi of /sup 68/Ge. The measurements are performed using large phantoms filled with water. Scattered radiation is substantially suppressed by selecting only chords through the actual position of the source instead of collecting all coincident events. Improved edge delineation and homogeneity of the attenuation images are achieved. >

Radioisotope therapy of cystic craniopharyngeomas

Eighteen patients suffering from cystic craniopharyngeoma were treated with intracavitary irradiation. The beta-emitting radioisotope 90y (2.25 MeV) was instilled into the cyst following stereotactic puncture of the space-occupying lesion. The surgical approach was planned using angiograms and reconstructed transmission computer tomography (TCT) coronal and saggital sections. Therapy was devised to deliver 20,000 rad to the cysts wall. Eleven patients received follow-up TCT examinations after four months. Eight of 11 patients had a significant volume decrease in the craniopharyngeoma cyst. In two patients, the cystic volume remained unchanged; one had progression of disease. It is concluded that the intracavitary treatment of cystic craniopharyngeoma will result in a reduction of the size of the space-occupying lesion.

Selective drug-induced reduction of blood flow in tumor transplants

The effect of a calcium antagonist and a physiologic amine on tumor and muscle perfusion was investigated with the aim of improving the preconditions for external hyperthermia treatment of cancer. Nisoldipine (0.04-4.0 mg/kg) and 5-hydroxy tryptamine (5-HT) (0.2-8.0 mg/kg) were administered i.p. in Sprague-Dawley rats bearing Walker 256 carcinoma, Yoshida sarcoma, or a homologous tumor transplant derived from a spontaneous leiomyosarcoma of the uterus. At the maximum dosage used, nisoldipine injection caused a decrease of the regional washout rate of Xenon-133 of 63 +/- 8% (SEM) in the Walker carcinoma and an increase of 80 +/- 41% in the muscle of the hind leg. 5-HT (8 mg/kg) caused a drop of 79 +/- 29% in the Walker carcinoma and only a slight fall of the washout rate in muscle of 14 +/- 4.8%. Tumor-to-muscle uptake ratios of 11C-butanol fell from 5.63 +/- 1.98 to 3.32 +/- 1.21, and from 5.3 +/- 0.56 to 2.98 +/- 0.30, after injection of 0.2 mg/kg nisoldipine and 4 mg/kg 5-HT, respectively. Similar reaction patterns and percentage changes were observed in different tumor lines at constant doses of 0.2 mg/kg nisoldipine and 4 mg/kg 5-HT. Both drugs representing two different rationales of vasomotor action were able to reduce blood flow specifically in transplanted tumors; nisoldipine increased muscle blood flow and decreased arterial blood pressure, whereas 5-HT acted without substantial systemic effects.

Effects of scintillation light collection on the time resolution of a time-of-flight detector for annihilation quanta

Monte Carlo methods were used to simulate the production and collection of scintillation light in a BaF/sub 2/ crystal. The calculated results were compared to measured values for detectors with different heights. Based on this simulation, a TOF (time-of-flight) detector consisting of two conical BaF/sub 2/ crystals (base radius 18 mm, top radius 15 mm, height 25 mm) was assembled. The total time resolution of this system was (210+or-5) ps FWHM (full width at half maximum) for 511-keV annihilation quanta and was found to be independent of the source position between the detectors. Since the time resolution of the TOF detector system consisting of two conical BaF/sub 2/ crystals is sufficiently constant in the volume of interest and does not change during operation, this detector meets the requirements for application in a clinical routine. >

11C-Butanol for imaging of the blood-flow distribution in tumor-bearing animals

Abstract1-11C-n-Butanol produced semiatomatically using a cyclotron was employed to investigate the whole-body distribution and kinetics of the label of this compound. Following the administration of 11C-butanol into the aorta of two dogs, more than 80% of the activity was cleared from the blood within 1 min. The activity distribution mirrored the cardiac output distribution as determined using 121I microspheres. Within 25 min p.i., a significant release of decay-corrected activity was only observed for the liver, spleen, and kidneys. Muscle and whole-body activity showed constant levels during this period. In 45 tumor transplants from rats, the dynamic behavior of the label was studied. The tissue retention of activity following injection into the a. femoralis was approximately 100% during the 1st 15 s for both tumor and muscle (n=6). The activity release by tumors during the 1st 10 min after intra-aortic injection was 18%±4.5% (n=39; decay corrected). In five different tumor lines (n=10), the initial 11C-butanol uptake was related to that of muscle, and the results were correlated with the tumor-to-muscle retention of 121I-microspheres (r =0.89). In 17 tumors, the correlation between 11C-butanol uptake and the washout rate of 133Xe resulted in a correlation coefficient of 0.96. Tumor-to-muscle uptake ratios could be equally determined using intra-aortic and intravenous injection, as evaluated by intraindividual comparison in 12 rats (y=0.01+0.98x;r=0.98). 11C-Butanol appears to be an appropriate radiotracer for the assessment of blood supply to malignant tumors relative to muscle.

Effects of Distorted PET Projection Data on the Reconstructed Image Using Different Reconstruction Algorithms

In a simulation study, the effects of multiplicative and additive distortions of the projection data on the reconstructed image quality and quantitation capability were investigated. Three different reconstruction algorithms were compared: (i) filtered back-projection (FBP), (ii) an iterative method with multiplicative correction term (ML-EM), and (iii) an iterative method with additive correction term (AR-AC). Distortions were uniformly applied to all projections with projection angles ranging from 72° through 108°. Multiplicative distortion factors ranged from 0.5 to 1.4, additive distortions were −30% to +30%. The reconstructed images and activity concentrations were not affected by the multiplicative distortions if ML-EM was applied, whereas additive distortions were best tolerated when FBP or AR-AC were used for image reconstruction.

Drug-specific /sup 19/F NMR and dynamic /sup 18/F PET imaging of the cytostatic agent 5-fluorouracil

The spatial distribution of the antineoplastic agent 5-fluorouracil (5-FU) has been mapped both with /sup 19/F NMR and /sup 18/F PET imaging techniques. For /sup 19/F NMR imaging of 5-FU and its major catabolite /spl alpha/-fluoro-/spl beta/-alanine (FBAL), a fast gradient-echo pulse sequence was employed. A chemical-shift selective saturation pulse was used to suppress either the 5-FU or the FBAL resonance before the other component of the /sup 19/F NMR spectrum was imaged. This approach yielded selective 5-FU and FBAL NMR images free of chemical-shift artifacts in readout and slice-selection direction. In phantom experiments, /sup 19/F 5-FU and FBAL images with a spatial resolution of 12.5/spl times/12.5/spl times/20 mm/sup 3/ were obtained in 32 min from model solutions, with drug and catabolite concentrations similar to those estimated in animals and patients undergoing i.v. chemotherapy with 5-FU. The PET experiments were carried out using /sup 18/F-labeled 5-FU. The biodistribution of 5-[/sup 18/F]FU in rats shortly after administration of the drug demonstrated the good vascularization of the transplanted tumors. The metabolic turnover of the cytostatic agent started about 10-20 min p.i. and was predominant in the tumor and liver tissue. The rapid adjustment of the /sup 18/F metabolite concentrations in the transplanted tumors to a steady state provides evidence of anabolic tumor activity, which supports the hypothesis of 5-FU trapping in malignant cells based on /sup 19/F NMR spectroscopy data. The high uptake of 5-[/sup 18/F]FU in the liver, on the other hand, mainly reflects the catabolization of 5-FU to the noncytotoxic FBAL, which leads to a reduced bioavailability of the drug. The chemical-shift selective /sup 19/F NMR technique and the /sup 18/F PET imaging method yield complementary metabolic and kinetic information on 5-FU. They are thus well suited for the noninvasive observation of the uptake and the turnover of the cytostatic agent in normal and neoplastic tissues. >