ORIGINAL ARTICLE 
Annals of Nuclear Medicine Vol.9, No.2, 81-87, 1995 

Clinical application of 62Zn/62Cu positron generator: Perfusion and plasma pool images in normal subjects 
 
Hidehiko OKAZAWA,* Yasuhisa FUJIBAYASHI,** Yoshiharu YONEKURA,* Nagara TAMAKI,* Sadahiko NISHIZAWA,* 
Yasuhiro MAGATA,* Koichi ISHIZU,* Tatsuro TSUCHIDA,* Norihiro SADATO,* Junji KONISHI,* 
Akira YOKOYAMA,** Ren IWATA*** and Tatsuo IDO*** 

*Faculty of Medicine **Faculty of Pharmaceutical Sciences, Kyoto University 
***Cyclotron Radioisotope Center, Tohoku University 

We have developed a new 62Zn/62Cu positron generator, and applied it for PET imaging of perfusion and plasma volume in 5 normal subjects. The generator makes it possible by a simple procedure to obtain 62Cu eluate and labeling compounds sufficiently every 40-60 minutes. 62Cu labeled pyruvaldehyde bis(N4-methylthiosemicarbazone) copper II (62Cu-PTSM) was employed for cerebral and myocardial perfusion imaging and 62Cu labeled human serum albumin-dithiosemicarbazone (62Cu-HSA-DTS) was used for plasma pool imaging. The images of cerebral blood flow, cerebral plasma volume and myocardial perfusion were excellent. In addition, the analysis of tissue activity and blood activity demonstrated the microspheric character of 62Cu-PTSM. Correction of arterial activity with the standard disappearance curve of 62Cu-PTSM suggested the possibility of quantifying blood flow. The results of this study indicate the capability of the 62Znf62Cu generator for wide clinical use without an in-house cyclotron. 
Key words: 62Cu-PTSM, 62Cu-HSA-DTS,PET, perfusion tracer, plasma pool imaging 

INTRODUCTION 

THE 62Zn/62Cu POSITRON GENERATOR has been proposed as a possible source of generator-produced radiopharmaceuticals for positron emission tomography (PET), that has been expected to play an important role in wide clinical application of PET imaging without the use of an in-house cyclotron.1 Although the physical half-life of the parent radionuclide is rather short (62Zn; 9 hr), a 10 min daughter half-life is thought to be suitable for PET imaging tracer. After copper (II) pyruvaldehyde bis(N4-methylthiosemicarbazone) labeled with 62Cu (62Cu-PTSM) had been applied as a perfusion tracer,2,3 excellent cerebral and myocardial perfusion images of human were obtained.4-7 Several quantitative approaches to the measurements of cerebral or myocardial blood flow with 62Cu-PTSM have also been reported.5-10 As blood-pool imaging agents, 62Cu labeled human serum album-indithiosemicarbazone (62Cu-HSA-DTS)11 and benzyl-1,4,8,11-tetraazacyclotetradecan-N,N',N",N"'-tetraacetic acid-albumin (62Cu-benzyl-TETA-HSA)12 were developed and investigated. These agents can provide cerebral and myocardial blood volume images and perfusion images on the same day. For wide clinical application, this generator should possess a simple milking technique. The automated remote synthesis of 62Cu-PTSM, 13 and the new generatorl4, 15 and labeling method by means of ligand exchange reaction11,16 can provide 62Cu-PTSM and 62Cu-HSA-DTS easily enough to use these tracers in clinical studies. The present study shows the human application of the newly developed 62Zn/62Cu generator in normal volunteers. In order to evaluate the clinical applicability of this generator for perfusion and plasma volume imaging with PET, PET scans were performed with 62Cu-PTSM and 62Cu-HSA-DTS with blood sampling to examine the kinetics of these tracers. 

MATERIALS AND METHODS 

Preparation of 62Zn/62Cu generator, 62Cu-PTSM, and 62Cu-HSA-DTS 
62Zn was obtained by 63Cu(p,2n)62Zn nuclear reaction with natural copper (63Cu: 69.2%) as a target material. A 62Zn/62Cu generator was prepared with 62ZnCl2 aqueous solution ( 1.1 GBq, pH 5.0) by the method reported previously. 14 In this generator, cation-exchange resin (CG-120, Amberlite) was packed into a column and 62Zn solution (1.1 GBq, in 2 ml water, pH 5.0) was loaded to adsorb 62Zn. A glycine solution (200 mM) was used as the eluant and 62Cu-glycine complex was obtained in the glycine solution. Elution efficiency was 60 to 70%, and the radiochemical purity was higher than 95%. Pyruvaldehyde bis(N4-methylthiosemicarbazone) was prepared as described previously.17 The 62Cu-PTSM was quantitatively obtained by simply mixing the generator eluate, 62Cu-glycine and PTSM solution for a few seconds by a ligand exchange reaction.16 The product was acquired in a total volume of 4 ml 62Cu-PTSM (0.1 mM PTSM in 5% DMSO) with glycine solution. 

HSA-DTS was also synthesized by the method reported previously.11 62Cu labeling of HSA-DTS was performed by simply mixing 4 ml of HSA-DTS solution (5 mg/ml in saline buffer (pH 6.0)) and 4 ml of 62Cu-generator eluate. 

Subjects 
The study involved 5 male normal volunteers (age range, 30-46 years). Four subjects were investigated with both 62Cu-PTSM and 62Cu-HSA-DTS on separate days, and the other subject underwent only a 62Cu-HSA-DTS study. The three subjects studied with 62Cu-PTSM were scanned for imaging of both the brain and myocardium with a single dose of the tracer. The other subject was studied only for myocardial perfusion imaging. The study was approved by the Ethical Committee of Kyoto University Faculty of Medicine, and written informed consent was obtained from all the subjects before the PET study. 

Positron emission tomography 
The PCT-3600W (Hitachi Medical Co., Tokyo, Japan) was used as a PET imaging device. 18 This system permits the simultaneous acquisition of 15 transverse slices with a center-to-center distance of 7 mm. All scans were performed at a resolution of 9 mm full width at half maximum (FWHM) in the transaxial direction and 6.5 mm in the axial direction. The field of view and pixel size of the reconstructed images were 256 mm and 2 mm for the brain, and 512 mm and 4 mm for the myocardium, respectively. 

The subject's head was immobilized with a head-holder. In the 62Cu-PTSM study, a small cannula was placed in the left brachial artery for serial blood sampling in two subjects. In the study of 62Cu-HSA-DTS, a small cannula was placed in the cubital vein contralateral to the administration side. Prior to all emission measurements, tomographic transmission data were obtained with a standard plate source of 68Ge/68Ga for attenuation correction. The tissue activity concentration in the PET images was cross calibrated against the well counter with a cylindrical phantom filled with 18F solution. 

For perfusion imaging, 370-777 MBq (l0-21mCi) of 62Cu-PTSM was injected intravenously over 5 seconds in a total volume of 4 ml. PET data acquisition of cerebral perfusion imaging w,as started at the time of 62Cu-PTSM injection and continued for 10 minutes, in 15-second frames for the first 120 seconds and in 60-second frames for the next 8 minutes. In addition to the serial dynamic PET images, static images of 2 to 10 min after injection were reconstructed from dynamic scan data. 

In the myocardial perfusion scanning, the data acquisition time course was in 10-second frames for the first 120 seconds, in 60-second frames for the next 6 minutes and finally in 240-second frames for the last 12 minutes (total 20 min). For the static images, dynamic scan data for 8 to 20 min after injection were used for reconstruction. 

In order to obtain plasma volume imaging, 296-740 MBq (8-20 mCi) of 62Cu-HSA-DTS was injected intravenously over 15 seconds in a total volume of 8 ml. PET data acquisition for the head was started 3 minutes after the administration of 62Cu-HSA-DTS and continued for l 6 minutes in 4-minute frames. Tissue activity images of 62Cu-PTSM reconstructed from the serial dynamic PET data and blood activity were corrected for physical decay of 62Cu (half-life 9.74 min). In order to examine the kinetics of 62Cu-PTSM, the regions of interest (ROIs) were placed on each serial dynamic PET image of the cerebrum and myocardium. For brain analysis, the ROI was placed to include the whole brain slice. For myocardial analysis, square ROIs were placed on the left ventricular (LV) wall and in the left atrium (LA) of the myocardium, 12 x 12 mm and 8 x 8 mm in size, respectively. 

Measurement of blood radioactivity 
In the 62Cu-PTSM study, arterial blood samples were obtained manually from the left brachial artery in two scans of two subjects' myocardium, from the time of the intravenous injection to the end of the scan. One ml of blood was sampled every 10 seconds for the first 2 minutes and then every 60 seconds for the next 8 minutes, and then sampled at 15, 20 minutes after the injection. The blood samples thus obtained were immediately measured in a scintillation counter to obtain the arterial radioactivity. The arterial radioactivity was corrected by means of the standard curve for the percent octanolextractable 62Cu-PTSM obtained from 10 subjects reported previously.7 

In the 62Cu-HSA-DTS study, samples of 2 ml of venous blood were taken from the cubital vein beginning at 1 min after the administration of the tracer to the end of the scan with an interval of every 4 minutes. Each sample of blood was divided into 0.5 ml and 1.5 ml and the latter samples were immediately centrifuged to measure plasma count. 

RESULTS 
Figure 1 shows the cerebral perfusion image reconstructed from the data for 2 to 10 minutes after the administration of 62Cu-PTSM. A cerebral blood flow (CBF) image with good contrast between gray and white matter was obtained. Figure 2 shows the cerebral plasma pool image obtained from the data for 3 to 19 minutes after the administration of 62Cu-HSA-DTS in the same subject. From the analysis of venous blood sampling, greater than 95% of total blood activity was found to exist in the plasma pool. 

Figure 3 shows the transaxial sections of myocardial perfusion images reconstructed from the data for 8 to 20 minutes after the injection of 62Cu-PTSM in another subject. The uptake of 62Cu-PTSM was homogeneous with the high contrast between the myocardium and the blood pool, but uptake in the liver was high and the contrast between the inferior wall of the myocardium and the liver was not so clear in the subject whose heart and liver located close to each other. 

Figure 4 shows a typical time-activity curve in the whole slice of the brain obtained from the tissue activity images reconstructed from the serial dynamic PET scan for 10 minutes after 62Cu-PTSM administration. The brain activity reached its maximum within 2 minutes and the tissue activity remained constant thereafter, which demonstrates that 62Cu-PTSM has the character of a microsphere tracer.2-10,19 Figure 5 shows a typical time-activity curve of the left ventricular myocardial wall and left atrium obtained from the serial dynamic PET scan for 20 minutes after 62Cu-PTSM injection. The sharp peak of activity in ROI placed over the left ventricular myocardial wall observed within one minute was considered to be due to the spillover of the intravascular radioactivity, and the tissue activity remained constant from 2.5 to 20 min, similar to the brain perfusion study. The arterial blood activity measured in the same study reached its peak value at same time as the activity in the left atrium, followed by rapid clearance, and was almost stable 3 minutes after the injection. The activity in the left atrium demonstrated slight underestimation of the peak activity and overestimation of the plateau value compared to the true arterial activity. 

DISCUSSION 

The 62Zn/62Cu positron generator and radiopharmaceu-ticals labeled with 62Cu have been looked forward to by PET centers without an in-house cyclotron for wide clinical use. The aim of this study was to evaluate the clinical applicability of our new generator for PET imaging, and this generator was considered to be applicable to a wide clinical use because of the short interval required to obtain sufficient eluate and the simplicity of the labeling method. Subjects were able to undergo sequential scanning of brain perfusion and myocardial perfusion with one dose of 62Cu-PTSM or studies before and after dipyridamole loading with an interval of one hour. These methods are available for clinical use when two or three scannings are needed in one patient on the same day. 

The quality of images was excellent with good contrast between gray matter and white matter in the cerebrum, and between the myocardium and the blood pool in the heart. As reported previously on the retention mechanism of 62Cu-PTSM, the first-pass extraction and back diffusion were completed within a few minutes,7 so that scanning between 2 and I O minutes seemed to be the best timing in a cerebral 62Cu-PTSM study. The quality of the brain image seemed to be better than with [99mTc]-d,1-hexamethylpropyleneamine oxime (HMPAO), but worse than 123I-isopropyl iodoamphetamine (IMP) in comparison with SPECT tracers because of the extraction ratio.7 In the myocardial study with 62Cu-PTSM, delayed data acquisition is considered to be better after sufficient clearance of blood activity to obtain better contrast images between the myocardium and the blood pool. Figure 5 shows that the radioactivity in the myocardial wall reached a plateau and the blood activity decreased to its minimum level about 3 minutes after the injection of 62Cu-PTSM. This result shows that the most suitable length of time for myocardial perfusion image scanning is between 3 and 20 min. In this graph, slight underestimation of the peak activity, and overestimation of the plateau value in LA was observed compared to the true arterial activity. The difference between arterial sampling and dynamic PET data in the counting interval was considered to be one of the reasons for the underestimation of the peak count. The partial volume effect could be another reason for underestimation. Overestimation of the plateau value was considered to be due to the spillover from the myocardial wall.20 The plasma pool image obtained with 62Cu-HSA-DTS represented less noise than the CBV image obtained by the C150 bolus inhalation method. The possible long scanning time is likely to be the reason for better image quality. 

Quantitative approaches to measuring blood flow by means of 62CU-PTSM have already reported in several articles.5-10 Although estimation of the accurate input function is essential for quantification of cerebral or myocardial blood flow, low level blood radioactivity remained constant beyond 3 minutes due to metabolites or other components. Herrero et al. estimated the true input function by using the standard curve of the octanolextractable 62Cu-PTSM ratio obtained from five dogs.6 They used this standard monoexponential curve for the correction of sampled blood activity. If the activity in ROI placed on left atrium is corrected, the true input function can be estimated from the dynamic data noninvasively. Figure 6 shows the percentage of octanolextractable 62Cu-PTSM in blood with respect to total blood activity as a function of time after intravenous injection of 62Cu-PTSM in 10 subjects in another study.7 The rate of disappearance of 62Cu-PTSM from blood was rapid and it could also be fitted monoexponentially. Figure 7 shows the time-activity curves for 62Cu-PTSM in the total arterial blood shown in Figure 5 (dashed line), and corrected radioactivity obtained with the mono-exponential curve shown in Figure 6 (solid line). The estimated PTSM-associated radioactivity cleared more rapidly because of the swift conversion of 62Cu-PTSM to the hydrophilic compounds. The corrected activity was almost negligible at 3 minutes after the injection despite remaining the total blood radioactivity thereafter, which was consistent with the results of our previous study.7 The estimated PTSM-associated radioactivity means the correction for the lipophilic part of blood activity that is considered to be the true input function.6,7 With this standard curve, arterial radioactivity can be corrected from the arterial sampling or dynamic data for the myocardial cavity to estimate the true input function, which indicates the possibility of quantitative analysis. 

The results of this study demonstrate the suitability of the new 62Zn/62Cu generator for wide clinical use. 62Cu-PTSM and 62Cu-HSA-DTS can be provided at intervals of 40-60 minutes by a simple method. The quality of the perfusion images and blood pool images was excellent 
and the possibility of quantification was indicated. 

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