A method for stabilising technetium-99m exametazime prepared from a commercial kit

A standardized method for the construction of tracer specific PET and SPECT rat brain templates: validation and implementation of a toolbox

High-resolution anatomical image data in preclinical brain PET and SPECT studies is often not available, and inter-modality spatial normalization to an MRI brain template is frequently performed. However, this procedure can be challenging for tracers where substantial anatomical structures present limited tracer uptake. Therefore, we constructed and validated strain- and tracer-specific rat brain templates in Paxinos space to allow intra-modal registration. PET [18F]FDG, [11C]flumazenil, [11C]MeDAS, [11C]PK11195 and [11C]raclopride, and SPECT [99mTc]HMPAO brain scans were acquired from healthy male rats. Tracer-specific templates were constructed by averaging the scans, and by spatial normalization to a widely used MRI-based template. The added value of tracer-specific templates was evaluated by quantification of the residual error between original and realigned voxels after random misalignments of the data set. Additionally, the impact of strain differences, disease uptake patterns (focal and diffuse lesion), and the effect of image and template size on the registration errors were explored. Mean registration errors were 0.70 ± 0.32 mm for [18F]FDG (n = 25), 0.23 ± 0.10mm for [11C]flumazenil (n = 13), 0.88 ± 0.20 mm for [11C]MeDAS (n = 15), 0.64 ± 0.28 mm for [11C]PK11195 (n = 19), 0.34 ± 0.15 mm for [11C]raclopride (n = 6), and 0.40 ± 0.13 mm for [99mTc]HMPAO (n = 15). These values were smallest with tracer-specific templates, when compared to the use of [18F]FDG as reference template (p<0.001). Additionally, registration errors were smallest with strain-specific templates (p<0.05), and when images and templates had the same size (p ≤ 0.001). Moreover, highest registration errors were found for the focal lesion group (p<0.005) and the diffuse lesion group (p = n.s.). In the voxel-based analysis, the reported coordinates of the focal lesion model are consistent with the stereotaxic injection procedure. The use of PET/SPECT strain- and tracer-specific templates allows accurate registration of functional rat brain data, independent of disease specific uptake patterns and with registration error below spatial resolution of the cameras. The templates and the SAMIT package will be freely available for the research community [corrected].

Evolution of Tc-99m in diagnostic radiopharmaceuticals

The progress in diagnostic nuclear medicine over the years since the discovery of 99mTc is indeed phenomenal. Over 80% of the radiopharmaceuticals currently being used make use of this short-lived, metastable radionuclide, which has reigned as the workhorse of diagnostic nuclear medicine. The preeminence of 99mTc is attributable to its optimal nuclear properties of a short half-life and a gamma photon emission of 140 keV, which is suitable for high-efficiency detection and which results in low radiation exposure to the patient. 99mTcO4-, which is readily available as a column eluate from a 99Mo/99mTc generator, is reduced in the presence of chelating agents. The versatile chemistry of technetium emerging from the 8 possible oxidation states, along with a proper understanding of the structure-biologic activity relationship, has been exploited to yield a plethora of products meant for morphologic and functional imaging of different organs. This article reviews the evolution of 99mTc dating back to its discovery, the development of 99Mo/99mTc generators, and the efforts to exploit the diverse chemistry of the element to explore a spectrum of compounds for diagnostic imaging, planar, and single photon emission computed tomography. A brief outline of the 99mTc radiopharmaceuticals currently being used has been categorically presented according to the organs being imaged. Newer methods of labeling involving bifunctional chelating agents (which encompass the "3 + 1" ligand system, Tc(CO)3(+1)-containing chelates, hydrazinonicotinamide, water-soluble phosphines, and other Tc-carrying moieties) have added a new dimension for the preparation of novel technetium compounds. These developments in technetium chemistry have opened new avenues in the field of diagnostic imaging. These include fundamental aspects in the design and development of target-specific agents, including antibodies, peptides, steroids, and other small molecules that have specific receptor affinity.

Studies on white blood cell labelling: (99)Tc(m)-HMPAO preferentially labels granulocytes

Mixed leucocytes have been labelled in small volumes of plasma using small aliquots of hexamethylpropylene amine oxime (HMPAO) from vials of Ceretec, which were fractionated after reconstitution and stored frozen under nitrogen. Labelling efficiency was found to be independent of HMPAO concentration >8.8 microg.ml-1 and independent of cell concentration at >8x10(7) ml-1. Results are presented from 145 patients referred for white cell scintigraphy. Labelling efficiency was 78.9+/-9.8%. The labelled mixed leucocytes were separated on a 3-step density gradient of Percoll/plasma which showed that (99)Tc(m)-HMPAO gave a granulocyte/lymphocyte specific activity ratio of 1.67; the ratio for (111)In-tropolone or oxine was 1.0, with no preference for granulocytes. It was noted that the labelling efficiency was reduced when there was a high proportion of lymphocytes present, and also that in one patient with non-Hodgkin's lymphoma the lymphocytes labelled as equally well as the granulocytes. The stability of the label was studied by fractionating the mixed cells after in vitro incubation and after 1 h in vivo; very little loss of label occurred.

Purification and stabilization of 99mTc-d,l-HMPAO: role of organic extractants

99mTc-d,l-HMPAO, an important SPECT agent for imaging cerebral perfusion, suffers from the disadvantage of an inherent instability and its shelf life has been reported to be 30 min. The latter is a harsh constraint and not compatible with Centralized Radiopharmacy procedures. During the attempts to improve upon the stability of 99mTc-d,l-HMPAO, preservation of product as an organic extract into suitable solvents like diethylether, ethylacetate, methylethylketone, chloroform was tried out. Chloroform extraction (yield: >90%) resulted in a product having stability not less than 5 hours. Gentle drying of the chloroform extract and reconstitution in normal saline resulted in quantitative recovery of 99mTc-d,l-HMPAO with acceptable radiochemical purity (>90%). This finding is thus of much significance, especially in the context of centralized large hospital radiopharmacy setting, by rendering convenience and flexibility in scheduling patients.

A trial study of leukocyte labeling with stabilized Tc-99m D,L-HMPAO by methylene blue and sodium phosphate buffer

We attempted to label leukocytes with stabilized Tc-99m D,L-HMPAO by methylene blue and sodium phosphate buffer (S-HMPAO).

METHODS

The results were compared with unstabilized Tc-99m D,L-HMPAO (U-HMPAO). U-HMPAO was obtained by reconstituting a commercial vial of D,L-HMPAO. Stabilization of the kit was performed by the addition of methylene blue. The leukocytes were labeled using a modified published method. The test samples of S-HMPAO and U-HMPAO were prepared immediately, and stood for 0.5, 1, 2, 4, and 6 h, respectively, at room temperature before analysis.

RESULTS

In comparison with U-HMPAO: (1) the radiochemical purity of S-HMPAO was higher; (2) the labeling efficiencies of S-HMPAO labeled leukocytes were higher and consistent; (3) the viability of S-HMPAO labeled leukocytes was as high as the viability of U-HMPAO labeled leukocytes at any time; and (4) the percentages of disintegrated from S-HMPAO labeled leukocytes in plasma were lower.

CONCLUSION

S-HMPAO is more stable than U-HMPAO and can provide higher leukocyte labeling efficiency. S-HMPAO, therefore, has the potential to replace U-HMPAO as a leukocyte-labeling agent.

Stabilization of Tc-99m D,L-HMPAO preparations as a leucocyte labelling agent

An attempt was made to use stabilized Tc-99m D,L-HMPAO (S-HMPAO) to label leucocytes. The radiochemical purity of Tc-99m D,L-HMPAO, labelling efficiency of leucocytes, cell viability of labelled leucocytes, and stability of S-HMPAO labelled leucocytes were calculated. In comparison with commercial Tc-99m D,L-HMPAO (C-HMPAO) without stabilization, immediately, at 0.5, 1, 2, 4, and 6 h after HMPAO preparation, the radiochemical purity of S-HMPAO and the labelling efficiencies of S-HMPAO labelled leucocytes were higher. S-HMPAO is more stable than C-HMPAO and can provide higher leucocyte labelling efficiency. S-HMPAO, therefore, has the potential to replace C-HMPAO as a leucocyte-labelling agent.

Radiopharmaceutical-related pitfalls and artifacts

The primary goal of this review article is to increase the reader's knowledge and understanding of problems associated with the radiopharmaceuticals commonly used in daily practice. To achieve this objective, problems related to the commonly used radiopharmaceuticals are divided into pitfalls and artifacts related to radiopharmaceutical preparation (technetium-99m [99mTc]-labeled and non-99mTc-labeled radiopharmaceutical) and those related to radiopharmaceutical administration. For the radiopharmaceutical formulation-associated pitfalls and artifacts, problems are discussed in terms of factor categories, such as factors associated with radionuclides, factors associated with components, factors associated with preparation procedures, and miscellaneous factors. As for the pitfalls and artifacts caused by radiopharmaceutical administration, these problems are categorized into errors associated with administration technique and nontechnical errors. Clinical manifestations (ie, appearance upon imaging) from the numerous literature-based examples are presented. The effect of the causative factors and the reason each factor can result in radiopharmaceutical preparation and administration problems are discussed. In addition, the possible preventive actions are presented for each group. However, the cause of some pharmaceutical related problems may not be easily recognized, and thus it is difficult to develop preventive and/or corrective plans for these cases.

Stabilisation of high-activity 99mTc-d,l-HMPAO preparations with cobalt chloride and their biological behaviour

It has been reported that the stability of a 1.11-GBq (30 mCi) technetium-99m d,l-hexamethyl-propylene amine oxime (HMPAO) preparation can be improved to up to 5 h by the addition of 200 micrograms CoCl2.6H2O within 2 min after reconstitution. However, it is not clear whether this method is also efficient for high-activity preparations (5.55 GBq) and whether this modified 99mTc-d,l-HMPAO has the same biological properties and can safely be used. We have now studied CoCl2-stabilised 99mTc-d,l-HMPAO preparations containing different amounts of "in-house" HMPAO ligand and SnCl2 and reconstituted with activities from 1.11 GBq to 5.55 GBq 99mTc. The characteristics of the generator eluates were also divergent, ranging from fresh eluates from a generator eluted less than 2 h previously to 4-h-old eluates from a generator not eluted during the preceding 72 h. Preparations containing up to 5.55 GBq 99mTc and as low as 2 micrograms SnCl2.2H2O can be efficiently stabilised for at least 6 h by the addition of CoCl2 shortly after reconstitution. Interestingly, it was found that the stabilisation method is not efficient if the cobalt ions are added prior to reconstitution of the preparation. This implies that the cobalt chloride cannot be incorporated in the labelling kit. Also, preparations with amounts of the ligand lower or higher than 0.5 mg formed the 99mTc-d,l-HMPAO complex with low radiochemical yield or showed rapid degradation. Therefore, combination of a subdivision and storage of Ceretec kits in fractions (as reported in the literature) is contra-indicated with this CoCl2 stabilisation method. CoCl2-stabilised Ceretec kits reconstituted with 5550 MBq 99mTcO4- and used 4-5 h after preparation retain the diagnostic usefulness of the fresh 1110-MBq preparation with regard to leucocyte labelling and brain imaging. Although baboon brain uptake of the stabilised preparation was 6%-9% lower, this small difference could not be distinguished in the tomographic images. The data obtained with both inhouse prepared d,l-HMPAO and Ceretec kits suggest that the eluate restrictions recommended by the Ceretec manufacturer can be neglected if the preparation is stabilised with Co2+ ions. Studies with 57Co-spiked CoCl2 added to d,l-HMPAO preparations demonstrated that the Co2+ ions clearly interact with the d,l-HMPAO ligand, probably to form one or more complexes. From biodistribution studies in mice it became evident that the toxicological profile of the Co2+ ions in the presence of d,l-HMPAO should be more favourable than that of cobaltous ions. For these reasons, it seems justifiable that CoCl2-stabilised 99mTc-d,l-HMPAO preparations should undergo rigorous studies to elucidate their clinical usefulness and pharmacological safety.