Samy Sadek

Attenuation-Correction Induced Artifact in F-18 FDG PET Imaging Following Total Knee Replacement

Purpose: The clinical use of PET FDG in the work-up of patients with bone and soft tissue malignant tumors is rapidly increasing. The recognition of any source of artifact, therefore, is important to avoid interpretation pitfalls.Procedures: Two patients with complete knee joint replacement by metallic prosthesis in the course of their treatment for malignant bone and soft tissue sarcoma were evaluated by PET F-18 FDG imaging using a dual head coincidence gamma camera.Results: Both studies demonstrated in the attenuation-corrected images intense increase activity at the joint space between the metallic prosthetic surfaces at the level of the knee joint. No uptake, however, was noted in the same location on the non-attenuation-corrected images. Subsequent bone and thallium-201 scans confirmed the absence of tumor recurrence in the first patient. The second patient had multiple follow up F-18 FDG scans over a period of 16 months that show no changes from the baseline study.Conclusion: In the F-18 FDG PET images of patients with total knee metallic prosthesis, an intense activity tends to be seen in the joint space, only in the attenuation-corrected images. Such pattern of uptake is considered artifactual and should always be verified in the non-attenuated images.

The hollow skull: a sign of brain death in Tc-99m HM-PAO brain scintigraphy.

The advantage of Tc-99m HM-PAO, a newly introduced compound for brain perfusion imaging, is illustrated in three patients with suspicion of brain death. With Tc-99m HM-PAO, the carotid flow study is not essential, as it is in the Tc-99m pertechnetate carotid angiogram; planar images are equally useful; and there is no need for SPECT images. For the diagnosis of brain death, Tc-99m HMPAO can be injected in the intensive care unit and planar images can be obtained at a later time using a mobile camera or whenever the patient can be moved to the nuclear medicine department.

Is 99Tcm hexamethyl-propyleneamine oxime uptake in the tissues related to glutathione cellular content?

The relationship between the concentration of tissue glutathione (GSH) content and uptake of 99Tcm HMPAO in Sprague Dawley rats was investigated. The GSH content of rat tissue was depleted with diethyl-maleate (DEM) and the ratio of GSH in control to GSH depleted rat was approximately twice that in the brain, liver, kidney, spleen and lung. The GSH content in all the organs studied except the liver had no statistically significant relationship with the uptake of 99Tcm HMPAO. The apparent increase of radioactivity in the liver was due to longer retention of 99Tcm HMPAO. This longer retention was due to stasis of bile flow as confirmed by subsequent experiments in which cholecystokinin (CCK).was administered to GSH depleted rats and compared to the uptake of GSH depleted rats without injection of CCK.

Comparison of 99Tcm-mercaptoacetyltriglycine and 131I-orthoiodohippurate in determination of effective renal plasma flow (ERPF).

Recent studies have suggested that 99Tcm-labelled mercaptoacetyltriglycine (99Tcm-MAG3) is a potential replacement for 131I-orthoiodohippurate (131I-OIH). The two radiopharmaceuticals were studied in 19 patients with varying degrees of renal impairment, for comparison of effective renal plasma flow (ERPF). All patients were injected i.v. with 2-3 mCi 99Tcm-MAG3 and 250-300 microCi of 131I-OIH simultaneously in separate arms to avoid any chemical incompatibility. Serial digital images were acquired simultaneously for 99Tcm-MAG3 and 131I-OIH at 15 s per frame for 30 min. The renogram curve was generated and ERPF value was calculated for each kidney using both radiopharmaceuticals and applying the same computer program (Schlegels program). In all cases, the 99Tcm-MAG3 and 131I-OIH renogram curves were quite similar. The values of ERPF of 99Tcm-MAG3 were not significantly different from those of 131I-OIH studies (paired t test, p less than 0.01). The correlation coefficient between the two methods was 0.95 (p less than 0.001) with a linear regression equation Y = -12.96 + 0.995X, where Y = ERPF determined by 131I-OIH and X = ERPF determined by 99Tcm-MAG3. This study indicates that 99Tcm-MAG3 is a useful 99Tcm replacement for 131I-OIH in the determination of ERPF and generation of renogram curves.

New 75Se-labeled radiopharmaceuticals: 2. Selenium derivatives of aralkylamines.

Two 75Se-labeled aralkylamines, 2-(3,4-methylenedioxyphenylseleno)ethylamine hydrochloride (75Se-6) and 1-methylseleno-1-phenylethylmethylamine hydrochloride (75Se-9), were prepared with high specific activity applying two different chemical means starting from [75Se]selenious acid. Tissue distribution studies in rats show high uptake in the lungs with lung/blood ratios of 27/1 and 2.3/1 at 10 min for compound 75Se-9 and 75Se-6, respectively. High adrenal uptake and adrenal-to-blood ratio (15/1 at 2 h) of compound 75Se-9 were observed. This study indicates that aralkylamines can accommodate a Se group and still show high uptake by the organs that contain appreciable amount of dopaminergic receptors.

Evaluation of 99Tcm mercaptoacetyltriglycine for the detection and localization of gastrointestinal bleeding in an experimental animal model.

Technetium-99m mercaptoacetyltriglycine (99Tcm-MAG3) was evaluated for the detection and localization of the site of gastrointestinal bleeding in a sheep model. Radioactivity was detected in both the stomach and lower abdomen. However, 99Tcm-MAG3 is partially excreted by the liver and in the bile. The radioactivity in the bile moved to the small bowel. This movement of radioactivity in the lower abdomen can lead to a misinterpretation of the site of bleeding. Hence, 99Tcm-MAG3 may not be an effective radiopharmaceutical for the localization of the site of bleeding in the gastrointestinal tract, particularly the lower abdomen.

Technetium-99m Hm-pao Images (planar and Spect) of the Human Brain

Tc-99m HM-PAO has a great potential in the assessment of regional brain perfusion in various neurologic disorders. An atlas of planar and SPECT images of the normal and abnormal human brain has been generated using Tc-99m HM-PAO as the tracer.

99Tcm hydroxyethyl starch: a potential radiopharmaceutical for lymphoscintigraphy.

The usefulness of 99Tcm hydroxyethyl starch (99Tcm HES) has been evaluated for visualization of lymphatic channels and lymph nodes. 99Tcm HES was prepared with a labelling efficiency higher than 95%. Its usefulness for imaging lymphatic channels and lymph nodes were compared to 99Tcm human serum albumin (99Tcm HSA), 99Tcm dextran (99Tcm DXT) and 99Tcm sulphur micro colloid (99Tcm SMC) in rabbits. 99Tcm HES showed a good visualization of lymphatic channels and lymph nodes and it had the highest clearance rate from the injection site (P < 0.01). 99Tcm HES showed higher uptake by the nodes than 99Tcm DXT (P < 0.001) at 90 min post injection. The concentration of 99Tcm HES in the lymphatic channels was higher than that of 99Tcm SMC at 90 min post injection (P < 0.001). Preliminary studies of 99Tcm HES in humans provided high-quality lymphoscintigrams of the leg, and the pelvic and para aortic lymph nodes in less than 10 min post injection. In addition, partially and completely obstructed lymphatics could be differentiated from the normal one. Hence 99Tcm HES is a potential radiopharmaceutical for visualization of lymphatic channels and nodes.