A comparative study of 99mTc and 131I in thyroid scanning

Thyroid scans performed with both 99mTc pertechnetate (99mTcO4) and (131I) were compared in 46 patients with palpable thyroid nodules to determine whether 131I scanning is any longer a necessary procedure. A discrepancy between the two types of scan existed in only three cases, in one of which the thyroid nodule showed uptake of 99mTcO4 but not of 131I. Subsequent surgery revealed a thyroid malignancy in this patient. In each of the other two discrepancies a nodule "cold" on 99mTcO4 scanning was apparently functioning on 131I scanning, and was found to be benign at surgery. As the convenience and lower radiation absorbed dose of 99mTcO4 patients compared with 131I make it a better scanning agent, it is recommended that 99mTcO4 scans of the thyroid be first nodules. If these nodules prove to be functioning equally with paranodular tissue, a 131I scan should also be performed to help exclude a possible thyroid malignancy.     

A comparative study of 99mTc and 131I in thyroid scanning

Thyroid scans performed with both ^99mTc pertechnetate (^99mTcO₄) and (¹³¹I) were compared in 46 patients with palpable thyroid nodules to determine whether ¹³¹I scanning is any longer a necessary procedure. A discrepancy between the two types of scan existed in only three cases, in one of which the thyroid nodule showed uptake of ^99mTcO₄ but not of ¹³¹I. Subsequent surgery revealed a thyroid malignancy in this patient. In each of the other two discrepancies a nodule cold on ^99mTcO₄ scanning was apparently functioning on ¹³¹I scanning, and was found to be benign at surgery. As the convenience and lower radiation absorbed dose of ^99mTcO₄ compared with ¹³¹I make it a better scanning agent, it is recommended that ^99mTcO₄ scans of the thyroid be first performed in the assessment of patients with thyroid nodules. If these nodules prove to be functioning equally with paranodular tissue, a ¹³¹I scan should also be performed to help exclude a possible thyroid malignancy.     

Discordant imaging of a thyroid nodule with 131I and 99mTc: concordance of 131I and fluorescent scans

A thyroid nodule, "hot" by 99mTc and "cold" by 131I scanning, was reimaged with a fluorescent scanner. The fluorescent scan was qualitatively similar to the 131I scan and demonstrated low iodine content in the nodule. This combination of scan patterns is compatible with an organification defect in the nodular tissue.     

Early kinetics of thyroid trap in normal human patients and in thyroid diseases

A six-compartment model is proposed for the study of early iodine kinetics (150 min) in the human thyroid gland including three compartments for extrathyroidal spaces. The results were compared with those obtained with an open two-compartment thyroid model. In six euthyroid subjects the values of the unidirectional thyroid clearance R41 = 86 +- 25 ml/min and of the irreversible clearance R65 = 22 +- 9 ml/min were consistent with those generally accepted. With TSH stimulation a clear increase in the thyroidal pump effectiveness was found without an increase in the irreversible clearance. In four untreated thyrotoxic patients a tenfold increase in the unidirectional thyroid clearance was found (p less than 0.01). The binding rate was much less increased and overlapped with the normal values; a large increase in the iodide compartment volumes was found. The unidirectional clearance remained elevated in seven patients after Carbimazole treatment and the binding rates were reduced. The six-compartment model demonstrated a rapid exit from the bound iodine compartment. In three of four goitrous patients studied, the thyroid unidirectional clearance was significantly increased ( p less than 0.001). The volumes of the iodide compartments were much higher than those observed in the other groups. The method was applied to analyse the kinetics of the different parts of the thyroid.     

Early kinetics of thyroid trap in normal human patients and in thyroid diseases

A six-compartment model is proposed for the study of early iodine kinetics (150 min) in the human thyroid gland including three compartments for extrathyroidal spaces. The results were compared with those obtained with an open two-compartment thyroidal model.In six euthyroid subjects the values of the unidirectional thyroid clearance R₄₁=86±25 ml/min and of the irreversible clearance R₆₅=22±9 ml/min were consistent with those generally accepted. With TSH stimulation a clear increase in the thyroidal pump effectiveness was found without an increase in the irreversible clearance.In four untreated thyrotoxic patients a tenfold increase in the unidirectional thyroid clearance was found (P<0.01). The binding rate was much less increased and overlapped with the normal values; a large increase in the iodide compartment volumes was found. The unidirectional clearance remained elevated in seven patients after Carbimazole treatment and the binding rates were reduced. The six-compartment model demonstrated a rapid exit from the bound iodine compartment.In three of four goitrous patients studied, the thyroid unidirectional clearance was significantly increased (P<0.001). The volumes of the iodide compartments were much higher than those observed in the other groups.The method was applied to analyse the kinetics of different parts of the thyroid.Research supported by DGRST (Contract 7371600) and INSERM (CRL 7750943 B)     

Radiation exposure due to 99mTc and 131I manipulated in syringes

Dose rates around and at the surface of disposable polypropylene syringes of 1 and 2 ml volume filled with ^99mTc and ¹³¹I solutions were determined by thermoluminescence dosimetry. Data for the efficacy of 0.5 and 1 mm lead shielding of ^99mTc and for the dose contribution due to ^– -radiation from ¹³¹I are given.     

Radiation exposure due to 99mTc and 131I manipulated in syringes

Dose rates around and at the surface of disposable polypropylene syringes of 1 and 2 ml volume filled with 99mTc and 131I solutions were determined by thermoluminescence dosimetry. Data for the efficacy of 0.5 and 1 mm lead shielding of 99mTc and for the dose contribution due to beta- -radiation from 131I are given.     

Comparison of 99mTc and 123I for thyroid imaging.

Thyroid images were obtained with 99mTc and 123I, using a scintillation camera and pinhole collimator, from 85 patients selected from over 1,000 patients predisposed to the development of thyroid neoplasms by prior radiotherapy. In 66 of 85 patients 99mTc and 123I gave essentially similar information, whereas in 19 patients some thyroid-image disparity was seen. In 11 of these 19, focal areas of increased 99mTc concentration were not seen with 123I. Radioiodide images showed slightly better contrast than the 99mTc images but required a longer imaging time and usually did not provide more information; in a few cases 99mTc showed abnormalities more readily. Regardless of which radionuclide was used, oblique views were needed to define some abnormalities. Currently 123I does not provide significant advantages over 99mTc for routine thyroid imaging. However, 123I is preferred in patients with very poor thyroid function and in those with suspected retrosternal thyroid tissue. In addition, areas of increased 99mTc concentration should also be studied with 123I.     

Pentavalent [99mTc]DMSA, [131I]MIBG, and [99mTc]MDP--an evaluation of three imaging techniques in patients with medullary carcinoma of the thyroid

Nine patients with histologically proven medullary carcinoma of the thyroid (MCT) were imaged using pentavalent [99mTc]dimercaptosuccinic acid [(V)DMSA], [131I] metaiodobenzylguanidine (MIBG) and [99mTc]methylene diphosphonate (MDP). Technetium-99m (V)DMSA demonstrated most of the tumor sites in eight patients with proven metastases, with an overall sensitivity of 95% in lesion detection. Iodine-131 MIBG showed definite uptake in some of the tumor sites in three of the nine patients imaged, with equivocal uptake seen in a further one patient, with sensitivity of only 11% for lesion detection. Technetium-99m MDP demonstrated bony metastases only, in four of the patients imaged yielding a sensitivity of 61%. Technetium-99m (V)DMSA has been demonstrated in this study to be a useful imaging agent in patients with MCT, showing uptake in significantly more lesions and with better imaging qualities than [131I]MIBG, and with the ability to detect soft tissue as well as bony metastases.     

Intercomparison of 131I and 99mTc activity measurements in Brazilian nuclear medicine services.

This work outlines the quality assurance program for the activity measurements of the most used radionuclides at Brazilian Nuclear Medicine Services (NMS). The program aims to guarantee that the patient is given the correct prescribed amount of activity in diagnostic or therapeutic applications. This accurate administration depends upon proper use and calibration of the activity meters by the NMS. Underestimation of administered activity in diagnostic practices could delay correct diagnosis disturbing the value of the investigation. On the other hand, the overestimation would be worse, mainly in therapeutic applications, because an unnecessarily high absorbed dose would be delivered to the patient. The preliminary results of intercomparison for 131I and 99mTc showed that many activity meters used at NMS's present problems giving results up to 41% greater than the reference values determined at the National Metrology Laboratory for Ionizing Radiation (LNMRI) which is recognized as the Brazilian authorized metrology laboratory for ionizing radiation. These results have demonstrated that the NMS should improve the accuracy of the activity measurements of the radionuclides administered to the patients and establish the traceability to the national standards of measurements. These standards are based on a pressurized well-type ionization chamber installed at LNMRI and calibrated with reference sources standardized by absolute methods. The protocol of the intercomparison and recommendations made in order to minimize errors in measuring procedures are described and results are discussed.     

A comparison of 99mTc and 123I scintigraphy in nodular thyroid disorders

Out of 450 patients referred for thyroid scintigraphy 83 consecutive patients with nodular goitre were selected for dual scintigraphy with ^99mTc and ¹²³I. Seventy-four patients had one or more palpable nodules and the other nine patients had palpably diffuse goitres with one or more nodules on the routine ^99mTc scintigram. For the comparison of scintigrams an eight level colour display was used, and regional differences between the ^99mTc and ¹²³I scintigrams were defined as a minimum of 25% (2 colour levels) variation of the relative activity in well defined areas.Thirteen patients, i.e. 16% of those with nodular goitre, had discordant scintigrams. All 13 patients had non-malignant thyroid disorders, (simple goitre, radio-iodine treated hyperthyroidism, autonomous adenoma, subacute thyroiditis, Riedel's and lymphocytic thyroiditis). The diagnosis was verified histologically in eight of these patients.Eight of the 13 patients had a high ^99mTc activity relative to ¹²³I, five patients showed the opposite discrepancy. Another 5 patients with a follicular or mixed follicular thyroid carcinoma had identical scintigrams.These findings indicate that ¹²³I does not offer such diagnostic advantages over the cheap and readily available ^99mTc, that its use for routine scintigraphy of the thyroid gland seems to be justified at present.     

Isolation and labelling of human leukocytes with 99mTc

The role of the leukocyte isolation procedure on cell labelling with ^99mTc has been evaluated. Separation of leukocytes was performed by two procedures: (1) sedimentation on methyl cellulose, followed by discontinuous gradient centrifugation; (2) methyl cellulose sedimentation and hypotonic haemolysis of residual red blood cells. After washing the cells in saline and incubation with a stannous pyrophosphate agent, the leukocytes were labelled with 5–10 mCi ^99mTc. Procedure 1 gave a higher purity but lower recovery of polymorphonuclear leukocytes, and a minor contamination of red blood cells. ^99mTc labelling of cells was slightly more efficient with this method, probably due to the presence of red blood cells. Procedure 1 is recommended for in vitro studies on cell kinetics and procedure 2 is recommended for clinical use.