20â MIN 99mTc THYROID UPTAKE: A SIMPLIFIED METHOD USING THE GAMMA CAMERA

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1 20â MIN 99mTc THYROID UPTAKE: A SIMPLIFIED METHOD USING THE GAMMA CAMERA H. P. Higgins, D. Ball, and S. Eastham St. Michael's Hospital and the University of Toronto, Toronto, Ontario, Canada Current methods of measuring the thyroid uptake of OsmTcpertechnetate have proved to be either too expensive or too time-consuming for wide clinical application. By affixing an in expensive lead shield to the face of a 4,000-hole collimator we can narrow the area of interest and obtain counts over the neck and mediasti num. These can be quantitated separately using the â œsplit-fieldâ option of a Nuclear-Chicago Pho/Gamma III camera. The percent osmtc thy roid uptake was calculated with the formula in this paper. In our studies 91 euthyroid patients had a mean 20-mm 99mTc uptake of 0.96% ± The mean uptake of 46 hyperthyroid pa tients was 1 1.5% of the dose, and there was no overlap with the euthyroid group. Fifteen pa tients on suppressive doses of thyroxine had a mean uptake of 0.06% ± 0.12, significantly less (p < 0.037) than the euthyroid group. As expected, some hypothyroid patients with Hashimoto's disease had tsmtc uptakes in the normal or hyperthyroid range. The test was very reproducible in the same patient with a technical error of ±0.069% of the dose in the euthyroid group. The results described com pare favorably with much more expensive and more time-consuming methods. Recent reports have attested to the value of 9omTc_ pertechnetate (OnmTc) for studying thyroid function as well as structure. ( 1â 5). Several authors have considered 9DmTc to be superior to 1311 for thyroid function studies because it results in a lower radia tion dose to the thyroid gland, it can be used for repeated tests on the same individual, it can be used in the presence of antithyroid drugs, it gives an ex cellent thyroid image, and the study can be com pleted at a single patient visit (2â 4). Moreover, the material is relatively inexpensive in a busy radioiso tope laboratory where technetium is used for brain scans, liver scans, etc. In spite of these advantages, ftomtchas not been widely used because the methods of quantitating up take by the thyroid were either too time-consuming or required very sophisticated and expensive modi fications to standard nuclear medical laboratory equipment. A simple, accurate, and inexpensive method was needed if the thyroid uptake of 9omTc was to become widely used as a test of thyroid func tion. The description of such a method is the sub ject of this report. MATERIALS AND METHODS A standard Nuclear-Chicago Pho/Gamma III camera with a split-crystal option and a 4,000-hole, low-energy collimator was used. A ½-in. lead sheet was affixed to the face of the collimator with a rec tangular aperture cut in the lead sheet. The dimen sions of this aperture could be varied (Fig. I ). After preliminary trials of different apertures, an aperture which was 10 cm wide and extended 7.2 cm above and 6.3 cm below the center of the crystal was used. The camera was positioned with the help of the oscilloscope so that the thyroid lay in that portion of the crystal above the division, and the mediasti num lay below the division (Fig. 2). Following in jection of 1 mci of nnmtc,the counts due to thyroid activity could be determined by subtracting the mcdi astinal area counts from the thyroid area counts. A standard containing an equivalent amount of OomTc ( 1 mciin a plasticbottlepositioned2.5 cm from the surface of a water bath) could be counted in Received May 26, 1973; original accepted July 18, For reprints contact: H. P. Higgins, Radioactive Isotope Laboratory, St. Michael's Hospital, 30 Bond St., Toronto, Ontario, Canada. Volume 14, Number

2 @ B. HIGGINS, BALL, AND EASTHAM Error due to distance from the collimator was de termined by measuring a source of 9omTc 10â 15cm from the collimator face, in a neck phantom filled with air or water, and with the neck phantom 10 cm from the collimator face (Fig. 3). The ability to :11@-â measure accurately the horizontal distance between two sources was tested by measuring two sources, 10 cm apart at different distances from the collimator face and the distance between the resulting images on Polaroid film was then compared (Fig. 4). CLINICAL STUDIES To test the validity of the neck background esti mation using the upper mediastinal counts, a total of 48 patients who had been referred to the Radio isotope Laboratory for brain scans were studied. FIG. 1. (A) Nuclear-ChicagoPho/GammaIll camerawith â /s-thesin.-thick lead shield affixed to collimator face and supported on brain scan had been given an oral dose of 500 mg patients, who as a part of the routine for the plywood frame (X). (B) Face-on view of aperture in lead shield (Y) 10 cm wide and of variable length. of potassium perchlorate 2 hr previously, were given a 1-mCi intravenous dose of 99mTc@pertechnetate. COUN THYROIDPHANTOM IN NECK PHANTOM PLACED 10 CMS. FROMCOLLIMATORFACE (counts at 2.0 cm. depth 100 p 0 1@ J@ 1'@.,. tt@ 1@ : A 20,261 11, â 0 neck phantom airfilled,tâ x neck phantom waterfilled cm depth of thyroid in neck phantom FIG.3. Thyroid phantom filledwithâ œmtc solution gavesame count when placed anywhere from 2.0 to 5 cm deep within air filled neck phantom. When neck phantom was filled with water, however, counting rate decreased with increasing depth. â FIG.2. Polaroid photof thyroidimagetakenwithlead shield in place, using split field option. Counts accumulated 21 mm after injection by each half of crystal are shown. the same manner. A correction was made for resid ual activity remaining in the syringe. The thyroid uptake was calculated from the for mula: % uptake = Thyroid area counts Mediastinal counts Counts of the dose (corrected for decay) DISTANCE FROM COLLIMATOR FACE POLAROID IMAGES OF 99m Ic 10cm APART AT VARYING DISTANCES FROM COLLIMATOR FACE SOURCES FIG. 4. Imageson Polaroidfilmof twommtcsourcesplaced 10 cm apart. Images remained 16 mm apart when parallel-hole x 100. collimator wasplaced 5,10,and15cmfromplaneofsources. 908 JOURNAL OF NUCLEAR MEDICINE

3 20-MIN nomtcthyroiduptake Neck and mediastinal counts were taken from 19 to 21 mm after injection (Fig. 5). Following comple tion of the thyroid study, the remaining 9 mci of OomTcpertechnetate was injected and brain scans performed in the usual way. Evaluation of 20-mm nomtcuptake. A total of 165 unselected patients referred to our nuclear mcdi cal laboratory for thyroid function studies had 9nmTc uptakes performed. In addition, all had complete history and physical examination, radioactive iodine uptake, serum thyroxine (Tetrasorb-125 ) and T3 resin sponge uptake (Trisorb-125). They were divided into four groups on the basis of combined clinical and laboratory findings. I. Control group : This group consisted of pa tients who were found on physical examina tion to have no evidence of goiter, who had normal serum thyroxine and T:@resin sponge uptakes, and who had had a normal radio active iodine uptake and scan. In a previous study (6) a large number of such patients were found to have thyroid function studies not significantly different from those obtained in chosen normals. 2. The hyperthyroid group : These were patients who were found to be clinically hyperthyroid and who have radioactive iodine uptakes, Se rum thyroxine, and T3 resin sponge uptakes compatible with this diagnosis. 3. Hypothyroid group: These were patients, some of whom had goiters and had evidence of Hashimoto's thyroiditis, who had serum thy roxine and T1 resin sponge uptakes well within the hypothyroid range. 4. Patients on suppressive doses of thyroxine: These were patients who had serum thyroxine levels in excess of and T resin sponge uptakes above 30% and who had 24-hr radio active uptakes below 5%. Reproducibility. Serial onmtc uptakes were done on 13 patients. A total of 33 tests were performed. The maximum interval between tests was 4 days, and each patient had from two to six tests per formed. RESULTS The change in counting rate for different distances from the surface of the collimator is shown in Fig. 3. There was no appreciable change in counting rate when the source was measured in air, but when it was measured in water there was a definite attenua tion. At 2.5 cm in depth a variation of 1 cm re suited in an 1 1% change in counting rate. The ability to measure distance between two FIG. 5. Lateralscintiphoto of neck20 mmafteri.v. injection of â Â mtc, showing method of estimating depth of center of thyroid. sources in the same plane is shown in Fig. 4. When two sources were placed 10 cm apart, the centers of the Polaroid images were 16 mm apart. This dis tance between the images remained constant when the perpendicular distance between the collimator face and the two sources were varied from 5 to IS cm. Therefore, a lateral view of the neck would per mit reasonably accurate measurement of the depth of tissue between the anterior surface of the neck and the center of the thyroid (Fig. 5). There was no significant difference between the mean mediastinal and the mean thyroid area counts in the 48 brain scan patients who had had their thy roid previously blocked by perchlorate (Fig. 6). Ninety-five percent of the mediastinal count values were between 85 and 1 15% of the neck count values. The reproducibility of the procedure was determined by calculating the technical error* of the 33 tests done on I 3 patients. It was 0.069% of the dose. The results of the onmtc thyroid uptake in the four clinical groups is seen in Fig. 7. The euthyroid group had a mean uptake of 0.96% with an ob served range between and 2.2% and a standard deviation of 0.52%. The distribution was slightly skewed. The hyperthyroid patients all had uptakes in excess of the normal range and in our series there was no observed overlap between the euthyroid group and the hyperthyroid group. In the hyperthy roid group there was better correlation of the 20- mm O9uuiTcuptake with the 10-mm 131! uptake (co * The technical error is the square root of the weighted average of the variances of each subject's tests. Volume 14, Number

4 HIGGINS, BALL, AND FIG.6. Scintiphoto ofthyroid andmediastinal areaswiththy roid blocked by perchlorate. Means of thyroid area counts and mediastinal area counts were not significantly different. efficient of correlation 0.8) than with the 24-hr 131J uptake (coefficient of correlation 0.4). In contrast, there was considerable overlap be tween the euthyroid group and the hypothyroid group. This was to be expected since some of the hypothyroid patients had goiters and might have intact or hyperactive trapping mechanisms although they were unable to produce adequate amounts of thyroid hormone since they had very low levels of circulating thyroxine. All the patients on suppressive doses of thyroxine had low or undetectable uptakes of pertechnetate at 20 mm. There was a significant difference between the mean of the euthyroid group and the mean of the group on suppressive doses of thyroxine (p = 0.037). liminary Polaroid exposure. In practice 95 % or 250 patients studied had thyroids whose centers were within 2 or 3 cm from the skin surface. Hence, a standard with its center placed 2.5 cm from the surface of the water bath eliminated the necessity of calculating the depth of the thyroid in the patient's neck. However, when a very large goiter is present or when the patient's neck is very thick, a correc tion for thyroid depth can be easily made by obtain ing a lateral view (Fig. 7). When combined with serum thyroxine and T:@resin sponge uptake, this test provides a complete assessment of thyroid func tion since a good thyroid image is obtained as an integral part of the procedure. The test gives a very good separation between normals and hyperthyroid patients and correlates better with the 10-mm uptake of 1311 than with the 24-hr uptake of 131!.This is not surprising because both the 10-mm 131! uptake and the 9nmTc uptake measure the activity of the â œtrappingâ mechanism. It does not give a good separation between hypo thyroid patients and euthyroid patients but does point out those patients who have a hyperactive trapping mechanism. As with the 10-mm uptake this may have prognostic significance with respect to the treatment of nontoxic goiters with thyroxine (7). OF DOSE 2OMINUTE â 9MTECHNETIUM THYROIDUPTAKE Euthyroid HyperthyroidHypothyroid N.T.G Thyroxine 91ftitients 46 Patients 13Patients 15Patients d(@ @ 15 DISCUSSION The method we described for measuring  uptake is one that can be readily duplicated in most well-equipped nuclear medical laboratories. It is simple and rapid enough to be used as a routine test of thyroid function. It takes approximately 25 mm of the patient's time and 8 mm of the technician's time. The chances of technical error are greatly re duced, since minor differences in distance from pa tient to collimator will not affect results. The position of the thyroid within the area of interest frame and the presence or absence of salivary gland activity in the area of interest frame can be readily checked with either a persistence oscilloscope or with a pre 10 5 FIG. 7. â œicuptakeof euthyroidgroupshowsnooverlapwith hyperthyroid group, and mean uptake of euthyroid group is sig. nificantly different from mean uptake of group on thyroxine. Nor mal and high uptakes in hypothyroid group were in patients with Hashimoto's disease. I. â 1 I JOURNAL OF NUCLEAR MEDICINE

5 20-MIN OOmTcTHYROIDUPTAKE The 20-mm 9DmTcuptake is suppressed to un detectable levels under normal circumstances by suppressive doses of thyroxine, so it will be useful as emphasized by Goolden, et al (3) in doing sup pression tests. It may be argued that the method used for meas uring neck background is insufficiently accurate. However, the results obtained in the clinical situa tions studied compare very favorably with those tients and patients suppressed by thyroxine. More over, any error estimation will be constant in a given patient since the volume of neck and mediastinal tissue â œseenâ by the â œthyroidâ area and â œmediastinalâ area of the crystal will be con stant. No test of iodine or nomtcconcentrating ability, i.e., no early uptake test, could be expected to dii ferentiate normals from hypothyroids. reported by other workers. The mean uptake of the Since errors in positioning the patient have been euthyroid group was 0.96% with an observed range reduced to an absolute minimum with this technique, from to 2.2%, which is nearly identical with serial studies can be obtained with an accuracy not that obtained by Hurley, et al (4) who reported a possible with a pinhole collimator no matter how mean for the normal group of 0.9% and a range sophisticated the associated equipment. from 0.24 to 3.4%, and with that obtained by This method should make possible the routine use Maisey, et al (5) for euthyroids without goiters of nnmtcfor thyroid function studies in most well who reported a mean of 0.9% and a range of 0.2â. equipped nuclear medical laboratories. 3.6%. It was very close to the results obtained by Any laboratory equipped with a gamma camera Goolden, Ct al using the dot scanning method (3). could use this method routinely for measuring the The mean for their normal group was 1.6% ±s.d. of 0.7%. Atkins also using a dot scanning method reported the mean uptake at 30 mm to be 1.73% ± 0.85% (2). None of these authors tested the accuracy of their method of estimating background in patients who had had their thyroids blocked with perchlorate be fore the administration of onmtc.burke showed that the administration of perchlorate after 119@'Tcwas an inaccurate method of estimating background, par ticularly in the hyperthyroid patients who had sig nificant residual radioactivity, presumably due to organic binding of the oonitc(6). Burke further mdi cated that counts obtained in areas above or below the thyroid really represented counts from volumes of tissue, and although the areas might be the same, the volumes were different and the resulting counts different. Our method using a mediastinal area smaller than the neck area overcomes, in part, Burke's objection to using counts from the mediastinal area to esti mate background. In the 48 brain scan patients who had their thyroids blocked by the prior administra tion of perchlorate, 45 had counts from the mcdi astinal area equal to the counts for the neck area ±10%. That this was adequate for practical clini cal purposes was born out by the results in the clini cal groups. Hyperthyroid and euthyroid patients could be clearly differentiated, as could euthyroid pa 20-mm  Tc thyroid uptake since the additional cost of the attachment ($45.00) is nominal in com parison with the cost of computerized attachments previously recommended for this test (4,7). REFERENCES 1. ANDROSG, HARPERPV, LATHROPKA, Ct al: Pertech netate-99m localization in man with applications to thyroid scanning and the study of thyroid physiology. I Clin Endo crinol Metab 25: 1067â 1076, A'riuNs HL: Technetium 99@ pertechnetate uptake and scanning in the evaluation of thyroid function. Sem NuciMed 1: 345â 355, GOOLDENAWG, GLASS HI, WILLIAMS EC: Use of 99 Tcm for the routine assessment of thyroid function. Br MedJ4: 396â 399, HURLEY PJ, MAISEY MN, NATARAJANTK, Ct al: A computerized system for rapid evaluation of thyroid func tion. J Clin Endocrino! Metab 34: 354â 360, MAISEYMN, NATARAJANTK, HURLEYPJ, et al: Vali dation of a rapid computerized method of measuring 99@ pertechnetate uptake for routine assessment of thyroid struc ture and function. I Clin Endocrinol Meiab 36: 317â 322, HIGGINSHP : The ten minute uptake of â 9: A clinical study and comparison with other tests of thyroid function. I Clin Endocrinol Metab 19: 557â 566, HIGGINSHP, ELKANI, DiosvA, et al: Prognosticvalue of high ten minute SM1uptake in nontoxic goitres. C.M.A.1. 91: 689â 693, BURKEG, HALKO A, SILVERSTEINGE, et al: Compara tive thyroid uptake studies with 1@@I Clin Endocrinol Metab 34: 630â 637,1972 Volume I 4, Number