Evaluation of 4 methods to localize parathyroid adenoma using 99mTc-MIBI

Full text: No radiopharmaceutical selectively accumulates in the parathyroid glands. Nevertheless, 201-Tl, 99m-Tc MIBI and 99mTc-Tetrofosmin are taken up by the parathyroid, as well as by the thyroid. Therefore different techniques were developed to separate parathyroid from thyroid gland. These include subtraction of the thyroid image from composite thyroid and parathyroid image using other radiopharmaceuticals that are taken up selectively by thyroid (Tc-99m or I-123). Dual phase imaging with MIBI is reported to be helpful as it shows faster washout from the thyroid compared to parathyroid. SPECT of the neck can also be performed to determine exact localization of increased tracer uptake. The aim of this study was to compare the sensitivity and diagnostic value of 4 methods of localising parathyroid adenoma using 99m-Tc-MIBI: (a) washout of MIBI from early and delayed planar images (b) image subtraction using early 99m-Tc-MIBI and 99m-TcO4- (c) image subtraction using late 99m-Tc-MIBI and 99m-TcO4- (d) late MIBI SPECT of the neck. 42 patients with clinically proven hyperparathyroidism were investigated. After an intravenous injection of 99m- Tc-MIBI (550 MBq) imaging was done as following: (i) LFOV image of the mediastinum at 5 min (ii) Zoomed planar image (early MIBI) of the neck region at 15 min (iii) SPECT of the neck (delayed MIBI SPECT), using zoom and body contour at 60- 90 min (iv) Zoomed planar image (delayed MIBI) of the neck region at 2 hrs. Thereafter 100 MBq of 99m-Tc pertechnetate was injected without moving the patient and zoomed planar image of the neck was acquired 10 min later (99m-Tc image). During data processing, washout image was obtained by subtracting the delayed MIBI image from the early MIBI image. Early MIBI/99m-Tc subtraction image was obtained by subtracting the 99m-Tc image from early MIBI image. For delayed MIBI/99m-Tc subtraction image, 99m-Tc image was subtracted from the late MIBI image. SPECT study of the neck was reconstructed using filtered back projection and displayed in transaxial, coronal and sagittal slices. All subtraction images ware calculated after normalisation and image realignment using dedicated Medicview software. All 4 methods were compared in all patients with thyroid/parathyroid palpation, ultrasound and surgical results. Sensitivity to detect parathyroid adenoma by early and late MIBI/99m-Tc subtraction scintigraphy was almost equal- 90% and 88% respectively (statistically insignificant). Dual phase 99m-Tc MIBI (washout subtraction) showed sensitivity of 73% (p < 0,05 vs. early and late subtraction scintigraphy and SPECT) but with possible higher specificity than both subtraction and SPECT methods. SPECT showed sensitivity of 86% but several cases not seen on subtraction scintigraphy were positive on SPECT. In one case of ectopic parathyroid gland in the mediastinum this was seen on both planar and SPECT images of the mediastinum, but SPECT allowed three-dimensional localisation. Combined sensitivity of all 4 methods was 96%. False positive results using all protocols were mostly due to thyroid nodules. In conclusion, combination of one, probably late MIBI/99m-Tc subtraction techniques with SPECT seems to give optimal sensitivity for the scintigraphic detection of parathyroid adenomas with no need for additional mediastinal image. (author)