**4.2 Tc-99m methoxyisobutylisonitrile (Tc-99m MIBI)**

Tc-99m MIBI, a lipophilic cationic molecules, was originally developed as a myocardial perfusion imaging agent. Today Tc-99m MIBI is also used for the study of many neoplastic diseases. The mechanism of Tc-99m MIBI accumulation in tumor has been reported to depend on the size of a tumor, the blood flow and the richness of mitochondria in the tumor

Post-Therapeutic I-131 Whole Body Scan in Patients with Differentiated Thyroid Cancer 243

Fig. 13. I-131, Tc-99m MIBI and F-18 FDG images. A 62-year-old woman with metastatic papillary thyroid cancer to the left proximal humerus, clavicle and sternum underwent I-131 therapy with 7.4 GBq (200 mCi). Her serum thyroglobulin level was 260 ng/mL. I-131 RxWBS (A) shows focal hot uptakes in the left clavicle and huemrus. On Tc-99m MIBI WBS, irregular hot uptakes are detected in the sternal area and faint uptake in the left clavicle (B). F-18 FDG PET/CT (C-E) shows multiple hypermetabolic lesions in the sternum, both hilar

Thallium-201 (Tl-201) is a monovalemt cationic radioisotope listed in same group with gallium in the periodic table. The membrane permeability for Tl-201 is almost equal to that for potassium (Elgazzar et al., 1993). It is suggested that intracellular accumulation of thallous ions is due to the transmembrane electropotential gradient (Mullins & Moore, 1960). The primary role of Tl-201 in nuclear medicine is for imaging of myocardial perfusion and viability (Pauwels et al., 1998). Also, Tl-201 chloride has affinity for various malignant tumors, although it is not specific for malignant tumor (Senga et al., 1982). Uptake of Tl-201 in tumor seems to be determined by blood flow, grade of malignancy, viability of tumor cells and tumor necrosis (Pauwels et al., 1998). There are several reports regarding Tl-201 and thyroid cancer. Shiga et al. compared FDG PET with I-131 and Tl-201 scintigraphy and reported that FDG uptake was concordant with I-131 in 38% and with Tl-201 uptake in 94% (Shiga et al., 2001). Senga et al. reported that when tumor showed a positive scan with Tl-

lymph nodes and lungs.

**4.4 Thallium-201** 

Fig. 12. Iodine negative and F-18 FDG PET positive case. A 65-year-old female underwent I-123 DxWBS (A) and F-18 FDG PET/CT (B,C) for elevated serum thyroglobulin level (3 to 17 ng/ml) after left modified radical lymph node dissection two years ago. I-123 DxWBS (A) does not show abnormal uptake. On F-18 FDG PET/CT (B), focal uptakes were shown in the left lateral neck (arrow). On axial view of fusion image (C), two hypermetabolic lesions were seen in the left cervical level II. Biopsy revealed recurred papillary carcinoma.

cells (Moretti et al., 2005; Piwnica-Worms et al., 1994; Saggiorato et al., 2009). MIBI irreversibly passes into the mitochondria using the electrical gradient generated by a high negative inner transmembrane mitochondrial potential of malignant cells (Chernoff et al., 1993; Moretti et al., 2005; Piwnica-Worms et al., 1994). Also, there is a report that TSH simulates both F-18 FDG PET and Tc-99m MIBI uptake in poorly differentiated papillary thyroid cancer in vitro experiment (Kim et al., 2009). Even though Tc-99m MIBI is inferior to I-131 scintigraphy in detecting I-131 avid lesions (Al Saleh et al., 2007), it can be applied for patients with elevated serum human thyroglobulin levels but negative I-131 whole body scan (Wu et al., 2003). However, there are several reports that F-18 FDG PET is more sensitive than Tc-99m MIBI SPECT in detecting metastatic cervical lymph node in differentiated thyroid cancer patients with elevated serum thyroglobulin level but negative I-131 whole body scan (Iwata et al., 2004; Wu et al., 2003) (Figure 13).
