**3.3.2 SPECT (Single Photon Emission Computed Tomography) or SPECT/CT imaging**

Although a radioiodine whole body scan is one of the excellent imaging tools for the detection of thyroid cancer, false negative results may be observed in cases with small recurrent lesions in an area of rather high background activity or in cases with poorly differentiated cancer tissues, which have low uptake ability for radioiodine (due to dedifferentiation).(Geerlings, van Zuijlen et al.) SPECT, which can provide cross-sectional scintigraphic images, has been proposed as a way to overcome the limitations of planar

Physiologic and False Positive Pathologic Uptakes on Radioiodine Whole Body Scan 9

diverticulum) or terminal oesophagus (Barrett's oesophagus). (Ma, Kuang et al. 2005) The

Uptake of iodine in the liver after radioiodine administration is related to the metabolism of radioiodinated thyroglobulin and thyroid hormones in the organ. The gall bladder also may occasionally be depicted with the biliary excretion of the radioiodine. (Shapiro, Rufini et al. 2000; Carlisle, Lu et al. 2003) A simultaneous hepatobiliary scan with Tc-99m DISIDA (Diisopropyl Iminodiacetic Acid) or mebrofenin is useful for characterizing the gall bladder uptake. Tracer accumulation in the oropharynx, nasopharynx and oesophagus is related to

Visualization of the oesophagus is extremely common and vertical linear uptake in the thorax that is removed by drinking water is characteristic of oesophageal uptake by swallowing of radioactive saliva. The oesophageal activity may also arise from gastric reflux. (Carlisle, Lu et al. 2003) Image acquisition after a drink of water is able to distinguish

Urinary or gastrointestinal anomalies can be responsible for false positive radioiodine uptake. (Ma, Kuang et al. 2005) Visualization of kidney and bladder after radioiodine administration is possible and this is known to be related to the urinary excretion of radioiodine into the urinary collecting system. Administered radioiodine is excreted mainly by the urinary system, and so all dilations, diverticuli and fistulae of the kidney, ureter and bladder may produce radioiodine retention.(Shapiro, Rufini et al. 2000) Visualizing the location of the renal pelvis of ectopic, horseshoe and transplanted kidneys is not usual and radioiodine at the pelvis may lead to misinterpretation. In fact, the renal pelvis and ureter are usually not visualized due to the rapid transit time of the radioiodine. (Bakheet, Hammami et al. 1996) A simultaneous renal scan with Tc-99m DTPA (Diethylene Triamine Pentaacetic Acid) or MAG3 (Mercapto Acetyl Triglycine) is useful for characterizing the urinary tract uptakes. (Shapiro, Rufini et al. 2000) Although the incidence is very uncommon, renal cysts are known to produce radioiodine uptake. The proposed mechanisms for the renal cyst uptake are a communication between the cyst and the urinary tract and radioiodine secretion by the lining epithelium of the cyst.

Tracer accumulation in the colon is very common. Incomplete absorption of the oral radioiodine administration is not considered as the reason of colonic activity due to the lack of colonic activity seen on the early images. Tracer accumulation is probably due to transport of radioiodine into the intestine from the mesenteric circulation and biliary excretion of the metabolites of radioiodinated thyroglobulin. (Hays 1993) Appropriate use of

Fig. 3. Physiologic uptake of radioiodine in the nasal cavity, the so called "hot nose". Intense tracer uptake was noted at the thyroid bed area (due to residual thyroid tissue), breast and

right lateral anterior left lateral

laxatives can be a simple remedy for the activity. (Shapiro, Rufini et al. 2000)

salivary gland (by the NIS expression of the glands).

ectopic thyroid and gastric mucosal tissues are able to take up radioiodine.

the activity from mediastinal node metastasis. (Shapiro, Rufini et al. 2000)

retention of salivary excretion of administered radioiodine.

(Shapiro, Rufini et al. 2000)

imaging and it is known to have higher sensitivity and better contrast resolution than planar imaging. Radioiodine SPECT has higher performance for detecting recurrent lesion compared to planar imaging in thyroidectomized thyroid cancer patients and it also changes the patients' management.

Radioiodine SPECT has excellent capability to detect thyroid cancer tissues, yet the anatomic evaluation of lesion sites with radioiodine uptake remains difficult due to the minimal background uptake of the radioiodine. The performance of SPECT with radioiodine may be further improved by fusing the SPECT and CT images or by using an integrated SPECT/CT system that permits simultaneous anatomic mapping and functional imaging.(Geerlings, van Zuijlen et al.; Spanu, Solinas et al. 2009) The fusion imaging modality can synergistically and significantly improve the diagnostic process and its outcome when compared to a single diagnostic technique. (Von Schulthess and Hany 2008) Therefore, SPECT/CT with radioiodine can demonstrate a higher number of radioiodine uptake lesions, and it can more correctly differentiate between physiologic and pathologic uptakes, and so it permits a more appropriate therapeutic approach to be chosen.(Spanu, Solinas et al. 2009) Despite its many advantages, SPECT/CT cannot be applied for routine use or whole body imaging due to the long scanning time and the additional radiation burden, and so the fusion image should be selected on a personalized basis for those who clinically need the imaging. (Oh, Byun et al. 2011)

### **3.3.3 PET (Positron Emission Tomography) or PET/CT imaging**

PET detects a pair of gamma rays produced by annihilation of a positron which is introduced by a positron emitting radionuclide and this produces three-dimensional image. Owing to its electronic collimation, I-124 PET gives better efficiency and resolution than in I-123 or I-131 SPECT, and so it offers the best image quality. (Rault, Vandenberghe et al. 2007) A fusion imaging modality with I-124 PET and CT can improve the diagnostic efficacy when compared to I-124 PET imaging by the same reasons of SPECT/CT over SEPCT only. I-124 PET/CT has superiority due to the better spatial resolution and faster imaging speed compared to I-123 or I-131 SPECT/CT.(Van Nostrand, Moreau et al. 2010) PET fused with MR is recently being used for research and in clinic fields and it will allow state of art imaging in the near future.
