metaiodobenzyl guanidine ( mibg ) is a derivative of guanethidine and acts as an analogue of nor - epinephrine . therefore , it is concentrated in the sympatho - adrenergic tissues , especially in the chromaffin tissue of the adrenal medulla . cellular uptake of mibg is followed by uptake in intracellular storage granules facilitated by vesicular monoamine transporters . mibg is labeled with either iodine-123 ( i-123 ) or iodine-131 ( i-131 ) . even though i-123 labeled mibg is the radiopharmaceutical of choice for single photon emission computed tomography ( spect ) and in pediatric patients owing to favorable photon energy and lack of beta radiation , the availability of this radiopharmaceutical is limited especially in india . most centers in india continue to use i-131 mibg due to its lower cost and ready availability . use of i-131 mibg is complicated by high energy photons and beta decay which leads to sub - optimal imaging properties and higher radiation dose to the patient . i-131 mibg has been shown to localize in pheochromocytomas including sporadic , benign intra - adrenal and extra - adrenal , familial and malignant neoplasms . further , it is also used for initial evaluation , predicting response to therapy and detection of recurrence in neuroblastomas . it is a useful tool in therapy of selected patients with medullary thyroid cancer and carcinoid tumor . i-131 mibg is also used in therapy of variety of tumors showing avidity to mibg . it was first developed for adrenal scintigraphy and its human use was first reported by sisson et al . the biodistribution of mibg is characterized by uptake in normal liver , myocardium , salivary glands , intestines , spleen and excretion into urinary bladder . the normal adrenal medulla can be visualized with i-123 mibg and more rarely with i-131 mibg . the finding of non - physiological uptake denote the presence of a neuro - endocrine tumor ( net ) . false - positives are very rare and can be due to obstruction to urinary tract or adrenal gland hyperplasia following unilateral adrenalectomy . scintigraphy using i-131 mibg has been reported to have a sensitivity of 77 - 90% and specificity of 95 - 100% in detection of pheochromocytomas / paragangliomas . in general , sub - optimal sensitivity of mibg might be associated with relative lack of affinity to mibg to the net , lack of storage granules or loss of vesicular monoamine transporter ( vmat ) due to tumor dedifferentiation . i-123 mibg is a very important investigation in management of children with neuroblastoma and has been shown to have sensitivity of about 90% and specificity close to 100% . better results are obtained for tumors originating from the midgut , whereas only 11% originating from the foregut are visualized with mibg . medullary thyroid cancer shows variable avidity to mibg with one study showing a sensitivity of 38.7% . lower sensitivity is because of very small size of the lesions . in spite of the lower sensitivity the availability of hybrid spect / single photon emission computerized tomography ( ct ) has allowed for fusion imaging . few studies have elaborated the role of spect / ct in mibg studies[1619 ] but the experience with i-131 mibg spect / ct is limited . i-131 mibg spect / ct patterns in various mibg avid lesions are pictorially depicted in this study . scintigraphic images of 11 patients [ figures 111 ] who underwent iodine-131 mibg are represented in this pictorial essay . a 25-year - old male presented with hypertension for 10 years . iodine-131 metaiodobenzyl guanidine ( mibg ) whole body scintigraphy and single photon emission computed tomography / ct showed physiological distribution of mibg . patient is being clinically followed - up without evidence of neuro - endocrine tumor a 35-year - old male presented with palpitation and headache for 2 years . iodine-131 metaiodobenzyl guanidine ( mibg ) whole body scintigraphy and single photon emission computed tomography / computerized tomography showed mibg avid enlarged right adrenal gland suggestive of functioning neuro - endocrine tumor . the tumor was surgically resected and histopathology revealed pheochromocytoma a 25 year with neck pain and a previous history of metastatic involvement of cervical lymph nodes by neuro - endocrine tumor was referred for iodine-131 metaiodobenzyl guanidine ( mibg ) study . the scan showed a focus of mibg uptake in the right side of the neck . single photon emission computed tomography / computerized tomography localized the uptake to the region of carotid artery bifurcation indicating carotid body tumor a 40-year - old female , known case of hypertension and type 2 diabetes mellitus presented with an adrenal mass . iodine-131 metaiodobenzyl guanidine ( mibg ) whole body scintigraphy and single photon emission computed tomography / computerized tomography showed mibg avid functioning left adrenal neuro - endocrine tumor . the patient was operated upon and the mass excised and was proven to be cystic pheochromocytoma a 30-year - old male suspected to be a case of hydatid cyst ( on ultrasonography ) in the tail of pancreas developed accelerated hypertension upon handling of the mass inside the operation theatre . iodine-131 metaiodobenzyl guanidine ( mibg ) whole body scintigraphy and single photon emission computed tomography / computerized tomography performed showed mibg avidity in the mass . patient was subjected to repeat surgery and was proven to have neuro - endocrine tumor post operatively a 19-year - old male presented with accelerated hypertension and right adrenal mass . iodine-131 metaiodobenzyl guanidine ( mibg ) whole body scintigraphy and single photon emission computed tomography / computerized tomography showed mibg avid focus in the right suprarenal area . histopathology revealed extra - adrenal pheochromocytoma a 33-year - old male presented with abdominal pain and headache . scan showed soft tissue density mass at the aortic bifurcation ( organ of zuckerkandl ) . iodine-131 metaiodobenzyl guanidine ( mibg ) whole body scintigraphy and single photon emission computed tomography / ct showed mibg avid lesion in a soft tissue lesion at the level of l3-l4 vertebrae . surgical removal was done and the mass proved to be paraganglioma a 24-year - old male with history of hypertension for 5 years presented with elevated urinary vma . iodine-131 labeled metaiodobenzyl guanidine ( mibg ) showed mibg avid mass lesion in the right para - aortic region suggestive of functioning neuro - endocrine tumor which was later confirmed a 50-year - old male with hypertension and bony pains was referred for metaiodobenzyl guanidine ( mibg ) study . iodine-131 mibg study with single photon emission computed tomography ( spect)/computerized tomography ( ct ) revealed mibg avid lesions in the para - vertebral region and multiple vertebrae . spect / ct showed multiple areas of lytic changes in the visualized bones indicating widely metastastic pheochromocytoma . the patient was subjected to chemotherapy and had partial symptomatic relief a 2-year - old male child , known case of spinal ganglioneuroblastoma with elevated vma and mri evidence of right paravertebral mass post chemotherapy was referred for restaging . iodine-131 metaiodobenzyl guanidine ( mibg ) whole body scintigraphy and single photon emission computed tomography / computerized tomography showed mibg avid tumor in the right paravertebral region opposite l1-l4 vertebrae suggestive of residual disease a 52-year - old female known case of men2a with status post excision of pheochromocytoma was referred for iodine-131 metaiodobenzyl guanidine in view of elevated calcitonin . hybrid single photon emission computed tomography / computerized tomography images localized the uptake to the enlarged left lobe of thyroid gland and calcified lesions in the anterior mediastinum , likely due to medullary thyroid carcinoma with mediastinal lymph nodal metastases which was confirmed later all patients underwent whole body scinitigraphy and hybrid spect / ct images of the involved sites . whole body images were acquired 48 h after intravenous injection of 0.5 mci to 1 mci of iodine-131 labeled mibg ( i-131 mibg ) . patients were asked to discontinue any medication interfering with i-131 mibg uptake before the study . patients were administered syrup collosol iodine ( 25 ml / day ; 8 mg/5 ml ) the day before and 4 days after i-131 mibg injection to block thyroid uptake of any free iodine . all images were acquired in infina hawkeye 4 spect / ct scanner ( ge healthcare , milwaukee , usa ) fitted with a medium energy general purpose collimator . spect images were acquired in 64  64 matrix , in a step and shoot method covering 360 degrees in 60 views and 30 s per view . following the spect acquisition , ct was acquired in helical mode and tube current of 2.5 ma covering the involved area . hybrid spect / ct images were examined by evaluating the individual spect images , ct images and the fused spect / ct images .
metaiodobenzyl guanidine ( mibg ) is a derivative of guanethidine and acts as an analogue of nor - epinephrine and is widely used in the imaging of tumors of neuro - endocrine origin . 
 iodine-123 mibg has ideal imaging characteristics but is expensive with limited availability . 
 iodine-131 mibg is widely used in india and is cheap . 
 hybrid single photon emission computed tomography ( spect)/computerized tomography ( ct ) allows for anatomico - functional imaging and is being tried in mibg studies . 
 however , the experience with i-131 mibg is limited . 
 we present a pictorial assay of i-131 mibg spect / ct findings in various mibg avid tumors .