الفهرس 
ANATOMY OF ADRENAL GLANDOnly 14 pages are availabe for public view
 |  | from | 101 |  |  |
| | | from | 101 | | |
Abstract
PET/CT offers a unique hybrid imaging technique that combines the attenuation and morphologic detail of CT with the metabolic information from PET. These images can be fused to allow accurate coregistration of anatomic and functional data, and the combination of the two types of images leads to more assured anatomic localization of areas of increased metabolic activity.
Accurate anatomic localization of foci of increased metabolic activity can be difficult or impossible at stand-alone PET, particularly in the abdomen and pelvis, which are characterized by a lack of reliable identifiable anatomic structures and variable physiologic FDG uptake.
Adrenal PET-CT allows determination of the attenuation characteristics and the metabolic activity of adrenal lesions with a single examination and, in the majority of cases, should provide information that is diagnostic. The characterization of adrenal lesions with FDG PET depends on increased glucose metabolism in malignancy. Lesions with activity that is less or much greater than that of the liver can confidently be diagnosed as benign or malignant, respectively lesions with slightly increased activity relative to the liver are classified as indeterminate. Authors have reported up to 100% sensitivity and specificity for FDG PET in distinguishing benign from malignant lesions.
In general, malignant lesions show 18F-FDG activity equal to or greater than liver activity, although benign adenomas may also show increased 18F-FDG uptake. Because of low 18F-FDG activity in the normal adrenal, visualization of the gland is difficult with stand-alone PET. The range of 18F-FDG activity of the normal adrenal gland could not, therefore, be characterized before anatomic coregistered PET and CT (PET/CT) became available.
Metastases to the adrenal glands are common and can have a variety of appearances at CT. Up to 50% of adrenal masses in patients with known malignancy may be benign ;thus, non-invasive characterization is important in preventing unnecessary biopsy. Findings that are suspicious for malignancy include a size greater than 3 cm; ill-defined margins; concomitant metastases in other anatomic locations; a thick enhancing rim; heterogeneity; and poor contrast material washout. Central necrosis and hemorrhage may occur.
Adenomas usually do not show abnormally increased FDG activity, although there have been reports of false-positive moderate FDG uptake Why some adenomas show relatively increased FDG uptake remains unclear.
The functional state of an adenoma is presumed to be a factor in determining the intensity of FDG uptake, with increased uptake in functioning adrenal adenomas In addition, pheochromocytoma of the adrenal gland has been reported to show increased FDG uptake at PET.
Adrenal lesions that show equivocal increased activity can be further characterized with the CT component of the PET-CT examination ; one author has suggested incorporating delayed contrast-enhanced CT for washout analysis as a useful adjunct in characterizing lipid-poor lesions. Because it makes use of the full capability of both modalities, fusion PET-CT can help characterize lesions as adrenal adenomas, particularly those that are deemed indeterminate with CT or PET alone.
Hypermetabolic lesions do not always indicate malignancy. In general, false-positive FDG uptake can occur at PET-CT due to granulomatous disease, abscess, surgical changes, foreign body reaction, or inflammation (e.g., in diverticulitis, gastritis, or arteriosclerosis) .
False-positive findings may occur at adrenal FDG PET, particularly with respect to pheochromocytoma and adrenal hyperplasia, although approximately 5% of adenomas and, rarely, myelolipomas may also yield false-positive findings.
False-negative PET findings can result if tumors are either too small or non -FDG avid. The latter group includes some neuroendocrine tumors, renal cell carcinoma (RCC), and certain types of lymphoma, although most lymphomas are FDG avid. High neighboring background activity can also obscure FDG uptake.
Metastatic lesions from pulmonary adeno-carcinoma with a predominantly bronchioloalveolar carcinoma component may also show little FDG uptake at PET-CT. Hemorrhage and necrosis have been reported to be the other common causes of false-negative FDG PET results.
Increased suprarenal FDG uptake due to brown fat, which may mimic an adrenal lesion, is another pitfall of adrenal PET.
In conclusion, PET-CT combines complementary modalities, thereby allowing precise structural and functional characterization of a variety of conditions affecting the adrenal gland, with subsequent significant impact on clinical management. A variety of pitfalls are inherent in the use of either modality alone, so that combined PET-CT provides added diagnostic value. However, PET-CT also has certain limitations. Benign entities (eg, lipid-poor adenomas) may show increased uptake at FDG PET and may be indeterminate at standard CT. Moreover, the combined information from PET-CT, although often useful, will not always obviate additional studies or biopsy.
A negative PET/CT image can be used to exclude metastases with a high degree of certainty. False-positive results, however, occur with subtle increases in FDG activity that may need clarification with alternative imaging methods or adrenal lesion biopsy. In the case of a positive PET scan, incorporation of delayed contrast-enhanced CT for washout analysis is a useful diagnostic adjunct when determination of the nature of the adrenal disease is essential for patient treatment.