Key points
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Positron emission tomography (PET) measures two annihilation photons of an emitted positron from a radionuclide-labeled tracer with a specific biochemical distribution, resulting in molecular imaging of biological nature. Moreover, reduction in patient radiation dose could be achieved as the positron emitter radionuclides are usually short lived. However, in spite of functional imaging on a biochemical level, PET alone suffers from lack of anatomical details. In order to overcome this disadvantage, hybrid scanners were introduced in which computed tomography (CT) or magnetic resonance (MR) images are registered with the PET images.
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Currently, 18 F-Fluorodexoglucose ( 18 F-FDG) is the most frequently used radiotracer for PET imaging. 18 F-FDG is transported into the intracellular space by facilitated transport similar to glucose. However, contrary to glucose, 18 F-FDG does not undergo further metabolism after phosphorylation. Thereby, this process allows mapping and measuring of the glucose metabolic activity. Despite the fact that elevated 18 F-FDG uptake could be observed in a variety of inflammatory or benign lesions, it remains as the cornerstone in the workup of many cancers . 18 F-FDG PET/CT imaging enjoys the potential of noninvasive preoperative determination of cardiac malignancies and detection of metastatic lesions originated from primary malignant cardiac tumors in a single study .
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Benign neoplastic cardiac masses are usually of no to low FDG avidity; therefore, PET/CT imaging can play an important role in discrimination of malignant from benign neoplastic cardiac masses. Be that as it may, particular attention should be paid to normal variations and potential causes of false positive findings for cardiac masses and malignancies in interpretation ( Boxes 15.1, 15.5, and 15.12 ).
Box 15.1
Normal variations.
Biodistribution of 18 F-FDG
18 F-FDG accumulation is observed in different body organs, including brain, heart, urinary system, gastrointestinal tract, liver, spleen, and bone marrow. Myocardial uptake follows glucose metabolism, which is defined by myocardial perfusion and function along with available substrate. As a result of elevated plasma glucose and insulin levels, 18 F-FDG myocardial uptake is increased after meal. Conversely, fasting state leads to decreased oxidative glucose metabolism while myocardial energy is supplied by fatty acids . Focal or diffuse myocardial uptake might occur after standard period of fasting for oncologic studies . Therefore, more prolonged fasting as well as consuming low carbohydrate diet is mandatory in order to decrease the normal cardiac background activity.
Ventricular uptake
Atrial uptake
Various left ventricular (LV) 18 F-FDG uptake patterns have been observed, including diffuse uptake throughout the LV myocardial walls, regional uptake in the lateral and posterior walls, and accumulation of radiotracer in the papillary muscles, which could sometimes be misinterpreted as thrombus or neoplasm when it occurs in the posterior muscle and in a globular pattern, or in base of the LV and posterolateral region, particularly in patients with a previous history of radiation therapy in an adjacent field to the heart ( Figs. 15.2 and 15.3 ) . On the other hand, increased uptake in the right ventricle (RV) has been reported to be correlated with RV dysfunction in patients with pulmonary hypertension .
The atrial uptake is uncommon but increased uptake in the right atrium is usually associated with cardiac disease, particularly atrial fibrillation, and is a source of false positive finding in cancer assessment . Moreover, 18 F-FDG uptake in the crista terminalis is variable but increased uptake might occur in this structure, mimicking cardiac mass or thrombosis . Right auricle is also a rare site of increased uptake with potential of misinterpretation as a pathologic mediastinal lymph node ( Fig. 15.4 ).
Box 15.5
Diagnostic pitfalls.
Different entities with the characteristics of increased accumulation of 18 F-FDG are known, leading to inaccurate diagnosis of a FDG-avid cardiac lesion. Therefore, the interpreting physician should be aware of potential causes of false positive findings for cardiac masses and malignancies.
Entity
Description
Lipomatous hypertrophy of interatrial septum
Lipomatous hypertrophy of interatrial septum (LHIS) is a relatively uncommon finding which is characterized by fat deposition in the interatrial septum. Despite the fact that this entity represents a histologic benign process, adverse sequels of supraventricular arrhythmias, syncope, and sudden cardiac death have been reported. Since this fatty infiltration can appear as a fat-containing mass-like lesion with 18 F-FDG avidity, other neoplastic processes, including myxoma, rhabdomyoma, rhabdomyosarcoma, and liposarcoma can be mimicked ( Fig. 15.6 ) .
Epipericardial fat necrosis
Epipericardial fat necrosis represents a benign entity of unknown etiology. The presenting symptom is usually pleuritic chest pain in a previously healthy subject. The CT component of PET/CT imaging allows characterization of the lesion as the presence of encapsulated pericardial fat density along with dense strands and/or thickening of the adjacent pericardium. The inflammatory infiltration of necrotic fat can explain the cause of 18 F-FDG accumulation in this lesion ( Fig. 15.7 ) .
Idiopathic hypertrophic cardiomyopathy
Glucose metabolism increases in hypertrophic myocardium as a result of decreased expression of beta-oxidation enzymes. Idiopathic hypertrophic cardiomyopathy (HCM) is a genetic disease in which 18 F-FDG uptake increases in the hypertrophic myocardium. The 18 F-FDG uptake could be even more prominent in obstructive HCM . HCM may present as a segmental FDG-avid mass-like lesion, which could be inseparable from cardiac metastasis in 18 F-FDG PET/CT imaging unless additional cardiac imaging is performed ( Fig. 15.8 ) .
Postradiation changes
Radiation therapy as a fundamental measure of multimodality therapy for some of the malignancies can lead to pericardial or myocardial injury. Both radiation-induced myocarditis and acute as well as chronic pericarditis have been introduced. In both situations, postradiation 18 F-FDG uptake may be falsely interpreted as malignant pericardial or myocardial involvement. Correlation with the radiation ports and CT component of PET/CT images as well as quantitative analysis by means of standardized uptake value (SUV) helps in differentiation of postradiation changes from malignant process ( Fig. 15.9 ) .
Cardiac sarcoidosis
Cardiac sarcoidosis is characterized pathophysiologically by accumulation of noncaseating granulomas in pericardium, myocardium, or endocardium of any of cardiac chambers, increasing the risk of morbidity and mortality in the process of the disease. Cardiac involvement can manifest as arrhythmias, heart failure, or sudden cardiac death. Despite the fact that active inflammatory process may pose diagnostic challenges in oncologic patients, the pattern of radiotracer uptake and the presence of hilar and mediastinal adenopathy or pulmonary involvement may help in differentiation between the two entities ( Fig. 15.10 ) .
Infective endocarditis
Infective endocarditis is a serious and potentially life-threatening condition. This condition can be visualized as a hypermetabolic focus in 18 F-FDG PET/CT imaging, which should be differentiated from a hypermetabolic cardiac mass ( Fig. 15.11 ) .
Box 15.12
Cardiac and pericardial masses.
Cardiac and pericardial masses encompass a heterogeneous broad spectrum of different entities, spanning from nonneoplastic to neoplastic processes of benign or malignant nature. In spite of rarity, they could pose significant diagnostic challenges . Moreover, the hemodynamic alteration caused by cardiac masses including blood flow blockage, valvulopathy, embolic events, arrhythmias, and functional dysfunction secondary to myocardial infiltration, results in a wide variety of clinical presentations . The patients’ symptoms are, therefore, nonspecific and are mostly related to the tumor location versus tumor type. The majority of primary cardiac tumors are benign in nature, whereas the main source of cardiac malignant neoplastic involvement is metastatic lesions from extra-cardiac malignant tumors. On the other hand, primary cardiac neoplasms are more often of benign nature and include myxoma , fibroelastoma, lipoma, fibroma, rhabdomyoma, and hemangioma in descending prevalence . 18 F-FDG PET imaging has been found to be advantageous to differentiate malignant from benign cardiac tumors .
Tumor type
Description
Myxoma
Cardiac myxoma is known as the most common primary cardiac tumor with benign nature. This tumor typically arises from the left atrium. Incidental detection of cardiac myxoma is not infrequent as a result of routine use of echocardiography and widespread utilization of CT . Cardiac myxoma can be detected with mild 18 F-FDG uptake in PET imaging ( Fig. 15.13 )
Fibroma
Cardiac fibroma is a fibroblast-derived benign connective tissue tumor which occurs predominantly in infants and young children. Cardiac fibroma is usually found in the septal and anterior walls of the left ventricle and might be visualized as cardiac masses of low to rather intense hyperactivity in PET images ( Fig. 15.14 )
Lipoma
Cardiac lipomas are well-circumscribed encapsulated tumors which are mainly composed of mature fat cells. This tumor can arise from subendocardium, subpericardium, or endocardium and is more frequently in the left ventricle compared to the right one. Unlike lipomatous hypertrophy of interatrial septum which can appear as a focal hyperactive area in PET/CT imaging, cardiac lipoma is not a FDG-avid lesion and is visualized as fat attenuation mass with no FDG uptake
Hemangioma
Cardiac hemangioma is a rare cardiac tumor that can occur in patients of all ages and at in any part of the heart. Hemangiomas are histologically classified as capillary, arteriovenous, and cavernous types. This tumor is usually visualized as mild FDG-avid mass in PET imaging ( Fig. 15.15 ) 
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