Thrombus accounts for the most commonly encountered intra-cardiac mass. It can occur in any of the cardiac chambers, though it most often involves the left-sided structures. Thrombus formation can be caused by hypercoagulable states, systolic dysfunction with wall motion abnormalities, atrial fibrillation, or artificial devices. It typically appears as a hypodense, low-attenuation filling defect in a contrast pool within a cardiac chamber and may be differentiated from primary and secondary tumors by knowledge of predisposing risk factors, attachment location, shape, and lack of mobility.

Cardiac CT has very high sensitivity for excluding thrombus of the left atrial appendage but findings of low attenuation in the left atrial appendage (LAA) are not specific to thrombus as this often represents circulatory stasis, an incomplete mixing of contrast material and blood. This “pseudo” filling defect may mimic thrombus, especially in low-flow states. However, delayed imaging of the LAA may significant- ly improve the specificity to distinguish thrombus from circulatory stasis.

Left ventricular thrombi are often located in an area of myocardial hypokinesis, dyskinesia or aneurysm formation. They are frequently crescent-shaped filling defects with broad based attachments. However, a pedunculated appearance has been observed and can mimic myxoma. Chronic thrombi may develop spotty calcifications, though this feature has not been shown to significantly differentiate thrombus from myxoma. Thrombus within the left ventricle may be distinguished from myocardium by lower attenuation characteristics with a threshold of 65 HU providing a sensitivity and specificity of 94 % and 97 %, respectively.

2 patients with LV infarcts with mural thrombus

”When confronted with a cardiac mass, the most important imaging considerations are the location of the tumor, the possibility of metastatic disease, and the clinical presentation. When considering the differential diagnosis for primary malignant cardiac masses, location is usually the most helpful feature. Myxomas tend to be left sided, and their overall frequency skews left-sided heart masses as more frequently benign. Cardiac lymphoma and angiosarcoma, the most common primary cardiac malignancies are predominantly right sided. Necrosis, surface enhancement (“sun-ray” appearance), and valvular involvement favor angiosarcoma, whereas homogeneity and vascular encasement favor lymphoma. Other cardiac sarcomas tend to be left sided, specifically arising from the posterior wall of the left atrium.”
Cardiac Neoplasms Radiologic-Pathologic Correlation 
John P. Lichtenberger III et al.
Radiol Clin N Am 59 (2021) 231–242