Clinical History
79-year-old male patient with chest pain radiating to the back. Hypertensive with concern for aortic dissection.
Imaging Findings
Figure 1a-c:
Axial arterial phase CECT demonstrates a round, hypoattenuating pseudothrombus (white arrow) with ill-defined margins that appears to be extending into the left atrium (LA) from the right atrium (RA). This “lesion” measures 89 HU.
Coronal reconstruction demonstrates the pseudothrombus (white arrow) within the IVC which appears to create a filling defect in the left atrium (LA). Anterior slices (not demonstrated here) show the defect continuing into the right atrium.
Sagittal reconstruction shows the pseudothrombus (white arrow) within the IVC and entering the right atrium (RA).
Figure 2 a-b:
15 minute delayed axial CECT images demonstrate resolution of the pseudothrombus which was previously seen within the right atrium (RA) and IVC (asterisk). Note the mitral annular calcification (white arrow).
Discussion
Flow-related artefact is a commonly encountered and sometimes overlooked phenomenon with early-phase contrast enhanced CT. Mixing and volume averaging of unenhanced blood with enhanced blood can imitate thrombus, referred to as pseudothrombus, or even masses. Flow artefacts can be problematic depending on their location. Mixing in the lower lungs can mimic pulmonary embolism while mixing within the heart, IVC or portal system may be mistaken for thrombus or neoplasm. [1, 2]
In this particular patient, mixing within the right atrium mimicked a mass or thrombus that appeared to cause a filling defect within the left atrium. Initially, it was called a left atrial mass originating from the interatrial septum. However, closer inspection of the images showed that this was only a flow artefact which appeared to extend into the left atrium. This was later confirmed by transthoracic echocardiography which demonstrated no mass or thrombus. Delayed images were overlooked which showed resolution of the “lesion” and would have confirmed that this was just an artefact.
Flow artefacts can be difficult to distinguish from real pathology. Mixing within the right atrium is thought to be accentuated by inspiration, during which negative intrathoracic pressures bring unopacified blood from the IVC. Avoiding exaggerated inspiration prior to scanning can help limit this phenomenon [3]. However, mixing may be unavoidable especially in situations where lung consolidation or atelectasis increases vascular resistance and causes focal slow pulmonary artery flow. [3] Atrial fibrillation also poses a problem due to stasis within the left atrium. Zimmerman looked at 402 patients with atrial fibrillation. 40 of which were found to have filling defects in the left atrial appendage mimicking thrombus. Only 9 of these 40 were confirmed to have true thrombus by transoesophageal echocardiography. [4]
Mistaking flow artefact for real pathology can result in unnecessary treatment, imaging or preclude procedures such as cardiac ablation for atrial fibrillation. Thus, it is critical for the radiologist to distinguish between the two. Imaging characteristics of ill-defined margins with Hounsfield units measuring greater than 78 confirms mixing artefacts. [1] Correlation with precontrast or delayed imaging can also identify flow artefacts in most situations. However, further imaging may sometimes need to be performed. [3, 5]
Differential Diagnosis List
Flow artefact simulating pseudothrombus/mass
Thrombus
Neoplasm
Flow Artefact
Final Diagnosis
Flow artefact simulating pseudothrombus/mass
