Left atrial thrombus – Contrast CT Scan: Contrast is seen filling the left atrium (LA) with a negative shadow (marked by red arrow). Negative shadow is due to the thrombus in the left atrium (LA thrombus). Circular shadow behind and to the left, seen adjacent the vertebral body is the descending thoracic aorta in cross section (Ao). Chamber anterior and to the right of the left atrium is the right atrium (RA). Large contrast filled chamber anterior and to the left of the left atrium is the left ventricle (LV). Small triangular contrast filled chamber just posterior to the sternum is the right ventricle (RV). Lungs are seen as black structures on either side of the heart due to the air content with low radiodensity. Small contrast filled shadows in the lung fields are the cross sections of branches of pulmonary vessels.
A meta analysis of 9 studies including 1646 patients showed that contrast CT scan has good diagnostic accuracy for detection of left atrial thrombi . The mean sensitivity for diagnosis of left atrial / left atrial appendage thrombus was 81% (95% CI: 70-90%). The mean specificity in the study was 90% (95% CI: 88-91%).
There may be difficulty in differentiating spontaneous echo contrast from solid thrombus in the left atrial appendage by transesophageal echocardiography (TEE). In a study of 102 patients with suspected acute cardioembolic stroke/transient ischemic attack without chronic atrial fibrillation, TEE indicated 10 spontaneous echo contrast and three thrombi. But a consensus reading of contrast CT and TEE indicated that all thrombi detected in TEE were false positive. Contrast CT detected three actual thrombi missed in TEE .
Another study aimed at devising an optimal protocol for cardiac CT based on TEE as the reference standard . 260 patients referred for radiofrequency catheter ablation of atrial fibrillation underwent contrast CT and TEE within 2 hours. Thrombi were defined as filling defects persisting at 6 min delayed acquisition. All the 10 thrombi detected by TEE were correctly identified by delayed contrast CT.