Echocardiogram is nothing but a ultrasound study (sonogram) of the heart. It can detect a blood clot in the left atrium. But TEE (trans esophageal echocardiography) is superior to trans thoracic echocardiography in detecting clots in left atrium. This is because left atrium is just in front of the esophagus and there is no interference by the air in the lungs which might cover the heart occasionally while imaging from the external surface of chest (trans thoracic electrocardiography). Moreover, since the distance is lesser, TEE probe can user higher frequency ultrasound imaging which gives better resolution of smaller structures. Higher frequency ultrasound has lower wavelength. The smaller the wavelength, better the resolution. But the attenuation of waves increases with distance. Hence smaller wavelengths cannot be used for larger distances.