Transthoracic echocardiography (TTE) is an essential tool during cardiac arrest because it identifies potentially reversible causes (e.g., tamponade, massive PE, etc.).
One of the limitations of TTE is that it is sometimes difficult to assess the heart in less than ten seconds (i.e., during a pulse check) and good views of the heart sometimes hard to obtain. Transesophageal echocardiography (TEE) offers the potential to overcome these obstacles.
TEE not only allows continuous visualization and better imaging of the heart during arrest, but it also allows the assessment of compression depth, and whether the heart is being correctly compressed during CPR.
Here is what a TEE probe looks like, here is an example of a TEE during arrest, and here is a podcast by @ultrasoundpodcast on the literature for using TEE during cardiac arrest.