  Echocardiography is the only technique capable of real-time imaging of the
heart and is vital for diagnosing the majority of cardiac diseases. However,
there is a severe shortage of experienced cardiac sonographers, due to the
heart's complex structure and significant operational challenges. To mitigate
this situation, we present a Cardiac Copilot system capable of providing
real-time probe movement guidance to assist less experienced sonographers in
conducting freehand echocardiography. This system can enable non-experts,
especially in primary departments and medically underserved areas, to perform
cardiac ultrasound examinations, potentially improving global healthcare
delivery. The core innovation lies in proposing a data-driven world model,
named Cardiac Dreamer, for representing cardiac spatial structures. This world
model can provide structure features of any cardiac planes around the current
probe position in the latent space, serving as an precise navigation map for
autonomous plane localization. We train our model with real-world ultrasound
data and corresponding probe motion from 110 routine clinical scans with 151K
sample pairs by three certified sonographers. Evaluations on three standard
planes with 37K sample pairs demonstrate that the world model can reduce
navigation errors by up to 33\% and exhibit more stable performance.
