During an endomyocardial biopsy, a small amount of tissue is removed from the internal lining of the heart for testing. The biopsy helps diagnose and treat heart muscle disorders and is also used to detect rejection of a new heart after a heart transplant. A long, flexible tube called a catheter is inserted into a vein and threaded up into the heart. The doctor can guide the catheter by watching its movement on a monitor showing an X-ray image of the area. The tip of the catheter is fitted with tiny jaws that the doctor can open and close. Once the catheter is in place, the doctor will take several small snips of muscle for microscopic examination.