Echocardiography is ultrasound imaging of the heart. It uses an ultrasound beam to scan the heart. The reflected ultrasound waves are analysed by a computer program to give the images of the heart. The device used to transmit and receive the ultrasound beam is known as the transducer. It generates ultrasound waves using piezoelectric crystals which oscillate according to the radiofrequency electrical signals fed into them. Typical ultrasound frequencies ranges from 2.25 – 10 Mhz. Lower frequencies have better penetration, but lower resolution. Higher frequencies have poor penetration, but better resolution. Higher frequencies are used to scan children and small babies while lower frequencies are used in adults. We need higher resolution, but less penetration in children and vice versa in adults. Highest frequency probes are used in new born babies. Higher frequencies can be used in adults in special modes of echocardiography like intracardiac echo and trans esophageal echo. In intra cardiac echo, a small tube with the transducer at its tip is introduced into the heart through a blood vessel. In trans esophageal echo, the transducer is at the tip of a device like an endoscope which is introduced into the food pipe (esophagus). The transducer in the esophagus is just behind the heart and very close to it. This avoids interference to the ultrasound beam from air in the lungs and allows high resolution imaging with higher frequency transducers.
The simplest mode of echocardiography available now is M-Mode or time-motion mode, which is graphical representation of the movements of various cardiac structures along a single imaging line. It has high temporal resolution (resolution in terms of time intervals) and is commonly used to take measurements of cardiac chambers in various phases of the cardiac cycle.
Two dimensional real time imaging gives the live images of the cardiac chambers and valves. This gives excellent anatomic information of the cardiac structures. Opening and closing of the valves can be visualized well. Structural abnormalities and abnormalities in the motion of the ventricular walls (walls of the lower chambers of the heart) are well studied by two dimensional echocardiography.
Three dimensional echocardiography is coming into vogue in more and more centres. It gives good spatial orientation of the cardiac structures. Real time three dimensional echocardiography is also called four dimensional echocardiography. Earlier devices used to acquire two dimensional images of the heart in multiple planes and produce three dimensional images by offline reconstruction. But increase in processor speed and evolution of advanced real time three dimensional transducers have made real time three dimensional (4D) scans feasible. Multiple special views like bird’s eye view and en face views of cardiac structures are possible with 4D scanning. It is useful in assessing detailed structure of the heart valves while planning valve repair. 4D echo is also good at delineating defects in the inter atrial and interventricular septa (partitions) of the heart.
Doppler interrogation measures the velocity of blood flow in various regions, especially across the valves and abnormal communications. Doppler information can be mapped on to the two dimensional image using colour flow mapping techniques (colour Doppler ehcocardiography) in a real time fashion. This give excellent information on valve functions.
Tissue Doppler imaging gives information on wall motion abnormalities. This helps in the delineation of regional differences in the contraction and relaxation of the heart muscles in those who have suffered a heart attack (myocardial infarction).
Contrast injection during echocardiography is done to assess defects in the walls separating the cardiac chambers as well as to assess the blood flow to various regions of the heart. While the contrast for visualizing septal defects is just agitated saline containing micro bubbles of air, specialized contrast material, again containing microbubbles are needed for assessing the blood flow to various regions of the heart muscle.
Advanced echocardiographic technologies include strain and strain rate imaging and speckle tracking. These are advanced technologies needing quite a lot of time and expertise in interpretation and hence not routinely done. Stress echocardiography is another advanced procedure which can assess the response of the heart muscle either to stress by exercise or with medicines. Dobutamine and adenosine are important drugs used in stress echocardiography.
Echocardiography is rapidly advancing and is a painless imaging procedure which can very well be done at the bedside, with even small hand held scanners.