Cardiac Imaging //
Recent advances in medical technology have enabled more information than ever before to be acquired non-invasively about your heart. Three of the most significant of these have been developments in Echocardiograms, Magnetic Resonance Imaging scanning (Cardiac MRI) and in Cardiac Computed Tomography (Cardiac CT or CAT scan). These give different, complementary information about the heart and can be used to provide a detailed and complete assessment of the heart structure and function.
CARDIAC CT SCAN
Although CT can be very useful particularly to look at any abnormalities of the pericardium, it is most commonly used to look for the presence of calcium within the coronary arteries. If patients have “hardening of their arteries” with cholesterol deposits being laid down, they quite quickly calcify, particularly in patients over 50, and this calcium can be picked up on a CT scan and the calcium score calculated. It was thought that this was an extremely useful test to screen for coronary disease but I think we are all a little disappointed with how it has turned out. Patients under 50 can have serious disease without calcification and we have seen this a few older patients too. In addition most older patients will have some natural calcification in their arteries and distinguishing critical narrowings can be extremely difficult. However a CT coronary angiogram can be performed when an iodine containing X-ray dye is injected into a vein in the arm and then the coronaries can be visualised with quite a high degree of accuracy. The patient is often given a small dose of oral or intravenous betablocker right at the beginning of the procedure to slow the heart rate down to make visualisation easier. The test is superb at excluding coronary disease, can be very good at showing critical coronary disease but unfortunately there are a group of patients in the middle who have moderate narrowings shown on the CT coronary angiogram and it may be difficult to tell whether these lesions are important or not and further testing may be needed.
CARDIAC MRI SCAN
MRI scans of the heart are difficult as the heart is a moving object but with special facilities cardiac MRI scans can be performed and can show us many different abnormalities. They do not involve the use of any radiation, unlike CT scans which use a moderate amount of radiation. We use them most frequently in looking for cardiomyopathies (disorders of the heart muscle) which can be particularly important in patients with rhythm disturbances particularly ventricular tachycardias. The scans can also be useful for evaluation of valve disease, congenital heart disease and can be used as a functional test to look at patients with suspected or known coronary artery disease, much in the same as echocardiography is used in a stress echo. An injection of contrast (Gadolinium) is usually used during these tests but this is quite different from Xray contrast used in CT scans which contains iodine. If a patient has had an allergic reaction to iodine, which does occur occasionally, they are unlikely to be allergic to Gadolinium.
Radionucleotide / Isotope Scans
These are not particularly used now within cardiac practice although used to be quite common. They were most commonly used as a functional test for those suspected of having coronary disease but now have been eclipsed by techniques such as stress echo and MRI scans which do not use radiation. I personally virtually never use these scans now.
TRANSOESOPHAGEAL ECHO (TOE / TEE)
Sometimes, to see the heart valves in particular very carefully, a transoesophageal echo (TOE, or in the USA, TEE) is needed. In a conventional echo there is quite a distance between the probe on the front of the chest and the valves and this can lead to a lack of resolution. During a transoesophageal echo the patient is usually sedated and a local anaesthetic sprayed on the back of the throat. The patient then swallows a small probe, rather like a gastroscope, and the echo probe on the tip of this gets much closer to the heart. It is particularly used to look at fine details of heart valves and their function and can be absolutely critical in patients suspected of having endocarditis (an infection of the heart).
An echocardiogram is an ultrasound test of the heart and is most frequently performed as a “transthoracic echo” (TTE) when the probe is placed in various positions on the chest including under the breast area and in the front of the chest and then different angulations are needed to visualise different parts of the heart. Echocardiography is particularly useful at looking at the function and size of the various chambers of the heart and also looking at the valves. It can also be used to be looked at the covering of the heart, the pericardium, and whether any abnormal fluid is contained within it. It is virtually always combined with Doppler which is a signal emitted by the same probe which helps to give us extra information, particularly on the function of the cardiac valves and how bad any narrowing or leakage is.
This is a specialised echo used particularly for looking at any abnormal communications or “holes” in the heart when some fluid is injected rapidly into a vein which then forms micro-bubbles in the heart and these can be seen travelling in an abnormal way if there is a faulty communication between various chambers of the heart.
A stress echo is a test where we are looking at changes in the function of the heart under an extra load. This is most commonly used when investigating patients suspected of having coronary artery narrowings or follow up of patients with already known coronary artery narrowings. Occasionally it is used to look at valve function. Most frequently we use “supine bicycle exercise” when the patient is lying and with a special exercise bike peddles to get the heart rate faster to put the heart under strain. Various drugs such as Dobutamine and Adenosine can also be used to stress the heart. Baseline images are taken of heart function and then the patient is put under stress and the function rechecked. If a patient has a significant narrowing in an artery, that part of the heart usually starts to malfunction as it is not getting its blood supply and that can be clearly seen on the echo.