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Non-invasive imaging: the gatekeeper to coronary procedures 

EUROECHO 2012 Congress News

Topics: Echocardiography (Non-invasive imaging)
Date: 08 Dec 2012

Luc Piérard, from the University of Liège, Belgium, outlined how for patients presenting with chest pain, but not an AMI, non invasive imaging is frequently used as the ‘gatekeeper’ to coronary angiography to determine whether an invasive procedure is necessary.

“The reality is that around 85% of patients complaining of chest pain in the emergency room haven’t suffered an MI, but have chest pain secondary to anxiety or thorax or oesophageal problems,” he said.

If these patients do not display overt ECG abnormalities or raised levels of troponin A levels, but algorithms suggest they may have an intermediate risk for CAD (defined as between 10 and 90%) they will be offered a non-invasive imaging procedure.

Non-invasive imaging also has a role to play for patients found to have CAD at angiography, says Sven Plein from the University of Leeds, UK. In these patients, non-invasive imaging provides prognostic information and guides clinical decision making regarding the need for percutaneous or surgical revascularisation.

“Non-invasive imaging shows whether a coronary stenosis is functionally significant and what the potential for recovery of contractile function in response to revascularisation is, and therefore whether it’s worthwhile undertaking such procedures,” says Plein.
SPECT stress-redistribution Thallium-201 images, demonstrating apical, inferior and inferolateral wall ischaemia. 

Luc PierardStress Echocardiography:  Luc Piérard, Liege, Belgium

What is it? Stress echocardiograms utilise ultrasound images of the heart to assess wall motion in response to physical stress. Patients able to exercise undergo exercise stress testing on treadmills or bicycles; while patients unable to exercise use pharmacological stressors (such as dobutamine or dipyridamole infusions) to increase myocardial oxygen consumption. The basic premise underlying use of echocardiography in ischaemic heart disease is that if a region of the left ventricle is rendered ischaemic it develops wall motion abnormalities.

History: In 1935 Tennant and Wiggers first demonstrated that coronary occlusion immediately resulted in instantaneous abnormalities of wall motion. The first studies in stress echocardiography were published in 1979 using dedicated exercise tables, with a key milestone being opacification of the left ventricle by a contrast agent (small microbubbles injected in a vein) permitting better delineation of the endocardium.

Use: Wall motion abnormalities are detected with echocardiography and serve as a marker of the location and extent of the ischaemic myocardium.

Advantages: Widely available (all hospitals have the equipment), inexpensive, and does not require ionizing radiation. Also able to identify problems with valves.

Disadvantages: Around one in 20 patients are poorly echogenic due to obesity or lung problems like emphysema. Extensive experience is needed for the interpretation of wall motion abnormalities.

What the future holds? Stress echocardiography using speckle tracking which would not be influenced by the angle.
Cameras and the use of stress-only protocols will reduce radiation exposure  and shorten the imaging time. It is also expected that use of selective vasodilator agents will increase, thus reducing the side effects of the older stressors. 



Sven PleinCardiac Magnetic Resonance (CMR):
Sven Plein, Leeds, UK

What is it? CMR uses strong magnetic fields and radio waves to interrogate structures in the body. Nuclei with an uneven number of protons aligned along the magnetic field, and when subjected to appropriate radiofrequency signals are transiently lifted to higher energy states. They subsequently emit this energy which is received and translated to images using multiple steps of signal encoding.

History: While the first magnetic resonance (MR) machines were introduced in the early 1980s, initial attempts to image the heart were confounded by respiratory and cardiac motion. These issues were solved by cardiac ECG gating and faster scanning protocols.

Use: In CAD allows accurate measurement of global and regional contractile function, detection of myocardial ischaemia and scar tissue.
Advantages: Generates images of the heart with good tissue contrast providing discrimination between blood and myocardium and between tissue states (i.e. viable versus infarction). Images are highly reproducible, and there is no ionizing radiation.

Disadvantages: Patients with some magnetic or ferromagnetic devices cannot undergo the procedure, and some patients may experience claustrophobia within the scanners. Little data is available on cost-effectiveness in comparison to other imaging modalities.

What the future holds? Studies showing whether use of CMR translates into better patient outcomes. Initiatives to make the technology more widely usable by non-experts.
 

Francesca PuglieseCardiac Computed Tomography: Francesca Pugliese, London, UK

What is it?
In Computed Tomography (CT) scanning, thin x-ray beams are projected into the body area under investigation and are detected by very thin detectors (0.5-0.6 mm) to create high resolution cross-sectional images.

History: The first CT scanners were introduced in the 1970s and since then this technology has become widely used to image all body parts (e.g. the head, the chest, the abdomen etc). The application of CT for imaging of the heart became possible in the late 1990s, with the introduction of multidetector CT scanners that were faster than previous generations of scanners and relied on cardiac ECG Gating.

Use: In Europe cardiac CT is used mainly to perform non-invasive coronary angiography in patients with chest pain in non-emergency situations.

Advantages:
Cardiac CT is fast, widely available, inexpensive and produces high resolution images of the coronary arteries non-invasively. It also allows the visualisation of earlier stages of disease.
 
Disadvantages: Cardiac CT requires exposure to x-ray radiation. With the latest CT scanners and good expertise, however, the amount of radiation to the patient is lower than invasive coronary angiography.

What the future holds? Current research is trying to establish if this technique is robust in detecting myocardial ischaemia as well as coronary artery disease. Also, large prospective studies are needed to show whether cardiac CT improves patient outcomes and can be used as a guide to treatment.


Anastasia KitsiouSingle photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI):
Anastasia Kitsiou, Athens, Greece.

Anastasia Kitsiou, Athens, Greece.

What is it? SPECT MPI uses radiotraces to produce tomographic images of the heart and assess myocardial perfusion and viability. Patients exercise on a treadmill, cycle ergometer or undergo pharmacological stress with vasodilators or dobutamine. The radioactive tracers used are Thallium-201, Tc-99m sestamibi and Tc-99m tetrofosmin. Abnormalities on SPECT images are seen as darker areas compared with normally perfused myocardial segments.

History: MPI has been used for detection of ischaemia and assessment of prognosis in patients with suspected or known CAD for more than 30 years.

Use: SPECT images may reveal normal perfusion or perfusion defects. The location and extent of defects reflect the location and extent of underlying CAD.

Advantages: Widely available, objective non-operator dependent, large patient populations have been studied both in terms of diagnosis and prognosis of CAD, making SPECT MPI a well established imaging technique.

Disadvantages: It requires ionizing radiation.

What the future holds? Advances in image-reconstruction algorithms, new generation cameras and the use of stress-only protocols will reduce radiation exposure and shorten the imaging time. It is also expected that use of selective vasodilator agents will increase, thus reducing the side effects of the older stressors.

Authors: Janet Fricker
EUROECHO 2012 Congress News