Cardiovascular disease (CVD) is the leading cause of death worldwide. In the US, for example, CVD kills more men than the next seven causes of death combined, and more women than the next 16 causes of death combined. Some of the most dreaded effects of CVD result from atherosclerotic coronary artery disease (CAD) and its sequelae, including chronic angina, acute myocardial infarction (MI), heart failure (HF), and sudden cardiac death. Half of all heart attacks occur in the form of sudden death, often in relatively young individuals with no known history of heart disease. The fundamental process leading to symptoms and clinical events in patients with CAD is atherosclerotic plaque formation. Non-obstructive plaque may develop for years without appreciable manifestations; plaque may progress to produce partial luminal narrowing (stenosis) with subsequent exertional chest pain (angina) or, as often occurs in women, atypical symptoms such as neck or jaw pain elicited by activity. When a plaque completely obstructs the vessel lumen, either due to progressive narrowing or more commonly acute plaque rupture, blood flow ceases with death of downstream heart muscle cells (myocytes) if blood flow is not restored in a timely fashion.
Smokers have a 2.5-fold increase in the incidence of atherosclerotic heart disease, but a frightening 10-fold higher risk of sudden cardiac death. Of note, this same landmark study demonstrated a nearly 64% reduction in the risk of coronary heart disease (CHD) simply with smoking cessation. While smoking alone has grave consequences, an analysis of over 300,000 men demonstrated that when combined with other risk factors, smoking is even deadlier: smokers in the highest quintiles of hyperlipidaemia and hypertension were shown to bear a 20-fold greater risk of death. This effect seems especially nefarious considering that smoking independently raises lipid levels as well as blood pressure.
Innovative discoveries in medicine, heightened public awareness and advances in the quality of clinical care provided to patients have reduced the morbidity and mortality associated with heart disease. While much work remains in arresting the disease process before clinical manifestations occur, co-operation among cardiovascular medicine specialists, basic science researchers and engineers has produced tremendous strides in the treatment of acute coronary events during the last few decades. Consider that in treating heart attack patients 25 years ago, there was little to offer beyond bed rest and supportive care resulting in long hospital stays (often six weeks or longer) and chronic disability.
With advances such as percutaneous coronary angioplasty (PTCA), stent technology, and antiplatelet and thrombolytic drug therapy, and greater access to cardiac catheterisation laboratories with interventional capabilities, todayÔÇÖs patient with acute MI who seeks medical attention soon after the onset of symptoms may be successfully revascularised before any myocardial injury occurs. In fact, it can be difficult to convince patients of the severity and chronicity of their underlying disease when their chest pain that began 30 minutes prior to presentation has completely resolved an hour later after PTCA/stent deployment in a previously occluded coronary artery. To successfully provide life-saving coronary interventional technology requires accurate identification and characterisation of coronary artery lesions; X-ray angiography serves as the clinical workhorse to accomplish these objectives.
X-ray coronary angiography is not new; developed in the 1960s, it remains the clinical standard for detecting coronary artery stenoses. Until a few years ago, very little had changed in the conventional process of recording analogue X-ray images of coronary arteries filled with radio-opaque contrast. Digital flat panel (DFP) detector technology has revolutionised this process by replacing the traditional image intensifier (II) chain to produce digital image data at the time of acquisition. The first DFP system optimised for coronary angiography was the Innova2000ÔÇØ system (General Electric). This detector consists of a caesium iodide scintillator that converts X-ray photons to light. Light-sensitive amorphous silicon elements generate an electrical charge, and finally readout electronics convert each charge into a digital picture element (pixel) in the image. Formal studies documented superior image quality, leading to rapid dissemination of this new technology. The impact is greatest for the individual patient.
With the ever-growing volumes of diagnostic and interventional coronary angiography procedures, cumulative radiation exposure to both patients and staff becomes significant, with associated dose-related risks. This was nicely reviewed in Business Briefing: Global Healthcare. Excess radiation dose may produce significant acute and late effects, ranging from cutaneous manifestations to carcinogenesis. Additionally, contrast-induced renal insufficiency is a common complication of catheterisation in patients with compromised renal function, further motivating the development of techniques that reduce radiation exposure and contrast volume.
The American College of Cardiology (ACC) has published guidelines regarding procedural safety and imaging standards. To address the issues of radiation dose and image quality with DFP technology, we used a National Electric Manufacturers Association (NEMA) X-ray phantom specifically designed for testing cardiac imaging systems and found significantly reduced radiation dose and concomitantly superior image quality; Table 1 summarises the study's findings.
By freeing the angiographer from the II chain, DFP imaging allows development of novel approaches to cardiovascular imaging. One such application is rotational or spin X-ray angiography. In this technique, the DFP detector is rotated in a pre-programmed trajectory around the patient during one injection of contrast. This enables rapid display of multiple views of the coronary tree using a single injection of contrast and continuous cine acquisition during the rapid rotation. In a rigorous comparison with conventional multi-plane, multi-acquisition coronary angiography, it was found that DFP spin angiography reduced radiation exposure by 45.6% ± 10.1% (p<0.0001), reduced contrast volume by 27.9% ± 12.8% (p<0.0001), and resulted in comparable image and coronary stenosis severity assessment.
While DFP represents a significant advance in X-ray angiography, other imaging modalities are improving in speed and spatial resolution, with faster and more intuitive offline processing algorithms that result in greater utility to the clinician. Technologies such as magnetic resonance imaging (MRI) and multidetector computed tomography (CT) have shown promise in the non-invasive visualisation of coronary arteries. Their role in the management of CVD may be one of earlier detection and referral for coronary intervention, particularly as advances such as drug-coated stents provide justification for intervening with lesions of intermediate severity of stenosis. For this to occur, non-invasive imaging must offer more than traditional 'lumenology' - the rational development of these technologies would be to offer some insight into the composition of atherosclerotic plaque with an assessment of rupture risk. Such technology may identify the 60% coronary stenosis that is likely to rupture and cause an acute event as a lesion that merits coronary intervention, potentially before a clinical event occurs.
By 2010, CVD is estimated to be the leading cause of death in developing countries due to risk factors such as tobacco consumption, high blood pressure and high cholersterol. Healthcare accounts for a significant proportion of societal expenditures; in the US, CVD excluding stroke produced over US$274 billion in direct and indirect costs in 1998, more than any other major diagnosis.
What is often overlooked is the return on investment of these healthcare dollars. Imagine a 42-year-old male in the prime of a productive career struck by an acute coronary event due to occlusion of the left anterior descending coronary artery (LAD); without rapid utilisation of procedures such as coronary angiography and PTCA/stent placement, the patient develops chronic, debilitating angina, progressive left ventricular dysfunction, recurrent hospitalisations and concomitant limitations on activity. Alternatively, with timely intervention facilitated by technology coupled with appropriate lifestyle changes, the patient returns to a productive career with a high quality of life afforded by good cardiovascular health. Fifteen years later, the cost of their timely therapy seems insignificant compared with the subsequent benefits. Better public health studies, as well as analyses driven by better hospital information technology systems, are needed to quantify the long-term financial and societal benefits of what is often deemed costly technology. While efforts aimed at preventing smoking and identification and treatment of modifiable risk factors continue, today's patient benefits from the synergistic efforts between engineers, scientists, and cardiovascular medicine that have produced great strides in how we treat CAD. Involving clinicians during design and throughout the development and deployment cycle can insure that technology such as digital imaging improves rather than complicates patient care.