Dr Sundeep Mishra, Prof of Cardiology, AIIMS, New Delhi and Editor-in-Chief, Indian Heart Journal elaborates on how innovation is tranforming radiology in India
The mission of the life sciences community is to prolong life, and to make it more healthy, comfortable and productive while it lasts. In this context, it can be argued that medical technological innovation has been the lifeblood for increasing life expectancy over the past century. Worldwide, the life-expectancy at birth was 31 years in 1900, 47 years in 1940, 61 years in 1980. This early improvement can be attributed to drug and chemical innovations: penicillin, streptomycin, vaccines, discovery of DDT, etc. But after this, between 1960 and 1997, there was a further 45 per cent increase in life expectancy in 30 developing and high-income countries, but now the focus shifted from infectious diseases to develop therapies for life-style diseases. For example during 1950 to 1995, the cardiac disease mortality in the US fell by more than half, increasing the overall life expectancy by three and half years. Nearly two-thirds of this could be attributed not to reduction in infectious diseases but to establish coronary care units, treatment of hypertension, and surgical treatment of CAD. Likewise, there was nearly 22 per cent decline in cancer deaths since the 1990s. Thus medical innovation has been the prime mover for mortality reduction for humans in recent times.
Innovation and medical imaging
The other goal of healthcare technology is to make healthcare management safer and less invasive. Since Roentgen discovered X-rays in 1895, there has been a near continuous innovation in medical imaging. After World War II, there has been a focussed interaction between computerisation and imaging technologies: computed tomography, magnetic resonance imaging, nuclear imaging, and ultrasound-positioned medical imaging, which has led to transformation of healthcare science. Currently, numerous techniques are available for imaging heart and provide valuable information for guiding diagnosis, patient assessment, and therapeutic intervention. They can be broadly divided into invasive and non-invasive.
NON-INVASIVE TECHNOLOGIES
Nuclear cardiology
Nuclear medicine is a branch of medical imaging that uses small amounts of radioactive material to diagnose and determine the severity, treat a variety of heart diseases. These techniques are minimally invasive and can reveal physiologically the blockage within the coronary arteries. Because nuclear medicine procedures are able to pinpoint molecular activity within the body, they offer the potential to identify disease in its earliest stages as well as after treatment. They are particularly useful in deciding if an invasive procedure like angioplasty or bypass surgery is required in a given patient. (See Figure 1) Cardiac SPECT (single photon emission computed tomography) scans assess the blood flow within heart vessels. The scans can use radio-sotopes like 201 thallium or 99mTc sestamibi or 99mTc tetrafosmin. MUGA (Multiple Gated Acquisition) Scan on the other hand is a test that is used to evaluate heart function. Positron emission tomography (PET) is the most recent advance in this area. It allows myocardial perfusion imaging i.e. assessment of regional myocardial blood flow more accurately than any other conventionally available technique like SPECT. The process involves intravenous injection of a positron-emitting perfusion tracer, such as 13N-ammonia, 15O-water, or 82Rubidium, and tracking the flow of the radiotracer passing through circulation and heart. This assessment both at rest and during various forms of vasomotor stress provides insight into early and subclinical abnormalities in coronary arterial vascular function and/ or structure, noninvasively. Further, it can throw light on myocardial flow reserve and even viability of myocardium, most accurately predicting whether patient will benefit from angioplasty/ bypass surgery and even monitor therapeutic interventions. It serves as a surrogate marker for coronary vascular health. (See Figure 2).
Echocardiography
The ultrasound technique, or echocardiography, is perhaps the greatest revolution in imaging technology for heart. It is a non-invasive modality which has figured most prominently in the structural diagnosis of a variety of heart conditions (other than coronary disease), such as valve disease, septal defects, myocardial and pericardial abnormalities. Echocardiography, including tissue Doppler imaging offers useful flow assessment and other functional information. (See Figure 3) Echocardiography employs high-frequency sound waves to generate visual images of cardiac anatomy and function like sonar. It utilises two approaches; trans-thoracic (TTE) and trans-esophageal (TEE). There have been several advances in the field of echocardiography. Portable ultrasound devices that weigh less than five pounds are capable of performing a complete bedside echocardiographic examination. The information provided has become so reliable and useful that some clinicians have even predicted demise of the stethoscope. (See Figure 4).
Stress echocardiography: It is the combination of echocardiography with a physical, pharmacological, or electrical stress. The diagnostic endpoint for the detection of myocardial ischemia is the induction of a transient worsening in regional function during stress. Stress echocardiography provides similar diagnostic and prognostic accuracy to nuclear imaging, but at a lower cost, without environmental impact, and with no biohazards for the patient and the physician.
3-D Echocardiography – One of the most significant developments of the last decade in this area is the evolution of 3-dimensional (3D) imaging which has for the first time made assessment of complex cardiac anatomy possible. Significant advances in three-dimensional echocardiography have made this modality a powerful diagnostic tool. It can provide accurate and reliable measurements of chamber size and function and offers novel views and comprehensive anatomic definition of valvular and congenital abnormalities, improving diagnosis and preoperative planning. Further, it can be extremely useful in monitoring the effectiveness of surgical or percutaneous transcatheter interventions so much so that 3-D echocardiography has become part of the routine clinical diagnostic armamentarium. (See Figure 4)
Contrast echocardiography: It has evolved rapidly in the last decade, with major developments in both contrast media and ultrasound equipment. This technology employing the use of microbubbles has been used predominantly for the detection of intracardiac shunts, but can also be utilised to optimise assessment of left ventricular function during stress echocardiography and when imaging is suboptimal. Further with refinement in technology it can be used as a tool for the routine clinical assessment of myocardial perfusion. The future holds the promise of new contrast agents capable of imaging specific abnormalities in the vasculature, such as thrombi or damaged endothelium, while the next stage in this rapidly developing field is likely to move ultrasound from diagnostic technique to therapeutics. (See Figure 4)
Magnetic resonance imaging
Cardiac magnetic resonance imaging (MRI) is a medical imaging technology for the non-invasive assessment of the function and structure of the heart and cardiovascular system. It uses a powerful magnetic field, radio waves and a computer to produce detailed pictures of the structures within the heart, to detect and monitor heart diseases and to evaluate the heart’s anatomy and function in patients with a variety of structural diseases. Unlike X-ray-based technologies, it does not use ionising radiation, and it may provide images of the heart that are better than other imaging methods for certain conditions. The advantages of MRI include its high spatial and contrast resolution, its ability to obtain images in virtually any plane and large fields of view. It may be beneficial for determining several parameters, including chamber size, global and regional left ventricle and right ventricle functions, volume and mass, cardiomyopathies, valvular function, pericardial disease, complex congenital heart disease and intracardiac shunts. However, the key disadvantages are limited availability, expense, requirement of special skills/ technical training needed to perform but most importantly interaction with metallic structures within body like metallic prosthesis, conventional pacemakers, neurostimulators, cochlear implants, bone growth stimulators, intracranial aneurysm clips, and women who are in the first trimester of pregnancy.
Magnetic Resonance Angiography: MRA is an application of MRI which can produce 3D and 4D images of blood vessels and the flow of blood through the vessels. This modality is used to generate images of arteries (and less commonly veins) in order to evaluate them for stenosis or dilatations (at risk for rupture). (See Figure 5)
Cardiac CT and CT Angiography
Cardiac computerised tomography CT is beneficial for determining a variety of structural abnormalities. They are coronary calcifications, pulmonary embolism, pericardial disease, congenital heart disease, cardiac thrombi and tumours, global and regional function and valvular heart disease. It utilises concentrated and focused X-rays for its evaluation. There are five important uses of cardiac CT; to screen patients with risk for heart diseases, to identify patients who do not need further cardiac evaluation, to consider serial imaging as ongoing management tool, to improve compliance and as a noninvasive angiography.
CT angiography is another imaging modality which has revolutionised the field of cardiology. It has a high negative predictive value which means that if negative it has a 98 per cent chance that there is no significant coronary artery disease. Thus it is a useful screening test and its use can help avoid unnecessary invasive coronary angiography. Further, it could be used for 3-D evaluation of not only heart vessels but entire cardio-vascular tree. While in more than 90 per cent of cases the quality is excellent, poor visualisation of small vessels (<1.5 mm), requirement for stable low heart rate, radiation exposure and high dose of contrast required may be important limitation of the procedure. (See Figure 6)
INVASIVE IMAGING
Intracardiac echocardiography
An important advance in echocardiography is intracardiac echocardiography wherein a catheter can be introduced into the heart via a vein and navigated within right heart chambers to obtain detailed anatomical landmarks that may guide catheter based interventional procedures such as intracardiac ablation and closure of atrial septal defects and patent foramen ovale.
Coronary angiography
A coronary angiography is a minimally invasive procedure using catheter to image coronary circulation. As a matter of fact, this investigation remains the gold-standard in the evaluation of the severity of coronary stenoses. Recently, the technique of rotational angiography has been developed to enhance the number of images available, to more fully assess the complex coronary vasculature as well as provide a 3D view. It has been found to be clinically useful in decreasing contrast dose, radiation exposure, and overall procedure time with an adequate safety profile and comparable image content. (See Figure 7)
Intravascular ultrasound
Intravascular ultrasound (IVUS) is a medical imaging methodology using a specially designed catheter with a miniaturised ultrasound probe attached to the distal end of the catheter. IVUS enables accurately visualising not only the lumen of the coronary arteries but also the atheroma (membrane/ cholesterol loaded white blood cells) ‘hidden’ within the wall. IVUS has thus enabled improvement in angioplasty outcomes as also benefit in clinical research. Virtual histology (VH) is a computer application of this technique which may permit plaque characterisation, which is shown in Figure 8.
Optical Coherence Tomography
Optical coherence tomography (OCT) is a novel invasive imaging technique that produces high resolution intracoronary images by a principle similar to IVUS, however, OCT uses infrared light, not ultrasound. The main applications of the OCT system are: atherosclerotic plaque assessment, stent struts coverage and apposition assessment, in-stent restenosis evaluation and for guiding angioplasty and its optimisation,which is shown in Figure 8.
Near infra-red spectroscopy
One of the most intriguing new technologies to help evaluate coronary atherosclerosis is near – infrared spectroscopy (NIRS). This modality provides real-time information regarding the lipid content of coronary plaques, which allows a new dimension of atherosclerosis imaging — chemical composition — to augment the physiologic and anatomic determinations and planning an interventional procedure, shown in Figure 8.
Cardiac thermography
Thermography is a new method for the evaluation of the inflammatory process locally and permits the identification of vulnerable plaque, one of the cherished goals in cardiology in recent times, which is shown in Figure 8.
How does medical innovation improve clinical practice
For translation of a new technology into improved patient outcomes, or in other words for a new technology to become effective there are three underlying requirements:
- A decision taken by healthcare delivery organisations to adopt these new technologies, looking at pros and cons of the technology.
- Deployment of these technologies within the complex organisational structure of healthcare providers.
- Monitoring the use of these new technologies.
Thus new technologies must not be only adopted by healthcare providers, but integrated into healthcare delivery systems adapting to their organisational structures to deliver the full benefit and impacting clinical outcome.
Conclusion
Within the course of last century, there has been a historic increase in life expectancy. This improvement in life-span can be closely linked to innovations in medical technology, drugs, vaccine or device. Cardiology remains a speciality where this marked reduction in mortality has co-related with medical innovations. Innovations in cardiac imaging are the latest ones which may in addition lead to a further improvement in quality of life. While development of echocardiography and coronary angiography represent a historic milestone, some new developments like CT angiography, MRI and PET imaging are redefining this field in altogether a new way.