Healthcare in India

Healthcare in India

The Present State and Evolution of Cardiac Imaging

The development and widespread use of cardiac imaging techniques have contributed to the improvement in evaluation of patients with known or suspected heart disease. Echocardiography is a well-established technique with a proven track record for the assessment of left ventricular function. Stress testing, coronary angiography and intravascular ultrasound are helpful in diagnosis of coronary artery disease. Cardiac computed tomography allows coronary calcium scanning along with noninvasive anatomic assessment of the coronary tree. It can be combined with functional imaging to provide a complete evaluation of the presence and physiological significance of the atherosclerotic coronary disease. Cardiac MRI offers a comprehensive cardiac evaluation, which includes wall-motion analysis, myocardial tissue morphology, rest and stress first-pass myocardial perfusion, as well as ventricular systolic function. Single-photon emission computed tomography (SPECT) provides evaluation of left ventricular assessment and myocardial viability with high sensitivity and specificity. Positron emission tomography (PET) has a high diagnostic performance but continues to have limited clinical use because of the high expense of the dedicated equipment and difficulties in obtaining adequate radionuclides. No single imaging modality has been proven to be superior overall. Choice of the imaging method should be adapted as needed based on clinical judgment of the risk of cardiac event, clinical history and local expertise.

Heart is made up of various structures like muscle, valve, conduction system, superior vena cava, inferior vena cava, great arteries, coronary arteries, coronary sinus, papillary muscle, chorda tendinea, pericardium and pulmonary veins.

In past, heart was evaluated by chest X-ray, fluoroscopy and barium swallow which had its own limitations. With advancement in cardiac imaging techniques, finer details of the heart are visible, leading to very accurate diagnosis. Use of new imaging techniques before, during and after cardiac intervention, has improved outcome of cardiac procedures substantially. Since a wide array of new imaging techniques are available, it is dilemmatic for general practitioners, physicians and patients to deicide the right investigation technique leading to right outcome. Use of non-judicious techniques affects the medical practitioners as well as patients.

For deciding the right imaging technique, prons and cons of each investigation should be considered before advocating its use. Some of the investigations may give anatomical information like CT and angiography whereas others give physiological information like Stress Thallium; some investigations are more useful for assessment of valve like echocardiography whereas others may be more useful for coronary arteries like coronary angiography.

Test for assessing cardiac Anatomy and function

Application
Test
Left ventricular function - Echocardiography
- Radionuclide imaging
- Gated MRI
Coronary artery disease diagnosis and prognosis - Exercise or pharmacologic stress testing with ECG, myocardial perfusion imaging, or echocardiography
- Magnetic resonance angiography
- Coronary angiography
- Intravascular ultrasonography
- MDCT coronary angiography
Myocardial viability - SPECT
- Stress testing with echocardiography
- PET
- Gated MRI

A few techniques may require radiation exposure like CT while others may not require radiation like MRI. Exposure to radiation is hazardous if repeated, and cause of concern in young patient and/or female with child bearing age. Some investigations are invasive like coronary angiography than others like echocardiography which is non invasive. Invasive investigations are not preferred by patients as they are associated with minor complications. Sensitivity and specificity of any diagnostic test advocates its use. Some investigations like ECG though simple, may not be very sensitive and specific for detection for detection of coronary artery disease like coronary angiography (though invasive) which is more reliable. In this article, prons and cons of investigation techniques to be used for diagnosis in day to day practice are discussed.

Various Cardiac Imaging Techniques

Chest X-ray though available everywhere may not be very reliable investigation compared to other newer investigations. Fluoroscopy and barium swallow are outdated techniques because of their definite limitation.

Electrocardiography (ECG)
A standard ECG provides 12 different vector views of the heart’s electrical activity as reflected by electrical potential differences between positive and negative electrodes placed on the limbs and chest wall. By convention, the ECG tracing is divided into the P wave, PR interval, ST segment, T wave and U wave. It is used especially to assess arrhythmias and myocardial ischemia.1,2 It can be very useful in setting of acute chest pain in emergency department for the diagnosis of acute coronary syndrome (ACS).3 ECG must be repeated after 1 hr and 8 hr if there is strong suspicion of ACS and it should be used along with cardiac marker (troponin and CKMB) for precise diagnosis of ACS. It is not very sensitive (50%) and specific for stable angina. It is also used to assess atrial enlargement, ventricular hypertrophy and conditions that predispose to syncope or sudden death (Long QT syndrome, Brugada syndrome, Wolf-Parkinson-White syndrome).

P Wave
The P wave represents atrial depolarization. An increase in amplitude of either or both components occurs with atrial enlargement. Right atrial enlargement produce a P wave > 2 mm while left atrial enlargement produces a P wave which is broad and double peaked.

PR Interval
PR interval is the time between onset of atrial depolarization and onset of ventricular depolarization. Normally, it is 0.10 to 0.20 sec; prolongation defines 1st-degree atrioventricular block.

QRS Complex
The QRS complex represents ventricular depolarization. Normally, the QRS interval is 0.07 to 0.10 sec. An interval of 0.10 to 0.11 sec is considered incomplete bundle branch block or a nonspecific intraventricular conduction delay, depending on QRS morphology; ? 0.12 sec is considered complete bundle branch block or an intraventricular conduction delay. Normally, the QRS axis is 90° to ?30°. An axis of ?30° to ?90° is considered left axis deviation and occurs in left anterior fascicular block (?60°) and inferior MI. An axis of 90° to 180° is considered right axis deviation; it occurs in any condition that increases pulmonary pressures and causes right ventricular hypertrophy (cor pulmonale, acute pulmonary embolism, pulmonary hypertension), and it sometimes occurs in right bundle branch block or left posterior fascicular block.

QT Interval
QT interval is the time between onset of ventricular depolarization and end of ventricular repolarization. where QTc is the corrected QT interval; R-R interval is the time between 2 QRS complexes. All intervals are recorded in seconds. QTc prolongation is strongly implicated in development of torsades de pointe ventricular tachycardia. QTc is often difficult to calculate because the end of the T wave is often unclear or followed by a U wave with which it merges.

ST Segment and T Wave
Though ST changes are classic of ischemia and infarction, resting ECG can be absolute normal in triple vessel disease. ST-depression and T-inversion is not specific in myocardial ischemia as found in various other diseases, also.
ST- segment elevation can be caused by early repolarization, myocardial ischemia and infarction, left ventricular aneurysm, pericarditis, hyperkalemia and pulmonary embolism.

ST- segment depression can be caused by hypokalemia, digoxin, subendocardial ischemia, reciprocal changes in acute MI, LVH, HOCM and aortic stenosis.

Specialized ECG Tests

Enhanced techniques like additional precordial leads, signal averaging, continuous ST- segment monitoring, QT dispersion heart rate variability, holter monitor, event recorder etc. provide additional information.

a) Holter monitor
Holter monitoring is continuous monitoring and recording of the ECG for 24 to 48 hour and is useful for evaluating intermittent arrhythmias and silent ischemia.4 The Holter monitor is portable, enabling patients to participate in normal daily activities; it may also be used for sedentary hospitalized patients if automated monitoring is unavailable. Patients are asked to record symptoms and activities so that they may be correlated with events on the monitor. The Holter monitor does not automatically analyze the ECG data; a physician does so at a later date.

b) Event recorder
Event recorders are worn for up to 30 days and can detect infrequent rhythm disturbances that 24-hour Holter monitoring may miss.5 The recorder does not operate continuously but is activated by the patient when symptoms occur. A memory loop enables information to be stored for seconds or minutes before and after activation. The patient can transmit ECG data by telephone to be read by a physician. If patients have serious events (eg, syncope) at intervals of > 30 days, an event recorder may be placed subcutaneously; it can be activated by a small magnet.

Echocardiography
Echocardiography uses ultrasound waves to produce image of the heart and great vessel. Compared with other noninvasive techniques, it is the most versatile and provides additional information at the lowest cost.6 Ventricular systolic function, valvular disease and hemodynamics can be assessed quickly and with reasonable accuracy. Echocardiography is particularly useful in the evaluation of diastolic dysfunction and pulmonary hypertension in patients with dyspnea. The portability of cardiac ultrasound systems permits evaluation of patients in the intensive-care unit. It does not use radiation. One can have real time assessment, allow rapid interpretation and can be repeated. The disadvantage is that the success of imaging varies from laboratory to laboratory and is, in part, dependent on the sonographer expertise, patient mix and physicians' proficiency and tolerance for technically difficult studies (e.g., obese patients). Another disadvantage of echocardiography is that visual assessment of wall motion and thickening requires significant expertise and involves some inter-observer variability, especially in lower quality images.
a) 2D Echocardiography and Color Doppler

2D Echocardiography gives better idea about anatomy of heart, size and shape of various structures of heart and is more useful in etiology of various disease whereas Color Doppler gives more information about severity of disease.

It remains the first choice in the assessment of LV function.7 Heart failure patients undergoing echocardiography have a better outcome than those managed without the performance of this test by understanding systolic function.8 Echocardiography can be the first line of investigation for the evaluation of patients with breathlessness, chest pain and syncope. Detailed evaluation of cardiac patient by echocardiography can give precise idea about the etiology, diagnosis, severity, prognosis and management of the patients. Newer addition to echocardiography is stress echocardiography, transesophageal echocardiography (TEE), tissue doppler and strain imaging, 3D Echocardiography and contrast echocardiography.

b) Stress Echocardiography

During stress echocardiography, increased oxygen demand can be achieved through exercise or administration of adrenergic ?-agonist (dobutamine) or a vasodilator (typically, dipyridamole).9 With increasing doses of dobutamine, viable tissue exhibits a biphasic response with improved contractility at low doses and regression to abnormal wall motion at higher doses. Dipyridamole leads to transiently increased coronary flow, which leads to improved contractility in viable myocardium.10 Stress echocardiography (Dobutamine) has 84% sensitivity and 75% specificity for diagnosis of coronary artery disease - better than exercise treadmill test (Table 2). For single vessel disease and multivessel disease accuracy of stress echocardiography is 54-94% and 85-100%, respectively. It is very useful when exercise treadmill can’t be done because of joint or spine problem or in a bed ridden patient. Normal stress echo has good long term prognosis.
The current standard for analysis of echocardiography is visual assessment of wall motion and endocardial systolic thickening. During stress echocardiography in a stable patient, one should withdraw drugs like beta blocker and diltiazem. Another factor for false negative study include a small increase in myocardial oxygen demand due to low wall stress, detection of single vessel disease and failure to detect limited wall motion abnormalities in patients with suboptimal echocardiographic images.
Sensitivityand specificity of various cardiac assessment modalities
Modality Source Studies Number of patients Sensitivity (%) Specificity (%)
Exercise Echocardiography ACC/AHA Stable Angina Guidelines 13 741 83% 84%
Dobutamine Echocardiography ACC/AHA Stable Angina Guidelines 13 436 75% 83%
Exercise ECG ACC/AHA Exercise Testing Guidelines 147 24,074 68% 77%
Excess SPECT NICE, UK
ACC/AHA Stable Angina Guidelines 21
16 2.360
4,430 81%
89% 65%
72%

c) Transesophageal Echocardiography (TEE)
It is widely available and semi invasive investigation which is a very useful imaging technique in specific situations like pre operative, intra-operative assessment during cardiac surgery especially valve surgery.12 It is an essential tool during certain intervention like ASD closure, PFO closure and septal puncture. Certain posterior structures of heart like left atrium, inter atrial septum, pulmonary vein, descending aorta, LA appendage are better seen with TEE compared to transthoracic echocardiography (TTE). It is specifically useful for better assessment of prosthetic valve function, prosthetic valve endocarditis, and finding out the source of embolism in a patient with TIA like PFO, LA appendage, aortic atheroma. In a patient with COPD with poor echo window and patient on ventilators when TTE may not give adequate information, TEE is a better option.

d) Tissue Doppler and Strain Imaging
Tissue Doppler imaging which records the motion of tissue or other structures with a velocity or frequency shift much lower than that of blood flow; is available in certain high end echo machine. It addresses motion of cardiac muscle during cardiac cycle and provides more accurate assessment of diastolic function (Figure 6). It gives better assessment of regional wall abnormalities. Relaxation abnormality of heart is detected by tissue Doppler and has been correlated with long term prognosis. It can be very useful to differentiate restrictive heart or constrictive heart disease. Strain imaging is an emerging technique in echocardiography. It provides valuable tools in the understanding and assessment of the basics of myocardial properties and mechanics.
e) 3D Echocardiography

Newer matrix transducers with more than 3000 elements allow 3D or multidimensional images of heart (Figure 7). This shortens the examination time and minimizes variation in acquisition of cardiovascular images.

3D echocardiography provides more accurate volumetric measurement of left ventricular ejection fraction (LVEF).14 With advancement and clinical experience in 3D and multidimensional echocardiographic imaging, visualization and quantitation of cardiovascular structure, function and hemodynamics will improve.

f) Contrast Echocardiography
Contrast echocardiography can be carried out by two methods. One uses agitated saline to identify a right-to-left intracardiac shunt and to improve the doppler signal from the right side of the heart. The other uses micro-bubbles of perfluorocarbon gas, which bubbles passage through pulmonary and myocardial capillaries. It enhances LV border detection (in patient on ventilator), LV volume and LVEF determination. It is indicated in patients with suspected LV thrombus and is very useful for patients suspected with PFO (Right to Left Shunt).15

Treadmill Test (TMT)/ Stress Testing
A treadmill test used diagnostically is considered to have a positive result if the patient develops signs and symptoms of ischemia during stress, i.e., ST-segment depression and angina.16 In stress testing, the heart is monitored by ECG and often imaging studies during an induced episode of increased cardiac demand so that ischemic areas potentially at risk of infarction can be identified. Heart rate is increased to 85% of age-predicted maximum (target heart rate) or until symptoms develop, whichever occurs first.

Stress testing is used for diagnosis of coronary artery disease (CAD) and for risk stratification and monitoring of patients with known CAD. In patients with CAD, a blood supply that is adequate at rest may be inadequate when cardiac demands are increased by exercise or other forms of stress.

Stress testing is less invasive and less expensive than cardiac catheterization, and it detects path physiologic abnormalities of blood flow; however, it is less accurate for diagnosis in patients with a low pretest likelihood of CAD. It can define the functional significance of abnormalities in coronary artery anatomy identified with coronary angiography during catheterization. Because coronary artery plaques that are not significantly stenotic (i.e., do not result in ischemia during stress testing) may nonetheless rupture and cause an acute coronary syndrome, a normal stress test result does not guarantee future freedom from MI.

Risks of stress testing include infarction and sudden death, which occur in about 1out of 5000 patients tested. Stress testing has several contraindications.

TMT is absolutely contraindicated in acute coronary syndrome (MI within 48 h or uncontrolled unstable angina), aortic dissection (acute), aortic stenosis if symptomatic or severe, arrhythmias if symptomatic or hemodynamic ally significant and heart failure if decompensated .
TMT is relatively contraindicated in atrioventricular block if high-degree, bradyarrhythmias, electrolyte imbalance, hypertension (systolic > 200 mm Hg or diastolic > 110 mm Hg), hypertrophic obstructive cardiomyopathy, inability to exercise adequately due to mental or physical impairment, stenosis of heart valve if moderate or severe and stenosis of left main coronary artery.

Cardiac Catheterization
Cardiac catheterization is the passage of a catheter through peripheral arteries or veins into cardiac chambers and coronary arteries. It can be used to perform angiography, intravascular ultrasonography, measurement of cardiac output, detection and quantification of shunts, endomyocardial biopsy and measurement of myocardial metabolism.17

Left heart catheterization
Left heart catheterization is commonly used to assess coronary artery anatomy. It is also useful for assessing aortic BP and systemic vascular resistance, aortic and mitral valve function.18

Right heart catheterization
Right heart catheterization is most commonly used to assess right atrial, right ventricular and pulmonary artery pressure occlusion pressure and LV end diastolic pressure.19 It may help in the diagnosis of cardiomyopathy, constrictive pericarditis and cardiac temopnade when noninvasive testing is nondiagnostic.

Injection of radiopaque dye into coronary or pulmonary arteries, the aorta and cardiac chambers is useful in certain circumstances. Digital subtraction angiography is used for nonmoving arteries and for chamber cineangiography.

Coronary angiography
Coronary angiography via left heart catheterization is used to evaluate coronary artery anatomy in various clinical situations, as in patients with suspected coronary atherosclerotic or congential disease, valvular disorders before valvular replacement or unexplained heart failure (Figure 9). Coronary angiography through radial root is easily accepted by patients. It has less chance of heamatoma and the patient can be discharged within 2-3 hours.20

Pulmonary angiography via right catheterization is used to diagnose pulmonary embolism.21 Intraluminal filling defects or arterial cutoffs are diagnostic. Radiopaque dye is usually selectively injected into one or both pulmonary arteries and their segments. CT angiography has largely replaced right catheterization for diagnosis of pulmonary embolism.

Aortic angiography via left heart catheterization is used to assess aortic regurgitation, coarctation, patent ducuts arterious and embolism. Ventriculography is used to visualize ventricular wall motion and ventricular outflow tracts, including subvalvular and supravalvular regions. After LV mass and volume are determined from single planar or biplanar ventricular angiograms, end-systolic and end-diastolic volumes and ejection fraction can be calculated.

Contraindications
Relative contraindications for cardiac catheterization include renal insufficiency, coagulopathy, fever, systemic infection, uncontrolled arrhythmia or hypertension and uncompensated heart failure.

Complication
Mortality rate is 0.1 to 0.2%. MI (0.1%) and stroke (0.1%) may result in significant morbidity. Other complications include allergic reaction- anaphylaxis and dye induced nephropathy in old age and diabetic patients.

Intravascular ultrasound
Miniature ultrasound transducers on the end of coronary artery catheters can produce image of coronary vessel lumina and walls and delineate blood flow.22 This technique is increasingly along with coronary angiography. It gives better assessment of composition of plaques (Ca++), and stent expansion which determines long term outcome after angioplasty.

Coronary artery flow measurement
Coronary angiography shows the presence and degree of stenosis but not the functional significance of the lesion. Extremely thin guidewires are available with pressure sensors or Doppler flow sensors. It can be useful to estimate coronary flow (expressed as Fraction Flow Rate).23 These flow measurements are most useful in intermediate lesions (40 % -70% stenosis) with multiple lesions to identify those that are clinically significant.

Computed Tomography

Two different computed tomography modalities can be used to assess cardiovascular system. One uses nonmechanical movement of the x-ray source (i.e., electron beam CT, EBCT) and the other involves the motion of the x-ray source and table, along with multiple detectors to obtain the data in spiral or helical fashion (i.e., multidetector-row CT, MDCT).24

Electron-Beam CT
For detection of coronary calcium EBCT was used but because of spatial resolution and high cost, it has been replaced by MDCT.

Multidetector-Row CT
Since the introduction of MDCT in the early 1990s, acquisition time, detector number, spatial and temporal resolution have continuously improved with each new generation of scanners, resulting in excellent image quality and diagnostic accuracy in the detection of CVD.25 The spatial resolution of MDCT is higher than that of EBCT (Figure 11). It also offers higher signal-to-noise ratio with shorter scan time. However, continuous x-ray radiation during systole and diastole with MDCT means significantly higher exposure than with EBCT. Patients with left bundle branch block may be better served by MDCT for evaluation of suspected CVD.26 In MDCT, the time for one 360-degree tube rotation has been decreased to 500 milliseconds or less.

Cardiac multi–detector row CT imaging is performed either in a sequential mode, where the patient table is moved incrementally between successive rotations of the x-ray tube, or more often in a spiral mode, where the patient table is moved continuously during continuous rotation of the x-ray tube.27 The 4 slice MDCT allows for scanning with one x-ray tube and 4 detector rows in a single gantry rotating twice/second around the patient. The projection is obtained in spiral or helical path through continuous gantry rotation and table movement. The 16-slice multidetector computed tomography (MDCT) scanners have demonstrated good diagnostic accuracy for the detection of significant coronary stenosis in patients with known or suspected CVD.28-30 Excellent feasibility and diagnostic accuracy make 64-slice MDCT an ideal imaging modality for the anatomic evaluation of coronary circulation.31 64 slice MDCT has 98% sensitivity, 93% sensitivity, 74% positive predictive value, and 98% negative predictive value. The extremely high negative predictive value suggests its use to exclude coronary artery disease.

CT angiography can be very useful in setting of acute chest pain in an emergency department as it can help to diagnose certain life threatening disease like pulmonary embolism, dissection of aorta along with CAD.

Radiation exposure during MDCT is about 15 mSv which is significant whereas it is 0.1 mSv for simple X-ray and 7.0 mSv for coronary angiography. Hence, MDCT technology improvisation is focused on reduction of the radiation dose. In addition, increased coverage has been achieved with the introduction of 320-slice MDCT. These systems allow volumetric data acquisition of the entire heart within a single gantry rotation, thereby eliminating over sampling and stair-step artifacts.32

Limitation of CT Angiography
i) Radiation exposure is much higher than coronary angiography
ii) Iodinated contrast agent induced nephrotoxicity
iii) CTA image quality depends on diastolic window (heart rate and R- R interval)
iv) Visualization of distal coronary artery segments is suboptimal in many patients
v) Coronary calcium obscures true lumenal diameter (“blooming effect”)
vi) Estimation of severity of stenosis is problematic

Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is another useful technique for noninvasive assessment of cardiac structure and function MRI evaluates the presence of CVD using three very different techniques:

a) Evaluation of wall motion and thickening during stress,
b) Dynamic first-pass perfusion imaging, which assesses for inducible perfusion defects,
c) Anatomical assessment (coronary MRA), which provides noninvasive visualization of the coronary tree.
During a magnetic resonance examination, the patient is subjected to a high local magnetic field, usually 1.5 Tesla, which aligns the protons in the body. These protons are excited by a radiofrequency pulse and subsequently detected by receiver coils. A recent proliferation of 3T magnetic resonance (MR) system has been driven by the potential for better signal-to noise ratio, image contrast and spectral separation.33 Abnormal soft tissue can be better differentiated through measurement of these four parameters than through any other previous technique. In addition, scans can be performed in any plane, because the spatial orientation of the image is determined by manipulation of magnetic fields. It has been used in the chest to evaluate the heart, major vessels, mediastinum and hilar structures because of the natural contrast provided by flowing blood.34
MRI has the advantage of being a highly versatile modality because it provides multiple techniques in one imaging system. It does not involve exposure to radiation. The excellent resolution enables the assessment of subendocardial perfusion abnormalities. In addition, MRI is well validated for quantifying the volumes, mass and function of the ventricles. Other clinical applications of MRI include assessment of myocardial infarction. Ejection fraction and cardiac output are assessed better with MRI compared to MDCT.35 It has been shown that three-dimensional MR coronary artery imaging is feasible at 3 T, with increase in Signal to noise ratio compared to 1.5 T.36

Disadvantages of cardiac magnetic resonance include high cost and limited availability. Monitoring during testing can be suboptimal and ECG tracings and ST segments can be altered by the strong magnetic field. The entire acquisition and image processing can be time consuming (30 min or more on average). MRI cannot be used in patients with severe claustrophobia. It is also absolutely or relatively contraindicated in patients with pacemakers, defibrillators, certain aneurysm clips and other indwelling ferromagnetic materials. Newer pacemakers are MRI compatible. MRI is feasible only in cooperative patients.

Tilt Table
Tilt table testing is simple, very useful test, used to evaluate syncope in younger, apparently healthy patients and, when cardiac and other tests have not provided a diagnosis, in elderly patients.37 Tilt table testing produces maximal venous pooling, which can trigger vasovagal (neurocardiogenic) syncope and reproduce the symptoms and signs that accompany it (nausea, light-headedness, pallor, hypotension, bradycardia). With positive test, HR and BP falls significantly with syncope.

Sensitivity varies from 30 to 80% depending on the protocol used. The false-positive rate is 10 to 15%. Relative contraindications include severe aortic or mitral stenosis, hypertrophic cardiomyopathy, and severe coronary artery disease (CAD).
Nuclear Imaging
Echocardiography, CT and MRI have great advantages in spatial resolution compared to nuclear imaging, but imaging using nuclear techniques has high sensitivity for tracer detection.38 The major advantage with nuclear imaging is that various radiotracers enable the evaluation of not only perfusion and metabolism but also neuronal function, myocardial flow reserve, and receptor density. Nuclear imaging techniques include single photon emission computed tomography and positron emission tomography.

Single-photon emission computed tomography (SPECT)
Single-photon emission computed tomography is used with radiopharmaceuticals, mainly thallium (201Tl), to assess perfusion and cell membrane integrity as hallmarks of viability.39 Thallium is a potassium analog that is introduced into myocytes (Figure 13). Myocardial thallium is then exchanged continuously with the systemic thallium. Thallium kinetics are directly proportional to tissue blood flow. Hence, normal tissue has more rapid uptake and washout than underperfused, viable tissue. Thallium redistribution in regions that initially had a thallium defect is the hallmark of viability by this technique. Thallium reinjection after stress or 3- to 4-hour redistribution imaging significantly improves viability assessment, as does semiquantitation of thallium activity.

SPECT is more sensitive but less specific in predicting functional recovery compared to echocardiography.40 Its drawback is the long duration of testing (2–4 h). SPECT is also limited to facilities certified in the handling of nuclear radioactive tracers. Performing SPECT requires anticipatory coordination and planning to maintain a schedule for testing and patient preparation prior to the test and to obtain the radioactive tracer in a timely manner.

Positron Emission Tomography (PET)
Positron emission tomography is an accurate method to assess myocardial perfusion and metabolism in detection of coronary artery disease.38 Like SPECT, PET also uses exercise or dobutamine (or vasodilator) to induce stress. Three positron emitting radiotracers are used for cardiac PET namely 13N and 82Rb for evaluation of myocardial perfusion and 18F deoxy-D-glucose (FDG) for assessing myocardial glucose metabolism. 82Rb and 13N imaging is limited due to high cost and requirement of on-site cyclotron for its production respectively. The use of PET in assessing myocardial perfusion is expected to increase in near future with emergence of low cost PET systems and regional distribution of positron emitting radiotracers. A meta-analysis has showed better sensitivity and specificity of PET for the diagnosis of coronary artery disease with 92% and 85%, respectively.41

There are several advantages of PET(Figure 14).Image uniformity is the most important property of PET compared to SPECT because of its excellent spatial and temporal resolution PET with FDG evaluates atherosclerotic plaques.42 Studies have shown that macrophages are responsible for the FDG accumulation in vulnerable plaques and as these plaques are rich in macrophages, FDG imaging was shown to be useful in detection of atherosclerotic plaque.43,44 The disadvantages of PET including high cost and availability of technique which hinder the use of PET for myocardial perfusion.

Conclusion
The availability of multiple techniques presents the clinician with the challenge of knowing the relative utility of each method in order to choose the appropriate technique(s) for each clinical setting. Available tests all have advantages and drawbacks, and none can be considered suitable for all patients. The information obtained from tests should be accurate, reliable and reproducible. It should also provide incremental prognostic value to the risk predicted by clinical assessment. Hence, the clinician must select investigative technique judiciously.