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|Year : 2010 | Volume
| Issue : 2 | Page : 125-130
Cardiac imaging: Current and emerging applications
B Jankharia1, A Raut2
1 Piramal Diagnostic Services (Formerly Wellspring), Piramal Diagnostics - Jankharia Imaging, Bhaveshwar Vihar, India
2 Department of Radiology, KEM Hospital, Parel, Mumbai, India
|Date of Submission||13-Jan-2009|
|Date of Decision||12-Mar-2009|
|Date of Acceptance||20-Feb-2010|
|Date of Web Publication||8-Jul-2010|
Piramal Diagnostic Services (Formerly Wellspring), Piramal Diagnostics - Jankharia Imaging, Bhaveshwar Vihar
Source of Support: None, Conflict of Interest: None
Cardiac magnetic resonance imaging (MRI) and computed tomography (CT) scan have made big inroads as modalities used for evaluation of various pathologies of the heart. Cardiac MRI is typically used for perfusion and viability studies as well as to study various cardiomyopathies, valvular diseases and the pericardium. It has been used in the evaluation of congenital heart diseases over the last two decades. Cardiac CT is used mainly for the evaluation of the coronary arteries, typically in the setting of "to rule out coronary artery disease".
Keywords: Cardiac computed tomography, cardiac magnetic resonance imaging, computed tomography, congenital heart disease, coronary artery disease, magnetic resonance imaging, review, viability
|How to cite this article:|
Jankharia B, Raut A. Cardiac imaging: Current and emerging applications. J Postgrad Med 2010;56:125-30
Cardiac magnetic resonance imaging (CMR) and cardiac CT (CCT) are the new kids on the block as far as cardiac imaging is concerned. This review describes the indications for these two modalities.
| :: Cardiac MRI|| |
A 1.5T scanner is required with high-speed gradients and specialized scanning sequences.
Image acquisition is done in a particular phase of cardiac cycle to avoid image blur and cardiac motion artifacts. This is called as electrocardiographic gating.
Pulse sequences used for CMR can be broadly divided into dark-blood and bright-blood techniques. A motion picture loop throughout the various phases of cardiac cycle can be produced with gradient echo (GRE) and balanced steady-state sequences to get rapid cine imaging. Cine imaging is useful in the functional assessment of ventricles.
Congenital heart disease
CMR is useful in understanding complex anatomy in Congenital heart disease (CHD) and gives information not obtained by echocardiography.  It is especially useful in evaluating the functional significance in various types of CHDs, e.g. in evaluating right heart function in Ebtein's anomaly.  CMR not only detects intraventricular shunts such as atrial and ventricular septal defects, but can also calculate shunt size with phase-velocity mapping.  CMR plays an important role in evaluating complex surgical shunts and baffles and has a very important role to play in the postoperative evaluation of patients with tetralogy of Fallot. 
Valvular heart disease
CMR can demonstrate the presence and quantify the severity of valvular heart disease.  Using phase-velocity mapping as well as ventricular chamber volumes, aortic stenosis and regurgitation as well as mitral disease can be accurately evaluated. The valve areas can be accurately measured using planimetry. 
Arrhythmogenic right ventricular dysplasia (ARVD) [Figure 1] is characterized by fatty or fibrous infiltration of the right ventricle (RV) walls with thinning and pseudo-sacculations.  The presence of fat can be identified on T1W images. The cine images show thinning of the anterior and inferior walls, enlargement and dilatation of the RV, areas of dyskinesia or focal bulging, reduced ejection fraction and impaired ventricular filling during diastole. 
The diagnosis is usually made on echocardiography. CMR is better than echocardiography for diagnosing apical hypertrophic cardiomyopathy.  CMR also helps in assessing the functional sub-valvular narrowing in patients with hypertrophic obstructive cardiomyopathy.  The presence of abnormal areas of enhancement suggests necrosis and fibrosis and may indicate an adverse prognosis. 
Inflammatory and infiltrative cardiomyopathy
CMR is an excellent tool for diagnosing myocardial inflammation as in viral myocarditis  or sarcoidosis.  Abnormal areas of mid-myocardial and epicardial enhancement are seen often with wall motion abnormality. Infiltrative conditions like Fabry's disease  and amyloidosis  can also be diagnosed reliably.
In patients with thalassemia, iron deposition in the myocardium is a significant cause of death.  Using the T2* parameter, the myocardial iron content can be quantified and then followed up regularly to see efficacy of chelating agents. 
CMR is more accurate than 2D echocardiography in the functional assessment of the heart.  CMR can measure ventricular ejection fraction and end-diastolic and end-systolic volumes.
Coronary artery imaging
Despite reports in the literature, , CMR is still not a robust modality for the evaluation of the coronary arteries. 
Myocardial perfusion and viability
Perfusion: First pass perfusion studies can be performed using intravenous gadolinium and stress with either adenosine or dipyridamole.  The technique is sensitive for diagnosing areas of hypoperfusion, corresponding to the presence of ischemia. 
Myocardial viability [Figure 2]
Using special sequences, delayed hyperenhancement can be studied with CMR. It has been shown that all infarcts  show delayed hyperenhancement, 5-10 min after the injection of intravenous gadolinium. The extent of infarction correlates with the ability to recover function after revascularization  and thus this study can help decide whether revascularization will be of help in patients with myocardial infarction.
Cardiac and pericardial masses and thrombi
CMR is an accurate means to evaluate cardiac and pericardial masses.  Gadolinium enhancement differentiates thrombus from neoplasm. 
CMR is an accurate tool for diagnosing pericardial effusion and constrictive pericarditis.  Pericardial thickening of more than 4 mm in a patient with a restrictive physiology is usually diagnostic.  The presence of paradoxical septal deviation in early diastole in the first expiratory breath on real-time CMR has been shown to be a useful sign to diagnose constriction. 
| :: Cardiac CT|| |
At the bare minimum, a 64-slice CT scanner is required for accurate CT angiography (CTA).  Faster, 256- and 320-slice  and dual-source  CT scanners are now available as well.
Irrespective of the speed of the scanner, the best images are obtained with a lower heart rate  and thus beta-blockers are routinely used to bring down the heart rate to around 65 beats per min or less. Intravenous contrast is used at a high injection speed to opacity the coronary arteries. A calcium score study is often performed prior to the intravenous injection and in some centers if the score is more than 800, a CTA is then not performed. Cardiac CT is contraindicated in those patients with severe allergy to iodinated contrast or in those suspected to be pregnant.
Coronary artery imaging
Cardiac CT has a negative predictive value of around 97-99% , in the evaluation of coronary artery disease [Figure 3]. The positive predictive value varies from 44-93%, depending on the patient selection criteria.  In view of this, the main indication of cardiac CT is still in the setting of "to rule out coronary artery disease", i.e. to screen for coronary artery pathology in the setting of medium to high-risk but asymptomatic patients or in patients with atypical symptoms. 
Post-bypass [Figure 4]
Cardiac CT is an excellent tool to study bypass grafts and to assess their patency  and is currently used as the first tool in the evaluation of patients suspected to have graft pathology.
The results in patients with stents to diagnose in-stent pathology are quite variable  due to difficulties in in-stent lumen visualization, and cardiac CT is typically used mainly in patients with larger stents and to look at disease elsewhere in the coronary tree. Catheter angiography remains the gold standard for in-stent lumen visualization.
Coronary artery anomalies
Cardiac CT is the gold standard in the evaluation of coronary artery anomalies. 
Cardiac CT reconstruction in multiple phases allows the assessment of ejection fraction and end-diastolic and end-systolic volumes with considerable accuracy. 
Cardiac CT has been found useful in the evaluation of aortic and mitral valve areas and the extent of calcification.  The evaluation of valve areas may help in decision-making as far as management is concerned.
Pericardium and cardiac and pericardial masses
Cardiac CT is useful in diagnosing pericardial thickening, calcification, effusion and masses  as well as cardiac masses. 
Congenital heart disease
The role of cardiac CT in CHD is mainly to evaluate the pulmonary arteries, veins and aorta.  It is especially useful in patients with pulmonary atresia to evaluate the presence and extent of major aorto-pulmonary collaterals  [Figure 5]. It also helps in the evaluation of Glenn and Fontan shunts  and in other postoperative situations.
| :: Conclusion|| |
Both cardiac CT and CMR have become extremely useful tools to study various facets of the heart as described above. They both have different strengths. Cardiac CT is best suited to study the coronary tree. CMR is best used to evaluate the myocardium, pericardium and the valves.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
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