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MARCH / APRIL 2014 :: 75(2)
Medical Imaging

Medical imaging has revolutionized screening and diagnosis, but this technology is not risk-free. As use of advanced imaging has grown, attention has increasingly focused on the risks of radiation exposure, the anxiety associated with incidental findings, and the costs of such imaging. This issue of the NCMJ will address the pros and cons of medical imaging and will discuss how this technology can be used more safely and effectively.

INVITED COMMENTARY

Noninvasive Cardiovascular Imaging

Robert J. Hartman

N C Med J. 2014;75(2):146-148.PDF | TABLE OF CONTENTS



Over the past 2 decades, use of noninvasive cardiovascular imaging has increased dramatically. This article provides a brief synopsis of the current state of several technologies—echocardiography, cardiac magnetic resonance imaging, and cardiac computed tomography—as well as a glimpse at future possibilities in cardiac imaging.

Echocardiography remains the mainstay of noninvasive cardiovascular assessment and diagnosis of heart disease; however, many recent advances have revolutionized this imaging modality. Machines and processors have dramatically improved to the point where handheld cardiac ultrasound devices are now available [1], and many medical schools train students in ultrasonography—not just of the cardiovascular system but also of many other organ systems. As access to echocardiographic devices has improved for primary care providers and certain specialists, the question has arisen of how to adequately train noncardiologists in the use of these devices. In response, recommendations have been proposed for the number of training hours and/or number of studies clinicians should perform when learning to diagnose a particular condition; these recommendations vary depending on the type of clinician and the diagnosis in question [1-4]. It remains to be seen whether these recommendations are adequate and whether clinicians will comply with them.

There have also been further advances in 3-dimensional (3D) echocardiography, which now allows volumetric assessment to be performed during the image-acquisition step rather than during the post-processing phase. This real-time imaging is helpful for determination of left ventricular volume and mass, assessment of mitral valve anatomy and pathology, guidance of devices used to close atrial septal defects [5], and monitoring of transcatheter aortic valve implantation [6].

Stress testing with echocardiography (stress echo) provides additional helpful information that cannot be gained using stress testing with electrocardiography. Subtle wall-motion abnormalities seen following peak exertion can help cardiologists decide whether the patient would benefit from cardiac catheterization for revascularization. A meta-analysis of studies examining the accuracy of stress echo compared with basic stress testing for the evaluation of patients with an intermediate pretest risk of coronary artery disease (CAD) found that stress echo had a higher sensitivity (76% versus 68%) and a higher specificity (88% versus 77%) [7]. Another meta-analysis found that stress echo has a sensitivity similar to that of radionuclide myocardial perfusion imaging for the diagnosis of CAD (85% versus 87%), but the specificity of stress echo was higher (77% versus 64%) [8]. However, when it comes to interpretation of findings, stress echo has the disadvantage of being more subjective, as results are based solely on human interpretation; in contrast, radionuclide myocardial perfusion imaging uses automated computer quantification in addition to human interpretation, allowing for increased accuracy and reproducibility [9].

With all of these advances in echocardiography plus the current emphasis on appropriate use of health care resources, clinicians should be aware of the guidelines for appropriate use of echocardiography in adults, which now encompass transthoracic, transesophageal, and stress echocardiography [10]. According to the third iteration of these guidelines, which were published in 2011, clinicians should no longer perform imaging in adults for infrequent atrial or ventricular premature contractions or for perioperative evaluation of ventricular function in patients with no symptoms or signs of cardiovascular disease, nor should patients with a low global risk of CAD undergo stress echocardiography. Criteria for appropriate use of echocardiography in children have not yet been established.

Cardiac Magnetic Resonance Imaging
The noninvasive imaging modality that has gained the most attention is cardiac magnetic resonance imaging (MRI). Cardiac MRI has become the gold standard for assessment of cardiac anatomy, interventricular volumes, and cardiac function; for the identification and diagnosis of different types of cardiac tumors; and for the evaluation of coronary vascular anomalies [11]. It has been shown to be helpful for the identification and management of coarctation of the aorta and interrupted aortic arch, and for evaluation of postoperative complications following surgery to correct transposition of the great arteries [12]. Cardiac MRI is also helpful in the follow-up of congenital heart defects in patients for whom imaging windows are insufficient due to scarring or body habitus.

Future areas of study include using electrocardiogram-gated MRI to generate a fetal electrocardiogram to better identify heart rhythm abnormalities in utero [13]. There is anticipation that cardiac MRI could also be used for assessment of myocardial perfusion and ischemia, for atherosclerotic plaque imaging, and as a type of therapeutic imaging for electrophysiology and interventional procedures [11]. Some health care centers currently have hybrid suites that combine cardiac catheterization and cardiac MRI [12]; however, there have been no published clinical trials showing that using these modalities in combination yields a better clinical outcome than using them in tandem.

A benefit of cardiac MRI is that, unlike most other noninvasive imaging modalities, it does not expose the patient to ionizing radiation. However, it still has several disadvantages: acquisition of images takes a long time, often more than 30 minutes; the patient must be able to comply with instructions to hold his or her breath periodically; and the patient should not be claustrophobic. Most children younger than 7 years cannot perform serial breath holding, and children aged 7–12 years have difficulty with serial breath holding; therefore, children in either age group require anesthesia and endotracheal intubation. Also, cardiac MRI is currently contraindicated if the patient has an implanted device, such as a pacemaker or aneurysm clip, although MRI-compatible devices and leads have recently come to market, which could make this contraindication moot within a generation.

Cardiac Computed Tomography
Cardiac computed tomography (CT) has several advantages: It is a much faster modality; it can accommodate patients with implanted devices; and it is the imaging modality of choice for the assessment of vascular rings or slings [12, 14]. A meta-analysis of studies that compared cardiac multidetector (multislice) CT with cardiac MRI for the identification of CAD found that cardiac multidetector CT had greater weighted-average sensitivity (85% versus 72%) and specificity (95% versus 87%) [15]. Cardiac multidetector CT is therefore recommended for assessment of symptomatic patients who are at intermediate risk of CAD after initial risk stratification [16].

In terms of limitations, cardiac CT requires the use of contrast agents, which pose concerns about both renal toxicity and anaphylaxis; adequate coronary imaging requires slow heart rates (60 beats per minute or slower), making its use in infants almost impossible; and most importantly, cardiac CT exposes the patient to ionizing radiation. The projected lifetime cancer mortality rate attributable to such radiation exposure is higher in children [16].

Table 1 provides a comparison of cardiac MRI and cardiac CT. As with echocardiography, there are criteria for the appropriate use of cardiac MRI [17] and cardiac CT [18] in adults, the latter of which have undergone 2 revisions. However, criteria have not yet been established for appropriate use of these modalities in the pediatric population.

Conclusion
As these noninvasive imaging modalities have become more widely used in adults, they have gained the attention of pediatric cardiologists and are now being used more often for patients with complex congenital heart defects. The result has been a decrease in the number of catheterizations at some institutions [19]. A retrospective analysis of data from a single institution [20] compared use of cardiac MRI with use of cardiac catheterization to plan surgery in children with congenital heart disease; those who underwent cardiac MRI were found to have fewer complications during recovery from the diagnostic procedure, and the rate of mortality within 30 days following cardiac surgery was not significantly different between the 2 groups.

It may seem that one imaging modality is superior to another, that only one modality is indicated for the diagnosis and treatment of a specific cardiac condition, or that the role of cardiac catheterization has dramatically decreased as noninvasive cardiac imaging technology has continued to improve, but none of these statements is true. For example, an echocardiogram should be sought after cardiac CT is used to diagnose a vascular ring, because more than 12% of patients have additional congenital heart defects [14]. As temporal resolution of 3D echocardiography has improved, some have come to believe that this modality could surpass cardiac MRI for certain indications. Others have suggested combining 3D echocardiography with cardiac MRI and/or cardiac CT to create a set of “fusion” images [5]. Guidelines now exist for multimodality cardiovascular imaging for specific cardiac conditions, including hypertrophic cardiomyopathy [21] and pericardial disease [22], and it is anticipated that similar guidelines will be published for other conditions in upcoming months and years.

Acknowledgments
Potential conflicts of interest. R.J.H. has no relevant conflicts of interest.

References
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21. Nagueh SF, Bierig SM, Budoff MJ, et al. American Society of Echocardiography clinical recommendations for multimodality cardiovascular imaging of patients with hypertrophic cardiomyopathy: endorsed by the American Society of Nuclear Cardiology, Society for Cardiovascular Magnetic Resonance, and Society of Cardiovascular Computed Tomography. J Am Soc Echocardiogr. 2011;24(5):473-498.

22. Klein AL, Abbara S, Agler DA, et al. American Society of Echocardiography clinical recommendations for multimodality cardiovascular imaging of patients with pericardial disease: endorsed by the Society for Cardiovascular Magnetic Resonance and Society of Cardiovascular Computed Tomography. J Am Soc Echocardiogr. 2013;26(9):965-1012.


Robert J. Hartman, MD clinical assistant professor of pediatrics, Section of Pediatric Cardiology, Brody School of Medicine, East Carolina University, Greenville, North Carolina.

Address correspondence to Dr. Robert J. Hartman, Brody School of Medicine, East Carolina University, 115 Heart Dr, Greenville, NC 27834 (hartmanr@ecu.edu).