Other imaging in the assessment of infective endocarditis

Introduction

Current European Society of Cardiology (ESC) and American College of Cardiology (ACC)/American Heart Association (AHA) guidelines for the diagnosis and management of infective endocarditis (IE) require the association of an infective syndrome and confirmation of endocardial involvement to establish the diagnosis [ , ]. Although the “standard modified Duke criteria” are commonly used, these have a lower diagnostic yield in patients with prosthetic heart valves, implantable electronic devices, or nondiagnostic echocardiography [ ]. Modification of these criteria by the ESC in 2015 incorporated additional imaging modalities to improve diagnosis in these situations ( Table 5.1 ), including (1) incorporation of paravalvular lesions detected by computed tomography (CT) (major criterion), (2) abnormal ¹⁸ F-FDG positron emission tomography (PET)/CT at the site of a prosthetic heart valve >3 months following implantation (major criterion), and (3) detection of recent embolic events using imaging alone (minor criterion).

These additions were not intended to diminish the relevance of echocardiography in the setting of IE but to enhance overall diagnostic certainty in patients with suspected IE and limited echocardiographic information. Transthoracic echocardiography (TTE) retained a Class I indication as the first-line imaging modality, while transesophageal echocardiography (TEE) received a Class I recommendation for all patients with a clinical suspicion of IE and a negative or nondiagnostic TTE, and in patients with a prosthetic heart valve or intracardiac device (reflecting reduced sensitivity of TTE to detect vegetations on prosthetic valves vs. native valves [50% vs. 76%] and improved sensitivity of TEE in both settings [native valves 96%, prosthetic heart valves 92%]) [ ].

While the ESC guidelines include CT and PET imaging within the modified Duke diagnostic criteria, the ACC/AHA guidelines highlight the value of these imaging modalities in the detection of extracardiac complications of IE [ ]. Notwithstanding this subtle difference, both bodies acknowledge the ability of novel imaging techniques to provide supplementary diagnostic information beyond the anatomical and hemodynamic findings of echocardiography [ ].

Computed tomography—angiography

Although cardiac CT initially focused on coronary and vascular imaging, multiple studies have shown increasing utility in suspected IE. Technological advances have led to significant improvement in scanner spatial resolution to <0.5 mm, enhancing the capability to visualize valve morphology (vegetations/perforations) and perivalvular complications (abscesses/pseudoaneurysms) [ , ].

Diagnostic performance

Native-valve endocarditis

Given the high diagnostic accuracy of TEE in the detection of IE-related lesions in native valves, there are minimal data concerning the diagnostic performance of alternative imaging methods in this setting. Multidetector CT angiography has shown comparable diagnostic performance compared to TEE as the gold standard for IE imaging, particularly in detection of valvular and paravalvular complications. In a study of 37 patients with clinically suspected IE, cardiac CT showed a diagnostic accuracy of 96% (sensitivity 97%, specificity 88%) with regard to intraoperative findings, and high levels of agreement with TEE with respect to vegetation size and mobility (but not leaflet perforation) [ ]. In a further study of 19 patients with aortic valve IE requiring surgery, CT was highly accurate in the detection of aortic valve pseudoaneurysms (sensitivity 100%, specificity 87.5%) or perivalvular extension (sensitivity 100%, specificity 100%). Although all vegetations >10 mm were accurately detected, CT failed to recognize smaller lesions. As a result, diagnostic performance was poorer in overall vegetation recognition (sensitivity 71.4%, specificity 100%) [ ]. In a later series of 71 affected valves (70% native), CT imaging yielded a sensitivity and specificity of 81% and 90% for the detection of abscesses and pseudoaneurysms. Conversely, TEE was superior to CT in the detection of vegetations and leaflet perforation. The combination of both modalities allowed detection of all abscesses and pseudoaneurysms confirmed at surgery compared to echocardiography or CT alone [ ].

On the basis of these observations, cardiac CT has a primary role in native-valve IE as an adjunctive imaging test where perivalvular complications are suspected but cannot be adequately excluded by echocardiography alone.

Prosthetic-valve endocarditis

Echocardiographic imaging is often limited in the setting of prosthetic heart valves where acoustic shadowing limits visualization of leaflets and periannular structures. In a study involving 27 patients with suspected prosthetic valve (16 of whom underwent surgery), CT imaging agreed closely with TEE findings in patients with aortic wall thickening, but less closely in those with abscess or valve dehiscence (and only moderately in detecting vegetations) [ ]. In a small study of 28 patients with suspected prosthetic-valve IE, use of CT as adjunct imaging beyond TTE and TEE demonstrated additional findings (predominantly mycotic aneurysms) resulting in a recommendation for surgery rather than conservative treatment in 21% of patients [ ]. In a further study of 67 patients with aortic prosthetic-valve IE, perivalvular abscesses providing an indication for surgery were detected on CT but not on TEE in 14% of the cohort ( Figs. 5.1 and 5.2 ) . Conversely, lesions providing an indication for surgery were detected by TEE but missed by CT in 19% of patients (with complete agreement in 85% of cases) [ ]. Diagnostic performance of CT was superior with respect to perivalvular complications and inferior with respect to vegetations, valve perforation, and perivalvular leaks, as confirmed in multiple subsequent studies [ ].

These findings suggest that CT should be a complementary investigation to TEE for the assessment of suspected prosthetic-valve IE. Combined imaging leads to higher rates of detection of IE-related pathology and allows comprehensive evaluation of potential complications and alternative diagnoses [ ].

Additional benefits of computed tomography—angiography and key limitations

Cardiac CT has a well-defined role in surgical procedural planning in patients who have undergone previous cardiac surgery ( Fig. 5.3 ) and is also the preferred technique for noninvasive evaluation of the coronary arteries in patients with aortic valve IE and aortic root pathology to mitigate the risk of catheter-induced aortic trauma and systemic embolism of aortic valve vegetations [ , , ]. CT is also commonly recommended for the detection of embolic complications of IE or the exclusion of other occult sources of infection when cardiac imaging is normal ( Fig. 5.4A and B ) [ ].

Key limitations of cardiac CT are the use of ionizing radiation, need for iodinated contrast, and the related risk of acute kidney injury and allergic reactions. Furthermore, imaging of the coronary arteries and heart valves requires slow and stable heart rates (owing to its limited temporal resolution) which may be problematic since intravenous or oral beta blockade is relatively contraindicated in patients with congestive cardiac failure or severe valvular incompetence. Prosthetic heart valves and cardiac implantable electronic devices (CIEDs) can also create substantial hard beam artifacts that limit evaluation of key structures [ ].

Molecular imaging

Whereas TTE and TEE provide excellent visualization of native heart valves, they are limited in the presence of prosthetic heart valves and CIEDs. In these instances, molecular nuclear imaging techniques provide complementary information to localize potential sources of infection when vegetations are not clearly seen [ , ].