Hybrid Imaging: Calcium Score and Myocardial Perfusion Imaging

Coronary heart disease (CHD) remains the top cause of death due to cardiovascular conditions worldwide, with someone suffering a myocardial infarction every 40 seconds. This highlights the importance of non-invasive imaging technologies like myocardial perfusion imaging (MPI), which are crucial for detecting coronary artery disease (CAD) early, even before symptoms appear. However, the reliance solely on MPI has shifted due to its limitations in definitively ruling out atherosclerosis, leading to the adoption of hybrid imaging techniques. Hybrid imaging combines computed tomography (CT) with MPI techniques such as positron emission tomography (PET) and single photon emission computed tomography (SPECT). This integration, often within a single gantry system, enhances the diagnostic accuracy by allowing for attenuation correction (AC), acquisition of the coronary artery calcium score (CACS), and more precise tracing of radiotracer uptake. The built-in CT in modern MPI systems assists in these functions, which is essential for better diagnosis and risk assessment in patients. The addition of CACS to MPI, a method involving the assessment of calcified plaque in coronary arteries, notably enhances diagnostic and prognostic capabilities. CACS helps in identifying atherosclerosis and predicting potential cardiac events, facilitating personalized risk management and the initiation of tailored interventions like statins and aspirin. Such comprehensive imaging strategies not only improve the accuracy of detecting CAD but also help in stratifying patient risk more effectively. In this paper, we discuss how the incorporation of CAC into MPI protocols enhances the diagnostic sensitivity for detecting obstructive CAD, as evidenced by several studies where the addition of CAC to MPI has led to improved outcomes in diagnosing CAD. Moreover, CAC has been shown to unmask silent coronary atherosclerosis in patients with normal MPI results, highlighting its incremental diagnostic value. We will discuss the evolving role of hybrid imaging in guiding therapeutic decisions, particularly the use of statins for cardiovascular prevention. The integration of CAC assessment with MPI not only aids in the early detection and management of CAD but also optimizes therapeutic strategies, enhancing patient care through a more accurate and personalized approach. Such advancements underscore the need for further research to fully establish the benefits of combining CAC with MPI in the clinical assessment of cardiovascular risk.

[What actually happens … in hybrid imaging?]

CLINICAL/METHODOLOGICAL ISSUE

The goal of this article is to shed light on innovations in perfusion imaging and the fields of application that have opened up in hybrid imaging of the heart.

STANDARD RADIOLOGICAL METHODS

As before, the most commonly used modalities in hybrid imaging are single photon emission computed tomography (SPECT) and positron emission tomography/computed tomography (PET/CT). Perfusion tracers and the radioactively labeled glucose analog ¹⁸F‑fluorodeoxyglucose (FDG) are commonly used for vitality imaging.

METHODICAL INNOVATIONS

Use of PET/MRI (magnetic resonance imaging) is becoming increasingly widespread. In addition, FDG is also increasingly applied in imaging infectious and inflammatory myocardial diseases. Furthermore, novel tracers are used, such as the amyloid-specific tracers in cardiac amyloidosis.

PERFORMANCE

Overall, this development has led to an increasing use of hybrid imaging techniques. These still include myocardial perfusion imaging, but are also used in inflammatory and infectious diseases such as endocarditis, myocarditis and sarcoidosis, as well as in underestimated diseases such as cardiac amyloidosis. The use of tracers has led to the creation of new fields of application in hybrid imaging.

PRACTICAL RECOMMENDATIONS

Hybrid imaging combining myocardial perfusion and coronary visualization seems to be particularly advantageous in complex cases such as multivessel disease. In infectious and inflammatory myocardial diseases, FDG PET/CT or PET/MRI has clearly demonstrated its added value. New fields of application are very promising, but their significance has yet to be clearly demonstrated.

Recent advances in cardiac SPECT instrumentation and imaging methods

Cardiac SPECT continues to play a critical role in detecting and managing cardiovascular disease, in particularly coronary artery disease (CAD) (Jaarsma et al 2012 J. Am. Coll. Cardiol. 59 1719-28), (Agostini et al 2016 Eur. J. Nucl. Med. Mol. Imaging 43 2423-32). While conventional dual-head SPECT scanners using parallel-hole collimators and scintillation crystals with photomultiplier tubes are still the workhorse of cardiac SPECT, they have the limitations of low photon sensitivity (~130 count s^-1 MBq^-1), poor image resolution (~15 mm) (Imbert et al 2012 J. Nucl. Med. 53 1897-903), relatively long acquisition time, inefficient use of the detector, high radiation dose, etc. Recently our field observed an exciting growth of new developments of dedicated cardiac scanners and collimators, as well as novel imaging algorithms for quantitative cardiac SPECT. These developments have opened doors to new applications with potential clinical impact, including ultra-low-dose imaging, absolute quantification of myocardial blood flow (MBF) and coronary flow reserve (CFR), multi-radionuclide imaging, and improved image quality as a result of attenuation, scatter, motion, and partial volume corrections (PVCs). In this article, we review the recent advances in cardiac SPECT instrumentation and imaging methods. This review mainly focuses on the most recent developments published since 2012 and points to the future of cardiac SPECT from an imaging physics perspective.

Hybrid Imaging in Ischemic Heart Disease

Hybrid imaging for ischemic heart disease refers to the fusion of information from a single or usually from multiple cardiovascular imaging modalities enabling synergistic assessment of the presence, the extent, and the severity of coronary atherosclerotic disease along with the hemodynamic significance of lesions and/or with evaluation of the myocardial function. A combination of coronary computed tomography angiography with myocardial perfusion imaging, such as single-photon emission computed tomography and positron emission tomography, has been adopted in several centers and implemented in international coronary artery disease management guidelines. Interest has increased in novel hybrid methods including coronary computed tomography angiography-derived fractional flow reserve and computed tomography perfusion and these techniques hold promise for the imminent diagnostic and management approaches of patients with coronary artery disease. In this review, we discuss the currently available hybrid noninvasive imaging modalities used in clinical practice, research approaches, and exciting potential future technological developments.

Relationship between Calcium Score and Myocardial Scintigraphy in the Diagnosis of Coronary Disease

Half the patients with coronary artery disease present with sudden death - or acute infarction as first symptom, making early diagnosis pivotal. Myocardial perfusion scintigraphy is frequently used in the assessment of these patients, but it does not detect the disease without flow restriction, exposes the patient to high levels of radiation and is costly. On the other hand, with less radiological exposure, calcium score is directly correlated to the presence and extension of coronary atherosclerosis, and also to the risk of cardiovascular events. Even though calcium score is a tried-and-true method for stratification of asymptomatic patients, its use is still reduced in this context, since current guidelines are contradictory to its use on symptomatic diseases. The aim of this review is to identify, on patients under investigation for coronary artery disease, the main evidence of the use of calcium score associated with functional evaluation and scintigraphy.

MicroRNAs Regulate Vascular Medial Calcification

Vascular calcification is highly prevalent in patients with coronary artery disease and, when present, is associated with major adverse cardiovascular events, including an increased risk of cardiovascular mortality. The pathogenesis of vascular calcification is complex and is now recognized to recapitulate skeletal bone formation. Vascular smooth muscle cells (SMC) play an integral role in this process by undergoing transdifferentiation to osteoblast-like cells, elaborating calcifying matrix vesicles and secreting factors that diminish the activity of osteoclast-like cells with mineral resorbing capacity. Recent advances have identified microRNAs (miRs) as key regulators of this process by directing the complex genetic reprogramming of SMCs and the functional responses of other relevant cell types relevant for vascular calcification. This review will detail SMC and bone biology as it relates to vascular calcification and relate what is known to date regarding the regulatory role of miRs in SMC-mediated vascular calcification.