Current Medical Imaging - Volume 9, Issue 3, 2013

Cardiac single photon emission computed tomography (SPECT) and positron emission tomography (PET) are increasingly being used in the evaluation of patients with suspected or known coronary artery disease by assessing myocardial viability and perfusion and providing invaluable diagnostic and prognostic information. With their capacity to quantify left ventricular function as well as coronary flow reserve, these myocardial perfusion imaging techniques are superior to other methods for the detection of multivessel coronary artery disease and potentially, for risk stratification and prediction of cardiac events. Hybrid SPECT/CT and PET/CT scanners allow identification of flow-limiting coronary lesions and therefore offering great potential for both diagnosis and management. Advances in molecular biology of the cardiovascular system have helped to develop the molecular imaging which may be useful to evaluate targeted molecular and cellular abnormalities in the future. In this review, we will discuss the current state of the art in cardiac nuclear imaging, which include SPECT and PET evaluation of myocardial viability and perfusion. Radiopharmaceuticals used in cardiac SPECT and PET imaging are described, and radiation dose associated with the radiopharmaceuticals is briefly discussed. Clinical applications of hybrid imaging methods in coronary artery disease are presented and future directions are highlighted.

Coronary CT angiography is increasingly used in the diagnosis of coronary artery disease due to rapid technological developments, which are reflected in the improved spatial and temporal resolution. High diagnostic accuracy has been achieved with 64- and more slice CT scanners, and in selected patients, coronary CT angiography is considered a reliable alternative to invasive coronary angiography. Quantifying the amount of coronary artery calcium with coronary CT angiography has been widely accepted as a reliable non-invasive technique for predicting risk of future cardiac events. Coronary CT angiography is able to detect variable densities in the coronary atherosclerotic plaques and demonstrate the corresponding intraluminal changes due to presence of plaques, thus providing prognostic value of disease extent. Coronary CT angiography is an accurate method to measure coronary bifurcation angles and demonstrate dimensional changes, hence, providing relevant features of left coronary atherosclerosis. This article provides an overview of the diagnostic and predictive value of coronary CT angiography in coronary artery disease.

CT of the heart has evolved rapidly over the last decade. Adjunct medication in order to obtain the best possible images is a frequent requirement of coronary and perfusion CT examinations of the heart. Heart rate control and coronary dilation are the key objectives for coronary CT investigations; heart rate control is usually achieved with cardio-selective short acting B Blockers, where B Blockers are contraindicated as in severe asthma. Ivabradine an If channel blocker is an alternative drug this is usually prescribed for a week before the CT examination and therefore requires a significant amount of preparation and prior knowledge of the patient. Coronary dilatation is usually achieved by the use of short acting nitrates. Cardiac perfusion examinations by CT require the administration of adenosine or an adenosine agonist. In all cases of medication usage care must be taken to minimise drug interaction and mitigate the side effects of the drugs used as well as deciding on the order and timing of the use of medication [1, 2].

The application of magnetic resonance imaging (MRI) in evaluating the heart and its plethora of diseases has grown over the years. Abundant research and advancements in this modality had been made, largely due to the perceived benefits of MRI. These accepted advantages of MRI include absence of ionizing radiation, reproducible technique and does not require iodine-based contrast media. It had even been touted as the ‘one-stop shop’ for cardiac imaging. Cardiac magnetic resonance (CMR) has the ability to assess the heart comprehensively, examining cardiac morphology, ventricular function, perfusion, myocardial viability, as well as delineation and quantification of tissue constituents. Other benefits lie in the intrinsic properties of MRI which provide high temporal and spatial resolution, superb tissue contrast and possibility of imaging in any plane. In view of these superior advantages of CMR, it is currently the modality of choice in a number of cardiac diseases.

The purpose of quantitative evaluation of cardiac images is to acquire more objective, accurate, and delicate numeric data than from the results of qualitative observation. By quantitative analysis, the prediction of cardiovascular risk and the depiction of subtle changes have become more reliable. Until cardiac computed tomography (CT) and magnetic resonance imaging (MRI) were introduced 10 years ago, echocardiography had been traditionally used for cardiac morphology imaging. Based on technical advancements, cardiovascular CT and MRI started to be used for quantitative evaluation of cardiovascular structures. In this article, quantitative evaluation techniques in cardiovascular imaging will be discussed with an emphasis on CT and MRI.

The development and applications of quantitative imaging biomarkers are essential goals for modern biomedical imaging science. Imaging biomarkers are growing in their diversity and impact, and are of particular importance in the evaluation of novel drugs and treatments. In cancer, molecular imaging using PET and optical methods report directly on cellular events and characteristics, and are complemented by MRI, CT and US methods that measure downstream effects such as changes in tumor volume, cell density, tissue vascular properties and blood flow. These are being applied in developing new drugs and in clinical trials and are proving useful in cancer management, especially to evaluate treatment response. Integrating multiple data sets from different modalities such as PET and MRI can provide a more comprehensive view of tumor metabolic and physiologic state. In neuroscience, quantitative brain morphometry is used to characterize and distinguish subject groups and identify structural variants in individuals which correlate with behavior and function. PET studies of neurotransmitters and their transporters are well established and provide direct evidence of whether drugs hit specific targets, along with their in vivo binding properties. Functional MRI based on BOLD (blood oxygen level dependent) signals provides unique insights into neural circuits and inter-regional functional connectivities, which may be quantified to assess changes with treatment, development, degeneration or as an index of severity of disease. Pharmaceutical MRI uses similar measurements to evaluate the actions of drugs and signaling pathways in the brain. In diabetes and metabolic disorders, measures of tissue composition, physiology and metabolism provide quantitative indices of disease risk and progression. Overall, there are numerous such biomarkers under development in many different areas of application in each modality, and these activities define much of the research in current imaging science.

Advances in neuroimaging techniques have facilitated the study of anatomical and functional changes in the brain. Image analysis can aid precise diagnosis and treatment by providing quantitative measures. Despite the extensive research, automated analysis of neuroimages still remains a challenging problem. Integration of prior knowledge based on anatomical features can help to improve the accuracy. Brain's bilateral symmetry and its association with pathology could work as a priori when interpreting neuroimages for clinical diagnosis. This paper brings some of this research together to analyse symmetry integrated methods in neuroimaging and reviews various state-of-the-art symmetry plane detection techniques, applications, and key issues.