Hybrid and Multimodality Imaging



Hybrid and Multimodality Imaging


Albert J. Sinusas



INTRODUCTION

Multimodality imaging is routinely used to facilitate the diagnosis and evaluation of cardiovascular diseases, along with risk stratification, and directing and monitoring therapy. Hybrid imaging permits the integrated assessment of structure and physiologic indices. There is now a focus on developing multimodality imaging strategies to assess the underlying cellular and molecular processes that underlie and/or participate in modulating the structural and pathophysiologic changes in the cardiovascular system. Thus, the future of multimodality imaging involves the simultaneous evaluation of structure, physiology, and molecular processes.

With the critical advancement in hybrid imaging technology, a parallel development of imaging probes for improved evaluation of these physiologic and molecular changes has occurred. The most widely used approaches for molecular imaging include radiotracers that allow for high sensitivity in vivo detection and quantification of molecular processes with single-photon emission computed tomography (SPECT) and positron emission tomography (PET).1 Although SPECT and PET imaging provide high sensitivity for detection and quantitative assessment of molecular and physiologic processes, they suffer from limited resolution. Therefore, these radiotracer-based imaging approaches have been integrated with high-resolution anatomic imaging approaches such as x-ray computed tomography (CT) or magnetic resonance imaging (MRI) to provide anatomic co-localization and improved quantification.

Multimodality imaging can also be used to guide interventional and surgical procedures. The integration of three-dimensional (3D) imaging with cone beam tomography and real-time fluoroscopy is now part of clinical practice.

This chapter reviews the development of hybrid imaging technology for evaluation of critical physiologic and molecular processes in the cardiovascular system, including inflammation, cell death, autonomic regulation, angiogenesis, and myocardial vascular remodeling. The evolving practice of image-guided interventions is also addressed. These advancements in multimodality imaging technology should lead to improved image quantification and diagnostic accuracy, thereby promoting “precision medicine,” and a more personalized approach to health care delivery.


FUNDAMENTALS OF HYBRID IMAGING


Technologic Advantage

The primary aim of multimodality hybrid imaging is to take advantage of the different physical properties of image formation from each modality to create an image set with complementary (or even synergistic) information, to produce a superior image set for research purposes, clinical applications, or a combination of both. A hybrid imaging methodology might simply improve quantification of a targeted molecular probe or imaging approach or provide anatomic localization, as would be the case for hybrid SPECT/CT, PET/CT or PET/MRI. Hybrid imaging technology may also provide new insight into a molecular pathway or pathophysiologic process or provide a link between molecular processes and associated anatomic or physiologic events. Clinical application of a hybrid imaging technology may result in improved diagnostic accuracy, improved prognostication, or monitoring a disease process or therapeutic intervention.

Therefore, the advantage of any multimodality imaging approaches is dependent on acquiring information that is more valuable than that from individual components, or even the composite sum of the components. Ideally, one should obtain unique information that can only be derived using a truly integrated hybrid technology, as opposed to acquiring images on two separate imaging systems and fusing the datasets following acquisition.


IMAGING MODALITIES AND PROCEDURES

There are many multimodality and hybrid imaging technologies currently under development for the evaluation of the cardiovascular system. This chapter cannot address all of these modalities; however, a recently published textbook that provides a very comprehensive review of hybrid imaging in cardiovascular medicine is available.2 This chapter focuses on the most clinically relevant modalities, along with those that have translated to routine clinical application.


Single-Photon Emission Computed Tomography/Computed Tomography

The use of hybrid SPECT/CT scanners allows the patient to be imaged on the same equipment table in the exact same position, which minimizes changes in the patient orientation
between the CT and SPECT images. Thus, hybrid SPECT/CT imaging systems automatically co-register the two sets of image data, which facilitates the use of CT for SPECT attenuation correction. The CT component may be a slow-rotating nondiagnostic scanner or a fast-rotating multidetector diagnostic scanner. The slow-rotation systems generally mount the CT x-ray tube and detector on the same gantry as the SPECT detectors. Although the CT images are suboptimal for diagnostic purposes, they provide sufficient image quality for attenuation correction.3 The hybrid SPECT/CT systems that incorporate a fast-rotating multidetector CT scanner use separate gantries for SPECT and CT imaging. Decoupling the SPECT and CT gantries allows for rapid-rotation and diagnostic CT imaging. Hybrid SPECT/CT systems have incorporated as many as 64-slice detectors. These more expensive, fast-rotation systems are capable of diagnostic CT imaging for either calcium scoring or CT angiography. However, the fast-rotation speed of the CT compared to the SPECT acquisition can result in image mismatch because of respiratory motion and introduce additional artifacts in the attenuation correction scans when performing cardiac SPECT/CT imaging. Reconstruction artifacts may also be introduced because of metal implants, or truncation of the attenuation CT scans.

Although a high-quality CT introduces some complications, these high-end hybrid systems provide valuable complementary information that improves risk stratification of patients undergoing stress perfusion imaging.4,5,6 The fusion of anatomic information from CT and physiologic information from SPECT can better identify hemodynamically significant stenoses, and physiologically guided intervention may be beneficial.7 The use of hybrid SPECT/CT imaging may be particularly helpful in the setting of complex congenital heart disease, or in the evaluation of anomalous coronary arteries. Figure 38.1 illustrates the value of hybrid SPECT/CT imaging in evaluation of the physiologic significance of an anomalous right coronary artery that originates from the ostium of the left main coronary artery.






Other novel clinical applications of hybrid SPECT/CT include the noninvasive diagnosis of transthyretin cardiac amyloidosis (Figure 38.2) and the evaluation of peripheral arterial disease (Figure 38.3). In both clinical conditions, the registered anatomic information from CT is used to improve localization and quantification of SPECT radiotracer uptake.


Positron Emission Tomography/Computed Tomography

Hybrid imaging has become the convention for cardiac PET imaging, and hybrid PET/CT systems are generally equipped with diagnostic CT scanners. The use of CT images for attenuation correction of PET images offers the same advantages and limitations associated with hybrid SPECT/CT. Some hybrid PET/CT scanners are even capable of dual-energy CT imaging, which offers some unique advantages related to correction for beam hardening and metal artifacts, and material decomposition. Higher resolution CT scanners (64-slice and above) and hybrid imaging with PET and contrast CT imaging can provide a detailed anatomic assessment of the cardiovascular system complemented by either functional or metabolic information provided by PET, particularly if the CT images are acquired with iodinated contrast. Contrast CT images provide definition of the endocardial and epicardial surfaces of the heart, allowing for partial volume correction of the PET images and more accurate estimation of tissue activity and quantification of myocardial blood flow.

Coronary computed tomography angiography (CCTA) can characterize the epicardial coronary vessels and provide

information on the degree of stenosis within the vessel, as well as atherosclerotic plaque extent and morphology.8 Investigators have developed algorithms that provide automated analysis of plaque features.9,10 Although CCTA has excellent negative predictive value for ruling out obstructive disease, it tends to overestimate the severity of the stenosis.11 Therefore, the combination of anatomic information from computed tomography angiography (CTA) and the physiologic information from PET can provide a more comprehensive assessment of coronary artery disease. Alternatively, the functional significance of a coronary stenosis detected by CCTA can be estimated with computational fluid dynamics and determination of the fractional flow reserve (FFR). The accuracy of the combined assessment with both modalities is better than with standalone PET or CCTA in several observational studies.12,13,14 This improved accuracy is due to improved specificity, with either normal perfusion in the territory of a stenosis thought to be obstructive on CCTA, or normal coronary anatomy in an area thought to demonstrate abnormal perfusion. Figure 38.4 provides an example of how hybrid PET/CT imaging can guide management in a patient with complex congenital heart disease. There is also evidence that hybrid PET-CCTA imaging can reduce referral for unnecessary downstream coronary angiography.15 Although the evaluation of both anatomy and function with hybrid PET/CT can increase overall diagnostic accuracy, PET-CCTA is associated with both greater cost and increased radiation exposure to the patient.