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ESC Textbook of Cardiovascular Imaging - sample

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Chapter 6 Hybrid

Chapter 6 Hybrid imaging: combination of pet, SpeCT, CT, and MRI Juhani Knuuti and Philipp A. Kaufmann Contents Introduction 89 Definition of cardiac hybrid imaging 89 Rationale for cardiac hybrid imaging 89 Integrated scanners versus software fusion 92 Imaging protocols for hybrid imaging 93 Image analysis and interpretation of hybrid imaging 94 Radiation safety aspects 95 Clinical impact of cardiac hybrid imaging 95 Future perspectives 96 Conclusion 97 References 97 Introduction Hybrid scanners combining PET or SPECT with high resolution multi-detector CT are becoming the standard for almost all commercially available nuclear imaging systems. In addition, the newest generation of scanners offers combination of PET with MRI. Hybrid scanners offer the ability to assess the anatomy of the heart and coronary arteries, and the functional evaluation either at stress (for assessment of induced ischaemia), or at rest (for viability) in association with the left ventricular systolic function. Therefore, combining functional information with anatomy by using hybrid systems is appealing [1]. Definition of cardiac hybrid imaging The term cardiac hybrid imaging has been proposed if images are fused combining two datasets, whereby both modalities are equally important in contributing to image information. Mostly this refers to combining CT with a nuclear myocardial perfusion imaging technique. Some reports have referred to X-ray based attenuation correction of perfusion imaging as hybrid imaging, raising confusion about its exact meaning because in such settings the CT or MRI data do not provide added anatomical information, but are simply used to improve image quality of the PET or SPECT modality. Similarly, the parametric maps obtained from low-dose CT do not provide image information beyond that needed for attenuation correction, although it could be used to obtain calcium scoring [2, 3]. Others have used the term hybrid imaging for the mere side-by-side analysis of perfusion and CT or MRI images. To avoid confusion we suggest using the term hybrid imaging to describe any combination of structural and functional information beyond that offered by attenuation correction or side-by-side analysis, by fusion of the separate datasets, for example, from CT coronary angiography and from SPECT or PET into one image. Similarly, separate acquisition of structural information, as well as functional data such as, for example, perfusion on two separate scanners or on one hybrid device, would allow mental integration of side-by-side evaluation but only fusion of both pieces of information would result in what should be considered a hybrid image (› Fig. 6.1). Rationale for cardiac hybrid imaging The field of cardiac imaging has witnessed an enormous development in the past years and is now offering an ever-increasing spectrum of tools and options to the clinicians. The potential disadvantage is that the patients may now be exposed to multiple, sequential, timeconsuming, and costly diagnostic tests and procedures, which may deliver occasionally

Chapter 7 New technical developments in cardiac CT Stephan Achenbach Contents Introduction 99 Historical development 99 Areas for improvement 101 Prospectively ECG-triggered axial acquisition 101 Single-beat acquisition 102 Iterative reconstruction 102 Dual-energy CT 103 Functional assessment of coronary artery lesions 103 CT myocardial perfusion imaging 103 Transluminal attenuation gradient 104 FFR CT 104 Automated plaque analysis 104 References 105 Introduction Computed tomography (CT), in the context of cardiac imaging, faces numerous challenges. The heart is a complex, three-dimensional organ, which moves very rapidly and has small dimensions. Especially the coronary arteries, the main target of cardiac CT imaging, are difficult to visualize by any non-invasive technique. All the same, technology progress has made the use of CT for cardiac and coronary diagnosis possible. For selected applications, including ruling out coronary artery stenoses in low-risk individuals, CT has become a clinical tool [1, 2]. Historical development In order to be useful for cardiac imaging, several prerequisites must be fulfilled (see Table 7.1). The first commercially available CT scanner that permitted visualization of the heart with high temporal and spatial resolution was the ‘electron beam tomography’ system introduced in the late 1980s. Instead of an X-ray tube, which needs to rotate mechanically around the patient, it used an electron beam that was deflected by electromagnetic coils to sweep across semi-circular targets arranged around the patient where the X-rays were created. The radiation passed through the patient and attenuation was recorded by stationary detectors arranged on the opposite side. Temporal resolution was 100 ms, but slice thickness was limited to 1.5 or 3.0 mm, images were relatively noisy, and cost was high. The electron beam system is no longer available, but it demonstrated the utility of CT imaging for coronary artery calcium assessment and even early CT coronary angiography [3]. This prompted the development of cardiac applications for mechanical CT systems. Around the year 2000, the first multi-detector row spiral (or ‘helical’) CT systems were introduced, permitting acquisition of up to four cross-sections with sub-mm thickness simultaneously along with ECG-synchronized image reconstruction. A relatively low pitch (table feed) was used, so that every level of the heart was covered during the entire cardiac cycle by at least one of the four detectors (this acquisition mode is called ‘retrospectively ECG-gated spiral acquisition’ and is still in use today). It allows all data acquired in systole to be discarded and images to be reconstructed based solely on X-ray attenuation data acquired during phases of slow cardiac motion in diastole. Multi-row acquisition was necessary to cover the complete volume of the heart within one breathhold, and with four-slice systems, acquisition typically required 35 to 40 s. Imaging the coronary arteries was cumbersome, but possible [4, 5]. It was soon reported that low heart rates substantially improve image quality (since the diastolic phase of slow motion is

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