Chapter 3 Cardiac CT—basic principles Filippo Cademartiri, Erica Maffei, Teresa Arcadi, Orlando Catalano, and Gabriel Krestin Contents Introduction 47 Basic cardiac CT technique 47 Definition of CT parameters 48 Patient selection 49 Scan parameters 49 Retrospective gating 49 Prospective triggering and radiation dose 51 Image reconstruction 52 Image evaluation 52 Limitations in MDCT coronary angiography 53 Future developments and outlook 53 References 53 Introduction Since 2004 multi-detector computed tomography (MDCT) scanners have been available, which enable the simultaneous acquisition of 64 slices per rotation. The additional improvement in number of slices per rotation, spatial and temporal resolution that followed has already provided excellent results in the field of cardiac imaging. Nowadays we further optimize technical aspects that may provide new clinical insight and applications. Basic cardiac CT technique The most important components of a CT system are the X-ray tube and the system of detectors ( Fig. 3.1). The combination of a fast rotation time and multi-slice acquisitions is particularly important for cardiac applications [1, 2]. The latest generation of 64- MDCT scanners meets these requirements. They are able to acquire 64 sub-millimetre slices per rotation and routinely achieve excellent image quality and the visualization of small- diameter vessels of the coronary circulation, combining isotropic spatial resolution (0.4 mm 3 ) with gantry rotation speeds of 330 ms. They also redefine the MDCT methodology of analysing coronary plaque and evaluating stent lumens. Until a few years ago, CT systems had only a single row of detectors, which meant that for each rotation they were able to acquire only one slice. These systems were followed by others known as multi-slice or multi-detector-row, featuring many detector rows positioned in a two-dimensional array. During a rotation, numerous contiguous slices are acquired. As a result, a broader region of the body can be acquired in the same timeframe, with an improvement in image quality. This also has the advantage of drastically reducing examination times, which is an important factor given that thoracic and abdominal examination requires the patient to maintain breath-hold to guarantee that image quality is not compromised by chest motion. The clinical impact of the new technology lies in the improvement in image quality in terms of both spatial and temporal resolution. The improvement in spatial resolution includes numerous features of non-invasive coronary imaging: ◆ it increases the ability to visualize small-diameter vessels (e.g. the distal coronary branches) ; ◆ it increases the ability to quantify calcium, in that it reduces blooming artefacts; ◆ it enables the reduction of blooming artefacts in stents and therefore enables the visualization of the stent lumen; ◆ it improves the definition of the presence of coronary plaques and better quantifies their characteristics (volume, attenuation, etc.).
Chapter 4 CMR—basic principles Jeremy Wright and Jan Bogaert Contents Introduction to MRI physics 55 Contraindications to MRI 55 MRI contrast agents 56 Pulse sequences 56 Cardiac motion 56 Respiratory motion 57 Cardiac function 57 Cardiac morphology and tissue characterization 58 Delayed contrast enhancement 58 Myocardial perfusion 60 Velocity encoded CMR 60 MR angiography 61 Conclusions 61 References 62 Introduction to MRI physics Magnetic resonance imaging (MRI), formerly called nuclear magnetic resonance (NMR), relies on the physical properties of hydrogen nuclei (protons). These protons, abundantly present in the human body, have an intrinsic ‘spin’. When a patient is brought into a highstrength magnetic field, the ‘spins’ of the human body align with the direction of the magnetic field . Application of a radiofrequency (RF) pulse can excite the spins and perturb their alignment, with vector components in line with the magnetic field (longitudinal magnetization) and perpendicular to the field (transverse magnetization). These spins gradually return to their resting state (relax), and in the process create RF signals, which are used to create an image. The magnitude of signal arising from the tissue is mainly influenced by two relaxation times (T1 and T2), proton density, and movement of the protons (blood flow) . T1 is the time constant describing the return of longitudinal magnetization to baseline, and T2 is the time constant describing return of transverse magnetization to baseline. Note that T1 and T2 of a proton are independent, and vary according to the local environment of the proton (i.e. the tissue). This phenomenon enables the excellent soft-tissue discrimination seen in MRI images. Fat and water are at the extremes of T1 and T2 relaxation times. Fat has short T1 and T2, whereas water has long T1 and T2 times. T1-weighted images exploit the differences in T1-relaxation behaviour between tissues. For instance, fat has a hyper-intense (‘bright’) appearance, fluid has a hypo-intense (‘dark’) appearance, while myocardial tissue is iso-intense (‘grey’). In comparison, on T2-weighted images, fluid has a bright appearance, while fat has a less bright appearance. Image formation also requires understanding of the origin of a particular signal in the patient. This is achieved by application of magnetic field gradients in a process called spatial encoding, a detailed discussion of this can be found in any basic MR textbook. For any image ‘slice’ the raw data acquired are called ‘K-space’ , and consist of multiple ‘lines’ of data (typically between 128 and 256). To generate an image, the K-space data undergo a complex mathematical process called Fourier transformation. The key concept of this transformation is that the centre of K-space contains image contrast information, while image resolution is governed by the periphery of K-space . Contraindications to MRI The main contraindications to MRI relate to the presence of metal implanted within the patient. Non-magnetic material has a risk of heating and electric current induction, and ferromagnetic material may move in the magnetic field. An implanted programmable device (neurostimulator, insulin pump, cochlear implant, etc.) can malfunction when exposed to magnetic fields and RF pulses. Sternal wires, most prosthetic cardiac valves, coronary stents, orthopaedic implants, and surgical clips are not contraindications to