(BQ) Part 2 book Cardiovascular Imaging presents the following contents: Cardiovascular magnetic resonance (clinical applications, emerging applications of cardiovascular MRI), future prospects of cardiovascular (molecular imaging, specific cardiovascular applications of molecular imaging, multidisciplinary cardiovascular imaging programs).
120 CARDIOVASCULAR MAGNETIC RESONANCE IMAGING André Schmidt Joao AC Lima INTRODUCTION Cardiac magnetic resonance imaging (CMRI) (Lund 2001) is a promising imaging modality with substantial clinical applications thanks to its unique diagnostic versatility CMRI provides detailed anatomical information about the heart and also allows for the assessments of global and regional cardiac function, volumes, and mass, and the assessments of myocardial perfusion, valvular function, and tissue characterization In this chapter various established and emerging clinical applications of CMRI will be discussed MRI principles Paramagnetic substances with an odd number of protons and/or neutrons, such as 1H, 14N, 31P, 13C, and 23Na, have the property of spinning (precession) around their axes and can be used for ‘imaging’ by MRI When exposed to a magnetic field, these atoms will align with the magnetic field and continue to precess Hydrogen is the atom most widely used in MRI because of its abundant presence in the human body and optimal signal strength Therefore, unless stated otherwise, the MRI described in this chapter is referred to 1H MRI From the basic physics it is known that a moving charged particle generates a magnetic field When exposed to an intense magnetic field, such as that generated by MRI equipment, all individual magnetic fields are aligned and a resultant vector is obtained Another important concept of MRI is the Larmor equation: f = γM, where f is frequency in revolutions per second of the precessing substance, γ is the gyromagnetic ratio of the substance (e.g hydrogen) which is a constant, and M is the strength of the magnetic field expressed in Tesla (T) According to this equation, the frequency of precession is directly proportional to the strength of the magnetic field, and since the gyromagnetic ratio is a constant being specific for each substance, the frequency of precession is unique The strength of the magnetic field in a specific location can be manipulated to obtain information from which images can be generated Radiofrequency (RF) pulses are used to manipulate the strength of the magnetic field and to generate tomographic images of the body, since they can be emitted in precise dimensions An important distinction to be made is that the continuous magnetic field in a MRI scanner is due to a direct current, while RF pulses are generated by alternating currents located in the coils inside the MRI scanner These pulses induce an electromagnetic wave that affects the precessing, making it tip over its direction The maximal effect is obtained when the nuclei are deflected by 90° When the RF pulses cease, a return to its original position begins, but in order to so, the protons have to release the energy gained from the RF pulses By collecting this information and using a mathematical procedure called ‘Fourier Transform’, a computer can generate and display an accurate image with degrees of intensity (gray levels) and a precise spatial location By using the distinct patterns of release of this energy from tissues with distinct percentages of hydrogen, a precise map of the tissues can be obtained CMRI (Lund 2001) has some unique characteristics Usually, a static image can be obtained by repeating RF pulses during data acquisitions Due to the fact that the heart is beating, electrocardiographic Cardiovascular Magnetic Resonance Imaging gating is needed to define precisely the time point in the cardiac cycle where the RF pulses are applied This ECG-gating procedure should be repeated until the image of the particular point is completed so that image of the next point can be acquired without motion interference Several consecutive cardiac cycles are needed to produce an image of each particular point As such, long image acquisition time is expected for CMRI Also, it should be understood that the patient should not breathe during the image acquisitions, since it would otherwise change the position of the heart, causing another image artifact Thus, CMRI is an examination that requires careful monitoring in order to obtain good-quality images Nonetheless, continuous improvements in techniques have substantially reduced the effects of cardiac and respiratory motion MRI scanner MRI scanners consist of a superconducting magnet that produces a strong magnetic field, expressed in Tesla For cardiac applications a 1.5 T MRI scanner is usually used A uniform magnetic field can be generated and can also be manipulated with the use of gradients to produce small differences, providing spatial location In addition, the scanner has coils to generate the specific RF pulses that resonate the specific atoms at a specific location, and antennae located in the coils will receive the signals that can be analyzed and organized by a computer to reconstruct images The computer is also needed in order to generate the sequence of RF pulses and gradient adjustments during the examination MRI safety Besides the usual precautions applicable to any MRI scan, it should be remembered that the MRI suite is a tight place, usually not suitable for emergency care Patients who are clinically unstable should not undergo a MRI examination, unless they are supervised by trained personnel and there is emergency equipment near the MRI suite where the patients can be rapidly removed to if needed The main practical limitation for MRI is currently the imaging of patients with vascular clips, used for cerebral aneurysm surgery; there is a potential risk of dislodgement (Fenchel et al 2005) Patients with implanted cardiac pacemakers, defibrillators, cochlear implants, and neurologic stimulators also should not undergo a MRI examination due to possible malfunctioning of these devices or the potential risk caused by the generation of currents Recent data has shown that MRI examinations in a 1.5 T scanner are feasible and safe for most modern pacemakers and intracoronary stents, although some image artifacts occur Prosthetic heart valves, unless they have a large amount of alloy, such as the StarrEdwards pre-6000 series (Edwards et al 2000), present no problem Other metallic material that often exists in patients who had cardiac surgery, such as sternal wires, clips, and epicardial pacing leads, have not been reported to cause complications (Shellock & Kanal 1994) MRI has no known effect on the fetus Nevertheless, an MRI examination of pregnant women is usually postponed until the second trimester Safety concerns should be balanced against the benefits expected in each individual clinical situation Claustrophobia can occur in 1–5% of patients, but the use of light sedation, without compromising cooperation from the patient, may solve this problem There are internet sites (e.g www.mrisafety.com) that provide accurate and updated information on safety issues related to MRI which can be helpful in specific situations The increasing use of contrast agents has raised the question of intolerance and risks Gadolinium, a rare metal, tends to accumulate in tissues due to its high affinity for membranes It is used in the form of chelates, which are water soluble and are not nephrotoxic Most of the injected gadolinium is excreted quickly by the kidneys Some fecal excretion also occurs It is rarely associated with allergic reactions (Ungkanont et al 1998) Metallic taste is the most common side-effect, followed by headache, nausea, and vomiting The side-effects are usually mild and rarely require medical intervention Severe allergic reactions are rare (