Ebook Cardiovascular Imaging: Part 2
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Ebook Cardiovascular Imaging: Part 2
5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2 imaging modality with substantial clinical applications thanks to its unique diagnostic versatility. CMR1 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 perfus Ebook Cardiovascular Imaging: Part 2ion, valvular function, and tissue characterization. In this chapter various established and emerging clinical applications ofCMRI will be discussed.MEbook Cardiovascular Imaging: Part 2
RI principlesParamagnetic substances with an odd number of protons and/or neutrons, such as *H, |4N, 11 p, l3C, and 23Na, have the property of spinnin5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2d 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 *H .MRI.From the basic physics it is known that a moving charge Ebook Cardiovascular Imaging: Part 2d particle generates a magnetic field. When exposed to an intense magnetic field, such as that generated by MRI equipment, all individual magnetic fieEbook Cardiovascular Imaging: Part 2
lds arc aligned and a resultant vector is obtained. Another important concept of MRI is the Larmor equation: f - yM, where f is frequency in revolutio5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2themagnetic 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 t Ebook Cardiovascular Imaging: Part 2he magnetic field in a specific location can be manipulated to obtain information from which images can be generated.Radiofrequency (RF) pulses arc usEbook Cardiovascular Imaging: Part 2
ed to manipulate the strength of the magnetic field and to generate tomographic images of the body, since they can be emitted in precise dimensions. A5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2 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 positi Ebook Cardiovascular Imaging: Part 2on begins, but in order to do so, the protons have to release the energy gained from the RF pulses. By collecting this information and using a mathemaEbook Cardiovascular Imaging: Part 2
tical procedure called ‘Fourier Transform’, a computer can generate and display an accurate image with degrees of intensity (gray levels) and a precis5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2tissues 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, electrocardiographicCardiovascular Magnetic Resonance Imaginggating is needed to define precis Ebook Cardiovascular Imaging: Part 2ely 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 particulaEbook Cardiovascular Imaging: Part 2
r point is completed so that image of the next point can be acquired without motion interference. Several consecutive cardiac cycles are needed to pro5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2ld 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 subst Ebook Cardiovascular Imaging: Part 2antially reduced the effects of cardiac and respiratory motion.MRI scannerMRI scanners consist of a superconducting magnet that produces a strong magnEbook Cardiovascular Imaging: Part 2
etic field, expressed in Tesla. For cardiac applications a 1.5 T MR1 scanner is usually used. A uniform magnetic field can be generated and can also b5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2pecific 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 adjust Ebook Cardiovascular Imaging: Part 2ments during the examination.MRI safetyBesides the usual precautions applicable to any MRI scan, it should be remembered that the MRI suite is a tightEbook Cardiovascular Imaging: Part 2
place, usually not suitable for emergency care. Patients who arc clinically unstable should not undergo a MRI examination, unless they arc supervised5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2mitation for MRI is currently the imaging of patients with vascular clips, used for cerebral aneurysm surgery; there is a potential risk of dislodgemcnt (Fcnchcl Ct al. 2005). Patients with implanted cardiac pacemakers, defibrillators, cochlearimplants, and neurologic stimulators also should not und Ebook Cardiovascular Imaging: Part 2ergo a MRI examination due to possible malfunctioning of these devices or the potential risk caused by the generation of currents. Recent data has shoEbook Cardiovascular Imaging: Part 2
wn that MRI examinations in a 1.5 T scanner are feasible and safe for most modem pacemakers and intracoronary stents, although some image artifacts oc5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2oblem. 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).MR1 has no known effect on the fetus. Nevertheless, an MRI examination of pregnant wo Ebook Cardiovascular Imaging: Part 2men is usually postponed until the second trimester. Safety concerns should be balanced against the benefits expected in each individual clinical situEbook Cardiovascular Imaging: Part 2
ation. Claustrophobia can occur in 1-5% of patients, but the use of light sedation, without Compromising cooperation from the patient, may solve this 5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2helpfill 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 arc water soluble and arc not nephrotoxic Ebook Cardiovascular Imaging: Part 2. Most of the injected gadolinium is excreted quickly by the kidneys. Some fecal excretion also occurs. It is rarely associated with allergic reactionEbook Cardiovascular Imaging: Part 2
s (Ungkanont Ct al. 1998). Metallic taste is the most common side-effect, followed by headache, nausea, and vomiting. The side-effects are usually mil5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2dolinium should be avoided in pregnant or lactating women.CLINICAL APPLICATIONSC.MRI is an emerging diagnostic methodology. Clinical applications arc expanding because of its potential for evaluating various aspects of cardiovascular diseases.122Cardiovascular Magnetic Resonance ImagingAnatomical ev Ebook Cardiovascular Imaging: Part 2aluationCMRI allows for imaging of the heart and the great vessels in any desired planes in order to explore anatomic relations and surrounding structEbook Cardiovascular Imaging: Part 2
ures (165). Thanks to this flexibility it is possible to perform GMR1 in planes that are familiar to cardiologists trained in X ray GT and echocardiog5. CardiovascularMagnetic Resonance ImagingAndré Schmidt Joao AC LimaINTRODUCTIONCardiac magnetic resonance imaging (CMRI) {Lund 2001) is a promising Ebook Cardiovascular Imaging: Part 2 used when the heart itself is being evaluated (Dinsmore el al. 19X4). The accuracy of plane orientation is extreme, allowing for precise orientation and selection of the best plane to demonstrate the point of interest. Standardization of myocardial segmentation has been recommended in order to faci Ebook Cardiovascular Imaging: Part 2litate the comparison between cardiovascular imaging modalities (Cerqueira er al. 2002).Slice thickness, image resolution, and application of sequenceEbook Cardiovascular Imaging: Part 2
s arc selected as needed to acquire the desired information. As discussed later, small structures such as the walls of coronary arteries can be defineGọi ngay
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