Cardiovascular Magnetic Resonance Imaging



Cardiovascular Magnetic Resonance Imaging


Joã L. Cavalcante



I. INTRODUCTION.

Cardiovascular magnetic resonance imaging (CMRI) has undergone rapid developments over the last two decades and is now an important imaging technique of the heart and great vessels. Advantages of CMRI include its large field of view, high spatial and temporal resolution, and ability to do tissue characterization. In contrast to nuclear imaging and cardiac computed tomography (CT), magnetic resonance imaging (MRI) does not involve exposure to ionizing radiation. Applications of CMRI include acquisition of anatomic-quality still and cine images of the heart and great vessels in multiple planes, precise measurement of cardiac chamber volume and function, assessment of myocardial perfusion and fibrosis, quantification of blood velocity and flow, and noninvasive magnetic resonance angiography (MRA). As CMRI is not a “push-button” technique, clear communication between the ordering physician and the imaging staff is important, indicating the reason for the CMRI examination so that adequate pulse sequences and imaging planes are obtained aiming to answer the desired clinical question.



II. INDICATIONS.

Common indications and components of the CMRI evaluation are listed in Table 51.1.


III. CONTRAINDICATIONS.

Contraindications to CMRI are listed in Table 51.2.


IV. BASICS OF CARDIAC MRI


A. MRI Physics.

Hydrogen is the most abundant atom in the body, and it is the excitation of hydrogen nuclei, often referred to as protons, that forms the basis for clinical MRI. Atoms behave like tiny bar magnets, aligning parallel to an external magnetic field while wobbling about the magnetic field at a certain frequency (precessional frequency) that creates the longitudinal magnetization. Application of a short radiofrequency (RF) pulse with the same precessional frequency as that of the atomic nucleus will cause excitation or resonance of the nucleus, temporarily changing its alignment within the magnetic field (transverse magnetization). However, this is an unstable state of higher energy. As the RF pulse is switched off, the spins quickly return to their resting state, i.e., aligned with the field, as this is energetically

the most favorable situation. The newly established transverse magnetization starts to disappear (a process called transversal relaxation), and the longitudinal magnetization grows back to its original size (a process called longitudinal relaxation). During this process, an RF signal is generated, which can be captured by the receiving coil and readily measured. This process constitutes the underlying principle of MRI.








TABLE 51.1 Cardiovascular Magnetic Resonance Imaging Indications and Applications
































Indication(s)

Applications


Aortic disease

Aortic aneurysm morphology and size; acute aortic pathology (dissection, intramural hematoma, penetrating ulcer); coarctation of the aorta; branch vessel disease; evidence of vasculitis; postoperative graft stenosis, infection, or leak; assessment for aortic regurgitation or other associated pathologies


Ischemic heart disease

Ventricular volumes and function; myocardial scar and viability; quantification of mitral regurgitation; assessment for LV aneurysm, thrombus, VSD, and other complications


Nonischemic cardiomyopathies

Ventricular volumes and function; myocardial wall thickness; LV outflow tract obstruction in hypertrophic cardiomyopathy; presence and patterns of myocardial scar/fibrosis; assessment for myocardial iron deposition in suspected hemochromatosis; quantification of mitral regurgitation; evaluation for ARVD in patients with ventricular arrhythmias or syncope


Pericardial disease

Pericardial effusion; pericardial thickening with or without calcification; pericardial tethering; signs of constrictive physiology including conical/tubular deformity of the ventricles, diastolic septal bounce, early cessation of diastolic filling, and dilated IVC


Congenital heart disease

Anatomic definition; ventricular volume and function; valve morphology and function; shunt calculation; assessment for anomalous origin of the coronary arteries; anomalies of the aorta, pulmonary arteries, and systemic and pulmonary veins


Valvular heart disease

Valve morphology; regurgitation and/or stenosis etiology and severity; ventricular size and function


Cardiac masses

Size and extent of mass; tissue characterization


Pulmonary veins

Pulmonary vein anatomy and stenosis; cardiac anatomy and function


ARVD, arrhythmogenic right ventricular dysplasia; IVC, inferior vena cava; LV, left ventricular; VSD, ventricular septal defect.









TABLE 51.2 Contraindications to Cardiovascular Magnetic Resonance Imaging












































Specific devices

Special issues


Cerebral aneurysm clips

Certain cerebral aneurysm clips pose a danger due to potential for displacement when exposed to a magnetic field. Aneurysm clips classified as “nonferromagnetic” or “weakly ferromagnetic” are safe.


Cardiac pacemakers and ICDs

The presence of a pacemaker/defibrillator is a strong relative contraindication to MRI owing to several potential problems, including (1) movement, (2) malfunction, (3) heating induced in the leads, and (4) current induced in the leads. In addition, artifact from the leads will often cause significant image degradation.

FDA has recently approved the first MRI-safe pacing system (Revo MRI by Medtronic, Inc.) that allows patients to undergo, for example, brain and knee MRI scans. Currently it is not safe for the area of coverage to include the chest, although ongoing work is being done toward that goal.


Cardiovascular catheters

Catheters with conductive metallic components (e.g., pulmonary artery catheters) have the potential for excessive heating. Hence patients with such devices should not undergo MRI.


Cochlear implants and hearing aids

Most types of implants employ a strong magnet or are electronically activated. Consequently, MRI is contraindicated because of potential injury or damage to the function of these implants. External hearing aids can and should be removed before the MRI procedure.


Intravascular coils, stents, and filters

These devices typically become incorporated securely into the vessel wall within 6-8 wk after implantation; hence, most are considered MRI safe. However, specific information on the type of device should be obtained before MRI is planned (www.mrisafety.com). Intracoronary stents have been shown to be safe during MRI, even when performed on the day of implantation, although many stent manufacturers recommend waiting 6-8 wk.


ECG electrodes

MR-safe ECG electrodes are strongly recommended to ensure patient safety and proper ECG recording.


Foley catheters

Certain Foley catheters with temperature sensors have the potential for excessive heating. They are generally safe if positioned properly and disconnected from the temperature monitor during MRI.


Heart valve prostheses

All types of heart valve prostheses have been shown to be safe during MRI. However, prosthetic material may lead to image artifacts.


Metallic foreign bodies

All patients with a history of injury with metallic foreign bodies such as a bullet or shrapnel should be thoroughly evaluated, as serious injury may result from movement or dislodgement of the foreign body.


Metallic cardiac occluders (e.g., management of PDA, ASD, or VSD)

MRI is safe for nonferromagnetic devices immediately after implant. Weakly ferromagnetic devices are safe from approximately 6-8 wk after placement, unless there is concern about retention of the device.


Retained epicardial pacing wires

MRI in patients with retained epicardial pacing wires after cardiac surgery appears safe. Retained transvenous pacing wires are a contraindication to MRI.


ASD, atrial septal defect; ECG, electrocardiogram; ICD, implantable cardioverter-defibrillator; MR, magnetic resonance; MRI, magnetic resonance imaging; PDA, patent ductus arteriosus; VSD, ventricular septal defect.


The signal generated by an excited proton is dependent on its molecular environment, such that the magnetic resonance (MR) signal from a hydrogen atom in blood can be discriminated from the MR signal from a hydrogen atom in fat or other tissue types. An MRI machine, therefore, includes a strong magnet that creates a continuous magnetic field and RF coils for transmitting the excitation pulses and receiving the radio signals generated by the excited protons. Application of predictable variations or “gradients” in the magnetic field, using gradient coils within the magnetic bore, allows three-dimensional (3D) spatial localization of each signal. The raw data are initially mapped in “k-space” before a Fourier transformation to generate the final MRI image.


B. T1, T2, and image contrast.

The rate of relaxation of an excited proton along the longitudinal axis (i.e., the direction of the external magnetic field) is described by its T1 time, whereas the transverse axis is described by its T2 time. T1 and T2 times depend on the molecular environment of the protons (intrinsic to the tissue characteristics) and the magnetic field strength. T1 and T2 relaxation times of differ ent tissues are important determinants of image contrast and, although not measured directly, images can be either T1 or T2 “weighted” to facilitate tissue characterization.


C. Issues specific to CMRI.

Cardiac and respiratory motion poses significant chal lenges to CMRI. In contrast to echocardiography, which is based on real-time imaging, CMRI sequences usually acquire a single image over several heart beats to optimize the spatial and temporal resolution. It is, therefore, necessary to gate images to the cardiac cycle with either an electrocardiographic or pulse signal. Electrocardiographic gating is usually retrospective, although prospective gating is sometimes useful, particularly in patients with arrhythmias. Respiratory motion is typically negated by performing breath-holds during the examination. In patients who are unable to maintain a breath-hold, averaging multiple MR signals may help to decrease the noise created by respiratory motion, at the expense of increasing the examination time by a factor of the number of signals averaged. Respiratory naviga tor sequences that coordinate imaging with a particular phase of diaphragmatic and hence respiratory motion are also effective, and they are typically used for pulse sequences that are too long for a single breath-hold, such as free-breathing wholeheart 3D coronary MRA sequences. Finally, real-time imaging using newer ultra-fast pulse sequences can be used in the absence of electrocardiographic or respiratory gating, at the expense of a significant decrease in temporal and spatial resolution.


D. CMRI pulse sequences and applications


1. Spin echo.

Spin-echo sequences are characterized by a refocusing RF pulse after delivery of the initial excitation pulse. Rapidly flowing blood appears dark, hence they are also known as “black-blood” sequences. Spin-echo sequences provide still images, which are typically used for anatomic delineation of the heart and great vessels owing to their excellent tissue contrast and high signal-to-noise ratio (SNR). They are relatively insensitive to magnetic field inhomogeneities and artifacts related to ferromagnetic objects such as sternal wires and prosthetic heart valves. Turbo spin echo is a newer technique that provides faster acquisition times than standard spin echo does. The main disadvantage of spin-echo sequences is the relatively long time it takes to acquire an image, making them more susceptible to motion artifacts and unsuitable for cine imaging.


2. Gradient echo.

Gradient echo sequences are characterized by the use of refocusing gradients after the delivery of the initial excitation pulse. Rapidly flowing blood appears bright, hence they are also known as “bright blood” sequences. Gradient echo is a fast imaging technique that is relatively insensitive to motion artifacts,
making it ideal for cine imaging. However, it has less tissue contrast and increased susceptibility to magnetic field inhomogeneities and ferromagnetic-related artifacts than spin-echo imaging but less than balanced steady-state free precession (B-SSFP). A variety of gradient echo sequences are widely used in CMRI for cine imaging, myocardial perfusion and scar assessment, coronary imaging, and MRA.


3. Cine imaging.

The most widely used pulse sequence for cine imaging is a gradient echo sequence called balanced steady-state free precession (B-SSFP), which is characterized by high SNR, high image contrast between blood and myocardium, and low sensitivity to motion artifact. However, B-SSFP is relatively insensitive to blood flow and, therefore, can be suboptimal for imaging of valve dysfunction or intracardiac shunts, which can usually be better illustrated using other gradient echo pulse sequences, such as echo planar imaging or phase velocity mapping. In addition B-SSFP is also more susceptible to magnetic field inhomogeneities which can be problematic in patients with mechanical valves or other cardiac implants.


4. Myocardial tagging.

RF pulses can be applied before the excitation pulse to generate dark saturation lines or grids on cine images, which are then tagged to the myocardium and further used to assess myocardial deformation. The tags can be used to help qualitatively assess myocardial motion and pericardial tethering or to quantitatively measure myocardial strain.


5. Perfusion imaging.

Very fast gradient echo sequences are used for dynamic imaging of left ventricular (LV) myocardial perfusion during the first pass of a gadolinium contrast agent during rest and stress states. Fast gradient echo techniques are commonly used, such as fast low-angle shot or B-SSFP with a prepulse to null or darken the myocardium. Normally perfused myocardium shows an increase in signal intensity due to gadolinium contrast, whereas abnormally perfused areas remain dark or hypoperfused.


6. Delayed imaging.

Delayed hyperenhancement imaging for myocardial scar or fibrosis is performed 10 to 30 minutes after injection of gadolinium contrast using gradient echo sequences with an inversion recovery prepulse to null signal from the myocardium. Areas of myocardial scar or fibrosis have a larger extracellular space with a greater accumulation and slower washout of gadolinium and, therefore, appear bright compared with dark, normal myocardium on delayed imaging.


7. Phase-contrast velocity mapping.

The phase difference in the spin of protons in moving blood compared with nonmoving protons within a magnetic gradient is called the “spin phase shift” and is proportional to the velocity of the moving protons. A phase-encoded image is constructed, with the gray level of each pixel coded for velocity. Phase-contrast velocity mapping could be considered analogous to pulse wave Doppler echocardiography. It can be used to measure blood velocity and hence quantify cardiac output, shunts, and valve dysfunction. There are, however, limitations, given that the accuracy of this method is highly dependent on factors such as flow pattern, flow velocity, size, and tortuosity of the vessel. Flow-related signal loss can be a result of loss of phase coherence that can occur in cases of significant flow acceleration and even in higher orders of motion present in complex flow patterns.


8. Magnetic resonance angiography.

MRA of the great vessels typically involves a 3D fast gradient-echo acquisition after injection of gadolinium contrast. The image resolution is typically 2 × 2 × 3 mm, making MRA an excellent option for imaging of large to intermediate size arteries, but less optimal for imaging of smaller vessels.


9. Parallel imaging.

A number of parallel imaging techniques make use of multiple receiving body coils to acquire extra data after each excitation pulse. This helps to decrease the imaging time and improve temporal resolution, but at the small relative cost of decrease in the SNR.


E. Contrast agents.

A number of gadolinium chelates are used as contrast agents in clinical MRI. Gadolinium significantly shortens the relaxation time of nearby
protons, thereby increasing their signal intensity

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Jun 7, 2016 | Posted by in CARDIOLOGY | Comments Off on Cardiovascular Magnetic Resonance Imaging

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