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Published online September 17, 2002, 10.1148/rg.e6
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(Radiographics. 2002;22:e6-e6.)
© RSNA, 2002

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Cardiovascular MR Imaging: Technique Optimization and Detection of Disease in Clinical Practice1

Ron C. Gaba, MD, Ruth C. Carlos, MD, MS, William J. Weadock, MD, Gautham P. Reddy, MD, MPH, Michael B. Sneider, MD and Philip N. Cascade, MD

1 From the Department of Radiology, University of Michigan, 1500 Medical Center Dr, Ann Arbor, MI 48109 (R.C.G., R.C.C., W.J.W., M.B.S., P.N.C.); and the Department of Radiology, University of California, San Francisco (G.P.R.). Presented as a scientific exhibit at the 2001 RSNA scientific assembly. Received March 15, 2002; revision requested May 16; revision received and accepted July 19. R.C.C. supported in part by the GERRAF program. Address correspondence to R.C.C. (e-mail: rcarlos@umich.edu).


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Magnetic resonance (MR) imaging has emerged as an important and growing means of cardiovascular imaging, with many advantages over other radiologic modalities, including excellent spatial and temporal resolution, lack of ionizing radiation, and noninvasiveness. In this article, the utility of MR imaging in cardiovascular imaging and in the diagnosis of cardiovascular disease will be discussed. MR techniques for evaluating the heart and vasculature will be described, and troubleshooting techniques will be presented. Imaging findings in congenital anomalies such as septal defects, patent ductus arteriosus, transposition of the great arteries, and tetralogy of Fallot will be identified. Valvular lesions and methods for evaluating valvular function will be discussed. MR imaging findings in acquired disorders such as aneurysms and pericardial disease will be described.

© RSNA, 2002

Index Terms: Aneurysm, aortic, 56.73 • Aorta, abnormalities, 56.15, 56.16 • Aorta, dissection, 56.74 • Aorta, MR, 56.12141, 56.12142, 56.12143 • Aorta, stenosis or obstruction, 56.151 • Heart, abnormalities, 51.14, 51.17 51.73 • Heart, MR, 51.12141, 51.12142, 51.12143 • Heart, valves, 53.12141, 53.12142, 53.12143 • Heart, ventricles, 52.12141, 52.12142, 52.12143


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Cardiovascular disease, including hypertension, coronary heart disease, stroke, congenital cardiovascular defects, congestive heart failure, and peripheral vascular disease, is the leading cause of morbidity and mortality in the United States. These conditions affect more than 60 million people and are responsible for the death of almost 1 million Americans per year (1).

Imaging represents an important component of the diagnosis and evaluation of cardiovascular disease. Modalities that are routinely used to image the heart and vasculature include standard radiography, echocardiography, nuclear imaging, cineangiography, and computed tomography (CT). Magnetic resonance (MR) imaging has recently emerged as an important and growing means of cardiovascular imaging, with many advantages over other radiologic modalities. Advantages include excellent spatial and temporal resolution, lack of ionizing radiation, and noninvasiveness (2). Current advances in MR imaging technology allow a new and wide range of cardiovascular applications. Although currently underused, cardiovascular MR imaging growth will likely be explosive, enabling a comprehensive MR examination to answer a wide variety of clinical questions.

In this article, the utility of MR imaging in cardiovascular imaging and in the diagnosis of cardiovascular disease will be discussed. The indications for the use of MR imaging for cardiovascular imaging will be presented, along with elements of patient management before and during MR studies. MR imaging techniques for evaluating the heart and vasculature will be described, and troubleshooting techniques will be presented. The MR imaging features of various cardiovascular abnormalities will be reviewed.

Throughout this article, place the mouse cursor over an image to highlight the findings in color. Advanced applications such as detection of wall motion abnormalities, cardiac ischemia, and coronary arterial imaging are beyond the scope of this review.


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Patient Management
Patient management, including both the preparation of the patient prior to an imaging procedure and the care of the patient during an imaging procedure, represents an important consideration in MR imaging. Prior to any MR imaging examination, the patient must be screened for contraindications to MR imaging. Advance knowledge of the presence of clinical disease, in particular cardiac arrythmias and respiratory disorders, will increase the likelihood of a successful examination. Absolute contraindications include the presence of a cardiac pacemaker, cardiac defibrillator, Swan-Ganz catheter, or other conductive metallic devices. A relative contraindication is the presence of a new coronary stent placed within 6 weeks prior to the MR study.

Intravascular devices, including stents made from nonferromagnetic materials, are considered safe for MR imaging procedures in static magnetic fields up to and including 1.5 T immediately after implantation (3,4). Weakly ferromagnetic devices that may exhibit magnetic qualities typically become securely attached to the vessel wall approximately 6–8 weeks after implantation due to tissue growth; hence, a 6–8 week delay is recommended to ensure that stents remain in position (3). Further, manufacturers may not distinguish between ferromagnetic and nonferromagnetic devices in their documentation; therefore, a 6–8 week delay is recommended if full information regarding the implanted device is not available. Cardiac ischemia and infarct quantitation are becoming important indications for cardiac MR imaging. Many of these patients may have received stents more recently than the 6-week waiting period. Identification of a nonferromagnetic stent allows immediate MR imaging. If the stent is ferromagnetic or the composition is unknown, the relative value of performing the MR imaging procedure must be weighed against the small risk of stent migration during the 6–8 week window.

During MR imaging, a cardiac or torso phased-array coil for adults should be used. In infants, the head coil is appropriate. Electrocardiograph (ECG) leads should be placed on the patient for gating. ECG gating is superior to peripheral gating, since there is an intrinsic delay between the ECG R-wave and peripheral detection of the gating signal. Variations in the temporal relationship between the R wave and peripheral detection of the gating signal may produce inconsistencies in the timing of image acquisition (5). Skin contact of the ECG leads should be ensured by the placement of a conductive gel and by skin shaving if necessary. Anterior chest positioning of the ECG leads is physically easier than posterior chest positioning; however, posterior chest positioning of the ECG leads yields less motion artifact during the imaging study. The ECG leads should not be allowed to form loops, to reduce the hazard of superficial burns. The cables should be aligned parallel to the bore to reduce electrical interference. Although wider spacing between leads yields a better ECG signal, narrower spacing results in fewer artifacts due to magnetic gradients.

During the imaging study, the patient should be monitored and supported with MR-compatible equipment if necessary, including oxygen and intravenous (IV) sedation. Supplemental oxygen aids breath holding. Sedation can decrease motion artifact and patient anxiety and is essential in infants and young children. Sedation guidelines vary from institution to institution and with the experience of the individual practitioner; therefore, we do not wish to recommend specific doses. Appropriate screening and monitoring of the sedated patient by trained personnel is imperative. For children less than 2 years old, our institutional guidelines recommend oral chloral hydrate, with the dose titrated by weight. Diphenhydramine may be used in addition, again titrated by weight and response of the child to previous medication. For children older than 2 years, pentobarbital can be administered intravenously, titrated by weight to a maximum dose of 150 mg. A wider range of sedation options exist for adults and can include both oral and intravenous medications, depending on the preference of the practitioner. General anesthesia is a last resort for individuals who fail conscious sedation.

ECG Troubleshooting
In the event that problems due to ECG gating arise during an MR imaging study, certain troubleshooting techniques may be used. As the patient is placed within the magnet, an electrical current, termed magnetohydrodynamic effect, is induced in flowing blood, which results in distortions of the ECG signal (6). Distortion is most pronounced during systole and may result in tall or inverted T waves (Fig 1), potentially masking ischemia occurring during MR imaging. Repositioning the leads may decrease the size of the T waves or result in their normalization. An increased heart rate due to T-wave elevation may be countered by increasing the trigger level. Increasing the trigger level also aids in evaluating tachycardic patients.



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  		Figure 1.  Sample ECG tracings.

 
Lead placement may result in an inverted R wave (Fig 1) and missed triggers. Swapping positions of the leads to the right arm and left leg may be necessary. Alternatively, one may cycle through lead choices, taking care not to rely on the automatic lead selection generated by the MR unit. Changing field gradients during the cardiac examination also may induce current on the ECG tracing, which manifests as noise. Although most modern gating systems have filtering mechanisms that decrease the likelihood of inappropriate or missed triggers, switching leads may also help. Repeated missed triggers may be avoided by decreasing the trigger level. If all maneuvers fail, ECG gating may be turned off and the examination attempted with use of peripheral gating. This can be useful in imaging infants or patients with extremely rapid heart rates.

MR Imaging Sequences and Troubleshooting
Basic cardiovascular MR imaging sequences include black blood imaging and bright blood imaging. Black blood imaging is used to depict anatomy, pericardial and mediastinal abnormalities, and extraluminal aortic disease. Black blood imaging includes ECG-gated true spin-echo or fast spin-echo imaging or inversion-recovery (IR) half-Fourier single-shot turbo fast-spin-echo sequences. Spin-echo sequences generate black blood due to time-of-flight effects of flowing blood that vary with the echo time (TE). Longer TEs permit the blood to exit the section before generating a signal. With slow flow rates, the blood may remain in the section long enough to generate signal, giving an incomplete black blood effect (7,8). Lengthening the TE may suppress signal from slowly flowing blood, at the expense of T1 weighting and longer imaging times. For optimum image production, ECG gating is essential. For spin-echo or fast-spin-echo sequences, cardiac gating requires a repetition time (TR) around the R-R interval. Although cardiac gating is preferred, IR half-Fourier single-shot turbo fast-spin-echo techniques can be used without cardiac gating. Furthermore, IR sequences can be performed in a shorter time and can be completed in a breath hold. In a double IR sequence, an initial inversion pulse nulls blood magnetization, giving a black blood effect, while tissue that remains in the imaging plane is unaffected. A second inversion pulse accentuates the contrast between long T1 and short T1 tissues and nulls fat. Both pulses occur prior to the readout pulse. T1 or T2 weighting can result by selecting the appropriate effective TE (9). IR sequences are preferably performed in a breath hold; if breath holding is not possible, the sequence can be performed during free breathing with an increased number of acquisitions.

Bright blood imaging is used to demonstrate flow and motion and to image valvular disease. Bright blood cine sequences include segmented-k-space small-flip-angle gradient-echo sequences or fast imaging with steady-state precession or refocused steady-state free precession (SSFP). Segmented-k-space gradient-echo sequences produce blood-myocardium contrast by saturating stationary tissues and producing flow-related enhancement due to flowing blood. This technique involves a fast gradient-echo sequence with a relatively short TR (2.5–10 msec), very short TE (1–2 msec), and a small flip angle (8°–20°) (10). This sequence can be performed at a single location in the cardiac cycle (single-section multiphase acquisition) for temporal resolution of wall motion. The sequence can also be performed at multiple locations regardless of the cardiac cycle (multisection single-phase acquisition) for anatomic delineation. In both acquisition methods, the R-R interval is divided into several cine frames corresponding to different cardiac phases; the number of phases is roughly less than or equal to 0.85 (R-R/TR). The number of lines of k-space that can be acquired during a single frame is referred to as the number of views per segment (VPS). The temporal resolution of the sequence is equal to VPS x TR (11). As TR decreases with improvements in gradient performance, increasing VPS to maintain desired temporal resolution is possible. Increasing VPS requires fewer heartbeats for image generation, leading to faster imaging times. However, increasing VPS comes at the expense of image blurring, particularly of rapidly moving structures, and decreased number of frames available for cine display. Segmented-k-space gradient-echo imaging should be performed during breath holding to eliminate artifacts caused by respiratory motion. If breath-hold imaging is not possible, multiple signal averages should be obtained. ECG gating is paramount for optimal sequence performance. VPS and the total number of lines of k-space acquired should be adjusted so that imaging does not extend beyond the trigger window, as missing ECG triggers wastes imaging time and prolongs the acquisition. Patients with rapid arrythmias may require a reduction in the number of cardiac phases or sections acquired, a reduction in k-space resolution, or an increase in VPS. Further, patients with irregular heartbeats may be difficult to gate. Increasing the trigger window yields more robust triggering but decreased acquisition time.

Refocused SSFP imaging or fast imaging with steady precession is an important alternative to segmented-k-space gradient-echo imaging. The refocused SSFP sequence has recently proved valuable for cardiac imaging because of its high signal-to-noise ratio and excellent blood-myocardium contrast. Refocused SSFP imaging relies on the ratio of T2 to T1 for image contrast rather than inflow enhancement (12). Although improved gradient performance permits shorter and shorter TRs decreasing imaging times, segmented-k-space gradient-echo imaging cannot take advantage of shorter TRs. As TR decreases, the time available for in-flow decreases with a commensurate increase in blood saturation and loss of blood-myocardium contrast. Refocused SSFP imaging exploits the shorter TR available with higher gradient performance for shorter imaging times, with a small trade-off in spatial and temporal resolution. Further, despite shorter imaging times with refocused SSFP compared with segmented-k-space cine gradient-echo imaging, the refocused SSFP sequence attains higher contrast-to-noise ratios (13). The relative flow independence of refocused SSFP makes the sequence particularly appropriate for individuals with low flow states such as poor left ventricular function, where blood-myocardial contrast is diminished. Compared with segmented-k-space gradient-echo imaging, the refocused SSFP sequence is more susceptible to field inhomogeneity and metal susceptibility artifacts that manifest as a dark stripe or flow ghosts, accentuated at longer TRs (12). The refocused SSFP sequence is also susceptible to pulsatility artifact due to the aorta and great vessels.

Other useful bright blood imaging techniques include velocity-encoded cine (phase-contrast) gradient-echo imaging, time-of-flight imaging, and three-dimensional (3D) gadolinium-enhanced MR angiography. Phase-contrast gradient-echo imaging can be used for quantification of flow and pressure gradients. Encoded within the raw data is information about the phase angle of the magnetization vector. Moving spins have markedly different phase angles from those of stationary structures; these phase angles are dependent on flow velocity. Velocity mapping permits production of phase-velocity maps in which pixel intensity is proportional to the velocity of flow. Additional mathematical postprocessing allows derivation of physiologic flow information such as flow or pressure gradient measurements (14). Time-of-flight imaging relies on blood inflow for contrast between the vessel wall and the lumen and is best reserved for visualization of the aorta. 3D gadolinium-enhanced MR angiography can used for aortography, pulmonary angiography, or evaluation of coronary bypass grafts. Gadolinium-enhanced MR angiography can be performed with gadopentetate dimeglumine, which leaks out of the intravascular space rapidly; therefore, imaging must be rapid to maintain contrast between the coronary arteries and the myocardium. To maintain a short imaging time, spatial resolution and coverage are traded. Intravascular contrast agents can be used in place of gadopentetate dimeglumine, permitting prolonged imaging time (15).

A summary of the imaging protocols used in this review is presented in Tables 1 and 2.


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  		Black Blood Imaging Protocol

 

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  		Bright Blood Imaging Protocol

 
MR Planes of Imaging
Important MR imaging views include the true planar, two-chamber scout, short axis, long axis, four-chamber, and true two-chamber. The true planar view may be in seen in axial, sagittal, or coronal sections. To achieve a two-chamber scout view, a true axial view through the left ventricle should be obtained, and then an oblique coronal scout view should be positioned parallel to the interventricular septum (Fig 2).



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  		Figure 2.  Normal two-chamber view.

 
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For a short-axis view, a double oblique image should be prescribed from a true axial image and a two-chamber scout image depicting the left atrium and left ventricle. The planes of imaging prescribed should be along the long axis of the left atrium and left ventricle on both images (Fig 3).



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  		Figure 3.  Normal short-axis view.

 
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To obtain a long-axis left ventricular view, an imaging plane should be positioned from the left ventricular apex through the mitral valve by using a two-chamber view depicting the mitral valve (Fig 4).



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  		Figure 4.  Normal left ventricular outflow view.

 
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A four-chamber view may be obtained by prescribing an imaging plane orthogonal to the short-axis view (Fig 5).



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  		Figure 5.  Four-chamber view.

 

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   Footnotes
 
Abbreviations: ECG = electrocardiograph, IR = inversion recovery, IV = intravenous, SSFP = refocused steady-state free precession, TE = echo time, TR = repetition time, VPS = views per segment, 2D = two-dimensional, 3D = three-dimensional.


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