Diagnostic Imaging Europe
August/September 2004

Report: Cardiovascular

Cardiac MR imaging anticipates bright future

Cardiac MR has developed enormously in the past decade, emerging as a fast, high-quality imaging procedure. It offers potential for higher spatial resolution in static imaging and greater temporal resolution in dynamic imaging.

A successful cardiac examination requires careful preparation. Patients undergoing cardiac MR imaging are likely to have pacemakers and cardiac defibrillators, which can have safety implications. Patients should be informed about the examination procedure and made to feel relaxed and comfortable. If a dedicated cardiac coil is used, operators should ensure that it is placed correctly.

Image quality is affected by the strength of the ECG signal, which triggers the MR sequence to the heartbeat. Acquiring a good ECG signal inside a powerful magnetic field requires careful preparation of the patient's skin, including shaving if necessary, and use of an abrasive cream such as NuPrep to remove dead skin cells. Patients who practice holding their breath outside the magnet are more likely to cooperate with breath-hold imaging during the scan.

MR examinations generally include T1- and T2-weighted imaging sequences, but this is not the case in cardiac MR, where operators are aiming for good contrast between blood and myocardium. Evaluation is performed on so-called black blood and white blood images, except in the case of a few specific pathologies. Black blood imaging using spin-echo or dual inversion recovery sequences is commonly used to assess anatomy. White blood images acquired with gradient-echo sequences reveal details of cardiac function.

Thin bands of myocardium may be presaturated prior to imaging, in a technique known as tagging. Because the bands follow the heart's movement, they can be used to track motion of the underlying myocardium.1 Operators can generate phase/flow maps for flow quantification by selecting a phase contrast sequence. This can be useful in assessing restricted heart valves or a constricted aorta. MR perfusion, used to view myocardial dynamic enhancement, involves administration of gadolinium-based contrast at rest and stress, together with a vasodilatation agent. The technique permits reliable assessment of myocardial perfusion reserve and differentiation of viable from nonviable myocardium.2

Perfusion MR imaging has been compared to echocardiography, SPECT, and PET for evaluation of patients with coronary artery disease. A relative or absolute myocardial perfusion reserve determined noninvasively with MR perfusion imaging can provide good agreement with invasive studies.3

The technique additionally addresses the issue of viability. Normal viable myocardium shows rapid signal enhancement when contrast is administered, and the contrast washes out quickly. Delayed hyperenhancement corresponds to nonviable myocardium (Figure 1).

Coronary MR angiography remains a major challenge. A number of methods have been tested: 2D and 3D, breath-hold and free breathing, white blood and black blood, with and without extracellular or intracellular contrast, spiral and echo-planar imaging.4-14 It is not yet clear which of these methods and/or sequences will prove optimal for clinical practice. The 3D approach, however, is a likely favorite because of its capability for multiplanar reconstructions and maximum-intensity projections (Figure 2).

Contrast-enhanced MRA of the large vessels is an established examination.15,16 Because a 3D volume can be obtained in just five or six seconds, approximately four phases can be acquired in a single breath-hold and contrast can be followed through the heart and great vessels.

Scanners equipped with up-to-date hardware and software enable real-time imaging. Cardiac MR examinations are faster, and more sequences can be performed in an acceptable time.

Operators should always tailor the imaging protocol to the specific problem addressed. The following is a suggested routine protocol.

- three-plane survey: axial, coronal, sagittal;

- axial sequence covering heart and great vessels: spin-echo or echo-planar imaging;

- long-axis view (left ventricle, left atrium, and mitral valve) using single-slice multiphase cine breath-hold sequence: gradient-echo or TrueFISP (true fast imaging with steady-state precession)/B-FFE (balanced fast field echo)/Fiesta (fast imaging employing steady-state acquisition);

- short-axis view (planed on long-axis view, perpendicular to long axis of left ventricle) using single-slice multiphase cine breath-hold sequence: gradient-echo or TrueFISP/ B-FFE/Fiesta; and sigma four-chamber view (planed on basis of long axis and short axis images) using single-slice multiphase cine breath-hold sequence: gradient-echo or TrueFISP/B-FFE/Fiesta.

This protocol would allow relevant views to be planed, depending on the problem to be solved. These could include views of the aortic arch, short-axis volume (where left ventricular volume and ejection fraction can be measured through postprocessing), left ventricular outflow tract, flow measurements in the aorta or restricted heart valves, right ventricular outflow tract, pulmonary artery flow, coronary artery imaging, contrast-enhanced MRA of the great vessels, rest and stress perfusion, and viability assessment.

TECHNOLOGICAL ADVANCES

Technological advances have improved the image quality of cardiac MR and have accelerated patient throughput. The four-lead vector ECG system, for example, finds the heart's electrical axis automatically. It produces a well-defined R peak and a stable signal to trigger the MR examination, making prescan preparation much faster.

Additional advances, including stronger and faster gradient systems, segmented k-space techniques (such as segmented echo-planar imaging),17 and spiral imaging, coupled with faster computers, have improved scanning and reconstruction times.18 Real-time scanning is available on some MR systems. Parallel imaging techniques such as SENSE (sensitivity encoding) and SMASH (simultaneous acquisition of spatial harmonics) boost scan speeds further.

Use of TrueFISP/B-FFE/Fiesta can also cut scanning times and improve image quality. This sequence, which employs balanced gradients, uses the stimulated echo to enhance image contrast between blood and myocardium.19 Gradient-echo cine imaging formerly took several minutes to complete, but a modern MR scanner with real-time imaging capability produces a cine that covers the entire left ventricle, with several slices and phases, within a single breath-hold.

MR imaging of the coronary arteries and heart valves is complicated by the heart's beating motion and diaphragmatic motion caused by respiration. Compensation techniques have been developed to combat respiratory motion; the so-called navigator gating technique, for instance, uses a radio-frequency kernel to create a 1D image and monitor the diaphragm's motion.

Operators can set a movement margin in millimeters, within which all MR imaging data are accepted. This yields extremely clear images with well-defined detail. But quality comes at the expense of scan time, as all data outside the millimeter-scale margin are rejected. Navigator gating can also be used with slice-tracking, in which imaging slices are moved along a certain factor with diaphragmatic movement. The combination of navigator gating and segmented k-space techniques enables coronary angiography with patients breathing freely.7 Breath-hold imaging, with and without contrast, is also used for coronary MRA.5-13

The spatial resolution of coronary MRA remains lower than that of x-ray or CT coronary angiography, but development in the area is ongoing. Navigator gating and slice tracking have helped improve valve imaging by preventing the heart valve from moving out of the imaging plane during systole. Fast techniques have made perfusion studies possible, with high sensitivity and specificity.2,18 Some researchers suggest that cardiac MR is the gold standard for determining myocardial thickness, mass, systolic and diastolic wall thickening, ejection fraction, and global and regional function.20

Installation of 3T MR scanners has shown that higher field strengths can improve image resolution. This is of great benefit in cardiac imaging, and especially coronary artery imaging, where improved signal can be traded for spatial or temporal resolution.

TRAINING AND COOPERATION

Radiographers involved in cardiac MR should not only receive training in patient preparation, but they should also have onsite application training using the department's chosen scanner. Knowledge of cardiac anatomy and pathology is essential for choosing the optimum sequence for a specific condition and for acquiring diagnostically useful views. Radiographers can supplement their training by attending a dedicated cardiac MR course (see www.leedsscmr.org) or seeking out educational material (see www.scmr.org).

Close cooperation among cardiologists, radiologists, and radiographers is critical. All three should ideally discuss the expected diagnosis and the views the radiologist and cardiologist require before the cardiac patient is scanned.

MR radiologists must maintain an interest in cardiac MR if the procedure is to be performed in the radiology department. Cardiac MR scanners may be placed in cardiology departments if radiology departments fail to meet cardiologists' demands.

Cardiac MR as a complete examination for evaluating cardiac disease is progressing toward clinical reality. It assesses cardiac anatomy as well as regional and global function and offers the option of flow quantification. Perfusion and viability studies performed with MR have even better resolution than those using SPECT or PET.21

Contrast-enhanced MRA can be performed quickly to highlight abnormalities in the larger vessels (Figure 3).15,16 Coronary MRA admittedly has lower resolution than competing techniques, but researchers are addressing this issue. Indications for coronary MRA are currently limited to visualization of abnormal coronary arteries and bypass graft control, as well as cases where x-ray angiography is impossible, such as proximal left main stem stenosis. Three-D coronary MRA, an exercise-independent approach that allows accurate detection of proximal and midcoronary artery disease, can identify and exclude left, main, and three-vessel disease reliably.7

Coronary MRA could become the preferred angiographic examination method if spatial resolution is improved. It is noninvasive, involves no ionizing radiation, and offers the option of 3D reconstructions.

The field of cardiac MR is continually evolving. Researchers have trialed MR spectroscopy as a tool to monitor myocardium metabolism22 and have developed a method of displaying cardiac MR imaging data interactively.23-25 The latter technique employs 3D MR data to create a virtual reconstruction of the heart (Figure 4). Surgeons equipped with two joysticks can turn the heart and view it externally from any direction or inspect internal cardiac features. This promising tool could be used for preoperative planning in patients with congenital heart disease. Such reconstructions could eventually be used as training aids for cardiac surgeons.

MS. BLANKHOLM is a research radiographer in the department of neuroradiology at the Center for Functionally Integrative Neuroscience at Aarhus University Hospital, Denmark.

References

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