Paaladinesh Thavendiranathan, MD, MSc, FRCPC and Scott D. Flamm, MD, MBA, FACC, FAHA

1. How does cardiac magnetic resonance imaging (CMR) produce images?

    CMR uses a strong magnet (1.5 to 3.0 Tesla; equivalent to 30,000 to 60,000 times the strength of the earth’s magnetic field), radiofrequency pulses, and gradient magnetic fields to obtain images of the heart. When placed in the bore of a magnet, positively charged protons, mainly from water, are aligned in the direction of the magnetic field, creating a net magnetization. Radiofrequency pulses are used to tilt these protons away from their alignment, shifting them to a higher energy state. These protons then return to their equilibrium state through the process of relaxation and emit a signal. The relaxation consists of two components: T1 and T2 relaxation. Magnetic gradients are applied across the tissue of interest to localize these signals. The signals are then collected using a receiver coil and placed in a data space referred to as k-space, which is then used to create an image. CMR uses differences in relaxation properties between different tissues, fluids, and blood, and changes that occur due to pathological processes to create contrast in the image.

2. What is unique about CMR?

    Similar to echocardiography, CMR allows the generation of images of the heart without exposure to ionizing radiation. Although the spatial resolution of CMR is comparable to echocardiography (approximately 1 mm), the contrast-to-noise and signal-to-noise ratios are far superior (Fig. 10-1). The latter allows easier delineation of borders between tissues and between blood pool and tissue. The contrast between blood pool and myocardium (see Fig. 10-1) is generated using differences in signal properties of the different tissues without the use of contrast agents. CMR is also not limited by “acoustic windows” that may hinder echocardiography, and images can be obtained in any tomographic plane. Finally, CMR can provide information about tissue characteristics using differences in T1 and T2 signals, and with addition of contrast agents.

3. What are the limitations of CMR?

    The major limitation of CMR is availability. Given the cost, the special construction necessary to host a CMR system, and the technical expertise and support necessary, CMR is not widely available at all centers. CMR is limited with respect to portability, unlike echocardiography where imaging can be performed at the patient’s bedside. There is also a set of contraindications (listed later) that limits the use of this technology in selected patient populations. Image acquisition can be challenging in patients with an irregular cardiac rhythm or difficulty holding their breath, though newer real-time techniques provide an opportunity to overcome these barriers (Fig. 10-2). Finally, because patients have to lie still in a long hollow tube for up to an hour during imaging, claustrophobia may become an important limiting factor.

4. What are the common imaging pulse sequences used in CMR?

    Pulse sequences are orchestrated actions of turning on and off various coils, gradients, and radiofrequency pulses to produce a CMR image. In very simple terms, the pulse sequences are based on either gradient echo or spin echo sequences. The most common sequences used are bright blood sequences (where the blood pool is bright); dark blood sequences (where the blood pool is dark); steady state free precession sequences (most commonly used for function or cine images); and inversion recovery sequences (e.g., delayed enhancement imaging used to assess myocardial scar).

5. What are the appropriate uses of CMR?

    Although echocardiography is usually the first-line imaging modality for questions of left ventricular (LV) function and assessment of valvular disease, CMR still has many important indications. The following questions and answers pertain to the appropriate use of CMR in the clinical setting, as recommended in the multisociety appropriateness criteria published in 2006.

6. What is delayed enhancement (DE) CMR imaging?

    One of the unique aspects of CMR is the ability to identify myocardial scar and/or fibrosis. This is most commonly performed using DE imaging. Gadolinium-based contrast agents are first administered, then after waiting approximately 10 minutes and allowing time for the contrast to distribute into areas of scar and fibrosis, an inversion recovery sequence is performed. This sequence nulls (makes it black) normal myocardium and anything that is bright within the myocardium is most likely myocardial scar or fibrosis (Fig. 10-3).

7. Does CMR have a role in the evaluation of chest pain?

    In patients with chest pain syndrome, CMR stress testing can be performed to assess flow-limiting coronary stenosis. This is most appropriate in patients with intermediate pretest probability of CAD with uninterpretable ECG or who are unable to exercise. The two stress methods used are vasodilator perfusion CMR or dobutamine stress function CMR. Vasodilator stress testing is performed identically to nuclear stress testing with the use of adenosine. With peak coronary vasodilation, gadolinium contrast agent is administered and perfusion of the myocardium during the first pass of the contrast agent is captured. Areas of perfusion abnormality can be detected as dark areas (Fig. 10-4). Resting perfusion is also commonly performed, mainly to differentiate imaging artifacts from a true perfusion defect. This is then followed by DE imaging to assess for myocardial scar. Alternatively, graded doses of dobutamine can be administered to provide a sympathetic stress with cine imaging to identify regions of wall motion abnormality, in a manner similar to echocardiography.

8. Can CMR coronary angiography be used to assess chest pain?