CMR for malignant cardiac tumors: Diagnosis, approach, and follow-up

Key points

• Cardiac magnetic resonance (CMR) is the gold standard noninvasive imaging modality to assess tissue characteristics in vivo which gives it a unique advantage in discriminating benign cardiac masses from malignant tumors. CMR also provides visualization of tumor invasion, hemodynamic effects, and location relative to surrounding cardiac and extracardiac structures. These features make CMR an essential tool for diagnosis and management of cardiac tumors.

• The following sequences form part of a comprehensive CMR exam: cine imaging (e.g. balanced steady state free precession (bSSFP)), T1/T2-weighted black-blood (BB) images, T1/T2 mapping, first-pass perfusion, and delayed enhancement imaging. The characteristics of the tumor are often described as hyper/iso/hypo-intense, meaning higher, equal to, or lower signal intensity compared to normal myocardium. For instance, the extensive vascular networks associated with malignant tumors often present as hyperintense on first-pass perfusion and on late gadolinium enhancement (LGE) images. The high volume of intracellular free water content in malignant tumors and the frequently observed surrounding edema may lead to longer T1 and T2 relaxation times. At the same time, necrosis and hemorrhage within the tumor often result in heterogeneous signal on T1W and T2W BB images. The newer T1 and T2 mapping techniques provide quantitative T1 and T2 values, instead of the relative grayscale obtained through T1W and T2W BB imaging, providing an opportunity to further advance the diagnosis of malignant cardiac tumors.

• CMR limitations include absolute and relative contraindications for imaging in patients with devices that are not MR compatible (e.g., noncompatible pacemakers, internal defibrillators, mechanical circulatory support devices, etc.). Breath-holds are often used in clinical protocols and patients that cannot hold their breath often have decreased image quality. Similarly, the need for ECG gating makes CMR a challenge in patients with irregular heart rhythm as it can lead to acquisition artifacts and poor image quality.

Abbreviations

Introduction

Cardiac masses entail a heterogeneous group of disorders that can be broadly divided into benign and malignant tumors. Malignant cardiac tumors are exceedingly rare (compared to benign masses) and often present with significant diagnostic and therapeutic challenges. Cardiac magnetic resonance (CMR) imaging has emerged as a key noninvasive technique in their evaluation, primarily due to the superior tissue characterization without ionizing radiation exposure. Moreover, CMR provides visualization of cardiac tumor extension/invasion, relationship with surrounding cardiac and extracardiac structures, and the evaluation of hemodynamic effects. Malignant tumors can be further divided into primary and secondary cardiac malignancies ( Table 14.1 ). Among malignant primary cardiac tumors, the most common are sarcomas which account for about 75% of the cases, while primary cardiac lymphomas, mesotheliomas, neuroendocrine tumors, and others account for the remainder 25% of cases ( Table 14.1 ) . Secondary tumors can invade the heart by direct extension (e.g. pleural mesothelioma), intracavitary spread (e.g. renal cell carcinoma via inferior vena cava or lung carcinoma via pulmonary veins), and hematogenous or lymphatic, metastatic spread. The reports about the prevalence of cardiac metastases vary widely, from 2% to up to 18%; however, these estimates are largely based on autopsies and may not reflect the cases seen in cardiac imaging centers .

In this chapter we describe current clinical indications for CMR and detailed CMR imaging protocol recommended for the comprehensive assessment and management of cardiac tumors. We then provide examples of applied CMR imaging and specific CMR findings with primary and secondary cardiac tumors.

Role of CMR imaging in establishing the diagnosis

The initial identification and evaluation of a cardiac mass most often starts with the readily available transthoracic echocardiogram (TTE). Echocardiographic findings, in turn, frequently represent an indication for CMR to (a) confirm abnormal cardiac mass (vs normal anatomical structure as described in Table 14.2 ) and (b) further differentiate between benign, malignant, and nontumorous masses (e.g. thrombus). CMR imaging protocols can be employed to evaluate hemodynamics, morphology, pericardial invasion, size, location, homogeneity, and signal characteristics of cardiac masses, and thus aid in differentiation between benign and malignant tumors . Features concerning for malignancy include large tumor size, involvement of the pericardium and the right heart, tissue heterogeneity, and high extracellular volume (ECV) determined by T1 mapping before and after gadolinium contrast infusion. A position paper published in 2020 by the Society of Cardiovascular Magnetic Resonance (SCMR) outlines clinical indications for the use of CMR in the evaluation and management of cardiac masses . Beyond diagnosis and differentiation of malignant masses, CMR is recommended for guidance of surgical therapy, assessment of treatment effect, as well as posttreatment surveillance ( Table 14.3 ) .

Most clinically indicated CMR images are obtained on 1.5T field strength magnets, though in recent years 3T imaging has been making a significant advance in quality and availability. The signal intensity of a neoplastic lesion is dependent on the tissue morphology and CMR parameters employed to obtain the images. Fig. 14.4 illustrates a CMR protocol recommended for the evaluation of cardiac masses . Axial plane images with T1-weighted (T1W) and T2-weighted (T2W) sequences are used for native tissue characterization and are the first step in CMR evaluation. In recent years, the evolution of CMR parametric mapping has provided us with the ability to quantify myocardial tissue alterations based on T1, T2, and T2*(star) relaxation times and extracellular volume (ECV) measurements . In turn, these values can be used in conjunction with qualitative T1W and T2W images in differentiating between cardiac tumors. For instance, a hypointense mass on T1W (or low T1 on T1 mapping) may represent a calcified tumor, meanwhile, hyperintensity on T1W images (high T1 on T1 mapping) can be seen in masses with high-fat content (e.g. lipomas and liposarcomas), hemorrhagic or highly vascularized tumors (e.g. angiosarcoma and hemangioma), as well as in melanomas ( Table 14.5 ). On T2W images, highly vascularized tumors such as angiosarcoma present with hyperintense signal (high T2 on T2 mapping), in contrast to fibrous tumors which are hypointense (low T2 on T2 mapping). Importantly, the most common benign primary tumor, cardiac myxoma, also presents as bright/hyperintense on T2W and with high values on T2 mapping and in instances may present a diagnostic challenge ( Fig. 18.8 ).

Table 14.5

CMR characteristics of malignant cardiac tumors.

Tumor	Preferential location	T1W	T2W	LGE	Cine images
Primary cardiac sarcomas
Angiosarcoma	Right atrium	Heterogeneous hypo- and hyperintensity due to hemorrhage and necrosis	Heterogeneous hyperintense, “cauliflower” appearance	Heterogeneous enhancement pattern with marked surface enhancement (“sunray appearance”) and central necrosis	Fixed on the wall with a broad base, does not move with the heart Impaired right atrium and ventricular motion
Undifferentiated sarcoma	Left atrium	Isointense	Slightly hyperintense	Heterogeneous enhancement	Hypo- or isointense
Rhabdomyosarcoma	Frequently involves the valves Commonly not confined to one chamber	Isointense	Hyperintense	Homogeneous enhancement with areas of low central intensity due to necrosis	Isointense
Osteosarcoma	Left atrium	Heterogeneously hypointense	Hyperintense	Nonspecific due to calcification	Hypo- or isointense
Leiomyosarcoma	Posterior left atrium, pulmonary vein and mitral valve	Isointense or hypointense to myocardium	Hyperintense	Markedly enhanced	Hypo-or isointense
Primary cardiac lymphoma	Right atrium (often involves more than one chamber)	Isointense	Heterogeneously hyperintense	Heterogeneous enhancement, with area of low enhancement in the center of the lesion	Isointense, large soft tissue mass
Pericardial mesothelioma	Pericardial space	Homogeneously isointense	Heterogeneously hyperintense with areas of necrosis	Markedly heterogeneous enhancement	Hypointense nodule
Metastatic melanoma	Right heart	Hyperintense	Hyperintense	Heterogeneous enhancement	Nodular hyperintense appearance in the myocardium
Other metastatic involvement	Pericardium	Hypointense	Hyperintense	Heterogeneous enhancement	Varies depending on the primary
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In contrast to qualitative assessment on T1W and T2W images, mapping permits quantification and may allow visualization of the disease process related to intra- and extracellular disturbances. Though current clinical evidence is scarce, mapping techniques can be used to characterize masses, pericardial effusion, and fatty lesions within the heart. For example, vascularized tumors with high water content have long T1 and T2 relaxation times, while they have low T1 values postcontrast update due to significant gadolinium contrast uptake . Although significant advances have been made in parametric mapping techniques, future research and validation are needed before routine clinical application for diagnoses of cardiac tumors. When describing T1W and T2W imaging with hyperintense or hypointense findings for various tumors, this information can also be obtained using T1 and T2 mapping.

Steady-state free precession (SSFP) technique is a valuable sequence to assess the relationship between the tumor with the myocardium, pericardium, blood pool, valves, and adjacent tissues. Assessment of tumor movement with respect to normal cardiac structure during cardiac cycle provides information on tumor attachment and whether tumor growth is invasive. Additionally, the hemodynamic effects on cardiac pumping and valvular function can be evaluated by cine SSFP images. Also, phase contrast flow imaging can be used to quantify hemodynamic effects with, e.g. increases in flow velocity if the tumor is affecting ejection or filling of blood.

Beyond native tissue characteristics, gadolinium contrast infusion can be employed to discern cardiac masses from normal cardiac structure through first-pass perfusion and late gadolinium enhancement (LGE). First-pass perfusion is conducted with dynamic imaging during infusion of contrast and is an important method to identify tumor vascularity and presence of necrosis. Masses such as hemangioma and to a lesser degree angiosarcoma tend to show early enhancement after contrast infusion and are easily distinguishable from other lesions . As malignant tumors frequently cause tissue necrosis by obliteration of capillary beds, first-pass perfusion images may exhibit dark central areas (necrosis) with hyperenhancement of the surrounding tissue, an important diagnostic measure to differentiate benign from malignant tumors. Late gadolinium enhancement (LGE) sequences are obtained 10 min postcontrast administration. Benign cardiac masses more often present as markedly homogeneous enhancing lesions (with the exception of myxoma), while malignant tumors present heterogeneous contrast enhancement due to extensive vascularity and necrosis. To discriminate normal myocardium from neoplastic masses, the inversion time is set to null normal cardiomyocytes as the exact inversion time varies depending upon patient physiology, timing postcontrast administration, and type of sequence.

Primary malignant tumors