https://orcid.org/0000-0002-4763-7805
Introduction
Cardiac masses represent a heterogenous groups of primary and secondary cardiac tumours, benign and malignant masses, and tumour-like lesions. About 75% of primary cardiac tumours are benign lesions, with myxoma, fibroelastoma, and lipoma being the three most frequently diagnosed lesions at adult age.1 Of the malignant lesions, sarcomas and lymphomas are most frequently seen. Malignant cardiac tumours are about 20 times more often cardiac metastases than primary cardiac tumours. Tumour-like lesions include thrombi, infectious, or inflammatory lesions but also anatomic or congenital variants. To guide clinical decision-making, correct characterization of a cardiac mass is important.
Cardiovascular magnetic resonance (CMR) imaging is considered the gold standard technique to evaluate cardiac masses non-invasively. CMR provides a large field of view and high spatial and temporal resolution, and, importantly, it provides the ability of tissue characterization. Tissue characterization is provided by means of contrast enhancement, classic T1- and T2-weighted imaging techniques, but also with the more recently introduced parametric mapping techniques. Cine imaging provides first assessment of the mass: (i) presence and localization, (ii) relation with surrounding structures, (iii) potential haemodynamic consequences, and (iv) accompanying abnormalities. The addition of myocardial tagging reveals absence of contractility and delineates the borders of a mass to evaluate invasive growth. With T1- and T2-weighted imaging, it can be identified whether the mass is high fat- or fluid-containing. First-pass perfusion provides assessment of vascularity of the mass. Late gadolinium enhancement (LGE) is the technique used to detect focal areas of fibrotic tissue or myocyte necrosis and is also related to the vascularity of the mass. More recently, parametric mapping techniques have become available enabling direct assessment of tissue magnetic resonance properties such as T1, T2, and T2*. In this ‘how to’ paper, we outline the use of parametric mapping in the assessment of cardiac masses.
CMR in the evaluation of cardiac masses
Trans-thoracic echocardiography is the first-line imaging modality for patients with suspected cardiac masses. Trans-oesophageal and/or adding 3D imaging can be of value to assess mass localization and connection with surrounding tissues into more detail. While contrast echocardiography enhances assessment of the vascularity of the mass, tissue characterization options are more limited when compared to CMR. Moreover, CMR has a larger field of view and may overcome image quality issues. CMR has evolved as the reference standard for assessment of suspected tumours, although for infectious (vegetations and abscesses) and calcified lesions, CMR is not the imaging modality of choice. Clinical indications include assessment of a suspected cardiac mass and differentiation between benign/malignant/non-tumourous origins and follow-up.2 Depending on the indication and/or with contra-indications, cardiac computed tomography (CT) and positron emission tomography (PET) are alternative imaging methods. Fluor-fluorodeoxyglucose PET images can be combined with CMR to improve the differentiation of benign and malignant masses.
Imaging protocols
The standardized CMR protocol includes the following images3:
Imaging of left and right ventricular structure and function by means of balanced steady state free precession (bSSFP) long- and short-axis cine imaging, with additional slices through the mass and perpendicular to each other.
T1-weighted fast spin echo (FSE) with slices through the mass and surrounding structures with and without fat suppression to assess whether the mass is high fat-containing.
T2-weighted FSE with fat suppression through the mass and surrounding structures to assess whether the mass is high fluid-containing.
First-pass perfusion after an intravenously administered gadolinium-based contrast agent with slices through the mass to assess vascularity.
LGE (∼10 min after contrast injection) to assess distribution of contrast within the extracellular space and including images with the inversion time (TI) set to null thrombus (∼500–550 ms at 1.5 T; 850–900 ms at 3 T).
Optional: early gadolinium enhancement (EGE) 1–3 min after contrast administration with TI set to null thrombus.
Examples of available preparatory modules for fat suppression include, but are not limited to, the Fat-Sat technique. Fat-Sat suppresses the signal of fat by applying a 90° radiofrequency-pulse tuned specifically to the resonance frequency of fat followed by a spoiler gradient de-phasing the fat protons. It is fast and commonly used but sensitive to magnetic field inhomogeneities which can lead to incomplete fat suppression near metal. Dixon technique is less sensitive to B0/B1 field inhomogeneities but requires longer imaging time. Another technique is the hybrid method SPAIR. SPAIR uses a 180° adiabatic inverting pulse tuned to the resonance frequency of fat followed by a spoiler gradient destroying any transverse magnetization. It requires longer imaging time but is less sensitive to B1 field inhomogeneities resulting in more homogeneous fat suppression. A complete T2-weighted sequence including fat suppression is short tau inversion recovery (STIR). STIR is an inversion-based technique and is not fat-specific, i.e. also suppressing signal of other tissues with short T1 times (e.g. mucoid/protein-rich tissues and melanin). It is insensitive to magnetic field inhomogeneities.
In contrast to conventional tissue characterization, which relies on relative variations in signal intensities, parametric mapping provides direct assessment of T1, T2, and T2* relaxation times and may improve tissue characterization. Therefore, the following CMR sequences may be added prior to contrast administration:
Native T1 mapping using modified Look Locker Inversion recovery (MOLLI) or shortened MOLLI (ShMOLLI) with slices through the mass.
Native T2 mapping using either T2 prepared single-shot bSSFP or gradient spin echo of at least three source images, with slices through the mass.
Multi-echo gradient recalled echo T2*-weighted imaging with slices through the mass.
Imaging report
Images should be post-processed, interpreted, and reported according to the guidelines.4 Reporting should include:
Presence, number, location, extent, and size;
Morphology, margin, attachment, and invasion in surrounding structures if present;
Mobility and obstruction if present;
Signal intensity on bSSSF when compared to the blood pool;
Signal intensity on T1- and T2-weighted FSE when compared to the myocardium;
Enhancement on first-pass perfusion, EGE, and LGE;
T1, T2, and T2* relaxation times of the mass;
Pericardial effusion if present.
Interpretation of results
The localization and morphology of the tumour may already provide clues of its nature. For example, a myxoma is often localized in the left atrium and attached to the atrial septum by a thin stalk. In contrast, cardiac lipomas and metastases can be found in all cavities, with the latter mostly showing invasive growth.1 Several imaging features can be helpful to discriminate benign from malignant tumours. In general, benign tumours have well-defined margins, are located intra-cavitary, are mobile, and do not show invasion of surrounding structures. In contrast, malignant tumours have poorly defined, irregular margins, may be localized intra-myocardially, may show invasion of surrounding structures, and are often associated with pericardial effusion.5 Tissue characterization provides further assessment of the composition of the lesion. Lesions with short native T1 relaxation times and consequently with high signal intensity on T1-weighted images include fatty tumours (lipoma, liposarcoma), recent haemorrhage (methaemoglobin), and melanomas (melanin). In contrast, native T1 relaxation times increase with free water, fibrosis, or amyloid infiltration. Therefore, long T1 times represent tumours like cysts (with low protein fluid) or vascular malformations and most malignant masses. Long T2 relaxation times are associated oedema. Masses like cysts and tumours with high vascularity like haemangiomas exhibit long T2 relaxation times and thus high signal intensity on T2-weighted images. T2* time is shortened in the presence of iron as with cardiac thrombus/haemorrhage. Highly vascularized masses show enhancement on first-pass perfusion and LGE images. While CMR is highly effective in distinguishing benign from malignant lesions, histopathologic assessment remains the gold standard to determine the type of cardiac mass. An overview of generally reported imaging characteristics of cardiac masses in relation to CMR pulse sequences is presented (Figure 1).
Imaging characteristics of cardiac masses in relation to CMR pulse sequences. Signal intensity (SI) of tumours and T1, T2, and T2* relaxation times are described when compared with normal myocardium. Imaging findings may differ depending on the characteristics of individual tumours. Second row: example of a caseous calcification of the mitral annulus (MAC). *Melanin-rich tumours. Created with BioRender.com.
Conclusions
CMR is the reference standard method for non-invasive assessment of cardiac masses owing to its unique ability to provide tissue characterization non-invasively. Addition of parametric mapping may improve tissue characterization and, if available, should be added to local CMR protocols.
Funding
None declared.
Data availability
No new data were generated or analysed in support of this research.
References
Author notes
Conflict of interest: None declared.
