CT and MRI give answers in cardiac neoplasms

Noninvasive cardiac imaging is gaining widespread acceptance. Both CT and MRI can determine the absence or presence of coronary artery disease accurately and reliably. This is done by either assessing the coronary artery morphology or by offering detailed insight into functional aspects and myocardial perfusion.

Cardiac MRI has also demonstrated its potential in the assessment and differential diagnosis of inflammatory disease, storage disease, and cardiomyopathies. Storage disease is related to metabolic disorders and/or accumulation of excessive metabolic products (e.g., Fabry's disease, amyloidosis, hemochromatosis).

Information from CT and MRI can also help in the assessment of cardiac masses. It is rare for referring physicians to request a CT or MRI examination solely for the assessment of a primary or secondary cardiac neoplasm. Confirmation or exclusion of cardiac thrombi by imaging, however, is of growing interest. Both may be requested after an initial echocardiographic screening.

DIAGNOSING MASSES

The parameters needed for cardiac CT and MR imaging in visualizing tumors differ little from those required in other clinical applications, such as imaging of coronary artery disease.1,2

For cardiac CT, the extremely high spatial resolution used to assess coronary pathology is not mandatory in tumor imaging. Using wider collimations (1 to 2 mm) or reconstructing thicker slices (2 to 3 mm) could make it possible to lower the radiation dose substantially for diagnostic assessments. The majority of CT referrals, however, are intended to confirm, locate, differentiate, or simply exclude a potential cardiac mass and to provide detailed information on vessel structures and vascular anatomy near a confirmed mass for therapy planning. The applied scanning protocol consequently usually matches a coronary CT angiography protocol.

Modern cardiac CT can include unenhanced scanning and delayed imaging, as well as first-pass CTA. The former two are used to delineate potential mass calcifications and the latter to assess lesion uptake of contrast in relation to surrounding myocardium.

Addition of one or both of these techniques to the standard protocol will raise patients' overall radiation burden. Exposure can be lowered by using a moderate slice thickness (2 to 3 mm). Delayed scanning is typically initiated five to seven minutes after contrast injection.3 Consideration of possible dose-reducing strategies, including ECG-related dose-saving strategies or even prospective ECG-triggered modes, should be a prerequisite.

The regimens used for contrast application should be revisited when imaging cardiac masses on CT. The latest generation of scanners can cover the heart in just five to 12 seconds. Contrast injection is also tailored to optimize delineation of the coronary artery tree.

The depiction of masses within the right heart is hampered by the use of dual-syringe power injector systems and saline flushing, which leave little or no enhancement in the right atrium and right ventricle. Modern injection systems avoid this limitation by allowing for an individual dilution of contrast agents at arbitrary values. Sufficient contrast for mass delineation within the right atrium and ventricle can be achieved by flushing the initial contrast agent bolus with a contrast agent/saline mix (20% to 30%/80% to 70%) instead of pure saline. Inflow of unenhanced blood from the inferior vena cava still may mimic a cardiac mass within the right atrium. Therefore, patients suspected of having right-sided masses should undergo MRI instead.

Cardiac MRI offers a far wider variety of techniques for the investigation of tumors than does cardiac CT. Cardiac morphology is depicted on MRI with outstanding soft-tissue contrast. MRI can additionally assess tumor function by providing information on perfusion and the distribution of contrast. Proper planning of imaging protocols is strongly recommended.

A substantial part of cardiac mass evaluation on MRI can be performed without gadolinium contrast. Unenhanced images demonstrate excellent soft-tissue contrast and intrinsic flow contrast for blood signal delineation and suppression. Any cardiac MRI examination requested for mass delineation should start with a dedicated and precise morphological overview that covers the entire heart. Fast spin-echo (FSE) techniques, such as HASTE, or single-shot gradient-echo (GRE) techniques, such as steady-state free precession (SSFP), may be applied to reduce examination time.

Initial morphological screening may be followed by more dedicated techniques for soft-issue differentiation, such as T1- and T2-weighted FSE sequences. These single-slice techniques should be restricted to suspicious areas or masses already identified. T1-weighted slice positions need to be repeated after contrast injection to check for contrast uptake and to delineate possible regions of necrosis.

Flow artifacts may mimic cardiac masses even in black-blood prepared FSE. Gradient-echo techniques may provide further information, while cine SSFP techniques may show the flow patterns. The latter technique also provides detailed insight into mass-based functional impairment, such as in- or outflow obstruction, as well as global and regional myocardial function. The use of parallel imaging makes it possible to use multislice cine approaches.4

Plain FSE techniques are helpful in differentiating soft-tissue components of tumors. Contrast-enhanced techniques give further information on tumor vascularization, delineation, and potential invasion of adjacent structures, such as the pericardial space. Additional information can be provided by techniques that are used routinely in the investigation of coronary artery disease, such as myocardial perfusion imaging and delayed-enhancement imaging. The perfusion technique can be applied during contrast injection and does not interfere with any other postcontrast technique.

Dedicated postprocessing demonstrates any mass contrast uptake that is present (Figure 1). Delayed-enhancement GRE sequences give an overview of mass extent.

EPIDEMIOLOGY

Cardiac tumors may be classed as primary neoplasms or secondary neoplasms. Primary tumors originate within the heart and are mainly located there. Secondary tumors are most commonly due to metastasis or extension of malignancies located outside of the heart. Primary tumors are far less common than secondary cardiac neoplasms. Autopsy studies have demonstrated a prevalence of 0.0017% to 0.33% for this type of cardiac lesion.5 Secondary tumors are reported to occur 20 to 40 times more often than primary cardiac neoplasms.5,6

Secondary tumors are almost exclusively malignant lesions that affect the heart through metastatic or direct spread, while the majority (over 60%) of primary cardiac neoplasms are benign.5 Neither MRI nor CT will be sufficient for the ultimate diagnosis of a cardiac tumor, though features seen on imaging may support a differential diagnosis.

The patients' age, history, and symptoms; the site of the mass; and specific imaging features will all contribute to the final diagnosis. CT and MRI can help determine whether a mass's location is intracavitary, intramyocardial, pericardial, or paracardial. Differentiation of masses into atrial or ventricular and distinction between left- and right-sided locations may also assist with the final differential diagnosis.7

Cardiac myxomas, for example, may be identified from their typical imaging features and tendency to be found in certain sites. This benign tumor entity accounts for 30% to 35% of all primary cardiac tumors and for approximately 50% of all benign primary cardiac masses.5 Among these tumors, 95% are located within the atria (75% left-sided). They are typically attached to the oval fossa with a stalk (Figures 1A and 2). Myxomas may rarely occur at multiple locations or be part of the Carney complex.8 Prolapsing myxomas may impair ventricular filling. Myxomas exhibiting these typical features can usually be diagnosed on multislice CT or MRI, with the latter modality providing additional information on mass enhancement (Figure 3).9

Papillary fibroelastomas are the second most common benign cardiac tumor and are typically revealed by echocardiography. Most lesions are smaller than 1 cm in diameter and attached to the fast-moving cardiac valves (90%).10 Multislice CT and MRI play a smaller role in the diagnosis of this tumor entity.

Rhabdomyomas and fibromas each account for approximately 5% of all primary cardiac lesions. Their differential diagnosis is of particular interest with regard to treatment options. While rhabdomyomas may present as multiple intramyocardial masses, they tend to regress over time. Fibromas may show progressive growth.11 Rhabdomyomas are the most common cardiac tumor seen in childhood and may be associated with tuberous sclerosis.12,13 The tumors mimic the myocardium on T1-weighted plain FSE. Homogeneous enhancement that exceeds the level of normal myocardial enhancement is seen after contrast injection. Fibromas, on the other hand, may be identified by a typical hypointense presentation on T2-weighted MRI or as coarse calcifications seen on CT (Figure 4).9

Primary malignancies account for 25% to 30% of all primary cardiac masses and include various entities of mesenchymal origin. These lesions represent a clinical dilemma because they are often asymptomatic when small, then produce nonspecific symptoms when they are larger.14 Cardiac angiosarcoma is the second most common primary cardiac tumor (9%). Approximately 75% to 80% of these masses are located within the right atrial wall extending into the lumen.14,15 Large angiosarcomas can be diagnosed easily on CT, though smaller lesions may be obscured by high-density influx artifacts during contrast application.

The masses are typically heterogeneous on T1-weighted and T2-weighted MRI and after contrast injection. Central tumor necrosis with heterogeneous peripheral enhancement is often referred to as a sunray appearanc).16 Another clue is the presence of partially hemorrhagic effusion caused by invasion of the pericardial space.

Rhabdomyosarcoma is the most common malignant childhood cardiac tumor. It mimics normal myocardium on unenhanced T1-weighted MRI but may show a varying level of enhancement, indicating necrosis, after application of contrast.16 Rhabdomyosarcoma is not more or less likely to be found in any particular chamber, unlike angiosarcoma, and may even involve valvular structures.

Other primary cardiac malignancies include malignant fibrous histiocytoma, leiomyosarcoma, lymphoma, and primary cardiac osteosarcoma.5 The latter typically arises at the left atrial roof and may not show any tumor calcification at all.

Secondary involvement of the heart by external primary malignancies is far more common than any primary cardiac tumor.5,6 Hematogenous spread, retrograde lymphatic involvement, direct contiguous involvement (e.g., bronchogenic carcinoma), and transvenous extension (e.g., renal cell carcinoma) can all cause extracardiac malignancies to affect the heart. Presentation will depend on the associated primary malignancy and the nature of cardiac involvement.

PSEUDOMASSES

Cavitary thrombus is by far the most common mass. This pseudomass is responsible for about 15% of all ischemic strokes. Early identification of thrombi, which may occur in any cardiac chamber, is consequently of paramount importance. Clot development may be due to changes to surface properties or flow dynamics, or changes in blood coagulation. Patients with artificial devices, such as prosthetic valves or pacemakers, with atrial fibrillation or dysfunctional ventricles, are particularly at risk.

Thrombi are typically recognized on cardiac CT as having a hypodense appearance. The signal intensity seen on MRI will vary according to the clot's age and the applied imaging technique (Figure 6). CT studies have shown that atrial and ventricular clots can be identified accurately after a relatively short examination.17 MRI provides more detail on underlying pathologies, while adding information about regional myocardial function and the extent of possible myocardial infarction. The time needed to acquire this information, however, is greater than that associated with cardiac CT.

MRI may not perform as well as a transesophageal echocardiogram when imaging thrombi within atrial appendages.18 FSE sequences can lead to flow artifacts. Contrast-enhanced inversion recovery GRE techniques provide accurate results and fast cardiac coverage and are preferred.16,19 MR perfusion imaging can demonstrate the presence or lack of contrast enhancement if cardiac CT results are inconclusive (Figure 6C). Although echocardiography remains the modality of choice for thrombus screening, even a nongated chest CT may reveal unknown cardiac thrombi.

Caseous calcification of the mitral valve annulus represents another pseudomass. This entity arises in patients with annular calcifications and is found in about 3% of autopsies.20 It is typically seen on CT as an area of high density at the posterior aspect of the mitral valve annulus. This same area is hypointense on all MRI sequences and does not enhance.

State-of-the art cardiac CT and MRI can provide exquisite, adequate delineation of cardiac tumors and pseudomasses. Cardiac MRI is a superior tool for mass differentiation owing to the detail it provides on soft tissue. Assessment of cardiac thrombi, however, can be made reliably and rapidly with CT. Detailed knowledge of possible anatomical variants is mandatory to avoid misinterpretation, given the high spatial resolution of images.

The examination of cardiac masses remains a niche application for both CT and MRI.

DR. WINTERSPERGER and DR. HUBER are associate professors of radiology and cardiac imaging experts in the department of clinical radiology at the Grosshadern Clinic at the University of Munich. DR. BAUNER is a radiology resident, and PROF. REISER is a professor of radiology and chair of the department of clinical radiology at the same institution.

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