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1 Department of Diagnostic and Interventional Radiology, University Hospital
Essen, Hufelandstraße 55, D-45122 Essen, Germany.
2 Department of Cardiology, University Hospital Essen, D-45122 Essen,
Germany.
3 Department of Cardiology, Elisabeth Hospital Essen, D-45138 Essen,
Germany.
Received April 5, 2002;
accepted after revision May 17, 2002.
Abstract
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MATERIALS AND METHODS. Twenty-four patients with known or suspected intracardiac thrombi were examined using MR imaging and echocardiography. All MR examinations were performed on a 1.5-T MR scanner using dark-bloodprepared half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences, fast imaging steady-state free precession (trueFISP) cine sequences, and inversion recovery gradient-echo fast low-angle-shot (inversion recovery turbo FLASH) sequences after injection of 0.2 mmol/kg of gadolinium diethylene triamine pentaacetic acid.
RESULTS. MR imaging and echocardiography revealed 12 thrombitwo in the right atrium, one in the right ventricle, three in the left atrium, and six in the left ventricle. Compared with echocardiography, MR imaging revealed three additional thrombi in the left ventricle; these thrombi were confirmed at surgery. All 15 thrombi appeared as filling defects on early contrast-enhanced inversion recovery turbo FLASH MR images. Only seven thrombi were detected on HASTE images, and 10 thrombi were seen on trueFISP images. Four thrombi showed enhancement 10-20 min after contrast material injection and were characterized as organized clots.
CONCLUSION. Contrast-enhanced inversion recovery turbo FLASH sequences were superior to dark-bloodprepared HASTE and trueFISP cine MR images in revealing intracardiac thrombi. Compared with transthoracic echocardiography, MR imaging was more sensitive for the detection of left ventricular thrombi. The characterization of thrombi may be used to predict the risk of embolism, which is higher for subacute clots than for organized thrombi.
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However, distinguishing normal myocardium from clots may be difficult on echocardiography, hampering the diagnosis of thin mural thrombi. In addition, because of the lack of diagnostic criteria, differentiating subacute thrombi from organized thrombi on echocardiographya distinction that is important in predicting the risk of embolic complications [4]is challenging.
Within the last 10 years, MR imaging has emerged as a new noninvasive cardiac imaging technique that can provide complementary information to the data obtained by echocardiography in patients with various cardiac diseases. MR imaging can be considered the first-line imaging technique in patients with congenital heart disease and in those with suspected cardiac tumors. Recently, several new cardiac MR techniques have been introduced to improve spatial and temporal resolution, robustness, and contrast properties [5, 6] of the modality. These rapid technical developments have expanded the indications for cardiac MR imaging.
The aim of our study was to evaluate the diagnostic accuracy achieved with different unenhanced and contrast-enhanced MR sequences for the detection of intracardiac thrombi and to compare the results with those achieved using transthoracic and transesophageal echocardiography.
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A 1.5-T scanner (Magnetom Sonata; Siemens Medical Solutions, Erlangen, Germany) was used for all MR imaging. The MR imaging protocol included an ECG-triggered dark-bloodprepared half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequence (TR/TE, 2 heartbeats/60 msec; flip angle, 160°) covering the entire heart in the axial orientation. Thereafter, four-chamber and two-chamber views as well as contiguous short-axis images of the entire heart were acquired with a fast imaging steady-state free precession (trueFISP) cine sequence (3 msec/1.5 msec; flip angle, 60°). Images in the oblique orientation were obtained to further investigate suspicious areas.
Immediately after the injection of 0.2 mmol/kg of gadolinium diethylene triamine pentaacetic acid (Magnevist; Schering, Berlin, Germany) (flow rate, 2 mL/sec), breath-hold ECG-triggered 2D inversion recovery turbo FLASH images (8/4; flip angle, 25°) of four- and two-chamber views of the heart were acquired. Repeated three-dimensional (3D) inversion recovery turbo FLASH sequences (4/1.4; flip angle, 10°) in the short-axis orientation were then performed. In patients with suspicious findings, additional oblique slices were obtained, using either the 2D or the 3D inversion recovery turbo FLASH sequence. Images were acquired both immediately after injection of the contrast material and as long as 20 min after the injection. To optimize the image contrast, we varied the inversion time (nonselective inversion pulse) for both sequences between 180 and 300 msec.
The typical in-plane resolution was 1.6 x 1.3 mm2 for both sequences. Whereas the 2D sequence is a single-slice technique (slice thickness, 8 mm), the 3D sequence can acquire as many as 24 slices with a slice thickness of 4 mm in one breath-hold of reasonable length, using a shorter TR, partial Fourier reconstruction (6/8), z-axis interpolation, and longer data acquisition window, with 77 k-space lines per heartbeat to improve speed. The total imaging time required, including patient positioning, was 45-60 min.
All MR images were interpreted by consensus by two experienced radiologists who were unaware of the diagnosis and of the results of the echocardiographic examinations. Hard copies were used for the interpretation of the HASTE and inversion recovery turbo FLASH images, whereas the trueFISP images were reviewed as cine loops on a workstation. The interpretations for the HASTE, trueFISP, and inversion recovery turbo FLASH MR images were performed separately.
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The thrombi appeared isointense or slightly hyperintense relative to the myocardium on dark-bloodprepared HASTE images (Fig. 1A). Both of the right atrial thrombi, one of the left atrial thrombi, and four of nine left ventricular thrombi were detectable on HASTE images. The other eight thrombi were not visible on HASTE images because of poor contrast between the thrombus and the myocardium and because of slow-flow artifacts, which hampered the establishment of the correct diagnosis, especially in patients with impaired ventricular function or left ventricular aneurysms (Fig. 2C).
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Thrombi showed low signal intensity on trueFISP cine images (Figs. 1B, 3A, 4A, and 4B). Therefore, the differentiation of the myocardium from the thin mural thrombus was not possible on trueFISP images (Figs. 2A, 2B, 4A, and 4B). Both right atrial thrombi, the only right ventricular thrombus (Fig. 3A), two left atrial thrombi, and five of the nine left ventricular thrombi were detectable on trueFISP images.
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Delineation of thrombi was optimal on early contrast-enhanced inversion recovery turbo FLASH images. All thrombi appeared as lowsignal-intensity filling defects in the cavity. Four of 15 thrombi showed significant contrast enhancement 10-20 min after the injection of contrast material (Fig. 4D).
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Among the imaging techniques routinely used to visualize intracardiac
thrombi, 2D transthoracic echocardiography is the modality of first choice.
Reasons include availability, high accuracy, and low cost. The technique is
characterized by sensitivity and specificity values for left ventricular
thrombi of approximately 90% compared with aneurysmectomy or autopsy
[7]. Several studies have shown
transesophageal echocardiography to be more sensitive for the detection of
atrial thrombi than transthoracic echocardiography
[3,
8], particularly for the
detection of thrombi in the left atrial appendage. However, transesophageal
echocardiography is semiinvasive. The sensitivity of angiocardiography for the
detection of intracardiac thrombus is unacceptably low, and indium-111
platelet scanning is time-consuming and expensive
[7]. Contrast-enhanced CT is
more sensitive for ventricular and atrial thrombi than is transthoracic
echocardiography, but the technique has been shown to be inferior to
transesophageal echocardiography for revealing atrial thrombi
[9,
10]. These imaging techniques
are of limited use in distinguishing between a thrombus and a cardiac tumor or
in characterizing a thrombus as acute or
subacute.
,
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Over the years, cardiac MR imaging with different types of sequences has emerged as a noninvasive alternative for the detection and characterization of intracardiac masses [11,12,13,14]. In agreement with earlier studies, our data show that the value of spin-echo and turbo spin-echo MR imaging sequences for revealing intracardiac masses is limited by artifacts caused by slow-flowing blood [11, 15]. Because of the high signal intensity inherent to blood, gradient-echo sequences are robust and more sensitive for the detection of intracardiac thrombi. The differentiation of a mural thrombus from the myocardium can be challenging, however [11]. The recently developed trueFISP sequence used in our study improves the contrast between the myocardium and blood [6], but thrombi are isointense relative to the myocardium, so the diagnosis of mural thrombi may be hampered.
The IV administration of gadolinium diethylene triamine pentaacetic acid was shown to enhance the contrast between the myocardium and thrombi, thereby improving our ability to detect and characterize thrombi [4, 14]. Our data suggest an ECG-triggered inversion recovery turbo FLASH sequence originally developed for imaging myocardial infarction [5] is optimally suited for imaging cardiac thrombi also. Immediately after the injection of contrast material, the myocardium and the cardiac cavity show high signal intensities that allow easy detection of cardiac thrombi (Fig. 2A,2B,2C,2D). Inversion times can be varied to improve the contrast between thrombi and the myocardium. The signal intensity of myocardial infarctions, which are frequently associated with left ventricular thrombi, increases over time, permitting easy differentiation between nonviable infarcted tissue and subacute thrombi (Fig. 1A,1B,1C). A recently developed ECG-triggered 3D inversion recovery turbo FLASH sequence even allows full cardiac coverage in a single breath-hold and seems ideally suited for the detection of intracardiac thrombi. However, use of ECG triggering is a prerequisite for obtaining high-quality images. Although not present in our study, artifacts may occur in patients with atrial fibrillation or other arrhythmias, which are frequently associated with intracardiac thrombi.
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Intracardiac thrombi are associated with a variety of diseases. Functional disorders such as myocardial infarction or atrial fibrillation are a common cause of cardiac thrombi [7, 16]. The frequency of left ventricular thrombi is approximately 30% in patients with an acute or healed myocardial infarction. Most thrombi develop within the first week after the infarction and are most often noted at the apex of the left ventricle in patients who have had an anterior myocardial infarction [7]. Deep vein thrombosis or the use of central venous catheters may lead to development of thrombi in the right heart, which should be considered pulmonary emboli in transit [1, 17,18,19]. Chronic anabolic steroid abuse as well as several disorders and diseases such as Behçet's syndrome, coagulopathies, Löffler's endocarditis, Churg-Strauss syndrome, or right atrial aneurysm should be considered potential causes of intracardiac thrombi [20,21,22,23,24]. In these rare clinical settings, the differentiation between thrombi and cardiac tumors can be challenging [21, 24], and MR imaging may provide additional information to that obtained from echocardiography.
Intracardiac thrombi may cause arterial or pulmonary embolisms and should
be regarded as life-threatening. It is estimated that in approximately 30% of
cases of cerebral infarction, the causative thrombus originated in the heart
[2,
3]. Cardiac diseases or
abnormalities that may cause arterial emboli include atrial and ventricular
thrombosis, cardiac tumors, thrombosis on heart valve prostheses, and
endocarditis [8]. Our data
indicate that contrast-enhanced MR imaging depicts intracardiac thrombi more
accurately than transthoracic echocardiography does andin a limited
number of casesis as sensitive as transesophageal echocardiography.
Cardiac MR imaging is the most accurate imaging modality in patients with
suspected cardiac tumors. However, cardiac MR imaging cannot replace
echocardiography in patients who have had a cerebral stroke because
endocarditis, thrombosis on heart valve prostheses, and other potential
sources of embolismpatent foramen ovale, valve strands, atrial septum
aneurysm, and dystrophy of mitral annulusare difficult to visualize on
MR imaging.
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In patients with a pulmonary embolism, the rare finding of right heart thrombi may be underdiagnosed. Clinical studies using echocardiography and autopsy studies suggest that these thrombi occur in 6-18% of patients with a pulmonary embolism [25,26,27]. Most of these patients cannot undergo MR imaging because they present with severe dyspnea or cardiogenic shock [1, 26]. Transesophageal echocardiography is unquestionably the imaging technique of first choice in these patients. Use of MR imaging should be restricted to clinically stable patients who have either no symptoms or only minor ones and in whom potential differential diagnoses like intracardiac tumors and congenital structures such as Chiari's network or persistent eustachian or thebesian valves cannot be excluded with certainty. In addition, cardiac MR imaging may be combined with pulmonary MR angiography [28] to provide a single test for the pulmonary vasculature and heart in patients with suspected pulmonary embolism.
The risk of embolism depends on morphologic parameters that can easily be assessed on echocardiography and MR imaging. The embolic risk is approximately 50% for mobile or protruding thrombi compared with an embolic risk of approximately 10% for nonmobile or flat thrombi [7]. MR imaging can distinguish subacute clotswhich do not enhance after contrast material injectionfrom organized thrombi [4]. Four of the 15 thrombi in our study showed significant enhancement and could be considered organized clots. Such characterization provides more information than echocardiography does and may be of great clinical interest because embolic complications are more likely to occur with subacute clots than with organized thrombi.
Echocardiography remains the imaging modality of choice in patients with suspected intracardiac thrombi. However, our data show that ECG-triggered contrast-enhanced cardiac MR imaging is emerging as an accurate noninvasive alternative technique for the detection and characterization of intracardiac thrombi.
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