The goal of this article is to describe how cardiac magnetic resonance can be used for the evaluation and diagnosis of a suspected cardiac thrombus. The method presented will describe data acquisition as well as the pre-procedure and post-procedure protocol.
We present the conventional cardiac magnetic resonance (CMR) protocol for evaluating a suspected thrombus and highlight emerging techniques. The appearance of a mass on certain magnetic resonance (MR) sequences can help differentiate a thrombus from competing diagnoses such as a tumor. T1 and T2 signal characteristics of a thrombus are related to the evolution of hemoglobin properties. A thrombus typically does not enhance following contrast administration, which also helps differentiation from a tumor. We also highlight the emerging role of T1 mapping in the evaluation of a thrombus, which can add another level of support in diagnosis. Prior to any CMR exam, patient screening and interviews are critical to ensure safety and to optimize patient comfort. Effective communication during the exam between the technologist and the patient promotes proper breath holding technique and higher quality images. Volumetric post processing and structured reporting are helpful to ensure that the radiologist answers the ordering services' question and communicates these results effectively. Optimal pre-MR safety evaluation, CMR exam execution, and post exam processing and reporting allow for delivery of high quality radiological service in the evaluation of a suspected cardiac thrombus.
Cardiac magnetic resonance (CMR) imaging is an important diagnostic modality for the evaluation of cardiovascular function and pathology. Technological advances allow for reduced acquisition time, improved spatial and temporal resolution, as well as higher quality tissue characterization. These advances are particularly useful in the evaluation of cardiac masses.
Echocardiography remains the first line imaging modality for the initial evaluation of cardiac masses, specifically with respect to mass location, morphology, and physiologic impact. However, echocardiography is limited by poor tissue characterization, a restricted field of view, and operator dependent image quality. Cardiac computed tomography (CT) is often utilized as a second-line imaging modality for assessing cardiac masses. Advantages of cardiac CT over other modalities include excellent spatial resolution and a superior ability in detecting calcifications. The main disadvantage of cardiac CT is patient exposure to ionizing radiation. Additional limitations include decreased temporal resolution and soft tissue contrast resolution. CMR is emerging as a valuable tool in the characterization of cardiac masses detected on echocardiography or CT. Compared to CT, CMR does not expose patients to ionizing radiation. In addition, CMR can be useful in treatment and surgical planning1,2.
A thrombus is the most common cardiac mass. The most common locations for cardiac thrombi are the left atrium and left atrial appendage, especially in the setting of atrial fibrillation or a dysfunctional left ventricle1,3. The diagnosis of thrombus is important for the prevention of embolic events as well as establishing the need for anticoagulation. CMR can aide in determining the acuity of a thrombus. Acute thrombus typically demonstrates intermediate T1- and T2-weighted signal intensity relative to the myocardium due to high amounts of oxygenated hemoglobin. Increased methemoglobin content in the subacute thrombus results in lower T1-weighted signal intensity and intermediate or increased T2-weighted signal intensity. With a chronic thrombus, methemoglobin and water are replaced with fibrous tissue leading to decreased T1- and T2-weighted signal intensity1,2,3.
The avascular composition gives a cardiac thrombus intrinsic tissue characteristics that can be exploited by contrast enhanced CMR, to aide in the differentiation of a thrombus from other cardiac tumors4. An organized thrombus does not enhance while true cardiac lesions enhance on post contrast imaging due to the presence of intratumoral vascularity3. Arterial perfusion imaging allows real time assessment of vascularity within a mass and is critical to differentiate a thrombus from a tumor. Perfusion within a mass can also be useful in the delineation of a bland thrombus from a tumor thrombus. Cine imaging provides advantages over other modalities that can be subject to motion artifact, and the temporal resolution provided by real time gated perfusion imaging increases sensitivity in detecting enhancement5.
T1 mapping is a MR technique that allows pre-contrast native T1 relaxation times and post-contrast extracellular volume calculation to detect pathologic alterations in tissue. By adding a quantitative dimension to CMR, T1 mapping can help differentiate various disease processes from the normal myocardium. An emerging application is the characterization of cardiac masses and delineation of masses from cardiac thrombi. Previous studies performed on a 1.5 T Aera XQ scanner have reported native T1 relaxation times of a recent thrombus (911 ± 177 ms) and a chronic thrombus (1,169 ± 107 ms)6. Other pertinent native T1 relaxation times include lipoma (278 ± 29 ms), calcifications (621 ± 218 ms), melanoma (736 ms), and normal myocardium (950 ± 21 ms). This data suggests that T1 mapping can add quantitative information to a non-contrast exam which in the setting of contraindication to IV gadolinium could be extremely useful6,7.
Contrast-enhanced CMR has been well validated for the detection of a left ventricular thrombus. It has been shown to provide the highest sensitivity and specificity (88% and 99%, respectively) for detection of a left ventricular thrombus compared to transthoracic (23% and 96%, respectively) and transesophageal (40% and 96%, respectively) echocardiography8. Currently, there are no large-scale studies validating the utility of CMR for assessing a thrombus in other chambers of the heart3.
Despite the many advantages of CMR over other imaging modalities for evaluating cardiac masses, there are also limitations. CMR, like cardiac CT, relies on electrocardiographic gating. This can cause artifact and image degradation in patients with significant arrhythmias. Image quality can also be degraded when scanning patients who have difficulty complying with breath hold requirements. However, faster acquisition times and respiratory gating techniques allow for quality images during free breathing. The presence of certain implanted devices is a contraindication for CMR and poses as a major disadvantage, although the number of MR compatible implantable devices is increasing1,2.
In summary, specific CMR sequences can be utilized to develop a dedicated MR imaging protocol for the evaluation of a suspected cardiac thrombus. The method presented here will provide instructions for the acquisition of CMR data for evaluation of a suspected thrombus. Pre-procedure screening, sequence selection, troubleshooting, post-processing, volumetric analysis, and report generation will be discussed.
The following protocol follows the departmental clinical guidelines and is adherent to the institution’s human research ethics guidelines.
1. Prepare for MRI Data Acquisition
2. Acquire the MRI Data [Cardiac MR without and with IV Contrast Limited] Focused Scan to Evaluated Potential Cardiac Thrombus
NOTE: Basic scan sequences are often loaded by the MRI technologist from scan libraries that are present on each MRI scanner. Standard cardiac scan prescription and orientations are also considered routine operating tasks for MRI technologists.
3. Analysis of the MRI data
The CMR protocol designed for the evaluation and diagnosis of cardiac thrombus encompasses patient screening and preparation, data acquisition utilizing specific sequences, data post-processing, and report generation. Specific signal characteristics on given sequences can infer with high accuracy the diagnosis of a cardiac thrombus and differentiate these from the competing diagnosis of a cardiac tumor. Table 1 highlights the conventional and emerging CMR sequences that are commonly used to evaluate for cardiac thrombus.
A cardiac thrombus has a low SSFP signal with absent internal perfusion and absent delayed enhancement (Figure 1 and Figure 3). The T2 signal on dark blood imaging can vary depending on the age of the blood products within the thrombus. In subacute thrombi, mildly increased T2w signal can be encountered (Figure 3B); whereas in chronic thrombus, low T2w signal is expected. Alterations in native T1 signal are also expected with chronic thrombus having elevated T1 relaxation times (Figure 1D,E and Figure 3F).
Pazos-Lopez et al. showed that CMR can differentiate a thrombus from other cardiac tumors with excellent accuracy22. Cardiac thrombi were smaller, more homogenous, and less mobile than tumors22. Higher or isointense signals compared to normal myocardium on T2w, first pass perfusion, and LGE sequences were more common in tumors vs. thrombi (85% vs. 42%, 70% vs. 4%, and 71% vs. 5%), respectively22.
Figure 1: A 71 year-old male with history of prostate cancer and a left ventricular mass seen on CT. CMR demonstrates an intraluminal LV mass compatible with thrombus within an LV apical aneurysm with associated chronic LV infarct (A) Axial SSFP demonstrates LV apical wall thinning with an aneurysmal configuration at the apex. There is a low signal intraluminal structure within the LV apex. (B) Axial first pass arterial perfusion image: There is no perfusion within the LV apical structure. (C) 3 chamber LGE image: no LGE within the LV apex mass. LGE within the apical wall is >50% wall thickness compatible with previous infarct. (D) Color native T1 map demonstrates native T1 relaxation time within the LV apex mass of 1105 ms suggesting chronic bland thrombus. (E) Enlarged color native T1 map at LV apex: There is a thinned LV apex wall with the blue-green ROI T1 relaxation time measuring 1,268 ms which is compatible with a prior infarct. Please click here to view a larger version of this figure.
Figure 2: A 70 year-old male with hepatocellular carcinoma metastatic to the IVC and right atrium. This right atrial intraluminal metastasis is shown to provide comparison to intraluminal thrombus in other figures (A) Axial SSFP: A cavoatrial junction mass demonstrates low signal. (B) T2 dark blood: The high T2 signal within the mass (arrow) is nearly iso-intense to nearby hepatic tumors seen on the same image. (C) Axial Native T1 map color image (Siemens myomaps, Erlangen, Germany): the mass (arrow) demonstrates a native T1 relaxation time of 724 ms. (D) Coronal MRA: the mass is contiguous with adjacent hepatic tumor extending through the IVC into the right atrium (arrow). Please click here to view a larger version of this figure.
Figure 3: A 61 year-old male with metastatic urothelial carcinoma with a right ventricular mass seen on CT which is compatible with thrombus on CMR. (A) Axial SSFP: A low signal mass near the RV apex is noted. (B) Axial T2 dark blood: there is isointense to mildly hyperintense T2 signal within the mass related to the presence of subacute blood products. (C) Axial dynamic arterial perfusion: no perfusion is seen within RV mass. (D) Axial post contrast CT: there is no enhancement within the RV mass. (E) Axial LGE: the non-enhancing RV mass is compatible with thrombus. (F) Grayscale pre-contrast native T1 Map demonstrates an elevated T1 relaxation time within the mass of 1,094 ms, which is compatible with thrombus. Please click here to view a larger version of this figure.
With the increasing quality and frequency of diagnostic imaging, it is not uncommon to discover incidental cardiac masses when performing imaging for unrelated indications. Patients with cardiac masses are often asymptomatic, and if present, symptoms are typically nonspecific.
The diagnosis of cardiac thrombus is important not only for differentiating thrombus from benign or malignant cardiac tumors, but also for determining the need for anticoagulation and prevention of embolic events1. In patients with a suspected cardiac thrombus, the option for a single imaging modality with a specific protocol can provide for accurate and efficient diagnosis.
The protocol described includes specific CMR sequences designed for optimal localization and characterization of a suspected cardiac thrombus. For structural and functional evaluation, cine SSFP images are acquired in two-chamber, three-chamber, four-chamber, and short-axis views. SSFP imaging provides high spatial resolution and is not dependent on flow effects. This allows for a short time to repetition (TR), which improves temporal resolution. This is particularly useful for patients with breath-holding difficulty, and it aids in assessing for any mobility of a suspected thrombus. SSFP also provides a high signal to noise ratio (SNR) and contrast to noise ratio (CNR) due to intrinsic contrast properties between the myocardium and blood pool. For tissue characterization, black blood T1-weighted and T2-weighted double and triple inversion recovery FSE images are acquired with and without fat saturation. The T1-weighted images provide excellent contrast resolution for determining size and extent of the thrombus, as well as providing information on the presence or absence of recent hemorrhage or melanin due to T1 shortening. T1-weighted images also serve as a basis for comparison to post-contrast images. The fat-saturated images are useful for determining the presence of fat in a cardiac mass. The T2-weighted images are useful for characterizing myocardial edema associated with a mass, or to assess for a cystic component. Post gadolinium enhancement images are acquired during the injection of contrast (first pass perfusion) and repeated at approximately 10 minutes post-injection (LGE). The perfusion images are useful for distinguishing vascular tumor from a thrombus. For LGE, a phase-sensitive inversion recovery sequence is utilized, and the inversion time is set to null thrombus. This aids in differentiating a thrombus from a tumor. If there is a known tumor, this aids in delineating a thrombus surrounding or associated with a tumor1,2,3,4.
We also highlight the emerging role of T1 mapping in the evaluation of thrombus which can add another level of support in diagnosis. T1 mapping is potentially helpful in distinguishing a thrombus from a tumor by providing quantitative assessment of pre-contrast T1 relaxation times. T1 mapping can also potentially differentiate between an acute and a chronic thrombus. More recent (<1 week) thrombi have been shown to have shorter T1 values compared to older (>1 month) thrombi6. Additionally, T1 mapping in addition to T2 mapping have shown to be useful for differentiating masses such as cardiac myxomas from myocardium23.
Multiple imaging modalities can be employed to comprehensively evaluate cardiac masses, each possessing strengths and weaknesses. CMR is emerging as the imaging modality of choice for evaluating cardiac masses. CMR allows for the qualitative and quantitative assessment of cardiac anatomy, function, perfusion, and tissue characteristics in a single examination. Unlike CT, CMR does not expose patients to ionizing radiation. In contrast to echocardiography which suffers from poor tissue characterization and limited field of view, CMR offers superior tissue characterization, high spatial and temporal resolution, multiplanar imaging capabilities, and a larger field of view1,2,3.
Prior to any CMR exam, patient screening and interviews are critical to ensure safety and to optimize patient comfort. Effective communication during the exam, between the technologist and the patient, promotes proper breath holding technique and high quality images. Volumetric post processing and structured reporting are helpful to ensure the radiologist answers the ordering services' question and communicates these results effectively. Optimal safety screening evaluation, CMR exam execution, exam post-processing, and reporting allow for delivery of high quality radiological service in the evaluation of suspected cardiac thrombus.
The authors have nothing to disclose.
The authors acknowledge support from the Department of Diagnostic Imaging at the H. Lee Moffitt Cancer Center and Research Institute.
MRI Scanner | Siemens Healthcare Erlangen, Germany |
Magnetom Aera 1.5 Tesla | MRI scanner that will be used for the demonstration |
Post processing software | Medis The Netherlands |
Qmass software | post processing software for ventricular volumetric and T1 mapping analysis |
Scanner processing software | Siemens Healthcare Erlangen, Germany |
Myomaps | Scanner sequence package and post processing software |