New MR imaging contrast agents.

Many new MR contrast agents are undergoing laboratory development, are in clinical trials, or are being used for routine clinical applications. These agents function by targeting specific receptors that allow the agent to accumulate within the reticuloendothelial system, the hepatobiliary system, or the blood pool. Each agent has unique properties that offer advantages over unenhanced and gadolinium chelate-enhanced MR imaging. Use of these agents, however, requires an understanding of their current and potential clinical indications and inherent limitations. This article reviews the development, evaluation, and clinical application of a number of these agents, including ferumoxides, Mn-DPDP, AMI-227, SHU-555A, Gd-EOB/DTPA, Gd-BOPTA, and MS-325.     

Diagnostic accuracy of arterial phase 64-slice multidetector CT angiography for left atrial appendage thrombus in patients undergoing atrial fibrillation ablation

Introduction: Multidetector CT (MDCT) is used prior to atrial fibrillation ablation (AFA) to anatomically guide ablation procedures. Whether 64-slice MDCT also can be used to diagnose left atrial thrombus is not known.
Methods: We sought to determine the accuracy and interobserver variability of MDCT in the evaluation of left atrial thrombus prior to AFA. We enrolled 50 patients scheduled for AFA who underwent 64-slice MDCT scan and transesophageal echocardiography prior to the procedure. Three experienced observers reviewed all the MDCT images for the presence of a left atrial thrombus, and two different readers interpreted the transesophageal echocardiograms (TEE), which were used as the gold standard. All observers were blinded to clinical data and each other.
Results: Interobserver variability between the three MDCT readers was poor (highest kappa statistic 0.43, P = 0.001). Diagnostic accuracy was highly variable, with sensitivities ranging from 100% to 50% and specificities ranging from 85% to 44%. TEE reader agreement was 98%.
Conclusion: MDCT demonstrates high interobserver variability and has only modest diagnostic accuracy for the detection of left atrial thrombus in patients undergoing AFA procedure. Potential factors affecting the accuracy of MDCT include image quality and the difficulty of distinguishing clot from pectinate muscle. MDCT likely is not the optimal method to detect left atrial thrombus using current techniques and standards of interpretation.     

Regional infarction identification from cardiac CT images: a computer-aided biomechanical approach

Purpose

Regional infarction identification is important for heart disease diagnosis and management, and myocardial deformation has been shown to be effective for this purpose. Although tagged and strain-encoded MR images can provide such measurements, they are uncommon in clinical routine. On the contrary, cardiac CT images are more available with lower costs, but they only provide motion of cardiac boundaries and additional constraints are required to obtain the myocardial strains. The goal of this study is to verify the potential of contrast-enhanced CT images on computer-aided regional infarction identification.

Methods

We propose a biomechanical approach combined with machine learning algorithms. A hyperelastic biomechanical model is used with deformable image registration to estimate 3D myocardial strains from CT images. The regional strains and CT image intensities are input to a classifier for regional infarction identification. Cross-validations on ten canine image sequences with artificially induced infarctions were used to study the performances of using different feature combinations and machine learning algorithms.

Results

Radial strain, circumferential strain, first principal strain, and image intensity were shown to be discriminative features. The highest identification accuracy (\(85\pm 14\) %) was achieved when combining radial strain with image intensity. Random forests gave better results than support vector machines on less discriminative features. Random forests also performed better when all strains were used together.

Conclusion

Although CT images cannot directly measure myocardial deformation, with the use of a biomechanical model, the estimated strains can provide promising identification results especially when combined with CT image intensity.

     

Role of computed tomography and magnetic resonance imaging in assessment of acute aortic syndromes

Acute aortic syndromes refer to the spectrum of aortic emergencies that include nontraumatic diseases of the aorta, such as aortic dissection, intramural hematoma, penetrating atherosclerotic ulcer, aortic aneurysm leak, as well as traumatic aortic transection. Patients presenting with nontraumatic acute aortic syndromes usually have a similar clinical profile; hence, clinical diagnosis is difficult. Computed tomography (CT) and magnetic resonance imaging (MRI) allow for specific diagnosis of the underlying condition. Traumatic rupture of the aorta is one of the most dreaded complications of blunt chest trauma; therefore, in patients with high-risk deceleration injuries, radiographic assessment of the aorta is crucial. Imaging methods should detect even subtle aortic wall disruption and should provide a mechanism for communicating the findings to the surgical team. Noninvasive, cross-sectional imaging techniques have proven efficacy in the diagnosis of aortic pathology and have largely replaced aortography. Both CT and MR imaging provide aortogram-like reconstruction of the original data sets, and in addition to assessing the aortic lumen, permit detailed evaluation of the aortic wall, as well as comprehensive assessment of thoracic and abdominal viscera. This article addresses the role of different imaging modalities in assessment of acute aortic syndromes, with focus on CT and MRI, and with discussion of the key imaging findings that allow distinction among the various aortic pathologies.