This work presents a method that allows for the assessment of 3D murine myocardial motion in vivo at microscopic resolution. Phase-contrast (PC) magnetic resonance imaging (MRI) at 17.6 T was applied to map myocardial motion in healthy mice along three gradient directions. High-resolution velocity maps were acquired at three different levels in the murine myocardium with an in-plane resolution of 98 m, a slice thickness of 0.6 mm, and a temporal resolution of 6 ms. The applied PC-MRI method was validated with phantom experiments that confirmed the cor-rectness of the method with deviations of <1.7%. Myocardial in-plane velocities between 0.5 cm/s and 2.2 cm/s were determined for the healthy murine myocardium. Through-plane velocities of 0.1-0.83 cm/s were measured. Velocity data was also used to calculate the myocardial twist angle during systole at different slices in the short-axis view. Magn Reson Med 55: 1058-1064, 2006. In recent years transgenic and knockout mice have gained increasing importance in cardiovascular research. Murine models of cardiovascular disease with gene overexpres-sions, mutations, or knockouts can easily be produced. An examination of the consequences of these modifications adds to our understanding of the function of certain gene products. These facts have been the driving force behind the development of suitable methods for assessing the cardiac function of mice in vivo. Echocardiography and ventriculography have proven to be feasible tools for evaluating heart function in small animals (1-5). However, over the past few years MRI has emerged as the most powerful imaging modality for measuring a broad spectrum of functional parameters and thus allowing for accurate detection of the pathophysiological state of the murine heart (6-8). Traditional cine MRI allows assessment of wall thickening and the ejection fraction. If the entire left ventricle (LV) is imaged by acquiring different slices, global cardiac functional parameters such as LV mass, end-diastolic volume (EDV), and end-systolic volume (ESV) become accessible (9). In combination with spatial modulation of magnetiza-tion (SPAMM), local functional parameters (e.g., myocar-dial motion) can be measured (10,11). By generating a grid of dark bends within an image and thus tracking the deformation of the myocardium during the heart cycle, motion and strain parameters of the myocardium can be calculated. An intrinsic drawback of this method is the large difference between spatial image resolution and the tag spacing, which limits the resolution of motion information. Recently published studies achieved a resolution of 133 267 m in the image plane by applying a tag spacing of 0.7 mm (12). With respect to the size of the murine heart (inner diameter of about 10 mm and myocardial wall thickness on the order of 1-2 mm), the tagged cine MRI approach yields restricted information about local myocar-dial motion. Aletras et al. (20) introduced a combination of SPAMM with an additional unencoding gradient module called displacement encoding wi...
This work introduces an MR-compatible active breathing control device (MR-ABC) that can be applied to lung imaging. An MR-ABC consists of a pneumotachograph for respiratory monitoring and an airway-sealing unit. Using an MR-ABC, the subjects were forced to suspend breathing for short time intervals, which were used in turn for data acquisition. While the breathing flow was stopped, data acquisition was triggered by ECG to achieve simultaneous cardiac and respiratory synchronization and thus avoid artifacts from blood flow or heart movement. The flow stoppage allowed a prolonged acquisition window of up to 1.5 sec. To evaluate the potential of an MR-ABC for segmented k-space acquisition, diaphragm displacement was investigated in five volunteers and compared with images acquired using breath-holding, a respiratory belt, and free breathing. Respiratory movement was comparatively low using the breath-hold approach, a respiratory belt or an MR-ABC. During free-breathing diaphragm displacement was comparatively large. To demonstrate the potential of an MR-ABC, lung MRI was performed using whole-chest 3D gradient-echo imaging, multislice turbo spin-echo (TSE) imaging, and short tau inversion recovery TSE (STIR-TSE). Cardiorespiratory synchronization was used for each sequence. None of the volunteers reported any discomfort or inconvenience when using an MR-ABC. Flow stoppage of up to 2.5 sec per breathing cycle was well tolerated, therefore allowing for a reduction of the total imaging time as compared to usage of a respiratory belt or MR navigator. Magn Reson Med 58:1092-1098, 2007.
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