MR imaging provides anatomic and functional information that is superior to that provided by conventional cardiac imaging modalities such as echocardiography and angiography. Contrast material–enhanced MR angiography is particularly useful for the assessment of deep anatomic structures such as the pulmonary arteries, which are difficult to see on echocardiograms and difficult to access at selective angiography. Furthermore, cine MR images can provide additional information about cardiac function, valve patency, and the hemodynamic significance of vascular stenosis.
Contrast-enhanced MR Angiography
With the progress in MR imager development and the ability to acquire 3D MR images within a single breath hold (or during the first passage of a bolus of contrast material), contrast-enhanced MR angiography has become the method of choice for visualization of the great vessels of the chest and abdomen. Imaging parameters and partition dimensions should be carefully adjusted to achieve the smallest possible voxel size while allowing sufficient spatial coverage of the target vessels within a single breath-hold acquisition. Imaging time can be decreased by using partial Fourier imaging, fewer partitions, fewer phase encoding steps, or a rectangular field of view.
Cardiac MRI and Contrast-Enhanced MR Angiography
Cardiac MRI was performed using a Sonata 1.5-T or an Avanto 1.5-T scanner (both by Siemens Medical Solutions). Multislice single-shot spinecho images using HASTE were obtained in three orthogonal planes to define the cardiac anatomy. In some patients, steady-state free precession (SSFP) multislice images were also obtained to improve definition of the blood-tissue borders. Turbo spin-echo imaging was often undertaken to delineate the edges of the sinus venosus defect. Ventricular volumes, ventricular mass, and systolic function were calculated using a stack of short-axis ECG-gated cine images. Cine phasecontrast velocity flow maps were obtained in accordance with established protocols and practice to better detect the anomalous flow patterns and to calculate the pulmonary-to-systemic (Qp/Qs) blood flow ratio Contrast-enhanced MR angiography (CE-MRA) was also performed in some patients. The decision to perform CE-MRA was made by the individual performing the scanning. In those cases, the coronal orientation was used during breath-hold at end-inspiration before and after the IV administration of gadopentetate dimeglumine (Magnevist, Schering) . The bolus was timed to the arrival of the contrast agent in the ascending aorta.
Postprocessing and Volume Rendering
The acquisition of 3D data in contiguous slabs allows the reformatting of images in an oblique orientation. From the 3D data set from MR angiography, for example, it is possible to obtain two-dimensional images in any oblique plane across the volume of data. With the high target-to-background contrast on MR angiographic images, it is often desirable to vary the section thickness to obtain two-dimensional reformatted images from 3D data sets that contain the required anatomic structures. This is particularly true for angiographic data where, by increasing the thickness of the reconstructed orthogonal or oblique plane, one may obtain a better view of multiple vascular branches and their course. The maximum intensity projection technique is the simplest and most widely used technique for visualization of 3D MR angiography data. It is based on a simple algorithm of projecting all the data on to one plane by selecting the highest intensity data element (voxel) in the data set along the projection lines. The resulting image is similar in appearance to images obtained from traditional X-ray angiography. Because maximum intensity projection does not differentiate the front from the back, overlap between adjacent structures makes it difficult to visually appreciate the exact spatial location of a given structure. An alternative rendering technique called surface rendering introduces a degree of opacity and thus allows better perception of the anatomic structures proximal to the viewing point and obscuring of structures that are located behind them. The surface rendering technique requires a selection of a surface, or threshold, between the object of interest and the surrounding structures. More advanced rendering techniques rely on sophisticated combinations of transparency and opacity of different anatomic structures. Under the common rubric of volume rendering, there are a variety of sophisticated algorithms that assign different degrees of opacity and even different colors and textures to objects in the volume of interest.
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