Isotropic reconstructions use cubes, while in anisotropic methods one side is longer than the two others. If the voxel can be kept smaller, less structures will be represented by one single pixel, and therefore spatial resolution and contrast will be better.ĭata acquisition and reconstruction methods define different voxel shapes. In the final image pixel, they will be indistinguishable. If the voxel has a large volume, it can contain many different structures and tissue types. If the FOV is smaller (e.g., 12.8 cm) and the same matrix size is used, the spatial resolution is 0.5 mm.Ġ9-03 Spatial Resolution and Partial Volume EffectsĪs in other digitized imaging methods, voxel and pixel size influence spatial resolution and thus contrast.Īll anatomical structures within one voxel add to its averaged signal intensity in the final image. If the FOV is the whole head with an edge length of 25.6 cm and a matrix size of 256×256 is used, then a single pixel represents 1 mm. Commonly, individual voxels and pixels are larger in body imaging than in head imaging. Usually in MR imaging, the field-of-view is at least 256×256. In this case, we have an image matrix of 6×6, i.e., a grid of 6 rows and 6 columns with a total number of 36 pixels. Both combined represent the field-of-view (FOV), as shown in Figure 09-03. It is defined by the steepness of the x-gradient (the frequency-encoding gradient) and the number of phase-encoding steps in the y-gradient. The image matrix is characterized by the number of pixels in the x- and y-directions. These 256×256 picture elements are called the image matrix. Therefore, for instance, 256×256×1 voxels are created of a slice of an object and turned into pixels. In reality, however, voxel size depends on a number of limiting factors, with computer capacity and the signal obtained from an individual voxel being the main obstacles. In principle, voxels could be as small as a single cell. These numbers are used to create a picture consisting of pixels. In each volume element, the signals are averaged and turned into a number which represents a certain level on a gray scale. We want to image an entire person who for this purpose is mathematically divided into volume elements.
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