Back-projection imaging

From a set of projections acquired under different angles (p in polar coordinates, a spin density map is reconstructed in back-projection imaging (cf. Section 6.1). For a 2D spin density Mq(jc, y) the projection onto an axis r which is at an angle (p with respect to the j -axis follows by integration over the space variable s orthogonal to r (Fig. 5.4.1),... 

Figure Bl.14.4. k-space representations of the (a) spin-echo imaging pulse sequence in figure Bl.14.1. (b) echo-planar imaging sequence in figure Bl. 14.3 and (c) back-projection imaging. In (a) and (b) the components of the wave vectors and represent frequency and phase encoding, respectively. For back-...

The transverse images of cold and hot regions insert were obtained by the filtered back projection image reconstruction technique. It was corrected for uniformity and center of rotation (COR). Chang s attenuation correction method was applied by choosing 0.111, 0.121, 0.131, 0.141 and 0.151 cm linear attenuation coefficient values. Butterworth filter with cut off frequency 0.35 cycles/cm and order 5 was used. Scatter correction technique was not applied due to non-availability with this gamma camera system. 

Figure 1 Back-projection imaging. (A, B) Sequence form used for acquiring a back-projection data set for an image in the X-/plane. At this time the slice selection procedure is simply shown as a block. It will be described later in the text. The X and /gradients are constrained to define a series of vectors (given by 0 = tan- (Y/X) of constant magnitude R (= (x + (A) shows the variant in the...

To create a two-dimensional image, two gradients are applied along the X- and y-directions, and a series of one-dimensional images recorded in different directions in the xy-plane. A technique known as back-projection... 

One-dimensional images of toluic acid were obtained with a stationary sample. For two-dimensional images, the 4-bromobenzoic acid crystal was rotated about an axis orthogonal to the gradient direction in constant increments of either 3° or 6° over a range of 180° to collect sets of 60 or 30 one-dimensional projections, respectively. The two-dimensional images were calculated with the filtered back-projection reconstruction algorithm (36). 

Fig. 3 Transaxial slices reconstructed with filtered back projection (top row) and iterative reconstruction (bottom row). Negative reconstructed values have been set to zero for the filtered back projection reconstruction. The streak artifacts and count defects caused by focal activity deposition in the central airways are clearly evident on the reconstructions with filtered back projection. The iterative reconstruction provides artifact free images.

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