Imaging Errors

The imaging errors of the gamma camera appear among errors occurring during SPECT reconstruction, since the consistent errors of the projections are strongly amplified during reconstruction. For example, the uniformity error results in ring structures (as visible in the image below).

Besides the imaging errors occurring in gamma cameras, further errors appear in the SPECT due to the movements of the head, the longer time of recording and the fact that it has to be quantitative. Remedies for these types of errors are listed below.

1. The movement of the gamma camera causes the sinogram of a point source to differ from the expected sine function. Therefore, the correction of the centre of rotation (COR) is needed. However, an even bigger problem is the head getting displaced from the ideal trajectory in each measurement, so general corrections cannot be applied. Fortunately this does not cause as great problems in human applications as it does in cases when our aim is to achieve a resolution below one millimetre.
   

   Figure 2.
2. Movement correction (MC) needs to be applied because of the movement of the object being examined while taking the image. A reason of the movement may be the movement of the patient. In this case the projection images need to be displaced by the extent of the displacement of the patient during monitoring. The movement of the chest during breathing can be corrected with monitoring it with a CCD camera, with observing the movement of the reference points placed on the body and with the anatomical model of respiration. The movement of the heart can be corrected with gated images by dividing the cardiac cycle into 8-32 parts. In this case we get a weaker image statistics and the reconstruction will be noisy, that is why a four-dimensional reconstruction is preferred where every slice is reconstructed at the same time without dividing them into time slices. The image below shows the correction of an axial movement in case of the heart, calculated in projection images based on an integral linogram (to the left: before MC, to the right: after MC).
   

   Figure 3.
3. There are more types of tests the aim of which is quantitative imaging. Examples include the measurements of thyroid uptake, cardiac ejection fraction and renal function. In order to obtain quantitative information partial-volume effect (PVE) correction needs to be applied. When the reconstructed activity is divided into more voxels and the size of the voxel is significant compared to that of the object to be imaged (e.g. in case of the wall of the heart), then the activity that can be measured in the voxel belonging to the object is notably lower. A solution applied in clinics is modelling the scattering and the resolution during reconstruction, which is only a partial solution because of the presence of the noise. Another solution may be to transfer the high-frequency information of the high resolution co-registered images of the MR or the CT into the SPECT image, thus adding further anatomical information to the reconstruction. This technique is still in the research stage.

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