Phantoms and Cadavers

Unlike people, phantoms designed for medical imaging can remain perfectly still and can be displaced and sometimes even rotated with considerable accuracy. For imaging modalities with low resolution, such as PET, phantoms can produce images that are indistinguishable from real images by casual inspection [12]. Aside from biological factors (e.g., violations of rigid-body constraints or absence of realistic high-resolution partial volume effects), images of phantoms will reproduce many factors that may limit registration accuracy. These include some (though in some modalities, not all) of the spatial distortions produced by the imaging equipment, and the noise introduced by the imaging equipment and by image reconstruction. Phantoms are most effective for estimating the accuracy of intramodality registration. For intermodality registration involving different scanners, phantoms suffer from the same difficulties as real images, since it is usually not possible to know the precise absolute position of the phantom. For intermodality registration obtained with the same scanner (e.g., two different MRI pulse sequences), only the relative position of the phantom is required. However, this case suffers from the limitation that the phantom must mimic reality in two different imaging contexts where different physical properties account for the observed tissue contrasts. In this situation, cadavers may provide an effective alternative so long as the imaging modality does not depend on physiologic processes.

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