Basic nonlinear acoustics: an introduction for radiological physicists.

Presented is a brief introduction to nonlinear acoustics, a topic of increasing importance in modern diagnostic ultrasound. Specifically treated is shock wave and harmonic production in lossless media. We also present a description of linear attenuation mechanisms in soft tissue and finally nonlinear propagation in soft tissue.

Positronium: review of symmetry, conserved quantities and decay for the radiological physicist.

The PET-CT scanner, which overlays a high spatial resolution CT image on a relatively low resolution physiologic PET image, has greatly increased the efficacy of the PET imaging modality. With the resulting increase in the presence of PET imaging in the radiologic and medical physics communities, medical physicists can expect professional responsibilities in acceptance testing, quality assurance, radiation safety and resident teaching of this new modality. With this in mind we present a review of a subtle aspect of PET: the formation, symmetry properties and decay of positronium; the hydrogen like positron-electron complex which forms prior to annihilation.