Comparison of Transcranial, Scalp-to-Brain, Layered Phantoms

Ultrasound in the brain suffers from the major drawback of acoustic deterioration through the skull. Treatment or diagnosis is heavily dependent on skull thickness and diploe scattering. Our objective is, therefore, to examine ultrasound signals in laboratory controlled experiments using transcranial phantoms. A comparison is presented of scalp-to-brain, three layer phantoms. Soft tissue phantoms are usually made from gelatine/agarose often with a variety of additional substances to act as scatterers. Bone phantom materials are typically comprised of: calcium, plaster and coral. However, plaster can form impenetrable boundaries, and coral, while it has similar cavities to trabecular bone, these cavities do not completely fill with a gelatine based mixture. Air pockets can develop in both plaster and coral boundaries leaving impenetrable air cavities. Transcranial layers were mimicked using gelatine with scattering additives for the soft tissue, scalp and brain, and calcium based mixtures for skull bone. Different consistencies were investigated and two combinations were found to give acceptable tissue-equivalent layers. In both cases, gelatine (with scattering additives) was poured into a container to mimic the brain. When this had thickened, a calcium-gelatine layer was added to acoustically resemble the trabecular bone. Then, a highly dense calcium-water (first instance) or gelatine-calcium-water (second instance) layer was thinly coated on top to form the cortical skull surface. Finally, a scalp equivalent soft tissue layer was added using a gelatine (with scattering additives) mixture. Satisfactory transcranial bone phantoms were developed with similar acoustical properties to human tissue, confirmed with a commercial ultrasound scanner.