TissueDB/Materials/Silica Dioxide
License: CC-BY-SA-3.0 by KENPEI
Silica dioxide - also written silicon dioxide, and shortened to silica or SiO2 - is a fine white powder stirred into hot agar to make the gel scatter ultrasound. It is the scattering half of the standard three-part agar phantom: silica dioxide supplies the scattering and evaporated milk supplies the absorption, so the two can be dialled independently to hit a target attenuation coefficient.[1] Plain agar with no silica is "homogeneously hypoechoic due to the lack of scattering"; the same gel at 3% w/v silica "appeared with increased echogenicity due to the significant number of scatters".[1] The particles are small compared with the ultrasound wavelength, so they are treated as Rayleigh scatterers and are randomly oriented in the gel, which is why the doped gel shows no directional dependence of attenuation.[1] Sold as: silica dioxide or silicon dioxide powder, laboratory grade. Both characterisations used Sigma-Aldrich (St Louis, Missouri, USA) powder; only the earlier one states a particle size, "between 0.5 and 10 µm".[1][2]
Tissues
| Tissue | Visual | Tactile | Simulator | Notes |
|---|---|---|---|---|
| Brain | 2% w/v agar + 1.2% w/v silica dioxide + 25% v/v evaporated milk - the source's stated brain recipe, giving a total attenuation coefficient of 0.59 ± 0.05 dB/cm-MHz, of which 0.34 ± 0.04 is the scattering the silica contributes and 0.25 ± 0.03 is the absorption the milk contributes.[1] | |||
| Muscle | 2% w/v agar + 2.1% w/v silica dioxide + 40% v/v evaporated milk - the source's stated muscle recipe, 0.99 ± 0.08 dB/cm-MHz total (0.59 ± 0.07 scattering, 0.40 ± 0.04 absorption). Measured against freshly excised porcine muscle under the same 1.14 MHz sonication, "the produced temperatures in the muscle phantom and in real muscle are very close".[1] Note: the paper's abstract prints the muscle silica figure as 2% while its Results, Table 2 and Conclusions all print 2.1%; 2.1% is used here because three places carry it against the abstract's one. A second characterisation reaches muscle from a different direction and without milk. Its stated optimum, 6% w/v agar + 4% w/v silicon dioxide, measured 1.10 ± 0.09 dB/cm-MHz and was chosen as "the recipe that was found ... to possess an attenuation coefficient close to that of human muscle"; its table for matching freshly excised rabbit muscle (1.18 ± 0.46 dB/cm-MHz) gives a band of 2-4% w/v silicon dioxide in 6% agar.[2] | |||
| Liver | 6% w/v agar + 0-4% w/v silicon dioxide, no evaporated milk - the recipe band the source gives for matching freshly excised rabbit liver, whose attenuation coefficient it measured at 0.86 ± 0.20 dB/cm-MHz at 1.1 MHz. The milk-free recipes were preferred "due to their increased durability".[2] | |||
| Kidney | 6% w/v agar + 2-4% w/v silicon dioxide, no evaporated milk - the source's recipe band for freshly excised rabbit kidney, measured at 1.46 ± 0.44 dB/cm-MHz at 1.1 MHz.[2] | |||
| Tumor (and the healthy tissue around it) | Silica dioxide does two different jobs in one phantom here. 4% w/v silicon dioxide in 6% w/v agar is the opaque block standing in for the healthy tissue surrounding the tumour; 1.1% w/v silicon dioxide goes into the transparent polyacrylamide sphere that is the tumour itself, where it acts as an MR contrast agent. The source attributes "the excellent contrast achieved between the TMPs and the TUMPs" on T1- and T2-weighted images to "the lowered MR relaxation times" produced by the silicon dioxide and glycerol in the sphere.[3] |
Simulation Requirements
Visual. Under ultrasound this is the whole point of the additive. A silica-free agar gel reads as homogeneously hypoechoic; adding silica raises echogenicity, and both characterisations imaged their doped gels on a clinical scanner to confirm it - 3% w/v silica in 2% agar on a Philips HD7,[1] and 4% w/v silicon dioxide in 6% agar on a Mindray UMT-150, where the sample "appeared with increased echogenicity due to the ability of silicon dioxide to scatter ultrasound waves".[2] Out of the scanner the powder simply tints the gel opaque white; it carries no colour or surface detail of its own.
Tactile. Not characterised. Both characterisations measure acoustic and thermal properties, and neither reports a stiffness, modulus or handling figure for the silica fraction. The one handling limit either of them records belongs to the other additive: above 30% v/v evaporated milk the gel "lost its stiffness" and "became too difficult to handle".[2]
Troubleshooting
More silica stops helping above 4% w/v, and then reverses. Up to 4% the attenuation coefficient rises by 0.101 dB/cm-MHz for every 1% of silicon dioxide added (R2 = 0.995). Above 4% it falls: the measured coefficient of a 6% agar gel is 1.10 ± 0.09 dB/cm-MHz at 4% silicon dioxide but 1.01 ± 0.10 at 6%. The source's explanation is that the extra scattering "further lowered the ultrasonic absorption at a significant level", and absorption is the larger term. Do not treat the additive as a dial that only goes up.[2]
Silica alone will not heat a phantom. In a gel of 2% agar and 2.1% silica with no evaporated milk, sonicated at 20 W for 30 s, "the produced temperature is very low (attributed mostly to agar)". The same gel with milk and no silica heats "drastically". For a thermal or focused-ultrasound phantom the absorber is what makes the temperature; silica sets what the image looks like.[1]
Stir gently. The powder has to be mixed into hot gel without whipping air into it - "care was taken by stirring the solution gently to avoid the creation of air bubbles that are known to reflect ultrasound waves". An air bubble is a far stronger reflector than the particle you added on purpose.[2]
The gel it goes into is the perishable part. The nine characterised phantoms were measured within 24 hours of being cast "to avoid decomposition by microbial growth"; no preservative was added, and the source notes that one would be needed to hold the properties over time.[2]
Alternatives
| Alternative | Best For | Trade-offs |
|---|---|---|
| Evaporated milk | Raising attenuation by ABSORPTION rather than scattering - the half of the recipe that makes a phantom heat realistically under focused ultrasound. | About 0.01 dB/cm-MHz per percent added, against silica's 0.28 ± 0.03, so it is a far weaker lever per unit; and above 30% v/v the gel gets too soft to handle. It is less an alternative than the other half - every published recipe here uses both.[1][2] |
| Talc / talcum powder | A scattering agent in gelatin and magnesium-silicate recipes, where it is added at 2.4% to 13% by weight. | A 2026 characterisation of 13 candidate phantom materials uses talc as its scattering agent in three recipes - agar/glycerol/talc at 2.4%, agar/n-propanol/talc at 13%, and magnesium silicate with "3% talc as a scattering agent" - and reports that talc "influences attenuation properties, but to a lesser extent" than the graphite powder another group used in its place. It names aluminium oxide and silicon carbide as the traditional scattering agents it replaced. It never measures talc against silica dioxide, and the two are not compared in the same matrix.[4] |
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Menikou G, Damianou C. Acoustic and thermal characterization of agar based phantoms used for evaluating focused ultrasound exposures. Journal of Therapeutic Ultrasound 2017;5:14. DOI: 10.1186/s40349-017-0093-z. PMID 28572977. PMC PMC5452295.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Drakos T, Antoniou A, Evripidou N, Alecou T, Giannakou M, Menikou G, Constantinides G, Damianou C. Ultrasonic attenuation of an agar, silicon dioxide, and evaporated milk gel phantom. Journal of Medical Ultrasound 2021;29(4):239-249. DOI: 10.4103/JMU.JMU_145_20. PMID 35127403. PMC PMC8772477.
- ↑ 3.0 3.1 Sofokleous P, Damianou C. High-quality agar and polyacrylamide tumor-mimicking phantom models for magnetic resonance-guided focused ultrasound applications. Journal of Medical Ultrasound 2023;32(2):121-133. DOI: 10.4103/jmu.jmu_68_23. PMID 38882616. PMC PMC11175378.
- ↑ Mencarelli M, Puggelli L, Virga A, Furferi R, Volpe Y. Characterization of soft tissue-mimicking materials for ultrasound training phantoms. Journal of Materials Science: Materials in Medicine 2026;37(1). DOI: 10.1007/s10856-026-07020-7. PMID 41760986. PMC PMC12960330.