Introduction: Tutorial for Silicon Phantom Simulating an Arm With Vessels for Ultrasound Research.

In this tutorial a way to make silicon phantom simulating an arm comprising two layers: muscle and fat and some vessels.
The purpose of this phantom is for research in medical imaging (ultrasound or other modalities). The described produce a silicon phantom with similar stiffness properties to a real arm.

Image phantom, or simply phantom, are designed objects that are scanned or imaged in the field of medical imaging to evaluate, analyze, and tune the performance of various imaging devices (see Fig.1).

The reason why we do this is because in this way it is possible to have an artificial object which can be treated in a such a ways that could hurt a real person. Dead body or animals could be used, but experiments with them requires long clearance process, are expensive and also rise several ethical questions. This is the reason to use at least for the initial experiments artificial phantom simulating part of the body.

[1] “An Easily Made, Low-Cost, Tissue-Like Ultrasound Phantom Material.” R.0.Bude,MD, and R.S.Adler,MD, PhD, Clin Ultrasound 23:271-273, May 1995
[2] “Addition of Metamucil to Gelatin for a Realistic Breast Biopsy Phantom” Helen Morehouse, MD, Harshada Pranav Thaker, MD, Chandowti Persaud, RT J. Ultrasound and medicine August 1, 2007; 26 (8)
[3] A Wave Equation Approach to Ultrasound Elastography Ali Baghani, PhD. Thesis, University of British Columbia.

Step 1: A Commercial Phantom

Fig. 2 depicts a phantom simulating a baby in the womb.

Step 2: Gelatine Phantom

An easy way to do a phantom is using common agar or gelatin as in Fig.3:
Gelatin phantoms require gelatin and other ingredients that can be found in a local supermarket. They are not corrosive, cancerous, or expensive. There are several tutorial which suggest also the addiction of other organic material such as olives and rigatoni [1], or Metamucil to simulate breast tissue [2]. The approach is quite simple, you use the right ration of gelatin and hot water to simulate the level of stiffness of the tissue. A reader can refer to [1] for a simple and effective procedure. Fig.4. shows the ultrasound scan of a gelatin phantom.

Step 3: Materials

To build a more resistant phantom the following material is necessary:
• A silicon compound.
• A silicon thinner.
• Some cellulose.

Silicon compound are generally sold as A+B, two different bottles that when mixed produce the silicon.
Smoothon is worldwide brand which sells a product called Ecoflex suitable for this
purpose. Ecoflex is sold in different level of hardness. To simulate the human tissue a
reasonable choice is Ecoflex 30 (Fig.5).

Step 4: Silicon Thinner

A Silicon thinner is an expensive solvent which can be bought also through the
Smoothon network (see Fig.6).

Step 5:

The last ingredients is the cellulose. It is necessary since the silicon itself is almost
transparent to the ultrasound. Cellulose introduces speckles which are visible
through the ultrasound. In [3] is suggested that the Sigma cell Type 50 should be used
(see Fig.7) , however performed experiments with different cellulose did not
show any difference.

Step 6:

To produce the two kinds of tissues different ratios of Ecoflex and Silicon thinner must be used. The reported ratios produce silicon phantom with similar stiffness properties to a real arm.
For the muscle, smaller amount of thinner is used:
Compound A, compound B and thinner in a ratio 3:3:2. (e.g. 30g compound A, 30g compound B, Thinner 20g).
For the fat layer, more thinner is used:
Compound A, compound B and thinner in a ratio 2:2:3. (e.g. 20g compound A, 20g compound B, Thinner 30g).
The amount of cellulose required to allow the ultrasound probe to see something is 1% of the total amount. E.g. 0.7g and 0.8g respectively for the aforementioned example of the muscle and fat.
To simulate the vessel, balloon can be used but a penrose latex drain is more suitable since it is thinner and therefore softer than the balloon skin.
The casting procedure to produce each layer requires 4 hours. Therefore, the procedure for the entire arm structure requires 8 hours.
First spray some slide spray on the surface where the cast will be performed (olive oil is also ok), this allow to remove the cast in an easier manner as in Fig.8.

Step 7: Stirr and Pour the Muscle Layer

Then weight and stir in a separate bowl the ingredients for the muscle tissue for about 5 minutes. Pour this first mixture in the cast-bowl. Wait 4 hours. Fig.9 show the result after 4 hours.

Step 8: The Vessel

Put the balloon/latex drain in the cast-bowl, eventually fixing it with mini wrench or glue (see Fig.10).

Step 9: The Fat Layer

Prepare the second mixture for the fat layer and pour it on top of the already prepared (Fig. 11 and 12).
Skin can be neglected or simulated by a thing gauze, bones can be simulated by woods, and therefore the fable itself can be used as bone.

Step 10: The Overall Setup

We have produced a tissue-like ultrasound phantom using a silicon compound, a silicon thinner, a penrose latex drain and cellulose. The echo texture of this material simulates the fat and bone tissue which are comprised in an arm and the latex drain simulates the vessel which pump blood. Fig.13 shows the overall setup to simulate an arm and Fig.14 shows what an ultrasound machine can see.

Step 11: Let's See What the Ultrasound Can See

Let's see what the ultrasound can see

Step 12: Safety

Silicon compound are not considered hazardous, but it is corrosive and it could cause some problem at some airport screenings. Though it is not seriously corrosive, wear chemically protective gloves to prevent prolonged or repeated skin contact. In case of inhalation, remove source(s) of contamination and move victim to fresh air. In case of eye contact, flush eyes with plenty of water. In case of skin contact, wash thoroughly with soap and water; remove contaminated clothing and launder before reuse. Never eat, drink, or smoke in work areas. Practice good personal hygiene after using this material, especially before eating, drinking, smoking, using the toilet, or applying cosmetics.

Moreover, do the final shape using the cast-bowl , I have personally noticed that it is almost impossible to remodel afterwards the silicon, even cutting with surgical lancet or scalpel. See Fig.15