MRI phantoms - are there alternatives to agar?

Abstract

The suitability of different gelling agents as MRI phantoms was evaluated in terms of homogeneity, gel stability and reproducibility. Time and effort for preparation were also taken into account. The relaxation times of various gel compositions were estimated. Carbomer-980 and Carbopol-974P were determined to be promising novel phantom materials. These gelling agents are readily available, inexpensive and easy to handle given that thermal treatment is not required. Furthermore, the viscoelasticity of their polymer network is pH-dependent. With such characteristics, it was even possible to embed sensitive objects and retrieve them after testing. This was demonstrated with a fiber phantom for Diffusion Weighted MRI applications. Since Carbomer-980 and Carbopol-974P are non-hazardous, they are also suitable for multimodal setups (e.g., MRI as well as ultrasonic imaging).

Figure 1. Test gels.

A sodium alginate gel, B PAC-300 gel, C xanthan gel, D xanthan gel with mold growth after 2 weeks, E Carbomer-980 gel, F Carbomer-980 gel prepared with warm water (37°C, shows less bubble formation and better homogeneity). G Carbopol-974P gel prepared with warm water (37°C, no bubbles visible). H PNC-400 gel prepared with warm water (37°C, bubble formation is still visible).

Figure 2. T₁- and T₂-weighted images.

A+B PNC-400 gel, C+D sodium alginate gel, E+F PAC-300 gel, G+H xanthan gel, I+J Carbopol-974P gel, K+L Carbomer-980 gel.

Figure 3. Estimation of T₁ and T₂ relaxation times from three samples.

A Fits of T₁ relaxation time for PNC-400 phantom mixed with 0.20 ml Magnevist, Carbopol-974P phantom mixed with 0.012 g manganese nitrate and Carbomer-980 phantom mixed with 0.017 g manganese nitrate. B Fits of T₂ relaxation time for PNC-400 phantom mixed with 0.20 ml Magnevist, Carbopol-974P phantom mixed with 0.012 g manganese nitrate and Carbomer-980 phantom mixed with 0.017 g manganese nitrate.

Figure 4. Effect of contrast agents on image homogeneity.

T₂ image of Carbopol-974P phantom (0.5% Carbopol-974P, spin echo sequence with TE = 50 ms and TR = 3000 ms). A Phantom without contrast agent. B Phantom with 0.20 ml Endorem and C phantom with 0.012 g manganese nitrate.

Figure 5. Measurements of the diffusion phantom.

A b0-image of DTI phantom measured in aqueous solution B b0-image of the fiber phantom in Carbomer-980 gel (pH = 13 and concentration = 0.8%). C Color-coded FA map of the same DTI phantom measured in aqueous solution and D embedded in gel. Phase encoding direction was A–P. Colors indicate direction of polyester fibers. They refer to the standard DTI color scheme (red: left- right (x-direction), green: anterior – posterior (y-direction), blue: inferior-superior (z-direction)).