Convection Enhanced Delivery (CED) is a powerful technique to deliver drugs locally for the treatment of degenerative neurological diseases and brain tumors. Testing of CED in large animal models is time consuming and expensive and to alleviate this burden we sought to develop a suitable tissue mimic for the putamen (a target for delivering drugs for the treatment of neurodegenerative diseases)¹. We recognize that the model might be useful in other applications such as phantoms for MR imaging as well.We tested the mechanical and imaging characteristics of agarose gels of different composition. Gels were made from agarose powder of varying strengths {500 g/cm², 950 g/cm² and 1200 g/cm²} by dissolving agarose powder in saline and heating to 90⁰C prior to pouring into receptacles. The strength of the gel is a measure of its resistance to fracturing. Gels with higher gel strength are stiffer. One liter blocks of gel were used to test the distribution of 0.1% safranin O dye when infused using CED through a 15 port Codman catheter. One hundred microliters (Vi) of dye solution was infused at 0.2 μL / min while the infusion pressure and volume were monitored. From photographs of the infusion cloud taken at the completion of the infusion we estimated the volume of distribution (Vd). Samples of gels were imaged to estimate several properties including diffusion coefficient, T1rho, mechanical stiffness and free volume of water. The diffusion coefficient was estimated using a standard double echo Echo Planar-spin echo sequence with b-values of 0 and 1000 s/mm². To measure T1rho we imaged samples with a spin-locking spin echo sequence at multiple spin lock durations². Gel samples were placed on a Resoundant™ transducer and elastographic images were acquired to measure the gel stiffness. To estimate the volume of free water, we used a MOLLI technique to measure T1 in five samples of each gel strength to which we added 100 μL of varying concentrations of Gd-DTPA³. We then calculated the relaxivity 1/T1/[Gd] per unit of concentration of Gd.Figure 1 shows an infusion of 100 μL of 0.1% Safranin O dye into a 500 gm/cm² gel sample. Table 1 lists the measurements performed on the gel samples. From the Vd estimated from photographs of the infusion in each sample we calculated the average ratio Vd:Vi. Measurements of Vd:Vi demonstrate larger ratios for higher gel strengths. The values of Vd:Vi were significantly different between the different gel strengths. There was no obvious trend for the maximum infusion pressure measured during the infusion. From the MOLLI-derived T1 values we estimated the relaxivity of the samples. The measurements of the water diffusion coefficient (ADC), relaxivity (1/T1/[Gd]), and gel density were equivalent for the three gel strengths demonstrating that the volume for free water was equivalent for the three gel strengths. Measurements of the shear modulus from the elastographic images demonstrated an increase commensurate with increasing gel strength. The ratio Vd:Vi for the different gel materials increased with increasing gel strength and the values for different gel strength were significantly different from one another. This response did not arise from a difference in the volume accessible to the water as the gels had identical density and identical relaxivity which could only result from identical free volume. Additionally, the ADC for the three materials was also similar indicating that the pore space was large enough that it did not restrict the free diffusion of water. The variation of Vd:Vi must reflect instead, a difference in the mechanical properties of the gels and indeed, the elastographic images indicates a significant difference in the shear modulus of the materials. We speculate that the difference in Vd:Vi reflects a difference in the resistance of the material to strain from the infusion pressure leading to a different rate of progression of the dye through the gel.These results indicate the ability to select the mechanical and MRI features of a gel to mimic those of tissue. The Vd:Vi ratio observed in these gels is similar to that observed in the CED infusion of neurotrophic factors into the putamen in rhesus monkeys. The difference of Vd:Vi for the different gel strengths appears to arise primarily from differences in the tensile strength of the gels rather than from the differences in available free space which appears to be the same for all gels. These results guide us in the development of suitable tissue mimics to represent the response of tissue in various surgical procedures.