Spatial Measurement of Tumor Interstitial Fluid Pressure: A Method to Measure the Interstitial Fluid Pressure

Interstitial Fluid Pressure or IFP is the force exerted by the fluid in the spaces between the cells in a tissue. First, anesthetize a tumor-bearing mouse with 2% isofluorane mixed with oxygen and immobilize it on the computed tomography or CT-IFP robot platform. Now, inject in the lateral tail vein of a mouse with liposomes encapsulated with a contrast agent to visualize the tumor. Return the animal to its cage and let it recover.

Between 48 to 72 hours post-injection, secure the anesthetized mouse on the CT-IFP platform and select a few evenly spaced positions on the tumor for measuring the IFP. Next, flush the IFP system needle with a heparin saline solution before insertion. Bring the needle on the selected position and insert it within the tumor tissue.

Now acquire images at all positions and measure the IFP using the software. The IFP in the tumor is usually high due to its dense nature, abnormal blood vessels, and defective lymphatic system. This may lead to the accumulation of liposomes in the tumor periphery. In the following protocol, we will demonstrate the technique to measure the IFP in a solid tumor.

To measure the IFP, tape the animal on the CT-IFP robot platform, such that the tumor is immobilized and accessible to the CT-IFP robot system. Obtain an anatomic micro CT scan as just demonstrated, then load the preneedle insertion data into the CT-IFP robot alignment software and adjust the window and level to visualize the tumor. Click on the rim of the tumor in any image, followed by the selection of a second rim location on the adjacent side of the tumor.

The software will calculate a series of positions along a linear line between the two points. Then select the x, y, and z-coordinates for a series of five to eight evenly spaced positions from the list. Next flush the IFP system needle with a saline heparin solution and enter the first predetermined needle positions into the XYZ coordinate windows of the CT-IFP robot control software.

Press the Go button to move the robot to the desired location. Then for each needle position in turn, click the Insert Needle button to insert the needle into the tissue. Pinch and release the PE-20 tubing to confirm a good fluid communication between the IFP needle and the tissue and observe that the IFP measurement increases and returns to the pre-pinching value on the IFP acquisition software. Finally, acquire an anatomic CT scan with the needle inserted, clicking the Retract Needle button at the end of the scan to remove the needle from the tissue.