LED-Based Photo-Acoustic Computed Tomography

           Photo-acoustic imaging (PAI) is a non-invasive, high-resolution hybrid imaging modality that combines optical excitation and ultrasound detection. PAI can image endogenous chromophores (melanin, hemoglobin, etc.) and exogenous contrast agents in different medical applications. It offers the contrast and spectral behavior of optical imaging combined with the high penetration depth and spatial resolution of acoustic imaging due to weak ultrasound scattering in biological tissue. In PAI, tissue is illuminated by a nanosecond laser pulse. Endogenous or exogenous molecules will absorb this incident energy to produce a fast local temperature increase; acoustic waves then propagate inside the tissue due to thermal expansion. Wideband ultrasound transducers can detect photo-acoustic signal via trans-abdominal or infra-vascular settings, and the combination of PAI with ultrasound can offer functional details as well as anatomical and structural information. This high-resolution modality has been used to image endogenous chromophores (melanin, hemoglobin, etc.) and exogenous contrast agents in different medical applications.

Traditionally, researchers and scientists use Laser as the heat source to stimulate the sample in order to create a propagation of ultrasound waves, after which a ring transducer all around the sample is used to receive the signal and create a 360 degree tomography of the sample. Our project sought to provide a cheaper and more portable device of PAI compared to the traditional set-up where we replaced the Laser with an array of LED for the lighting mechanism and reduced the ring transducers into one single linear transducer. To create a similar tomography, we designed a rotational mechanism to the device so that the linear transducer can receive signals from any positions desired. Our design aims to provide a potentially better imaging tool for the researchers where it is more affordable and user-friendly while producing quality tomography just as capable as the ones in the current market.

Project Objectives:

        Our project aims to use a simple LED light source, instead of laser, to excite the object with diffused light, causing slight thermal expansion within the object which induces a propagation of ultrasound waves. Then a transducer is used to detect the waves from which a topography is computed. Right now their research needs a device to rotate the transducer all around the tested sample to experiment whether getting a thorough 360 degree scan will increase the resolution of the device. Our team’s main goal is to build a device that can rotate the transducer around the sample and at the same time support the LED array on top of the sample.

Final Design: 

Key Component: Water Tank        

        A water tank that is able to rotate with a gear plate and a sample plate, holding ultrasound transducer during imaging process.

       The rotating water tank has one crucial component which is the contacting ring bearing which is press-fitted onto the tank. The friction between the bearing and the support table solely determined the difficulty for our drive train to rotate the system. Therefore, thorough researches were made to finally decide on using Delrin as the ring material. The first iteration on the design failed because of the inaccurate measurements made on the tank perimeter resulting in a failure of the press-fit. The second iteration, however, was successful where the final friction between the tank and the table was minimal.

Key Component: Transducer Gate

        A system of drive train includes a stepper motor, gears and timing belt which rotates the water tank during imaging process.

        The final version of the drive train satisfies all the four functional requirements. The 24 V power supply integrated with the stepper motor can precisely control the motion of water tank. A gear cover which is made of PLA can prevent users’ fingers from hurting. The electronic enclosure at the bottom works as a motor mount which could fix both the power supply and the stepper motor. The electronic enclosure is made of fiberglass which has good quality of heat and electrical insulation.

Key Component: Gantry

        A gantry to combine all components together as a system, which hold and adjust LED device in certain vertical position, as well as combining all other components into a complete system.

        The combination of gripper and PVC rods is the final design decision since they are easy to manufacture and adjust. Two  PVC rods with large cross section area was used as the vertical supports since they can be fixed onto the base board. The gripper was connected to a horizontal thin steel rod through a joint so that the horizontal and vertical position of LED can be adjusted. This LED gantry can be fabricated by drilling a hole on PVC rod(vertical support) and pressing fit the steel rod through this hole. Then fix the vertical rods onto the base board by using the screw.