s19Ultrasound

Abstract:

In this experiment a 5MHz transducer that has a wavelength of 0.3mm in water was used to create ultrasound images. It was determined that the transducer has a focal point 2.5±0.1 inches away and that the greatest image resolution occurs at in this region.

Introduction:

One of the most critical aspects in medicine is to be able to non-invasively diagnose a patient. There are multiple techniques used to do this; one of the most common being a CT scan. CT scans use an x-ray beam that rotates around a patient to generate images with greater detail than traditional x-ray scans. While this is an extremely useful technique, there are some downsides that make it less than ideal in certain instances. These machines are very expensive, limiting availability in small rural hospitals. CT scans also use x-rays which have a high enough energy to ionize atoms and molecules and these ionized molecules are known to be carcinogenic$\cite{rad}$. It is for these two reasons that we need a different diagnostic technique, the ultrasound. Ultrasounds are low cost and pose no known health risks in a clinical setting$\cite{safe}$. Because of these traits, we studied how an ultrasound works in this experiment.

Theory:

Ultrasound imaging works by creating an ultrasonic wave that propagates through a medium. When this wave interacts with an object that has a different impedance, a fraction of it gets reflected back. The amplitude of the reflected wave is then measured and converted into 8-bit grayscale values to create an image. To create an ultrasonic wave a piezoelectric is used. A piezoelectric is a crystal that converts mechanical compression into electric potential or vice versa.

By supplying a voltage to the piezoelectric the crystal will contract and expand at whatever rate the voltage is switching.

This alternating voltage is applied for a set amount of time and it's in this time-frame that the ultrasound is created from the expanding and contracting piezoelectric. In order to detect an ultrasound, the original ultrasonic wave must be reflected off an object and come back to a receiver, typically the same piezoelectric used to create the wave. It's known that the fraction of the original wave reflected, R, is dependent on the acoustic impedance of the original medium through which the wave was propagating Z₁ and the impedance of the object being imaged Z₂ given by

From eq(1) it's clear to see that if the impedance of the medium is close to the impedance of the object being imaged only a small fraction of the original wave will be reflected.

This is helpful for imaging something inside an object because by matching the impedance of the medium to the impedance of an object, say a fruit, the fruit part of the image will be ignored and the only things that show up will be the seeds.

The impedance of a material is given by

where ρ is the mass density and c is the speed of sound in the material.

Once a wave is reflected it travels back to the transducer that houses the piezoelectric. If the speed of sound $c$ is known for the medium and we measure the time Δt it takes for the echo to return to the transducers, the distance to the object d is calculated by

Experimental Set Up:

The experimental apparatus consisted of a piezoelectric that acted as sender and receiver for our signals. The piezoelectric sent a voltage spike to our single element transducer, causing it to emit a sound wave at its natural frequency of 5MHz. An object to be studied was placed in a container fill with water and the transducer was placed vertically above it via optical mounting. Because the piezoelectric is both a sender and receiver, it generated the initial wave and read the reflections off the object under study. The signal was read by an oscilloscope and sent to a computer via a driver. An oscilloscope was used because in order to prevent aliasing the data must be sampled at a minimum of 10MHz. A signal reading was taken then a stepper motor was used to move the transducer horizontally over the top of the object. Then a new signal reading was taken at the new location. This process was repeated until an entire cross section of the object had been taken. The stepper motor is interfaced with the computer via a DAQ. The entire data collection process is controlled by a labview program.

• Here is a block diagram of the experimental apparatus

This is an image of the transducer on optical mounting connected to the stepper motor

Here is an image of the picopulser. Port TX goes to transducer, Trigger goes to trigger channel on oscilloscope, RF goes to main channel on scope. The pulser is set to single mode with internal triggering. It was found that the best signal was observed with the following settings on the picopulser: Gain - 60dB, Probe Frequency - 5, Pulse Voltage - 210V.

Here is are images showing the motor control with wires out and connecting to the corresponding pins of the DAQ

Results:

Two wires were set with a separation of 0.3mm. The focal point of the transducer was given to be 2.5in. An image was taken with the focal point aimed directly at the wires. The two separate wires were resolvable. This image is shown below:

The transducer was then moved so that the focal point was 0.1 inches above the wires. The two wires were still resolvable. This image is shown below:

The transducer was moved up again so that the focal point was 0.2 inches above the wires. The two separate wires were longer resolvable. This image is shown below:

Theses images shown that we found we had a vertical resolution of 0.3mm at the focal point. The focal point was found to be 2.5 +/- 0.1 inches below the transducer.

Additionally an image of an apple was taken. The apple was mounted to the bottom of the container with a screw to keep it from floating. The top of the screw is visible in the image. Additionally, a small grey line is visible at the bottom of the image, this was confirmed to be a worm by cutting the apple open.