Hi. My name's Dr. Jill Clarke. I'm a Senior Lecturer in Medical Radiation Sciences at the University of Sydney. I'm also a qualified, accredited, and experienced sonographer in general cardiac and vascular ultrasound. I'm here in the ultrasound lab at the Cumberland campus of the University of Sydney, where we use a clinical quality ultrasound machine to teach basic sonography to our radiography students, physics students and research students from a variety of disciplines who wish to use ultrasound in an informed way in their research projects. Today, I'm going to talk to you about this very popular non-ionizing imaging modality. An ultrasound works differently to the other medical imaging techniques you've learned about, as it does not use ionizing radiation. Instead, it creates an image of the inside of the body using high frequency sound waves. These sound waves are of much higher frequency than those we can hear, thus the name "ultrasound". The normal range of human hearing is about 20 hertz to 20 kilohertz. Ultrasound used for medical diagnosis uses waves between about one and 20 megahertz. For example, this probe is a one to five megahertz probe, which is useful for scanning the abdomen and pregnancies. This is a 1-5 megahertz cardiac probe with a smaller space to scan through the ribs to the heart. This is our highest frequency probe, a 5-12 megahertz probe, good for scanning superficial structures such as the thyroid. So let's discuss how ultrasound scans work. Ultrasound uses a small handheld probe which are called an ultrasound transducer, or just an ultrasound probe. This transducer emits high-frequency sound pulses into the body, and acts both as transmitter and receiver. It's a form of echolocation, using mechanical sound waves. Not electromagnetic waves, just like sonar. Sound waves need a medium to travel through, and the gel we use seals the gap between the skin and the transducer. Each probe contains multiple piezoelectric crystals, or ceramics which vibrate in response to electricity producing the sound waves. The reverse happens to the crystals when the probe receives the sound back again. So the crystals create an electric current, and it's this signal that gets turned into an image. In medical imaging, ultrasound is used to image internal tissues and organs. Each time the sound waves hit a boundary between different types of tissue, or say between tissue and bone, some of the waves are reflected back to the transducer. This in turn sends a signal to the machine. The machine then calculates the distance of the boundary from the transducer using the time the signal arrived, combined with the speed of sound in soft tissue. Using this information, we construct the image. Ultrasound is particularly good for imaging soft tissues and is typically used to examine organs, tendons and ligaments, and lumps and bumps. They are also used for monitoring blood flow. Here, we see my radial artery, that long black tube on the screen. Now I can show you the blood flow using a mode called color Doppler. Now we can hear the blood flow using a mode called pulse Doppler. Like ultrasound, a CT could also show this anatomy. However, a general X-ray would not be able to detect this with great clarity. Both X-ray and CT use ionizing radiation while ultrasound does not. Since it's not based on ionizing radiation, ultrasound is particularly safe as a diagnostic tool in trained hands. So it's often used in pregnancy, to monitor the development of the fetus and screen for any complications. Let's talk briefly about the history behind ultrasound. Ultrasound was first used in medicine by Dr. Karl Theodore Dussik in Austria in 1942. He used it to investigate the brain. Pioneering clinical work was carried out in the mid 1950s in Glasgow. The obstetrician Ian Donald teamed with engineer Tom Brown to develop a prototype system. The development of diagnostic ultrasound in Australia commenced in 1959 with a research project to build an ultrasound instrument at the Commonwealth acoustic laboratories which then became the Ultrasonic Institute which built the pioneering UI Octasun, a water bath scanner, nowadays in the Powerhouse Museum. From the 1970s onwards, the use of ultrasound in hospitals became widespread. We now have the technology and computing power to generate amazing 3D images like these 3D baby faces. Ultrasound is very safe. In its short history of medical use, no independently confirmed adverse effects have been caused by exposure from contemporary diagnostic ultrasound machines in humans. The World Federation of Ultrasound in Medicine and Biology states that ultrasound is safe when used prudently in the hands of trained health professionals on properly maintained equipment. Biological effects such as heating do occur. So ultrasound should be used only by a qualified health professional to provide medical benefit to patients. Ultrasound exposures during examinations should be as low as reasonably achievable. That is, using the ALARA principle. In Australia, check your sonographer is an accredited medical sonographer. Undergoing an ultrasound is usually non-invasive and painless. There are rarely any effects from the procedure. But ultrasound does have a heating effect and mechanical effects from the waves traveling through the tissues. The heating effect can lead to an increase in temperature of body tissue. Nonetheless, under normal circumstances blood flow through the tissue removes this heat and cools somebody down. There has been some concern over whether or not this heating effect is enough to damage developing fetuses. Various studies have been carried out on this. For example, laboratory experiments have shown that thermal effects from high-powered ultrasound can damage tissue in the spinal cord of a mouse fetus. So it's technically possible that such a temperature increase could occur with the newest diagnostic ultrasound devices that use Doppler ultrasound. Care must be taken if these devices are used. Safety indices are usually displayed on the ultrasound screen. These act as a guide as to how much heating or mechanical pressure is taking place. The sonographer will keep these to a minimum, while still obtaining a good image. All in all, ultrasound is very safe. There is no evidence for any causal relationship between ultrasound scans and long-term adverse effects on fetuses.
