Ultrasound is the use of high frequency sound waves to produce pictures of tissues and organs within the human body - including unborn babies and the heart. It is also used to treat patients with muscular sprains and swelling, kidney stones, and some types of cancer. Meanwhile non-ionising radiation, which includes electro-magnetic radiations such as visible light, infrared (heat), radio waves, and near ultra-violet, can also be used to both treat and image patients. All of these techniques have the advantage that their energy levels are not sufficiently high to cause ionisation of atoms within tissues. However, the principle of reducing any exposure to as low as reasonably practicable is just as applicable as it is in ionising radiation.
Who needs ultrasound or non-ionising radiation?
Most pregnant women will have an ultrasound scan, to check on the progress of their unborn child. Patients with blood flow difficulties can be scanned with ultrasound as it reveals narrowing or blocking of blood vessels.
Heart disease can also be detected with ultrasound, using a technique known as echocardiography in which the various parts of the heart are scanned and watched on a monitor as they are working. In addition, patients with a wide range of other problems including gall stones, tumours, and cysts filled with fluid can be sent for an ultrasound scan. As a treatment, many of us will have had debris removed from our teeth by ultrasonic dental instruments that use the sound waves they generate to chip away at the deposits. Higher power ultrasound can be used in a similar way to pulverize kidney and gall stones.
Infrared can be used to relieve pain and improve circulation for physiotherapy patients, while light can be used to image inside the body. For example stroke victims and premature babies can be scanned using light-based techniques including diffuse optical tomography that can monitor blood volume, oxygen content and flow. Another technique based on light, Optical Coherence Tomography, is used to diagnose retinal disease.
How do these techniques work?
In an ultrasound scan, a handheld sensor that produces ultrasound waves is moved over the skin above the part of the body that needs to be looked at. The ultrasound waves that are bounced back by the different vessels and organs within the body that they hit are detected by the sensor, and turned into an image by computer. Optical Coherence Tomography (OCT) is the optical equivalent of ultrasound, and uses a beam of infrared shone from a laser instead of sound waves.
In diffuse optical tomography, boundaries between different internal structures including cell walls reflect and scatter the infrared light of different wavelengths that is shone at them. Different amounts of oxygen in tissues absorb particular wavelengths of light, so the reflected light is processed by computer to produce a 3D image that reveals areas starved of oxygen - for example during a stroke - and with too much oxygen - as with a growing cancerous tumour.
High intensity focussed ultrasound (HIFU) is being developed for use in the treatment of cancer, and is currently undergoing clinical trials in the UK. In the future, it may be used to kill cancer cells in single tumours or parts of larger tumours in prostrate, kidney, liver, and bladder cancer. There are also prototype optical mammography systems undergoing testing, and another light based technique known as photoacoustic imaging - which can reveal signs of disease via unusual patterns in blood oxygen content and flow - is in the early stages of clinical trials. OCT may become routinely used in many areas of medicine within the next few years, and has been proven to detect the difference between cancerous and normal tissue in the gut and skin.
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