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Teaching Medical Physics
Classroom Activity for 14-16
The Teaching Medical Physics resources are designed for teaching 14-16 science using examples from medical physics. The resources consist of six sets of presentations, worksheets and teacher notes that complement the Institute of Physics 2011 schools lecture “From X-rays to Antimatter: The Science of Seeing Inside your Body”:
Find out how physicists build machines that do what our eyes cannot – see inside the human body.
This inspiring lecture will reveal how:
- over the past hundred years physicists have developed increasingly sophisticated techniques to see inside the body
- these techniques use x-ray, radioactive molecules and magnetic fields to produce images of the body
- these images allow doctors to better diagnose and treat illness and disease
An interview with Michael Wilson:
Pulse oximeters use red and infra-red light to monitor pulse rate and the oxygenation of a patient’s blood. The body scatters and absorbs visible and near infra-red wavelengths significantly so that in order to have a measurable signal thin parts of the body must be used.
A typical pulse oximeter consists of light emitting diodes (LEDs) mounted opposite light sensors in a clip that can be attached across a finger or earlobe.
As the light produced by the LEDs travels through the body it is absorbed by an amount that is dependent on its wavelength and the average number of oxygen atoms attached to each haemoglobin molecule.
The amount absorbed also fluctuates as the arteries expand and contract in response to each heart beat allowing the pulse oximeter to determine the pulse rate as well as blood oxygen saturation from the transmitted signal.
Ultrasound imaging systems uses piezoelectric transducers as source and detector. Piezoelectric crystals vibrate in response to an alternating voltage, and when placed against a patient’s skin and driven at high frequencies produce ultrasound pulses that travel through the body.
As they travel outwards and encounter different layers within the body the ultrasound waves are reflected back towards the source.
The returning signal drives the crystals in reverse and produces an electronic signal that is processed to construct the image. Compared to MRI, ultrasound has the advantages of low cost and portability.
It is also preferred over x-ray imaging for procedures in which ionising radiation poses a significant risk, such as checking foetal development during pre-natal care.
X-ray imaging utilises the ability of high frequency electromagnetic waves to pass through soft parts of the human body largely unimpeded.
For medical applications, x-rays are usually generated in vacuum tubes by bombarding a metal target with high-speed electrons and images produced by passing the resulting radiation through the patient’s body on to a photographic plate or digital recorder to produce a radiograph, or by rotating both source and detector around the patient’s body to produce a “slice” image by computerised tomography (CT). Although CT scans expose the patient to higher doses of ionising radiation the slice images produced make it possible to see the structures of the body in 3D.
Electrocardiograms (ECGs) record the activity of the heart through electrodes placed on the patient’s skin.
Cardiological contraction is caused by changes in electrical potential in the hearts muscle cells; electrical activity that the body conducts to its surface. Although it is altered by the intervening tissue, the resulting signal at the skin accurately reflects the cardiological cycle and can be used to identify any anatomical and physiological anomalies in a completely non-invasive manner.
The gamma camera is an imaging technique used to carry out functional scans of the brain, thyroid, lungs, liver, gallbladder, kidneys and skeleton. Gamma cameras image the radiation from a tracer introduced into the patient’s body.
The most commonly used tracer is technetium-99m, a metastable nuclear isomer chosen for its relatively long half-life of six hours and its ability to be incorporated into a variety of molecules in order to target different systems within the body. As it travels through the body and emits radiation the tracer’s progress is tracked by a crystal that scintillates in response to gamma-rays.
The crystal is mounted in front of an array of light sensors that convert the resulting flash of light into an electrical signal. Gamma cameras differ from X-ray imaging techniques in one very important respect; rather than anatomy and structure, gamma cameras map the function and processes of the body.
Positron emission tomography (PET)
Positron emission tomography (PET) is a gamma imaging technique that uses radiotracers that emit positrons, the antimatter counterparts of electrons.
In PET the gamma rays used for imaging are produced when a positron meets an electron inside the patient’s body, an encounter that annihilates both electron and positron and produces two gamma rays travelling in opposite directions.
By mapping gamma rays that arrive at the same time the PET system is able to produce an image with high spatial resolution.
Another advantage of PET over procedures that employ gamma emitting tracers is the greater availability of suitable isotopes. Positron emitting isotopes of biologically active elements such as fluorine, carbon and oxygen are all available. Fluorine-18 in particular, can be used to make a radioactive analogue of glucose which is preferentially taken up by brain and cancer cells making an ideal tool for detecting tumours. PET can also be used to map brain function and the diagnosis of conditions such as Alzheimer’s disease.