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Medical Imaging
Physics Unit 3…read more

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X-rays are high frequency, short wavelength
electromagnetic waves. The wavelength is roughly the
same size of an atom.
They are transmitted by healthy tissue, but are absorbed
by denser materials like bones and metal.
They affect photographic film in the same way as light,
which means they can be used to take photographs.
X-ray photographs can be used to diagnose many medical
conditions such as bone fractures or dental problems.
X-ray images can be formed electronically using charge-
coupled devices (CCDs). CCDs are silicon chips about the
size of a postage stamp, divided up into a grid of millions
of identical pixels. They detect X-rays and produce
electronic signals which are used to form high resolution
images. This technology is also used in digital cameras.…read more

Slide 3

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CT Scans
Computerised axial tomography (CT) scans use X-rays to produce high resolution
images of soft and hard tissue.
The patient is put into a cylindrical scanner and an X-ray beam is fired through the
body from an X-ray tube and picked up by the detectors on the opposite side.
The X-ray tube and detectors are rotated during the scan
A computer interprets the signals from the detectors to form an image of a two-
dimensional slice through the body. Multiple two-dimensional CT scans can be put
together to form a three dimensional image of the inside of the body.…read more

Slide 4

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X-rays can cause ionisation, high doses will kill living cells. This means they can be used
to treat cancer. However, X-rays must be carefully focused at the right dosage so that
not many normal cells are killed.
Treating Cancer:
X-rays are focused on the tumour using a wide beam.
The beam is rotated round the patient with the tumour at the centre.
This minimises the exposure of normal cells to radiation, and so reduces the chance
of damaging the rest of the body.
Radiographers who work with X-ray machines or CT
scanners need to take precautions to minimise their X-
ray dose.
They wear lead aprons, stand behind a lead screen or
leave the room while scans are being done.
Lead is used to shield areas of the patient's body that
aren't being scanned, and the exposure time to the X-
rays is kept to a minimum.
Lead absorbs X-rays so protects the radiographer.…read more

Slide 5

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Electrical systems can be made which produce electrical oscillations of any frequency.
These can be converted into mechanical vibrations to produce sound waves of a higher
frequency then the upper limit of human hearing (20,000Hz). This is ultrasound.
When a wave passes from one
medium into another some of the
wave is reflected off the boundary and
some is transmitted and refracted.
This is partial reflection.
This means whenever you point a
pulse of ultrasound at an object where
it reaches a boundary some will be
reflected back.
The time it takes for the reflections to reach the detector can be used to measure how
far away the boundary is. This is how ultrasound imaging works.…read more

Slide 6

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Ultrasound in Medicine
Ultrasound has a variety of uses in medicines, from investigating blood flow in organs to
diagnosing heart problems or checking foetal development.
Breaking kidney Stones:
Kidney stones are hard masses that can block the
urinary tract. An ultrasound beam concentrates
high-energy waves at the kidney stone and turns it
into sand-like particles. These particles then pass
out of the body in the urine. The patient doesn't
need surgery and it's relatively painless.
Pre-natal scanning of a Foetus:
Ultrasound waves can pass through the body but
whenever they meet a boundary between two
different media some of the wave is reflected back
and detected. The exact timing and distribution of
these echoes are processed by a computer to
produce a video image of the foetus.…read more

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