X Ray REVISION

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  • Created on: 04-05-13 15:22
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X-rays
· Describe the use of X-rays in imaging internal body structures
X-rays belong to the short-wavelength, high frequency end of the electromagnetic
spectrum. They have wavelengths in the range of 10-8m to 10-13m. They are produced when
fast-moving electrons are rapidly decelerated; their kinetic energy is transformed to photons
of electromagnetic radiation as the electrons slow down.
When using X-rays in imaging, a beam is produced via an X-ray tube and passes through the
body structure. The X-Ray tube contains two electrodes: a cathode from which electrons are
emitted and an anode, which is made of hard metal such as tungsten. An external power
supply produces a high voltage between the two electrodes which accelerates a beam of
electrons between the gap. An incoming electron may collide with an atom on the anode
which removes an electron and provides a vacancy in one of the atom's electron shells. This
may be filled by another electron which simultaneously releases an x-ray photon of a specific
energy. X-rays are emitted when electrons hit the anode providing they have enough energy.
The x-ray radiation passes through the body structure; as it does so, the atoms and
molecules of the biological material are ionised. As this happens, the X-rays transfer some of
their energy to the material thus the beam of X-rays is gradually absorbed as it passes
through the material. Different materials within the body structure have different densities
and thus they will absorb the radiation at varying levels, producing a contrast so that the
different components of the internal structure are distinguishable from each other; this is
attenuation of the X-ray beam. A clinical image is produced by the electronic detectors or
photographic film capturing the attenuated X-rays on the other side of the body.
· Describe the use of contrast media and how soft tissue can be imaged using an
X-ray

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Contrast media are used to distinguish between different soft tissues that absorb X-rays
equally and would otherwise show up the same on an image. They are elements with high
values of the atomic number; they have a high number of electrons in their atoms which
interact with the x-rays resulting in a higher level of absorption. Iodine and barium are both
examples of contrast media as they are efficient at absorbing x-rays.…read more

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Computerised axial tomography consists of an X-ray tube which rotates round a patient
whilst the detectors remain stationary. The CAT scanner is operated when the patient lies in
a vertical ring of X-ray detectors. The x-ray tube rotates around the ring which exposes the
patient to a beam of X-rays from all directions. The detectors opposite the tube send out
electronic records which are received by a computer which then proceeds in constructing a
three-dimensional image of the patient.…read more

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