AQA A2 Physics - Inside the Body Revision Notes

Notes for AQA A2 physics, spec b: physics in context. Covers pretty much everything needed in the chapter i think, pretty much straight for the textbook - x-rays, ultrasound, endoscopy etc

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  • Created by: tom
  • Created on: 27-04-11 22:21
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Inside the Body
Lower energy x-rays are absorbed readily by the body, which makes them harmful as
they could ionise atoms in the body. A filter is used to remove these. X-Rays go
through the body and fall on a photographic plate. This forms an image.
Intensity passing through the body depends on
Thickness of the tissue and bones
Density of the tissue and bones
X-rays are not diffracted or scattered significantly as they travel through the body,
due to their short wavelength. Any scattering that does occur will blur the image,
therefore the contrast of the image is improved by using a lead grid. This absorbs the
scattered x-rays, meaning only x-rays travelling in a straight line will reach the
photographic plate
The density of bone is much greater than the surrounding tissue, so clear images
with greater contrast are formed when looking at the bone. Where there is a small
difference in density between the organs it can be harder to get a clearer image. In
this case, a contrast medium is used. In the stomach, a Barium Meal is taken. The
barium sulphate absorbs x-rays, therefore improving the image. In the blood vessels,
a solution of iodine is introduced to the blood stream to improve the contrast
between the blood and the surrounding tissue. Image intensifiers are also used to
improve the image by magnifying the intensity of the output image. This means
lower doses of x-rays can be used, which reduces the risks associated with x-rays.
CT Scanners (Computerised Tomography)
In the same way a flatbed office scanner makes copies of documents by detecting
the intensity of the light reflected from the document as the light source and
detector moves along it, a CT scanner detects the intensity of x-rays transmitted
through the body.
- Data is recorded as an x-ray source rotates around the patient
- At the same time the patient travels slowly though the x-ray beam
- Beam fans out from the x-ray source and sensors on the other side of the
body record the intensity of the transmitted beam
- Each rotation of the beam provides data for a thing cross section of the body
- A three dimensional image is then produced by a computer
X-rays are ionising radiations, therefore can cause damage to cells. Safety
precautions include radiation badges, screens and leaving the room when the
procedures take place.

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X-ray Production
X-rays are produced when high velocity electrons are decelerated (slowed or
stopped) or by a nucleus of an atom especially by high atomic number material, such
as the anode target in an X-ray tube. An electrically heated filament (cathode) within
the X-ray tube generates electrons that are accelerated from the filament to the
anode target by the application of a high voltage to the tube. The energy gained by
the electron is equal to the potential difference (voltage) between the anode and
cathode.…read more

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Ultrasound consists of high frequency longitudinal waves above the range of human
hearing. Medical Frequencies are around 1 ­ 5MHz, and are used for diagnosis,
cleaning surgical instruments and cleaning teeth. Scanning equipment is very
portable, making it very useful for vets etc.
The production process uses a ceramic crystal called lead zirconate titanate, which
like quartz, exhibits the piezoelectric effect.
When compressed, a potential difference is produced across it.…read more

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Producing the image
An image is produced by waves reflected from parts of the body that are made of
different tissues. The image is built up by sending out pulses of ultrasound and
changing the beam direction so that it scans the body. Some energy is reflected
from any interface the beam meets as it travels through the body.
An image consists of shades of grey.…read more

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Magnetic Resonance Imaging (MRI):
Why is MRI Used
MRI produces a detailed image of the inside of the body. Images are much clearer
due to the image depending on the density of hydrogen nuclei in different parts of
the body, not on absorption or reflective properties of radiation or sound. This
means it can be used to look at parts covered by bone.…read more

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Producing the field
The magnetic flux density inside the coil used in the MRI machine is given by
B= 2r
o r = Radius
o N = Number of turns
o I = Current
o 0 = Permeability of free space
Resistive coils produce the require fields would generate energies of more than
50kW, meaning cooling is needed. They would also be costly and wasteful of energy.
Super conductors have overcome this problem. Most MRI machines use super
conducting magnets.…read more

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Advantages and Uses
Endoscopy can be used for direct observations inside the body using light
Flexible versions used to examine the gastrointestinal tract and the colon
Rigid versions are used in arthroscopy for examining and carrying out
operations on damage knee cartilage
The operation of an endoscope depends on the use of fibre optics to transmit light
from an outside source into the patient and carry the reflected light from the inside
of the patient back to the outside to form an image.…read more

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Charge Coupled Devices (CCD's):
Much more sensitive to light
Image can built up over a long time - can be hours or even days
Image consists of electrical signals so it can be enhanced electronically
Can be used to produce moving images without the need for chemical
Now used in medicine and digital cameras
How they Work
A photon of light falls onto a capacitor called a photosite (pixel)
The photons that fall onto a photosite liberate electrons from the atoms.…read more


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