Ionising Radiation: X-ray and CT

Question 1

You will be expected to demonstrate understanding of the principles, but not to memorise or use the equations given in the slides or to describe in detail the structure of the equipment used to create and detect X-rays.

You will not be expected to recognise or interpret the abnormal images shown in the later slides.

Answer 1

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Question 2

Outline how medical X-rays are produced

Answer 2

CATHODE

• Filament heated
• electrons leave the filament (lots of energy vibrate)
• overall positive charge produced so electrons attracted back

ANODE

• make anode more positive than cathode (lost electron)
• electrons will want to go to the anode
• potential difference
• anode more positive and therefore attract electrons from the cathode to the anode
• electrons decelerated by the target (hits anode and slows down)
• energy of electrons is converted mainly to heat (98%) and 2% is emitted as x-rays

Question 3

Explain how X-rays (and gamma rays) interact with matter

Answer 3

• x-rays or gamma rays pass through matter they may be…

–> transmitted unchanged (all energy in X-rays passes through body and end up with the same amount of energy coming out the other side)

–>absorbed partially (into the matter and don’t leave the other side)

–> total absorption (xrays completely absorbed)

–> attenuated (scattered) [direction originally is changed]

Question 4

Explain how the interaction of the X-ray with matter allows for the formation of an image

Answer 4

• totally absorbed by receptor it would show white as nothings come through
• fully transmitted it would show black
• partially absorbed it will be a grey
• no attenuation means energy it has when enters stays unchanged
• attenuation is basically when it bends slightly causing partial transmission or large bending causes no transmission (white)
• attenuation caused by different body structures are responsible for visualisation of the structures on the image
• dependent on attenuation coefficient

Question 5

Interpret a normal PA chest radiograph, labelling the diaphragm, costo-phrenic and cardio-phrenic angles, right atrium, left ventricle

Answer 5

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Question 6

Explain why lungs, ribs and heart appear more or less bright in the X-ray image

Answer 6

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Question 7

Interpret a lateral view of the upper respiratory tract, labeling the oropharynx, larynx, trachea, epiglottis and hyoid bone (no need to explain what happens when someone is strangled!)

Answer 7

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Question 8

Briefly describe how lung cancer, pulmonary oedema, pneumonia and emphysema would be recognised on a chest X-ray

Answer 8

Pulmonary Oedema = bat wings sign/ bi-lateral / result of pulmonary vessels enlarged also you can only see right main bronchus below bifurcation/ enlarged left ventricle/ heart booth shaped

Pneumonia = solid airless portion of lung fluid is drawn into the lungs/ globular pneumonia only affecting one lobe

Emphysema = more pronounce hyla as enlarged pulmonary arteries/ diaphragm is flattened/ lungs are darker from breakdown in alveoli/increase in blackness

Question 9

Explain how and why contrast agents are used to image cardiac vessels

Answer 9

• Contrast agent is used which is based on iodine so high atomic number is introduced via catheter into blood vessels of heart
• Cardiac vessels more prominent as filled with contrast you can selectively catheterise individual blood vessels

Question 10

Explain how CT differs from a standard X-ray, and the advantages of CT over X-ray

Answer 10

• The colours reflect characteristics of the material
• The things which attenuate bone/lungs/vessels all have different colours
• Purely reflecting the attenuation characteristics of the structures
• uses a motorised table and rotating detectors numerous splices
• each shade of grey is a different radio opacity (made out of some material which will attenuate xrays differently)

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