Medical physics Flashcards
9702 42 FM 20
Explain why an aluminium filter may be placed in the X-ray beam when producing an X-ray image of a patient.
(aluminium filter) absorbs (most) low energy X-rays
X-ray beam contains many wavelengths
so low energy X-rays are not absorbed in the body
9702 42 FM 20
In an X-ray image, white regions show greater absorption of X-rays than dark regions.
State and explain the difference between the X-ray image of bone compared to that of muscle.
bone is seen as lighter
bone has a higher µ value so absorbs more
9702 42 FM 20
Explain why ultrasound used in medical diagnosis is emitted in pulses
allows the reflected signal to be distinguished from the emitted signal
detection occurs in the time between emitted pulses
(reflection of ultrasound) detected by same probe / transducer / crystal
cannot emit and detect at same time (hence pulses)
9702 42 FM 20
Explain the principles of the detection of ultrasound waves used in medical diagnosis.
pulses of ultrasound
ultrasound incident on piezo-electric crystal
waves make crystal oscillate
oscillations (of crystal) generates an e.m.f. (across the crystal)
9702 42 FM 20
Why gel is applied to the surface of the skin during an ultrasound scan.
without the gel most of the ultrasound is reflected
Z values more similar so less (ultrasound) is reflected
9702 42 FM 19
Outline the principles of computed tomography (CT) scanning.
X-rays (are used)
(object is) scanned in sections / slices
scans taken at many angles / directions
slices are combined
(to give) 3-dimensional image (of whole structure)
9702 42 MJ 20
By reference to specific acoustic impedance, explain why there is very little transmission of ultrasound waves from air into skin
specific acoustic impedances of air and skin are very different
intensity reflection coefficient depends on difference between acoustic impedane
most ultrasound reflected so little transmission
Definition
Attenuation
Gradual decrease of intensity of radiation when it passes through a medium
Formula
Attenuation of X-ray equals
Max Intensity * exp^(-attenuation coefficient * thinkness)
Definition
Piezo-electric effect
Production of emf across a crystal by putting the crystal under stress
Definition
Aucostic impedence
Product of density and speed of ultrasound going through a medium
Formula
Reflected intensity
(Z1 - Z2)^2 / (Z1 + Z2)^2
Definition
Impedence matching
Use a gel to reduce the intensity of unwanted reflected ultrasound between 2 materials
Definition
Radiotracer
Substance consists of radioactive material that is attached to a natrual chemical
