Remote Sensing Flashcards
Explain the need for Remote sensing
It is a non invasive diagnosis technique that poses less risk and is less traumatic for the patient
main principles of production of X-rays
metal target bombarded by high-speed electrons
electromagnetic emosion on hitting target
whole energy turnet to energy of one photon with no K.E at cut-off point
K.E = eV = hf0 = hc/λ
spikes correspond to emission line spectrum ( excitation and de-ecitation of electrons )
Feature of X-Ray tube
heated filament (cathode)
accelerating voltage
evacuated chamber (vacuum)
cooled metal anode
- majority of energy turned to heat
- anode held at earth potential
x-ray window
What is intensity and how is it controlled
rate of arrival of electrons on target i.e Tube current
greater heater current = hotter filament = greater rate of emission = greater intensity
what is hardness and how is it controlled
Hardness is the penetration
controlled by accelarating voltage
- longer wavelength x-rays are softer and are unable to pass through patient
- contribute to overall radiation without useful purpose
- aluminium filter used to absorb soft rays
How x-rays produce internal images
x-ray radiation affects photographic plates
shadow image of body obtained
different materials in body absord x-rays to different extents
what is sharpness and how is it improved
ease with which edges of structures can be seen.
improved by:
- limiting size of apperture
- reducing anode target area
- lead grid to absorb scattered rays
what is contrast and how can it be improved
difference betweeen darks and lights of x-ray image
contrast affected by:
- absorption properites of materials
- using contrast medium to increase differenced in absorption coefficient
- exposure time
Purpose of CT scanning
To give 3D image of body that can be viewed from many angles
- i.e can be rotated
Principles of CT scanning
- many X-rays of one slice
- from different angles
- (all images in same plane)
- images combined to give image slice
- repeated for succesive slices
- 3D image formed
- by high-power computer
- image can be viewed from different angles / can be rotated
Generation and detection of ultrasound waves
- piezo-electric transduces used
- opposite sides of crystal coated with layers of silver to act as electrodes
- crystal is quartz with complex lattice structure
- positions of ions not fixed
- movement encouraged by applying electric field
- alternating voltage gives rise to resonance to give max amplitude
- oscillations produces sound waves in ultrasonic range (20kHz+)
- same transducer used to receive waves
- incoming wave alters positions of ions inducing a potential difference which is then amplified and processed
- coaxial cable used
- up to 600MHz
Piciples behind use of Ultrasound
- pulse of ultrasound
- reflected at boundaries / boundary
- received / detected (at surface) by transducer
- signal processed and displayed
- time between transmission and receipt of pulse gives (information about) depth of boundary
- reflected intensity gives information as to nature of boundary
define specific acoustic impedance
Z=pc
density of medium x speed of wave in medium
What is the intensity reflection coefficient and what it shows
for eave at 90 degrees to medium of impedances z1 and z2
IR / I = (Z2 - Z1)2 / (Z2 +Z1)2
higher difference in Z means more of the wave reflected.
gel used between transudecer and sking because coefficient between skin and air is large
absorbed rays goves heating effect used in physiotherapy
attentuation of ultrasound and x rays
I = I0 e-ux
x1/2 = ln2 / u
