MEDICAL IMAGING 2 Flashcards
What is acoustic Impedance
Z is defined by the equation Z=pc, where p is the density of the material and c is the speed of sound in the material. the unit is kgm^-2s^-1
what is impedance matching
is the reduction in intensity of reflected ultrasound at the boundary between two substances, achieved when the two substances have similar or identical acoustic impedances.
explain how the A scan works
amplitude scan.
- a short pulse of ultrasound waves are sent into the body at the same time an electron beam travels across the screen of a cathode ray oscilloscope.
the transducer receives the pulses which causes vertical spike on the CRO screen.the x axis shows the time that the echo took to be detected by the transducer and can be used to work out the depth/thickness of the material (reflecting tissue). No photo is produced, but measurement scan be taken.
why do you have to half your time when calculating distances in A scans
they are reflections!! take twice as long to get back to the reciever.
what is a B scan
a real time 2d or 3d image is built up using returning echoes recorded from several transducers in an array. or a transducer is moved from many different positions or angles around the patient.the greater the amplitude of the reflected pulse, the brighter the white dot will be.
how is speed in a material affected by density?
if density is increased, so it speed
what happens when a wave meets a boundary
some of the wave is reflected, some is transmitted/absorbed. incident energy=reflected +absorbed
what happens to the reflected sound wave is there is a huge difference between the acoustic impedances of the materials?
much more is reflected than transmitted.
what does the ratio of transmission to reflection depend on
the acoustic impedances of the materials
(Z2-Z1) ^2 Ir
___________ = ______
(Z2+Z1) ^2 Io
why is it important that we remove low energy x rays from patients
they are absorbed by the skin and cause damage.
what are the advantages of MRI scans
no radiation
no risk of radiation poisoning
gives better soft tissue contrast than CT scans
Generates 3D data simultaneously
what are disadvantages of MRI scans
cardiac pacemakers may be affected by MRI scans
metallic objects may be heated
strong magnetic field may draw in other magnetic bodies
the doppler effect for a moving reflector
.
c
f’= ____________ x f
(c-2v)
c is the velocity of the electromagnetic radiation from the speed gun, v is the speed of the moving object and f is the frequency of the transmitted waves. this means that the speed of the moving reflector can be calculated from the change in frequency of the waves.
how do you determine speed of blood flow
ultrasound transducer emits and detects ultrasound, the transducer is placed at an angle to the artery , ultrasound is reflected by the blood . there is a change in frequency.
What are energy levels
Discrete energy of electrons in an atoms
