Chapter9-HOMI-Med Imaging

Ultrasound = piezoelectric crystals transform electrical energy into acoustic energy and back. MAJOR ADVANtAGE - NO HARMFUL SIDE EFFECTSThe crystals vibrate with frequencies btw 2 and 10 MHz. These frequencies are higher than audible soundHow it works: Pulsed sound waves are emitted --> energies and arrival times of the received echoes caused by reflections are measured. Need to determine source and possible reflectors distance. Distance = between source and reflector = (Time Interval btw emission of sound wave pulse  and echo from reflector ) X (Sound velocity)/ 2Accuracy of distance- depends on the length of pulse, the shorter the pulse, the more accurate it is

A-Mode: = Amplitude mode = energy of each echo is displayed as a function of time interval between pulse and echo: Amplitude is displayed on vertical axis and time is on the horizontal axisA Mode Provides 1D information about the location of the reflecting boundaries

M-Mode= Motion mode- 1D information , Amplitude of the echo from each pulse is represented as brightness of the point located along the time axis, corresponding to the arrival time of the echo Sometimes M-Mode is called Brightness Mode - B-Mode. The other axis now used to display the echos of each pulse. Therefore coordinates indicates the depth in athe patient and the other coordinate indicates the time so it is possible to  visualize the movement of objects that are crossed by  the ultrasound beam

C-Mode - Compound Mode = angle of the beam is moved. the crystal is moved, connected to a flexible arm linked to a fixed point of reference - rotation transducers are present in the arm

Sector SCan - speed up the movement of the transducer, parallel scanning--> the crystal is moved in a way that each beam is continuously shifted in a direction until a complete area is scanned. ( produces a 2D image) Sector Scan - each successive bean makes a small angle with previous one --> Produces a 2 D image In  both cases - the intensity of the image represents the amplitude of the echoInstead of moving a single crystal, can also use crystal array of 100 crystals and sequentially activate each crystal 

Doppler Effect: = used to measure flow velocities - When a sound wave is reflected by a target that has a velocity component in the direction of the sound beam, the frequency of the reflected sound wave is higher is when the target approaches the sound source and lower when the target recedes from the sound source. The shift in frequency is proportional to the frequency of the incident beam and the velocity of the targetUsed to measure with 2 transducers --> Today it is more likely due to pulsed sound --< only need 1 transducer --> can overlay motion with anatomic image 

X-ray - detected using a fluoroscopic screen that produces visible light that in turn exposes film or screen Bone attenuates X-rays more more than LungFluorescent screen in the image intensifier transforms x-ray distribution into charge distribution of electrons, electrons are then accelerated and directed against a second smaller fluorescent screen on which the amplified light image appears.  

Images can be generated on a storage phosphor plate and read out by a laser scanner --> digital radiology or Computed Radiology

Digital Subtraction Angiography - contrast agent used and image is obtained, contrast injected, shadow image subtracted, vessels are seen .

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