Module 5 : Real-Time Considerations Flashcards
real time scanning
- real time scanning consists of multiple frames per second and each frame is made up of multiple scan lines
real time scanning - limitation
- each scan line has to wait for all echoes to be received from selected depth prior to next pulse being sent out
- depth is limiting factor along with number of focuses
frame rate
- number of times the sweep of sound is produced by the transducer (frames per second)
frae rate and temporal resolution
- frame rate increases = temporal resolution increases
temporal resolution
- gives us ability to image moving structs, overcome motion artifact
- survey quickly
what frame rate can we detect flicker
- 15-20 fps
frame rate in imaging
- 30-60 fps
- 120 scan lines
factors affecting frame rate
- depth of field
- sector angle
- number of lines
- number of focal zones
frame rate - depth
- higher frame rate = less depth
- lower frame rate = deeper depth
frame rate - sector angle
- higher frame rate = narrower sector
- slower frame rate = wider sector
frame rate - scan lines
- higher frame rate = less lines
- lower frame rate = more lines
frame rate - unsimplified equation
depth (cm) x LPF x # of foci x FR = 77000cm/sec
- 77000 is 1/2 154000
frame rate - simplified equation - RANGE AMBIGUITY
FR = 77000 cm/sec / LPF x depth x # of foci
reciprocal of frame rate
- the time it takes to produce one frame
FR = 1/ T frame
scan line density
- number of scan lines per degree or number of scan lines per centimeter
- 1 line/degree = good resolution
low line density
- more interpolation
high line density
- over writing occurs
spatial resolution
- scan lines are and aspect of spatial resolution
- things that effect scan line density have and impact on the details of the image we see
factors affecting spatial resolution
- number of scan lines
- number of focal zones
- size of sector
cine loop
- system can store many frames into memory that can be reviewed after freezing
RAM memory
- the cine loops are stored in RAM memory of machine (Random Access Memory)
- it is short term and is erased when we un freeze
advantage to cine loop
- can improve efficiency of seeing tiny sturcuters missed during real time interrogations
freeze frame
- continuous display of a single Fram in the memory on the monitor
- same image is read over and over and the raster display continuous to write image to display
- a CRT refreshes 30 frames per second even though image is frozen
- if machine stops writing phosphor stops glowing and image disappears
side lobes
- secondary projections of ultrasound energy that radiate away from main beam
- limited to single disc mechanical probes and radial mode vibration is key contributor
side lobes - issue
- can falsely place reflectors in the image as if they came from main beam
- because machine assumes all reflections come from centre beam
side lobes - correction
- insulator ring is added to element to suppress the radial mode vibration
grating lobes
- just like side lobes but are specific to array probes
- same assumption is made by machine with result being and artifact
- result of crosstalk between crystals
grating lobes - correction
- apodizaiton
- sub dicing
apodizaiton
- weakening voltages to outer crystals to reduce grating lobes
sub dicing
- each element divide into smaller pieces or sub elements that will act as one crystal
- if cut to dimension of less than one wavelength the grating lobes will project at 90’ or greater
+ then transmitted into air between crystals so reduce effect go cross talk
channels
- refers to number of elements in the array
are more channels better
- more channels means more precise control over beam characteristics
+ like dynamic aperture
+ receive focus
