Module 5 : Real-Time Considerations

Question 1

real time scanning

Answer 1

• real time scanning consists of multiple frames per second and each frame is made up of multiple scan lines

Question 2

real time scanning - limitation

Answer 2

• 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

Question 3

frame rate

Answer 3

• number of times the sweep of sound is produced by the transducer (frames per second)

Question 4

frae rate and temporal resolution

Answer 4

• frame rate increases = temporal resolution increases

Question 5

temporal resolution

Answer 5

• gives us ability to image moving structs, overcome motion artifact
• survey quickly

Question 6

what frame rate can we detect flicker

Answer 6

• 15-20 fps

Question 7

frame rate in imaging

Answer 7

• 30-60 fps

- 120 scan lines

Question 8

factors affecting frame rate

Answer 8

• depth of field
• sector angle
• number of lines
• number of focal zones

Question 9

frame rate - depth

Answer 9

• higher frame rate = less depth

- lower frame rate = deeper depth

Question 10

frame rate - sector angle

Answer 10

• higher frame rate = narrower sector

- slower frame rate = wider sector

Question 11

frame rate - scan lines

Answer 11

• higher frame rate = less lines

- lower frame rate = more lines

Question 12

frame rate - unsimplified equation

Answer 12

depth (cm) x LPF x # of foci x FR = 77000cm/sec

• 77000 is 1/2 154000

Question 13

frame rate - simplified equation - RANGE AMBIGUITY

Answer 13

FR = 77000 cm/sec / LPF x depth x # of foci

Question 14

reciprocal of frame rate

Answer 14

• the time it takes to produce one frame

FR = 1/ T frame

Question 15

scan line density

Answer 15

• number of scan lines per degree or number of scan lines per centimeter
• 1 line/degree = good resolution

Question 16

low line density

Answer 16

• more interpolation

Question 17

high line density

Answer 17

• over writing occurs

Question 18

spatial resolution

Answer 18

• 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

Question 19

factors affecting spatial resolution

Answer 19

• number of scan lines
• number of focal zones
• size of sector

Question 20

cine loop

Answer 20

• system can store many frames into memory that can be reviewed after freezing

Question 21

RAM memory

Answer 21

• the cine loops are stored in RAM memory of machine (Random Access Memory)
• it is short term and is erased when we un freeze

Question 22

advantage to cine loop

Answer 22

• can improve efficiency of seeing tiny sturcuters missed during real time interrogations

Question 23

freeze frame

Answer 23

• 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

Question 24

side lobes

Answer 24

• secondary projections of ultrasound energy that radiate away from main beam
• limited to single disc mechanical probes and radial mode vibration is key contributor

Question 25

side lobes - issue

Answer 25

• can falsely place reflectors in the image as if they came from main beam
• because machine assumes all reflections come from centre beam

Question 26

side lobes - correction

Answer 26

• insulator ring is added to element to suppress the radial mode vibration

Question 27

grating lobes

Answer 27

• 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

Question 28

grating lobes - correction

Answer 28

• apodizaiton

- sub dicing

Question 29

apodizaiton

Answer 29

• weakening voltages to outer crystals to reduce grating lobes

Question 30

sub dicing

Answer 30

• 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

Question 31

channels

Answer 31

• refers to number of elements in the array

Question 32

are more channels better

Answer 32

• more channels means more precise control over beam characteristics
  + like dynamic aperture
  + receive focus