72. ULTRASOUNDS: IMAGING WITH ULTRASOUNDS

Question 1

1. What do Ultrasound images display?

Answer 1

• they display the differences in the Acoustic
  Independence (Z) of different tissues

Question 2

2. What are the differences in Acoustic Independence caused by?

Answer 2

THEY ARE CAUSED BY:
- variations in density of different structures
- variations in compressibility of different tissues

Question 3

3. In its simplest form, how is a Ultrasonic Pressure wave transmitted?

Answer 3

• it is transmitted from the Transducer
• it is transmitted along a straight line of sight
• it is transmitted into the body

Question 4

4. What happens as the Pressure Wave propagates?

Answer 4

• the echoes are reflected from objects
• along the line of sight

Question 5

5. What are the returning echoes received by?

Answer 5

• they are received by the Transducer

Question 6

6. What do we need to map in order to display the image?

Answer 6

WE MAP:
- the echo magnitude as brightness in the image
- the arrive time as distance along the line of sight

Question 7

7. What produces the 2D image?

Answer 7

• repeating the mapping of the echoes and the arrival
  time along many lines of sight

Question 8

8. What does the circled part of this image show?

Answer 8

• this is the First Sound Wave

Question 9

9. What does the circled part of this image show?

Answer 9

• this is the Last Sound Wave

Question 10

10. In order to create an Anatomic image, what must happen to the Sound Pulse?

Answer 10

THE SOUND PULSE MUST TRAVEL
- to the Reflector
- this is located in the body

THE SOUND PULSE MUST THEN RETURN
- to the Transducer

Question 11

11. What does the travel time of the Sound Pulse allow us to do?

Answer 11

• it allows us to accurately position the Reflector

Question 12

12. How do Ultrasounds determine reflector depth?

Answer 12

• they measure a pulse’s time-of-flight

Question 13

13. Define “time-of-flight”?

Answer 13

• this is also called the “go-return-time”
• it is a specific elapsed time from the Pulse Creation to
  the Pulse Reception

Question 14

14. When is the “go-return-time” longer?

Answer 14

• it is higher when we have a detector deep inside the
  body

Question 15

15. What happens when a Reflector is located superficially?

Answer 15

• the time from the Pulse Creation to the Pulse
  Reception is brief

NB:
- this is the basis of measuring distance in diagnostic
imaging

Question 16

16. What information are Ultrasound system Computers programmed with?

Answer 16

• they contain the average speed of sound
• this is found in tissues

Question 17

17. What is the “go-return-time” directly related to, when we already know the average speed of sound in tissue?

Answer 17

• it is directly related to the distance that the pulse
  travels

Question 18

18. How can we calculate the depth of a reflector
    (in mm) ?

Answer 18

• the speed of the sound waves in the soft tissue
  (in mm / µs)
  MULTIPLIED BY the “go-return-time” (in µs)
  __________________________________________________________
  2

Question 19

19. Which rule always applies when sound travels through Soft Tissue?

Answer 19

• the 13 µs rule

Question 20

20. What is the 13 µs rule?

Answer 20

FOR EVERY 13 µs OF GO-RETURN-TIME:
- the object that creates the reflection
- is 1 cm deeper in the body