Module 6 - 6.5.3 Using Ultrasound

Question 1

Ultrasound (definition)

Answer 1

Longitudinal waves with frequencies greater than 20000Hz (the upper limit of the audible range)

Question 2

Audible Range of Frequencies

Answer 2

20Hz - 20kHz

Question 3

Benefits of Using Ultrasound (4)

Answer 3

• no known dangers with low intensity
• real time, live images/videos produced
• can tell different tissues apart
• ultrasound machines are portable and inexpensive

Question 4

Ultrasound is reflected when

Answer 4

there is a change in density of the material

Question 5

Denser materials reflect – ultrasound

Answer 5

more

Question 6

Ultrasound cannot pass through – , so

Answer 6

• air spaces
• cannot be used to look at organs behind the lungs, for brain imaging, to detect fractures

Question 7

Using Ultrasound (3)

Answer 7

• pulse of ultrasound is sent into body from transducer
• time taken for reflected echoes to return to transducer can be used to determine depth of material
• must halve the time taken

Question 8

A Transducer (definition)

Answer 8

A device which converts a non-electrical signal into an electrical signal, such as a microphone

Question 9

A Transducer contains

Answer 9

a piezoelectric crystal

Question 10

A Transducer acts as

Answer 10

both a transmitter and receiver of ultrasound

Question 11

How a Transducer Works (5)

Answer 11

• when a positive p.d. is applied to piezoelectric crystals, the crystals contract and change shape
• when a negative p.d. is applied, the crystals revert back to their original shape
• when a high frequency alternating p.d. (signal) is applied, the crystals oscillate at that frequency to produce ultrasound waves
• if the waves reflect back and hit the crystals, a p.d is produced
• when the transducer picks up the reflected sound, a computer works out how far the sound has travelled

Question 12

Ultrasound Scanning: A Scan (5)

Answer 12

• Amplitude Scan
• short pulse of ultrasound waves sent into body at same time an electron beam travels across screen of cathode ray oscilloscope
• transducer receives reflected pulse causing vertical spikes on screen of CRO
• x axis shows time the echo took to be detected
• no image produces but measurements taken to determine dimensions

Question 13

Ultrasound Scanning: B Scan (5)

Answer 13

• Brightness Scan
• much more common
• real time 2D/3D image of scanned area built up from many echoes recorded from several transducer/one transducer moved to different angles
• the greater the amplitude of reflected pulse, the brighter the dot
• a range of brightnesses is shown corresponding to different material being scanned

Question 14

Impendance Matching (definition)

Answer 14

The reduction in intensity of reflected ultrasound at the boundary between two substances
(achieved by matching substances with similar impendances)

Question 15

Ultrasound is reflected back if the boundary between 2 materials have (3)

Answer 15

• different acoustic impendances
• therefore most of the ultrasound is reflected at a air-skin boundary
• so a gel is used to reduced the amount of ultrasound reflected back

Question 16

The Doppler Effect with Ultrasound (3)

Answer 16

• can be used to determine the speed of blood flow in the heart/in arteries
• red blood cells reflect ultrasound
• the greater the change in frequency of transmitted and received signal, the greater the speed of the blood

Question 17

Equation for The Doppler Effect for a Moving Reflector using Ultrasound

Answer 17

Δλ/λ ≈ Δf/f ≈ v/c

Question 18

The Change in Frequency of a Doppler Scan can be colour coded (5)

Answer 18

• if a vein and an artery are close to one another, then the blood will be flowing in opposite direction within them
• a flow of blood towards the source of ultrasound waves will result in an increase in the frequency of the received waves
• this could be colour coded red
• blood flowing in the opposite direction will result in a decrease in the frequency
• this could be colour coded blue