Ultrasound

Question 1

what is ultrasound?

Answer 1

USS is a form of imaging relying on the use of sound waves in frequencies above that of human hearing (>20Hz)

it has a number of uses for
* diagnostic e.g. FASTs scans, abdo USS, cranial haemorrhage in neonates, ECHO
* in obstetrics to monitor fetus
* regional anaesthesia to improve effectiveness and safety
* doppler effect in CO monitoring
* therapeutic - physiotherapy

Question 2

how does USS work?

Answer 2

It works via the piezoelectric effect.
The USS probe contains multiple crystals that vibrate when current is applied and then produce sound.

USS emits sound waves to tissue
sound is reflected off tissue boundaries when there is a change in density of tissue.
sound waves are detected by USS and again via piezoelectric effect transduced into current for interpretation.

the amount of wave transmitted back and the length of time taken will give info on properties of tissue boundary and depth. this is taken to microprocessor which analyses to create image on screen.

the probe alternates between the modes of firing sound waves and picking up sound waves = pulse echo technique

Question 3

what frequency sound waves are used by an USS? are used by the human ear?

Answer 3

human hearing 20Hz to 20KHz

USS anything above 20KHz
however in medical USS usually 2.5 to 15 MHz

Question 4

how do USS waves travel through tissue depending on their wavelength/ frequency?

Answer 4

short wavelength, high frequency = lower depths, high resolution

long wavelength, low freq = deeper but reduced resolution.

Question 5

which type of current is applied to the crystals for the piezoelectric effect?

Answer 5

very high frequency 5Mhz (i.e. much higher than mains of 50Hz)

the frequency of sound waves produced relates to the frequency of this current and USS relies of sound freq in MHz.

Question 6

what colours are bone, tissue and fluid?

Answer 6

urine /fluid = black
tissues = grey
bone = white (strongly reflected sound waves)

Question 7

define frequency …

Answer 7

the number of waves passing a point per second. measured in hertz
inversely related to wavelength

Question 8

define wavelength of a wave?

Answer 8

the distance between 2 peaks of a wave
or the length for one complete wave cycle.

Question 9

what is the equation relating velocity of a wave to wavelength and velocity?

Answer 9

v=λ×f

v velocity
λ wavelength
f frequency

the speed of sound in the body is constant and thus wavelength and frequency are inversely related.

Question 10

what is meant by the attenuation?

Answer 10

the ability for a tissue to absorb sound waves. the lower the attenuation, the easier to visualise as less sound waves absorbed and lost e.g. blood and water.

higher attenuated structures such as bone / air - absorb or scatter the sound waves so harder to visualise.

Question 11

why is gel used in USS?

Answer 11

improve contact between the probe and surface and reduce air in between these.

air has high attenuation and thus will absorb sound waves and reduce image quality.

gel is a low attenuation material - allows sound waves to pass through without loss of energy.

Question 12

define resolution

Answer 12

the ability to distinguish between 2 points
hence improves quality of an image

Question 13

what type of wave is a sound wave

Answer 13

sound waves are longitundal waves
do not belong to EM spectrum (transverse waves)

Question 14

what modes of USS are used in medical imaging?

Answer 14

A mode = amplitude - uses a single wave, assesses depth of tissue. no 2D image. e.g. opthal to look at globe size or bladder scanner

B mode = brightness - linear array of waves to create 2D image. most common mode e.g. regional, vascular acsess

M mode = motion - B mode used in series to look at changes to image and hence motion. used for heart valves / echo

doppler mode = uses the doppler effect looking at a frequency shift in sound waves to determine velocity of a moving object e.g. blood.

Question 15

define the doppler effect…

Answer 15

The doppler effect is the phenomena whereby the sound of a moving object changes its frequency depending on if its moving towards or away from sound detector

e.g. motorbike approaching have higher frequency as it gets closer.

the shift in frequency is proportional to velocity of the moving object.

as sound waves hit RBC they wil be reflected off them, the frequency of the reflected waves will depend on velocity and direction of RBC movement.

can be used to quantify the direction and velocity of blood flow via the following formula

velocity = change in frequency x speed of sound in blood/ 2x original freq x cos angle.

Question 16

how is the probes angle important in doppler effect?

Answer 16

if at 90 degres
cos 90 = 0
so needs to be angulated and constant to measure velocity.

Question 17

what happens to frequency shift of blood moving towards/ away from detector?

Answer 17

towards = increase in frequency
away = decrease in freq

Question 18

what factors affect the image quality in an USS?

Answer 18

Frequency of waves - the higher frequency, the higher the resolution. hence to view deeper structures, this will have to be reduced which will reduce image quality.

Attenuation of the material - low attenuation helps not loose energy from the sound waves hence use of gel to reduce attenuation (air has high attenuation)

gain - increasing brightness to optimise the contrast seen at tissue bounfaries.

type of probe - a linear probe creates higher frequency than curvilinear probe.

Question 19

what is colour doppler?

Answer 19

on modern USS machines you can select a mode which looks at blood flow via doppler effect

red = blood moving towards probe
blue = blood moving away from probe

can help with vascular access

Question 20

what are the pros and cons of USS?

Answer 20

PROS:
* safe , non ionising
* cheap
* relatively portable / more convienient
* simple to use
* many uses - cardiac output, regional, obstetrics

cons:
* not good for all tissue types e.g. bone
* not good for deep structures
* user dependant - requires some training

Question 21

define depth, gain and focus..

Answer 21

depth = how deep the sound waves go and image will show. lower frequency, longer wavelgnth

gain = how bright image is - increases noise

focus = increasing resolution - focal zone has highest resolution - can adjust where this is.

Question 22

compare the amount USS is reflected at different tissue interfaces?

Answer 22

tissue and air interface = most reflected
bone-fat
soft tissue to water
muscle to fat = least reflected

hence difficult to distinguish between muscle and fat
and because almost all is reflected between air and tissue again it is impossible to visualise structures below the air.