Ultrasound and more

facts about ultrasound

• frequency is greater than 20000 Hz

• non ionising

• non invasive

• quick

• medical imaging ultrasound has freq of 1-15MHz

• can be refracted/reflected/diffracted at boundaries

• can identify small features

what is the piezoelectric effect?

• when some crystals are compressed/stretched/twisted, they produce an emf and get a charge between opposite faces

• this process is reversible so applying a pd across the crystals can compress or stretch them

what and why are ultrasound transducers?

• generates and receives ultrasound

• changes electrical energy to/from sound

• to generate, high frequency alternating pd is applied across opposite faces of crystal, compressing and expanding it

• the frequency is the same as the frequency of oscillation of the crystal

• the crystal resonates and produces an intense ultrasound signal

• usually 5000 pulses of ultrasound are emitted per second

• to detect ultrasound, any ultrasound incident on crystal makes it vibrate (compress/expand) so generates an alternating emf, which can be detected by circuit

• the crystals are lead zirconate titanate or polyvinylidene fluoride instead of quartz nowadays

what is an a-scan?

• a stands for amplitude apparently

• transducer records along a straight line through the patient

• can be used to determine the thickness of bone or the distance between two tissues like lens and retina

• when ultrasound pulse sent into patient, it is partly reflected and partly transmitted at boundary

• reflected pulse has less energy and is detected at transducer

• oscilloscope displays pulses as voltage against time

• distance can be calculated by multiplying time difference between initial pulse by speed of ultrasound (and dividing by 2 probably)

what is a b scan?

• multiple a scans in different directions (b stands for brightness apparently)

• the transducer is moved over patient’s skin so that 2D image produced

• output of transducer is connected to high speed computer

• for each position of transducer, dots produced on screen, showing boundary between tissues

• brightness of dot is proportional to intensity of reflected ultrasound

what is acoustic impedance?

• the fraction of ultrasound intensity reflected at boundary depends on it

• defined as product of density of substance and speed of ultrasound in that substance

• Z=pc (Z is measured in kgm^-2s^-1)

what is the intensity reflection coefficient?

• the proportion of incident ultrasound reflected at the boundary, also known as \frac{Ir}{I0}

• \frac{Ir}{I0}=\frac{\left(Z2-Z1\right)^2}{\left(Z2+Z1\right)^2} , where Z₁ and Z₂ are the acoustic impedances of the two substances

• only works when angle of incidence is 0

• the greater difference in Z, the more reflection happens

what is acoustic matching?

• acoustic matching is when two substances have similar impedance values so negligible reflection occurs at their boundary

• when the transducer is placed on skin, there are often air pockets between transducer and skin

• the air skin boundary means 99% of ultrasound will be reflected before it enters patient

• coupling gel is smeared onto skin because it has acoustic impedance similar to skin

• the gel fills the air gaps and ensures that almost all ultrasound enters body

what is the Doppler effect in ultrasound?

• when ultrasound is reflected off a moving object, its frequency changes

• non invasive technique

• range of frequencies 5-15MHz

• uses reflection of ultrasound from blood cells to help doctors evaluate bloodflow through major arteries and veins

• can reveal blood clots, fatty deposits

what happens during a doppler scan?

• ultrasound transducer pressed lightly over skin

• sends pulses of ultrasound and receives the reflected pulses from inside the patient

• ultrasound reflected off tissues has same frequency, but ultrasound reflected off moving blood cells has different

• when blood moving towards transducer, reflected wave has higher frequency

• when blood moves away from transducer, reflected wave has lower frequency (and high wavelength)

• change in f is directly proportional to v

• only works for speed>1cm/s because lots of background ultrasound in tissues

• computer colour codes direction and speed of blood

how to determine speed of blood with doppler effect?

• \Delta f=\frac{2fv\cos\theta}{c}

• f is original frequency, v is speed of blood, c is speed of ultrasound in blood

• \theta is angle between skin and transducer, can’t be at right angles because cos 90=0