Session 6 Vascular Studies Flashcards
Main disadvantage of ultrasound in general?
highly operator dependent
What does a duplex scanner do? (duplex ultrasonography)
does 2 things:
- grey-scale (brightness mode)= visualise architecture of a body part, atheromatous plaque can be directly visualised in blood vessel.
- colour flow imaging- doppler ultrasound, visualise flow or movement of a structure
how does sound travel?
as a longitudinal wave, as sound is a pressure wave and when an object vibrates, it creates a mechanical
disturbance in the medium in which it is
directly adjacent to (usually air.) The medium then carries the disturbance in the form of oscillating and propagating pressure waves.
Sound travels at different velocities in different
mediums.
Air molecules compressed and stretched in order to allow wave propagation
what quality of sound is relied upon in ultrasound to produce an image?
reflections and echoes in path of beam, so sound goes back to transducer
frequency of ultrasound?
> 3 million Hz, so greater than our threshold for hearing
how does an ultrasound investigation work?
100s of piezoelectric crystals along face of transducer. When an electric current is applied to these crystals, they change shape rapidly. The rapid shape changes, or vibrations, of the crystals produce sound waves that travel outward, with electrical to acoustic energy conversion. Conversely, when sound or pressure waves hit the crystals, they emit electrical currents. Therefore, the same crystals can be used to send and receive sound waves.
basis of B mode (brightness) imaging?
2D image produced from linear array of transducers simultaneously scanning a plane through the body. Specular reflection occurs at large interfaces e.g. atheromatous plaque, scattering occurs at rough surfaces and small structures- smaller than wavelength of US beam. Resistance to ultrasound passage= acoustic impedence. Adjacent US beams formed across transducer face.
how can distance from a reflector to transducer be measured?
as know speed of US in tissue, and can measure how quickly echoes return, so D=VxT
how does impedence mis-match in B mode imaging affect reflection?
higher impedence mismatch, means a bigger difference in resitance to US passage between 2 adjacent materials, and this causes a stronger reflection from boundary e.g. plaque appears white, and blood black
colour on B mode imaging display if high intensity reflection?
white e.g. plaque
what is attenuation?
gradual loss of energy of US beam as it passes through deeper structures
problem of high-frequency transducer?
doesn’t penetrate as far into body, so would be useful at looking at superficial structures
advantage of high-frequency transducer?
good resolution
disadvantage of wide shaped transducer?
poorer resolution, but may be used for aortic aneurysms to be able to look at whole area of aneurysm
advantage of low-frequency transducer?
can penetrate more into body, so look at deeper structures
what do colours mean of Doppler colour flow imaging?
brighter hue= faster velocity
red= flow towards transducer, blue= flow away from
How can arterial stenoses be graded?
Using doppler indices: velocity increases across a stenosis, so more narrowing, higher velocity of blood flow, which is visible as a brighter hue on colour Doppler US
advantages of duplex scanning compared to CT scanning?
non-invasive, portable, low cost, can repeat, no radiation, real time
limitations of duplex scanning compared to CT scanning?
very operator dependent, so operator error
poorer resolution when looking at deeper structures
limitations of Ankle brachial pressure index? (continuous wave doppler)
contraindicated if calcification of arteries e.g. in diabetes mellitus as less or incompressible arteries, will produce falsely elevated ankle pressure, so giving false -ves
lower blood pressure in leg compared to arm is indicator of what?
peripheral vascular disease
how is ankle BP index calculated?
Highest Ankle Systolic Pressure /
Brachial Systolic Pressure
clinical applications of doppler ultrasound?
carotid stenoses AAA ankle brachial pressure index peripheral assessments- DVT, varicose veins transcranial doppler
process of ankle brachial pressure index measurement?
Use a Doppler and a sphygmomanometer
Measure dorsalis pedis and posterior tibial pulses
Compare pressure to brachial
Calculation: Ankle systolic/brachial systolic
Less than 1 – abnormal
The smaller the value the worse the disease
Greater than 1 - normal
usefulness of a flat/linear probe and a set probe?
Flat/linear • High frequency • Higher resolution • Poor penetration Set • Lower frequency • Lower resolution • Greater penetration
how to tackle loss of signal from deeper structures?
Deeper structures
Produce a weaker signal
Could lead to an unbalanced image
But can use Gain controls
Boost the intensity of returning signal
Intensity is the same regardless of depth of tissues
• Balances images
Doppler effect: when sound approaches an object, how is the frequency?
it is higher
what does a continous wave doppler achieve?
picks up all vessels- arteries and veins, at the same time
function of duplex scanning in looking at stenoses?
Identify position of blockage
Extent and severity of disease
Better than angiogram
• Cheaper
• No radiation
• Less invasivehow can varicose veins be sealed?
laser tment
what will veins not do if DVT?
won’t collapse if clot present
