Ultrasound flow detection

The application of ultrasound techniques to the investigation of peripheral vascular disease has produced a significant improvement in our understanding of peripheral vascular physiology. The difference between the arteries and veins is that the arterial blood flow is regular and phasic and more susceptible to mathematical analysis than venous blood flow which is irregular and, though ultimately controlled by the heart, is profoundly affected by many other factors (e.g. respiration, abdominal pressure and skeletal muscle contraction).
Doppler flow detection depends upon the principle that the frequency of a sound-wave reflected from a moving object is changed in proportion to the speed movement of the reflecting object. An object moving away from the source of a sound reflects the sound at a lower frequency; an object moving towards the source of a sound increases the frequency of the sound. The frequency change can be used to detect movement and to measure the velocity of the movement.
The simplest ultrasound probe consists of two piezoelectric crystals; one crystal is excited to transmit ultrasound, the other crystal becomes excited on receipt of the reflected ultrasound.
The ultrasound is directed towards a blood vessel by coupling the probe to the tissues with a coupling jelly to stop all the sound being reflected at the air-skin interface. Although other tissue interfaces of different density will reflect some of the sound waves, the majority are reflected from the red cell because they have a much higher density than their surrounding plasma. Any change in reflected frequency indicates movement of the red cells.
As Doppler ultrasound instrumentation has improved, it has become possible to focus the ultrasound to a precise depth, examine a known volume of blood (the sample volume), use pulsed as well as continuous wave sound, use a zero crossing frequency-proportional-to-voltage converter to give a signal to drive a...

Daugiau informacijos...

★ Klientai rekomenduoja

Šį rašto darbą rekomenduoja mūsų klientai. Ką tai reiškia?

Mūsų svetainėje pateikiama dešimtys tūkstančių skirtingų rašto darbų, kuriuos įkėlė daugybė moksleivių ir studentų su skirtingais gabumais. Būtent šis rašto darbas yra patikrintas specialistų ir rekomenduojamas kitų klientų, kurie po atsisiuntimo įvertino šį mokslo darbą teigiamai. Todėl galite būti tikri, kad šis pasirinkimas geriausias!