I- Conventional ultrasound imaging :
ultrasound, the probe is used for both transmitter and receiver. The images are constructed from reflections (echo) on different body structures.
The ultrasound is emitted by successive wave trains of short duration to avoid at the transducer, the interference between the waves it emits and those it receives.
The soft tissue / bone interface lets through about 30% of the incident energy. This means that an echogenic structure located behind a bone may be hidden.
Since the transmitter and receiver are combined, only approximately normal echogenic surfaces beam path will be correctly detected. It is therefore important for medical diagnosis, placing the probe as perpendicular as possible to the contours of bodies.
- Ultrasound A mode: this is the basis of ultrasound.
The signal which will form the image represents the amplitude of the echo based on the time elapsed from the issuance.
SI1: distance from the probe to the interface 1
SI2: distance from the probe to the interface 2
For the probe receives the echo of the interface 1, the sound wave must travel 2xSI1 and 2xSI2 for the interface 2.
The echo time is :
t1 = 2xSI1 / c, and t2 = 2xSI2 / c
If S1 = 7.5cm and SI2 = 15 cm, c1=c2=1500 m/s (neighbors in reality but not equal)
Found t1 = t2 = 0.1ms and 0.2ms
The program must be stopped to analyze the echo, avoiding having interference.
The area of each peak depends on the energy received, So when the echo time increases, the peak decreases as the reflections that have followed have made with energy losses.
The shape itself of the peak is spread due to the dispersion of the vibration.
Echo is represented as a point of light even brighter than the reflection coefficient is high, so that the energy received is great.
If the probe is moved, each interface is described line by line (scanning). If these are several sensors that are placed side by side on a bar, a plane is described and directs tomography.
By activating the turn transducers on the array, an image obtainable by 1/1000 th of a second and one can thus recommence the sequence for performing dynamic imaging of a moving member. Example of a fetal ultrasound.
– Echographie TM :
The probe is fixed and the scroll support function of time. If the interface is stationary, one obtains a straight line parallel to the traveling direction, if the interface is movable, a curve representing the relative motion of the point is obtained referred. This mode is used to assess the movement of the heart valves.
II- Echographie Doppler :
This effect was proposed by Christian Doppler 1842. Il est le décalage entre la fréquence de l’onde émise et de l’onde reçue lorsque l’émetteur et le récepteur sont en mouvement l’un par rapport à l’autre ; it also appears when & rsquo; wave is reflected from a moving object relative to the transmitter and receiver. C & rsquo; is a fairly common phenomenon that & rsquo; we encounter almost every day without you noticing.
A person is standing in the water, on the shore. Waves arrive on his feet every ten seconds. Walking person, then runs towards the sea : she goes to meet the waves, they reach it with a higher frequency (for example, every eight seconds, then every five seconds). The person is then turn around and walk then runs towards the beach ; the waves reach with a lower frequency, for example, every twelve, then fifteen seconds.
When a sound source is closer to & rsquo; an observer there is a compression of sound waves ; the length d & rsquo; perceived wave is shorter and making the sound more acute.
When a sound source s & rsquo; d & rsquo away; an observer there is a decompression of sound waves ; the length d & rsquo; perceived wave is longer and making the sound more serious.
- Applied to the hemodynamic, Doppler analysis blood flow velocity.
- This examination is still useful in cases of cardiovascular risk factor.
- It gives the real age of the patient's arteries.
- It also allows easily locate clots (in the case of venous thrombosis e.g.), a narrowing or an abnormal dilation of the arteries.
The apparatus comprises:
- A probe comprising a transmitter and an ultrasound receiver.
- Graphic chart recorder (curves and spectra).
A gel is used to obtain good contact between the probe and the patient's skin.
The variation in frequency between the incident signal and the reflected signal is given by the relation:
Δf = 2f.cos θ.v/c from where v = c Δf/2f.cos θ
v: red blood cell velocity
c: Ultrasonic velocity in the tissues
f:frequency of the incident signal
i:angle between the speed of the globules and the direction globule probe.
- If the blood cells away from the probe, the angle θ is greater than 90O, and cos θ is less than zero.
- If the cells are close to the probe, the angle θ is less than 90O, and cos θ is greater than zero.