Doppler
The main purpose of the Doppler examination is to study the circulation of red blood cells in the vessels (arteries or veins). The doppler does not use X-rays and, in principle, the Doppler examination does not require an injection of contrast product. Doppler is therefore a non-invasive technique for studying vessels!The procedure for a Doppler examination is identical to that for an ultrasound examination. Moreover, the ultrasound examination and the Doppler examination are performed with the same device.
Preparation for a Doppler examination varies depending on the region studied. The study of the vessels of the neck or the limbs does not require special preparation. In general, a period of fasting is required for a doppler examination of the abdominal vessels.
The formation of Doppler and ultrasound images is based on the properties of the sound waves reflected by the different structures studied. A beam of sound waves is emitted by an ultrasound probe. When the beam hits a moving structure, the reflected sound waves have a different frequency. This frequency will be increased if the moving structure moves towards the ultrasound probe; it will be reduced if the moving structure moves away from this probe. The information collected during a Doppler examination is based on the frequency differences between the emitted sound wave and the reflected sound wave.
As the Doppler is a reliable and non-invasive technique, this examination is found in the front line in the decisional algorithms of vascular pathologies.
The fields of application are vast:
• Permeability of the vessels (deep vein thrombosis of the leg, carotid or vertebral dissection).• Presence of stenosis (Renal arteries, carotid arteries for example)
• Vascularization / rejection of a transplant (liver, kidneys for example)
• Demonstration of hypervascularisation (tumor angiogenesis)
The doppler makes it possible to study the flow in the arteries as well as the veins, whether in the abdomen, the neck or the limbs. Naturally, the caliber of the vessel has an influence on the reliability of the Doppler study: the smaller this caliber, the more complicated the study becomes and therefore less reliable. The deeper the vessels studied are located, the more difficult the Doppler study can become.
There are a few variants of the doppler technique that allow you to have additional or different information. These variants are listed below.
Color Doppler: kidney
Pulse Doppler: normal arterial flow in the kidneys
Color Doppler
The color visible inside the vessel reflects both the speed of the blood and its direction relative to the probe. The color indicates the direction of flow relative to the probe. By convention, everything that goes to the ultrasound probe is colored red and everything that goes away from it is blue. Therefore, under normal conditions, a vein and an artery with a parallel path will appear different colors. The higher the speed, the more the colors turn yellow or white and the presence of a turbulent flow causes the appearance of yellowish areas of color.Pulsed Doppler
The pulsed doppler provides a graph describing the speed / frequencies as a function of time. We can draw information such as resistivity, pulsatility indexes, instantaneous speeds, ...The morphology of the arterial flow is characteristic since it varies according to the cardiac cycle. The morphology of the venous flow is characteristic since it varies according to respiration: increase during inspiration, decrease during expiration.