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Color Doppler Imaging/Study​

  1. When ultrasound is reflected from a moving surface, the frequency of the sound is altered slightly in a manner that depends on the speed of movement of the surface. This is due to the Doppler effect.
  2. In Color Doppler Imaging areas of blood flow are represented as color within image.
  3. It has become common practice to represent flow towards transducer as red and flow away as blue.
  4. Velocity information in special form and color flow helps in diagnosis.
  5. Many indices of waveform analysis have been devised including resistance index and pulsatilty index, resistivity index, systolic : diastolic ratio are the commonly used ones.

Ultrasound contrast agents
  1. Ultrasound contrast agents are microbubbles whose diameter ranges from 0.1 to 10 µm. They are made of inert gas enclosed in a phospholipidic, albumin or polymer shell.
Mechanism of contrast enhancement
  1. The microbubbles act as echo-enhancers by increasing the backscattering echo intensity, which is proportional to the change in acoustic impedance between the blood, and the gas making the bubbles. The different acoustic impedance at this interface is very high and in fact all of the incident sound is reflected, even though not all of it will go back to the transducer.
  2. The increased echogenicity is seen as a increase in color, power and spectral Doppler signal strength or gray scale image intensity.
The ideal contrast agent should be non-toxic, injectable by a peripheral vein, and small enough to pass through the pulmonary, cardiac and capillary system. It should be stable enough to undergo the shear forces and pressure changes. Its half-life should be sufficient to allow a complete examination of the vessel or organ of interest.
E.g. Albumex, levovist, echogen
Applications of ultrasound contrast agents
Vascular applications
  1. Vascular ultrasound contrast agents enhance the backscattered Doppler flow signals by up to 25 dB (about a factor of 20) in both color and spectral modes. Hence contrast agents can increase the speed and confidence of arterial examinations of the extremities and in the abdomen
  2. They are useful in evaluation of carotid arteries especially in the presence of calcific plaques and vascular tortuousity.
  3. Transcranial Doppler (TCD) studies suffer from a poor signal-to-noise ratio and so contrast-enhanced TCD is receiving attention.
  4. In the liver, UCA can help detect flow in main portal vein or collaterals in patients with cirrhosis, improves evaluation of TIPSS stent for patency and stenosis and help in definitive exclusion of hepatic vein or IVC patency, and demonstrate small intrahepatic collaterals in patients with Budd- Chiari syndrome.
  5. Evaluation of renal artery stenosis and peripheral vascular disease.

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