Question

If a cardiologist sees no color in a region on a color Doppler map of the heart, does that mean that there is no blood flow in that region? Explain why or why not, using a drawing if necessary. Q4.6. Q4.7 Describe the ideal ultrasound contrast agent. Give the ideal properties of its acoustic impedance, speed of sound and half intensity-length, compared, for example, to those for soft tissue, blood, etc.

Answer 1

Q.4.6.If a cardiologist sees no color in a region on a color Doppler map of the heart, does that mean that there is no blood flow in that region? Explain why or why not?

Doppler color flow imaging is a method for noninvasively imaging blood flow through the heart by displaying flow data on the two-dimensional echocardiographic image. This capability has generated great excitement about the use of the technique for identifying valvular, congenital, and other forms of heart disease, as the color flow image imparts spatial information to the Doppler data.

All Doppler flow imaging systems encode the directions of flow into two primary colors: red and blue. Any number of color assignments could be made, but red and blue are chosen because they are primary colors of light (together with green).

The colors displayed on the flow map image contain useful information. By convention, Doppler color flow systems assign a given color to the direction of flow; red is flowing toward, and blue is flowing away from the transducer. Three typical color bars from a color flow imaging device give an initial frame of reference to the meaning of colors. Such color reference bars always appear on the screen of Doppler flow imaging devices. The center of the standard color bar on the left is black (white center reference mark) and represents zero flow. In addition to simple direction, velocity information is also displayed. Progressively increasing velocities are encoded in varying hues of either red or blue. The more dull the hue, the slower the velocity. The brighter the hue, the faster the relative velocity.

There is also relative flow velocity information in the color hues; the brighter the color the higher the velocity detected. Thus, high velocities away from the transducer will appear as lighter shades of blue, and higher velocities toward the transducer will be represented by lighter shades of red or even yellow. Low-velocity flow will be represented by darker shades of these colors. The absence of flow is always represented by black.

Thus we can say that.,If a cardiologist sees no color in a region on a color Doppler map of the heart, it means that there is no blood flow in that region.

Q 4.7. Describe the ideal ultrasound contrast agent.Give the ideal properties of its acoustic impedance, the speed of sound and half-intensity-length?

Ultrasound contrast agents are :

• Used to enhance anatomic structures
• Injected into the body
• Initially used in echocardiography, now branching into other areas of ultrasound
• In combination with advances with equipment and technology, this markedly improves the capabilities of diagnostic sonography.

Functions :

- Help to eliminate some ultrasound limitations, such as contrast resolution on grayscale (B-mode), detection of slow flow or flow in very small vessels
- Use of ultrasound contrast agents that area administered intravenously has been shown to improve the evaluation of blood flow through both large and small vessels as well as through cardiac chambers.

#Types of agents:

Agitated saline, encapsulated gas bubbles, free gas bubbles, colloidal suspensions emulsions, and aqueous solutions

It can be administered via slow IV infusion, typically 3 cc's of IV bolus injected in the upper extremity vein will provide several minutes of enhancement or provide prolonged enhancement lasting as much as 12 minutes or more.

*Microbubbles of air or other gases are ideally suited as echo-enhancers. The ideal one is perfluorocarbons

*The high difference in acoustic impedance at the gas fluid/tissue interfaces provides strong acoustic backscatter, and gas microbubbles approximately 1 to 5 microns in size resonate in the acoustic beam at frequencies used in diagnostic ultrasound. This "lucky coincidence" substantially increases the echo-enhancing potential of microbubbles.

These are stronger enhancers than air and are less soluble in water (and plasma). They not only provide stronger Doppler enhancement but also have a longer duration of about 10 minutes after bolus injection.

An additional and usually undesirable physical property of microbubbles is attenuation of the ultrasound beam, which occurs at high microbubble concentrations. Attenuation increases disproportionately with concentration and can be a problem, particularly in contrast-enhanced echocardiography.

The bubbles in these agents are protected by a stabilizing layer or shell consisting of various chemicals that allow the bubbles to withstand several passes through the pulmonary and peripheral capillary systems. After intravenous injection, they achieve enhancement of the entire vascular system, including the left heart and large and small arteries, as well as peripheral veins and the portal venous systems. It also enhances gray-scale echoes from the parenchyma of highly vascular organs such as the kidney or liver.

The gas contained by the agents is exhaled via the lungs over the course of several minutes following the injection. The shells or protective layers vary in their chemical constituency and undergo different metabolic pathways.