Left Ventricular Contrast Imaging for Echocardiography

Echocardiography–Using Left Ventricular Contrast

LV contrast

Intravenous left ventricular contrast imaging is used either when despite all of the sonographer’s skills, the resultant images are not sufficient for the adequate assessment of chamber geometry and wall motion or to rule out an apical thrombus.

Ultrasound contrast works by introducing a large quantity of highly reflective microspheres into the blood, thus increasing the strength of reflected ultrasound in the blood pool. Agitated saline contrast works in the same fashion, but is not able to pass through the lungs and circulate in the arterial system, thus only being useful as a right-sided contrast and for detection of right to left shunts.

Since intravenous contrast is a controlled substance, it can only be used on order of a physician and must be administered within safe parameters by someone who is trained in its use and authorized to use it. The policies concerning the handling and administration of ultrasound contrast vary by state and the institution where the sonographer works. The sonographer usually initiates the request for contrast by contacting the person who is authorized to prescribe it, usually a physician, physician’s assistant or nurse practitioner. This person is responsible for interviewing the patient to screen for possible contraindications and for administering the contrast.

The dosage characteristics of ultrasound contrast are an important factor for the sonographer to be familiar with. There is an optimal concentration level and rate of infusion when using ultrasound contrast. If the bolus of contrast agent causes too high of a concentration in the blood, the result is, “white-out” conditions and the borders of the left ventricular chamber are difficult to discern, while too small of a dose results in only minimal opacification of the chamber. Once the proper dose is administered, the initial “white-out” will be of minimal duration, gradually followed by a period of time of up to several minutes in which good images are obtainable.

The administration of ultrasound contrast requires that the power level of the ultrasound machine be lower than for routine imaging because the microspheres are subject to rapid destruction at standard power settings. All ultrasound machines provide a preset which is usually labeled, “LV Contrast,” or with the actual name of the contrast medium such as,

“Definity,” or “Optison.”   If no preset is available, the machine should be set to .5 MI initially, and can be adjusted in a range of between 0.1 to 0.8 MI in order to get the desired results.

Ultrasound contrast must be repeatedly administered in small doses as needed to opacify the chamber throughout the exam. In most cases, an initial small dose is better because if the chamber fills with too high of a concentration level, it can take a long period of time (up to a few minutes), for the microspheres to clear, resulting in a lengthened exam time. When using contrast for a stress test, an overly high concentration of the contrast can delay the acquisition of images and can ruin the test. The sonographer therefore must learn by experience how much and how frequently ultrasound contrast should be given and to customize its use for each individual patient.

Doppler can be used for brief periods of time while ultrasound contrast is circulating through the heart but also uses a higher power level, causing rapid destruction of the microspheres, thus affecting the intensity and longevity of the contrast effect. Ultrasound contrast can strengthen a weak spectral Doppler signal when needed; much like saline contrast is used to enhance the jet of tricuspid regurgitation.

The major machine adjustments that can affect image quality while using contrast include,Power (Mechanical Index), Gain, Focus, Frequency, Dynamic Range and Compression. These adjustments are usually only necessary if the preset is not producing a crisp image or if swirling of contrast is seen.

Initially, the focus should be set at the mitral valve level because a focused ultrasound beam in the area of interest will cause a more rapid rate of microsphere destruction, (seen as insufficient opacification and swirling of the contrast that does not improve no matter how much more contrast is injected).

Lowering the frequency can help if, despite the use of contrast, the image of the chamber is still faint. Dynamic range and compression are useful for adjusting the contrast of the image. Image colorization, also known as B-Color can make it easier to view the image.

The use of ultrasound contrast does not automatically guarantee an accurate image of the left ventricular chamber.  Shadowing, caused by ribs and lung tissue can create a false impression of chamber geometry and myocardial motion and so it is important to recognize and to try to prevent it from happening.

If shadowing is seen on the lateral wall and changing the position of the transducer does not help, increasing the Power (Mechanical Index) may eliminate the shadow.  The initial recommendation for Power setting is at about 0.3 MI.  Increase the MI until the wall is no longer shadowed.  Remember that the higher the power level, the faster the microspheres are destroyed.

It is important to realize while using ultrasound contrast, that whenever the machine is placed in normal imaging mode or Doppler, the machine will not switch back to the LV contrast preset and so, the sonographer must lift the transducer off of the patient’s chest while adjustments are made so that the microspheres will not disintegrate any more than necessary.

The use of ultrasound contrast does not automatically guarantee an accurate image of the left ventricular chamber.  Shadowing, caused by ribs and lung tissue can create a false impression of chamber geometry and myocardial motion and so it is important to recognize and to try to prevent it from happening.

The ultimate result of using intravenous ultrasound contrast is totally dependent on patient body characteristics, a sonographer’s experience and the use of techniques which produce good images. The optimal image will demonstrate a distinct border where the contrast outlines the endocardial border of the left ventricular chamber, making accurate assessment of wall motion and volume measurements possible.

Left Ventricular Thrombus:

 The apical views are very useful for the detection of left ventricular thrombus because the most common site for a thrombus is at the left ventricular apex. The sonographer should always be alert for and investigate anything at the apex that has similar appearance to a thrombus. The presence of apical thinning and scaring, a wide left ventricular apex and/or hypokinesis to dyskinesis and aneurysm should cause the sonographer to consider giving the area special attention for the presence of an apical thrombus.

Recording a suspected left ventricular thrombus:

  1. Decrease the depth or use the zoom so that the apical region is featured.
  2. Move the focus to the level of the apex.
  3. Use the highest frequency that has sufficient penetration and contrast.
  4. Use 2-D Colorization to aid in visualization.
  5. Repeat these steps for all of the apical views and the parasternal short axis at the apex.
  6. Ask the physician to authorize the use of LV contrast for a more definitive assessment and diagnosis.

A suspected left ventricular thrombus needs to be distinguished from normal endocardial trabeculations which are often seen at the left ventricular apex. Trabeculations can be distinguished from a thrombus by demonstrating their anatomical relationship to the left ventricular wall with a long digital clip, showing a seamless connection to the myocardium, that it has the same tissue characteristics as the myocardium and that it thickens during systole. Thrombi will usually have a different textural appearance from the myocardial tissue and are usually more spherical in shape, or if of the laminated type, will appear as an akinetic segment, distinct and motionless against the left ventricular wall.

The administration of left ventricular contrast can be very helpful in distinguishing a thrombus from myocardial tissue. As the contrast fills the left ventricular chamber, a thrombus will appear darker than the surrounding tissue.

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