Gray scale [also grayscale, grey scale = brit.] produces basically black and white images with series of shades of gray. Solid areas appear white and fluid areas appear black, varying from black at the weakest intensity to white at the strongest. Gray scale resolves artifacts as small as 1 mm. The display is made by transmitting bursts of energy and analyzing the returning signal. Gray scale pictures are limited to the gray scale tones; color pictures display more information because the color is added to the gray scale.
Most ultrasound contrast agents also improve gray scale visualization of the flowing blood to such a degree that the tissue echogenicity increases. Gray scale enhancement of flow in an organ promises to improve lesion detection, along with the ability to differentiate between normal and abnormal areas, using many of the criteria already routinely used in CT and MRI.

See also Compress, Densitometry, Triplex Exam and QB-Mode.

The earliest introduction of vascular ultrasound contrast agents (USCA) was by Gramiak and Shah in 1968, when they injected agitated saline into the ascending aorta and cardiac chambers during echocardiographic to opacify the left heart chamber. Strong echoes were produced within the heart, due to the acoustic mismatch between free air microbubbles in the saline and the surrounding blood.
The disadvantage of this microbubbles produced by agitation, was that the air quickly leak from the thin bubble shell into the blood, where it dissolved. In addition, the small bubbles that were capable of traversing the capillary bed did not survive long enough for imaging because the air quickly dissipated into the blood. Aside from agitated saline, also hydrogen peroxide, indocyanine green dye, and iodinated contrast has been tested. The commercial development of contrast agents began in the 1980s with greatest effort to the stabilization of small microbubbles.

The development generations by now:
To pass through the lung capillaries and enter into the systemic circulation, microspheres should be less than 10 μm in diameter. Air bubbles in that size range persist in solution for only a short time; too short for systemic vascular use.
The first developed agent was Echovist (1982), which enabled the enhancement of the right heart. The second generation of echogenic agents, sonicated 5% human albumin-containing air bubbles (Albunex), were capable of transpulmonary passage but often failed to produce adequate imaging of the left heart. Both Albunex and Levovist utilize air as the gas component of the microbubble.
In the 1990s newer developed agents with fluorocarbon gases and albumin, surfactant, lipid, or polymer shells have an increased persistence of the microspheres. This smaller, more stable microbubble agents, and improvements in ultrasound technology, have resulted in a wider range of application including myocardial perfusion.

See also First Generation USCA, Second Generation USCA, and Third Generation USCA.

The term hyperechogenic or hyperechoic is used if there are many internal echoes. Hyperechoic tissues appear bright in ultrasound imaging. Tendons are hyperechoic because of the fibrillar pattern. Ligaments appear hyperechoic when the beam is perpendicular to the tissue. Peripheral nerves are hyperechoic relative to muscle. Liver angiomas, tumor cells, blood vessels, fibrosis, and liver steatosis appear diffuse hyperechoic.

(HyCoSy) Hysterosalpingo contrast sonography is used for evaluation of fallopian tube patency in patients with fertility problems who underwent transvaginal sonography. HyCoSy compared to more invasive techniques such as chromo-laparoscopy is rapidly becoming the screening test of choice to determine tubal patency.
Any body cavity that can be accessed can, in principle, be injected with vascular contrast. The contrast agent is instilled into the uterine cavity via a small Foley type catheter and, using transvaginal echography, the passage of the echogenic contrast along the tubes and into the adnexal peritoneum is tracked.
Hysterosalpingo contrast sonography does not offer the same anatomical and false negative results, e.g., because of tubal spasm, are possible so conventional X-ray salpingography is needed when tubal surgery is an option.

See also Endocavitary Echography, Transvaginal Sonography.

Contrast microbubbles can be destroyed by intense ultrasound and the scattered signal level can increase abruptly for a short time during microbubble destruction, resulting in an acoustical flash (sudden increase in echogenicity).
Intermittent imaging with high acoustic output utilizes the properties of contrast microbubbles to improve blood-to-tissue image contrast by imaging intermittently at very low frame rates.
The frame rate is usually reduced to about one frame per second, or it is synchronized with cardiac cycles so that enough contrast microbubbles can flow into the imaging site where most microbubbles have been destroyed by the previous acoustic pulse. Because bubbles are destroyed by ultrasound, controlling the delay time between frames produces images whose contrast emphasizes regions with rapid blood flow rate or regions with high or low blood volume.