(TCCS) Transcranial color coded sonography is a combination of B-mode and pulsed wave Doppler. TCCS is used to study morphological and functional assessment of the circle of Willis, intracranial hemodynamics caused by extracranial artery stenosis, collateral flow and the vascular supply of intracranial lesion. Color imaging of the intracranial vessels allows placing the spectral Doppler volume correctly. This modality has encouraged the widespread use.
Contrast enhanced TCCS analysis of cerebral arteriovenous transit time (cTT) is used as a measure of cerebral microcirculation.
The windows that are used for transcranial Doppler examinations include regions where the skull bones are relatively thin or where naturally occurring gaps allow proper penetration of the sound beam.

See also A-Mode, Cranial Bone Thermal Index, Transcranial Doppler and Transcranial Window.

A transducer is a device, usually electrical or electronic, that converts one type of energy to another. Most transducers are either sensors or actuators. A transducer (also called probe) is a main part of the ultrasound machine. The transducer sends ultrasound waves into the body and receives the echoes produced by the waves when it is placed on or over the body part being imaged.
Ultrasound transducers are made from crystals with piezoelectric properties. This material vibrates at a resonant frequency, when an alternating electric current is applied. The vibration is transmitted into the tissue in short bursts. The speed of transmission within most soft tissues is 1540 m/s, producing a transit time of 6.5 ms/cm. Because the velocity of ultrasound waves is constant, the time taken for the wave to return to the transducer can be used to determine the depth of the object causing the reflection.
The waves will be reflected when they encounter a boundary between two tissues of different density (e.g. soft tissue and bone) and return to the transducer. Conversely, the crystals emit electrical currents when sound or pressure waves hit them (piezoelectric effect). The same crystals can be used to send and receive sound waves; the probe then acts as a receiver, converting mechanical energy back into an electric signal which is used to display an image. A sound absorbing substance eliminates back reflections from the probe itself, and an acoustic lens focuses the emitted sound waves. Then, the received signal gets processed by software to an image which is displayed at a monitor.
Transducer heads may contain one or more crystal elements. In multi-element probes, each crystal has its own circuit. The advantage is that the ultrasound beam can be controlled by changing the timing in which each element gets pulsed. Especially for cardiac ultrasound it is important to steer the beam.
Usually, several different transducer types are available to select the appropriate one for optimal imaging. Probes are formed in many shapes and sizes. The shape of the probe determines its field of view.
Transducers are described in megahertz (MHz) indicating their sound wave frequency. The frequency of emitted sound waves determines how deep the sound beam penetrates and the resolution of the image. Most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency, but multi-frequency probes are also available.
See also Blanking Distance, Damping, Maximum Response Axis, Omnidirectional, and Huygens Principle.

Echoes of deep lying structures within the body do not always come from the latest emitted sound pulse and can produce an aliasing artifact. Aliasing lowers the frequency components when the pulse repetition frequency is less than 2 times the highest frequency of a Doppler signal. This artifact can be problematical at Spectral or Color Doppler examinations.
Aliasing of the data displayed in pulsed wave technology is utilized as a benefit in determining transitions from laminar to turbulent flow.

See also Ultrasound Imaging Modes.

The mirror artifact is similar to the reverberation artifact. Mirror image artifacts (mirroring) can occur if the acoustical impedances of the tissue is too much different and the ultrasound is reflected multiple times on tissue layers. The echo detected does not come from the shortest sound path, the sound is reflected off an angle to another interface so that like a real mirror, the artifact shows up as the virtual object.
An empyema or lung abscess can be simulated by a mirror image artifact of a hepatic cyst. This liver lesion can appear like a lesion within the lung because the wave is reflected off the diaphragm back into the liver. The angle of reflection is equal to the angle of incidence. The sound pulse hits the interfaces within the liver lesion and is reflected back to the diaphragm once again with an angle of reflection equal to the angle of incidence and then back to the transducer.
Also by a pelvic ultrasound scan the sound can be reflected off the rectal air at an angle so that the deep wall of an artifactual cyst represents the mirror image of the inferior and anterior walls of the bladder. Mirror image artifacts can cause other strange appearances such as invasion of a transitional cell carcinoma through the bladder wall.
Also called Cross Talk.

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