Ultrasonic Encephalography Health Article

Definition

Ultrasonic encephalography, or echoencephalography, is the use of ultrasound to produce a noninvasive diagnostic image of the brain and its structures, including the alignment down the midline, the size of ventricles, and the presence of bleeding or tumors.

Purpose

Ultrasonic encephalography is a noninvasive way to create images of the brain. Also called intracranial ultrasound or head ultrasound, the test is most commonly used on children under the age of two to diagnose hemorrhage or hydrocephalus (enlargement of the head due to accumulation of fluid). It is particularly useful in the neonatal intensive care unit to provide bedside monitoring of premature babies who are at higher risk for hem orrhage. A series of tests are commonly ordered for babies born earlier than 34 weeks of gestation.

Ultrasonic encephalography can also detect the swelling inside the head (cerebral edema), as shown by an increase in the size of the lateral ventricles, sometimes seen in diabetic children. The test can be used in adults to monitor the size of the ventricles or to determine a shift in the structure of the brain from midline due to swelling or a tumor. However, for adults and older children, this test has been largely replaced by computed tomography (CT).

Precautions

There are no contraindications to ultrasonic encephalography.

Description

Ultrasonic encephalography uses ultrasound to produce diagnostic images of the brain. Ultrasonic waves are sound in the range above what normally can be heard by the human ear, anything above 20,000 Hertz (cycles per second) in frequency. Ultrasonic encephalography generally uses high frequency sounds waves, in the ranges of 5 to 10 MHz.

Sound waves can produce an image of the brain because of the different densities present in the tissue of the brain, blood, or tumor and the cerebrospinal fluid within the ventricles. Matter of different density reflects, or echoes, the sound waves differently, allowing the machine to distinguish between the structures.

The fineness of the distinguishing process is known as resolution. Resolution is affected by the frequency of sound waves used. As frequency increases, resolution increases. However, an increase in frequency reduces the ability of the sound waves to penetrate into the brain. Because of this relationship, successful ultrasonic encephalograms often zero in on the structures of interest, maximizing the resolution by using the highest frequency that penetrates sufficiently into the head.

A main reason why ultrasonic encephalography is used in newborns and children under the age of two is the presence of the anterior and posterior fontanelle, triangular structures at the top and back of the head where bones of the skull have not yet fused. As bone is a poor conductor of ultrasonic waves, the fontanelles provides convenient conduits into and out of the brain for the ultrasound pulses. Once the bones have fused together, the resolution of the ultrasound is greatly reduced by having to pass through bone in order to visualize the brain.

Ultrasonic encephalography involves sending ultrasonic waves through the top of the head, bouncing them off the brain structures, and recording the resulting echo. The results of the test can be produced in a plotted graphic form, known as an A-mode echo or in a two-dimensional mode. In A-mode, one axis represents the time required for the return of the echo and the other corresponds to the strength of the echo. A 2-D echo produces a cross-sectional image of the brain. As of mid-2000, 3-D imaging of the neonatal brain was still in experimental stages, with poor visualization as compared to 2-D images.

The ultrasound unit used for echoencephalography includes a TV monitor (cathode ray tube or CRT), a transducer for sending and receiving the ultrasonic waves, the transmitter, the receiver, the amplifier, and recording devices. The transducer is a hand-held instrument that is generally used both to transmit sound waves and to receive the echoes. The transducer includes the element, electrode connections to the transmitter and the receiver, backing material, a matching layer, and a protective face.

The element is the core of the transducer, the material that actually produces the sound waves. Elements are built around piezoelectric ceramic (e.g. barium titanate or lead zirconate titanate) chips. (Piezoelectric refers to electricity that is produced when pressure is put on certain crystals such as quartz.) These ceramic chips react to electric pulses by producing sound waves (they are transmitting waves) and react to sound waves by producing electric pulses (receiving). Bursts of high-frequency electric pulses supplied to the transducer by the transmitter cause it to produce the scanning sound waves. The transducer then receives the returning echoes, translates them back into electric pulses, and sends them to the receiver. The backing material helps to focus the sound energy into the element, while the matching layer helps to reduce reflection of the sound from the transducer surface. The protective face shields the internal components of the transducer. Electrodes connect the transmitter and the receiver to the transducer. The amplifier boosts the returning signals and prepares them to be displayed on the TV monitor (CRT).