Dandy-Walker syndrome

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 hemorrhage. 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).

Preparation

The patient who is undergoing an ultrasonic encephalogram is laid on his or her back or side and must be still during the test. It is suggested that children two months to one year of age do not eat or drink for three hours before the test, so a bottle can be drunk during the exam. Particular care must be taken if the child is connected to a respirator. Warmed conducting gel is placed on the head to ensure an air-free contact between the transducer and the head (air is a very poor conductor of ultrasound) and to allow the transducer to slide easily.

The area that provides the least amount of interference with the ultrasound waves is called the acoustic window. For infants and young children, the best acoustic windows are transfontanelle, that is, through either the posterior or anterior fontanelle. Some standard views from the anterior fontanelle include midline sagittal (viewed from the side, through the midline, or middle of the head), lateral sagittal (viewed from the side, displaced from the midline), and coronal views (viewed from the front, angled toward the back, middle, and front). Axial views (across the temple) can be used, despite the reverberation artifacts caused by the skull, to follow lateral ventricle size.

An ultrasonic encephalogram is noninvasive, causes no pain, and takes about 20-30 minutes.

Aftercare

After the test, the patient can return to regular daily activities and meals.

Complications

There are no complications or side effects of ultrasonic encephalography.

Results

Ultrasonic encephalograms are mainly performed for the diagnosis and follow-up of neonatal hemorrhage, hydrocephalus, and congenital malformations. Premature infants often develop bleeding in the germinal matrix of the caudate nucleus. The caudate nucleus is an elongated, arched gray mass in the center of the brain next to the lateral ventricles, and the germinal matrix is a group of brain cells in that area that is still developing. Bleeding can also occur in the choroid plexus (spongy tissue of the ventricles) and rarely, the cerebellum. If the bleeding is severe it can leak into the ventricle, a problem known as intraventricular hemorrhage (IVH). All of these bleeding problems can be seen initially as echogenic areas (white areas) that later can be replaced by fluid-filled cysts that scan as dark areas.

Bleeding in the neonate is sometimes associated with the later development of cerebral palsy, although other risk factors, such as bronchopulmonary dysplasia (BPD) appear to be equally predictive.

When looking for hydrocephalus, measurements of the ventricles are done. On a lateral sagittal view, the distance from the curve of the choroid plexus to the tip of the occipital horn generally should not be more than 16 mm. Using a coronal view, the body of the lateral ventricle should generally not be more than 3 mm. Finally, an axial view is often used to determine the lateral ventricular ratio, which is the lateral ventricular width divided by the hemispheric width (both widths measured from the outer border to the midline). The ratio is compared to previous measurements to see if swelling is developing.

There are many congenital malformations of the brain that can be either diagnosed or the severity determined with ultrasonic encephalography. Some representative examples include microcephaly, holoprosencephaly, Dandy-Walker Syndrome, and encephalocele. These conditions can have serious prognoses, so ultrasound is an effective means of determining what treatment, such as placement of a shunt or surgery, should take place.

Health care team roles

Ultrasonic encephalograms are often produced by specially trained ultrasound technologists. Training for such a position usually involves study at a two-year college or vocational program. A typical program would include:

• elementary principles of ultrasound

• ultrasound transducers

• pulse-echo principles & instrumentation

• ultrasound image storage & display

• artifacts (erroneous results)

• quality assurance

• bioeffects and safety

Certification of ultrasound technologists specializing in neurological work such as ultrasonic encephalography is available through the American Registry of Diagnostic Medical Sonographers as a registered diagnostic medical sonographer with a specialty in neurosonology. Certification requires passing both a general and a specialized test.

A physician such as pediatrician, neonatologist, or radiologist does the final review and diagnosis based on the results of an ultrasonic encephalogram. The doctor can be present for the exam or may review saved images.

Key Terms

Acoustic window

Area through which ultrasound waves move freely.

Congenital malformation

A deformity present at birth.

Echogenic

Highly reflective of ultrasound waves; these tissues show as a white area in the scan.

Hemorrhage

Bleeding, the escape of blood from the vessels.

Hydrocephalus

A congenital or acquired condition characterized by an increase in size of the cerebral ventricles. Without treatment it can cause enlargement of the head, brain shrinkage, mental deterioration, and convulsions.

Intracranial

Inside the skull.

Ventricle

A small cavity in the brain. Humans have two lateral ventricles, a third ventricle, and a fourth ventricle.