Authors: DR. ASHISH GUPTA, (H.O.D.), VINAY GUPTA, Dept.of ORAL & MAXILLOFACIAL SURGERY, SUDHA RUSTAGI COLLEGE OF DENTAL SCIENCES AND RESARCH, Ghaziabad

ULTRASONOGRAPHY

The phenomenon perceived as sound is the result of periodic changes in the pressure of air against the eardrum. The periodicity of these changes lies anywhere between 1500and20,000 cycles per second(hertz).Hz. By DEFINITION, ULTRASOUND has a periodicity greater than 20KHz.Thus it is distinguished from other mechanical waveforms simply by having a vibratory frequency greater than the audible range. Diagnostic ultrasonography, the clinical application of ultrasound, uses vibratory frequencies in the range of 1 to 20MHz.

MECHANISM OF ULTRASONOGRAPHY

Scanners used for sonography generate electrical impulses that are converted into ULTRA-HIGH frequency sound waves by a TRANSDUCER, a device that can convert one form of energy into another-in this case ELECTRIC ENERGY into SONIC ENERGY.

TRANSDUCERS

convert the electrical energy into ultra high frequency sound waves, these waves pass through the object and whenever they meet a surface of changed density they are reflected back partly, a part of the transducer also acts like a sensor and converts this returned energy into images.

The most important component of the transducer is a thin PIZOELECTRIC crystal or material made up of a great number of dipoles arranged in a geometric pattern. The most widely used piezoelectric material is LEAD ZIRCONATE TITANIUM.

Sonic waves reflected back(echoes) cause a change in the thickness of the crystal, which in turn produces a n electrical signal that is AMPLIFIED,PROCESSED AND DISPLAYED. In this system the transducer serves as both a TRANSMITTER and a RECEIVER.

Current techniques permit ECHOES to be processed at a sufficiently rapid rate to allow perception of motion; this is referred to as REAL TIME IMAGING.

In contrast to X-Ray imaging, in which the image is produced by transmitted radiation, the reflected portion of the beam produces the image in sonography. The fraction of the beam that is reflected back to the transducer depends on the acoustic impedance of the tissue, which is a product of its density (and thus the velocity of sound through it) and the beam’s angle of incidence.

Because of its acoustic impedance, a tissue has a characteristic internal echo pattern. Consequently, not only changes in echo patterns delineate different tissues, but they also can be correlated with pathologic changes in a tissue. Interpretation of sonograms, therefore, relies on knowledge of both the physical properties of ultrasound and the anatomy of the tissues being scanned.

APPLICATION IN ORAL AND MAXILLOFACIAL SURGERY
DIFFERENTIATION :

The main application of ultrasound is the differentiation between solid and cystic masses. Solid masses are echogenic producing internal echoes. Cystic or fluid filled lesions are echo free with enhancement of the deep wall.

OBSTRUCTIONS :
Ultrasound is useful in the evaluation of sialoliths in patients with acute OBSTRUCTIVE SIALOADENITIS or in whom CONTRAST SIALOGRAPHY was contraindicated because of known history of iodine allergy. In these cases, the SIALOLITH appears as a focal echogenic density exhibiting acoustic shadowing.

FRACTURES :

A comparison between SUBMENTOVERTEX VIEW RADIOGRAPH and REAL TIME ULTRASONOGRAPHY in the diagnosis of ZYGOMATIC ARCH fractures. One single submentovertex view radiograph was not confirmatory of zygomatic arch fractures whereas ultrasound was confirmatory.
If the invention of ultrasound had been before radiograph, we would opt for ultrasound more than radiographs in cases of FRACTURES.

TUMORS :

PAVLOV IP reported that the accuracy of ULTRASOUND in differentiating between BENIGN and MALIGNANT LESIONS of the SALIVARY GLAND is amazingly high at 91.8%.

BIOPSY :

ULTRASOUND guided FINE NEEDLE ASPIRATION BIOPSY is a very acute diagnostic procedure. It can be used for DEEP LYMPH NODES, SALIVARY GLAND PATHOLOGY and for DEEP SEATED HAMARTOMATOUS LESIONS.

VASCULAR LESIONS : DOPPLER ULTRASOUND is particularly useful in identifying SOFT TISSUE VASCULAR LESIONS.

ADVANTAGES OF ULTRASOUND

Non-invasive
Economical
Painless
Quick
No adverse reaction
Mobile equipment
Easier repeatability
Low cost of storage

LIMITATION IN ORAL AND MAXILOFACIAL SURGERY

Ultrsonography has been used as a diagnostic tool in other areas of body, however, it has not been used extensively in the evaluation of inflammatory lesions involving the head and neck.

One major LIMITATION of the ultrasound is its inability to penetrate OSSEOUS STRUCTURES such as the maxilla and mandible. Consequently its use is restricted to areas of the MAXILLOFACIAL REGION where the sound wave doesn’t have to penetrate bone (eg. superficial lobe of parotid, lower maxillofacial region {submandibular triangle and neck}).

Another major limitation of ULTRASOUND is detailed knowledge of SONOSTRUCTURE is required. Operator dependant In short, ULTRASOUND is one modality which is not being utilized to its fullest potential by the ORAL PHYSICIANS and ORAL IMAGEOLOGISTE in the diagnosis.

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