Overview
The ultrasound examination uses high frequency sound waves (greater than 20,000 Hz) and a computer to construct images of the various organs of the human body.During the ultrasound examination, a probe is moved on the skin. This probe emits sound waves which propagate in different tissues. Due to reflection phenomena, part of the sound wave beam returns to the ultrasound probe. These reflected sound waves (echoes) are collected by the probe and transmitted to a computer. Thanks to the analysis of these data, images will be obtained.
Organs such as the liver, gallbladder, spleen or kidneys are analyzed in detail by ultrasound. A simple manipulation of the probe makes it possible to have images in any plane of space and in real time.
Image formation
The ultrasound probe contains a transducer. Under the effect of electrical impulses, the crystals of this transducer will deform and produce vibrations (this is the piezoelectric effect). The sound waves emitted by the ultrasound probe propagate through the tissues. The speed of propagation of these sound waves essentially depends on the characteristics of the tissue. The impedance of a fabric accounts for its acoustic characteristics. The acoustic impedance is the product of the specific mass of the tissue by the speed of propagation of the sound waves. When the beam of sound waves crosses fabrics of different acoustic characteristics, part of this beam is reflected. The proportion of reflected sound waves depends on the difference in acoustic impedance of the tissues crossed.The reception of echoes by the transducer will generate an electric current. These signals undergo a succession of processing operations to result in the formation of a gray scale image. The information collected is therefore based on the differences in acoustic impedance of the various structures crossed.
Risks of an ultrasound examination
No deleterious biological effect has been observed to date. The ultrasound examination is therefore safe and it is for this reason that this method is used as a diagnostic tool in pregnant women. The presence of a pacemaker or an insulin pump is not a contraindication to an ultrasound.Diagnostic information
Since ultrasound does not use X-rays or magnetic fields, it is easy to see that the information provided by this modality differs from that obtained by computed tomography (CT) or MRI. Ultrasound images of a kidney are quite different from those obtained by computed tomography or MRI. These three radiological techniques can be complementary.The diagnostic information obtained by ultrasound is taken from the differences in acoustic behavior between normal tissue and pathological tissue. The greater this difference in acoustic behavior, the more easily the lesion is detectable. Conversely, if the pathological tissue has acoustic properties similar to that of healthy tissue, the lesion can be difficult to identify.
Ultrasound appearance of the lesions.
The ultrasound appearance of a lesion is described in terms of the intensity of the echoes visible on the image and of the adjacent structures.The appearance of lesions can be classified into characteristic categories:
• hypoechoic lesion;
• hyperechoic lesion
• the cyst
These three types of lesions are illustrated in the image gallery below. Click on the thumbnail below to access large images. Use the arrows on your keyboard or the arrows on the side edges of the images to scroll through the illustrations.
Ultrasound of the liver. Hyperechoic lesion. Image 1.
