SCANNING RULES - ORIENTATION of the image , Background of the image | ACOUSTIC enhancement and SHADOWING, ULTRASOUND BEAM | Frequency and resolution


 Orientation of the image
Woman left shocked by what she discovered in her ultrasound photo ...

Orientation of the image
     
It is possible for teh images on the monitor to be reversed so that, on transverse scans, the left side of the patient is seen on the right side of the screen. Although there may be an indicator on the transducer , it is essential before scanning to check visually which side of the transducer produces which side of the image . This is the best done by putting a finger at one end of the transducer 180° and check again .on the longitudinal scans,the head of the patient should be on the left side and the feet on the right side of the screen.



(Fig:1a). Two axial images
Of the same fetal head, but aligned 180° differently .


Background of the image
    The image on the screen may be predominantly black or predominantly white. It may have a white background with black echoes or a black background with white echoes showing as spots or lines . There is usually a switch to make this change; if not, an engineer should adjust the machine so that it always shown a black background with white echoes.


     
         (Fig:1b). Transverse image of an enlarged uterus with background change .

 Acoustic enhancement and shadowing

 Clear liquids allow ultrasound to pass directly through without much alteration, so that echoes that come from tissue behind liquid are usually enhanced (brighter). This is known as "acoustic enhancement". Drinking enough water to fill the stomach will displace gas-filled bowel, providing an acoustic window. This is particularly useful in visualizing the body and tail of the pancreas. 

• Gas in the bowel or elsewhere can present a variety of sonographic patterns. The beam can be scattered, reflected, absorbed and refracted, making it very difficult to image underlying structures. For this reason, ultrasound can not be used to image the normal lungs or to demonstrate lung disease other than peripheral masses. A chest X-ray provides much better information.


     (Fig:2a). A liquid-filled structure, the gallbladder, with posterior enhancement due to low ultrasound attenuation. The walls of the gallbladder cause two lateral shadows.

    (Fig:2b). When liver and ascites are imaged through the ribs, the ribs cast two shadows and there is a layer of reverberation in the ascites .

 Focus of the ultrasound beam

Because the organs or parts of the body that are of interest will be at different depths, the focous of the transducer should ideally be adjustable. If the focal distance is fixed, the most suitable transducer must be chosen for the particular examination. The best choice is described in each section of the manual.

Frequency and resolution

The higher the frequency of ultrasound, the better the resolution . this mean that smaller details become visible when higher frequency is used. however, the penitration of the ultrasound into the body will be less.

    
         (Fig:2c). Images of uterus made at different scanner frequencies. Quality is best judge by comparing the details of the endometrium. There is reverberating in the liquid-filled urinary bladder............

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