1.1. Physical

 

Ultrasound(US)-waves move in human tissues and liquids as (pre-dominantly) longitudinal waves with an average velocity of 1540m/sec. All acoustic interfaces in soft tissues only partially reflect sound beams, and this allows further propagation of a more or less high proportion of the initial acoustic energy. Reflected parts of the beam can be received and measured according to their strength and to the time interval between emission and reception, thus giving information relating to a) the acoustic properties of the reflecting interface (strong or weak reflector) and b) the distance of this interface to the unit producing and receiving the initial and the reflected beam(s).  These units are the scanning probe`s piezoelectric crystals.

 

Highly reflective interfaces give rise to a “loud“ echo which is represented on the screen as a bright spot, whilst the opposite is true of weakly reflective interfaces. Areas without acoustic interfaces - such as the lumen of vessels and other cavities containing liquid (be it blood, bile, pancreatic juice, ascites, or urine) - give no reflection and no spot on the screen ie a black space on the monitor.

 

The scanner-to-interface-distance is represented on the monitor by the distance of the brightness and modulated spots away from the screen`s upper margin.  ie The further away they are the later they arrive back. The resulting array of numerous piezo-elements finally enables us to have a realtime insight into the abdomen and indeed other parts of the body providing they are not compromised by completely reflective interfaces such as bones, gas filled structures or metallic implants.  The scanning-plane formed by the sector array can be swept freely across the abdomen.

 

High ultrasound-frequencies (>7.5. Mhz) give a high resolution, although penetration depth is limited - and vice versa. Lower frequencies (< 3 Mhz) render limited resolution but with a deep penetration. A suitable compromise for abdominal work is to use routine probes with frequencies of 3.5 - 5.0 Mhz.

1.2. Apparatus

 

1.2.1. Scanner types

For abdominal ultrasound, array scanners of the curved type are used as the best compromise of two other standard-type probes: the linear and the sector scanner. The linear scanners have some disadvantages when applied to  regions of interest covered by non-flat regions of the body surface (e.g., the spleen behind the left costal margin), whilst sector scanners with a minimal contact surface result in limited information for the near field only. Both the scanning plane and the received ultrasound-signals are modified and improved with increasingly sophisticated software backup. The price class of the ultrasound system (low, middle, high-end) is maybe less important as compared to the skills and the experience  of the examining doctor; however, too cheap an equipment will be less fruitful. The combined use of abdominal and cardiac ultrasonography demands different probes and different software provided by more sophisticated (and more expensive) systems only. The use of colour/Doppler-equipment is helpful in the abdomen as well.

 Figure 1. Scanner-types

 

 

1.2.2. Adjustment and orientation

 

The monitor brightness and contrast should be  adjusted to clearly show  all sections of the greyscale.

 

In longitudinal sections, the left screen margin corresponds to the cranial region whilst in transverse sections, the left screen margin corresponds to the right side of the patient (and this convention should be followed by all examiners).

 

The peritoneal layers are too thin to be visualized directly. Therefore, the intra-abdominal and retroperitoneal organs must be differentiated according to the known anatomy and with the aid of respiratory maneuvers (with the intestine gliding along the abdominal wall structures).

 

Since different regions/organs in the same patient would have different acoustic properties, the panel settings may need to be corrected continuously during the same ultrasound procedure to achieve a picture which is both subjectively and objectively optimal.

 

 

Scanner controls

 

Modern scanners are  sophisticated and any number of menu driven options may be available.  For routine abdominal ultrasound scanning however, a limited number of control adjustments are necessary. 

 

1.  Gain:  The echo signal returning to the body is converted into an electronic signal by the transducer.  This electronic signal has to be amplified to produce images on the monitor.  This signal amplification is called Gain and will regulate the strength of the echo’s depth.  Most scanners will have controls for “near gain” amplification and “far gain” amplification.  Some scanners will have “time gain compensation” which is essentially an adjustment for the sensitivity at each depth and to allow compensation for signal loss from the far field.  This can be set so that the solid organs such as the liver will have uniform brightness at all depths.

 

2.  Zoom:  This will allow magnification of selected areas of the ultrasound

 

image usually up to about five times normal. To get an idea of the (non-) magnification status of a given picture, note the cm-bars at the screen`s margin.

Looking at zoomed areas of interest has the advantage of a more detailed view with the drawback of lesser an orientation, especially for colleagues who are not used to look at variable plane sections. A loss or lack of orientation in anatomy and in pathological findings is - apart from possible malreactions ("ultrasound is not readable", "pictures like weatherforecast", "I prefer CT") - harmful for medical selfconscious-ness and not beneficial for intercolleague com-munication.

 

3.  Freeze frame allows the image to be held. Measurements can then be taken of organs shown on the frozen image by applying a cursor and using a trackball to measure to a second mark.   The distance travelled will then be displayed on screen measured in cm. In addition to (zoom-) freeze-control, cineloop options are helpful.

1.2.3. Artefacts

 

The realtime ultrasound picture that appears on the screen is influenced by numerous components such as attenuation, dispersal and refraction of the ultrsound beams (both the initial and the reflected beams) which can cause artefacts that don’t represent  true information. Moreover, the ultrasound machine`s software can calculate only  an average sound velocity.  Nevertheless, the more sophisticated an ultrasound system is, the less artefacts will be visible.

As a rule, most artefacts can be easily recognised due to their non-biological properties such as strange geometry, repetitive appearance or failure to be reproduced in different scanning sections.   Thick muscle layers are apt to produce rather diffuse artefacts, worsening the whole picture  but  this is not  true of fatty layers, which can give good results even in obese patients.