Starting Abdominal Ultrasound

Beginners problems with Ultrasound investigation comprise:
Technicalities, Perception, imaging techniques and Anatomy.

The nearest analogy to ultrasound is the ground Radar, used at an airport. It is in one plane. The two main qualifiers of this analogy are:
1. The much greater reduction in signal strength as the ultrasound goes deep into the patient and back again to the probe.
2. The variation in speed of the ultrasound as it traverses gas, rather than solid tissues or fluid.

The speed of sound in air is roughly 300m/sec and in tissue 1530m/sec. Sound is reflected from boundaries that mark changes in density or elasticity, thus air tissue boundaries block visualisation of some regions.

Since sound energy is attenuated and it is scattered by the tissue, common sense would propose compensating for the loss of energy of the more distant returning echoes. Since the position or character of the tissues in the patient cannot be predicted, the machine cannot compensate completely and the resulting images might have brighter or darker echoes than the beginner might expect.
Manufacturers are attempting to write systems that can recognise areas of altered ultrasound characterisics. The problem isn't solved yet and remains an interesting area for research.

The technique of examination is based on using a large solid organ or a fluid filled organ as a conduit for the sound, an ultrasound window.

With ultrasound, 'it's not what it looks like, it's where it is' that matters. The diagnostic process is a little different from plain film assessment. Abnormalities are easier to see when the observer has some idea of the anatomy and pathology that is probable. Mental differential lists and a progressively refined diagnosis are helpful. Ultrasound assessment is aided by constructive thinking about the whole patient from the very start. British Radiologists take a history as they go. An important refinement of this process is that the observer must not be tempted to force a coherent picture of the observations. Any slight discrepancy or unexplained variation must not be explained away. Some pathologies can give rise to very subtle changes in the final images.

Perception

The retina seems to be arranged on the premis that objects are bounded and edges in an image mean a new object. Changes in an ultrasound images due to differences in transmission of sound can be interpreted, falsely, as structures.

Some my earlier research at Central Mid. in 1983 involved a primitive digitiser and satellite image analysis system. When applied to an ultrasound image of a simple renal cyst, the software identified the increased echoes behind the cyst as another structure. The clear fluid in the cyst did not attenuate the sound as much as the compensating gain in the processor expected it to be reduced. Put simply, Ultrasound interpretation requires that the position of echoes is much more important than their superficial appearance . The investigator is using ultrasound images and the position of the probe to produce a mental construct of what is inside the patient. It is that mental image that is the product of ultrasound assessment. Incidentally machines are not the only things to be fooled by false boundaries. The tendancy of the human visual system to make solids out of bounded rings cannot be overestimated. The cartoons in newspapers depend on this.

Real time ultrasound presents much visual information at a high rate. Eye saccides are a way of expressing the need for the eye to point the image on that part of the retina, which has the greatest resolution. The eye ficks to a new position roughly 3 times a second. If your eye saccidic fixation speeds are below average, then you will have to take more time over your observations. The repeat rate of the image display is affected by the ultrasound hardware and may affect the position of your fixations. Not all spatial information in an image is appreciated at once. Of course, you will be unaware of what you have missed. Deliberate reassessment pays dividends. Because of the need to make observations, you may not be aware of long periods without blinking. Remind yourself to blink occasionally and do not spend more than one hour continuously at the screen. This will reduce eyestrain.

Tasks in three dimensional space are performed in the parietal lobe. Moving the probe and constructing a mental image, complicated by moving the patient, are all processed in the parietal lobe, to some degree, and will interfere with each other. Cut down the parietal processing load for better interpretation. If you have a cine playback, that can sometimes help you to observe and assess things which move quickly on the screen. Perusal of any criminal court proceedings will tell you that even the most gifted people can make unpredictable witnesses of events that happen in a short time. ( polemic warning ;-) ) There is an argument for not increasing the work-stress in ultrasound, since any diversions or undue hurry can degrade diagnostic performance.

Practical hints

For abdominal ultrasound, examine all structures initially with the patient supine. The position of mobile structures is then more predictable. Later you may turn the patient for a closer examination of individual structures. Breath holding changes the position of things so much that I recommend you allow the patient to breathe normally for the initial assessment. Place the probe over each site you wish to examine and allow the images to come to you. Normal breathing moves the organs and you don't have to burden your parietal processor while trying to chase the organ with the probe.

Bowel gas can be persuaded to move. Smooth muscle does not respond immediately, but persistent gentle pressure can work wonders, moving ultrasound-opaque bowel gas. Think of each normal anatomical structure that you would expect to see and wait for it to appear. You may not see all of an organ or vessel, but a neighbouring structure will provide a clue. Re-examination with real-time colour doppler can prove very helpful. Examination of the same structure can be helped by changing the patient's position, erect to see the pancreas for example. Once you have made a differential diagnosis, look with the ultrasound for the consequences of each diagnosis. Additional information will then appear under your ultrasound probe.

Explanation to the patient takes time, but a co-operative patient will enable you to perform the techniques that get better images.

Anatomy

To be expanded later:
For the time being, comparison of the CT cuts and the first and second set of ultrasound images may help your studies.

For further sources, try the Medical Ultrasound WWW Directory by Don Christopher from the University of Toronto.

Technicalities

BASICS

Put very simply, no matter what machine that you may have, the total activity of the machine to process the original information remains the same. You may only change how that machine effort may be allocated. Main controls are:
Focus, compensation gain, contrast/grey-scale and sound frequency.

It is appropriate to make a simplistic analysis of the generation of a sound beam.
In tissue a 5 MegaHerz sound will travel 1530 meters in one second and 5 million wavelengths will occupy 1530 meters. Let us examine neighbouring regions of a flat probe surface that are emitting sound at the same time and are separated by a half wavelength, or half cycle.

In front of the probe, at every half wavelength into the patient, the wavelets will add up. To one side, in the plane of the surface of the probe, when the sound has traveled half a wavelength, its power will be cancelled by the equal and opposite energy emitted by its neighbour one half wavelength away.

By timing the emission of sound by neighbouring sources on the working surface on the probe, the angle or direction in which the wave front builds-up can be changed. By changing the timing difference between sources in the middle and at the edges of the probe surface, the position of the narrowest part of the beam within the patient, the focus, may be be changed.

Much processing discriminates slight differences in the phasing, or phase angle, of the returning sound so as to separate small echoes. By setting the machine to coarser discrimination, the faint specular echoes within an obese patient may be recognised. Alteration of amplification of the returning echoes can be used to adjust the brightness of the echoes in the final images. Lower pitch, lower frequency sound will be subject to less scatter, when used for deep echoes. There is at least one focus zone so that you can make the ultrasound beam at its most compact in the area that you most wiish to image.

Further Reading

Cases on this server

• gall-stone in CBD.
• pleural metastasis from Ca bronchus
• Appendicitis
• chronic pancreatitis.
• iatrogenic ovarian cystosis

• ultrasound anatomy
• ultrasound sites on the WW. Web

Ian Maddison July 1998