Ultrasound Imaging

 


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Lesson #5

Lecture Objective - To become familiar with the advantages and limitations of ultrasound imaging and to develop some visual experience with the most common image presentations.

 

For the previous discussions of medical imaging, the density variations on the film image were produced by the recording of "x" or gamma photons with the image showing the relative frequency of photons passing through or emanating from an anatomic site. To interpret ultrasound images your thinking must change about what is causing the film density variation. In ultrasound, the black and white variations on the film are not related to the density of tissue as expressed by the relative passage of "x" photons through that tissue or by the localization of radioactive material within organs. For ultrasound images, the film gray tone variation is produced by the difference in echo production of the various organs (and their internal interfaces). When there are more echoing interfaces, that part of the image will either be increased black or increased white depending on how the machine is set.

Individual echoes can be recorded as either black or white on the opposite background. There is no convincing evidence that images are more accurately interpreted in either display pattern. You should probably get used to accepting either the black or white echo display and, for that reason, there are examples of both in this lesson. Most imaging ultrasound machines have the capability of displaying the echo pattern either in black on a white background or white echoes on a black background. Once you gain some experience, you will have no difficulty determining which color represents the echo productive area.

Echoes are produced at any tissue interface where a change in acoustical impedance (you can consider this to be the speed of sound) occurs. On these images, the film density is proportional to the intensity of the echo (a more energetic echo would produce a darker or lighter dot on the film). The intensity of the returning echo, that is the energy returned to the transducer, is determined by, (1) the magnitude of the change in the acoustical impedance at the echoing interface, (2) the characteristics of the intervening tissue, and (3) the normality (perpendicularity) of the interface to the transducer. The appearance of the echo on the film is also determined by the degree of amplification (gain) applied after the echo has been received by the transducer.

The images in this lesson are all recorded in the "B" mode. The "B" stands for brightness modulation meaning that the brightness of the dot is proportional to the intensity of the echo. (Do you know what an "A" mode display would look like? What does "A" mean)?

ANSWER #1

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When the transducer is stationary, only a single dimension can be recorded. That dimension is the distance of the echoing interface from the transducer. This distance is calculated by determining the length of time required for the echo to return to the transducer after the transducer sends the original pulse of sound energy. The calculation is based on the average speed of sound in tissue which is 1540 meters per second. If the transducer is moved across the patient's body while all echoes are recorded and maintained on a video screen, a two-dimensional image ("B" scan) will be generated.

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"B" scan images are sometimes referred to as contact compound scanning because the transducer is in contact with the patient's body and the technologist must use a compound or "rocking" motion to create an adequate anatomic display. Another method of creating a two-dimensional image in "B" mode is the "real time" method. In this method, an array of multiple transducer elements or a rotating transducer in a water bath container is held in contact with the patient's body and this allows a two-dimensional image to be created.

The real time display produces a usefully large area which is continuously imaged and this allows you to evaluate some organ dynamics, but the field of view is not as global as with the contact compound scanner. As you look at the images in this lesson, try to determine which were made with the contact compound scanner and which were made with the "real time" machine. "Real time' images are usually made with a sector scanner so the images will be pie shaped.

 

Safety

Ultrasound has been shown to be relatively safe but no imaging method which deposits additional energy into the body should be considered entirely risk free. When the decision   to make a diagnostic image is made, the physician should always make a conscious judgement about whether the potential benefits of the imaging procedure are greater than any potential risk. There are possible risks from ultrasound imaging and if you would like to know more about these, the reference below gives a discussion of safety issues.

Miller DL. Update in Safety of Diagnostic Ultrasonography. J Clin Ultrasound 1991;19:531-540.

 

Image Examples

This first image is a trans-axial view of the heart which is sometimes referred to as a four-chamber view. Sometimes it is difficult to obtain this view because of the configuration of the patient's chest wall. In most individuals there is an area in which the heart is in direct contact with the anterior chest wall and this provides an acoustic window through which the sound beam can be directed without being obstructed by intervening, air bearing lung. As you can see, if this image were displayed in real time it would be possible to evaluate ventricular wall motion as well as valve dynamics.

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Cross-sectional image of the heart

 

Before real-time imaging, the typical method of displaying echocardiography was called "M" mode. The "M" stood for motion mode and this was really a modification of a "B" mode display using a stationary transducer but using a moving recording device. The technique either scanned the ultrasound pattern across a TV screen or used a moving paper strip chart recorder so that in one axis you could see the position of the echo returning structure relative to the transducer and on the other axis you would have the elapsed time. Using that kind of display, it was possible to study the dynamics of moving structures such as the ventricular wall or the valve leaflets. The image below is an example of an "M" mode display and you will still see these in textbooks.

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"M" mode display

 

The image below is a longitudinal (para-saggital) image of the gallbladder. The gallbladder is normal but there is an echo productive structure adjacent to the posterior (far) wall of the gallbladder. Although this echo productive area could be produced by a mucosal irregularity such as a polyp, the presence of a strong acoustic shadow directly below the echo productive structure indicates that it is a gallstone. In this image, the echoes are displayed as black spots on a white background.

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1st gallbladder with gallstone

 

This is another image of the gallbladder. Did you notice that this one is made with a sector scanner on real time equipment (the area covered by the image is not as large as the previous image and the image is "pie-shaped")? This image is made with the echoes being displayed as white spots on a dark background. The gallbladder in this study has an abnormally thick wall which is very echo productive. Again, there is an echo productive structure adjacent to the posterior wall of the gallbladder and there is a prominent acoustic shadow deep to this gallstone. The thickened gallbladder wall is reliable evidence of chronic cholecystitis.

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2nd gallbladder with gallstone & thickened wall

 

The image below is the anterior view of a nuclear medicine biliary tract scan (remember, this was one of the two methods to image the liver). This image is frequently obtained when an ultrasound image of the gallbladder is made. If acute cholecystitis is present, the gallbladder will not fill with radioactive material because the cystic duct is too edematous for retrograde flow from the common bile duct. Usually when the gallbladder does not fill with radioactivity on this study you will find that the gallbladder wall is edematous on the ultrasound image.

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Normal

Abnormal

Anterior view of the liver

Anterior view of the liver

 

The images below are longitudinal (para-saggital images of the liver and right kidney. The image on the left shows the normal echo relationship of the liver and kidney. Usually the hepatic parenchyma and the renal cortex have about the same the degree of echo productivity. Of course, there can be many concentrated areas of echo production within either organ simply related to anatomic structures. A good example of this is the heavy echo return from the central portion of the kidney caused by the collecting system and vascular elements. The image on the right shows increased echo production throughout the liver. This is usually the result of fibrotic change and is a manifestation of diffuse liver disease.                                                                                                 

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Normal liver/kidney

  Increased echo production

 

The image below shows a longitudinal representation of the kidney with a large renal cyst. You can see the cyst as an area which has no internal echoes. In fact, there are three criteria to determine if a mass lesion is cystic:

1) There are no internal echoes.

2) The far wall (away from the transducer) is just as clearly displayed as the near wall.

3) There is a zone of increased echo recording deep to the mass.

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Renal cyst

 

As you can see, this mass satisfies all three criteria so we are sure it is a renal cyst.

Take a close look at the zone of increased echoes deep to the cyst. This is exactly the reverse phenomenon of the acoustic shadow which was seen with the gallstone. In this case, we see increased echoes deep to the mass because there is no attenuation of the sound energy as it travels through the fluid filled mass. Therefore, the beam arrives at the far wall of the cystic mass having more residual energy than it would if it passed through the solid organ adjacent to the mass. Just remember, a zone of increased echoes deep to a mass is suggestive evidence that the mass is fluid containing.

Did you notice the "A" mode display along the left margin of the image?   You can see that the area of the cyst has a relatively flat amptitude tracing.

The image below is another picture of the kidney but this time you can see an echo productive structure which is a kidney stone and this is confirmed by the presence of the acoustic shadow deep to the stone.

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Kidney stone

 

 Obstetrics

Probably the best known use of medical ultrasound imaging is related to obstetrics. Most pregnancies in the United States have ultrasound imaging at some time during the gestation whether they need it or not. Although these images are popular with prospective mothers, you might want to again review the article on the safety of ultrasound. The first image below is a trans-axial image through the fetal head. You can see that the sidewalls of the fetal head have been marked and the distance (the bi-parietal diameter) between the marks has been measured. The bi-parietal diameter can then be compared to a chart to determine gestational age.

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Trans-axial fetal head

 

This image shows the entire fetus and is a good demonstration of the clarity with which fetal parts can be shown. This is an ideal imaging situation since the object of interest is surrounded by fluid. In this image you can see the placenta along the anterior wall of the uterus. Then there is a clear space with no echoes which is the amniotic fluid. The well-defined fetus appears to be lounging on the posterior wall of the uterus with his feet propped up on the placenta, sort of like relaxing in a reclining chair.

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Entire fetus

 

This trans-axial image very clearly shows the edge of the placenta which is sometimes referred to as the chorionic plate. In this image, I think it is easy to understand how, with careful positioning, it would not be difficult to determine the external genital pattern of the fetus.

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Trans-axial - chorionic plate

 

From the images below you can see that it is not only relatively easy to determine the sex of the fetus but even the state of sexual arousal.

 

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Sex of fetus (a)

Sex of fetus (b)

                                                       

Issues

1.  How safe is ultrasound imaging?

2.  What are the limits of spatial resolution in ultrasound imaging?  Are ultrasound images as statistically reliable as conventional radiographs?

3. Technical artifacts are common on ultrasound images.  Do these artifacts limit the clinical usefulness (see reference below)?

 

 

Reference Citations

1. Miller DL. Update in Safety of Diagnostic Ultrasonography. J Clin Ultrasound 1991;19:531-540.

2. Scanian KA. Sonographic Artifacts and their Origins. AJR: June 1991;156:1267-1272.

 

If you are interested in learning more about this subject, the websites listed below contain useful information and a number of ultrasound images.

Related URLs

1.  http://www.radiologist.com/usound1a.htm

2.  http://home.earthlink.net/~terrass/radiography/medradhome.html

 


Send us comments: Dr. David Adcock, DAVID@uscmed.sc.edu.

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