Ultrasound Technique

  • General

  • Technique
    - Transducers
    - Frequency
    - Different planes
    - Reflection/deflection/absorption/scatter
    - Color Doppler
    - Duplex Doppler

  • Artifacts

General

Ultrasound is a convenient and accessible tool for examination. It is relatively cheap and fast. Additionally, patients are not exposed to ionizing radiation. 
Figure 1 gives some idea of the many applications of ultrasound technology. The list includes only tests performed by the radiologist; prenatal ultrasound tests in pregnant women, for instance, are performed by specialized obstetricians.

Figure 1. Broad outline of ultrasound indications.

A significant benefit of ultrasound is that in some cases the clinical picture, e.g. local pressure pain or palpable swelling, can immediately be correlated with ultrasound findings. Additionally, it is a dynamic procedure with moving images. This may be useful, e.g. to demonstrate an inguinal hernia during Valsava or assess compressibility of the gallbladder or vessels (fig. 2).

Figure 2. Inguinal hernia (performed during Valsava followed by compression). 

Unfortunately, ultrasound also has its drawbacks. Not all patients are suitable for ultrasound. In adipous patients, it can be difficult to image everything clearly (fig. 3). Additionally, the quality of the examination largely depends on the experience of the person performing the ultrasound.

Figure 3. Difference in image quality in an adipous versus slender patient.

For additional details on specific ultrasound examinations, see the respective courses, e.g. General Abdominal Ultrasound (= in progress).

Technique 

Ultrasound uses sound waves. They are reflected, deflected or absorbed in the body. The reflected sound waves produce the ultrasound image. The more sound waves are reflected, the more hyperechogenic (= whiter) the tissue is imaged. With reduced reflection, the image will be more hypoechogenic, and anechogenic if there is no reflection (= black). 
Both the speed of sound through the tissue and tissue density impact the quality of the ultrasound image. High-density tissue generates multiple echo reflections (e.g. bone/calcareous structures), producing hyperechogenic images. Fluid reflects no sound waves and therefore is anechogenic (= black). Soft tissue (e.g. organs) is somewhere between hyperechogenic and anechogenic. Isoechogenic is when the tissue has the same echogenicity as the surrounding tissue (fig. 4/5). 

Figure 4. Echogenicity with corresponding terms.

Figure 5. Sample abdominal ultrasound examination. Note the different echogenicities of the various structures.

Transducers

In general, three different transducers are used (fig. 6): sector, linear and convex. The sector transducer emits sound waves in a fan-shaped beam. The transducer head is small and the beam close to the transducer is narrow.  As the beam travels away from the transducer, it widens, imaging more of the deeper structures. This transducer is used in particular in neonatal skull ultrasound. The small transducer head can see the brain parenchyma through the unfused skull sutures. The linear transducer emits parallel sound waves, achieving high resolution of surface structures (including skin lesions). The convex transducer emits parallel sound waves from a convex surface. Sound waves are emitted in a fan-shaped beam as in the large convex transducer, only there is more space between the sound waves close to the transducer. This is the transducer commonly used in abdominal ultrasound.

Figure 6. The convex, linear and sector transducers with different sound beams.

Figure 7 shows an ultrasound examination of the right kidney using a convex transducer.

Figure 7. Ultrasound examination of the right kidney (convex transducer). Note the kidney moves during relaxed breathing. The passing black (hypoechogenic) vertical bands are the ribs.

Frequency

In addition to transducer shape, frequency also impacts image quality. Frequencies between 2.5 and 7.5 MHz are used for diagnostic ultrasound.  High frequency enables a higher image resolution, but depth is reduced (= lower penetration depth). Low frequency reduces resolution, but increases penetration depth. 

Various planes

A transducer is used to perform transversal and sagittal assessments. By moving the transducer over the skin, a parallel series of ultrasound images is obtained, allowing systematic assessment of each part of the body.  Another technique is to tip the transducer. The transducer is held in place but is rotated ninety degrees; only the sound beam changes direction.  In this way structures can be evaluated in two directions.
For instance, in the craniocaudal direction (= transversal plane) and left-right direction (= sagittal plane).
Important: location and direction of the transducer on the patient's skin determine anterior/posterior and left/right on the imaged obtained. 

As a general rule, in the transversal plane (fig. 8):

  • the top of the ultrasound image is the anterior side and the bottom is the posterior side. 
  • left on the image is actually right and vice versa.  The body is seen from below as it were (as in a transversal section of a CT scan).
     

Figure 8. Left kidney in the transversal plane.

As a general rule, in the sagittal plane (fig. 9): 

  • the top of the ultrasound image is the anterior side and the bottom is the posterior side. 
  • right on the image is towards the feet (= caudal) and left is towards the head (= cranial).

The images can be read from the screen during the examination.
Orientation tip when attending a live examination: the top of the image is the location where the sound waves enter the patient first. So irrespective of position and tipping, the top is the skin side.

Figure 9. The left kidney in the sagittal plane.

Reflection/deflection/absorption/scatter

When sound waves move on the boundary surface between two media with different densities, part of the beam is reflected to the transducer. This phenomenon is called reflection. The remainder of the beam continues on into the tissue, but under a different angle. This is called deflection. As sound waves penetrate the tissue, part of the energy is converted into heat. This energy loss is called absorption. Finally, part of the sound waves are lost in scatter. This takes place when sound waves move through inhomogeneous tissue or in a 'hard’ boundary surface (= large density difference between two media). Part of the sound waves are reflected in random directions, a small part of which towards the transducer. For a summary see figure 10.

Figure 10. Transmission, reflection, deflection, absorption and scattering of sound waves on a boundary surface between two media.

Color Doppler

Blood stream patterns may be evaluated using echo Doppler. One of the applications of echo Doppler is color Doppler. This technique can be used to evaluate the presence of flow and flow direction in a blood vessel. 
Sound reflections of a moving object undergo frequency changes. During the examination the difference between the emitted and received frequencies is measured; the frequency shift/Doppler shift (fig. 11).

Figure 11. Doppler shift; difference between emitted and received frequencies.

The Doppler shift and Doppler angle (fig. 12) are then calculated, allowing determination of blood circulation patterns. Note: the specific Doppler calculation will not be explained further in this course.

Figure 12. The Doppler sound wave traveling at an angle through the flowing blood in the blood vessel; the Doppler angle.

As explained above, moving objects undergo a change in frequency. In color Doppler, frequency changes are converted into color on screen. Blue means the blood is moving away from the transducer; red means the blood is moving towards the transducer (note: blue and red does not necessarily mean low-oxygen and high-oxygen blood respectively). Explanation: when blood moves towards the transducer, the wave length of the sound wave shortens, the sound frequency increases (positive Doppler shift). The opposite happens in blood moving away from the transducer (= negative Doppler shift). See also figure 13.

Figure 13. Various frequencies and wave lengths in flowing blood.

Duplex Doppler

The flow signal of a blood vessel can also be represented in a spectrum. 
The Doppler shift is shown on the vertical line, time on the horizontal line (fig. 14/15). Blood flowing towards the transducer has a positive Doppler shift and is shown above the line.  Flow under the line has a negative Doppler shift (= flow away from the transducer).

Figure 14. Image without and with color Doppler flow of the aorta (sagittal direction).

Figure 15. Duplex Doppler of the aorta (sagittal direction).

Artifacts

Ultrasound examinations are associated with a diversity of ultrasound artifacts and can be encountered during the examination. Unfortunately, these artifacts cannot all be discussed in this course.
Two important artifacts are explained here: acoustic shadow and posterior sound transmission. Even though these are artifacts, they are valuable in practice. 

Acoustic shadow

Acoustic shadowing is caused by two different phenomena, total reflection or absorption. Total reflection occurs on the boundary surface between gas/tissue because of the large difference in density between gas and tissue. Total absorption occurs when the sound waves are absorbed by calcareous structures (= including stones, bone). Sound waves are (virtually) all reflected/absorbed; no sound waves reach the area behind these structures, making this part of the ultrasound image entirely anechogenic (= black). This is termed acoustic shadow (fig. 16).

Figure 16. Acoustic shadowing caused by reflection by intestinal gas.

Acoustic shadowing is important in detecting disorders including tendon calcifications, stones or free air. The artifact is also used to differentiate solid and calcified masses, e.g. gallbladder polyp (fig. 17) from bile stones.

Figure 17. Acoustic shadowing by a bile stone caused by absorption of sound waves by the calcareous stone.

Posterior sound transmission 

In order to differentiate a cyst from a solid lesion, two artifacts are use: posterior wall amplification and increased sound transmission.  These phenomena occur when sound waves move through an anechogenic structure, usually a cyst. The sound wave loses little energy as it passes through the fluid in the cyst. That is why there is more energy left in the sound wave in the posterior wall and behind the structure than at the same level in the surrounding area (note: the surrounding tissue is more solid). More energy will therefore be left to reflect to the transducer. This results in a echogenic posterior wall and echogenic area behind the cyst (fig. 18).

Figure 18. Renal cyst with posterior wall amplification and increased sound transmission.

Posterior sound transmission is a good tool to differentiate a cyst from a solid lesion (fig. 19).

Figure 19. An hepatic cyst with posterior wall enhancement and sound transmission versus solid hepatic lesion.  Note the cyst is anechogenic, as opposed to the echogenic solid liver lesion (the solid lesion was shown to be an hemangioma on a CT scan).

Sources

  • B. Block. Abdominal Ultrasound: Step by Step (2004).
  • W.D. Middleton et al. The Requisites – Ultrasound (2004).
  • J. Bates. Abdominal Ultrasound: How, why and when (2011).

Authors

Text:

drs. F.Y. Jiang (resident radiology LUMC)
dr. R. van den Boom (abdominal radiologist & education coordinator LUMC)

Image editing:

drs. A. van der Plas (resident radiology LUMC)

20/02/2016 (translated 11/09/2016)