Ultrasound

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.

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,

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.

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). 

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.

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