Ultrasound Artifacts and Resolution Techniques

An artifact is an image feature that does not accurately represent the actual anatomy.
Typically arises from various issues in the image acquisition process.

The machine combines information from multiple reflectors into a single object, leading to the creation of artifacts.
Core Axial Resolution
- Usually observed with low-frequency transducers.
- To counteract start effects, sonographers should utilize the highest frequency necessary for adequate penetration into the anatomical depth.

Lateral Resolution
- Defined as equal to the width of the sound beam.
Elevational Resolution
- Related to the thickness of the ultrasound beam.
- When the beam thickness is excessive, other echo information can interfere with the slice being imaged, leading to artifacts.
- Known as:
- Slice Thickness Artifact
- Partial Volume Artifact
- Section Thickness Artifact
Example: The gallbladder may show echoes from the duodenum if the beam width is too thick.

Slice Thickness Artifacts manifest prominently in areas where echoes are not expected but are present due to beam thickness.
One-Dimensional Array Transducers
- Suggested to mitigate slice thickness artifacts by representing echoes from adjacent structures more accurately.

Artifacts where anatomy appears incorrectly positioned in the image.
Causes of Position Artifacts include:
- Changes in sound direction due to refraction.
- Echoes returning directly to the transducer, producing:
- Reverberation Artifact
- Ring Down Artifact
- Comet Tail Artifact
- Mirror Artifact
- Multipath Artifact
Sound energy can interact with reflectors not aligned with the primary beam:
- Gradients and side lobes contribute to inaccuracies.
- Example: The aorta's position may appear altered due to inaccurate routing by the machine.

Mirror Artifact
- Occurs when sound reflects off an oblique angle specular reflector creating a duplicate image deeper than its actual position.
Comet Tail Artifact
- Often appears in the presence of gas or other small structures, simulating echoes that return from the surrounding anatomy incorrectly.

Results from sound bouncing between multiple strong reflectors.
Characteristics of Reverberation
- Equally spaced reflectors deeper in the image.
- Strongly observed in needles or mechanical valves.
- Example: Needle used in biopsy showing step-like spacing in the image.

Caused when the sound’s interaction with tissues affects its attenuation significantly.
Types of Attenuation Artifacts:
- Shadowing
- When a strong attenuating structure blocks sound from producing echoes from behind it, leading to anechoic areas.
- Edge Shadow
- Occurs from refraction and divergence at curved interfaces, resulting in hypoechoic areas behind the structure.
Enhancement
- Excess sound energy leads to brighter echoes behind less attenuating structures, aiding in the diagnosis of fluid-filled cysts versus solid masses.
Focal Enhancement
- Increased echogenicity at focal point due to the strongest part of the sound beam.

Speckle Artifact
- A grainy appearance due to interference in echoes, mainly observable in the near field.
- Techniques available to reduce speckle including using higher frequency transducers.
Electronic Interference
- May arise from machinery or components operating on similar frequencies; utilizing different frequency transducers can help mitigate this.

Use of Spatial Compounding and Frequency Compounding to improve image clarity by combining images from different angles or frequencies.
Regular adjustments of gain, depth, and focus are crucial for optimal imaging.

Results from incorrect assumptions about tissue propagation speeds (typically set to $1540 ext{ m/s}$ ). Faster mediums may result in shallower placements, while slower mediums can cause deeper placements, distorting anatomical features.
Example: If the diaphragm appears discontinuous in an image, it may be due to variations in propagation speed through adjacent tissues.

Artifacts play an essential role in ultrasound imaging, understanding them is vital for accurate interpretation. Proper techniques, depth settings, and imaging adjustments can mitigate their effects significantly.