Mind Map
Chapter 21
Chapter 22
Artifacts
Image characteristics
Definition: An error in imaging, altering the image to be less accurate.
Aneachoic- An extreme form of hypoechoic, meaning entirely without echos.
Isoechoic- Describes structures with equal echo brightness
Hypoechoic- Portions of an image that are not as bright as surrounding tissues
Homogeneous- A portion of tissue or an image that has similar echo characteristics throughout.
Hyperechoic- Portions of an image that are brighter than surrounding tissues.
Heterogeneous- A portion of tissue or an image that has differing echo characteristics throughout.
Basic assumptions of imaging systems
- Sound travels in soft tissue at exactly 1,540 m/s.
- Reflections arise only from structures positioned in the beam's main axis.
- Sound travels directly to a reflector and back
- The imaging plane is very thin.
- Sound travels in a straight line
- The strength of a reflection is related to the characteristics of the tissue creating the reflection.
Types of artifacts.
Reverberation- Multiple, equally spaced echos caused by the bouncing of the sound wave between two strong reflectors. Assumption 2 is invalid.
Comet tail- A solid hyperechoic line directed downward. Produced when two objects are spaced closely together and the sound beam bounces between them. Also known as ring down artifact. It is parallel to the sound beam's main axis and t appears as a single long hyperechoic echo. Assumption 2 is invalid.
Shadow- A hypoechoic or anechoic region from a highly attenuating structure. Assumption 6 is invalid.
Edge Shadow- A form of shadowing where a hypoechoic region extends from the edge of a curved reflector. Assumption 6 is invalid. Also called shadowing by refraction.
Enhancement- A hyperechoic region beneath tissues with abnormally low attenuation. Assumption 6 is invalid.
Focal enhancement- A special form of enhancement in which a side to side region of an image appears brighter than tissues at other depths. Assumption 6 is invalid. Also called focal banding.
Mirror Image- A second copy of a reflector. The artifact is deeper than the true reflector. Assumptions 1 and 2 are invalid.
Speed Error- Created when a sound wave propagates through a medium at a speed other than that of soft tissue. Assumption 3 is invalid. The artifact appears as a step-off, as if the structures are split. This is also known as a range error artifact.
Lobes
Lobe artifacts appear when sound energy is transmitted in a direction other than along the beam's main axis. Lobe artifacts degrade lateral resolution. Assumption 2 is invalid. Side lobes are created by mechanical transducer, while grating lobes are made by array transducers.
Reduced via sub dicing and apodization.
The lobes are a second copy of the true reflector. The artifcat and true reflector are side by side.
Refraction- Created when a sound pulses changes direction during transmission. Occurs when a sound waves trikes a boundary obliquely and the media on either side of the boundary have different propagation speeds. Assumption 1 is invalid
Slice Thickness artifact.- Also known as section thickness artifact or partial volume artifact. It causes reflections from the flaring of the imaging plane. Assumption 5 is invalid.
Multipath artifact- Created when sound pulses glance off a second structure on the way to or from the primary reflector. Assumption 2 is invalid.
Lateral resolution- The ability to differentiate objects that are side by side and perpendicular to the sound beam's main axis.
Axial resolution- The ability to differentiate objects that are parallel to the beam's main axis.
More artifacts
Temporal resolution artifact- The inability to precisely locate a moving structure at all times.
Spatial resolution artifact- The inability to tell the different of objects that are close together.
Noise- small amplitude echos that result from many sources. Harmonic imaging reduces noise.
Speckle- Noise that results from destructive and constructive interference.
Clutter- A form of noise that is the presence of false echo signals arising from locations outside of the main sound beam.
Quality Assurance- The routine periodic evaluation of an ultrasound system to guarantee optimal image quality.
Devices used for QA: Tissue equivalent phantom, Doppler phantom, and beam profile phantom
Tissue equivalent phantom- Used to test gray scale, tissue texture, multi-focus and adjustable focus
Doppler phantom- Tests Doppler via a vibrating string, moving belt, and the use of a flow phantom.
Slice Thickness Phantom-A phantom made to assess the slice thickness and elevational resolution.
Performance Measurements
Sensitivity- The ability of a system to display low-level echoes. It is evaluated in two forms: Normal and maximum.
Normal sensitivity- The setting where all structures in the phantom are displayed. This is the baseline for the subsequent measurements.
Maximum sensitivity- The output power and amplification of the system are set to maximum.
Dead zone- The zone for the image where the transducer is inaccurate. It is the most superficial area of the image. A standoff is used to create distance in order to take the image.
Registration accuracy- The ability of the system to place reflections in proper positions while imaging from different orientations.
Range accuracy- Also known as vertical depth calibration, describes the system's accuracy in placing reflectors at correct depths located parallel to the sound beam.
Horizontal calibration- The system's ability to place echoes in their correct position when the reflectors are perpendicular to the sound beam.
Focal zone- The depth where the intensity is the highest.
Compensation Operation- Also known as Uniformity, it describes the system's ability to display similar reflectors in the phantom with echoes of equal brightness.