In Service US Flashcards
Does harmonic imaging improve lateral or axial resolution?
increased lateral resolution and reduction of side lobe artifacts and reverberation artifacts caused by tissues close to the transducer. Harmonic imaging utilizes harmonic frequencies generated by the interaction of the incident ultrasound beam with the patient’s tissues. The wave distortions generating harmonic frequencies increase with increasing depth and are localized within the central part of the beam. The transducer sends a lower frequency pulse, but only receives the 1st order harmonic (double the incident frequency) echoes. This leads to increased lateral resolution and reduction of side lobe artifacts and reverberation artifacts caused by tissues close to the transducer.
How is acoustic impedance defined?
Acoustic impedance is defined as the product of the density (in kg/m^3) and speed of sound (in m/s) in the tissue.
The ACR recommends quality control on US transducers be performed how frequently:
quarterly
When does reverberation artifact occur?
When the US signal reflects repeatedly between highly reflective interfaces. It has horizontally positioned linear echoes that are spaced evenly.
What does the focal point on the US machine represent?
Depth of highest lateral resolution
Lateral resolution (resolution perpendicular to the ultrasound beam) is determined by the beam diameter. For unfocused or single element transducers, beam diameter decreases with increasing depth in the near field, being the smallest at the junction between near and far field (about 1.2 the transducer diameter or the effective transducer diameter) and increases with increasing depth in the far field. Lateral resolution can be changed by not activating all transducer elements at one time, decreasing the effective transducer diameter. In addition, with phased array transducers, transmit and receive focusing allows to maximize lateral resolution. The focal zone is indicated by a little triangle at the side of the image and can be changed by the user. Elevational resolution is the slice thickness and depends on the transducer element heights.
What does the damping block on the nonpatient side of the piezo crystal do?
The damping block on the non-patient side of the piezo crystal absorbs backwards directed ultrasound waves. More dampening results in a larger bandwidth of the ultrasound pulse but decreased pulse duration.
What is beam width artifact?
Beam-width artifact refers to the lateral blurring of a point target that occurs as echoes from the same target are insonated at adjacent beam positions
Beam width varies with depth and is narrowest at the focal zone. Beam-width artifact is a manifestation of lateral resolution, which refers to the ability to discriminate two closely spaced points at the same depth within the imaging plane as distinct.
US artifact Radiographics article
How can beam width artifact be reduced?
- Dynamic receive focusing
- multiple transmit focal zones (FZs), FZ placement
- Increasing frequency
- local speed of sound selection
reverberation artifact
