Terminology Flashcards
What is axial resolution?
Axial resolution in ultrasound refers to the ability to discern two separate objects that are longitudinally adjacent to each other in the ultrasound image.
What is Lateral resolution?
Lateral resolution in ultrasound refers to the ability to discern two separate objects that are adjacent to each other.
What is GAIN and how it affects the appearance of the image?
Alters the amplification of the overall received signal. Increasing gain boosts the signals and makes the overall image brighter. Increasing the gain too much, inherent noise within the system will be amplified leading to poor image quality.
What is TGC and how it affects the appearance of the image?
Time gain control/depth gain control/swept gain: Alters the gain at a specific depth range. Compensates for variation in attenuation.
What is DEPTH and how it affects the appearance of the image?
Depth: Alters the depth of the image. Greater depths require
more time, resulting in a decreased frame rate.
Controls the depth on the image, optimal depth depends on the beam penetration, which is determined by transducer frequency.
What is SECTOR WIDTH and how it affects the appearance of the image?
Sector width: refers to the sector angle, it’s important in determining the frame rate at a given angle. Narrow sector width increase line density and more scan lines over the same area thus improving better detail and increase image resolution (lateral resolution). Fewer scan lines to process hence increases frame rate. Increase temporal resolution.
What is POWER and how it affects the appearance of the image?
Power can be defined as increasing the output power to the transducer produces high intensity ultrasound pulses. This increases the amplitude of the electrical signal applied to the transducer, which has the effect of making returning echo signals from all reflectors appear brighter. The disadvantage of increasing the power is that acoustic exposure of the patient increases.
What is DYNAMIC RANGE and how it affects the appearance of the image?
This allow the range of echoes or shades of gray displayed on the screen to be decreased. This will remove low-level echoes form the display and results in an image with more contrast.
What is HARMONICS and how it affects the appearance of the image?
Harmonic imaging is a technique to reduce haze or scatter, this will produce a cleaner image with higher contrast resolution.
What is COMPOUND IMAGING and how it affects the appearance of the image?
Compounding: This technique combines electronic beam steering with conventional linear array technology to produce real-time imaging acquired from different view angles. Between 3 and 9 sector images are rapidly acquired and combined to produce a compound real-time image. Compound imaging improves image quality by reducing speckle, clutter and acoustic artefacts. Gives better definition on boundaries in structures. with the improve contrast resolution it is useful in the application of breast, peripheral blood vessels, and muscoskeletal imaging.
What is ZOOM and how it affects the appearance of the image?
Zoom/Res: Magnifies an area of interest in the screen. Two forms of zoom, read zoom and write zoom. Read zoom magnifies the image. Write zoom increases ultrasound information content within the image, i.e. improves image resolution by increasing scan line density and the number of pixels per square centimetre.
What is DOPPLER GAIN and how it affects the appearance of the image?
Gain: Determines the sensitivity of the system to the flow. Doppler gain is adjusted by increasing the gain until random noise appears in the image ‘background speckling’ and then lowering the gain until only a very few noise pixels are present. By lowering the gain, noise, blooming and motion artefacts are reduced but may result in only the centre of the large vessel being filled up, mimicking wall swelling, as slow flow signals alongside the vessel wall will go undetected. Too high gain settings result in random noise or blooming where colour pixels may cover the artery walls.
What is ANGLE CORRECTION CURSOR and how it affects the appearance of the image?
Angle correction: Tells the scanner the direction of flow relative to the ultrasound beam so that velocity measurements can be made.
Doppler angle correction refers to an imaging post-processing method used to adjust for the effects of insonation angle on the Doppler shift.
Measurement of flow velocity with Doppler imaging is dependent on the angle between the ultrasound beam and the target (insonation angle), with the maximum and true velocity achieved at 0 degrees (parallel to the target). In most clinical scenarios, an insonation angle of 0 degrees is impractical and angle correction can still be applied to achieve an accurate velocity measurement 1.
Angle correction is considered accurate for diagnostic purposes at insonation angles less than 60 degrees 1.
At angles above 60 degrees, an error of up to 20-30% in calculated velocities can occur 2. If serially evaluating flow velocities in a vessel, it is recommended to maintain a similar Doppler angle between studies 3.
What is SCALE/PRF and how it affects the appearance of the image?
Pulse Repeition Frequency (PRF) controls the rate at which pulse of sound waves are produced and transmitted.
Alters the rate at which pulses are transmitted.
This affects sensitivity to low velocities and aliasing.
Adjusting the Depth, PRF, Transducer Frequency, Doppler angle, Baseline and CW Doppler can be adjusted to prevent aliasing.
What is Acoustic Impedance
Acoustic Impedance measures how much resistance the ultrasound beam when it passes through tissue. Tissue density x Speed of sound. Structures with greater difference in acoustic impedance produce stronger ‘echoes’ or reflected waves than two similar substances. For example, gallbladder and gallstone.
What is Acoustic Impedance Mismatch?
The difference in acoustic impedance between two structure is called acoustic impedance mismatch. It is responsible for the ultrasound waves that are reflected back towards the probe and being used to produce the ultrasound image. If there is a large acoustic mismatch, e.g. bone and muscle, or soft tissue and air, then a large portion of energy is reflected back. This results in a strong echo, which produce a bright image in the ultrasound display. However, very little energy are transmitted across the interface and any echoes produce beyond the image do not have enough energy to produce an image.
If the acoustic impedance mismatch is small, e.g. between soft tissues, then a small portion of the energy (1% or less) is reflected back. The rest of the energy is transmitted across the interfaces deep within the subject.
From the above it can be seen that where there is a large acoustic impedance mismatch, ultrasound will not produce a useful image beyond that interface. Therefore it is not practical to use ultrasound to produce images soft tissue subjects which contains gas or bone. However, ultrasound imaging is very good at discriminating between substances with small differences in acoustic impedance and is therefore excellent at differentiating between different types of soft tissues.
What is Speed of Sound/Propagation velocity?
Sounds travels at different speed through different medium. This is known as velocity propagation. The Propagation velocity is related to frequency and wavelength. Clinical ultrasound is transmitted through tissues of the body at an average velocity of 1540 m/s.
The propagation velocity of sound waves through tissues is affected by density and elasticity of the medium through which it is travelling. Higher propagation velocities are seen with tissues that have increased stiffness and density, i.e. Bone 3000 to 5000 m/s.
Propagation velocity is inversely proportional to density and direct proportional to stiffness. In other words, the less density the tissue the slower the propagation velocity it is, and the stiffer the tissue the higher the propagation velocity it is. i.e. Propagation is speed is slowest at air but fastest with bone.
What is ultrasound frequency?
Frequency is the number of cycles of acoustic waves per second. The unit of frequency is hertz (Hz). One cycle per second is 1Hz. Diagnostic ultrasound has a frequency 1 MHz to 20 Mhz.
Frequency is closely related to period and wavelength.
Period is the time required for one complete wave to pass.
Wavelength is the distance travelled by one complete wave.
Frequency is inversely proportional to period and wavelength. The shorter the period, the higher the frequency; the shorter the wavelength, the higher the frequency.
Higher frequency results in greater image resolution but poor penetration.
Lower Frequency results in good penetration but less detail seen.
What is ECHO-LOCATION/pulse echo principle?
It’s the measures of time between the transmission of the transmit pulse and the reception of a given echo, the ultrasound machine can calculate the distance between the probe and structure that caused that echo.
What is SPATIAL PULSE LENGTH?
Length of the ultrasound pulse is know as spatial pulse length. A shorter SPL improves axial resolution. SPL can be reduce by heavier damping or use of a high frequency transducer.
