Flashcards in Module 5 questions Deck (11)
Describe the harmonic imaging mode and indicate the advantages compared to conventional B-mode imaging. In your answer discuss the problems with bandwidth and how this is overcome using inversion harmonics.
Harmonic imaging allows the use of echo detection at a higher frequency (exactly twice the normal frequency) with associated better spatial resolution than conventional B-mode imaging. It also allows slightly deeper imaging than normal B-mode (at the same doubled frequency) as the attenuation of the beam is less – you can sort of think of the harmonic imaging done with the incident beam at the transducer frequency (lower) and the echoes at twice this frequency. The main problem with standard harmonic imaging is that there is likely to be an overlap in the bandwidth of the outgoing ultrasound pulses and the bandwidth of the returning second harmonic echo signals. This is a problem as the circuitry in standard harmonic imaging uses frequency selection to separate the echo signal components frequency components. This is overcome using inversion harmonics where two pulses are produced for each line-of-sight; the second pulse is an inverted version of the first. In this case when the two echo signals from a specific depth are added the first harmonic components exactly subtract, while the second harmonic components add – thus separating the first and second harmonic components in the. The disadvantage here is the doubling of the frame time (2 pulses per scan line instead of one).
What are contrast agents in diagnostic imaging, and how is the contrast effect enhanced using harmonic imaging?
Contrast agents are usually encapsulated micro-bubbles of air about 5 μm in size. When present in tissue they provide vast numbers of scattering features due to their small size with a significant acoustic impedance mis-match (air/tissue). This produces a higher echo signal strength from that region. The micro-bubbles also interact with the incident ultrasound pressure pulse in a non-linear manner; the fractional change in bubble volume is different for the same fractional pressure change in compression and in rarefaction. This gives rise to an added second harmonic component to the echo signal. For this reason contrast agents are often used in harmonic imaging.
What is meant by the following: streaming, and cavitation? How do these arise and what is the potential for these effects to do harm?
A potentially hazardous effect of the ultrasound pulse pressure wave is a mechanical effect where the atoms and molecules are literally ‘rattled’ around by the passing pressure pulse. This is the mechanical effect, of which there are two main components:
Streaming – where the tissue (usually fluids) are physically moved along by the pressure pulse (and can be imaged in some instances using Doppler imaging). For fluid close to solid boundaries this force can set up large velocity changes that causes rapid fluid movement (micro-streaming) and can result in the fragmentation of molecules.
Cavitation – where rarefaction areas are rapidly ‘filled in’ by local molecules. In extreme cases this causes implosion and tissue damage. These oscillating micro-bubbles can lead to large forces that can damage molecules, and also lead to micro-streaming. If the gas bubbles grow rapidly and suddenly implode during a rarefaction cycle of the ultrasound pressure wave. This implosion causes a shock wave that can disrupt and damage molecules and even lead to the formation of highly reactive chemical radicals. Inertial cavitation can also cause haemorrhaging.
What does ALARA mean in practice? How would you answer a patient who asks: “Is this scanning procedure safe?”
ALARA stand for As Low As Reasonably Achievable. In answering such a question from a patient you should comment on:
• Your training
• Use of low power setting and high gain (if required)
• Limited time when scanning, especially in spectral Doppler and M-mode (where a single line-of-sight is used)
• Guided by the ALARA principle
• Understanding of bio-effects (TI and MI are now indicated on display screen)
• Studies on bio-hazards known (in a range of circumstances)
• Overall you can assure the patient your scanning procedure is safe
How would you go about using a tissue equivalent phantom to establish the accuracy for lateral measurements? If the true value is 6.0 cm and the measured value is 6.3 cm, what is the measurement error as a percentage? Is this acceptable? Justify your response.
Tissue equivalent phantoms contain regularly spaced fibres supported in a tissue equivalent matrix. These fibres are arranged in both vertical (axial) and lateral (horizontal) directions and are usually spaced 1 cm apart. In a test for lateral accuracy you would image the fibres and measure the distance (using crosshairs on the display) between two well-spaced (not adjacent) lateral fibres and compare this measured value with the known value. In the situation provided in the problem here, the measurement error, as a percentage, is given by: percentage error = 100 × (Measured value – True value)/(True value), so: Error% = 100 × (6.3 – 6.0)/6.0 = 5%. As a general guide, distance measurements from a tissue equivalent phantom should be accurate to within 2%, so the above value is unacceptable.
Indicate how each of the listed setting changes will affect the contrast resolution. Increasing the FOV. Increasing transducer frequency.
Increasing the transducer frequency will improve spatial resolution which will improve contrast resolution.
Increasing the FOV will reduce the size of the displayed image, degrading the perceived contrast resolution of small structures, as the human eye is less capable of perceiving small changes in contrast for smaller structures.
State the essential difference between normal harmonic imaging and inversion harmonic imaging.
Harmonc imaging uses 1 pulse down each line-of-sight and processes the echo data in the frequency domain. Inverse harmonic imaging sends 2 pulses down each line-of-sight (one inverted with respect to the other) and processes the echo data in the time domain. Inverse harmonic imaging has a slower frame rate.
Briefly explain the principles of writing multiple lines. What are the benefits of using this technique?
Array transducers use multiple elements to detect the echoes from each transmitted pulse of ultrasound. If these are added together with different time delays the echoes can be generated along different vectors within the transmitted beam area. Increased processing capability allows these vectors to be generated simultaneously, so up to five lines of echo information can be generated from a single transmit pulse. This increases the line density and improves the lateral resolution without degrading the temporal resolution.
When an ultrasound pulse is transmitted through tissue, harmonic frequencies are generated as a result of non-linear effects. Explain what this means.
Sound wave velocity depends on the density of the propagating medium, which means that as a sound wave travels through tissue, the velocity is variable, as the pressure associated with the sound wave introduces variation in the tissue density, and hence in the sound wave velocity. Specifically, the high-pressure crest of the wave travels faster, and the lower pressure trough travels slower. This results in distortion of the waveform shape, introducing additional frequency components that are exact multiples of the original frequency, more commonly referred to as harmonic frequencies.
Provide definitions of QA and QC in the specific context of diagnostic ultrasound scanning.
Quality control is the term used to describe the overall set of tests and measurements that are undertaken to compare the performance of a system with that of an ideal one. Quality control is about understanding the way a system is used and what needs to be measured, and how to go about doing those measurements.
Quality assurance is the overarching process that defines what needs to be checked and what measurements need to be taken to establish that a system is working with specifications. This process is required to provide the justification and evidence that the system is operating within acceptable tolerances.