DMUT2020- quality assurance and bioeffects/safety

What is quality assurance?

routine, periodic evaluation of an ultrasound system to guarantee optimal image quality and ensure excellence in healthcare

What is quality control?

Part of a quality assurance program that deals with instrumentation and equipment

How are ultrasound systems tested?

Using a tissue equivalent phantom

What is a hydrophone?

A small transducer element on a hollow needle is used to measure the acoustic output

What material is used for a hydrophone? Why?

Polyvinylidene fluoride (PVDF) because of its wide bandwidth

List the imaging systems from lowest acoustic output to highest

B-mode (2D), M-mode, colour doppler, pulsed wave doppler

What is a tissue equivalent phantom made of?

Water-based gelatin with graphite and nylon-line targets throughout to mimic tissue with a propagation speed of 1540 m/s

Tissue equivalent phantom: System sensitivity

The maximum depth that an echo signal can be detected and clearly displayed (maximum depth of visualization)

Tissue equivalent phantom: Image uniformity

Uniform brightness throughout when the system's gain settings are set properly

Tissue equivalent phantom: Dead zone

A. The distance from the front face of the transducer to the first identifiable echo at the phantom interface, no clinical data can be collected from this region. If the depth is too large, corrective action has to be considered.

How can dead zone be prevented?

By using an acoustic standoff

Tissue equivalent phantom: Resolution

The minimum reflector separation between two closely spaced objects which can be imaged separately. Poor resolution= small structures will appear as one

Tissue equivalent phantom: Axial resolution

The minimum VERTICAL reflector separation which can be distinguished

Tissue equivalent phantom: Lateral resolution

Resolution perpendicular to the sound beam (across). Dependent on the beam width: increased beam width=reduced lateral resolution

Tissue equivalent phantom: Vertical measurement accuracy

Used to measure depth calibration accuracy. The distance between several (vertically placed) rods are measured and compared with the actual distance given by the manufacturer. Difference >1mm warrants correction

Tissue equivalent phantom: Horizontal measurement accuracy

The distance between several rods (lined perpendicular to the sound beam: horizontal) are measured and compared with the actual distance given by the manufacturer. Difference >2mm warrants correction

Are vertical or horizontal measurements usually more accurate and why?

Vertical because axial resolution is generally superior to lateral resolution

Tissue equivalent phantom: Focal lesion resolution

The focal zone is the region around the focal point where the intensity and lateral resolution are the greatest. Cysts and tumours with in the phantom represent realistic imaging tasks and can demonstrate the system's resolution capability.

Tissue equivalent phantom: contrast resolution

Ability to distinguish echoes of slightly different intensities (dB)

Test objects

Do not simulate tissue but do provide specific measurements of instrument performance

How is doppler testing performed?

A Doppler test object use a moving solid object (usually on a string). A Doppler flow phantom uses a flowing blood-like liquid to evaluate the Doppler instrumentation

What is a true test result?

When the test matches the gold standard

True positive vs. true negative

TP test is when the test matches the gold standard and is positive for disease. TN test is when the test matched the gold standard and is negative for disease

False positive vs. false negative

FP test is when the test does not match the gold standard and shows positive for disease. FN is when the test does not match the gold standard and shows negative for disease

What is sensitivity?

The ability to detect disease when it is actually present (shows how sensitive a test is to the presence of disease)

What is specificity?

The ability to determine that a test is normal (negative) when there is no disease (shows how specific a test is in separating normals from patients with disease)

What is a positive predictive value?

The likelihood that a positive test is actually a true positive

What is a negative predictive value?

The likelihood that a normal test is actually a true normal

What is diagnostic accuracy?

An overall measure of how frequently the test and gold standard are in agreement (percentage of how often the test results are correct)

What is prevalence?

The percentage of patient evaluated who have disease present according to the gold standard

What system adjustments affect the TI?

Output power, focus (number), M-mode, Doppler

What is TI?

Thermal index- the potential for tissue heating (relative probability that a system could induce thermal injury)

When are temperature increases considered significant?

More than 2 degrees celsius

What tissue has the highest absorption coefficient?

Bone

What does tissue temperature increase depend on?

Output characteristics and tissue properties

TIS vs TIB vs TIC

Thermal index for soft tissue- when the beam travels through soft tissue and not bone, bone- focus is at or near bone, cranial- scanning with transducer near bone

What do mechanical (non-thermal) mechanisms depends on?

Radiation force (force exerted by the sound beam), streaming (shear stress on cells) and cavitation (production and behaviour of bubbles in a liquid medium)

Stable cavitation

bubbles oscillate in size in response to pressure changes, but do not burst

Transient cavitation

Bubble oscillations become large -> bubbles collapse -> shock waves -> high temperatures -> tissue damage

What is MI?

The likelihood of cavitation-related adverse biological effects

What is the maximum MI for tissues with gas bodies?

<1.9

What contemporary feature carries potential for non-thermal bioeffects?

Contrast agents

What form of intensity needs to be considered most for bioeffects?

SPTA- spatial peak temporal average

For an unfocused beam, what is the maximum SPTA intensity? For a focused beam?

Below 100 mW/cm^2. Less than 1W/cm^2

According to AIUM statements, what MI value for tissue with gas-bodies does not have bioeffect? Tissue without gas-bodies?

With= less than 0.4. Without= less than 4.0

What is the maximum TI value for unlimited time scanning of OB, neonatal transcranial and neonatal spine?

0.7

For obstetric exams, when is TIS vs TIB monitored?

Before 10 weeks from LMP -> TIS

What is the maximum TI value for unlimited time scanning of all adult and neonatal (except the eye)?

1.5

What is ALARA?

As Low As Reasonably Achievable

ALARA principles

Keep exposure to minimum, keep acoustic output to minimum, use only when medically indicated, potential risks/benefits should be considered

Electrical hazards may be due to?

The transducer holds the most likely component for electrical harm as it has direct contact with the patient