chapter 22: quality assurance

quality assurance

the routine, periodic evaluation of an ultrasound system to guarantee optimal image quality

how often do quality assurance evaluations have to be performed?

periodically and routinely

what are the 4 requirements for a quality assurance program?

• assessment of system components

• repairs

• preventative maintenance

• record keeping

what are the 5 goals of a quality assurance program?

• guarantee proper operation of the system

• detect gradual changes

• minimize downtime

• reduce the number of non-diagnostic exams

• reduce the number of repeat scans

what 3 devices enable the sonographer to perform quality assurance?

• tissue equivalent phantom

• doppler phantom

• beam profile/slice thickness

what are the proper methods for quality assurance programs?

• test under known, defined conditions

• use constant instrument settings

• use a phantom with measurable characteristics

• image in an identical environment

objective standards

• completely unbiased

• not affected by an individual’s previous experience, preference or taste

• factual, repeatable and able to be counted

subjective standards

• influenced by an individual’s experience or beliefs

• often cannot be verified using concrete facts and figures

what type of standards should quality assurance be based on?

objective standards

tissue equivalent phantom

has ultrasonic features similar to soft tissue

what are tissue equivalent phantoms used to evaluate?

used to evaluate gray scale and tissue texture, and multi-focus and adjustable-focus phased array transducers

tissue equivalent phantoms are similar to soft tissue in the following ways:

• speed of sound is 1,540 m/s

• attenuation

• scattering characteristics

• echogenicity

what are tissue-equivalent phantoms made out of?

• graphite-filled aqueous gels or urethane rubber materials

• graphite particles act as ultrasound scatterers in materials

doppler phantom

• evaluate doppler systems

• include a circulation pump which propels fluid through vessels embedded in a tissue equivalent phantom

what do doppler phantoms assess?

assesses the characteristics of all doppler modalities included pulsed, continuous wave, color and power mode

types of doppler phantoms:

• vibrating string

• moving belt

• flow phantom

slice thickness phantom

• contains a diffuse scattering plane that is at an angle to the incident sound beam

• mimics soft tissue

what do slice thickness phantoms assess?

the slice thickness and its effect upon image accuracy

sensitivity

the ability of a system to display low-level echoes

what are the 2 forms of sensitivity?

• normal

• maximum

normal sensitivity

• these settings are those at which all the pins, solid masses and cystic structures in the test phantom are accurately displayed

• output power, TGC and amplification are adjusted to establish this type of sensitivity

at what type of sensitivity are all subsequent quality assurance and performance measurements made?

at the normal sensitivity

maximum sensitivity

evaluated with the output power and amplification of the system set to the maximum practical levels

maximum visualization depth is used to assess:

sensitivity, and should not differ from one routine evaluation to the next

dead zone

• the region close to the transducer where images are inaccurate

• extends from the transducer to the shallowest depth from which meaningful reflections appear

• info within this region is unreliable and may not be used in the diagnostic setting

the dead zone results from:

the transducer ringing and the time it takes the system to switch from the transmit to the receive mode

what type of transducers have a thinner dead zone?

higher frequency transducers

an increasingly deeper dead zone may indicate:

• cracked crystal

• detached backing material

• longer pulse duration

registration accuracy

the ability of the system to place reflections in proper positions while imaging from different orientations

range accuracy

• AKA 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

digital calipers should be checked in both the:

vertical and horizontal directions

focal zone

• the depth at which the intensity is the highest and the beam is the narrowest

• the focus of phased array transducers must be carefully evaluated since they are adjustable by the sonographer

axial resolution

the smallest distance at which two pins positioned parallel to the sound beam are displayed as two distinct echoes

axial resolution is evaluated by:

scanning sets of successively closer spaced pins within the phantom

lateral resolution

the minimum distance at which two side-by-side pins are displayed as two distinct images

what is another method to evaluate lateral resolution?

measure the width of reflections on the display that are created by points targets in the phantom

compensation operation or uniformity

• uniformity describes the system’s ability to display similar reflectors in the phantom

• with proper TGC, identical reflectors should have the same appearance on the monitor, regardless of their depth

mock cysts and solid masses

• the tissue equivalent phantom is used to evaluate the dimension, texture and filling of cysts

• the system should accurately display hollow structures as anechoic and solid structures and hyperechoic

display, hardcopy output and gray scale dynamic range

• adjusting the system’s output power and amplification should produce changes in the gray scale display

• it is important to compare the relationship between the image on the system’s screen with the output of all other display devices such as remote viewing stations