C HA P T E R

14Ultrasound Equipment Quality

Assurance

James A. Zagzebski and James Kofler

K E Y T E R M S

axial resolutiondepth of visualizationhorizontal distance measurementlateral resolution

phantomscan image uniformitysensitivityslice thickness

string testvertical distance measurement

O B J E C T I V E S

At the completion of this chapter the reader will be able to do the following:• Discuss the importance of quality assurance for

ultrasound equipment• Describe the various phantoms used in ultrasound

quality assurance• Identify the basic quality control tests for ultrasound

• Explain the importance of documentation of qualityassurance testing

• Describe the basic quality control testing for Dopplercolor flow equipment

O U T L I N E

Components of an UltrasoundQuality Assurance Program 257Quality Assurance and PreventiveMaintenance 257

Tissue-Mimicking Phantoms 257Tissue Properties Represented inPhantoms 257

Typical Quality AssurancePhantom Design 257

Cautions About PhantomDesiccation 258

Basic Quality Control Tests 259Visual Inspection 259Transducer Choice 259System Sensitivity 260

Photography and Gray-ScaleHard Copy 261Monitor Setup and RecordingDevices 261

Routine Quality Assurance ofImage Recording 261

Scan Image Uniformity 262Distance MeasurementAccuracy 263Vertical DistanceMeasurements 263

Horizontal DistanceMeasurements 264

Other Important InstrumentQuality Assurance Tasks 264

Documentation 264

Spatial Resolution Tests 264Axial Resolution 264Lateral Resolution 265Cautions About Resolution Testswith Discrete Targets 265

Other Test Objects andPhantoms 266Anechoic Voids 266Objects of VariousEchogenicity 266

Spherical Object Phantom 267Doppler Testing 267

String Test Objects 267Doppler Flow Phantoms 267

Electronic Probe Tests 268

In an imaging facility, quality assurance is a process car-ried out to ensure that equipment is operating consis-tently at its expected level of performance. Duringroutine scanning each sonographer is vigilant for equip-ment changes that can lead to suboptimal imaging and

might require service. Thus in some ways, ultrasoundequipment quality assurance is carried out every day,even when it is not identified as a process itself.

Quality assurance steps to be discussed here gobeyond judgments of scanner performance that are

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made during routine ultrasound imaging. They involveprospective actions to identify problem situations,even before obvious equipment malfunctions occur.Quality assurance testing provides confidence thatimage data such as distance measurements and area esti-mations are accurate and that the image is of the bestpossible quality from the imaging instrument.

COMPONENTS OF AN ULTRASOUNDQUALITY ASSURANCE PROGRAM

Quality Assurance and PreventiveMaintenance

Various approaches are used by ultrasound facilitieswhen setting up a quality assurance program for theirscanners. Sometimes these programs include both pre-ventive maintenance procedures performed by trainedequipment service personnel and in-house testing ofscanners with phantoms and test objects. Some facilitiesrely on only one of these measures. For preventive main-tenance, emphasis is usually given to invasive electronictesting of system components such as voltage measure-ments at test points inside the scanner. Sometimes pre-ventive maintenance also involves an assessment of theimaging capability by scanning a phantom.

In-house scanner quality assurance programs usuallyinvolve imaging phantoms or test objects and assessingthe results. In-house tests may be performed by sonogra-phers, physicians, medical physicists, clinical engineers,or equipment maintenance personnel. Detailed recom-mendations from professional organizations and expertsin ultrasound on establishing an in-house quality assur-ance program are available elsewhere (ACR UltrasoundAccreditation Program, 2001; Goodsitt et al., 1998;Zagzebski, 2000).

Tissue-Mimicking Phantoms

In-house scanner quality assurance tests most often areperformed with tissue-mimicking phantoms. In medicalultrasound a phantom is a device that mimics soft tissuesin its ultrasound transmission characteristics. Phantomsrepresent “constant patients,” and images can be takenat different times for close comparison. Image penetra-tion capabilities, for example, are readily evaluated forchanges over time when images of a phantom are avail-able for comparison. Phantoms also have targets inknown positions, so images can be compared closely withthe region that is scanned. Examples include simulatedcysts, echogenic structures, and thin “line targets.”

Tissue Properties Represented in Phantoms

Tissue characteristics mimicked in commercially avail-able phantoms are the speed of sound; ultrasonic atten-uation; and, to some degree, echogenicity (i.e., the

ultrasonic scattering level). Phantoms cannot exactlyreplicate the acoustic properties of soft tissues.This is partially due to the complexity and variabilityof tissues. Instead, phantom manufacturers constructthese objects to have acoustic properties that representthe average properties of many different tissues. Some-times the term tissue-equivalent is used when phantomsare described; however, this term should not be inter-preted literally because most phantom materials arenot acoustically equivalent to any specific tissue.

Typical Quality Assurance Phantom Design

An example of a general purpose ultrasound qualityassurance phantom is shown in Figure 14-1. Such phan-toms are examined with scanner settings that are similarto those used when patients are being scanned.The phantom images have gray-scale characteristics thatare analogous to characteristics of organs, although theactual structures are not anatomically represented.

Figure 14-1, B shows the internal structure of thisphantom. The tissue-mimicking material within thephantom consists of a water-based gelatin in whichmicroscopic particles are mixed uniformly throughoutthe volume (Burlew et al., 1980; Madsen et al., 1978).The speed of sound in this material is about 1540m/sec, the same speed assumed in the calibration ofultrasound instruments. The ultrasonic attenuation coef-ficient versus frequency is one of two values: either 0.5dB/cm per megahertz or 0.7 dB/cm per megahertz(Box 14-1). Some users prefer the lower-attenuatingmaterial because they find it easier to image objects inthe phantom. However, standards groups recommendthe higher attenuation because it challenges machinesmore thoroughly (Zagzebski, 2000).

Attenuation in the gel-graphite material in the phan-tom is proportional to the ultrasound frequency andmimics the behavior in tissues (Lu et al., 1999; Madsenet al., 1978; Maklad et al., 1984). Other types of mate-rials have been used in phantoms, but only water-basedgels laced with powder have both speed of sound andattenuation with tissuelike properties (Madsen et al.,1978; Zagzebski, 2000).

Small scatterers are distributed throughout the tissue-mimicking material; therefore, the phantoms appearechogenic when scanned with ultrasound imagingequipment (see Fig. 14-1, C). Many phantoms havesimulated “cysts,” which are low-attenuating, nonecho-genic cylinders. These should appear echo free onB-mode images and should exhibit distal echo enhance-ment. Some tissue phantoms provide additional imagecontrast by having simulated masses or test objects ofvarying echogenicity. Such objects are evident in Fig-ure 14-1, C.

Most quality assurance phantoms also contain dis-crete reflectors such as nylon-line targets to be usedmainly for evaluating the distance measurement

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accuracy of a scanner. Tests of the accuracy of distancemeasurements rely on the manufacturer of the phantomto have filled the device with a material with a soundpropagation speed of 1540 m/sec or at least closeenough to this speed that no appreciable errors are

introduced in calibrations. These phantoms also relyon the manufacturer having defined the reflectorpositions accurately. With the correct speed of sound(1540 m/sec) and precisely known distances betweenpointlike reflectors, it is easy to check the accuracyof distance measurements with calipers, as describedlater.

Phantoms often contain a column of reflectors, eachseparated by 1 or 2 cm, for vertical measurement accu-racy tests. One or more horizontal rows of reflectorsare used for assessing horizontal measurement accuracy.Additional sets of reflectors may be found for assessingthe axial resolution and the lateral resolution ofscanners.

Cautions About Phantom Desiccation

When a phantom made of water-based gels is used, lossof water (desiccation) may become a problem as thephantom ages. If this occurs, the speed of sound inthe phantom may have changed. A scanning surface thathas become concave is an indication of severe

A

B C

FIGURE 14-1 Example of a general-purpose quality assurance phantom. A, Phantom being imaged with an ultrasound scanner. B, Close-upof phantom, with diagram of interior contents. C, B-mode image of the phantom.

BOX 14-1 Tissue Attenuation Coefficients

Attenuation coefficients are normally specified in decibels per cen-timeter. To include the dependence of attenuation on frequency,phantom manufacturers divide the attenuation coefficient by thefrequency at which the measurement is done. This yields units ofdecibels per centimeter per megahertz. Strictly speaking, thisapproach should be used only when attenuation is directly propor-tional to the frequency, as we often assume for tissues. The valueof 0.7 dB/cm per megahertz is representative of the attenuationcoefficient in difficult-to-penetrate fatty liver.* The depth thatstructures can be visualized within tissue-mimicking material hav-ing this amount of attenuation more closely correlates with clinicalpenetration.

*Lu ZF, Lee FT, Zagzebski JA: Ultrasonic backscatter and attenuation indiffuse liver disease, Ultrasound Med Biol 25:1047, 1999.

258 CHAPTER 14 Ultrasound Equipment Quality Assurance