**Abstract**

Ultrasound techniques have become a gold standard, both as a complement to conventional diagnostic methodologies and as a technique of choice. This has led to the need for quality control procedures, as happened in other sectors of diagnostic imaging. The aim of this chapter is to propose a series of physical parameters related to quality, both in B-mode and in Doppler velocimetry that could be measured with the use of commercial or "homemade" phantoms, following a protocol that considers both innovative proposals and indications published in the literature. In order to do this the different parameters are described and their physical meaning is discussed. Tests on different equipment are performed to evaluate the robustness of the protocol and the chosen parameters. The main results obtained with high clinical significance are presented. This allows both the acceptability of the equipment from a clinical ultrasound point of view and the consistency of diagnostic performance over time.

**Keywords:** ultrasound, quality control, patient protection, optimization, radiological imaging

#### **1. Introduction**

Ultrasound is a very popular diagnostic imaging technique. The technology has been known since the eighteenth century, and its use in medicine was first suggested by Austrian physician Karl Theodore Dussik in 1941. Usually, however, obstetrician Ian Donald and engineer Tom Brown are cited as pioneers of ultrasound, as in the 1950s they developed a prototype of an ultrasound device, called ultrasound or sonography scanner, to be used for medical purposes, mainly to identify any fetal malformations.

Ultrasound has spread throughout the world since the 1970s. Unlike methods such as X-rays, it does not use ionising radiation but rather creates an image of the inside of the body using high-frequency mechanical waves, which cannot be picked up by our ears: ultrasound. The sounds that we can normally hear have a frequency ranging from 20 to 20,000 Hz, whilst ultrasound for medical diagnostics usually has frequencies from 1 to 20 MHz.

To carry out an ultrasound scan, the sonographer uses a small probe: this is held in hand, contains many transducers (order of magnitude 101 , 103 ) and is placed on the

skin in correspondence with the volume to be examined. This device emits highfrequency sound impulses into the body, and acts both as a transmitter and a receiver, picking up the impulses reflected by the organ or tissue to be examined. More specifically, every time the sound waves encounter a separation surface between different types of tissue (i.e. acoustic impedance), part of them is reflected backwards and is picked up by the transducer; the unreflected part of the wave continues its travel.

To "seal" the space between the probe and the skin, and thus ensure better propagation of the mechanical waves, a dense water-based gel is spread on the skin immediately before the exam.

Each probe contains numerous piezoelectric crystals, i.e. ceramic or other materials that vibrate in response to the passage of electricity, producing ultrasonic waves. Vice versa, when the probe captures the waves reflected from the examined volume (echoes), the opposite happens: the transducers convert the pressure of the waves into current, and this current forms a signal which is transformed into an image by the sonographic scanner.

The instrumentation is actually able to calculate the position (depth) of the different tissues starting from the time it takes for the waves to return to the probe (being the speed of sound through the body's tissues constant, 1540 m/s). Thanks to this information the sonographic scanner is able to reconstruct a digital tomographic image of the volume being analysed obtained by merging the several lines representing the amplitude of the single line waves received (**Figure 1**, real-time brightness mode, B-mode). Today, with this technology, we have the opportunity to view images in great detail [1–5].

The frequency of the sound waves used varies depending on the type of analysis. For example, for superficial structures such as the thyroid, muscles, tendons, ligaments and breast glands, higher frequency ultrasounds with high resolution but low penetration are used (7–18 MHz), whilst to examine deeper organs such as the liver, kidneys and the heart, or to visualise the inside of the abdomen, a probe that produces lower frequency waves with low resolution, high penetration is more suitable (1–6 MHz).

**Figure 1.** *From ultrasound reflected echoes to pixels of a line of the image.*

#### *Perspective Chapter: Quality Assurance in Diagnostic Ultrasound DOI: http://dx.doi.org/10.5772/intechopen.114115*

Ultrasound is also used to monitor the flow in the blood vessels, through an analysis method called Doppler ultrasound or echo-Doppler. The underlying physical principle is the Doppler effect: when the receiver and the source are moving relative to each other, the sound waves perceived by the former have a different frequency than that of the sounds emitted by the latter.

In particular, if the wave received by the probe has a higher frequency than the one emitted, the receiver and source are getting closer, whilst if they are moving away, the received frequency will be lower. Doppler ultrasound is widely used in the analysis of blood flow and can provide information regarding the speed and resistance to blood flow, allowing the highlighting of vessel narrowing (stenosis) or enlargement (aneurism), increasing (narrowing, stenosis) or slowdowns (enlargement, aneurism) of the regular flow velocity.

This information can be displayed either through a velocity spectrum referring to a sampled volume (**Figure 2**, spectral Doppler) or by colouring the approaching and receding parts of the flow differently in the panoramic image (typically the former red and the latter blue), (**Figure 3**, colour Doppler).

The use of ultrasound techniques has become increasingly widespread in recent years, both as a complement to conventional diagnostic methodologies and as a technique of choice. This has led to an ever-increasing diffusion of ultrasound equipment, with the consequent need to apply quality control procedures on the performance of these instruments by the radiologist and medical physicist responsible of the scanner, as has happened in other sectors of diagnostic imaging.

However, it is necessary to point out an important and peculiar characteristic of ultrasound techniques: the diagnostic performance is never linked only to the characteristics of the equipment but is strongly influenced by the operator. The large number of variables involved, the numerous possibilities for different machine setups, and the direct interaction of the sonographer-probe-patient determine the impossibility

#### **Figure 2.**

*Doppler velocity spectrum as a function of time referring to a sampled volume in a vessel. Pixel brightness = blood mass. Positive speed = moving towards the probe. Negative speed = moving away the probe.*

#### **Figure 3.**

*Colour Doppler image. Red pixel = moving towards the probe. Blue pixel = moving away the probe. Brightness = higher speed.*

of separating the quality component linked to the machine from the one linked to the operator. Therefore, the results of quality control must be interpreted in this light.

A complete quality control programme allows to:


It is therefore necessary to draft a complete quality control protocol which includes, in addition to the definition of the parameters being evaluated, the identification of the limits of acceptability of these parameters. The extreme variability of the setup configurations, the different technological quality of the scanners (and cost) and the conditions of use have not allowed, at present, to agree in setting these values. We will refer to the limits proposed by the AAPM (American Association of Physicists in Medicine) working group protocol, keeping in mind that the approach commonly accepted in centres with more experience in the field of quality controls on ultrasound equipment is to set their own acceptability limits relating to the individual equipment in every foreseen configuration.

Quality checks on ultrasound equipment can be classified into clinical checks and physical checks.

Clinical checks are based on a subjective evaluation carried out by the sonographer/radiologist on a series of clinical images, which can therefore also be performed during the examination.

Physical checks are based on assessments carried out on physical parameters that can be obtained using test objects and/or phantoms (**Figures 4**–**8**).

This last type of control can also be divided into two further categories: a first level, which usually produces qualitative indications, and a second, which provides quantitative information. In the first case, we proceed only with the visual investigation of the image obtained, whilst in the second level, it is necessary to have access to the numerical content of the image, downloading it to obtain numerical values of parameters. This second type of approach is advisable as it minimises errors and is also much simpler and quicker to perform. There is also software on the market that allows you to completely automate the management of secondlevel controls. The parameters subject to physical quality control will be listed below: each of these parameters can therefore be evaluated with a first or secondlevel procedure.

Clinical quality control consists of a subjective evaluation by the operator of a clinical image. The resulting judgement is obtained through the qualitative evaluation of some parameters, described below, applicable to both real-time two-dimensional ultrasound (B-mode) and Doppler velocimetry.

In this scenario and considering that sonography exams performed in 2022 as stated by the World Health Organisation were 2,8 billion worldwide (vs 3,6 billion exams using X-rays), a quality assurance programme becomes mandatory also with this technology. A set of clinical and physical parameters to be used to understand their meaning and weight has to be proposed, a test of them performed in order to assess the feasibility of their use, and some comments about acceptance limits, so different in their setting from diagnostic X-ray, discussed.

**Figure 4.** *B-mode quality control phantom.*

#### **Figure 5.** *B-mode quality control phantom image.*

**Figure 6.** *Doppler quality control phantom.*

**Figure 7.** *Doppler velocity spectrum of a Doppler phantom with laminar flow directed towards the probe.*

**Figure 8.** *Colour Doppler image of a Doppler phantom with laminar flow directed towards the probe.*
