Introduction to ultrasound

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Chapter 17 Introduction to ultrasound

Rita Phillips

KEY POINTS

Diagnostic ultrasound uses high frequency sound waves in the megahertz (MHz) range.

Ultrasound is a form of non-ionising radiation.

Sound waves are created by the piezoelectric effect.

The sound waves are transmitted into the body and reflected back in varying amounts from an anatomical interface and these reflected waves are detected to produce an image.

Modern ultrasound machines interpret the returning signals to generate images, which they send either to film or to a picture archiving communication system.

Ultrasound transducers have different frequencies and are usually in the range of 1–20 MHz.

The higher the frequency, the better the resolution, but the poorer the penetration.

Ultrasound is a cross-sectional study and therefore each organ can be examined in several planes.

Ultrasound is operator dependent and improper use of ultrasound techniques and equipment can lead to missed or misdiagnosis.

In order to aid diagnosis, a full clinical and medical history must be available.

Ultrasound studies include motion mode; spectral; colour and power Doppler; two-, three- and four-dimensional imaging.

Although ultrasound has been in use for several decades with no proven evidence of damaging effects, high levels of ultrasound energy can cause bio effects in tissue; therefore prudent use of ultrasound is recommended.

Ergonomics play an important part in minimising ultrasound work related disorders, especially affecting the upper limbs due to repeated movements, stretching and twisting.

Training, quality assurance and audits are all important factors for maintaining high quality patient-focussed ultrasound examinations.

INTRODUCTION

Ultrasound was originally developed during World War I to track submarines as SONAR technology (Sound, Navigation And Ranging). Ultrasound was first used medically in the 1950s, with very early applications in fetal biometry; nowadays, it is used in just about every field of medicine. Furthermore, it is also now practised by a wide variety of professionals, in a multidisciplinary setting.

SOUND WAVES

Sound is a wave that is created by vibrating objects and propagated through a medium from one location to another, via particle interaction (Fig. 17.1). These particles move in a direction parallel to the direction of the wave; that is, longitudinally. Each individual particle pushes on its neighbouring particle and propels it in a forwards direction whilst restoring its original position at the end of the interaction. This backwards and forwards motion of particles in the direction of the wave creates regions of high pressure within the medium, where the particles are compressed together (compressions), and also regions of low pressure, where the particles are spread apart (rarefactions). The wavelength of sound is the distance between two successive high pressure pulses or two successive low pressure pulses.

Figure 17.1 Sound waves.

The frequency of a wave refers to how often the particles of the medium vibrate when a wave passes through it and is measured as the number of complete back-and-forth vibrations (cycles) of a particle of the medium per unit of time. The hertz (Hz) is a unit for frequency where 1 Hz is equivalent to one cycle per second. Humans can hear sound with frequencies of 20 to 20 000 cycles per second (i.e. 20–20 000 Hz).

ULTRASOUND

Ultrasound is high frequency sound beyond the hearing of the human ear. The frequencies of ultrasound required for diagnostic medical imaging are in the range 1–20 MHz. These frequencies can be obtained by using piezoelectric materials (particularly crystals). When an electric current is applied and reversed across a slice of one of these materials, the material contracts or expands. So a rapidly alternating electric field can cause a crystal to vibrate. These vibrations are then passed through any adjacent materials, or into the air as a longitudinal wave is produced – a sound wave (Fig. 17.2).

Figure 17.2 Sound production from a piezoelectric transducer.

The piezoelectric effect also works in reverse. If the crystal is squeezed or stretched, an electric field is produced across it. So, if the ultrasound energy makes contact with the receiving crystal, it will cause the crystal to vibrate in and out and this will produce an alternating electric field. The resulting electrical signal can be amplified and processed in a number of ways. The piezoelectric effect occurs in a number of natural crystals, including quartz, but the most commonly used substance is a synthetic ceramic: lead zirconate titanate. The crystal is cut into a slice with a thickness equal to half a wavelength of the desired ultrasound frequency, as this thickness ensures most of the energy is emitted at the fundamental frequency.

Normally the transmitting and receiving crystals are built into the same hand-held unit, known as an ultrasonic transducer or probe. The transducer emits ultrasound in rapid pulses and also acts as a receiver most of the time.

It is important to note that the minimum interval between ultrasound pulses equals the time for the deepest echoes to return to the transducer, and the distance that each echo takes to return from the interface depends on the distance of the particular interface from the transducer and the speed of sound within that material. The thickness, size and location of various soft tissue structures in relation to the origin of the ultrasound beam are calculated at any point in time using this ‘pulse echo technique’.

Equation for the calculation of the distance of tissues from a transducer:

The speed of sound itself varies from one material to another (Table 17.1) and is dependent on temperature, pressure and other factors.

Table 17.1 Speed of sound through various mediums

Medium	Speed (m s⁻¹)
Air	330
Water	1497
Fat	1440
Blood	1570
Metal	3000–6000
Soft tissue	1540

Through electronic processing of the returning sound waves, a two-dimensional image can be created that provides information about the tissues and objects within the tissues. Real-time B-scans allow body structures that are moving to be investigated. This is done by allowing a rapid series of still pictures to be built up to capture the movement. The faster the frame rate, the quicker the image will be updated and the better the resolution of the image. More sophisticated systems have an array of transducers rather than just one pair of transmitter and detector.

Images can be then be displayed on a screen monitor (via what is known as a scan converter) and can be recorded on videotape, thermal paper, laser imaging or digitally on a picture archiving and communication system (PACs) or DVD. The thickness, size and location of various soft tissue structures in relation to the origin of the ultrasound beam are calculated at any point in time using this ‘pulse echo technique’.

ACOUSTIC IMPEDANCE

The strength of the reflected sound wave depends on the difference in ‘acoustic impedance’ between adjacent structures. The acoustic impedance of a medium is related to its density and the speed of sound through that medium (Table 17.2). The greater the difference in acoustic impedance between two adjacent structures, the more sound will be reflected, refracted or absorbed at their boundary rather than transmitted.

Table 17.2 Typical acoustic impedance of various mediums found in the body

Medium	Acoustic impedance (in acoustic ohms)
Air	0.000429
Water	1.50
Blood	1.59
Fat	1.38
Muscle	1.70
Bone	6.50

Example

Air and bone have such very different impedances to those of fat, muscle or water that a beam of ultrasound wave meeting bone or air is almost entirely reflected, refracted or absorbed; consequently there will be no transmission of sound beyond this layer. Similarly, air and skin have very different acoustic impedances; therefore a coupling medium is needed to match the impedance of the crystal in the probe more closely to the impedance of the skin of the patient and thus allow transmission of sound through the skin surface. The most common coupling medium, acoustic gel, is applied on the patient’s skin before an ultrasound examination. Coupling gel also has the advantage of reducing air bubbles between the skin surface and the transducer, thereby improving contact and minimising friction.

On the other hand, body layers such as fat, muscle and many body organs have very similar acoustic impedances, enabling most of the beam to pass from one layer into the next, with only a small fraction being reflected, and making this modality ideal for imaging soft tissue organs (Table 17.3).

Table 17.3 Different types of ultrasound scan

Type of scan	How echo is received	Example of scan
A mode	Amplitude mode	Amplitude mode
A mode	Each layer producing a reflection shows up as a peak on the trace. The larger the echo, the higher the peak
B mode	Brightness mode	Musculoskeletal detail
B mode	Each reflecting echo is registered as a bright spot; the larger the amplitude of the reflecting echoes, the brighter the spots
M mode	Motion mode	Fetal pole (B mode) + waveform (M mode)
M mode	Moving echoes are recorded to give traces of fetal heart pulsations

ULTRASOUND ENERGY

There are two types of ultrasound energy used in diagnostics: continuous energy and pulsed energy.

1 Continuous sound energy uses a steady sound source and, when ultrasound is reflected from a moving surface, the frequency of the sound is altered slightly in a manner that depends on the speed of movement of the surface. This is due to the Doppler effect.

2 Pulsed sound energy is based on a pulse echo technique, whereby a pulse of sound is transmitted and then followed by a pause, during which time an echo has a chance to bounce off the target and return to the transducer. Pulsed ultrasound, because of its high frequency, can be aimed in a specific direction and obeys the laws of geometric optics with regard to reflection, transmission and refraction.

DOPPLER ULTRASOUND

The Doppler effect was discovered by Christian Andreas Doppler (1803–1853) and is now commonly used in ultrasound imaging to examine the movement of liquids, such as blood flow in arteries and veins, allowing the location of blockages to be determined precisely. It is also used in fetal echocardiograms (ECG) in detecting and listening to the fetal heartbeat. Pulsed wave Doppler can also be used to evaluate blood flow through different structures and measure the velocity of the blood flow.

TRANSDUCERS

Ultrasound transducers, also called probes, come in different shapes, sizes and frequencies to allow use in different scanning situations (Fig. 17.3). For example, in an obstetric or upper abdominal scan the transducer used is known as a ‘convex-array’ or curvilinear transducer. This contour allows the transducer to be moved across the abdomen whilst maintaining good contact with the abdominal surface and also giving the wide field of view needed to see the upper abdomen or the whole fetus. Examples of Doppler studies are spectral, colour and power Doppler.

Figure 17.3 A selection of ultrasound probes.

For thyroid, breast or musculoskeletal scans, a transducer with a flat surface – a ‘linear-array’ transducer – is commonly used.

In some applications, where the window to the organ of interest is small, transducers with a small footprint (i.e. the surface in contact with the patients) are used. These include:

• neonatal head scans – the fontanelles are used as a window

• inter-cavity scans – the transducer head is inserted into the vagina or rectum

• cardiac scans – the space between the ribs is used as a window.

FREQUENCY SELECTION

Higher frequency ultrasound waves have a shorter pulse waveform (wavelength), longer near field and less divergence in the far field; they allow better resolution of superficial structures. More energy, however, is absorbed and scattered by the soft tissues so that higher frequencies have less penetrating ability. On the other hand, low frequency ultrasound waves are longer in length and allow better penetration to image deeper organs. So the ultrasound operator has to weigh up the risk and benefit of selecting resolution as the expense of the penetration, depending on the organ being imaged.

Example

A low frequency transducer will be able to image structures as deep as 15–20 cm, but may not give as good a resolution and image quality as a higher frequency probe such as a 10 MHz or 12 MHz probe.

Typically, for general use of ultrasound in obstetrics and upper abdominal scans, a range of 3.5–6 MHz probe is suitable, and a high frequency transducer (7–12 MHz) can be used for scanning the thyroid gland, breast and testes. It can also be used to examine paediatric and neonatal patients.

SAFETY

A vast amount of research has been carried out in laboratories and animal studies to investigate the effect of using high intensity ultrasound. These studies have found that there are two main changes occurring within the body tissues:

• Heating effect, which can result in localised rise in tissue temperature (prolonged temperature rise, typically above 41 °C, can cause damage to tissue cells).

• Cavitations effect can occur in the presence of very high ultrasound pressures, causing oscillation of microbubbles, which can result in the destruction of tissue cells.

It has been over 35 years since ultrasound was first used on pregnant women and some earlier studies found increased frequency of left handedness in boys, dyslexia, and low birth weights in babies who had had excessive prenatal scans. However, these results were not reproduced in larger later studies.

Nevertheless, the prudent use of ultrasound should be recommended. Guidelines on the safe use of ultrasound are available and these recommend working within the safe levels of primary regulated metrics such as TI (thermal index), a metric associated with the tissue heating bio-effect, and MI (mechanical index), a metric associated with the cavitation bio-effect.

Ultrasound power and scanning times should be kept ‘as low as reasonably practicable’ to achieve an adequate image for interpretation. This is the ALARP principle and all trained operators should adhere to it. It is also very important for the operator to assess the risk/benefit of the ultrasound examination to minimise the unnecessary exposure of patients to ultrasound.

ULTRASOUND MACHINES

Nowadays, advanced technology has enabled ultrasound machines to be smaller, more compact, more mobile and portable (Fig. 17.4). So ultrasound can be used in different settings – from departments with dedicated ultrasound rooms to bedside examinations where ultrasound can be used without moving the patient to another place. Wherever it is used, it important to select a machine that is fit for the purpose for which it is required (i.e. the needs of a department, particular clinic or practitioner).

Figure 17.4 A range of ultrasound machines.

APPLICATION OF ULTRASOUND TECHNIQUE

Ultrasound is usually utilised as a first line investigation, due to its advantages, but may have to be complemented with other cross-sectional imaging studies (e.g. MRI or CT). Information can be obtained by using different imaging planes, such as sagittal, coronal and transverse (Box 17.1).

ADVANTAGES OF ULTRASOUND COMPARED WITH OTHER MODALITIES

• Ultrasound examinations are generally non-invasive and more acceptable to patients – the studies do not generally involve use of contrast and there is a lesser degree of discomfort compared to other cross sectional imaging modalities.

• Ultrasound methods are relatively inexpensive, quick and convenient – compared to techniques such as X-rays, CT, MRI scans. This makes it an ideal modality to monitor changes over a period of time; for example in tumour growth or a progressive disease. It is also very suitable as a screening tool in applications such as ovarian screening and screening for abdominal aortic aneurysm or fetal anomalies. The equipment can be made portable for convenience and the images can be stored electronically.

• Ultrasound does not involve ionising radiation – this makes it suitable for obstetrics and paediatric examinations.

• An ultrasound study is dynamic – this means movements and physiological changes can be observed to aid diagnosis.

• It has no known long-term side effects.

DISADVANTAGES OF ULTRASOUND COMPARED WITH OTHER MODALITIES

• It is highly dependent on the operator – as ultrasound examinations are interpreted during the scan, operators need to be highly skilled to obtain accurate images and then interpret them.

• Ultrasound cannot be used for examinations of areas of the body containing gas or bone – this means the method is of very limited use in diagnosing digestive or skeletal problems, such as bowel pathology, lung lesions, fractures and adult brains.

• Ultrasound is not specific in diagnosing all pathology – there may be many pathological conditions that can look the same on the image. Previous and current medical history of the patient is vital to enable accurate interpretation.

• Images are dependent on the characteristics of the patient – therefore it is not always possible to obtain diagnostic images from patients with high body mass indices as excessive fat layers can weaken the signal on transmission and reflection back to the transducer.

ROLE OF AN ULTRASOUND DEPARTMENT

• To provide first line investigations; e.g. in right upper quadrant pain to exclude biliary colic, renal colic, for pelvic pain to exclude ovarian cysts or ectopic pregnancies.

• To obtain additional information to aid diagnosis in cases of abnormal liver function tests, palpable lumps, pain.

• As a valuable tool in one-stop clinics (haematuria, one stop breast clinics, early assessments clinics).

• To speed up patient management and improve patient pathway in cases of definitive diagnosis.

• For reassurance purposes; e.g. during pregnancy.

• To screen for abnormalities such as ovarian cancer, aortic aneurysm and fetal anomalies.

• To provide a good monitoring/surveillance modality; for example, fetal well-being and aortic aneurysms.

• To provide a research tool to aid technological advances in this field and improve diagnostic efficiency of ultrasound.

PATIENT PREPARATION

The patient should eat and drink normally before and after the scan unless otherwise instructed; for example for gall bladder scans the patient will have to avoid fatty foods for 6 hours before the scan; for renal and bladder scans the patient will be required to attend with a full bladder. In obstetrics and gynaecology a full bladder may also be necessary to displace the bowel gas upwards and laterally to allow good visualisation of the pelvic structures. Usually patients are advised to carry on taking their usual medication.

The other important patient preparation is to obtain informed consent from the patient, by providing an explanation of the scan, who is going to do the scan, and when the results will be available. This is because in most cases the result can be available immediately and it is important to make the patient aware of this in case of bad news.

OPERATOR SKILLS IN ACQUIRING AND INTERPRETING AN ULTRASOUND IMAGE

There are many factors that affect the accuracy and safety of ultrasound; operator skills are probably the most important of these.

By using the correct techniques to allow adequate visualisation of the organ investigated, the operator can be sure that any potential pathology cannot be missed. Equally, knowledge of anatomy (especially cross-sectional anatomy), and understanding of physiology is vital to recognise physiological and pathological appearances that may be apparent on an ultrasound image.

Knowledge of ultrasound equipment technology is crucial to enable the sonographer to manipulate the controls on the ultrasound machine. By using the equipment appropriately the sonographer can ensure that the image obtained can be optimised to give the best possible quality for diagnosis.

The sonographer should be aware of artefacts, which are appearances that are falsely created by the reflection, refraction and absorption of the sound waves. These appearances can confuse the operator, leading to misdiagnosis. It is important for the operator not only to recognise but also to try to minimise them. Examples of these are mirror images and reverberation echoes.

Excellent interpersonal skills are needed to establish a good relationship with the patient, which enables the acquisition of adequate images for interpretation. In some cases counselling skills are necessary for the sonographer to support the patient initially in case of bad news, such as fetal abnormalities or fetal death.

COMMON APPLICATIONS OF ULTRASOUND

OBSTETRIC ULTRASOUND

Early pregnancy

Diagnosis and confirmation of on-going early pregnancy can be done by a high frequency trans vaginal scan. A gestation sac can be seen as early as 5 weeks of pregnancy (Fig. 17.5). It is very important to initially confirm the site of the pregnancy within the cavity of the uterus, thereby excluding the possibility of an ectopic pregnancy (although in very rare cases, an intra- and extrauterine pregnancy can occur together). A fetal pole and a visible heartbeat can be detectable by ultrasound by about 6–7 weeks. For women with vaginal bleeding an ultrasound scan is important to exclude miscarriage. Complicated conditions such as molar pregnancies (a pre-cancerous condition of the placental tissue) can also be detected in time for effective management. Multiple pregnancies can be excluded or confirmed, and the type of twins (identical or non-identical – ‘chorionicity’) can also be determined by ultrasound (Fig. 17.6).

Figure 17.5 An early intrauterine gestation sac of approximately 5 weeks (3 weeks from conception).

Figure 17.6 Multiple pregnancy with lambda sign indicating non-identical twins.

Dating pregnancies

The establishment of correct gestational age and assessment of fetal size is very important for pregnancy management in terms of delivery and the timing of further tests; therefore ultrasound dating scans provide an accurate estimated date of delivery (EDD) by evaluating the fetal size. Fetal measurements (Table 17.4) include crown rump length (CRL), biparietal diameter (BPD), head circumference (HC) and femur length (FL). In later scans, the abdominal circumference (AC) is used in the assessment of fetal growth. Growth trends can be monitored by serial scans to exclude intrauterine growth retardation, or macrosomia in high-risk women (Fig. 17.7).

Table 17.4 Fetal measurements

Measurement	Example of scan
Crown rump length (CR length)	Bi-parietal diameter (head circumference, HC)
Femur length (FL)	Abdominal circumference (AC)

Figure 17.7 Fetal growth charts.

Diagnosis of fetal anomalies

The National Screening Committee recommends that every woman should be offered a routine scan to allow her the choice to screen her baby for abnormalities. This scan is usually performed at about 18–20 weeks of pregnancy (Fig. 17.8). Although ultrasound is sensitive for the detection of a variety of fetal anomalies, such as spina bifida, skeletal dysplasia, abdominal hernias, renal problems, it cannot detect all abnormalities as some problems may either be too subtle to be seen on a scan or develop late in pregnancy. Some fetal organs may be more challenging to assess; for example the detection of certain heart defects in the fetus is made more difficult, owing to its size and movement. Doppler studies of the fetal heart can be useful in some cases.

Figure 17.8 Obstetric scans. A Longitudinal section of fetal spine. B Cross section of fetal chest to show 4-chamber view of the heart. C A low lying placenta.

The placenta can also be assessed for its site to exclude complications in later pregnancies, such as placenta praevia (see Fig. 17.8C). Amniotic fluid volumes (liquor) can be assessed to exclude excessive (polyhydramnios) or reduced (oligohydramnios) liquor volumes in cases of certain fetal abnormalities or growth disorders. Fetal presentations (breech, oblique) and fetal intrauterine death can also be confirmed. A pregnancy scan may also reveal other pathology, such as uterine fibroids and ovarian cysts.

Fetal chromosomal disorders

Ultrasound is not very sensitive in the detection of chromosomal abnormalities; the most promising feature so far that is used in the assessment of Down syndrome is fetal nuchal translucency (fluid area behind the fetal neck; Fig. 17.9). This measurement can be used alongside blood results, fetal age, and maternal age to determine the risk of Down syndrome. Invasive diagnostic tests such as amniocentesis (sampling the liquor); chorionic villus sampling (CVS; sampling the placenta) andfetal blood sampling (from the umbilical cord) can be performed under ultrasound guidance to obtain fetal cells to determine the genetic composition (karyotype).

Figure 17.9 Nuchal translucency.

GYNAECOLOGICAL ULTRASOUND

Ultrasound is primarily used in the assessment of uterine and ovarian structures (Figs 17.10 and 17.11). The uterus can be examined to exclude normal uterine variants such as bicornuate or didelphic uterus, or pathologies in the presence of pelvic pain, abnormal vaginal bleeding or palpable masses within the pelvis. Pathologies such as fibroids, simple or complex ovarian cysts, endometrial polyps, tubo-ovarian abscess and suspected endometrial cancer can be detected by ultrasound. This modality is also very extensively used in the assessment and monitoring of patients with infertility, by monitoring and tracking follicular and endometrial development and determining correct timings for infertility procedures. The role of ultrasound in screening for ovarian cancer is still being researched, as the ultrasound appearances are sometimes very subtle to interpret in non-postmenopausal women.

Figure 17.10 A Transabdominal image with full bladder and uterus. B Transvaginal uterus with endometrial measurement.

Figure 17.11 Ovary showing normal follicles.

ACUTE ABDOMINAL ULTRASOUND

In acute medicine, ultrasound is an ideal first line investigation (Figs 17.12–17.15); for example forright upper quadrant pain, biliary colic can be assessed by investigating the gall bladder and the bile ducts to exclude stones. In cases of renal colic, obstruction can be detected by the presence of pelvicalyceal dilatation and the presence of calculi, depending on their size. Acute pancreatitis and cholecystitis may well present with an oedematous appearances and wall thickening. Acute pelvic pain can be investigated to exclude ectopic pregnancy, ruptured ovarian cysts or inflammation caused by appendicitis. Pyrexia can be as a result of abscesses present within the abdomen or acute appendicitis, which ultrasound scans can confirm or exclude.

Figure 17.12 Gall bladder with sludge.

Figure 17.13 Normal right kidney.

Figure 17.14 Right kidney with hydronephrosis.

Figure 17.15 Ascites.

In cases of blunt abdominal trauma, FAST scanning (Focussed Assessment with Sonography for Trauma), is used in some accident and emergency departments to determine any organ injury. A scan is performed to detect abnormal interabdominal fluid collections, which can be a result of haematomas and interabdominal bleeding.

NON-ACUTE ABDOMINAL ULTRASOUND

In the presence of abnormal liver function tests (LFTs), ultrasound can be used to assess the liver for obstructive jaundice and detect any focal lesions (e.g. metastasis, Fig. 17.16) or generaldiffuse disease such as cirrhosis. Hepato- or splenomegaly can be assessed by measurements and reference to the normal ranges. In urogenital cases, congenital renal abnormalities can be excluded, such as ectopic or horseshoe kidney, and the kidneys can be examined to exclude any focal benign or malignant lesions, stones or obstruction. Urinary bladder volumes can be calculated if required. Inflammatory conditions like acute or chronic cholecystitis and pancreatitis can be excluded with ultrasound. Ultrasound is also a useful screening tool for the detection of abdominal aortic aneurysm by measuring the diameter of the aorta (Fig. 17.17). Depending on the size of the aneurysm, the patients can either be monitored over a period of time for any changes or referred for surgery. Ultrasound is an accepted tool to investigate and monitor patients after renal transplants to detect any signs of rejection.

Figure 17.16 Liver metastasis

Figure 17.17 Longitudinal section of abdominal aortic aneurysm.

HEAD AND NECK ULTRASOUND

Ultrasound can be useful in cases of abnormal thyroid function tests in detecting palpable thyroid lumps, enlarged thyroids and non-functioning thyroids (Fig. 17.18). Pathology such as nodular goitres and cystic disease can be detected in cases of thyroid lumps; it is not always possible to distinguish between benign and malignant tumours. Parathyroid glands are difficult to see on an ultrasound scan unless there is enlargement present due to disease. Salivary glands (i.e. parotid, submandibular and sublingual glands) can also be assessed with ultrasound to exclude pathology such as inflammation, stones and benign or malignant tumours. Doppler colour flow studies, fine needle aspirations (FNAs) and biopsies under ultrasound control may be necessary for further evaluation.