Introduction

Since the introduction of ultrasound to medical imaging in the 1960s its popularity has grown and applications widened into numerous subspecialties of medicine. Excluding plain radiography, ultrasound scans are the most commonly undertaken diagnostic imaging examinations in England. Figures from the Department of Health show that the numbers of ultrasound scans (including obstetric and non-obstetric) more than doubled from 1996 to 2010.¹ In fact, there were more ultrasound examinations performed than computed tomography (CT), magnetic resonance imaging (MRI), fluoroscopy and radio-isotope examinations combined.

Historically the use of ultrasound was confined to the radiology department, but advances in ultrasound technology made this modality more accessible to other healthcare professionals, resulting in a widening of its application across all fields of medicine: for example, obstetrics and gynaecology, general medicine, urology, orthopaedics, vascular studies, anaesthesia, paediatrics, etc.^2–5 Sonography is not currently recognised in the UK as a profession in its own right and there is potential for this imaging modality to be misused. The use of ultrasound in diagnosis is highly operator dependent, and the greatest danger to a patient is the risk of diagnostic misinterpretation by an inadequately trained healthcare professional or the failure of the trained professional to maintain competencies. Not all sonographers are currently required/able to register with a regulatory body such as the Health Professions Council (HPC). However, the majority of practising sonographers are registered under their primary profession, for example radiographer, midwife, nurse or clinical scientist. The postgraduate training of these non-medical healthcare professionals in the UK has long since been standardised, with courses ratified by CASE (the Consortium for the Accreditation of Sonographic Education). In 2005 the Royal College of Radiologists (RCR) also published ‘Ultrasound training recommendations for medical and surgical specialties’ which set out the minimum standards to be achieved by non-radiological medical staff undertaking ultrasound scans as part of their working practice.⁶

Equipment chronology

Physical principles

Sound is transmitted as series of longitudinal waves, vibrating the molecules within the medium through which it passes. The audible range of sound in humans is 20 Hz to 20 kHz; the waves used in ultrasound have a far greater frequency, typically >1 MHz, hence much greater frequency than audible sound.

As with sound and other wave propagation, the properties of an ultrasound wave are governed by the equation

where ν is the speed of sound. This is dependent upon the transmitting medium: in the case of ultrasound this will depend upon tissue density and the compressibility of its structure. In soft tissues ν has a relatively constant value of 1540 m/s, and this value tends to be assumed for all tissue.

f is the frequency. In ultrasound the frequency is determined by the transducer used, subject to some limited variation. In general higher frequency produces better resolution but reduced penetration.

λ is wavelength. In ultrasound penetration is proportional to wavelength, and wavelength is a determining factor for image resolution.

Ultrasound wave interactions

As the ultrasound wave travels through the patient it will interact with layers between different tissues. These interactions will cause the wave to be attenuated, i.e. energy is lost as the generated wave traverses the patient. There are several mechanisms by which the wave is attenuated.

• Absorption. As the wave passes through the patient some of its energy is lost in the tissues through which it passes. The rate of absorption is dependent upon the tissue type and the frequency of the wave. The tissues are vibrated by the wave, producting heat. It is this heat that is used to advantage in therapeutic applications of ultrasound. In diagnostic applications the induced rise in temperature is a potential hazard.

• Reflection. The wave is reflected at tissue interfaces; some of these reflected echoes will return to the transducer, where they will induce an electric signal, forming the basis for the ultrasound image. The degree of reflection is dependent on the acoustic impedance mismatch at the tissue interface.

• Acoustic impedance mismatch. The acoustic impedance (Z) is a measure of how the wave traverses a particular tissue:

where v is the speed of sound in the tissue and ρ is the tissue density.

The difference between the acoustic impedances at a tissue interface is termed the acoustic impedance mismatch. Minimising the mismatch (e.g. at the skin–tranducer interface) minimises reflection and maximises transmission. A large mismatch creates a lot of reflection with little transmission; the mismatch between soft tissue and air is such that this interface is almost impenetrable by the wave.

• Refraction. The wave may undergo refraction (change of direction) at a tissue interface if the wave front is not perpendicular to the interface. This phenomenon can cause misregistration and measurement artefacts.

• Diffraction. As the wave travels further from the transducer it becomes divergent, spreading the wave energy over a greater area and hence reducing its effective intensity.

Image formation

As the wave passes through the patient a proportion of it will be reflected back to the transducer. The vibrations caused when the reflected wave is incident on the transducer induce an electric signal, which can be used to form the image. The time taken for the echo to arrive indicates the depth of the tissue interface, and the size of the signal indicates the amount of reflection at the interface. This data is stored by the computer ready for processing.

When the signal is processed, many of the processing functions can be controlled by the sonographer to produce the required image. The quality of the image on screen is dependent on a competent practitioner manipulating the equipment controls and adapting their technique. However, the sonographer cannot influence the depth or the properties of the tissue that the waves must travel through, i.e. the characteristics of the patient, so there is usually a trade-off in the use of the best possible settings to optimise the image.

There are basic controls which can be used to optimise the ultrasound image. These include:

• Transducer frequency selection. A variety of transducers are available; on modern equipment they may be multifrequency. The higher the frequency of a transducer, the better the resolution, but at the expense of the depth of penetration.

The choice of transducer will depend on the case mix: high-frequency probes with a small footprint are required for small parts, cardiac, vascular and musculoskeletal applications (as these are superficial structures/organs); lower-frequency probes with a wider footprint are used for abdominal/pelvic assessments. An intracavity probe will be required for gynaecological or transrectal scans (Fig. 38.1).

• Time gain compensation. Amplifies signals that take longer to return (from deep interfaces) than those (from shallower interfaces) that return faster; this helps to counter the other effects of attenuation, which reduce the signal more from a more distant reflection.

• Overall gain. Amplifies all returning signals to provide a signal sufficient for display.

• Scan range/field of view

• Focus. Can be used to provide improved resolution of particular structures

• Dynamic range. Can be used to improve image contrast by manipulation of the displayed grey scale

• Magnification

• Zoom

Figure 38.1 Image of transducers.

Reproduced with permission from Toshiba Medical Systems Ltd.

Equipment and technology

There are a wide variety of ultrasound machines available commercially, from small handheld pieces to laptop-sized portable machines and on to the larger, static departmental machines (Fig. 38.2). It is essential that the correct machine is available to practitioners based on the case mix to be scanned, the throughput of patients expected and the location. It would be inappropriate to expect a sonographer to use a handheld device to scan a list of 20 patients on a machine not much bigger than a mobile phone, but these are ideal for use in ‘point of care’ situations where a large static machine would be impractical. Whenever machines are to be purchased, it is essential that sonographers are consulted to ensure that the most suitable equipment is acquired.

Figure 38.2 Image of portable ultrasound machine.

Reproduced with permission from Toshiba Medical Systems Ltd.

Imaging methods

Several different imaging methods are used in ultrasound imaging.

A-mode

Amplitude (A) mode was the first type of ultrasound to be used, but is rarely employed in imaging now. The received echoes are plotted with the x axis representing depth and the y axis the intensity of the reflected wave.

B-mode

In brightness (B) mode ultrasound the intensity of the reflected wave is represented on a monitor by the brightness of an individual pixel, rather than height on the y axis. An array of transducers simultaneously scanning a plane can be used to produce a 2D image of the scanned plane. Coupling this information with the position of the transducer enables real-time imaging to be produced. This is a dynamic process whereby images are updated at a fast rate to allow visualisation of movement. This is the most commonly used ultrasound technique. Real-time B-mode imaging gives information such as size, volume, shape, wall outline, internal architecture of organs and masses, relationship to adjoining structures, movement of organs and presence of fluid.

M-mode

Motion (M) mode is a variation in which rapidly generated pulses are imaged in succession: as interfaces move relative to the probe their velocities can be calculated. This mode is still used in echocardiography for assessing valve motion and timing.

Doppler mode

The Doppler principle is applied to evaluate blood flow in arteries and veins, and perfusion within an organ of interest. There are several types of Doppler ultrasound techniques in common use:

• Continuous wave (CW) Doppler. Electronically, the more basic CW Doppler involves using a transducer with two crystals in a simutanoeus transmission and reception of ultrasound waves; the difference between the transmitted and received frequencies is measured. This dual function can present a disadvantage as there is no information regarding the depth of the vessel being examined or the velocity of the blood flow, and may cause some difficulties in the interpretation of the results, particularly if the ultrasound beam encounters more than one blood vessel along its path. Typical examples of CW Doppler applications are fetal heart monitoring and echocardiography.

• Pulsed wave (PW) Doppler. PW is used to measure the velocity and direction of flow. PW uses a transducer that detects the shift in frequency between transmission and reception that results from the moving targets. The degree of frequency shift depends on the velocity and direction of flow relative to the transducer, the transmitted frequency, and the speed of sound of the tissue investigated. The main advantage of a PW transducer is that the operator can select a specific vessel, usually from a grey-scale image of a colour Doppler, to be investigated, called the sample volume, thus returning echoes from vessels outside this chosen sample volume are eliminated.

• Colour Doppler imaging. Generally used when additional information is required, such as a pattern of flow within a conventional B-mode image, e.g. perfusion of a specific organ, neovascularity, the direction of blood flow, or to highlight regions of interest such as jets and stenoses. A colour box (sample volume) is placed over the region of interest, the resultant flow is colour coded and is calculated by positive Doppler shift, shown as red for flow towards the transducer, and negative Doppler shift shown as blue for flow away from the transducer. There may also be shades of orange and yellow, either where there is turbulent flow or the Doppler settings on the ultrasound equipment are not set correctly. The transducer alternates between B-mode and colour Doppler imaging, updating each image. This is also known as duplex imaging

• Power Doppler. Power Doppler maps the magnitude of the Doppler signal rather than the Doppler shift. Duplex imaging is used to superimpose a colour box onto a B-mode image, similar to colour Doppler. The resultant colour image is in shades of yellow, orange and red, depending on the strength of the Doppler signal (Fig. 38.3). This imaging mode is sensitive and therefore useful in detecting slow flow and flow through smaller vessels; however, unlike colour Doppler imaging there is no information on the direction of blood flow, and because of its sensitivity it is prone to motion artefacts.

Figure 38.3 Power Doppler.

Harmonic mode

In this mode a broad bandwidth transducer (i.e. a transducer capable of receiving a relatively wide range of frequencies) is used. A wave is transmitted at an initial, fundamental frequency, and interactions within the deeper tissues cause frequency shifts to harmonic frequencies. Harmonic frequencies are a multiple of the fundamental frequency. In harmonic imaging the information from the fundamental frequency is filtered out, therefore only the higher harmonic frequencies are used for image formation. Harmonic imaging offers several advantages over conventional imaging, including improved contrast resolution, reduced noise and clutter, improved lateral resolution, reduced slice thickness, reduced artefacts and improved signal-to-noise ratio (Fig. 38.4A,B). In general, harmonic imaging is useful when examining deeper structures and obese patients.^13,14

Figure 38.4 Harmonic imaging demonstrating improvement in image of the gallbladder. (A) Image without harmonic imaging; (B) image with harmonic imaging applied.

Compound imaging

Real-time spatial and frequency compound imaging uses electronic beam steering to interrogate a structure from different viewing angles and at different frequencies. Several overlapping scans of the structure under investigation are obtained and averaged to form a multiangle compound image that is constantly updated in real time. Compound imaging improves image quality by reducing acoustic artefacts such as ‘speckle’, caused by coherent wave interference, and ‘clutter’, which can result from side lobes and reverberations. Applications include imaging of the breast, peripheral blood vessels and musculoskeletal injuries.^14,15

Extended field of view (EFOV)

The advantage of EFOV is that larger organs or pathology can be seen in one single panoramic image. A real-time transducer is slowly swept along the area of interest, and successive images are interpreted and processed relative to the probe movement. It is most useful for superficial organs such as thyroid or breast, and for musculoskeletal imaging, where the entire length of a muscle and surrounding organs can be visualised (Fig. 38.5). Unfortunately, EFOV imaging is limited when there are other movements present apart from the transducer, e.g. fetal movements.¹⁵

Figure 38.5 EFOV – scan of breast.

Reproduced with permission from Philips Medical Systems.

Very high-frequency imaging and intracavity transducers

With the development of very high-frequency imaging the current imaging frequency range of between 1 and 15 MHz is increased by miniature transducers that will offer very high-resolution imaging at frequencies ranging from 20 to 100 MHz. These transducers can be placed within cavities to give high-resolution images of structures close to the cavity walls; examples of applications include transurethral, transoesophageal and intravascular scanning.^14,15

Sonoelastography

Sonoelastography is an ultrasound technique for imaging the relative elastic properties of soft tissue and, in particular, for differentiating between benign and malignant tumours. Low-frequency (100–500 Hz) low-amplitude shear waves (a transverse wave that occurs when tissue is subjected to a change in shape without a change in volume) are transmitted into tissue and the resultant vibration is detected using colour Doppler. When a discrete hard mass, such as a tumour, is present in a region of soft tissue, a decrease in the vibration amplitude will occur at its location.^14,15

3D and 4D ultrasound imaging

3D ultrasound uses a dataset that contains a large number of B-mode 2D planes. Once the volume data is obtained it is possible to optimise the ultrasound image of the area of interest by rotating, reconstructing and rendering, allowing viewing in different planes and angles without further exposure of the patient to ultrasound, thereby reducing scanning times. Unwanted information can be ‘sliced’ out and the stored data can then be recalled and manipulated after the ultrasound examination.¹⁶

4D ultrasound is also known as ‘real-time 3D ultrasound’. The basic concept is that the ultrasound equipment can acquire and display the 3D datasets with their multiplanar reformations and renderings in real time. However, 3D or 4D can only build on the 2D B-mode images, therefore the limitations and artefacts that affect B-mode imaging, such as presence of gas and overlying structures, will also affect the quality of the 3D and 4D imaging. The main advantage of 3D/4D is that this technique gives better visualisation of spatial relationships by multiplanar imaging, which is useful for therapeutic and follow-up examinations, and rendering abilities to convey information in a different manner (Fig. 38.6). Quantitative volume estimations can also be calculated more accurately than with conventional 2D scans. Despite these advantages, 3D and 4D ultrasound imaging is still considered a complementary tool rather than a replacement for 2D B-mode imaging.

Figure 38.6 3D ultrasound.

Reproduced with permission from Ultrasound Now Limited.

Contrast-enhanced ultrasound (CEUS)

As in MRI, CT and conventional X-ray, the use of contrast media has enhanced the performance of ultrasound imaging. Ultrasound microbubble contrast agents are smaller than the mean diameter of a red blood cell, non-toxic, injectable intravenously, capable of crossing the pulmonary capillary bed after a peripheral injection, and stable enough to achieve enhancement for the duration of the examination. It is evident that CEUS facilitates improvement for the characterisation of focal liver lesions, detection of liver malignancy, guidance for interventional procedures and evaluation of treatment response after local therapies. However, the applications of CEUS have now expanded to other structures such as gallbladder, bile duct, pancreas, kidney, spleen, breast, thyroid, prostate and heart.

Ultrasound contrast agents are not only effective in ultrasonic imaging but are also important tools for the delivery of drug or gene therapy.^17,18 Furthermore, when the use of CEUS is combined with Doppler and harmonic imaging, sensitivity is greatly increased. The application of CEUS is continuously evolving; however, it should be noted that the insonation of gas-filled microbubbles has the potential to cause a number of biological effects, for example the induction of a physiological response to cardiac exposures (premature ventricular contractions), and damage at a microvascular level (microvascular rupture and subcutaneous haemorrhage).^19,20 The effect of insonation depends on the mechanical index (see section below on safety), the contrast agent used and the ultrasound imaging method. Although there is no proven evidence of harm resulting from clinical use of these agents, caution is recommended when contrast-enhanced imaging is undertaken.

Safety

It is important to highlight that although there is currently no absolute evidence that ultrasound imaging is harmful in humans, research has been carried out in laboratories and animal studies to investigate the effect of using high-intensity ultrasound.²¹ These studies have found that two main changes occur in body tissues:

• Thermal effect. There can be a localised rise in tissue temperature owing to the ultrasound energy being absorbed and converted into heat. This effect is displayed on the ultrasound monitor as a thermal index (TI). For example a TI of 1 indicates a temperature rise of 1°C. This is particularly important in obstetric scanning during development of the embryo and fetus. Three forms of the TI may be displayed:

The thermal index for soft tissue: this is used when ultrasound only insonates soft tissue, as, for example, during obstetric scanning up to 10 weeks after the last menstrual period (LMP)

The thermal index for bone: this is used when the ultrasound beam impinges on bone at or near its focal region, as, for example, in any fetal scan more than 10 weeks after LMP

The thermal index for cranial bone: this is used when the ultrasound transducer is very close to bone, as, for example, during transcranial scanning of the neonatal skull

• Cavitation effect. This can occur in the presence of very high ultrasound pressures, causing oscillation of microbubbles which can result in biological damage to tissue cells. The likelihood of cavitation occurring is related to the peak pressure and is referred to as the mechanical index (MI)

For all ultrasound imaging techniques, prudent use is advised. Comprehensive guidelines on the safe use of ultrasound have been developed by the Safety Group of the British Medical Ultrasound Society;²² these provide detailed advice on safe working levels for TI and MI. Adherence to these published guidelines and keeping exposure time ‘as low as reasonably achievable’ to produce an adequate image for interpretation and diagnosis will ensure that sonographers practise ultrasound imaging safely.

It is also important for the sonographer to assess the risk/benefit of not only the ultrasound examination, but also the imaging mode used, to minimise the unnecessary exposure of patients to ultrasound. For example, pulsed Doppler and colour Doppler imaging, with a narrow sample volume, carry a higher risk of thermal effects than a conventional B-mode examination, and the use of contrast agents can increase the potential for cavitation.

Clinical applications

Ultrasound is generally non-invasive and readily accepted by patients. It is relatively inexpensive, quick and convenient, and the absence of ionising radiation, or any other clinically significant biological effects, makes it an ideal modality to monitor changes over a period of time, e.g. in tumour growth or a progressive disease. It can be seen in real time, which is essential for dynamic studies, and it is especially useful during drainage and biopsy procedures. The ability to view several sections in one gentle sweep allows organs to be seen distinct from one another, and thus pathologies can be sited with accuracy. Structures can be seen in different planes, such as sagittal, transverse and coronal, and measured directly and accurately using linear, volume and circumferential measurements as well as measuring angles. It is also very suitable as a screening tool in applications such as antenatal screening and abdominal aortic aneurysm screening.

Ultrasound cannot be used for examining areas of the body containing gas or bone, making it of limited use in diagnosing gastrointestinal or skeletal problems, such as bowel pathology, lung lesions, fractures and adult brains. Ultrasound is not specific in diagnosing all pathology, therefore it is important to mention differential diagnoses; the previous and current medical history of the patient is vital to facilitate accurate interpretation. Images are also dependent on the characteristics of the patient: it is not always possible to obtain diagnostic images from patients with a high body mass index.

Upper abdomen

Ultrasound is often the first line of investigation for the diagnosis of upper abdominal pathology. For many patients an ultrasound scan will lead to a diagnosis, but a normal report may be just as useful to the referring clinician because the ability to exclude a large number of pathological conditions means that onward referral is made in a more appropriate and hence cost-effective manner.

A typical upper abdominal scan will include assessment of the liver, gallbladder, bile ducts, pancreas, kidneys, spleen, and the associated vasculature, and abdominal aorta.

Common clinical indications include:

• RUQ (right upper quadrant) pain

• Abnormal LFTs (liver function tests)

• Confirm/exclude gallstones/renal calculi/organomegaly

• Abdominal aortic aneurysm (AAA), haematuria and investigations into nature of palpable masses

The minimum images to be recorded should include:

• Left lobe of liver: sagittal section (SS) and transverse section (TS)

• Pancreas (ideally demonstrating head, body and tail): TS

• Abdominal aorta: SS/TS

• Right lobe of liver: SS/TS. The size of the liver is based on a measurement from the right hemi-diaphragm to inferior border in the midclavicular line

• Right lobe of liver: through the porta hepatis

• Gallbladder: SS/TS

• Common bile duct: measurement at widest point

• Both kidneys: SS/TS

• Spleen: SS/TS

Common pathology

The liver parenchyma is normally homogeneous and of a similar reflectivity to the renal cortex. Focal lesions can be readily identified, measured, and their blood flow assessed using Doppler. Diffuse pathology such as cirrhosis and fat infiltration, and focal lesions such as simple cysts, haemangiomas and metastases, can usually be confirmed or excluded (Fig. 38.7A,B).

Figure 38.7 (A) Normal liver showing right hepatic vein; (B) liver with tumour demonstrated.

Reproduced with permission from Toshiba Medical Systems Ltd.

Ultrasound should be the initial investigation for suspected focal pathology. It can demonstrate metastases with varying ultrasonic appearances, but despite there being some correlation between appearances and the primary site (such as highly reflective lesions possibly arising from a gastrointestinal primary), there are many differential appearances. Some metastases can show the same reflectivity (isoechoic) as normal hepatic parenchyma and may be missed.

In cases of jaundice, ultrasound can confirm surgical jaundice by the presence of dilated intra- and extrahepatic ducts, as distinct from medical jaundice which appears essentially normal. The level of obstruction can often be clearly demonstrated by the level at which the ducts or gallbladder are seen to be normal. For example, a dilated common hepatic duct and intrahepatic ducts with a normal or small gallbladder and common bile duct would demonstrate a high obstruction of the cystic duct or above, while a fully dilated biliary system would indicate an obstruction at the lower end of the bile duct. If the pancreatic duct was dilated as well, an ampullary/head of pancreas obstruction may be indicated. However, in early cases of obstruction changes to duct calibre may be subtle.

The common bile duct can be seen and assessed for normality, but the distal portion is often quite difficult to see owing to the gas-filled duodenum. As a consequence, calculi and other pathology in this section of the common bile duct are difficult to detect. Endoscopic ultrasound (EUS) can improve detection where distal pathology is suspected.

Ultrasound is an excellent tool to accurately and safely guide a biopsy needle to the area of tissue required for sampling. The needle tip is scanned as it enters the body, and can be seen in real time as it approaches the lesion or tissue required. Careful technique is required, but continual monitoring of the needle tip can avoid the unnecessary penetration of vessels or organs during the procedure.

Gallbladder

Ultrasound is the method of choice in the initial investigation of the biliary system and has long replaced the oral cholecystogram. Preparation for a hepatobiliary ultrasound scan involves the patient fasting for 4–6 hours in order to fully dilate the gallbladder. Gallbladder volume can be calculated using a formula based on three measurements, although most machines have built-in volume calculations. In addition, variations in shape (Phrygian cap, septate, double), position (intrahepatic, low lying), wall thickness and relevant pathology, such as gallstones, biliary sludge, polyps and tumours, can all be observed.

If the examination is urgent and there has been no time for fasting, a scan of the biliary system can still take place, albeit with the caveat that a contracted gallbladder can occur as a result of eating recently as well as because of pathology.

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