Ultrasound is the term used to describe sound frequencies above those audible to the human ear (> 20kHz). Although ultrasound has been in use in medicine since the 1920s, its use outside of hospitals is still rare. With low end machines costing approximately ?1,000, the reasons for this are not clear.

This may be due, in part, to a lack of knowledge around ultrasound and its capabilities and limitations. This article aims to provide a brief introduction to its use in general practice.

The physics behind ultrasound are well understood. An electric potential is applied to a material which alters in shape, creating a pressure wave. The frequency and shape of this wave depend on the electric potential applied and the material’s nature and shape. Modern machines use a single probe to transmit and receive. Rather than detecting attenuated waves, modern probes detect the echoes reflected from the tissues.

Current probes generate millions of pulses per second with frequencies varying between one and 20MHz. Although frequencies up to 80MHz are available, because of their limited ability to penetrate tissue these are generally only used in anterior eye surgery. Higher frequencies have better resolution; 400MHz machines are used in ultrasonic microscopy. 

For body cavities (eg. chest, abdomen, skull) frequencies between one and five MHz provide the best images while for the neck, limbs, abdominal wall, penis, scrotum, rectum and vagina, frequencies between five and 15MHz are the norm. Intravascular probes typically use between 20 and 50MHz.

Once the echo has been recorded, it is converted into an image. Initially the signal is converted into an amplitude known as the A mode. Later this is combined with continuous recording, or motion (M) mode. A two-dimensional image (brightness mode or B mode) is also developed. Images perpendicular to the probe are also added (C mode). Combining B and C modes allows the creation of surface rendered or tomographic three dimensional images. Combining these with continuous recording creates four dimensional images. Panoramic views have been developed in order to overcome the probe’s limited view.

Along with images, ultrasound can be used to detect the velocity and direction of fluid flow based on the Doppler principle.¹ When a source of a wave is moving toward or away from a receiver, the frequency at the receiver differs from that at the source of the wave. Outside of the clinical context, this effect has been used to attempt to measure the size of the universe. Medically, this effect is usually used to examine blood flow. Two modes are used: continuous and pulsed wave. 

Pulsed wave is used to determine flow velocity and direction. Direction is indicated by colour – blue away from the transducer and red towards it (BART). Turbulent flow is coloured yellow or green. For high flow velocities continuous Doppler is used. Spectral Doppler can be used to assess stenosis, dilations and regurgitations. Both can be combined with M mode.

Elastography is a method of examining how tissue deforms under pressure and is available on higher end machines.² Most commonly, probe pressure is varied and the resulting tissue deformation is recorded. Images are colour-coded with the most deformable tissues coloured green, the least deformable coloured blue and intermediate coloured red. While examining arteries, probe pressure is not required. While originally developed to improve cancer detection, elastography has found use in evaluating liver fibrosis. This technique is now available for MRI.

While the first contrast studies were carried out with agitated saline, current contrast agents use microbubbles (diameter <5 microns) with a gaseous centre coated with albumin or lipid; the composition depends on the manufacturer. These agents are considered to be safe, with a clinically significant adverse event rate of 60 million exposures.³

Because of these significant adverse reactions, contrast use, other than agitated saline, is best limited to hospitals. Contrast is used to define vessels, the endocardium and fallopian tube patency. Research applications include use in echocardiology, thrombolysis and targeted drug delivery.

Fusion ultrasound – simultaneous x-ray and ultrasound imaging – is used in interventional radiology.

Safety

Ultrasound safety has been a concern since it came into use. After extensive testing it is now considered to be extremely safe.⁴ Use of Doppler in pregnancy – except for foetal heart rate monitoring – should be limited. This is a particularly topical subject at present following the introduction of medical termination of pregnancy in general practice. Other modes of ultrasound are considered to present no significant risk.

One criticism of ultrasound is that it is operator-dependent. To reduce this, guidelines exist for echocardiography, obstetrics and musculoskeletal imaging.^5,6,7,8 Developing guidelines is difficult because of ultrasound’s continuing evolvement. Where guidelines exist, it is better to follow them to reduce interoperator variability.

The main physical limitations with ultrasound are transmission by air, reflection by bone and the presence of an acoustic window. Because sound is transmitted by air, gas-filled organs (larynx, lung, bowel) are difficult to visualise with external probes. Investigation of these organs is often better with endoscopy or a CT or MRI scan. Bone reflects almost all the sound that strikes it, making it almost impossible to visualise structures deep to the surface. Research in this area is ongoing.

While acoustic windows can be obtained for most structures, this is a particular problem for joints, the knee in particular. While many of the other joints can be imaged, the deeper parts of the cruciate ligaments and knee menisci cannot be seen. Because pathology of these structures is so common, knee imaging is generally done best with MRI, unless the lesion is superficial.

Uses

The range of uses for ultrasound is significant. Some uses, however, including intraoperative, intracardiac and endovascular, are unsuitable for use outside of hospitals.

Head/neck

Neck ultrasound provides superb thyroid, parathyroid and vessel visualisation. Lymph node assessment is superior to CT/MRI. Skin lesions such as lipomas, sebaceous cysts, neuromas and acne nodules can be easily diagnosed definitively. While the brachial plexus can be visualised, its deeper parts may be difficult to visualise. Depending on their anatomy, branchial clefts may be seen. Oesophageal and laryngeal imaging is inferior to that provided by CT/MRI. Cervical discs and facet joints are poorly visualised. MRI is preferred in this case.

Isolated facial bone fractures may be demonstrated but for complex head trauma, CT is the system of choice. Although floor of mouth and tongue pathology is well seen, the sinuses cannot be visualised. The pinna is easily examined but the temporal bone cannot be. While eye lesions can be seen, retinoscopy and optical coherent tomography are usually superior. 

Although intra-orbital lesions can be imaged, such lesions may extend through the orbital fissures and require CT and MRI. Brain imaging, done through the fontanelles, is in widespread use in infants and may be done through temporal and suboccipital windows in adults. Although CT and MRI provide excellent images, the use of transcranial ultrasound is growing; for some conditions it is actually the investigation of choice.

Thorax

Although MRI may provide superior images, ultrasound is the initial system of choice for cardiac pathology, other than for coronary artery disease. Ultrasound is not sensitive for pulmonary embolism but valve pathology, thrombus, tumours and cardiomyopathy are easily seen. While the main coronary arteries can be visualised, smaller branches cannot be well seen currently and angiography remains the gold standard. Stress echocardiography is used in ischaemic heart disease.

Although MRI is not widely used for respiratory disease, the use of inhaled helium in order to provide contrast for MRI may change this. Pulmonary pathology is less well seen on ultrasound, but it may be used in the assessment of lung fibrosis, pneumothorax and pleural effusions. 

Although pneumonias may be visualised with ultrasound, chest X-ray is superior. Ultrasound is the imaging system of choice for simple rib fractures, costochrondritis and xiphodynia but mediastinal pathology is not well seen. Although oesophageal lesions can be visualised with endosonology, it is not much used.

Ultrasound is the imaging system of choice for the male breast, and for the premenopausal female breast. Use in postmenopausal women generally depends on fat content; it is better to examine fatty breasts with mammography. Although the final diagnosis may depend on histology, many lesions, including simple cysts, can be diagnosed with imaging alone.

Abdomen

For lesions outside the bowel, ultrasound is the imaging system of choice, providing superb images of the liver, biliary tree, kidneys, the aorta and its main branches, para-aortic nodes, superior vena cava, and spleen. The adrenals are typically difficult to image, ureteric stones may be seen but CT is superior. The pancreas may be difficult because of bowel gas but its endoscopic imaging is superior to MRI and CT.

A fluid filled stomach provides contrast and may be used in functional studies. In general, however, endoscopy is preferred. Intestinal obstruction can be seen easily but the cause of the obstruction is rarely apparent. The terminal ileum and the appendix can normally be seen clearly. Mesenteric nodes may be seen but not consistently. Diverticulitis can often be seen but cannot be distinguished from carcinoma, a problem common to all imaging systems. For abdominal wall hernias, penile and scrotal lesions, ultrasound is the system of choice.

Pelvis

In obstetrics and the female pelvis, ultrasound is again the imaging system of choice. While MRI is the preferred modality for the prostate, in the presence of hip replacements, ultrasound is useful as MRI imaging is made difficult to use in this case. 

Ultrasound forms part of bladder assessment and for demonstrating ureteric reflux. Although ultrasound provides excellent imaging of rectal lesions, MRI is understandably preferred. The internal iliac and its branches are difficult to image. Therefore, other methods are preferred in this case.

Back

Although MRI is the imaging system of choice for imaging the spine, ultrasound may also be useful here. It can image interspinous ligament injuries and other injuries which are rarely visible on MRI scans, such as muscle tears typically involved in whiplash injuries. It is also of some assistance when dealing with costovertebral joint injections.

In infants, depending on the degree of spinal calcification, cord lesions may be seen.

Limbs

Other than for bone lesions, ultrasound is arguably the system of choice for limbs. Visualisation of muscles, nerves and ligaments is probably superior to MRI and skin and subcutaneous lesions are easily seen. Panoramic views can be used to examine the arteries and veins along their entire length with Doppler providing functional information. Caution should be exercised with ischemic legs as the causative stenosis may lie within the pelvis rather than within the leg.

Although the large and small joints may be examined with ultrasound, there are some specific problems. In the shoulder, while ultrasound will identify most lesions, the inferior acromial surface which may be responsible for supraspinatus tears is not well seen. Like MRI and CT, ultrasound cannot demonstrate intra-articular adhesions; arthroscopy is necessary to make this diagnosis. In the knee, the cruciate ligaments and menisci cannot been seen adequately. In the wrist the meniscus homologue may be difficult to visualise.

Some bone lesions, including stress fractures, are easily seen but plain film is generally superior. Scaphoid fractures are probably seen just as well with ultrasound as with plain film but MRI, CT and nuclear medicine all have better yields.

Certification

The ICGP has recently formed a group for those who are interested in learning more about ultrasound. Certification in various aspects of ultrasound is also available. Certification is not required to use ultrasound machines but the training provided by these courses is excellent and covers many aspects of its use that may be difficult initially. 

Part-time and full-time masters courses are offered by Trinity College and University College Dublin. These courses typically cover only one area out of the following: echocardiology, obstetric and gynaecology or general ultrasound.

Various national or international organisations, including the British Society of Echocardiology, the European Society of Echocardiology, the American Society of Echocardiology and the European League against Rheumatism offer training and certification in the various uses of ultrasound. These courses have three parts: practical hands-on training, submission of a log book with between 200-300 cases illustrating various pathologies, and an exam. The exam includes unusual but important conditions that should be recognised. 

Because of the transnational nature of many of these courses, logbooks are typically submitted online, while the practical sessions take place in various different cities.

Future

Ultrasound is a cheap, safe and an invaluable aid in diagnosis. It extends the range of diagnostic methods and treatments that can be offered. The main hurdle to its use is the detailed anatomic knowledge required by the operator. This burden may be eased by joining special interest groups including that of the ICGP. Patient satisfaction with ultrasound is considerable as a diagnosis can often be made immediately. 

Ultrasound use outside of hospitals is widespread in several countries, including Germany and South Africa. It seems likely that its use in outside of hospitals and in general practice in Ireland will become more widespread in the near future. 

References

1. Doppler JC (1842) Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels - Versuch einer das Bradley’sche Aberrations-Theorem als integrirenden Theil in sich schliessenden allgemeineren Theorie. Prague: Borrosch & Andrä
2. Ophir J, Céspedes I, Ponnekanti H, Yazdi Y, Li X. Elastography: a quantitative method for imaging the elasticity of biological tissues. Ultrason Imaging. 1991;13(2):111-34
3. Wei K, Mulvagh SL, Carson L, et al. The safety of definity and Optison for ultrasound image enhancement: a retrospective analysis of 78,383 administered contrast doses. Journal of the American Society Echocardiography. 2008; 21(11):1202-6
4. European federation of societies for ultrasound in medicine and biology. Clinical Safety Statement for Diagnostic Ultrasound [2018]  http://www.efsumb.org/blog/archives/885
5. American Society of Echocardiography: Guidelines & Standards http://asecho.org/guidelines/guidelines-standards/
6. Salomon LJ, Alfirevic Z, Bilardo CM, Chalouhi GE, Ghi T, Kagan KO, Lau TK, Papageorghiou AT, Raine-Fenning NJ, Stirnemann J, Suresh S, Tabor A, Timor-Tristsch IE, Toi A, Yeo G. ISUOG practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound in obstetrics & gynecology : the official journal of the International Society of Ultrasound in Obstetrics and Gynecology. 2013;41(1):102-13.
7. The Royal Australian and New Zealand College of Obstetricians and Gynaecologists. Mid-trimester fetal morphology ultrasound screening. Royal Australian and New Zealand College of Obstetricians and Gynaecologists. Clinical Guideline (C-Obs 49) https://www.ranzcog.edu.au/RANZCOG_SITE/media/RANZCOG-MEDIA/Women%27s%20Health/Statement%20and%20guidelines/Clinical-Obstetrics/Fetal-Morphology-Ultrasound-(C-Obs-57)-New-Mar-14.pdf?ext=.pdf
8. The 2017 EULAR standardised procedures for ultrasound imaging in rheumatology. Möller I, Janta I, Backhaus M, Ohrndorf S, Bong DA, Martinoli C, Filippucci E, Sconfienza LM, Terslev L, Damjanov N, Hammer HB, Sudol-Szopinska I, Grassi W, Balint P, Bruyn GAW, D’Agostino MA, Hollander D, Siddle HJ, Supp G, Schmidt WA, Iagnocco A, Koski J, Kane D, Fodor D, Bruns A, Mandl P, Kaeley GS, Micu M, Ho C, Vlad V, Chávez-López M, Filippou G, Cerón CE, Nestorova R, Quintero M, Wakefield R, Carmona L, Naredo E. Annals of the Rheumatic Diseases. 2017; 76 (12): 1974-1979