EDUCATION

Basics of ultrasonography

Maxine Murray

Sound and ultrasound waves consist of a mechanical disturbance of a medium such as air. The disturbance passes through the medium at a fixed speed causing vibration. The rate at which the particles vibrate is the frequency, measured in cycles per second or hertz (Hz). Sound becomes inaudible to the human ear above about 20 kHz and is then known as ultrasound. Diagnostic imaging uses much higher frequencies, in the order of megahertz. The higher the frequency used, the better the resolution (the ability to distinguish two adjacent objects). However, as the frequency increases, more of the ultrasound beam is absorbed by the medium and the beam cannot penetrate so far. For this reason, higher frequencies (for example, 7.5 MHz) are used to provide good detail of superficial organs such as the thyroid gland, testes, and breast, and lower frequencies (for example, 3.5 MHz) for examination of the abdomen.

Producing the image

Ultrasound probes are made from a piezoelectric ceramic substance that vibrates at a resonant frequency when an alternating electric current is applied across it. The resulting vibration is transmitted into the tissue in short bursts. The speed of transmission within most soft tissues is 1540 m/s, producing a transit time of 6.5 micro seconds/cm. When the waves encounter a boundary between two tissues of different density (such as soft tissue and bone) some will be reflected and return to the probe. The probe then acts as a receiver, converting mechanical energy back into an electric signal which is used to display an image on a video monitor. Because the velocity of ultrasound waves is constant the time taken for the wave to return to the probe can be used to determine the depth of the object causing the reflection. The object can then be displayed at an appropriate place on the monitor.

If the ultrasound beam meets a rough surface or small object the beam is scattered in all directions and only a small amount will be received by the probe. Air within the bowel also scatters ultrasound, and this is one of the main causes of non-diagnostic scans of the abdomen.

Types of scanner

Real time scanners sweep the ultrasound beam rapidly through the region of interest. The image is updated with each sweep so that you can see movement (for example, of a heart valve).

Doppler imaging depends on the fact that if the reflecting surface is moving in relation to the probe (for example, blood flowing in a vessel) the frequency of the received ultrasound wave will be different from that of the transmitted wave. If the reflector is moving towards the transmitting probe the frequency will be increased and vice versa. There is a constant relation between this change in frequency and the velocity of the moving reflector, and this can be used to calculate the velocity of flow within vessels. For example, in carotid arteries the velocity of flow increases with the severity of stenosis. Ultrasonography can therefore pick up critical stenoses which require surgery and display the site of the stenosis at the same time.

Conventional Doppler imaging produces a wave form that can be used to calculate the actual flow rate in a vessel, whereas colour flow Doppler displays the same information by superimposing the image of moving blood in colour on the usual real time image. The colour flow immediately draws the operator's attention to areas of high flow or disturbed flow, which can then be examined more thoroughly and quantitatively with conventional Doppler imaging

Advantages and problems

Ultrasonography has many advantages over conventional radiography, the main one being that it does not use ionising radiation (box). Diagnostic images sometimes cannot be obtained because of the patients' build (particularly in obese or kyphotic patients) or because of excessive bowel gas scattering the ultrasound beam. Such patients may require cross sectional imaging with computed tomography. Of all types of imaging in radiology, ultrasonography is the most operator dependent. In the hands of experts, the technique looks easy, but both performing and interpreting the examination can be extremely difficult, and if the result of the examination does not fit with the clinical picture, a more experienced operator may be the answer.

Very little patient preparation is required, but it is important to get it right. Patients must fast before examination of the abdomen to ensure that the gall bladder is full and therefore visible. A full bladder is required for examination of the pelvis in females. This pushes small bowel loops out of the pelvis, allowing the uterus and ovaries to be seen.

Advantages of ultrasonography

  • No ionising radiation
  • Safe in pregnancy
  • No known side effects
  • Cheap
  • Portable
  • Minimal preparation of patients
  • Painless
  • Non-invasive
  • Direct vision for biopsy

    • Applications

    Ultrasonography can be used to guide needle aspiration and biopsy of masses. The biopsy needle is highly echogenic and appears as a bright line entering the mass. The tip of the needle can be guided to avoid necrotic parts-which may not yield diagnostic material as well as adjacent structures such as blood vessels.

    The development of higher frequency probes which allow improved resolution has allowed ultrasonography of "small parts" such as the thyroid and parathyroid glands, salivary glands, breasts, testes, and eyes. The 1990s has also seen the development of tiny probes which can be attached to endoluminal devices to provide high resolution images. Examples include transvaginal ultrasonography of the uterus and Ovaries, transrectal examination of the prostate, and transoesophageal examination of the heart and aorta.

    In this article I will describe some of the main indications for ultrasonography. The Royal College of Radiologists' guidelines will give you a more extensive list.(1)

    OBSTETRIC ULTRASONOGRAPHY
    In early pregnancy ultrasonography is used to confirm intrauterine and exclude ectopic pregnancy. It can detect a viable fetus from about seven weeks' gestation and transvaginal probes will detect a fetus even earlier. Missed abortions and retained products after terminations can be readily identified.

    In later pregnancy ultrasonography is used to assess growth and to exclude anomalies. The number of conditions that can be identified antenatally with ultrasonography is continually increasing and includes renal abnormalities, diaphragmatic hernias, neural tube defects, and congenital heart defects. Ultrasound guided antenatal interventions are also increasing, including amniocentesis, chorionic villus biopsy, and intrauterine fetal transfusion.

    ABDOMINAL ULTRASONOGRAPHY
    Ultrasonography should be the initial investigation in patients with abdominal pain or a mass. If ascites is identified it can be tapped, and any mass can be evaluated for biopsy. It is important to check the patient's clotting status before intra-abdominal biopsy, and most patients will have to be admitted to a ward for monitoring for several hours afterwards. Ultrasonography can also localise collections in cases of sepsis, and drains can be inserted into subphrenic, subhepatic, and pelvic collections.

    The role of ultrasonography in the acute abdomen is less well defined. In equivocal cases of appendicitis, ultrasonography may show an appendix mass, stone, or a focal collection of free fluid next to the appendix. Acute cholecystitis and intussusception can also be diagnosed.

    Jaundice - Ultrasonography is good at detecting gall stones and can determine the level and cause of bile duct obstruction. Cirrhosis of the liver can be identified, as can its complications such as splenomegaly, ascites, varices, and portal hypertension (seen as reversed flow or thrombosis in the portal vein with Doppler ultrasonography).

    Pancreatitis - Ultrasonography is used to exclude gall stones as a cause of the pancreatitis and to exclude complications such as pancreatic abscess or pseudocyst. Large fluid collections or pseudocysts can be drained at the same time.

    URINARY TRACT
    Ultrasonography is the initial investigation of choice in conditions such as haematuria, a possible renal mass, and bladder outflow obstruction due to an enlarged prostate. In haematuria, ultrasonography can identify a tumour in the urinary bladder and exclude a space occupying lesion in the kidney. In a young patient or child with hypertension it will show relative renal size and evidence of parenchymal disease, but other imaging techniques are needed to exclude renal artery stenosis. This would normally be a nuclear isotope scan, but in future magnetic resonance angiography will probably be the investigation of choice.

    In renal failure ultrasonography will show the size and morphology of the kidneys and may demonstrate obstruction. The scan can appear normal even if the patient has an obstruction, either because urine output is low or because the obstruction is recent and dilatation has yet to develop.

    Transrectal ultrasonography and biopsy are useful for patients with prostatic malignancy. Patients should be given antibiotics before the investigation. Prostate volume can also be estimated and correlated with serum concentrations of prostate specific antigen to calculate the likelihood of malignant disease.

    Case history

    A 64 year old man was referred by his general practitioner to a one stop urology clinic with a 10 month history of urinary frequency, nocturia, and a poor stream, all of which suggested some bladder outflow obstruction. Urea and electrolyte concentrations were normal. On examination, there was dullness to percussion in the central abdomen, suggesting an enlarged bladder, and rectal examination showed a smoothly enlarged prostate. He had an ultrasound examination of the urinary system which showed an enlarged, thick walled bladder with several small diverticula. The prostate was smoothly enlarged and indented the bladder base. There was mild dilatation of both ureters and renal pelves with normal calyces and a normal thickness of renal cortex throughout both kidneys. The patient was then asked to empty his bladder into a urine flow meter. The flow rate was low at 9 ml/s. Repeat ultrasonography showed a residual urine volume of 150 ml.

    The investigations all suggested a serious obstruction to bladder outflow but no evidence of irreversible renal damage. The patient had transurethral resection of the prostate, which alleviated his symptoms.

    Comment - The role of ultrasonography in patients with symptoms of outflow obstruction is complementary to urine flow studies. A large residual volume with a low flow rate suggests serious outflow obstruction. Ultrasound examination may also help identify patients with very severe outflow obstruction leading to grossly dilated upper tracts and loss of functioning renal cortex. These patients often do badly after surgery, and symptoms may even be exacerbated. They are therefore referred for more detailed urodynamic studies before surgery. Before ultrasonography was readily available patients with symptoms of outflow obstruction were investigated with intravenous urography. This investigation gives less information than ultrasonography, takes longer, gives a high dose of radiation (roughly equal to 100-200 chest radiographs), and requires use of intravenous contrast, which can cause serious allergic reactions.

    CHEST
    Ultrasonography is not particularly useful in the lungs because air causes a great deal of artefact. It is used, however, to locate and drain small effusions in pleural disease (especially for patients in intensive care in whom supine chest radiographs may be difficult to interpret). Solid components within the pleura can be distinguished from loculated fluid and biopsy specimens taken under ultrasound guidance.

    VASCULAR
    Abdominal aortic aneurysms can be measured and followed up with conventional ultrasonography. I have already mentioned the role of Doppler ultrasonography in carotid artery disease. More recently, it has been used to follow up arterial limb grafts to predict and prevent graft stenosis. Ultrasonography is an effective method of detecting clots and reduced flow in larger vessels of patients with deep vein thrombosis. However, examination of the calf vessels is laborious and time consuming and can be technically difficult, particularly if the leg is swollen. As many clinicians give anticoagulation only to patients with above knee thromboses, examination of vessels below the knee is usually unnecessary.

    PAEDIATRICS
    Ultrasonography has several specific indications in children. Excellent images of the brain of neonates can be obtained by placing the ultrasound probe on the open fontanelle. This allows identification of haemorrhage in the ventricles or brain substance. Congential anomalies, including the presence and causes of hydrocephalus can also be shown.

    In acutely ill infants both intussusception and pyloric stenosis can be identified, avoiding the need for contrast media and ionising radiation. Children with irritable hip can have ultrasonography to identify effusion into the joint and allow aspiration of fluid to exclude septic arthritis. Ultrasonography is now used to screen infants to exclude congenital hip dislocation at an early and treatable stage.

    SMALL PARTS
    Scrotum - Ultrasonography is the best investigation for scrotal masses. If a possible malignant lesion is identified ultrasonography of the abdomen should be carried out to exclude para-aortic lymphadenopathy. Epididymitis can also be identified on ultrasonography. Testicular torsion is diagnosed clinically, but ultrasonography may help to show an abnormal lie of the testis or Doppler scanning may reveal absent perfusion.

    Thyroid - Ultrasonography is used to guide biopsy of thyroid masses. It will also differentiate between a multinodular goitre and a homogeneously enlarged gland and may confirm retrosternal extension.

    Breast - Ultrasonography is not sensitive enough to be used to screen for cancer at any age. It is useful, however, in patients with a palpable mass which cannot be seen on mammography, in differentiating a fibroadenoma from a cyst (which look identical on mammography), and in investigating painful lumps such as abscesses, which cannot be compressed for mammography.

    Eye - Ultrasonography of the eye is quick, painless, and simple. The ultrasound probe is placed directly on the closed eyelid, which is first covered in ultrasound jelly to provide good contact. This technique can show retinal detachments and vitreous haemorrhage and is also useful for detecting foreign bodies such as metal splinters in the eye or the retro-orbital tissues.

    Conclusion

    Ultrasonography may end up a victim of its own success. The absence of ionising radiation, coupled with the other advantages has led to an overwhelming increase in requests for investigations, and because it is inexpensive and non-invasive requests are difficult to refuse. Ultrasonography departments are therefore in danger of being overrun with inappropriate requests for an investigation which will have little effect on patient management.

    St George's Hospital,
    London SW17 0QT

    Maxine Murray,
    senior registrar in radiology

    REFERENCE

    1 Royal College of Radiologists. Making the best use of a department of clinical radiology. London: RCR, 1995.