Ultrasound has a great advantage over some of the other imaging modalities in that it does not involve ionising radiation. Hence it may be used quite freely for antenatal scanning, in children, and where repeated examinations are required.
- There is no evidence that it damages tissues or predisposes to malignant change. Unlike ionising radiation, it does not break chromosomes, at least at the power used for medical purposes.
- It is non-invasive and painless.
- The equipment tends to be much cheaper than eg MRI scanners and is potentially portable.
- The decision as to which imaging technique to employ is not based solely on safety or cost but also upon which gives the best images for that condition.
- Ultrasound is safe, comparatively cheap, and may possibly be the best imaging modality for soft tissues.
- Ultrasound may also be used for therapeutic purposes but this will not be discussed here.
- Obstetric ultrasound is discussed in a separate article.
How does ultrasound work?
- Ultrasound pictures depend upon reflection of very high frequency sound waves by interfaces between tissues. The frequency is typically 5 to 10MHz.
- Probes come in various shapes and sizes. They transmit a signal and receive the echoes.
- The electrical processor transforms the reflected sound signals into white pixels on a black background to give various shades of grey.
- When the sound waves travel easily through uniform substances such as water or urine, no echoes are produced and the screen is black.
- When the sound waves meet tissues of different densities, the sound waves are absorbed, reflected back to the probe, or transmitted through the tissue at different velocities. When this happens, the ultrasound image is white, or shades of grey, depending on the intensity of the reflection.
- The ultrasound probe is usually held in the hand and moved over the area to be examined. A gel or oil is employed to give a non-reflective acoustic link between the probe and the skin.
- Scanning may be performed at interfaces other than skin. Transrectal ultrasound is used for assessment of the prostate. Trans-vaginal ultrasound is used for some gynaecological examinations and devices placed in the oesophagus may be used to get certain imagines for echocardiography.
- It is possible to record ultrasound images but often the interpretation is done by the operator at the time, unlike an X-ray that is taken and read later.
- In the past 30 years, the technical advances in ultrasound scanning have been enormous, giving pictures of much greater resolution, as well as real-time scanning and colour doppler imaging. Nevertheless, the result is still dependent on the operator who performed and interpreted the examination. It is important to remember that artefacts can occur.
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The doppler effect
- Imagine a point source of sound. The sound waves may be portrayed as concentric circles radiating from that point. The longer the duration since that wave was produced, the further the circle will be from the centre.
- Now imagine that the point source is moving in a straight line. As it moves forward, the centre of the inner circles will be further forward than the centre of the more peripheral circles. Hence, the distance between lines in the direction of movement will be less than if the source was stationary, whilst the distance between the lines behind the moving source will be further spaced apart.
- This means that the sound heard in the direction of travel will appear to be of a higher frequency and behind it will appear to be of a lower frequency than if the source was stationary. We may recall the change in pitch from higher to lower of the whistle of a train as it speeds through a station or the siren of an emergency vehicle as it speeds past on the road. This is the doppler effect and the degree of change in pitch can be used to calculate the velocity of the moving source.
- However, the source may be static and the reflecting interface is moving. This produces the same effect. Hence it is possible to measure speed of flow. This may be displayed in the form of colour to give colour doppler pictures or the speed can be estimated to assess the adequacy of flow. This technique can be used to assess peripheral arteries, carotid or renal arteries, or in echocardiography. It can also be used to assess venous flow.
- X-ray examination of the abdomen gives very limited information. Only 70% of renal stones and 30% of gallstones are radio-opaque.
- Ultrasound has much to offer in the abdomen and is being used increasingly to help in the differential diagnosis of abdominal pain and the acute abdomen.
- It may even be used in the Accident and Emergency department for assessment of blunt trauma.
Liver and gall bladder ultrasound
- The liver appears as a fairly homogenous area. Tumours, whether primary or secondary, tend to be more echogenic. Cysts have a characteristic appearance with bright walls and dark centres.
- Ultrasound can be very useful in assessing cirrhosis and possibly malignancy. Hepatic cysts can occur with amoebiasis. Patients with cirrhosis are at risk of developing hepatocellular carcinoma and screening with alpha feto-protein and ultrasound is used by many.
- Gallstones are more readily demonstrated by ultrasound than any other technique. They are highly echogenic but impacted gallstones may be difficult to find. The presence of gallstones does not necessarily mean that any right upper quadrant pain is caused by them. Asymptomatic gallstones are common and the frequency increases with age. However, fluid around the gallbladder is highly suggestive of cholecystitis.
- The pancreas has always been a very difficult organ to obtain adequate images of but ultrasound can be a good technique.
- Ultrasound can be used in the diagnosis of carcinoma of the pancreas. The primary investigation is usually an ultrasound scan of the liver, bile duct and pancreas. It can show tumour mass, dilated bile ducts as well as any liver metastases. However, it is less good at demonstrating tumours in the pancreatic body and tail.
- In acute pancreatitis, the pancreas is poorly visualised in 25-50% of cases. Ultrasound may show a swollen pancreas, dilated common bile duct, and free peritoneal fluid. The presence of gallstones may also be important.
- In chronic pancreatitis, it may show calcification and dilatation of the pancreatic duct. Cysts may also be seen.
Assessment of appendicitis
- Appendicitis is a common condition but diagnosis can be very difficult, especially in females.
- The normal appendix is rarely seen, with a wall of no more than 6 mm thick. The normal appendix compresses when pressure is applied with the transducer.
- In acute appendicitis, wall thickness is 7 mm or more, and the appendix does not compress. There may also be tenderness when the area is pressed with the transducer.
- In female patients, a combination of transabdominal and transvaginal imaging may be very helpful in getting the correct diagnosis at an early stage.
- CT would appear to be slightly better than ultrasound in the diagnosis of difficult cases but CT gives a high dose of radiation and, especially in children, ultrasound assessment may be preferable and then CT if the situation is still uncertain.
Ultrasound scanning for abdominal trauma
- Blunt trauma may result in haemorrhage from the liver or spleen. A large clot will appear as a homogenous dark area and the size and position of the clot may be helpful to a surgeon who is contemplating operation to stem the bleeding.
- CT remains the "gold standard" for blunt abdominal trauma, but ultrasound also has its place, especially in the Accident and Emergency department where portable ultrasound can give quick diagnoses.
Assessment of abdominal aortic aneurysm
- This may be visible on plain X-ray, but only if there is calcification. However, ultrasound will give an accurate assessment of dimensions.
- As described in the article on abdominal aortic aneurysm (AAA), ultrasound can be used to measure an aneurysm and hence to stratify risk and aid management.
- It can be used for screening asymptomatic patients, although there is currently no routine screening in the UK.
- It can also be used in the emergency situation when abdominal pain may be due to imminent AAA rupture. A pulsating mass is an unreliable sign with both false positives and false negative results. Clinical signs may be unreliable until rupture has occurred when haemorrhage is dramatic and catastrophic. The operative mortality of repair of an AAA in the emergency situation is around 40%, compared with 5% or less for elective repair.
- Only about 70% of renal stones are radio-opaque.
- The dose of radiation from an IV urogram is very high but ultrasound is very good at showing stones and delineating the outline of kidneys and the collecting system.
- The renal cortex is seen as grey with some darker circles spaced uniformly around the edge. These darker circles correspond to the renal pyramids. In the presence of obstruction, dilatation of the renal pelvis and calyces will be seen. Renal size can be estimated.
- Glomerulonephritis or systemic illnesses such as diabetes, hypertension, arteriosclerosis, or autoimmune diseases result in kidneys that are hyperechogenic (and so a brighter grey) and the kidneys are often smaller than normal.
- The degree of greyness of the kidneys and the liver is often compared. The liver is used as the standard and looks more homogenous than the kidneys.
- If there is a stone in the kidneys or collecting system, it will show as a very bright echogenic area. Other causes of obstruction such as tumour are less intense.
- A volume of liquid such as a full bladder, shows as a black area that does not reflect sound. Ultrasound may be used to assess the residual capacity in a bladder after voiding.
- Abdominal CT is preferred to ultrasound for assessment of the adrenals.
Transrectal ultrasound of the prostate
- Ultrasound can be a useful means of assessing the prostate gland. Digital rectal examination gives limited information and other imaging modalities are not very useful.
- The normal prostate of a young man produces a fairly homogenous image.
- Benign prostatic hyperplasia often produces single, or numerous, cystic structures of various sizes. Cysts appear as anechoic (black) areas surrounded by hyperlucent areas in the walls of the cyst.
- Calcification may produce hyperechoic areas. This tends to follow prostatitis, even in a young man, but there may not be a clinical history of prostatitis.
- Malignant lesions in the prostate can be hypoechoic, isoechoic, or hyperechoic. Its value in the initial diagnosis of prostatic carcinoma is limited but it can guide biopsy.
- It is possible to use ultrasound to assess the volume of the prostate, that may be useful in planning both surgical or radiotherapy treatment.
Examination of testes using ultrasound
- Ultrasound can be used to assess the scrotum to distinguish between torsion and epidydimo-orchitis.
- It may be used to assess the testes in both suspected malignancy and in infertility.
- It can also be useful to detect an undescended testis in the inguinal canal.
- Ultrasound has shown itself to be especially useful in gynaecology.
- Clinically, it is often difficult to be sure of the nature of a pelvic mass. Ultrasound can distinguish an ovarian mass from an adnexal mass and a cystic from a solid tumour. Pedunculated or degenerating fibroids may be apparent.
- Colour doppler scanning to assess blood flow may be of value in differentiating benign tumours and carcinoma of ovary.
- Abdominal ultrasound of the pelvis is aided by the presence of a full bladder. This tends to push up the pelvic organs and the sound travels readily through a full bladder.
- Another technique is transvaginal ultrasound. The probe is nearer to the cavity of the uterus or the tubes and it can be very useful for the diagnosis of the viability of a pregnancy and if there are retained products of conception. It has become a standard technique for the diagnosis of ectopic pregnancy.
- During infertility treatment involving hyperstimulation of the ovaries to produce several ova, it is important to monitor the production of follicles carefully by ultrasound to achieve the desired result without excessive stimulation.
- If the threads of an IUCD are lost and it is feared that it has perforated the uterus, an X-ray will not show if it is in or outside the uterine cavity. An IUCD will show clearly on ultrasound and its precise position can be demonstrated. This is the investigation of choice.
- A classical ultrasound diagnosis is that of hydatidiform mole. Clinically the uterus is often large for dates and there may be excessive symptoms of pregnancy such as hyperemesis. The ultrasound picture shows no fetal parts but a "ground glass" appearance.
- This involves real-time ultrasound.
- It is the "gold standard" for the diagnosis of congestive heart failure as it shows a dilated and poorly emptying heart.
- Hypertrophy or cardiomyopathy may be demonstrated. Dilatation of the aortic outflow or atrial myxoma may be seen.
- Stenosis or regurgitation of valves may be demonstrated and severity assessed. Mitral valve prolapse can be assessed.
- There is a separate article discussing echocardiography in more detail.
- The thyroid gland is fairly superficial and amenable to ultrasound examination.
- This can help to assess the size of the gland and whether there are cystic portions or solid tumours in thyroid lumps.
- This is very helpful in the diagnosis of carcinoma of thyroid and it can guide fine needle biopsy.
- It may also be of value in demonstrating enlarged parathyroid glands in hyperparathyroidism.
Ultrasound of the brain
- Normally the brain is not a suitable tissue to assess by ultrasound as it is enclosed in a bony skull that is highly echogenic.
- In the newborn infant, the fontanelles are still open and it is possible to use this as a portal to assess the brain. Ultrasound can be used to assess deviation of the mid-line as may occur if there has been an intracranial haemorrhage.
- Ultrasound may be used to assess lumps in the breast.
- Mammography is also used but, especially in pre-menopausal women who have denser breasts, ultrasound may be a better technique. Hence it has a place in the diagnosis of both breast cancer and benign breast lumps.
- Ultrasound scanning is really a development from sonar that was developed during World War I for the detection of submarines and for the safety of navigation after the sinking of the Titanic in 1912. Further advances saw its use in industry to detect flaws in metal and the development of radar.
- In medicine, ultrasound was used as a therapeutic rather than diagnostic tool from the 1930s. Ultrasound was not promoted for diagnosis until about 1948. George Ludwig, who had been in the United States Navy, did much pioneering work on the properties of various tissues in relation to ultrasound. His first publication was in 1949.
- Advances in ultrasound diagnosis were very much dependent upon advances in physics and engineering to produce the equipment.
- In 1954, Ian Donald appreciated the potential of ultrasound in obstetrics.
- The 1960s saw an explosion of interest in ultrasound and its potential.
- Portable scanners, colour doppler scanners, echocardiography and many other techniques have blossomed. Physicians, physicists and engineers, working together, have revolutionised diagnostic techniques.
Further reading & references
- Valley VT, Fly CA; Ultrasonography, Abdominal. eMedicine. Updated: Nov 12, 2007.
- Peterson AC, Terris MK; Urologic Imaging Without X-rays: Ultrasound, MRI, and Nuclear Medicine.eMedicine. Updated: Nov 21, 2008.
- Woo J; A short History of the development of Ultrasound in Obstetrics and Gynecology
- Danta M, Barnes E, Dusheiko G; The surveillance and diagnosis of hepatocellular carcinoma. Eur J Gastroenterol Hepatol. 2005 May;17(5):491-6.
- Gandolfi L, Torresan F, Solmi L, et al; The role of ultrasound in biliary and pancreatic diseases. Eur J Ultrasound. 2003 Feb;16(3):141-59.
- Friday JH; Update on appendicitis: diagnosis and presurgical management. Curr Opin Pediatr. 2006 Jun;18(3):234-238.
- Doody O, Lyburn D, Geoghegan T, et al; Blunt trauma to the spleen: ultrasonographic findings. Clin Radiol. 2005 Sep;60(9):968-76.
- Earnshaw, JJ. Shaw, E. et al; Screening for abdominal aortic aneurysms in men.; BMJ 2004;328:1122-1124[full text]
- Cosford PA, Leng GC; Screening for abdominal aortic aneurysm. Cochrane Database Syst Rev. 2007 Apr 18;(2):CD002945.
- Boczko J, Messing E, Dogra V; Transrectal sonography in prostate evaluation. Radiol Clin North Am. 2006 Sep;44(5):679-87, viii.
- Nussbaum Blask AR, Bulas D, Shalaby-Rana E, et al; Color Doppler sonography and scintigraphy of the testis: a prospective, comparative analysis in children with acute scrotal pain.; Pediatr Emerg Care. 2002 Apr;18(2):67-71.
- Fleischer AC, Andreotti RF; Color Doppler sonography in obstetrics and gynecology. Expert Rev Med Devices. 2005 Sep;2(5):605-11.
- Haider Z, Condous G, Khalid A, et al; Impact of the availability of sonography in the acute gynecology unit. Ultrasound Obstet Gynecol. 2006 Aug;28(2):207-13.
- Amso NN, Griffiths A; The role and applications of ultrasound in ambulatory gynaecology. Best Pract Res Clin Obstet Gynaecol. 2005 Aug;19(5):693-711. Epub 2005 Oct
- Datta RV, Petrelli NJ, Ramzy J; Evaluation and management of incidentally discovered thyroid nodules. Surg Oncol. 2006 Jul;15(1):33-42. Epub 2006 Aug 28.
- Kamaya A, Quon A, Jeffrey RB; Sonography of the abnormal parathyroid gland. Ultrasound Q. 2006 Dec;22(4):253-62.
- Carson SC, Hertzberg BS, Bowie JD, et al; Value of sonography in the diagnosis of intracranial hemorrhage and periventricular leukomalacia: a postmortem study of 35 cases. AJNR Am J Neuroradiol. 1990 Jul-Aug;11(4):677-83.
- Irwig L, Houssami N, van Vliet C; New technologies in screening for breast cancer: a systematic review of their accuracy.; Br J Cancer. 2004 Jun 1;90(11):2118-22.
|Original Author: Dr Michelle Wright||Current Version: Dr Michelle Wright|
|Last Checked: 20/04/2011||Document ID: 4042 Version: 22||© EMIS|
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