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.
- Ultrasound is considered to be safe. 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 does cause a slight heating of tissues and potentially small cavitation in tissues. There is no evidence this causes any harm but, nevertheless, it is advised that imaging which is not medically necessary be avoided and exposure limited to the minimum level required.
- It is non-invasive and painless.
- The equipment tends to be much cheaper than, for example, MRI scanners and is potentially portable.
- Ultrasound may be considered the best imaging modality for soft tissues.
Ultrasound may also be used for therapeutic purposes but this will not be discussed here. Ultrasound is used for an ever-increasing range of diagnostic and therapeutic purposes and those discussed below are not a complete list.
See also the separate Obstetric Ultrasound article.
How does ultrasound work?
- Ultrasound pictures depend upon reflection of very high frequency sound waves by interfaces between tissues. The frequency is typically 5-10 MHz.
- 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. Transvaginal ultrasound is used for some gynaecological examinations, and devices placed in the oesophagus may be used to get certain images 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.
- 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. Artefacts can occur.
- Contrast enhanced ultrasonography further expands the range of use, as does endoscopic ultrasound.
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 were 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 were 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 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.
- Duplex ultrasound combines conventional ultrasound image with Doppler flow information. This is mostly used in vascular ultrasonography.
- X-ray examination of the abdomen gives very limited information. Many renal stones and most gallstones are not 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 is used in the Accident and Emergency department for assessment of blunt trauma.
Liver and gallbladder ultrasound
- The liver appears as a fairly homogeneous 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 liver cancer and six-monthly screening with alpha-fetoprotein and ultrasound is used in many parts of the world routinely.Contrast-enhanced ultrasound may be used where standard ultrasound is inconclusive in defining liver lesions.
- Gallstones are more readily demonstrated by ultrasound than by 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.
- Endoscopic ultrasound techniques are also be used for upper abdominal pathology.
- It has always been difficult to obtain adequate images of the pancreas but ultrasound can be a good technique.
- Ultrasound is often used in the initial diagnosis of pancreatic cancer.The primary investigation is usually an ultrasound scan of the liver, bile duct and pancreas. It can show tumour mass and dilated bile ducts as well as any liver metastases. However, it is less good at demonstrating tumours in the pancreatic body and tail. Endoscopic ultrasound may form part of further investigation. Further imaging is usually required when a pancreatic tumour is suspected.
- In acute pancreatitis, ultrasound may show a swollen pancreas, dilated common bile duct and free peritoneal fluid. The presence of gallstones may also be important. Standard ultrasound is limited as it does not assess organ perfusion; contrast-enhanced ultrasound may be more helpful in this situation.
- In chronic pancreatitis, ultrasound 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 in thickness. 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 obtaining 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, followed by CT if the situation is still uncertain.Some consider ultrasound the primary investigation of choice.
Ultrasound scanning for abdominal trauma
- Blunt trauma may result in haemorrhage from the liver or spleen. A large clot will appear as a homogeneous 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; however, ultrasound also has its place, especially in the Accident and Emergency department where portable ultrasound can give quick diagnoses.
Assessment of abdominal aortic aneurysm (AAA)
- This may be visible on plain X-ray but only if there is calcification. However, ultrasound will give an accurate assessment of dimensions.
- An ultrasound can be used to measure an AAA and hence to stratify risk and aid management.
- It now used for the national screening programme throughout the UK.[9, 10, 11, 12]
- 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-positive and false-negative results. Clinical signs may be unreliable until rupture has occurred when haemorrhage is dramatic and catastrophic.
- The dose of radiation from an intravenous 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 homogeneous 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. Some studies suggest that ultrasound should be the initial investigation of choice for renal stones.
- 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 homogeneous 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 prostate cancer is limited but it can be critical in guiding biopsy.
- It is possible to use ultrasound to assess the volume of the prostate; this may be useful in planning both surgical or radiotherapy treatment.
Examination of testes using ultrasound
- Ultrasound can be used to assess the scrotum in distinguishing between torsion of the testis and epidydimo-orchitis.It is, however, operator-dependent and age-dependent, and surgical exploration may still be required where the diagnosis is in doubt.
- 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 is of value in differentiating benign ovarian tumours and ovarian cancer.
- 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 intrauterine contraceptive device (IUCD) are lost and it is feared that it has perforated the uterus, an X-ray will not show if it is inside 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.
See separate Echocardiography article.
- 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 thyroid cancer 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 brain tissue is not suitable for assessment 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.
- Doppler can be used for assessing blood flow and stenosis within the carotid and main cerebral arteries.
- Ultrasound may be used to assess lumps in the breast. With clinical examination, mammography and biopsy, it forms part of the initial assessment of women with breast lumps in specialist breast clinics.
- Mammography is also used but, especially in premenopausal 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.
This is perhaps the area in which there has been the most expansion in the diagnostic (and therapeutic) use of ultrasound scanning. Conditions for which ultrasound is used in diagnosis include:
- Achilles tendonitis or rupture.
- Shoulder injuries and tendinopathies including tears, inflammation, thickening, ruptures, calcification of the:
- Supraspinatus tendon
- Infraspinatus tendon
- Long head of biceps
- Elbow: bursae, tendinopathy, effusion.
- Wrist: tendinopathy, tenosynovitis, carpal tunnel syndrome.
- Knee: effusion, bursae, Baker's cyst, tendinopathies.
- Hip: Screening for developmental dysplasia of the hip in neonates, bursae, effusion, tendinopathies.
- Muscle tears.
- Foot: neuroma, tendonitis.
Other uses for diagnostic ultrasound
Ultrasound is used to help distinguish the nature of lumps and lesions almost anywhere in the body. It helps in distinguishing lumps felt under the skin - for example, distinguishing lipomas from sebaceous or simple cysts. It may help in establishing whether a lump is a lymph node, and if enlarged gives an idea if this has an inflammatory or malignant cause. Ultrasound is widely used to guide biopsy. It is used as a first-line investigation to ascertain the nature of salivary gland swellings.It is used to assess vascular conditions, such as deep vein thrombosis and varicose veins. It can be used to visualise structures in the eye in ophthalmology. It can be used in the acute diagnosis of pneumothorax.It has been used for screening for osteoporosis, although dual-energy X-ray absorptiometry (DEXA) is superior.
The enormous list of possible uses for ultrasound in diagnosis discussed here is by no means complete but gives a flavour.
- 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 from the 1930s as a therapeutic rather than diagnostic tool. 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, use of contrast and many other techniques have blossomed. Physicians, physicists and engineers, working together, have revolutionised diagnostic techniques.
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