ULTRASONOGRAPHY: uses and contraindications of ultrasonography

Ultrasound

L'ultrasound, also called ultrasound scan or ultrasound, it is considered a first level exam, as it is cheap, widely available and does not expose patients to ionizing radiation. In addition to its diagnostic use, it can be used as a guide for biopsies or for interventional purposes. IS a simple exam to be performed but requires experience, operators sonographers more experts they will perform better and more accurate tests. This is what we mean when we say that ultrasound is an exam operator-dependent. Ultrasound is the only imaging exam in which the doctor creates the image to be interpreted.

Physical principles of ultrasound: ultrasound

What is ultrasound? How does it work? The operation of ultrasound, also called ultrasonography, it is based on the use of ultrasonic waves, ie mechanical waves. The ultrasound used have high frequency, above 20,000 Hertz. All other imaging methods instead use electromagnetic waves, therefore of the spectrum of light. Ultrasounds are obtained through piezoelectric crystals, contained in the probes, which are placed on the body tissue to be explored, suitably wet by a gel which allows the correct transmission of ultrasounds. These are crystals which, if crossed by electrical stimuli, enter into vibration generating ultrasounds.

The propagation of ultrasonic echoes depends onacoustic impedance of the tissues, that is from the resistance they oppose to the propagation of ultrasounds. Based on this, part of the echoes are reflected, another part continues its propagation in depth. The bone is not crossed by ultrasound, which is completely reflected.

It therefore appears completely colored black in areas not reached by ultrasound, e White on the surface, where there is maximum reflection. The presence of bone therefore prevents the underlying structures from being visualized: this prevents examining the brain contained in the skull. THE piezoelectric crystals they also function as detectors, as they transform the return vibrations reflected by the tissues into electrical stimuli which then allow the ultrasound image to be processed. The ultrasound image is in black and white, with varying shades of gray based on the quantity of reflected echoes. Air and liquid collections appear black (anechoic), extremely solid collections appear white (hyperechoic). The organs are usually gray in color, of varying intensity, defined as hypoechoic.

Each ultrasound has several probes ultrasound, they emit ultrasound to different frequencies and are suitable for the study of various apparatuses. For example for the study of abdominal organs are used convex probes, curved, which emit low frequency ultrasounds (around 3 MHz) and therefore penetrate better into deep tissues. Instead, they are used for the study of skin, subcutis and muscles linear probes, flat surface, which emit ultrasounds at higher frequencies (around 7.5 MHz) and allow the study of surface structures with greater resolution. There is also the so-called cardiological probe phased array probe.

Eco Color Doppler and contrast medium

Doppler ultrasound is based on the physical principles ofDoppler effect, so the sound waves undergo a frequency variation when they encounter a moving structure: the frequency increases if the object is approaching, and decreases if it moves away. These frequency variations are picked up by ultrasound probes, and in the resulting image the moving structures of approach towards the ultrasound probe take on a color red, while those who do move away they have color blue.

Ultrasound-color-doppler image

Image showing the Doppler effect used in ultrasound to identify the direction of flow of arterial and venous blood vessels.

These features make the ecoDoppler study particularly useful for visualizing vessels: arteries or veins, which are studied in an excellent way without the use of any type of contrast medium. Using the Doppler it is possible to process very precise data on the characteristics of the vessels under examination, for example the speed of the blood in motion, the presence of turbulence and the value of the resistances inside the vessels, defining any stenosis or other pathologies with precision. All these parameters are processed through software integrated in the latest generation ultrasound systems.

Even in ultrasound it may sometimes be necessary to use a half of contrast, to better visualize some lesions not even visible on Doppler. The most commonly used is the hexafluoride of sulfur (SonoVue). These are gaseous contrast media, consisting of micro bubbles aerial. These microbubbles are injected in a small quantity (1-2ml) intravenously, and allow to increase the echogenicity of the blood.

They are mainly used to determine whether some lesions such as liver nodules or kidney cysts may be malignant, i.e. related to the presence of a tumor. The ultrasound contrast medium is a gas and is eliminated through the lungs, so it can also be used in cases where the renal function is not optimal. Allergic / anaphylactic reactions are also possible for this type of contrast medium, however it is considered more tolerable of the contrast media used in CT and MRI. It is not administered to pregnant patients because it has not yet been sufficiently tested.

Uses of ultrasound

What is ultrasound used for? The uses of ultrasound are many:

  • In urgency, there is the so-called protocol ecoFAST, which allows a doctor with basic notions of ultrasound to recognize the severity of trauma by evaluating the presence of fluid or bleeding in the abdominal area and the health of the aorta artery.
  • In cardiology, to evaluate valvular pathologies, pericardial effusion, reduction of contractility of the heart linked for example to a heart attack. More accurate than transthoracic ultrasound (echocardiography or transthoracic echocardiography) in this area is trans-esophageal ultrasound (echocardiography or transesophageal echocardiography).
  • For the study of the liver by ultrasound of the upper abdomen, in patients suffering from steatosis, alcoholic or viral hepatitis, cirrhosis, liver tumors or benign lesions that require subsequent checks. It allows to evaluate the gallbladder, for any stones or malignant pathologies. The latter can only be viewed on an empty stomach. The study of the liver using elastometry, integrated in the most recent ultrasound scanners, is increasingly expanding.
  • For the study of spleen is lymph nodes, which can enlarge in case of infectious, haemolytic or lymphoma diseases. Trauma-related injuries to the spleen may also be evaluated. Also in this case the indicated examination is the abdomen ultrasound.
  • For the study ofurinary system, kidneys, bladder and prostate (the latter only in men) can be evaluated by ultrasound of the lower abdomen. In the kidneys, stones, cysts, solid lesions and dilations of the urinary tract can be recognized. Kidneys with impaired functioning appear smaller in size. In the bladder it is possible to recognize stones, clots, polyps or tumor vegetations, in fact even in case of hematuria (blood in the urine) ultrasound is recommended.
  • With particular probes it is possible to study the apparatus female reproductive like the uterus and ovaries in women, and the prostate in men. We speak respectively of transvaginal ultrasound or gynecological ultrasound and transrectal ultrasound.
  • With linear probes it is possible to study i soft tissue, for example for the appearance of cysts, lipomas or suspicious skin lesions.
  • Always using linear probes they can be studied muscles is tendons (not the bone, which is not visible on ultrasound). This is useful in injuries such as muscle sprains and the presence of calcifications.
  • In the children is newborns below the year, control examinations are carried out such as ultrasound of the hips after the first month of life (it is now a recommended screening test) and transfontanellar ultrasound, which deserves special mention as it is the only case in which you can study the brain with ultrasound. After one year of age, in fact, the baby's cranial fontanelles are welded making the brain inaccessible to ultrasound as it is completely surrounded by bone.
  • Breast echo can be performed in women who detect the appearance of swelling at the level of the breasts. Ultrasound is the examination of choice for the study of the breast before the age of 40. After the age of 40, mammography is preferred.
  • It is possible to use ultrasound and doppler for the study of blood vessels (angiological or vascular ultrasound), for example the carotids in case of plaques, or the aorta in case of aneurysm. The ultrasound venous study allows to recognize venous varices, incontinence and thrombosis.
  • Also there thyroid and the neck are analyzed with thyroid ultrasound. In the thyroid gland can be searched for the presence of goiter, nodules or cysts, which once identified can be checked according to the doctor's opinion, or biopsied to establish their nature, always with an ultrasound-guided procedure. Possible lymph node or major salivary gland pathologies (parotid and submandibular) can also be studied.
  • Ultrasound is used for performing biopsies ultrasound guided.
  • Ultrasound is also used to guide the placement of central venous catheters (CVC), in otherwise difficult to access veins such as the subclavian vein, which is located behind the collarbone. It is a useful procedure in anesthesia or when it is necessary to administer drugs in case of serious diseases.
Thyroid echo

Thyroid ultrasound, carried out by affixing a linear probe to the neck.

Comparison with other methods

  • X-ray: both are first-level and low-cost methods, however radiography exposes even if in a minimal dose to ionizing radiation, unlike ultrasound. While radiography allows a better study of the bone, ultrasound allows the optimal study of soft tissues. The two methods are complementary for articular and musculoskeletal pathologies, as the X-ray visualizes the bones, while the ultrasound allows to evaluate muscles, tendons and ligaments.
  • TC: CT is a second level method, therefore more expensive and which exposes to a high radiation load. Compared to ultrasound, it allows the study of all organs and systems in a way that is not dependent on the operator's experience, with greater resolution and detail.
  • RMN: magnetic resonance is also a second level method, very expensive but which does not involve exposure to ionizing radiation. It too has greater resolution than ultrasound, but longer times and some contraindications that, if present, prevent its execution (for example in patients with pace-makers).

Contraindications

There are no contraindications to performing an ultrasound.

Contraindications exist to the administration of the ultrasound contrast medium, and are known allergy and pregnancy, because the safety of these substances for pregnant women has not been studied.

 

Sources:

-Physical principles of ultrasound, Ass. Pr. Mirjan M. Nadrljanski et al.

-Contrast-Enhanced Ultrasound: what is the evidence and what are the obstacles ?, Stephanie R Wilson, Lennard D Greenbaum, Barry B Goldberg

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