Curio

Terminology

The word echography comes from two Greek roots, echo (an echo) and C-W communication (a drawing). It is thus defined as being “a drawing by the echo”.

The term echography indicates as well the medical act as the image which results from this. It is shortened in a current way in echo (with the female one: " it had a écho").

The apparatus allowing echography is a echograph .

The doctor who practices an echography is a echographist .

To note that the modern apparatuses comprise a whole a function Doppler. This is why one speaks about echography-Doppler (shortened in echo-Doppler or echo-DOP).

The material

The echograph is made up:

of a probe, allowing the emission and the reception of Ultrasound S;
of a computing system, transforming the time enters the reception and the emission of the ultrasound in image;
of an operator console, allowing the introduction of the data of the patient and the various adjustments;
of a display system: monitor;
of a system of recording of the data, either in an analogical way (video cassette, paper impression), or in a numerical way (format DICOM);

The whole is laid out on a mobile carriage, making it possible to carry out the examination with the bed even of the patient.

The needs are different according to the studied body. Most demanding is the Cœur, mobile essentially, which requires a good definition of the space but so temporal image.

The probe

The first studies on the ultrasounds were not applied to medicine, but aimed at allowing the detection of the submarines at the time of the First World War. In 1951, two British, JJ. Wild (doctor) and J. Reid (electronics specialist), submitted to the medical community a new apparatus: the echograph. It was intended for the research of the cerebral tumors but will make career in the Obstétrique. The use in obstetrics dates from the beginning of the year sixty ten with the apparatuses making it possible to collect the noises of the fetal heart (see Doppler effect).

The basic element of echography is a piezoelectric Céramique (PZT), located in the probe, which, subjected to electric impulses, vibrates generating Ultrason S. the echoes are collected by this same ceramics, which plays the part of receiver then: one speaks then about ultrasonic transducer. An echograph is provided with an echographic probe, named echographic bar, equipped with 64,96 even 128 ultrasonic transducers on line. The emission is done in a successive way on each transducer.

The ultrasounds are sent in a delimited perimeter (often trapezoidal), and the recorded echoes are signatures of the obstacles which they met. The echogenicity is the more or less great aptitude of a fabric for retro diffusing the ultrasounds. The frequency of the ultrasounds can be modulated: an increase in the latter makes it possible to have a more precise signal (and thus a finer image) but the ultrasound is then quickly deadened in the examined organization and does not allow any more to examine the underlying structures. In practice the echographist has, at his disposal, several probes with different frequencies:

2,5 to 3,5 MHz of everyday usage, with a definition about a few millimetres;
5 MHz for the structures close to the skin (heart of child for example), with a resolution lower than the millimetre;
7 MHz for the exploration of the small structures rather close to the skin (arteries or veins) with a resolution close to the tenth of millimetre;
10 MHz and more for example for the study, in research, of the small animal, but also, in the medical field, for the imagery surface (aiming at the structures close to the skin).

It is noted that this resolution also depends on the form of the examined structure: it is much better if it is perpendicular to the beam of ultrasounds that if it is parallel to this last.

The frequency of reception also exploits the quality of the image: in fundamental mode the transducer detects the signals of the same frequency as that of the emission. In harmonic mode , it detects the signals of a double frequency (second harmonic) of that of the emission. The advantage of this last system is that it detects primarily only the echoes returning in the same direction as the emission, drawing aside in fact the diffused echoes and making the signal much less disturbed. Nonlinear detection has a particular answer, it does not react to the first centimetres after the probe, which makes it possible to facilitate the imagery at a patient in overweight (whose layer of grease under the skin the passage of the ultrasounds complicates).

Freezing

For what is used the freezing which the doctor applies? For mechanical reasons, one considers that the contact between the probe and the belly cannot be perfect and that there thus exists a fine layer of air between the probe and the belly.

The acoustic impedances of the air and the skin (biological fabric), in Pa .s/m, are worth respectively:

(with 20°C) $Z_a = \ rho_a c_a = 1,204 \ times 343,4 = 413,5$

(with 37°C) $Z_p = \ rho_p. C. _p = 1047 \ times 1570 = 164,4.10^4$

They make it possible to calculate the value of the coefficient of transmission T of the interface air-skin:

$T = 4 \ frac {Z_a Z_p} {(Z_a+Z_p) ^2} \ simeq 10^ {- 3}$

This value is very low and thus generates an important attenuation of the signal between the emission and the reception of the Ultrason S by the probe. It is to cure this problem that the doctor applies a gel, to which the acoustic impedance is close to that of the skin, to obtain a weaker attenuation.

Treatment of the signal

The electronics of the echograph is given the responsability to amplify and treat these signals in order to convert them into video signal. The image in fact in levels of gray according to the intensity of the echo in return.

How do the various fabrics of the organization appear?

the simple liquids, in which there are no suspended particles, are satisfied to let cross the sounds. They are thus not announced by echoes. They will be black on the screen (Structures hypoechogenes)
the liquids with particles, blood, mucus, contain small echoes. They will thus appear in tons of gray, more or less homogeneous.
the solid structures, the bone for example, return much the echoes. One will thus see a white form with a shade behind. An exception however, the dome of the skull, very fine and perpendicular to the echoes, it lets some pass.
the soft fabrics are more or less echoic: the placenta is whiter than the uterus, which is whiter than the ovaries.
the gas and the air, are like the bone, very white.

Various adjustments

the power of emission is adjustable but plays only little in the quality of the image. It is theoretically necessary to use the acceptable minimum capacity in order to avoid a heating of examined fabrics. In practice current this risk is negligible.
the frequency of emission can be modified within the limits of the specifications of the probe.
the profit with the reception can be increased or decreased overall or in a variable way, according to the depth of the explored zone ( TGC for time profit compensation ).
Différents filters can be regulated: compression…
the imagery can be rocked of fundamental mode in mode of second harmonic (shortened in harmonic mode ) making it possible to have a better definition.
the beam of ultrasounds can be focused (acoustic lens by delay of emission regulated electronically) with more or less a great depth (exploits only little the quality of the image).
the zone of interest of the body explored can be widened, or on the contrary, narrowed. In this last case, the image has a better definition.
the rate of acquisition (in English frame spleen ) can be regulated. This parameter is not very important in the event of fixed bodies but must be appreciably increased to study the mobility of a structure (Cœur).

The operator console is provided with a keyboard making it possible to enter the identifiers of the patient and the comments. It gives access the various modes of echography and Doppler, like with the treatment and the storage of the images. It also makes it possible to take measurements (distance, surface…) and various calculations.

Visualization of the images

It is done via a screen.

Various modes are available:

more the current is the mode data base (for two-dimensional): it is about a representation out of cut of the studied body, the plan of this one being determined by the position which the inspector gives to the probe.
the mode Tm (for time motion in English) represents the evolution of a shotfiring cable (ordered) according to time (X-coordinate). This mode makes it possible to evaluate the mobile structures precisely (left ventricle for the heart, for example) and to evaluate the size of it. The latter depends however closely on the choice of the shotfiring cable and thus remains very inspector-dependant.

With these images in level of gray, can be associated with the data of the Doppler color.

Storage and distribution of the images

Theoretically, the data to be stored correspond to film of the duration of the examination (of a few minutes at more than one half an hour) what still poses problems as for the importance of the memory necessary. In practice only still images or short loops of images are preserved. The format is often owner (with a tool for conversion DICOM) or made in a native way in DICOM. This format, largely used in the field of the Medical imagery, makes it possible to preserve in the same document the identifier of the patient, the image and the characteristics of the acquisition of the latter.

In a simple way, the selected image is printed and united with the report. It has in this case only one role of illustration, the quality of the reproduction not making it possible to in no case to revalue, for example, a diagnosis.

The image can be also stored in an analogical way on a video cassette, involving a significant degradation of the definition, but making it possible to preserve sufficient information to be able to draw some from the information a posteriori.

Recent manner, the existence of recorder of DVD in real-time (at the same time) that the realization of the examination makes it possible to digitize several hours of examinations.

The images (or loops of images) can be transmitted in a numerical way, either by CDrom, or by Data-processing network.

Computerized treatment of the image

By interpolation of a loop of images, taken with a fast rate of acquisition, one can simulate a curved line Tm.
the recognition automated of contours remains the stone of obstacle of echography in 2005.
the parametric imagery consists in coding each following pixel of the parameters calculated on the image (evolution in time, dephasing…). It is a subject still in phase of research.
the three-dimensional imagery, until the beginning of this millenium, was made by superposition and interpolation of several successive images, made according to various plans of cut (either in a free way, or using a rotary probe). The process is relatively easy for the fixed bodies but much more complex for the mobile bodies (superposition of loops of images and either of simple images). Currently, certain echographs are provided with probes equipped with sensor-transmitters, either laid out in line but in the form of matrix rectangular, allowing a direct three-dimensional acquisition. The technical constraints and data processing however make that the standard image is then appreciably of less good definition, as well space as temporal, and that the volume of the directly displayable body remains reduced in the face.

Various types of apparatuses

the standard apparatuses, although laid out on caster carriages, are rather intended to be used in fixed telephone. They can be connected to a network, with a external Imprimante. Their cost spreads out between 50.000 and more than 150.000 €.
Of the smaller apparatuses is conceived to be used with the bed of the patient. The flat panel display is of less quality and they always do not have all the functionalities. They function on sector. Their price is lower than 100.000 €.
Of the echographs of the size and the weight of a portable PC was developed. The imagery is then of quite less quality and only the basic functions are available. They have the great advantage of being autonomous for their food. The interest is especially that of a " débrouillage" on the ground allowing to select the patients requiring of the thorough examinations.

Advantages and disadvantages of echography

Advantages:

the use of the ultrasounds is almost without danger: it is the only technique making it possible to have an image of the fetus with a good harmlessness. There is no allergy nor of counter-indication to this examination.
It is painless for the patient. It requires, except exceptions, neither hospitalization, nor Anesthésie. It can be repeated without problem.
echography is a technique of Medical imagery relatively inexpensive: it requires only one apparatus and the price of consumable can be negligible. The examination is carried out with only one person (doctor or manipulator).
the echograph is mobile, making it possible to carry out the examination with the bed even of a patient, in a resuscitation unit for example.
If it is carried out by a doctor, the result is immediate.

Disadvantages:

the image misses sometimes clearness, until being sometimes not exploitable: it is the problem of the echogeneicity , weak in particular in the event of obesity.
the examination, and thus its results, remains " inspector-dépendant". Measurements and the quality of the images depend much on the position of the probe (plan of cut), and thus, of the ability and the competence of the inspector. This manual positioning of the probe varies from one examination to another and is not known a priori, which makes complex any D interpretation of the examination and any retiming with another method of medical imagery. In other words, in case of doubt or of discussion, the examination must be remade entirely, ideally by another inspector.
the principal noise which comes to disturb the ultrasonic images is the " speckle" or granularity (because the image gives the impression to be made of grains). This noise is due to the fact that the ultrasonic imagery is a technique of coherent imagery, which authorizes the interferences between the waves and thus this granulous aspect of the image.

Realization of a standard echographic examination

According to the examined body, the patient must be with jeun or not. He is lengthened on a table of examination and the probe, covered with a gel, is posed directly on the skin compared to the structure to visualize.

Particular techniques of echography

Gynaecological echography and obstétricale

Within the framework of the medical supervision of the pregnancy, an echography makes it possible to obtain a monochromic image of a Fœtus inside the belly of his/her mother. Although it is the most known use of echography, one also uses this technology for the detection of the internal disorders of bodies (calculations, Kyste S, Cancer S).

To the Quebec, since 2004, certain private clinics of procreation and follow-up of pregnancy offer a service of echography in 3 dimensions which allows a more complete view of the fetus.

See articles:

Per-operational echography

The probe can be posed on the skin or directly in contact of the body. In this last case, the probe is covered with a sterile envelope.

Vascular echography

The examination is always coupled with the Doppler making it possible to analyze blood flows.

Cardiac echography (or echocardiography)

The examination of the Cœur comprises difficulties because it is:

mobile;
inserted in the Rib cage, in contact with the Lung S, its two structures (air and bone) preventing the transmission of the ultrasounds.

Echography with product of contrast

See too

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