The primary aims of the article, which is published under the heading "technical note" are: 1. to provide the practical description of trans-thoracic approach to coronary artery visualization and detection of the velocity profile in the coronary arteries, 2. to present basic transthoracic echocardiographic views of coronary arteries: the LAD, Cx and RCA, 3. to suggest a method of their division into segments. The secondary aim of the article is to describe a single centre experience with transthoracic visualization of coronary arteries. It is hoped that the paper will help to expand this interesting and promising technique.

With the patient in left decubitus position the transducer is placed at the left parasternal location and a parasternal short axis view of the great vessels is obtained. Slight change of the imaging plane so that it traverses the heart immediately below the pulmonary trunk allows for visualization of the left main (LM) and proximal LAD. These arteries may be relatively easily visualized in B-mode while Doppler examination is usually required to locate the more distal parts of the LAD (Fig. 6,7,8,9, movie 1, 2 [see Additional file 1, 2]). Depending on anatomical relations in a particular patient, sometimes it is easier to assess the proximal part of the LAD in a modified parasternal long axis views. The artery may be seen there as it traverses below the pulmonary trunk towards the interventricular groove (Fig. 10). The LM and proximal LAD may also be viewed using a modified apical five chamber view (traversing anterior wall of the heart) within the area lateral to the sinus of Valsalva (Fig. 11,12).

The middle and distal LAD may be assessed by placing the transducer in low parasternal position. The interventricular groove is located in short axis view and then the transducer is either tilted towards the base of the heart or rotated to obtain a modified long axis view (aligned parallel to the groove). Long axis view of the LAD can be obtained using either of these views (Fig. 13,14,15,16, movie 3 [see Additional file 3]). The middle and distal LAD can also be visualized using a modified apical 5-chamber view (traversing anterior wall of the heart). The entire LAD together with some diagonal branches can sometimes be assessed in this view (Fig. 12). The apical window is often used for examination of distal LAD at the apex (Fig. 17,18) and even on the inferior wall of the ventricle (Fig. 19,20). Apical window is usually the best to obtain low Doppler angles for velocity measurements in the LAD. The LM and the full length of the LAD may also be visualized in apical 3-chamber view, although the motion artifacts, proximity of the lung and winding of the LAD along its way down to the apex make examination difficult.

The patient may then be placed in supine position. The transducer is moved to the subcostal location and a long axis view of the heart is obtained. Then the transducer is tilted cephalad aiming at the anterior wall of the left ventricle where long segments of the LAD can be visualized. However, the quality of LAD images obtained in this view is usually low and Doppler angle is suboptimal (Fig. 21,22).

Since it may be assumed, that the circumflex coronary artery leaves the coronary sulcus at its lowermost point (as can be seen on parasternal short axis view), the left part of the sulcus between aorta and lowermost point of the sulcus may be divided into two sections of approximately the same length, and segments of the circumflex artery within respective sections of the sulcus may be considered proximal and middle Cx (Fig. 3,4).

The Cx is visualized in parasternal short axis view with imaging plane traversing the heart immediately below the pulmonary trunk (Fig. 23, movie 1, 2, 4 [see Additional file 1, 2, 4]). Sometimes it is better seen when the probe is directed just below the left atrial appendage (Fig. 24). Usually a slight change of probe orientation is necessary to visualize the middle Cx which runs down the coronary sulcus at the border between the atrium and the ventricle (Fig. 25, 26, movie 5 [see Additional file 5]). It is best visualized with the imaging plane traversing the mitral annulus. Depending on anatomical relations in a particular patient, sometimes it is easier to assess the proximal and middle Cx in a modified parasternal long axis views. The proximal and middle Cx may be seen in this view within the lateral wall of the left ventricle (Fig. 10, 27, movie 6, [see Additional file 6]).

The transducer may then be moved to the apex and a modified five chamber view (traversing anterior wall of the heart) is obtained. Area lateral to the sinus of Valsalva is carefully searched and one may find there the Cx originating from the LM. A reliable measurement of the flow velocity within the initial part of the Cx may then be performed. Further segment of the Cx can also be visualized in B-mode, but as this part of Cx with this ultrasonic window runs almost perpendicular to the ultrasonic beam and is located several centimeters away from the transducer, this particular view is of limited use for the assessment of the Cx (Fig. 28).

We have never managed to visualize the distal Cx – at the infero-lateral wall of the ventricle.

The parasternal short axis plane may be used to visualize the proximal RCA (Fig. 29). It may be relatively easily visualized in B-mode (Fig. 30) while Doppler examination is usually required to locate its middle and distal parts. Depending on anatomical relations in a particular patient, sometimes it is easier to assess the proximal RCA in a modified parasternal long axis views. The proximal RCA may be seen in this view running along the anterior wall of the aorta (Fig. 31). The proximal RCA can infrequently be visualized using apical window, but this view allows for recording of the flow velocity spectrum (Fig. 32, 33).

The middle RCA is visualized with the patient in supine position. The transducer is placed in subcostal location and the short axis view of the heart is obtained. The anterior wall of right ventricle at the coronary sulcus is carefully scanned and it is where good quality middle RCA images at low Doppler angles can be obtained (Fig. 34,35,36, movie 7 [see Additional file 7]).

The distal RCA can be assessed using apical window. An apical four chamber view is obtained and then the transducer is tilted more perpendicular to the chest so that the imaging plane traverses the inferior wall of the heart and the posterior interventricular groove. Good quality images of the right posterior descending coronary artery – usually distal part of the RCA – can be obtained in this way. Alternatively, a modified 2-chamber view (crossing the heart along the posterior interventricular groove) can be used (Fig. 37,38,39, movie 8 [see Additional file 8]).

There is a choice of ultrasound transducers available which may be used to examine the coronary arteries. The higher frequency transducers provide high resolution B-mode images and improved sensitivity of Doppler examination (both color and pulsed wave [PW] spectral Doppler). The high frequency transducers may be most successfully used when apical area is scanned – i.e. for examination of the distal LAD. The lower frequency transducers provide a good penetration and higher Nyquist limit for PW-spectral and color Doppler examination at the expense of lower resolution and lower sensitivity of Doppler exam. Nevertheless, they seem to be the optimal choice for scanning of RCA, Cx and proximal and middle LAD.

We suggest optimal quality B-mode image should be obtained before one starts to look for coronary arteries. When poor quality of B-mode image does not allow for visualization of coronary arteries one may decrease the dynamic range of B-mode scanning, change the ultrasonic window or use the harmonic mode for B-mode, which we believe greatly facilitates scanning of the coronary arteries.

Then the color Doppler is turned on and the Nyquist limit for color examination is in most instances adjusted to 0,4 – 0,5 m/s. Too little the Nyquist limit the more color Doppler artifacts obscuring the images are encountered. On the other hand, too high a Nyquist will make detection of low velocity flow within the coronary arteries impossible. Gain is increased until large amount of color artifacts almost obliterate the image. It is then turned down a little and the area where the artery should be located is carefully scanned. Further gain and Nyquist limit adjustments are performed when flow signal within the artery is detected. The size of the area where color Doppler examination is carried out should be kept at minimum. Some ultrasound machine manufacturers suggest a special color map should be used for coronary artery examination – a map, which does not discriminate between velocities to and away from the transducer. We, however, believe a regular color map is perfectly sufficient to this aim. If the artery cannot be localized, change of ultrasonic window (change of the distance between the vessel and transducer, change of Doppler angle), further optimization of B-mode image or readjustment of color settings (gain, size of the area, where color examination is carried out or Nyquist limit) or change of transducer frequency should be considered. If these fail an ultrasound contrast agent may be tried.

Usually a small part of the artery can be seen at first. Then, by moving step-by-step up and down the course of the vessel the entire artery (or its long part) may be assessed (Fig. 42, 43, movie 9 [see Additional file 9]).

Once the vessel is located velocity spectrum is recorded. As there is no general agreement on which Doppler angle is optimal for PW spectral Doppler (duplex) examination it is universally accepted that for the duplex scanning the angle should never exceed 60 degrees. The lower the angle the less the possible error introduced by any inaccuracies of Doppler angle estimation but at the possible price of sensitivity of recording of higher velocities. The latter may be particularly important in very tight stenoses where Doppler signal intensity may not be strong enough to allow for correct measurement of maximal velocity. In such cases it may be advisable to change the window so that the Doppler angle increases towards (but does not exceed) 60 degrees.

We usually use a 5 mm gate for pulsed Doppler (to narrow a gate would decrease the signal intensity and too wide would make the signal more "noisy"). Smaller gate (3 mm) may be used when distal segment of the LAD is examined. We generally use the manufacturer default Doppler filter setting (as for regular echocardiographic examination).

Artifacts pose an important problem which make the examination exceptionally difficult, especially when the Cx or RCA are scanned. Most of them are caused by movements of the heart.

Often the flow of physiological amount of fluid within the pericardial sac generates a strong signal which may be confused with flow within a coronary artery (Fig. 43,44,45,46,47, movie 10 [see Additional file 10]). The flow within the pericardial sac is most pronounced in systole, while the coronary artery flow is predominantly diastolic and this should allow for correct interpretation of images.

We have on a few occasions observed a characteristic artifact mimicking a very high velocity flow within the middle or distal LAD, which can be recorded both in color and spectral Doppler (Fig. 48,49, movie 11 [see Additional file 11]). When carefully examining the velocity spectrum one may notice then, that: 1. the signal is not confined to diastole – it is rather recorded in a random way, without any definite relation to the cardiac cycle and 2. the maximal velocity cannot be recorded due to aliasing.

When middle LAD is scanned some extracardiac vessels as well as a cardiac vein can be visualized on color Doppler examination, which may be confused with the LAD. However, spectral Doppler examination allows for proper distinction: the extracardiac arteries show a spectrum typical for peripheral arteries with predominantly systolic flow while veins show a steady, systolic-diastolic, low velocity flow which would cessate or significantly decrease on Valsalva maneuver. The left internal thoracic artery (LIMA) may be confused with the LAD in cases, in which the LIMA was used for a coronary artery by-pass. The velocity spectrum within the LIMA has then a prominent diastolic component typical for the coronary arteries which may be misleading for a beginner. However, the systolic part of velocity spectrum recorded in LIMA is typical for a peripheral artery. Although the systolic component successively diminishes between the origin of the artery internal mammary artery and its anastomosis with the coronary artery, it is more pronounced, than the diastolic component and therefore easily discernible.

The posterior descending coronary artery which usually is a distal branch of the right coronary artery can arise from the Cx when left coronary artery is dominant, which may cause diagnostic mistakes.

Some large branches of coronary arteries – the intermediate coronary artery (Fig. 8, movie 1 [see Additional file 1]), diagonal and marginal branches can sometimes be visualized and confused with principal coronary arteries, most notably the LAD and Cx. It is especially likely when the principal artery is occluded and a branch enlarges and takes over its function. We have occasionally encountered this problem with regard to proximal LAD and proximal Cx. It is easier to properly locate the LAD at its middle part, as it is clearly localized in the anterior interventricular groove. By moving step-by-step towards aorta or downwards to the apex without loosing the LAD from sight one may correctly identify the proximal and distal LAD. The fact, that distal LAD is the only artery which is visualized from a strictly apical transducer position and that it "winds" to the inferior wall of the heart is another clue (Fig. 20).

The differentiation between various vessels seen at the lateral and inferior wall of the ventricle may be challenging.

Improper machine settings, most notably choosing to high or too low Nuquist limit for color Doppler may result in failure to visualize the coronary arteries.