Technique of obtaining cardiac views

Transcription

1 Chapter 1 Technique of obtaining cardiac views Successful ultrasound diagnosis in any context depends first, on obtaining a series of defined crosssectional images and, second, on the correct interpretation of those images. The plane of cross-section is modified according to the structures identified, in order to demonstrate the structure of the threedimensional whole in an optimum fashion. The relationship between movements of the transducer, and alteration in the image obtained by those movements, is a complex one. Practice and intuition play a large part in the skilled manipulation of a transducer to obtain the desired views. However, a methodical and deliberate approach may improve the initial rate of learning and of continuing skill development, and also help in teaching those skills to others. There is no universal terminology to describe the axes of the transducer, or the different movements that the transducer can make relative to the abdomen. Figure 1.1 shows the three axes of an ultrasound transducer, which we have labeled X, Y, and Z. The following scheme may be helpful in directing others to scan and also in disciplining oneself into a methodical approach to scanning. Every transducer images in a plane or sector, which is usually evident from its shape or markings, and corresponds to the plane that includes the X- and Y-axes in Fig There are six possible movements of a transducer each in two possible directions. Three of these movements conserve the imaging plane, whilst viewing that plane from a different aspect, whereas the other three alter the imaging plane itself ( Figs. 1.2 and 1.3 ) In-plane slide In-plane slide describes the movement of the transducer across the abdomen in line with the initial plane of imaging, or movement in its X-axis. As the transducer slides, new structures will appear on the side of the image corresponding to the leading edge of the transducer and others will disappear from the trailing edge side. Structures in the image will appear to move laterally relative to the sector margins (and eventually X Z Fig The three axes of an ultrasound transducer, labeled to help with the description of the transducer movements that follows. Y Y disappear from the edge), but their appearance remains otherwise unaltered. Rocking Rocking the transducer describes the movement in which the sector remains in the same plane and the point of contact with the maternal abdomen is unchanged but the angle between the transducer and the abdomen is altered. This is movement in the X- and Z-axes of the transducer. In the image, new structures will appear on the side of the image corresponding to the leading edge of the transducer and others will disappear from the trailing edge side, as with an inplane slide. The difference between the in-plane slide and rock is best appreciated when the two are used Z X in this web service

2 (a) Fi g T h e t h re e t y p e s o f t r a ns d u ce r movements that keep the plane of imaging the same but alter the view within that plane are shown. In each diagram, the left-sided drawing is where the transducer starts, the middle panel represents the movement of the transducer and the right-hand panel is where the transducer ends up after the movement. (a). I n - p l a n e s l i d e. T h e transducer slides over the maternal skin in the line of its X-axis. The angles made with the skin by all three axes remain unchanged. (b)fi g (b). Rock. The point of contact of the transducer with the maternal abdomen remains unchanged. The angles that both the X- and Y-axes of the transducer make with the skin change, but the angle that the Z-axis makes with the skin remains the same. (c)fi g (c). Ch a n g e i n p re s su re. B ot h the point of contact of the transducer with the skin and the angle of contact with the skin in all three axes remain unchanged, but the transducer moves up or down in its Y-axis. The transducer moves nearer or further away from a given point in the fetus because of compression displacement or release of intervening tissue. 2 in this web service

3 (a)fi g T h e t h re e t y p e s o f t r a ns d u ce r movement that alter the imaging plane as well as the view. (a). O u t- o f p l a n e s l i d e. The transducer is moved along the line of its Z-axis. The angles that all three axes make with the skin are unchanged but the point of contact with the skin moves. (b)fi g (b). R ot at i o n. T h e t r a ns d u ce r rotates or spins around its Y-axis. The angles that all three axes make with the skin are unchanged as is the point of contact with the skin. (c)fi g (c). A n g u l at i o n. T h e p o i nt of contact with the skin remains unchanged, but the angle that the Y- and Z-axes makes with the skin alters. The angle between the X-axis and the skin remains constant. 3 in this web service

4 together in combination ( Fig. 1.4 ). An initial in-plane slide moves a central area of interest towards the edge of the image, but a subsequent rock of the transducer in the appropriate direction will bring the area of interest back to the center of the image. The difference between images before and after this combination of movements is that the path of ultrasound to the structure of interest is altered, so that shadows due to intervening structures may be avoided. Also, the angle with which the ultrasound beam strikes the structure of interest is altered, affecting the quality and appearance of the image. Change of pressure Change of pressure on the transducer allows a limited amount of vertical movement within the imaging plane. This movement is in the Y-axis of the transducer. Increasing pressure moves the transducer nearer to a distant area of interest, by displacing intervening fluid and soft tissue. Structures will appear to move up within the image. The quality of the image also changes. Within limits, image quality generally improves as the transducer moves nearer to the area of interest, but this may be counteracted by the displacement of amniotic fluid from between the transducer and the area of interest, a modest amount of which generally enhances image quality. Excessive pressure may be counterproductive, may cause discomfort to the patient, and can nearly always be avoided. Out-of-plane slide Out-of-plane slide describes the movement of the transducer across the maternal abdomen parallel to the initial imaging plane. This is movement in the Z-axis of the transducer. The images obtained are therefore parallel planes to the initial imaging plane. As the transducer moves, the images show new structures appearing within the body of the image, rather than appearing to slide in from the edge, as with an in-plane slide. Rotation Rotation of the transducer describes the movement whereby the central point of the transducer surface remains in a fixed position on the abdomen, but the transducer is spun about this point (around the Y-axis or the axis of the transducer cable). As the transducer rotates, structures in the center of the image remain in view but are cut in a different plane. Structures more peripheral in the image will disappear from view as the transducer rotates. Angulation Angulation of the transducer is when the point of contact of the transducer with the abdomen remains constant, but the angle that the transducer makes with the initial imaging plane is varied. This produces a similar change in the image to the out-of-plane slide, except that in the out-of-plane slide, near and far structures pass through the image at the same rate, whereas angulation has a greater effect on the appearance of distant structures than it does on near ones. This is movement in the Y- and Z-axes of the transducer. Transducer movements in practice Although the possible movements of a transducer have been described individually, they will often be used in combination. Indeed, a slide in any direction across the curved, rather than flat, surface of the maternal abdomen will inevitably involve an element of rock or angulation. The combination of an in-plane slide of the transducer with a rock in the opposite direction has already been described. A A A Fig Us e o f a co m b i n at i o n o f movements. A structure in the near-field casts an ultrasound shadow obscuring the deeper structure B of interest. An in-plane slide of the transducer removes structure A from the near field, but leaves the structure B at the periphery of the image. A subsequent rock of the transducer brings structure B to the centre of the image, without interference from structure A. 4 B B B in this web service

5 This is a useful method of obtaining clearer views of a structure, already in view in the desired plane, but partially obscured by intervening structures, such as a limb ( Fig. 1.4 ). Although the terms view and plane tend to be used interchangeably, an important distinction can be made between them. Any defined plane can be imaged from different directions to obtain different views. The actual structures within a given plane are fixed, but their appearance may vary considerably in different views (or approaches to the plane). This is because ultrasound, and therefore the image obtained using it, has a directional quality. The axial resolution of an ultrasound image is always greater than the lateral resolution. Structures consisting of reflective surfaces perpendicular to the ultrasound beam are generally demonstrated more clearly than those oriented parallel to the beam. Ultrasound shadows from dense structures interposed between the transducer and areas of interest may also detract from the image quality of some views within a plane. Usually more than one view of a particular plane will be required to demonstrate all of the anatomical features within that plane to best effect. Subtle adjustments to the angle of view within a plane explain at least in part the ability of an experienced echocardiographer to obtain clearer views than a less experienced pupil scanning the same patient. Methodical scanning dictates that the operator should understand how different movements of the transducer will alter the plane of section of the fetus and the effect of this relationship on the position and size of the fetus and its distance from the imaging probe. If the echocardiographer has already achieved the appropriate plane of section, but needs to change the view within that plane in order to optimize imaging of a particular structure, then this can be achieved by a combination of in-plane slide, rock, and change in pressure. These are the three movements that conserve the initial plane of imaging, whilst altering the view. If the desired plane of imaging has yet to be obtained, then a combination of out-of-plane slide, angle, and rotation will be required to move to the required plane. Once the desired plane has been obtained, further manipulation by in-plane slide, rock, and pressure will demonstrate the optimum view within that plane. Most of these movements are performed instinctively with experience, but understanding how they operate can accelerate the learning curve. Th e combination of movements required to move from one plane to another depends not only on the initial and final planes, but also the aspect from which the initial plane is being viewed. For example, consider the movements of a transducer required to move from a standard transverse four-chamber plane to the left ventricular outflow plane, passing through the right shoulder. With the fetus prone relative to the transducer, this change in plane can be achieved by an almost pure clockwise rotation of the transducer ( Fig. 1.5(a) ). However, if the same initial plane is viewed with the fetal right side uppermost, a combination of an out-of-plane slide towards the fetal head, combined with an angulation towards the abdomen, is required to achieve the same change in plane ( Fig. 1.5(b) ). Sweeps through the fetus to provide a series of approximately parallel image slices are an important element of fetal echocardiography. For a small fetus, some distance from the transducer, this can be achieved entirely by angulation of the transducer from a fixed point on the maternal abdomen. In contrast, for a larger fetus, or for one closer to the transducer, an out-of-plane sliding motion of the transducer is required to produce a similar series of image slices ( Fig. 1.6 ). Those whose prior experience is with pediatric echocardiography need to adjust their technique to the fact that fine movements of the transducer can produce large alterations in the plane of section of the small heart of the fetus. As a practical point, it is helpful in fetal cardiac scanning to support the hand and arm by maintaining direct contact between the hand and the maternal abdomen as well as with the probe, or by leaning or resting the hand and arm on the mother s thigh or abdomen, in order to facilitate the fine controlled movement which is necessary. Left right orientation Determination of the fetal position and establishing the left and right, anterior and posterior, and superior and inferior aspects of the fetus is vital. It is important at the outset to ascertain the right left orientation in the fetus, as this is indefinite in a cross-sectional image. In order to establish this, the operator must be aware of the relationship between the sector on the screen and the transducer in their hand and also that between the transducer on the abdomen and the fetus beneath. On most ultrasound systems, a marker on the side of the transducer corresponds to a symbol on the corresponding side of the sector image on the screen. Correct orientation of the transducer can also 5 in this web service

6 (a)fi g T h e t r a ns d u ce r m o ve m e nt s required to change from the fourchamber plane to the left ventricular outflow plane depend on the initial aspect of view. (a). With the fetus supine relative to the transducer, an almost pure (clockwise) rotation of the transducer is sufficient to move from the fourchamber plane to the left ventricular o u t fl o w p l a n e. (b) 6 Fi g (b). When the fetus is right-side up relative to the transducer, a combination of basic movements is required to shift from the fourchamber plane to the left ventricular outflow plane. Initially an out-of-plane slide towards the head gives a transverse imaging plane parallel to the four-chamber plane but close to the head. Subsequent angulation of the transducer to point back towards the heart gives an imaging plane passing through the right shoulder and demonstrating the left ventricular outflow. Note that the amount of initial out-of plane slide required will depend on the distance of the fetus from the transducer and the size of the fetus. in this web service

7 abd head abd head Fig T h e b ea m is p o si t i o n e d i n a transverse section about the level of the diaphragm. The beam is then swept or angled, down to image the stomach in the abdomen (abd) and then up towards the head end of the baby, to the inlet of the thorax. It can be seen that, in a small fetus, some distance from the transducer as in the left-hand panel, this is achieved by angulation of the beam but in a larger fetus, closer to the transducer (right-hand panel), by an outof-plane slide. Usually an element of both movements are involved. Note that the five important planes to be examined are not equidistant from each other. be confirmed by the effect of transducer movements on the image. Viewing the fetus in transverse section and keeping the transducer in the same plane of section, as the operator slides the transducer laterally (in-plane) to his or her left, the image on the screen moves appropriately, with new structures being revealed on the left of the screen and others disappearing from the right. (Should this not be the case then the transducer (a) (b) Figs. 1.7 and 1.8 E s t a b l ish i n g r i g ht l e f t orientation. The operator has already confirmed that the right side of the image as displayed on the screen is generated by the right side of the transducer as viewed by the operator. Subsequent small movements of the transducer, parallel to the four-chamber plane, will then allow the operator to decide if the image displayed corresponds to a section of the thorax viewed as though looking from above (Fig. 1.7(a) and 1.8(a)) or from below (Fig. 1.7(b) and 1.8(b)), whether the fetus is in a supine (Fig. 1.7) or prone position (Fig. 1.8). In Fig. 1.7(a) and 1.8(a), therefore, the head is in front of the screen (HF), whereas in Figs. 1.7(b) and 1.8(b), the head is behind the screen (HB). Many four-chamber views in the book will be labelled HF or HB to stress the understanding of this point. Look out for d is o rd e r s o f h ea r t p o si t i o n. 7 in this web service

8 (a) Fig. 1.7 and 1.8 Continued (b) 8 should be rotated 180 about the axis of its cable, to ensure that it is.) Alternatively, touching the edge of the transducer or adjacent abdominal skin should demonstrate distortion of the image at the corresponding side of the screen. This confirms the correct orientation of the transducer relative to the screen image. Then the transducer is moved parallel to the imaging plane (out-of-plane-slide), away from the operator. From the change in the structures seen, the operator should then be able to determine whether the transducer is moving up or down the fetus. If the appearance is consistent with the transducer moving down the fetus ( caudad) as it moves away from the operator, then the section of the fetus will be as though viewing a cut section from above ( Fig. 1.7(a) and 1.8(a)). Conversely, if the image moves up the fetus (cephalad) as the transducer is pushed away from the operator, then the section of the fetus will be as though viewing a cut section from below ( Fig. 1.7(b) and 1.8(b)). If one imagines the whole fetus viewed from either above or below as in Fig. 1.7(a) and (b) and 1.8(a) and (b), there should be no difficulty in ascertaining left and right, regardless of whether the fetus is supine or prone. Various other specific methods of establishing orientation have been in this web service

9 described, including that from Cordes as follows. In the long axis of the fetus, the head is positioned to the right on the screen (whichever side it truly lies). The transducer is turned 90 clockwise from this position to the four-chamber view. This effectively positions the head behind the screen. Thus, in the normal fetus, if he/she lies face up, this will display the liver on the left and the heart on the right of the screen, the reverse if face down. Each operator will have their own preferred method for establishing orientation with which they feel most secure. One cannot, however, establish the right left orientation of the fetus on the basis of the position of internal organs, such as the stomach or heart, since their position may vary from the normal. Moreover, abnormalities of laterality often affect both the stomach and the heart. Thus, the appearance of the stomach and cardiac apex on the same side of the fetus as each other in no way guarantees that both are normally positioned on the left. Fig. 1.9 demonstrates how an abnormality of position of the heart may be evident only if the operator ascertains right and left independently of the supposed position of the internal organs. However, once the left right orientation is determined and a normal (or otherwise) position of the stomach and cardiac apex is established, these structures are then useful points of reference to maintain appreciation of right and left throughout the rest of the scan, during which the fetus may move from its original position. After the first step on ultrasound examination of the maternal abdomen identifying the position of the fetus relative to the maternal pelvis, breech, cephalic or transverse lie, which will allow the correct identification of left and right in the fetus the transducer should then be lined up in a transverse section of the fetal trunk, approximately at the level of the diaphragm. Maintaining a more or less horizontal cut, the ultrasound beam is then swept from the level of the stomach, in a cranial direction, through the cardiac structures to the inlet of the thorax. This is achieved Fig T h e t w o u l t r a s o u n d i m a g e s are identical. It is only by understanding the relationship of the image slice to the whole baby that the operator can know if he or she is viewing a normally positioned heart as though from above (left-hand image), or mirror-image dextrocardia viewed as though from below (right-hand image). Instead of the right-hand fetus lying HF as would be expected, it is HB. L st Ao IVC R R IVC Ao IVC st L Fig O n t h e l e f t- h a n d p a n e l, t h e fetus lies with the head in front of the screen (HF). The stomach is seen in a cross-section of the abdomen lying on the left of the fetus in the usual position relative to the other abdominal structures. The abdominal aorta lies anterior and slightly to the left of the spine with the inferior vena cava anterior and to the right of the aorta. Note these normal spatial relationships. On the right-hand panel, the fetus lies with the head behind the screen (HB). The same positional relationships of the aorta, inferior vena cava and stomach are seen. 9 in this web service

10 RL Liver Thymus RV Fig This fetus is viewed from the front following removal of the anterior chest and abdominal walls to show the heart lying in the left chest with its apex pointing leftward. As a result of the large fetal liver extending to the left abdominal wall, the apex is pushed cranially. This renders the right heart structures anterior. The left atrium and ventricle lie posteriorly, so are hidden from view from the front. The arterial trunks in the upper thorax are covered by the thymus. Note that there is a good portion of right lung (RL) between the right atrium and right thoracic wall. LV mainly by angulation in the first half of pregnancy or by out-of-plane slide in the larger fetus ( Fig. 1.6 ). This sweep shows all the cardiac structures that are necessary to check during a fetal echocardiogram and, from the correct starting point, can be accomplished in seconds. Th e cross-section of the abdomen ( Fig ) shows the stomach on the left. As the beam is swept up to the heart, the inferior vena cava can be followed to its connection to the right atrium. The stomach is noted to be on the same side as the cardiac apex. The four-chamber plane is seen in a completely transverse section of the thorax just above the diaphragm. This is because the large fetal liver, extending to the left abdominal wall, tilts the apex cranially so that the base of the heart almost lies flat on the diaphragm ( Fig ). Note that this is in contrast to the situation in postnatal life and especially in the adult, where the apex points caudally and the four-chamber plane does not correspond to an orthogonal transverse plane. The correct technique of obtaining the four-chamber plane is vital, as the ultrasound beam must be positioned in the correct orthogonal plane at the right level in the heart, in order to analyze it accurately. Confirmation that the plane obtained is indeed a standard four-chamber plane should precede any attempt at detailed analysis. The correct plane is a completely transverse section, as evidenced by a round shape to the thorax and at least one complete rib in the image. When multiple ribs are seen, this indicates that the section is oblique, either laterally or antero-posteriorly ( Fig ). The correct level within the heart for assessment of the four-chamber plane shows the crux or center of the heart and is a precise point in the heart, which in the early fetus is quite small. If the level is too low in the heart, the 10 Spine Spine Fig I n t h e l e f t- h a n d p a n e l, t h e ultrasound beam is positioned correctly across the thorax in order to image the four-chamber view. The thorax has a round shape and one almost complete rib is seen. In the right-hand panel, the transducer beam cuts obliquely across the thorax, as can be seen by multiple ribs on one side of the image. Something of the four-chamber view is seen in the heart, but this is not adequate for either analysis of the view, nor is it the correct starting point to initiate a sweep to the great artery views (HF). in this web service