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How to… Practical advice on imaging-based techniques and investigations The Basic Training Sub-Committee (BTSC) of the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) was established in 2013 with the aim of creating a Basic Training (BT) program to support newly qualified or qualifying professionals entering the field of ultrasound in obstetrics and gynecology. Upon successful completion of the BT program, trainees can perform independently an obstetric and gynecological ultrasound examination and are competent to recognize potential structural abnormalities, which require referral for a second opinion1. Performing the routine second-trimester anomaly scan correctly is part of the BT obstetrics teaching program. The planes and measurements required for this scan have been defined in the ISUOG Practice Guidelines2-7. However, one of the difficulties faced by the ultrasound student or the newly qualified practitioner is understanding, and being able to apply, the correct qualitative and/or quantitative criteria when deciding whether the part of the fetal anatomy being examined, and/or its size, is ‘normal’ for the gestational age at which the scan is being performed. We have designed the 20 + 2-planes method as a practically focused addition to the BT program, and to provide a logical, comprehensive and time-efficient approach to the routine second-trimester scan in accordance with ISUOG guidelines. This method incorporates the tools required to make clinically relevant decisions. It thus helps to develop the confidence needed by the trainee and/or newly qualified practitioner to determine which findings require referral to a more experienced colleague following a routine second-trimester anomaly scan, and which do not. The structures requiring examination (of which some also require measurement) within each of the 20 planes in the 20 + 2-planes method are described herein. Members of the original ISUOG BTSC reviewed these data to agree: (1) qualitative and quantitative criteria for referral and (2) a list of common fetal anomalies that this method, when followed correctly, should exclude. Referral criteria relating to measurements were then agreed by the BTSC and were based either on internationally accepted published data or, when these were not available, on consensus within the group. In accordance with ISUOG guidelines, no recommendations are made as to which biometry charts should be used for reference, these being a matter of local and/or national preference1, 8. Herein, we provide an overview of the 20 + 2-planes method and how its correct application during a routine second-trimester fetal ultrasound examination assists in the identification or exclusion of 65 fetal structural anomalies (Table 1 (and Table S4 of Appendix S1)). A detailed and illustrated explanation of the topics outlined below is provided in the accompanying Appendix S1, ‘How to apply the 20+2-planes method for identification of 65 anomalies during routine second-trimester fetal ultrasound examination (extended version)’. This How To document is intended to reflect best practice at the time of publication. It is not intended to establish a legal standard of care because some deviations are inevitable depending on individual circumstances and available resources. The 20 + 2-planes method is so named because it includes two real-time sweeps of the fetus, together with 20 static planes of specific parts of the fetal anatomy. As shown in Table 2 (and Table S1 of Appendix S1), the 20 planes are grouped into seven anatomical areas for ease of reference, namely: spine (planes 1–3), head (planes 4–6), thorax (planes 7–10), abdomen (planes 11–13), pelvis (plane 14), limbs (planes 15–17) and face (planes 18–20). Plane 3, showing the full body length in coronal section, is included in the anatomical area of ‘spine’ to maintain the sequence of the scanning planes described. The sequential probe movements required to perform the structured examination through the 20 planes are described in Table 2 (and Table S2 of Appendix S1). The most common appearance of the 65 structural abnormalities excluded by identifying the normal appearance of the ultrasound landmarks associated with each of the 20 + 2 planes is shown in Table 3 (and Table S3 of Appendix S1). The examination is performed with the operator sitting on the left-hand side of the scanning couch, holding the probe in their right hand. The woman lies facing the operator; therefore, her right side is next to the operator. Understanding the relationship between both the orientation and the movement of the probe and the resulting images displayed is vital before any attempt to obtain images is made. There are only four probe movements: sliding, rotating, angling and dipping. It is important that only one movement is performed at a time; combining two or more together will not produce the result sought; for example, angling slightly when aiming only to slide will not produce the same result as that from sliding alone. Illustrative information relating to the four probe movements can be found in Appendix S1. Sliding describes the movement of the probe over the maternal abdominal wall. This is used to image a second section of the uterus that is parallel to the first. The probe should be held upright when the sliding movement is performed (Figure S1 of Appendix S1). Rotating describes the movement of the probe around its central axis and is used to change the plane of the fetal section being imaged; for example, changing from a transverse plane of the fetus to a longitudinal or oblique plane of the fetus, or vice versa. When the probe is rotated from a longitudinal position on the maternal abdomen to a transverse position on the maternal abdomen, the probe movement should be anticlockwise. This is because the relationship between one end of the probe and one side of the displayed image is constant, irrespective of where the probe is placed on the maternal abdomen. Therefore, rotating the probe incorrectly, that is, in a clockwise direction, will upset this important association (Figure S2 of Appendix S1). Angling describes changing the angle of the long face of the probe relative to the maternal skin surface. This is required when there is a lack of symmetry in the anteroposterior (AP) appearance of the anatomy being demonstrated, as a result of the current angle of the probe relative to the maternal abdominal wall being incorrect (Figure S3 of Appendix S1). Dipping describes altering the position of the whole image shown on the screen relative to the horizontal. It is achieved by pressing one short face of the probe gently into the maternal abdomen. Dipping, unlike sliding, rotating and angling, does not alter any of the information demonstrated within the image, but rather alters the orientation of that information (Figure S4 of Appendix S1). The 20 + 2-planes method begins with an overview of the uterus (sweep 1). Sweep 1 has two main purposes: first, to identify the position of the fetus within the uterus; and second, to confirm fetal cardiac activity. Sweep 1 has been achieved correctly when the operator has identified whether the fetal lie is longitudinal, transverse or oblique, has observed fetal cardiac pulsations, knows where the fetal head is within the uterus and can trace the fetal spine from neck to sacrum (Figure S5 of Appendix S1). An overview of planes 1 and 4–14 can be seen in the schematic display of the fetus shown in Figure 1 (Figure S6 of Appendix S1). For ease of demonstration of the other planes, the positions of coronal planes 2 and 3 are not included in this figure. The relationship between planes 1, 2 and 3 is shown in Figure 2 (Figure S7 of Appendix S1) and the correct ultrasound appearances of these planes are shown in Figure 3 (Figures S8–S10 of Appendix S1). Having ascertained the position of the fetus in the uterus from sweep 1, the longitudinal axis of the fetus can now be obtained. Plane 2 is achieved from plane 1 by angling the probe through 90°. Owing to the AP curvature of the spine, it is not always possible to obtain the correct coronal view of the complete spine in one image. It is therefore acceptable to demonstrate the required anatomy for plane 2 using two images. Plane 3 is achieved from plane 2 by minimal sliding of the probe away from the fetal spine, that is, in a ventral direction, towards the fetal body. The relationship between planes 1, 4, 5 and 6 is shown in Figure 1 (Figure S11 of Appendix S1) and the correct ultrasound appearances of planes 4–6 are shown in Figure 4 (Figures S12–S14 of Appendix S1). Plane 4 is achieved from plane 1 by siding the probe up the fetal body to visualize the upper sagittal spine and fetal head. The probe is then rotated through 90°. Dipping the probe may be required to bring the midline into the horizontal position. Plane 4, with the midline in the horizontal position, cannot be obtained by dipping the probe if the fetal head is in a direct occipitoanterior (OA) or a direct occipitoposterior (OP) position. Measurements of the biparietal diameter (BPD) and head circumference (HC) are made from this section, providing the reference charts being used require the transventricular section and not the ‘transthalamic’ or ‘thalami’ section. Measurement of the BPD using ‘outer-to-outer’ caliper placement is the method recommended by ISUOG3. Measurement of the Vp is made from this section and should be less than 10.0 mm from 18 + 0 weeks onwards to be considered normal. Plane 5 is achieved by a small sliding movement of the probe, towards the fetal neck, from plane 4. A small rotational movement may also be required to include both the CSP and the thalami in the section. Plane 5, with the midline in the horizontal position, cannot be obtained by dipping the probe if the fetal head is in a direct OA or a direct OP position. Measurements of the BPD and HC are made from this section, providing the reference charts being used require the ‘transthalamic’ or ‘thalami’ section and not the transventricular section. Measurement of the BPD using ‘outer-to-outer’ caliper placement is the method recommended by ISUOG3. Plane 6 is achieved by starting from plane 4 and then rotating the probe towards the fetal neck so that both the CSP and the cerebellum are included in the section. Measurement of the transcerebellar diameter is made from this section. Plane 6, with the midline in the horizontal position, cannot be obtained by dipping the probe if the fetal head is in a direct OA or a direct OP position. The relationship between planes 1, 7, 8, 9 and 10 is shown in Figure 1 (Figure S15 of Appendix S1) and the correct ultrasound appearances of planes 7–10 are shown in Figure 5 (Figures S17–S20 of Appendix S1). Confirmation of cardiac situs should be done before beginning the examination of the heart. The fetal lie is established from sweep 1, which, combined with plane 3, allows the operator to determine which side of the fetus is its left side and which is its right side. The apex of the heart should point to the left side of the fetal chest (Figure S16 of Appendix S1). Plane 7 is achieved by starting with plane 1 and then rotating the probe through 90° to obtain a transverse section of the fetal abdomen, then sliding the probe towards the fetal chest. The four-chamber view is best assessed, initially, with the apex of the heart ‘up’, that is, between the 11 o'clock position and the 1 o'clock position, as this provides optimal visualization of most of the required anatomy, and in particular, the positions and actions of the two atrioventricular (AV) valves (plane 7a). The ventricular septum (VS), however, is best assessed when it is lying at an angle of 90° to the ultrasound beam, that is, with the apex of the heart ‘lateral’, in the 9 o'clock or 3 o'clock position (plane 7b). Plane 7b is achieved from plane 7a by dipping the probe (Table 2 (and Table S2 of Appendix S1)). Once plane 7 has been obtained, it is important to observe the heart in real time to confirm equal contractility of the ventricles, opening and closing of both AV valves and the flickering of the foramen ovale in the left atrium (LA). Plane 8 is achieved from plane 7 by a small rotational movement of the probe towards the right shoulder (Table 2 (and Table S2 of Appendix S1)). The literature describes the RVOT in a number of ways, depending primarily on which level of the pulmonary arterial vasculature is included. The three most commonly described views are: (1) the main pulmonary artery (MPA) and pulmonary valve view; (2) the bifurcation view, in which the MPA is seen dividing into the left and right pulmonary arteries; and (3) the MPA and ductus arteriosus view at the point where the ductus arteriosus forms the ductal arch. In the normal heart, the vessel leaving the LV is the aorta. This vessel does not bifurcate, while the vessel leaving the RV, namely the MPA, does. Demonstrating bifurcation of the vessel leaving the RV, which has previously been confirmed as such from plane 7, thus confirms that the vessel leaving the RV is the MPA and, therefore, that the vessel leaving the LV is the aAo, so for this reason, the RVOT in the 20 + 2-planes method is described by the bifurcation view. Plane 9 is divided into two components: plane 9a describes the RVOT and 9b describes the ‘crossover’, specifically, the crossing of the RVOT over the LVOT. Plane 9a is achieved by returning to plane 7 and making a small sliding movement of the probe towards the fetal head. Plane 9b is achieved by sliding the probe between plane 9a and plane 8 to observe the RVOT ‘crossing over’ the LVOT when moving between the two planes in real time (Table 2 (and Table S2 of Appendix S1)). Plane 10 is achieved from plane 9a by making a small sliding movement of the probe towards the fetal head (Table 2 (and Table S2 of Appendix S1)). The relationship between planes 10, 11, 12 and 13 is shown in Figure 1 (Figure S21 of Appendix S1) and the correct ultrasound appearances of planes 11–13 are shown in Figure 6 (Figures S22–S24 of Appendix S1). Plane 11 is achieved by sliding the probe caudally from plane 10 down the fetal body. The transverse section is best assessed with the spine in either the 3 o'clock position or the 9 o'clock position (Table 2 (and Table S2 of Appendix S1)). Measurement of the abdominal circumference is made from this section. Plane 12 is achieved by sliding the probe caudally from plane 11 (Table 2 (and Table S2 of Appendix S1)). The cord insertion is best assessed with the spine in either the 3 o'clock or the 9 o'clock position. Plane 13 is achieved by sliding the probe caudally from plane 12 and then rotating the probe slightly (Table 2 (and Table S2 of Appendix S1)). The kidneys are best assessed in slightly different planes and for this reason plane 13 is subdivided into planes 13a and 13b: a transverse section of the left and right kidney, respectively, at the level of the renal pelvis. The maximum AP diameter of the renal pelvis should be measured using ‘inner-to-inner’ placement of the linear calipers, positioned at 90° to the axis of the kidney rather than to the axis of the abdomen. An AP diameter of less than 7.0 mm is currently considered to be normal for the second-trimester renal pelvis. The relationship between planes 12, 13 and 14 is shown in Figure 1 (Figure S25 of Appendix S1) and the correct ultrasound appearance of plane 14 is shown in Figure 7 (Figure S26 of Appendix S1). Plane 14 is achieved by a small sliding movement of the probe caudally from plane 12 (Table 2 (and Table S2 of Appendix S1)). The of the fetal be its appearance in plane 14 will depending on its When the cannot be seen initially, the findings should be with completion of the scan being for at 20 in to confirm normal and The relationship between planes and is shown in Figure 8 (Figure of Appendix S1) and the correct ultrasound appearances of these planes are shown in Figure 7 (Figures S26 and of Appendix S1). For of the long it is important to determine from the position of the fetus within the rather than on the which is the left or and which is the right or A correct cardiac situs and position can also be used to determine which is the left side and which is the right side of the It is more important to obtain and correct images of the three long of each together with the normal relationship of its or to than to obtain a image of the three long of a in which one or more of is not and/or the relationship of its or to the of that is Plane is obtained from plane 14 by sliding the probe Plane should demonstrate the full length of the that is to the on this the probe is the in the field of view, its full length is achieved (Table 2 (and Table S2 of Appendix S1)). sliding movements of the probe are also required to bring the into the of the Dipping of the probe may be required to bring the into an horizontal position, and to visualization of the end of the Measurement of the length is made from this section. Plane has two components: plane the three long of the left the left and its normal relationship to the left and plane the three long of the right the right and its normal relationship to the right The probe required to plane on the of the left and with to the left and to the left and for plane and the right that the measured in plane was the left a of sliding, rotating and angling of the probe is required to plane Dipping the probe may also be required to bring the into the horizontal position (Table 2 (and Table S2 of Appendix S1)). The relationship between the and is best from a sagittal view of the in which the full length of the is The full length of the should also be seen in this section. A minimal rotational movement of the probe may be needed if the is lying in a slightly or position. the and together a coronal section of the The and are of The is the more positioned Plane the three of the right together with the right and its normal relationship to the right is by returning to plane and the described to plane but for the right The same probe movements described for plane are to plane Plane has two components: plane the three long of the left the left and its normal relationship to the left and plane the three long of the right the right and its normal relationship to the right Plane is achieved by returning to plane 14 and sliding the probe to the upper chest. As the upper to be held to the it will then be to the probe through 90° to obtain a sagittal section of the then to slide the probe to the left side of the body to image the left rotational and possible angling of the probe may be required to obtain the correct image of the complete left in longitudinal section, depending on its position relative to the body. Dipping the probe may also be required to bring the into a more horizontal position (Table 2 (and Table S2 of Appendix S1)). the left and are lying in the same it is that the image of the full length of the will also include the full length of both the and together with the of the left hand. It is more that a small section of the left and/or will be seen with the In this the probe is rotated to obtain the full length of either the or the The is than the and the lies to the while the lies to the and will be together if the correct longitudinal section of the has been obtained. the together with some of the is a of this to of the Plane is achieved by returning to plane and the described to plane but for the right and hand. The relationship between plane 4 and planes and 20 is shown in Figure 9 (Figure of Appendix S1) and the correct ultrasound appearances of planes are shown in Figure 10 (Figures of Appendix S1). Plane 18 is achieved from plane 4 by sliding the probe caudally both are The should be positioned within the The probe is then rotated towards the of the fetal through The of will on the angle of the midline to the horizontal plane in plane 4. sliding movements are then required to plane 18 (Table 2 (and Table S2 of Appendix S1)). Plane 18 is best achieved with the coronal section of the face lying in the horizontal Plane is achieved by sliding caudally from plane 4 both are and/or angling of the probe may also be the correct section was obtained in plane 4, plane will demonstrate both lying in the horizontal one the The and will be if the probe is to bring the head into an OP position (Table 2 (and Table S2 of Appendix S1)). Plane 20 is achieved by sliding the probe caudally from plane 4 to the level of the The probe is then through 90° to demonstrate the face in sagittal section. Sliding and minimal of the probe may also be It is important to that a sagittal section of the face is obtained, as of is if the probe has been positioned slightly the face (Table 2 (and Table S2 of Appendix S1)). Once planes 1 to 20 have been achieved the examination is with sweep 2 a transverse sweep of the fetal body. The of sweep 2 is to both the spine, one at a time being by three and its skin in transverse section. This is achieved by returning to plane 1, then rotating the probe through 90° and sliding it to the level of the and/or angling may also be before starting to slide the probe caudally (Table 2 (and Table S2 of Appendix S1)). the 20 + 2-planes method correctly, the operator is with a and comprehensive approach to the routine second-trimester anomaly When the 20 + 2-planes method allows a of 65 fetal structural abnormalities to be the method provides the operator with the tools to make in between appearances that are normal and that are not. it provides the operator with the to between findings that to be to scanning and that are to a potential structural and therefore to correctly when referral for a second is We the of the ISUOG and for their and in and the the 20 + 2-planes We also the of the ISUOG and have the of this We are to and of Obstetrics and of and of Obstetrics and of Obstetrics and The for their in the of the probe movements slide and in providing the images shown in this and Appendix S1, the not have been we are to Ultrasound in Obstetrics for her to and in the of this The data that support the findings of this are available from the Appendix S1 How to apply the 20 + 2-planes method for identification of 65 fetal anomalies during routine second-trimester fetal ultrasound examination (extended The is not for the or of any information by the than should be to the for the
Chudleigh et al. (Mon,) studied this question.