Chapter 6. Image Capture and Orientation

Women’s Point of Care Ultrasound

Essential Concepts Made Clear

This chapter will focus on the basic techniques used to capture the best ultrasound image. Although it seems obvious, any assessment made by ultrasound depends on the quality of the image. Approach and orientation to the body and its structures, as well as how to maneuver the transducer, are integral to finding the best image. A detailed image leads to a more appropriate clinical decision.

Objectives:

By the end of this section you will be able to:

Apply the concept of tomography to ultrasound in how it obtains a slice of a structure for an image
Describe what is meant by scanning planes, or orientation to the body and its internal structures
Learn the terms of sagittal, longitudinal (or long), transverse, coronal, and how these are obtained
Demonstrate classic movements of a transducer: sliding, tilting, rocking, and rotating
Describe the relationship between the probe indicator (notch) and the color indicator or “dot” on the screen; recognize the clinical importance of this relationship

Tomography

Ultrasound is an imaging modality that uses the technique of tomography. Tomography is the technical term for how imaging acquires a single “slice” of the body to examine it. ACT procedure is where multiple slices of the body are taken by x-ray, and slices are reviewed separately and in sequence, looking for abnormalities.

Figure 1. Tomography examples

In ultrasound, the same thing is done, except it is done in real-time. Each movement of the transducer over the body changes the angle, direction, and depth of that slice. Once the ideal view is seen, the image is “frozen” for capture and measurement.

How the “cut” or slice is made, and where it is angled, determines what is seen. Imagine the transducer as a beam of light but instead slicing through a single plane as a thin vertical ‘cut-out’ instead of a round cylindrical beam.

But when looking at this single image slice, it is difficult to discern the location or general position of the structure without using certain guidelines on how we approach or capture the image. In other words, is the structure right or left? Is it cephalic (toward the woman’s head) or caudal (toward the woman’s feet)? Or is it anterior (toward the front of her body) or posterior (toward the back)?

This is why correct orientation is critical to accurate interpretation.

Sidebar 1. Tomography tips.

Orientation

This section describes how to orient the transducer to find structures, their location, and characteristics. This process is like using a compass to help find location. With ultrasound, it is critical to identify where you are in the body, and from which angle you are imaging. Locating a key body region as a frame of reference (such as the spine or other easily identified organ) helps to know where a particular part lies. Correct orientation to the woman’s anatomy is critical to obtaining an accurate representation of structures.

In defining orientation, the body is sectioned into planes or lines that divide the body into equal parts.

Orientation in ultrasound has to do with the placement and relationship of the transducer to the anatomy.

The transducer’s “notch” should always be pointing to the patient’s head in sagittal, and always to the patient’s right in transverse

Most common planes:^3,4

Sagittal (longitudinal)
Transverse
Coronal – used more with 3D and 4D scanning
Median plane has to do with midline of the body

Note: Orientation helps to identify the scanning region and helps to display the image correctly in relationship to other organs.

Figure 2. Body planes illustration.

Understanding how planes are defined is critical for ultrasound or any imaging modality, as it gives the frame of reference for describing the location of the body part being captured by the image.

Sagittal means:

Dividing the body into two longitudinal halves (right and left)

Transverse means:

Dividing the body into two sections – upper and lower halves

Coronal means:

Dividing the body into two sections – front and back (ventral and dorsal)

Figure 3. Planes of view for body structures.

When referring to a structure or organ, the orientation concepts remain the same, but some use the term “long” instead of “sagittal” when referring to a body structure as opposed to referring to the whole body.

Longitudinal (or “long”) means:

Dividing the organ or structure into two longitudinal halves (right and left)

Transverse means:

Dividing the organ or structure into two sections – upper and lower halves

Coronal means:

Dividing the organ or structure into two sections – front and back (ventral and dorsal)

We sometimes use the terminology of x, y and z axes.

Sidebar 2. Sagittal versus transverse slicing.

Think of the body part you are imaging as a familiar object, as in the above illustration of a loaf of bread. Slicing the object lengthwise gives you what is called the sagittal view. Slicing the object crossways gives you slices that are known as transverse.

Transducer Selection

Each type of transducer has a slightly different view, and this affects orientation. When choosing a transducer, select one with the appropriate frequency range for what is to be scanned. This was presented in the previous chapter. To review, to view a superficial structure more clearly, the most appropriate transducer requires use of a higher frequency range. For looking deeper into the body, a lower frequency is needed.

Positioning the transducer^8-10

Improper ergonomics can be a problem for those who perform regular ultrasound exams. It is important to hold the transducer correctly to prevent stress on wrists and hands. Hold the transabdominal transducer like holding a pencil or a paintbrush. If while scanning, the probe needs to be rotated in an awkward position, re-position the hand to avoid twisting the arm and wrist. But in general, holding the probe consistently as shown will cause less fatigue. When first learning to scan, many tend to hold the probe too tightly, which can cause muscle fatigue. Keeping the hand relaxed helps avoid this. Remember to loosen your grip so you don’t cause muscle fatigue while scanning.

Handedness can sometimes be a factor in how one scans. Those who are right-hand dominant may need to use the right hand to hold the transducer, since this hand has better fine motor control. And vice versa for the left-handed person. However, some find that even if left-handed, they accommodate by holding the transducer in the non-dominant hand. The room constraints, and limitations of whether the machine is on the right or left of the woman, can also affect which hand the practitioner uses to scan.

Sidebar 3. Holding the transducer.

Figure 4. Direction of scanning.

When placing the transabdominal transducer on the abdomen, the part of the screen image that is at the top is what is closest to the transducer. So “up” on the image is not up toward the maternal head, it is “up” toward the transducer.

When inserting a transvaginal transducer, the image perspective is different. What is seen on the screen as “up” is what is closest to the transvaginal transducer. But what is seen on the screen is really at right angles to how the probe is actually positioned. This requires a “re-orientation” of how the practitioner interprets the image. It becomes easier to understand with live scanning and visualizing how things change when the transducer is moved. This will be described in greater detail later.

Orienting the transducer with use of the notch.

A curvilinear abdominal transducer is an arced probe. Each probe has a notch or marker on the side to designate that this is the side you should be pointing either to the woman’s head or toward her right side.

Sidebar 4. Notch on curvilinear transducer.

The area toward the notch is called the “toe”, and the opposite end is called the “heel”.

Sidebar 5. Notch demonstration

Matching the notch to the screen

There is usually a “dot” or an indicator on the upper left corner of the screen that represents where the transducer notch is in relationship to the body. It will indicate direction toward the woman’s head in sagittal, or her right (in transverse). Sometimes, instead of a green or blue dot there will be some sort of symbol – it can also be an icon of the machine brand.

Figure 5. Direction indicator on the screen display.

Matching the Screen

There is usually a “dot” or an indicator on the upper left corner of the screen that represents where the transducer notch is in relationship to the body. It will indicate direction toward the woman’s head in sagittal, or her right (in transverse). Sometimes, instead of a green or blue dot there will be some sort of symbol – it can also be an icon of the machine brand.

Review: When the transducer is held to the body or structure being imaged, the notch should always be pointing toward the head (cranial or cephalad) (below)

Figure 6. Orientation - sagittal.

When the transducer is held transverse to the body (or the structure), the notch should always be directed to the patient’s right.

Figure 7. shows that when the transducer is rotated with the notch toward the maternal right, the image you see on the screen will be a transverse view of the pelvis.

Figure 7. Orientation - transverse.

When looking at an internal structure as the primary focus, such as a fetus, the terms sagittal or “long” can be used to describe the orientation to the fetus, and not the mother. This is because the fetus could be lying in a different plane than the vertical axis of the mother.

Figure 8. Sagittal to the FETUS.

Figure 9 demonstrates scanning a fetus in a transverse plane. It happens that we are also scanning the mother in transverse, but this is not always the case, so once again, the object of interest, and what is described as transverse, is the fetus.

Figure 9. Transverse to the fetal spine.

Transducer adjustments for Improved Images¹

Ultrasound positioning is not always either true sagittal or true transverse, just like body organs are not aligned either as exactly vertical or horizontal. When describing the above orientation concepts, this does NOT mean that those are the only angles that can be used. If there is a structure that lies in a different angle than perpendicular or parallel to the body planes, it may be necessary to turn the transducer slightly one way or the other to get a correct view.

If it is difficult to see a structure, additional movements may be needed to help bring it into better alignment. Changing angles can also affect artifacts.

This section describes how beam direction can be changed by moving the transducer.

The following slides describe movements of the transducer for the best image. The cardinal movements that can be used include sliding, tilting, rocking, rotating, and compressing.

Figure 10. Diagram of transducer adjustment options.

Sliding is the movement of the transducer in one plane – either up and down, or side to side in the transverse plane, as shown in Figure 11. Sliding over the entire pelvic area in both sagittal and transverse directions at the start of an exam enables the practitioner to get a view of the entire pelvis or gravid uterus before focusing on a single area.

Figure 11. Illustration of sliding the probe.

Tilting (or angling) involves leaving the transducer in the same place on the body, while angling it one direction or another. This allows changing the direction of the scanning plane without moving the probe (Figure 12).

Figure 12. Illustration of tilting the probe.

Rocking is a technique where the transducer is tilted forward or backward to view different angles (Figure 13). Angling the transducer heel to toe is most helpful in bringing an object of interest into the center of the screen. When only a part of the organ is seen in the screen, rocking the transducer toward the direction of the organ will bring it into a more central view, and allowing a more complete view of the organ or structure. This does not require the probe to be moved - it only changes the direction of the view.

Figure 13. Rocking the probe.

Rotating the transducer from sagittal to transverse or vice versa allows us to see the structure in those respective planes (Figure 14).

Rotating the transducer means that you leave the probe in place while turning it like turning a dial. Starting from the sagittal plane, the probe is turned so that the notch angles toward the maternal right. If already in a transverse plane and rotating back to sagittal, then rotation would typically be in a clockwise fashion.

When rotating, a key thing to remember is to avoid any movement of the probe other than rotating. The purpose of rotating is to keep the object of interest still in view, but to provide a transverse look at that object. Slight movement of the probe one way or another can make the object of interest no longer visible, requiring repositioning the probe to find it again.

Figure 14. Rotating the probe.

Compressing the transducer against the patient’s skin can improve contact or bring us nearer a structure. In general, the closer the transducer is to the structure, the better the image will be. However, care must be taken not to compress the imaged structure too hard, resulting in a distorted image or measurements. For example, when measuring pockets of amniotic fluid, too much compression can alter the measurements. Some fetal part measurements can also be altered by compression, such as the fetal abdomen.

The following are additional diagrams illustrating these maneuvers, with examples of ultrasound images that might be seen in those positions.

The illustration below indicates the transducer in a sagittal position with a transabdominal transducer. As stated before, sagittal means you have the probe angled parallel to the long axis of the body, with the notch pointed toward the woman’s head. The correlating image of the uterus shows that the uterus is also in a sagittal view.

Figure 15. Sagittal view of uterus with transabdominal probe.

The following illustration shows a transabdominal ultrasound of the pelvis in a transverse position. This means that the probe is angled perpendicular to the long axis of the body, with the notch pointed toward the woman’s right. The correlating image of the uterus shows a transverse “slice” of the uterus at that angle.

Figure 16. Transverse view of uterus with transabdominal probe.

Transvaginal Orientation

When using a transvaginal probe, orientation is different from transabdominal. It takes some additional learning time to understand the difference, and although it is discussed below, hands-on scanning experience helps increase understanding.

When using a transvaginal probe, the “notch” is at the top of the probe when held correctly. It should be pointing toward the anterior surface of the woman, or to the right side of her pelvis in the transverse, to keep orientation correct. This means that the dot on the screen indicates that the probe is either angling upward (toward the woman’s abdominal surface) or toward her right pelvis. This will be reviewed again in the GYN course.

When placing a transvaginal probe, the probe needs to be held straight “up and down”, with the notch pointed toward the anterior surface of the body (Figure 17). Observng the position of the probe in one’s hand allows the clinician to see which direction achieves a sagittal or a transverse view.

Figure 17. Correct position of transvaginal probe to achieve sagittal versus transverse views of the pelvis.

^{. **Photos are of ultrasound phantoms/manekins.}

As demonstrated in Figure 18, If the image on the screen is seen at the same angle as the insertion angle of the vaginal probe, the image would look like the view at the right. However, the display screen stays consistent with the transducer footprint represented at the top of the display, and the part of the body furthest from the transducer footprint at the bottom of the display. Since all ultrasound screen displays are in the same position (footprint at top and distance at the bottom) the configuration has to be reconciled in the mind of the practitioner as though the scan was being done at right angles.

Figure 18. Diagram of transvaginal probe in sagittal view.

The blue triangle at the end of the probe in Figure 18 represents the “toe” or the notch. It is angled toward the abdomen or anterior of the woman’s body. On the display to the right, the blue dot confirms that the ultrasound “toe” is pointed anteriorly, which is correct.

Figure 19. Orientation using a transvaginal probe.

Figure 20. Sagittal transvaginal illustration

And for a transverse transvaginal view, rotate the probe to the patient’s right and have the “toe” or top of the probe angled toward the patient’s right pelvis, as in Figure 20.

Figure 21. Transverse transvaginal illustration

Figure 22 View on screen of transvaginal scan.

Figure 23. Transperineal technique

The transperineal scanning technique is useful for several reasons. There are situations where transvaginal imaging is either not available, or transvaginal penetration is contraindicated for the woman. This approach can assist in visualizing structures that cannot be seen transabdominally, such as low pelvic organs (like a cervix). In this case, transabdominal imaging can be limited by the presence of the symphysis pubis, obstructing the view.

In this method, the curvilinear probe is used to visualize deeper structures in the pelvis using a lower frequency curvilinear transducer that can traverse deep into the pelvis. The sagittal view is obtained by placing the probe gently inside the intra-labial groove, with the notch pointing upward. The transverse view is accomplished by rotating the transducer so that the notch is pointing to the woman’s right side while pressing gently toward the introitus. This method will be reviewed in greater detail in the OB course.

Translabial imaging is used in many countries to evaluate progression of labor. The orientation is different since we are approaching the fetus or other pelvic structures in a different plane than the transabdominal. Translabial imaging will be further described in the obstetric POCUS course in this series.

NOTE: Whatever direction the notch on the transducer is pointing to will always be displayed as the left side of the screen on the monitor.

Orientation Summary

2-Dimensional Ultrasound images are captured as tomographic slices through the body
Correct orientation to the woman’s body as well as individual organs of interest is critical to obtaining an accurate representation of the structure(s) on screen
Principles of orientation of both transabdominal and transvaginal probes are described
Movement of the transabdominal probe include:
- Sliding
- Tilting
- Rocking
- Rotating

Foundations of Women's Point-of-Care Ultrasound

Chapter 6 Image Capture and Orientation