Guidance

AAA screening: ultrasound image quality guidance

Updated 1 May 2025

1. Introduction

As part of the NHS Abdominal Aortic Aneurysm (AAA) Screening Programme[i],  screening technicians use ultrasound imaging[ii] to measure the diameter of the abdominal aorta. 

NHS England provides guidance on this screening test and quality assurance (QA) processes. This ensures ultrasound measurements of the abdominal aorta are performed to an acceptable diagnostic standard and accuracy.

This document provides guidance for screening technicians, clinical skills trainers (CST’s) and Internal Quality Assurance (IQA) leads on specific aspects of image quality and image QA review. This is to promote consistency in performance across local services. Images should be quality assured within the appropriate timeframe as detailed in the AAA screening programme standards[iii].  

2. Image quality factors

Ultrasound image quality is influenced by factors including:

• operator skill, knowledge and performance
• body habitus and other patient factors
• ultrasound artefacts producing misleading features within an image
• condition and performance of equipment
• scanning environment

Personal preference and bias can also influence the assessment of image quality.

All the above factors will influence the overall appearance and acceptability of an image, with some factors beyond the control of the screening technician. Screening technicians should avoid overly focusing on capturing a perfect image at the expense of examining the visible aorta and missing an aneurysm.

3. Image quality categories

The categories set out below, aim to provide an objective and structured view of image quality.  This is to ensure consistency in the examination and subsequent QA review of captured images and in feedback together with recommended actions provided to screening technicians.

This document uses 3 categories of image quality. These are:

• satisfactory
• satisfactory with learning points
• unsatisfactory

3.1 Satisfactory

Satisfactory indicates the images are of diagnostic quality and have been captured in line with NHS AAA Screening Programme guidance[iv].  

3.2 Satisfactory with learning points

Satisfactory with learning points indicates the images are of diagnostic quality for accurately measuring the diameter of the aorta but have not been optimised. Allowances must be made for factors beyond the control of the screening technician, such as patient body habitus or other relevant limitations, particularly when these have been indicated on the AAA IT system SMaRT (Screening Management and Referral Tracking).

3.3 Unsatisfactory

Unsatisfactory indicates the images are not of diagnostic quality for the accurate measurement of aortic diameter due to non-optimisation of scanner controls, poor scanning technique or factors beyond the control of the screening technician.

If an image is in this category, the CST or IQA lead may decide the man needs to be rescreened.

Service providers should have a local process in place for managing performance issues, particularly if a screening technician has received a high number of unsatisfactory comments during image QA review and there are no mitigating reasons for this. Local services should refer to the IQA framework and competence assessments guidance for further support.

4. Image capture and review

4.1 Visibility of aorta

Do the images recorded in transverse plane (TP) and longitudinal plane (LP) demonstrate the abdominal aorta?

Yes

It looks like the aorta. Continue with image assessment.

No

The aorta is not visible, or cannot be clearly defined, or another structure has been mistaken for the aorta, or an insufficient LP length has been captured.

Guide for CST/IQA lead: attempt to identify the problem. Has the screening technician provided any comments or requested image QA? Give feedback to the screening technician, including a one-to-one review of the images. Consider further training if appropriate, with an action plan for improvement if there are concerns about a screening technician’s general performance. Recall man for further scan. Services should track these men for rescreen to encourage timely re-attendance.

4.2 Image gain

Satisfactory

For an image to be satisfactory, the gain settings should be optimised so the display demonstrates a range of grey scale echoes from white to black. It should also be noted that dynamic range setting can also affect the appearance of the B-mode image. The gain should allow clear differentiation of aortic lumen (dark grey and black) and aortic wall (light grey and white). This is to enable accurate placement of measurement callipers as well as identification of adjacent anatomy such as the lumbar spine and inferior vena cava (IVC) (see figures 1A and 1B below).

Figure 1A: A transverse image of the aorta with acceptable depth and gain settings and visible landmarks such as the inferior vena cava (IVC) and anterior border of the lumbar spine (marked by the star). The focus position is set too deep (see focus marker in white circle), but this has not had a significant impact on image quality and feedback can be provided in QA comments.

Figure 1B: A longitudinal image of the aorta showing the correct line of measurement.

Satisfactory with learning points

An image will be satisfactory with learning points if there is:

• high gain, resulting in the grey scale image having a disproportionate level of bright white echoes, with reduced differentiation between the aortic wall and lumen, and aortic wall and surrounding structures, including the lumbar spine
• low gain, resulting in the grey scale image display being predominately dark with mainly black and dark grey echoes

Guide for CST/IQA lead: a rescreen is not required provided the aortic walls are defined and accurate diameter measurements have been made. Provide feedback to screening technician.

Unsatisfactory

An image will be unsatisfactory if there is inadequate differentiation between the aortic wall and the lumen (see figures 2 and 3 below).

This is normally due to very low gain setting. However, excessive gain can result in bright white aortic wall echoes that lead to flaring or blurring with adjacent tissue. Accurate calliper placement cannot be verified.

Guide for CST/IQA lead: a re-screen is recommended with feedback given to the screening technician along with a one-to-one image review. Services should track these men for rescreen to encourage timely re-attendance.

Figure 2: A longitudinal image of an AAA. In this image, excessive gain would make accurate calliper placement at the inner aortic walls difficult as they are not clearly defined (marked by the arrow heads). In such situations, if the diameter was found to be close to surveillance or referral thresholds, a re-scan should be considered.

Figure 3: Transverse (A) and longitudinal (B) images of the aorta. The gain is too low, and no landmarks are visible, such as the anterior border of the lumbar spine. The focus position is high. It’s difficult to differentiate the longitudinal image from the transverse image, with both appearing roughly spherical. In this situation a re-scan would be appropriate.

4.3 Image display depth

Satisfactory

Whether an image is satisfactory or not will depend on the man’s body shape and body mass index (BMI). The centre of the aorta should ideally lie between the middle to lower half of the image screen with sufficient depth selected to include the identifying landmark of the anterior border of the lumbar spine.

TP and LP images should be captured with the same depth settings. This means the relative depth and size of the aorta, and characteristics of the 2 images, can be compared side by side.

Make allowances for men with low BMI, where the aorta can appear in the upper section of the image.

Satisfactory with learning points

An image will be satisfactory with learning points if:

• excess image depth is indicated when there is more than 4cm of image below the level of the anterior border of the lumbar spine or when the anterior surface of the lumber spine is positioned towards the middle of the image screen with respect to depth, as in figure 4
• too little depth is indicated (when the posterior wall of the aorta is positioned adjacent to the bottom of the image, with the level at the anterior border of the lumbar spine not visible)

Figure 4: Excessive depth has been used in this transverse image of the aorta. The anterior border of the lumbar spine is just about visible (upward arrowhead) but there is more than 7cm of depth below this point (the space between the 2 arrow heads). This means the image of the aorta only occupies a small part of the screen. The anteroposterior (AP) diameter has also been measured in an oblique (slanting) direction and the focus position is too low. This is an example of an image with multiple learning points.

Unsatisfactory

An image will be unsatisfactory if inappropriate depth setting has resulted in difficulty measuring the aortic diameter accurately.

This will be mainly applicable when excessive image depth has been used, resulting in a small image of the aorta within the display.

Guide for CST/IQA lead: if the aortic diameter cannot be accurately measured or is close to specific programme diameter thresholds (3cm, 4.5cm and 5.5cm), a rescreen is recommended. Services should track these men for rescreen to encourage timely re-attendance. This situation is uncommon, but the screening technician must be given feedback with a one-to-one review of the images.

4.4 Focus position

Satisfactory

The ideal focus point is between the anterior and posterior walls of the aorta. If more than one focus zone has been selected the aorta lies between the focal zones. The focal zone is where the beam converges to the focal point and diverges beyond it. Spatial resolution will be improved in this zone; that is, the ability to distinguish 2 closely adjacent reflectors as separate objects.

Satisfactory with learning points

An image will be satisfactory with learning points if:

• the focus position is set more than 3cm above the anterior wall of the aorta (see figure 3)
• the focus position is set more than 3cm below the posterior wall of the aorta (see figure 1A)

Guide for CST/IQA lead: provide feedback to the screening technician.

4.5 Landmarks

Satisfactory

For an image to be satisfactory the anterior border of lumbar spine should be demonstrated on TP and LP images (see figures 1A and 1B). When visible, the inferior vena cava (IVC) should be demonstrated in the TP. The IVC is not always visible. Compression of the IVC may occur in men with low BMI.

Satisfactory with learning points

An image will be satisfactory with learning points if the anterior border of the lumbar spine is barely visible within the image.

Allowances should be made when the screening technician has indicated difficulties due to body habitus or other factors.

Unsatisfactory

An image will be unsatisfactory if the anterior border of lumbar spine is not visible on any image (see figure 3). This can be due to a combination of factors such as orientation, depth, gain, focus or body habitus. Allowances should be made when the screening technician has made accompanying comments.

Guide for CST/IQA lead: check if the aorta has been captured and measured correctly. The absence of landmarks does occur in difficult scans. If there is significant doubt about the imaging of the aorta and its measurement, a rescreen is recommended. Services should track these men for rescreen to encourage timely re-attendance.

4.6 Orientation of aorta

Satisfactory

An image will be satisfactory if:

• the TP image shows the aorta in the centre of the image field, as opposed to one side
• the LP view shows the aorta orientated in a perpendicular direction, running across the image screen; this provides the best definition of aortic wall as the strongest reflections are obtained when the ultrasound beam is perpendicular to the walls of the aorta (see figure 5B)
• the correct aortic diameter measurement has been taken perpendicular to the walls of the aorta

Satisfactory with learning points

An image will be satisfactory with learning points if:

• the TP image shows the aorta positioned to one side
• the LP image shows the aorta is orientated in a diagonal direction across the screen (see figure 5A)

Guide for CST/IQA lead: there will be instances where the aorta will follow a tortuous (twisting) path or positioning of the probe on the abdomen is limited (by scarring or a stoma, for example). Allowances should be made in these circumstances. Advise screening technicians to record comments on the IT system at the time of the scan.

Unsatisfactory

An image will be unsatisfactory if the orientation of the aorta within the image has resulted in inaccurate diameter measurement. Poor orientation is also associated with oblique (slanting) planes of diameter measurement. In this case, the aorta can appear markedly oval in shape in the TP image.

Guide for CST/IQA lead: if the aortic diameter measurement is close to specific programme diameter thresholds (3cm, 4.5cm and 5.5cm), a rescreen should be considered. Services should track these men for rescreen to encourage timely re-attendance.

4.7 Calliper placement

Satisfactory

An image will be satisfactory if the measurement callipers are positioned correctly on the anterior and posterior inner walls of the aorta at the widest observed point on the image (see figure 1).

Satisfactory with learning points

An image will be satisfactory with learning points if the calliper placement is in the middle of the aortic wall, or within 2mm of the widest point on the longitudinal image.

Guide for CST/IQA lead: providing the aortic diameter is not close to specific programme diameter thresholds (3cm, 4.5cm and 5.5cm), a rescreen is not required. Provide feedback to screening technician with image review.

Unsatisfactory

An image is unsatisfactory if callipers are positioned outside the anterior or posterior aortic wall, or positioned adjacent to thrombus, or the measurement taken more than 2mm away from the widest point of the aorta in the LP (see figure 5).

Guide for CST/IQA lead: if the diameter is close to specific programme diameter thresholds (3cm, 4.5cm and 5.5cm), a rescreen is recommended.

Services should track these men for rescreen to encourage timely re-attendance. Provide feedback to screening technician with one-to-one image review.

Figure 5A: The anterior calliper has been placed outside the aortic wall. The white and black arrows demonstrate the anterior (white) and posterior (black) aortic walls. The image of the aorta is also orientated in a diagonal direction.

Figure 5B: There is better definition of aortic walls than in figure 5A, as the image of the aorta has been orientated perpendicular to the direction of the scan lines in the centre of the image. However, the posterior calliper has been incorrectly positioned adjacent to echoes from the lumbar spine, resulting in over-estimation of aortic diameter. The black arrowhead shows the position of the posterior aortic wall. In this example if the depth were adjusted more of the posterior landmark (spine) would be visible to enable technician to identify this better and avoid this error.

4.8 Orientation of calliper placement

Satisfactory

An image will be satisfactory if:

• the TP line of diameter measurement is taken in an anteroposterior direction
• the LP line of measurement is perpendicular to aortic lumen at the point of maximum aortic diameter

Satisfactory with learning points

An image will be satisfactory with learning points if:

• the TP line of diameter measurement is not quite in a true anteroposterior direction, but does not result in a change in the recorded diameter measurement
• the LP line of diameter measurement is not quite perpendicular to the lumen and runs in a slightly oblique (slanting) direction, but does not result in a change in the recorded diameter measurement (see figure 6)

Guide for CST/IQA lead: Provide feedback to the screening technician with image review.

Figure 6: The dotted line (A) shows the diameter of the aorta has been measured obliquely (in a slanting direction), as the direction of the aorta is slightly angled within the image. The correct line of measurement is shown by line B, as this is perpendicular to the walls and direction of the aorta. For reference, lines 10 degrees either side of the central axis of the image (90 degrees from top to bottom) are shown. Oblique lines of measurement close to or greater than 10 degrees from the correct line could result in over estimation of aortic diameter.

Unsatisfactory

An image will be unsatisfactory if:

• the TP line of diameter measurement is not in a true anteroposterior direction resulting in overestimation of aortic diameter

• the longitudinal line of diameter measurement has been taken obliquely and is not perpendicular to the lumen, resulting in overestimation of aortic diameter

Guide for CST/IQA lead: A rescreen should be considered if the diameter is close to specific programme diameter thresholds (3cm, 4.5cm and 5.5cm). Services should track these men for rescreen to encourage timely re-attendance. Provide feedback to the screening technician with one-to-one image review.

4.9 The aorta in the Longitudinal Plane

Satisfactory

The abdominal aorta is approximately 13cm in length. The suprarenal section can be difficult to image, as outlined in the NHS AAA screening programme non-visualisation policy[v].  

It is difficult to define a minimum length of aorta that should be shown on a captured image, but it must be possible to differentiate the longitudinal image (tubular) from the transverse image (circular). The only exception is if a large aneurysm is present, when the longitudinal image can show a more spherical shape.

It can be helpful to capture one or two extra images to show different sections of the aorta in the LP, providing this does not impede the scan. This can help if there are instances of bowel gas or scarring obscuring part of the image (see figures 7A-C).

Figure 7A: It can sometimes be difficult to demonstrate an adequate length of the aorta in a single longitudinal image, particularly if there is bowel gas present. In this image, only a short (4 to 5cm) length has been captured.

Figure 7B: A second image to accompany figure 7A showing a further section of the aorta.

Figure 7C: A composite of figures 7A and 7B, showing a longer section of the aorta.

Satisfactory with learning points

An image will be satisfactory with learning points if:

• the screening technician has made comments on the IT system due to difficulties capturing an image showing a long length of aorta, but they have indicated they have visualised the aorta at different levels. Ideally, the screening technician should attempt to capture more than one LP image with images taken at different levels as shown in figure 7C

Guide for screening technicians: there is a balance between trying to capture optimum images for possible image QA and focusing on the scan. On occasions where a section of the aorta was visible but became obscured before an image could be captured, a comment should be recorded on SMaRT.

Guide for CST/IQA lead: provide feedback with recommendations.

Unsatisfactory

An image will be unsatisfactory if:

• the screening technician has made no comments on the IT system regarding difficulty in providing a longitudinal image
• it is not possible to differentiate the longitudinal image from the transverse image (see figure 3)

Guide for CST/IQA lead: provide feedback with image review and use clinical experience to determine if a rescreen is necessary.

5. Additional guidance for screening technicians

5.1 Comment boxes in IT system

Screening technicians are encouraged to add comments at the end of each man’s screening episode on the programme’s IT system:

• if there are points that they would like to highlight or describe
• if there are limitations in imaging
• to request feedback about images
• if there are incidental findings
• if there is anything else relevant to that particular scanning episode

This enables the CST and IQA lead to better understand any aspects or limitations of the scanning episode during the image QA process. It will also encourage regular communication, including face-to-face meetings between CST/IQA leads and screening technicians. Difficult scans with learning points can be discussed at team meetings.

6. Additional guidance for CST/IQA leads

6.1 Remeasurement of images

The IT system does not currently allow for the direct remeasurement of static images of the aorta.  If the aortic diameter appears to be close to specific programme diameter thresholds (3cm, 4.5cm and 5.5cm), a rescreen is recommended.

6.2 Removal of a man from AAA surveillance

There will be instances when men undergoing surveillance will present with challenging imaging. For example, when it is difficult to define the walls of the aorta or there are large obscuring atheromatous plaques present. This is particularly the case if there have been inconsistent measurements over a number of surveillance scans and comments from screening technicians. A possible example is shown in figure 8, where there is irregular atheroma and thrombus present and difficulty defining the posterior wall and axis of measurement. In such cases the CST/IQA lead may recommend placing the man into imaging or vascular laboratory surveillance or discuss the images with the director/clinical lead for advice.

Figure 8. On-going surveillance of this aorta within the screening programme may present some challenges due to atheroma and tortuosity and definition of the posterior wall, especially as the AAA becomes larger. If the CST/IQA lead recommend the man remaining in AAA surveillance pathway, it would be appropriate to ask screening technicians to request image QA following each surveillance scan to ensure measurements are correct. In this example it appears an oblique line of measurement has been made.

7. References

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[i] NHS Abdominal Aortic Aneurysm (AAA) Screening Programme www.gov.uk/guidance/abdominal-aortic-aneurysm-screening-programme-overview

[ii] Ultrasound imaging
www.nhs.uk/conditions/ultrasound-scan/

[iii] AAA screening programme standards
www.gov.uk/government/publications/aaa-screening-quality-standards-and-service-objectives/abdominal-aortic-aneurysm-screening-programme-standards-valid-for-data-collected-from-1-april-2022

[iv] NHS AAA screening programme guidance www.gov.uk/government/publications/aaa-screening-clinical-guidance-and-scope-of-practice

[v] NHS AAA screening programme non-visualisation policy www.gov.uk/government/publications/aaa-secondary-ultrasound-screening/management-of-non-visualised-aortas