As 2020 begins and we think about personal resolutions and how they may help us, we should take some time to think about what resolution means in ultrasonography and how understanding it can improve the images we acquire.
When we talk about resolution in ultrasound imaging there are three ‘categories’ of resolution that we’re referring to: Spatial Resolution, Temporal Resolution and Contrast Resolution. The one that most of us instinctively think about when we hear the term ‘resolution’ is spatial resolution.
Spatial resolution is the ability to distinguish two separate entities as being separate. The higher the resolution, the closer together the entities can be and still be distinguished as being separate. For example, optimising spatial resolution can allow us to distinguish between two areas of pathology within the spleen rather than wrongly interpreting them as one lesion. Similarly, improving spatial resolution may allow us to visualise lesions adjacent to normal structures.
In ultrasonography, spatial resolution can be further subdivided into the three dimensions represented by an ultrasound beam:
Axial resolution – along the path of the ultrasound beam
Lateral resolution – perpendicular to the direction of the ultrasound beam
Elevational resolution – across the ‘thickness’ of the ultrasound beam.
Together, the three aspect of resolution form a three-dimensional ‘box’ representing the smallest object size that can be resolved.
Each of the three ‘dimensions’ of spatial resolution are determined by a different factor or factors.
Axial resolution is determined by two components: damping of the transmitted ultrasound pulse (over which we, as users, have no control) and the frequency of the transmitted pulse (over which we do have control). Increasing the frequency will improve axial resolution at the expense of reduced depth of penetration, and vice versa. Ideally, we should select the highest frequency available that gives enough depth of penetration to visualise the region on interest.
Two images of the left kidney taken from the same dog. In the left image the frequency is set at 7Mhz (Frq=7), In the right image the frequency is increased to 11MHz (Frq=11). The right image has improved axial resolution with better definition of renal cortex and medulla.
Lateral resolution is determined by the width of the ultrasound beam, which varies with depth. It is initially determined by the ‘aperture’ of the beam at the transducer face, narrowing within the focal zone where the lateral resolution is greatest, then diverging deep to the focal zone. Focusing of the ultrasound beam (either electronically or with an acoustic lens) can improve lateral resolution in the near-field at the expense of poorer far-field resolution. As most modern ultrasound machines have the ability to electronically set the focal zone (usually marked by an arrow or “hourglass” shaped marker visible on the depth scale of the screen), it should be set at the level of the region of interest or, for larger organs, at the deepest extent of the organ to be imaged.
Two sagittal plain images of the liver taken from the same dog. The focal zone – indicating the area of optimal lateral resolution – is marked by the two yellow “hourglass” shaped markers seen on the depth scale to the right of each image. The right image highlights the importance of setting the focal zone at an appropriate level. In this example, the far field focal setting allows us to appreciate more detail within the deeper areas of the liver parenchyma.
Elevational resolution is governed by the thickness of the ultrasound beam. It’s tempting to think of an ultrasound beam as being two-dimensional because the image we see on the screen is two-dimensional. However, this is not the case; the beam has a thickness that is related to the length of the transducer elements (piezoelectric crystals). The longer the elements, the thicker the beam. During image processing the ultrasound machine performs volume averaging across the thickness of the beam. The thicker the beam, the more the signal is ‘averaged,’ and therefore the lower the elevational resolution; the converse is also true. This oft-forgotten dimension of resolution plays a significant part in determining the quality of the image on the screen. However, elevational resolution is something we cannot specifically adjust as it related to the physical properties of the ultrasound probe. Modern ‘matrix’ array transducers, in which transducer arrays are arranged in more than one row (i.e. in a matrix), can steer the ultrasound beam dynamically in this plane and can improve elevational resolution. However, this is very demanding on the machine’s processing capabilities!
Temporal resolution relates to the ability of the ultrasound machine to pick up changes over time; the higher the temporal resolution, the quicker the changes can be whilst still being detectable. It is usually quoted as the frame rate, i.e. the number of times per second that the image is updated. A high frame rate gives a high temporal resolution, whereas a low frame rate gives a low temporal resolution. In practical terms, any frame rate below about 15-20 frames per second is useless, even for imaging stationary tissue, as it’s impossible to hold the probe (or the animal!) still enough to get an image that isn’t blurry. For structures that are moving rapidly (such as the heart of small animals) a high temporal resolution is critical and relies on specific designs of ultrasound probe - some high-end echocardiography equipment can achieve frame rates of around 400 frames per second!
Contrast resolution is, as the name implies, the ability of the machine to detect and display differences in echogenicity between adjacent tissues. As you might imagine, a machine that has high contrast resolution will be able to detect subtle differences in echogenicity of soft tissue structures and may produce quite a ‘contrasty’ (black and white) image. Conversely a machine with low contrast resolution will produce quite a ‘flat’, grey image.
Hopefully by discussing the factors above, we can all improve our image resolution in 2020.
Happy New Year from IMV imaging.
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