When we talk about resolution in ultrasound imaging there are actually 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. In ultrasonography, this 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; and elevational – across the ‘thickness’ of the ultrasound beam). These three form a three-dimensional ‘box’ representing the smallest object size that can be resolved.
Each of the three ‘dimensions’ of spatial resolution is 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 increase axial resolution (at the expense of reduced depth of penetration), and vice versa.
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.
Finally, 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. The effects of volume averaging on elevational resolution (and thus overall image resolution) can, to an extent, be mitigated by use of a fixed focal-length acoustic lens which maximizes elevational resolution (i.e. makes the beam most narrow in the elevational plane) at the focal zone. An acoustic lens functions in much the same way as an optical lens (such as in a pair of glasses) – it is a fixed structure that is built into the transducer and it focuses the ultrasound beam in the elevational plane. This comes at the cost of significantly poorer resolution close to the transducer face and in the far field. So-called ‘matrix’ array transducers, in which transducer arrays are arranged in more than one row (i.e. in a matrix), are able to steer the ultrasound beam dynamically in this plane and can thus further improve elevational resolution. However, this is very demanding on the machine’s processing capabilities!
Ultimately, the major input we can have on image resolution as ultrasound end-users is to increase frequency to increase resolution, and decrease frequency to decrease resolution (but gain added depth of penetration).
Fig.1 High frequency (A) and low frequency (B) images of canine small intestine – all other parameters were identical. Note the significantly better resolution in image A.
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 – 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.
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