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RESOLUTION

Image resolution refers to the clarity of the image or more precisely the ability to discern that two closely aligned structures are seperate. The better the resolution, the closer the structures can be and the machine will still resolve them as individual. 

Vertical or axial resolution of the image as discussed in the previous page is mainly affected by wavelength and pulse duration. Lateral resolution of the image is to do with beam width.

AXIAL RESOLUTION

This depends on the wavelength and the number of cycles in each pulse ie the spatial pulse length. The shorter the wavelength and the fewer the cycles in each pulse, the shorter the spatial pulse length and hence, the better the axial resolution. Most machines set the number of cycles per pulse between 2 and 3. High frequency transducers have shorter wavelengths. Thus, high frequency transducers have better axial resolution.

The machine is able to resolve two objects which are close together in the vertical plane as seperate if their separation is more than half the spatial pulse length.

LATERAL RESOLUTION

To understand lateral resolution visualise the US beam as a vertical beam like an hour-glass with a waist. Each piezoelectric crystal creates a beam. The lateral resolution is best at the narrow waist. The waist of the beam is the focal zone. You always want to increase or decrease the depth to ensure the structure you are visualising is within the focal zone of the US beam. Adjusting the focus to just below the structure you are imaging, moves the waist of the beam to the structure. 

When a beam passes through structures on the same plane, the machine will assume that the echoes belong to a single structure. 

3D visualisation of the US beam coming down from one crystal. As each crystal is activated a beam similar to this passes through the tissues. Note that the beam width is narrowest at the focal zone (centre of the beam). Thus the lateral resolution is best at this point.

High frequency transducers have a narrower beam width than low frequency transducers. Thus they have better lateral resolution allowing greater detail.

Low frequency transducer on the left has a wider beam. Thus the beam passes through two structures next to each other and is unable to resolve these. On the screen this is presented as a smear. On the other hand with the high frequency probe on the right, the beam is narrower and so only passes through one of the structures, the beam of the adjacent crystal passes through the adjacent structure. So the image on the screen is two structures next to each other.

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