Resolution

Chapter 7 Resolution



CONTENTS


Spatial resolution 39

Contrast resolution 42

Temporal resolution 42

Optimizing resolution 43


LEARNING OBJECTIVES



1. Define the term resolution as applied to an ultrasound image.

2. Explain what is meant by spatial, contrast, and temporal resolution.

3. Explain what is meant by axial and lateral resolution and list the factors affecting them.

4. Describe the factors that affect frame rate.

Resolution is a term which describes the ability of an imaging system to differentiate between structures, images, or events and display them as separate entities.


In this chapter we are going to examine the following categories of resolution and the factors that influence them:


• Spatial resolution – resolution in space

• Contrast resolution – resolution of gray shades

• Temporal resolution – resolution in time.


SPATIAL RESOLUTION


This is the ability to display two structures situated close together as separate images. When the structures are displayed as separate images we say that they are resolved (see Fig. 7.1).


image

Fig. 7.1 The difference between good resolution and poor resolution


In ultrasound imaging the spatial resolution depends ultimately on the wavelength of the sound used to produce the image. For example, the wavelength of a 5 MHz ultrasound beam is approximately 0.3 mm so it would not be possible to resolve objects less than 0.3 mm apart using a 5 MHz transducer.


In ultrasound imaging, spatial resolution is divided into two components:


1. Axial resolution – resolution down the screen

2. Lateral resolution – across the screen (see Fig. 7.2).

image

Fig. 7.2 Axial resolution and lateral resolution on the image produced by a sector scanner



Axial Resolution


This is resolution along the axis of the beam and depends upon the spatial pulse length – a short pulse length gives good axial resolution and the best resolution that can be achieved is half the spatial pulse length. For example if the pulse length is 1 mm then structures situated along the axis, which are less than 0.5 mm apart, will not be resolved (see Figs 7.3 and 7.4).


image

Fig. 7.3 Poor axial resolution and good axial resolution


image

Fig. 7.4 How spatial pulse length (SPL) affects axial resolution



Factors affecting spatial pulse length and therefore axial resolution


Frequency Each pulse of ultrasound is approximately two wavelengths long and therefore a shorter wavelength will reduce the pulse length. The wavelength of ultrasound depends on its frequency with the higher frequency waves having the shorter wavelengths. Therefore using ultrasound with a higher frequency for imaging will result in a shorter spatial pulse length and improved axial resolution.


Transducer design

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Default ThumbnailMultiple choice questions and answers The ultrasound beam The piezoelectric effect Instrumentation and controls Transducers Quality assurance and performance testing

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Mar 10, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Resolution

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