THE ULTRASOUND IMAGE: GENERATION AND DISPLAY

By

Dr/ Dina Metwaly

Bandwidth

All ultrasound transducers contain a range of frequencies, termed bandwidth

Broad bandwidth technology produces medical transducers that contain more than one operating frequency

for example: – 2.5 - 3.5 MHz for general abdominal imaging – 5.0 - 7.5 MHz for superficial imaging

Time Gain Compensation Operator controlled adjustment to compensate for the

attenuation of sound as it travels into the tissue Must be adjusted manually for each tissue type examined

and may be manipulated throughout an exam to optimize the image

RESOLUTION This is the ability to distinguish / identify two

objects. Wavelength (frequency) dependent. It includes Spatial and Temporal Resolution.

Spatial & temporal Resolution

Spatial Resolution is the ability to distinguish two separate objects that are close together and is itself divided into

a) Axial Resolution, which is this ability along the axis of the ultrasound beam

b) Lateral Resolution, which is in the direction perpendicular to the beam's axis.

All current equipment has an overall spatial resolution of 1.0 mm or less.

Temporal Resolution is the ability to accurately locate structures or events at a particular instant in time.

Lateral Resolution

The ability to distinguish objects that are side by side.

It is generally poorer than axial resolution and is dependent on beam width.

As the ultrasound machine assumes that any object visualised originates from the centre of the beam then two objects side by side cannot be distinguished if they are separated by less than the beam width.

The determinants of beam width are -a. Transducer frequency (beam width increases

with lower frequency transducers)b. Focusing of the beamc. Gain (increased gain will increase the beam

width and reduce resolution) To optimise lateral resolution therefore: -a. Use the highest frequency transducer (but

note that this is at the expense of penetration)

b. Optimise the focal zonec. Use the minimum necessary gain

Axial Resolution

The ability to recognise two different objects at slightly different depths from the transducer along the axis of the ultrasound beam

Axial resolution = spatial pulse length (SPL)      2

where SPL = λ × no. of cycles It is improved by higher frequency (shorter

wavelength) transducers but at the expense of penetration.

Higher frequencies therefore are used to image structures close to the transducer.

Temporal Resolution

This is dependent on frame rate. It is improved by -

a. Minimising depth - the maximum distance from the transducer as this affects the PRF

b. Narrowing the sector to the area of interest - narrowing the sector angle

c. Minimise line density (but at the expense of lateral resolution)

Display modes

Once the diagnostic information has been acquired and electronically processed, it has to be displayed for viewing and recording.

Different methods are used to display the information acquired from ultrasound examinations.

1. The amplitude mode (A-mode) 2. The brightness mode (B-mode static)3. The motion mode (M-mode)4. Real-time mode (B-mode Dynamic)5. The Doppler mode

A-mode – one-dimensional

Depends on: Distances between reflecting interfaces and

the probe are shown. Reflections from individual interfaces

(boundaries of media with different acoustic impedances) are represented by vertical deflections of base line, i.e. the echoes.

Echo amplitude is proportional to the intensity of reflected waves (Amplitude modulation)

Distance between echoes shown on the screen is approx. proportional to real distance between tissue interfaces.

Today used mainly in ophthalmology.

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A-mode – one-dimensional

B-mode – two-dimensional

A tomogram is depicted. Brightness of points on the screen

represents intensity of reflected US waves (Brightness modulation).

Static B-scan: a cross-section image of examined area in the plane given by the beam axis and direction of manual movement of the probe on body surface. The method was used in 50‘ and 60‘ of 20th century

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B-mode – two-dimensional - static

M-mode

One-dimensional static B-scan shows movement of reflecting tissues. The second dimension is time in this method.

Static probe detects reflections from moving structures. The bright points move vertically on the screen, horizontal shifting of the record is given by slow time-base.

Displayed curves represent movement of tissue structures chest

wall

lungs

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Comparison of A-, B- and M-mode principle

Real-time imaging (B-mode – dynamic)

Real-time imaging is rapid B-mode scanning to generate images of a selected cross section within the subject repetitively at a rate high enough to create the motion picture impression.

A rapid succession of images of the same plane are generated and viewed as they are acquired.

Although in reality each image in the series represents an independent static image, the effect of rapid acquisition and viewing at rates exceeding about 25 image frames per second creates the impression of continuity in time.

This impression arises due to limitations in human perception. We are unable to distinguish in time between events occurring at intervals shorter than about 40 milliseconds - they appear to us to occur "at the same time".

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B-mode - dynamic