Image characteristics and Optimization

IMAGE CHARACTERISTICS IN ULTRASONOGRAPHY AND OPTIMIZATION

J. J. John (DDR-CUHAS)

Ultrasound Image

In B-mode, it composed of a variety of bright dots that vary in intensity.

Intensity vary according to the strength of electricals signals hence, returning echoes from tissues or structures in the body.

The location of each dot corresponds to the anatomic location of each echo-generating organ or structure Provides anatomic and non-anatomic information (artifacts) An ultrasound image is composed of a variety of bright dots that vary in intensity according to the strength of the returning echoes from internal tissues and/or structures in the body.

The location of each dot corresponds to the anatomic location of each echo-generating organ or structure.

Each image provides a dot-for-dot correct anatomic representation of organs and structures in the body.

On the image there is also non-anatomic information, which is the result of things happening to the ultrasound beam before, during or after it strikes a reflecting surface. These phenomena, which are the result of the imaging technique itself, are called artifacts and are discussed elsewhere.

Image characteristics and Optimization

An ultrasound image is composed of a variety of bright dots that vary in intensity according to the strength of the returning echoes from internal tissues and/or structures in the body.

• Some of the returning echoes are strong and register as bright white dots on the image.
• The brightest areas with the highest intensity echoes are called hyperechoic.
• Areas with echoes of lesser strength are hypoechoic.
• There is no specific name for echoes of moderate strength.
• Areas with no echoes are anechoic and therefore black on the images.

When two structures generate the same kind of echoes, they are isoechoic.

Echogenicity

We are considering intensity of echoes and image apperance

ECHOGENICITY

• Focus on the intensity of echoes reflected by tissues or structures from inside the body.

Brightness of the dots generated by returning echoes from the body.

Echogenicity is the intensity of echoes reflected by tissues or structures from inside the body. Literally and simplistically, it means the ability of something to generate echoes.

ECHOGENICITY

Intensity of dots forming the image vary according to the strength of electricals signals processed and hence the returning echoes from tissues or structures in the body.

• Some of the returning echoes are strong and register as bright white dots on the image.

Some are weak and register as less bright dots on the image.

ECHOGENICITY

• The brightest areas with the highest intensity echoes are called hyperechoic.
• Areas with echoes of lesser strength are hypoechoic.
• There is no specific name for echoes of moderate strength.
• Areas with no echoes are anechoic and therefore black on the images.

When two structures generate the same kind of echoes, they are isoechoic.

ECHOGENICITY

• Word slide

Summary of previous page

ECHOGENICITY

Echogenicity of a structure is described relative to surrounding or adjacent tissue When we describe echogenicity of a structure, we describe it in relative terms comparing the echogenicity of the structure to that of surrounding or adjacent echoes.

• QUESTION: True or false?
• Echogenicity is an inherent property of a specific tissue.

ANSWER:

False. Echogenicity of a structure is an not absolute property. It is a relative thing, relative to the surrounding tissue.

ECHOGENICITY

Hyper echoic:- High intensity or bright echoes, which are more echogenic than the standard to which comparison is made, usually the surrounding tissue.

Hyperechoic means high intensity or bright echoes, which are more echogenic than the standard to which comparison is made, usually the surrounding tissue.

Image characteristics and Optimization

The round hyperechoic solid mass (arrow) under the capsule of this moderately echogenic liver is a hemangioma. The normal liver is the standard for comparison in this case.

The hyperechoic solid mass within this spleen represents a focal deposit of lymphoma.

Image characteristics and Optimization

Due to sickle cell nephropathy, the cortex of this kidney is hyperechoic relative to the adjacent liver parenchyma, which, due to its size, becomes the standard of comparison for most adjacent parenchymal organs.

Image characteristics and Optimization

The round mass (arrow) in this kidney is relatively hyperechoic when compared to the adjacent renal cortex (arrowheads). The echogenicity of a mass is compared to its organ of origin. Although the tumor itself is not very bright in echogenicity, it contains tiny hyperechoic foci due to microcalcifications scattered throughout the mass, which is a renal cell carcinoma (hypernephroma).

ECHOGENICITY

Hypoechoic:- Means low intensity echoes or less echogenic than the surrounding tissue.

Image characteristics and Optimization

This is an image the spleen, which is of moderate echogenicity, and contains several hypoechoic masses (arrows) representing deposits of lymphoma. The masses are hypoechoic relative to surrounding normal splenic parenchyma.

The relative echogenicity of a mass is compared to the echogenicity of the organ to which it relates.

L

In this case the large lobulated hypoechoic masses (arrows) of low echogenicity represent metastatic deposits within the liver (L). The masses are hypoechoic relative to adjacent normal liver parenchyma.

R

L

In this transverse scan of the scrotum, the normal right (R) testis is moderately echogenic, while the left (L) is enlarged and hypoechoic due to testicular cancer. The standard for comparison in this case is the contralateral normal-sized (right) testis.

ECHOGENICITY

Anechoic:- Means without echoes or having no internal echoes.

Simple fluid generally has no internal reflections due to absence of acoustic boundaries thus anechoic.

• Anechoic means without echoes or having no internal echoes.

Simple fluid generally has no internal reflections and is anechoic.

The elongated thin-walled anechoic structure dominating the picture is the gallbladder, which is filled with clear fluid, bile.

Image characteristics and Optimization

The round thin-walled anechoic structure on the left side of the picture(1) is a transverse section of the gallbladder, which is filled with clear fluid, bile. The smaller rounded anechoic structures in the right side of the picture are large blood vessels, the inferior vena cava (2) and the aorta (3).

ECHOGENICITY

• Isoechoic:- Means echoes of similar intensity.

QUESTION:

• Isoechoic means
• Echoes coming from the same transducer.
• Echoes of the same intensity.
• Echoes originating from the same depth of tissue.
• Echoes isometrically opposed to each other.

ANSWER: B

Image characteristics and Optimization

The dominant mass in this picture is an enlarged spleen. Adjacent to the spleen are two small round isoechoic masses representing splenules or accessory spleens. They are of the same echogenicity as the spleen itself.

Image characteristics and Optimization

Another example of an oval-shaped solid isoechoic mass adjacent to the lower pole of the spleen. The mass is of the same echogenicity as the spleen.

Echotexture

Uniformity of echoes on a particular area

ECHOTEXTURE

• Homogeneous
• uniform echoes
• Heterogeneous

non-uniform echoes

Echotexture is a different sonographic characteristic than echogenicity. It refers to the uniformity of echoes within a structure. It has nothing to do with the degree of brightness of the returning echoes.

The word echotexture may have come from a blend of “echo architecture” or from the semblance of a tactile quality one can get from looking at the uniformity of echoes on a sonographic image.

Echotexture can be homogeneous, in which case the echoes are uniform, or inhomogeneous, in which case the echoes are non-uniform.

QUESTION:

• What is the difference between echogenicity and echotexture?

ANSWER:

• Echogenicity refers to the intensity of the echoes reflected by tissues.
• Echogenicity refers to the amount of fluid in a mass.
• Echotexture refers to the uniformity of echoes within an area.
• Echotexture refers to the homogeneity of a tissue.

ANSWERS: A,C,D

ECHOTEXTURE

• Homogeneous
• Uniform echoes
• Fine, smooth texture

May includes slightly coarse texture

Homogeneous

Heterogeneous

This is a transverse sonogram through the scrotum. The right testis within the small box is smaller (actually normal in size) and homogeneous, surrounded by anechoic hydrocele. The left testis (larger box) is enlarged and heterogeneous consisting of a mixture of hypoechoic and hyperechoic areas due to a missed torsion.

Image characteristics and Optimization

The liver in this patient with chronic hepatitis has a very homogeneous echotexture. It has a fine grainy texture, like fine sand. No coarseness is detected.

Image characteristics and Optimization

This mass is almost entirely filled with low-level homogeneous internal echoes, which are very uniform in their appearance and distribution. The straight line in the mass is due to an interface of fluid and blood in an ovarian endometrioma, which is shown on this sagittal transvaginal scan.

ECHOTEXTURE

• Heterogeneous
• Non-uniform echoes

Irregular texture

Image characteristics and Optimization

A homogeneous liver parenchyma (left) is shown in comparison to a heterogeneous liver parenchyma (right, which was due to cirrhosis. The heterogeneous pattern has the appearance of numerous small nodules scattered throughout the parenchyma giving the liver a coarse, irregular echotexture.

Image characteristics and Optimization

This liver is very heterogeneous in its echotexture due to multiple small hyperechoic rounded masses scattered throughout the parenchyma due to a myriad of small hemangiomas in this case of diffuse hemangiomatosis.

Image characteristics and Optimization

This heterogeneous mass with echoes of variable intensities: high, medium and none, represents the internal contents of a hemorrhagic ovarian cyst.

focus.

Image Optimization in ultrasonoghaphy

Get the best of your ultrasound machine

Introduction

The quality of a diagnostic image is of the most importance determining its usefulness.

The overall quality of the ultrasound image is the end product of a combination of many factors originating not only from the imaging system but also from the performance of the operator.

All the components of the imaging system, including the transducer, the electronics, image processing, display, and recording devices, impact on the ultimate quality of the ultrasound image.

A discussion on the quality of the ultrasound image centres around image resolution.

Resolution

• Resolution refers to the ability to distinguish
• For the ultrasound image, components of resolution include
• Spatial resolution
• Temporal resolution and

Contrast resolution

Spatial resolution

Spatial resolution describes the ability to distinguish between objects located at different positions in space.

In reference to the ultrasound image, spatial resolution is concerned with the ability to distinguish between two reflectors in space.

It affects in a major way the capability of the imaging system to depict structural detail.

• Spatial resolution is divided into two components.
• Axial resolution

Lateral resolution

Axial resolution

Axial resolution is the ability to distinguish between echoes originating from two reflectors lying one behind the other along the axis of the ultrasound beam.

It is sometimes referred to as depth resolution.

Axial resolution

• Axial resolution is limited by the SPL.
• Reflectors closer to one another than half the SPL cannot be resolved.

The shorter the spatial pulse length, the better the axial resolution.

Axial resolution

Since:

Therefore, the higher the beam frequency, the better the axial resolution.

Lateral resolution

Lateral resolution is the ability to distinguish between two reflectors situated side by side in a direction perpendicular to that of the ultrasound beam.

Factors Affecting Lateral Resolution

In ultrasonic imaging, always axial resolution is better than lateral resolution, besides showing less variation.

This means that lateral resolution is the more limiting aspect of spatial resolution.

It is therefore important that the factors affecting lateral resolution be well understood.

• The factors affect lateral resolution in ultrasonic imaging include:
• Beam width
• Beam frequency

Scan line density

Effect of Beam width

Whereas axial resolution is limited by the length of the ultrasound pulse (SPL), The lateral resolution is limited by the width of the pulse.

Lateral resolution is limited by the beam width in the plane of the reflectors being resolved.

Reflectors closer to one another than the beam width cannot be resolved.

Effect of beam frequency

Frequency affects the beam shape, and hence has a major influence on lateral resolution.

The ultrasound beam can be made narrower at higher frequencies. Therefore, the higher the frequency, the better the lateral resolution.

Also, increased beam frequencies extend the Fresnel zone, although this advantage is partly counteracted by increased beam attenuation at higher frequencies.

Effect of scan line density

The image is formed by combining echo information from a large number of scan lines generated by scanning the beam across the selected plane of interest in the subject.

The number of scan lines contributing to the image affects lateral resolution.

Sampling the tissues at closer intervals improves resolution. Therefore, the higher the scan line density the better the lateral resolution.

Line Density

High Line Density

Low Line Density

A Line density Knob adjusts the number of scan lines in your ultrasound image.

Effect of scan line density

The line density may vary with tissue depth, depending on the type of scanner.

In particular, the line density decreases with distance from the transducer in sector scanners, hence the lateral resolution will also diminish with increasing tissue depth.

This effect is not present in systems in which the scan lines remain parallel to one another.

Effect of scan line density

High Line Density

Low Line density

Axial and Lateral resolution

Spatial resolution versus tissue depth

We have seen that higher beam frequencies are associated with better spatial resolution (both lateral and axial).

But higher frequencies also suffer more attenuation, thus limiting their useful practical range in tissue.

Investigation of small parts to tissue depths of up to about 4 cm can be undertaken using high frequencies of 5 MHz and above in order to maximize spatial resolution.

However, it is necessary to use lower frequency beams to achieve increased tissue depths, a measure which compromises resolution There is a conflict of interest between tissue depth and spatial resolution.

Use of Multifrequency Transducers

The dilemma of balancing spatial resolution with the requirements of tissue depth is partly addressed by the use of multifrequency transducers.

• There are two designs of multi frequency transducers. These are:
• Broad band transducers

Multiple center-frequency transducers

Broad Band Transducers

Broad band transducers are designed to provide a wide range of frequencies in the same beam.

• Broad band transducers, as the name suggests, are designed to offer a large bandwidth.

This translates into a wide range of frequencies in the beam Thus, combining the advantages of high spatial resolution from the higher frequency components with those of deeper penetration from the lower frequency components.

Multiple centre-frequency transducers

Transducers which offer selectable frequency rely on a change of crystal thickness.

The transducer is designed using a number of crystal layers stratified adjacent to one another.

A high frequency beam may be generated by connecting a single layer of the crystal material to the pulsing electrodes, while lower frequencies may be obtained by pulsing the transducer across a sandwich of 2 or 3 adjoining layers of the crystal.

Switching from one frequency to another then amounts to changing the effective thickness of the crystal.

The lower frequencies will be used for large tissue depths, while the higher frequencies will be for small parts.

Frame rates, Scan line density, and Tissue Depth

Lateral resolution is improved by having a large number of scan lines contribute to the image.

Also, high framing rates are desirable in the imaging of fast moving structures as in cardiology.

The total number of scan lines generated per second will be limited by the time required to produce one such scan line, depending on the desired tissue depth Therefore, high framing rates dictate a reduction in the number of scan lines contributing to each image frame, or a reduction in tissue depth.

For a given tissue depth, the higher the frame rate, the lower will be the line density

Frame rates, Scan line density, and Tissue Depth

Reducing the depth results in shorter time between pulses and this increases frame rate.

For high lateral resolution:

• Beam frequency should be high
• Number of scan lines should be large
• Tissue depth will be restricted

Frames per second may be limited

For large tissue depths:

• Low beam frequency is used
• Spatial resolution is reduced

Frames per second may be limited.

For very high framing rates (rapid motion)

• Number of scan lines per image frame may be reduced
• Lateral resolution may be reduced

Tissue depth may be restricted.

Contrast resolution

Contrast resolution is the ability to distinguish between signal sizes.

In the ultrasound image, this translates to differentiating between the intensities of the dots representing echoes of different size at the display.

The ability to detect small changes in the characteristic echo pattern of an organ may well depend on the level of contrast resolution.

The use of digital image processing techniques is contributing to improved contrast resolution of the ultrasound image.

Contrast resolution

• Poor contrast

Good Contrast

Temporal resolution

Temporal resolution is the ability to separate events in time.

The rate at which image frames are generated and viewed affects the visualization of moving structures.

The limit of temporal resolution for the human eye is about 40 milliseconds.

This means that events separated in time by more than 40 ms can be visually recognized as having occurred at different moments in time, whereas events occurring within a time interval of less than 40 ms are viewed as taking place "simultaneously".

Temporal resolution

This 40 milliseconds level of temporal resolution dictates that the framing rate required for real-time imaging to observe moving structures should be 25 frames per second (f.p.s) or more (if one image frame is generated every 40 ms, then in a total time of 1 sec, or 1000 ms, 25 image frames will be generated).

Framing rates below about 20 f.p.s are associated with a phenomenon called image flicker, which arises from the ability of the eye to distinguish the resulting image frames as being separate in time.