Introduction to Ultrasound Machine

• Ultrasound units description and image formation with proper orientation presented by Dr. Mona Nagah ELBeshbishi, a lecturer and consultant of radiology at DELTA University.

Learning Objectives

• Identify the parts of an ultrasound unit.
• Recognize machine keyboard options.
• Describe how ultrasound interacts with tissue.
• Explain image formation.
• Define image optimization.
• Recognize US echo patterns.

Parts of Ultrasound Machine

• Transducer/probe/scan head which sends and receives sound waves.
• Central Processing Unit (CPU): computer that does all of the calculations and contains the electrical power supplies for itself and the transducer probe.
• Display (Monitor) displays images from ultrasound data that has been processed by the CPU.
• Keyboard, used for data input and to take measurements.
• Disk storage device where acquired images are stored (hard, floppy, CD).
• Printer, used to print the image from the displayed data.
• PACS system.

Ultrasound Machine Keyboard

• Power Key: Turns the machine on and off.
• New Patient Key: Enters patient and study data.
• Probe menu key: Allows the user to choose the probe for exam,
• Freeze Key: Freezes or unfreezes the monitor.
• Store key: Uploads image to system to be printed later.
• Print Key: Prints image directly from printer.
• ABC Key: Used for labeling.
• Body Mark Key: Marks the organ examined.
• Caliber Key: Used for measurement.
• Track Ball: Moves the cursor.
• Image adjustment tools are used for; increasing or decreasing the depth of examined tissue, changing the image size and adjusting the gray scale.

Key Optimization Tools

• Operator controls optimize and adjust images.
• The tools include Frequency, Depth, Sector width (Line density), Gain, Focus/focal zones, and Doppler.

Interactions of Ultrasound with Tissue

• Acoustic impedance is a physical property of tissue that describes how much resistance an ultrasound beam encounters through tissue.
• Acoustic impedance (AI) depends on the density of material.
• The greater the density, the greater the acoustic impedance.
• Impedance determines how much incident sound is reflected back from the first medium, and how much is transmitted into the second.

Acoustic Boundaries

• Positions within tissue where values of acoustic impedance change.
• Tissue interfaces or acoustic boundaries are important in ultrasound interactions.
• An example of an acoustic boundary is urine and the urinary bladder wall.

Ultrasound Interaction with Tissue

• Reflection.
• Transmission.
• Refraction.
• Attenuation.
• Absorption.
• Scattering.

Reflection

• Redirection/return of a portion of sound beam back to the transducer.
• Reflections occur at the interface of differing AI's.
• Ultrasound image is formed from reflected echos.
• Reflection of a beam is called an echo.
• Reflection occurs at the interface between two mediums/tissues/organs where there is a change in acoustic impedance.
• With the acoustic impedance is the same, there is no reflection.
• For different acoustic impedances:
  • If the difference is small, there is weak reflection and most ultrasound waves pass to the next medium.
  • If the difference is large, there is strong reflection and a strong echo is produced.
• If there is a very large difference, or very dense materials, almost all waves are reflected, such as at an air/tissue surface.

Transmission

• Some ultrasound waves continue deeper into the body.
• Those waves will reflect from deeper tissue structures.

Refraction

• Occurs at the surface of different acoustic impedance.
• The proportion of the beam that is not reflected, but transmitted undergoes bending, the transmitted wave does not continue in the incident direction.
• Refraction leads to an image artifact, misplacement of the anatomy within the image, and loss of wave intensity.

Attenuation

• Progressive weakening of the amplitude or intensity of the sound wave as it propagates medium.
• Losses or weakenings include reflection, scattering, refraction, and absorption.
• Attenuation of ultrasound increases rapidly as beam frequency rises.
• The deeper the wave travels, the weaker it becomes.

Absorption

• This is the main form of attenuation.
• It is the process by which energy in an ultrasound beam is transferred to propagating a medium, and transformed into a different form of energy, mostly heat.
• The extent of absorption in a medium is affected by three main variables: viscosity, relaxation time and beam frequency.
• A medium absorbs energy from a beam.
• Viscosity of the medium is directly proportional.
• Relaxation time is directly proportional.
• Beam frequency is directly proportional.
• Relaxation time refers to the time it takes for particles to return to their original position.

Scattering

• Occurs when sound strikes a structure of different acoustic impedance to the wave surrounding tissue, that is smaller than the wave length of the incident sound wave.
• Scattering can be interaction of ultrasound with small or diffuse reflectors like capillaries, RBCs, bile ducts, etc.
• It is omnidirectional and of low energy.
• It produces a low level to mid grey on the display.

Image Formation

• Electrical signals are processed into "dots" of varying grades, from white to black, on the screen.
• Brightness of the dots are proportional to the strength of returning echoes ie., the brighter the echo the stronger the dot.
• Location of the dots determined by travel time ie., the earlier the returning echo, the higher the dot.

Image Optimization

• This depends on fundamental operator controls: frequency, depth, sector width (line density), gain, focus/focal zones, and Doppler.

Frequency

• High frequencies should be selected to interrogate the MSK structures.
• Deeper structures: hip, shoulder and knee may require reduced frequency in order to obtain adequate penetration.
• Frequency will increase axial resolution but decrease penetration.

Depth

• It is used to determine area of scan field
• It should be dependent on the structure of interest.
• Less depth is needed for superficial structures.
• More depth is needed for deeper structures.

Sector Width (Line Density)

• Sector width is the scan area that can be reduced or extended.
• Increasing scan lines in a smaller area increases image quality.
• Image quality and frame rate will increase with smaller sector width
• Image land-marking is essential for retrospective review to assist in anatomical area recognition.
• Using a trackball adjusts sector width and direction.

Gain

• Sound is attenuated by tissue.
• This is compensated for by adjusting gain.
• The more tissue to penetrate the more attenuation of the signal leading to difference in echogenicity of the image between near and far fields of views.
• Gain adjusts how bright or dark the image appears by increasing or decreasing the strength of the returning echo signals.
• Overall Gain is similar to a stereo dial control, as you turn it up it brightens the image but darkens it when you turn it down.

Time Gain Compensation

• Most ultrasound machines allow further adjustment of gain in more specific areas.
• It compensates for ultrasound attenuation inside tissue, and controls the amount of echoes received by the transducer.
• Time Gain Control (TGC) is generally a slider bar, which allows the operator to select specific image depths for amplification.
• It helps to amplify deep returning echoes, which would otherwise appear brighter due to increased attenuation.

Focus

• The focus control is often a dial or toggle button with position and number indicated on screen.
• The ultrasound focus produces a thinner beam.
• Adjusting the focal zone to the area (or just behind the area) of interest will increase lateral resolution and image quality.
• The focal point is where resolution is highest.
• By moving the focal position up and down, one can improve image resolution in different areas of the image.
• Focal point: narrowest portion of the beam.
• Far field: region distal to the focal point where the sound beam diverges.
• Near field: region between the transducer and the focal poing.

Image Optimization Summary

• Utilize the highest frequency possible for penetration.
• Small sector width.
• Adjust gain/TGC appropriately.
• Employ adequate depth for structure.
• Position focus and adjust the number of focal zones as appropriate.
• Adequate image quality is not always achievable
• After the imaging is done, images can be stored, displayed, recorded and printed, and archived as DICOM (Disc or PACS).

Ultrasound Echo Patterns

• Strong Reflections = White dots = Hyperechoic or Echogenic which are more echogenic than surrounding tissue.
• Examples are: gallstone, renal calyx, and bone.
• Weak Reflections = Grey dots = isoechoic (intermediate echogenicity with most solid organs like the liver and spleen).
• Weaker reflections = dark grey to black dots = hypoechoic which are less echogenic than surrounding organs with low echogenicity as most solid masses (focal lesions or tumors.)
• No Reflections = Black dots = Hypoechoic- Echofree - Anechoic / Sonolucent (Devoid of echos that appear black).
  • Examples includes blood vessels, gall bladder & cysts.
• Enhancement is increased through sound transmission that travels into an anechoic (fluid-filled) structure and is not attenuated. Acoustic Shadowing occurs when the sound beam hits a calcified object which causes a sharp shadow posterior to boarder.
• Echotexture can be homogeneous.
• Homogenous is completely uniform in texture.
  • Examples includes the liver and thyroid gland.
• Echotexture can be heterogeneous.
• Heterogeneous is non uniform in texture or composition.
  • Tumors have cystic and solid patterns.

Explore the basic workings of an ultrasound machine and how its components work together to produce an image. Understand the importance of acoustic impedance and attenuation. Learn about reflection and how the dots form the image.