ULTRASOUND PHYSICS MADE EASY

1. ULTRASOUND MACHINE
Dr Prajwith Rai

2. BASIC COMPONENTS
• Transmitter / pulser to energize transducer.
• Transducer
• Receiver and processor – to detect and amplify the backscattered energy
and manipulate the reflected signals for display.
• Display – presents ultrasound image or data in a form suitable for analysis
& interpretation.
• A method to record / store ultrasound image.

3. TRANSDUCER
• A transducer is a device that converts one form of energy to another.
Ultrasound transducers convert electrical energy to ultrasound energy
& the reflected ultrasound waves from tissues are converted back to
electrical energy, which in turn gives rise to ultrasound image.

4. Principle
• Ultrasound transducer work on the principle of piezoelectric effect.
This is the unique phenomenon by which certain materials respond to
an applied electric field by changing shape. Changing the polarity of
the applied voltages causes compression and expansion of the
material which gives rise to ultrasound waves.

6. Components of the transducer :
• The piezoelectric crystal, about 0.5mm is thickness and located near
the face of the transducer is the critical component. Modern
transducers contain lead zirconium titanate crystal.
• On either side, this crystal element is coated with conducting film to
ensure contact with the electrodes. The electrodes are made of gold
or silver and are two in number. The outer one is grounded and
insulated and the inner one abuts the thick backing block that
absorbs the sound waves reflected back from the tissues.
• The whole transducer unit is housed in a strong plastic case with
rubber or cork as the acoustic insulator.

7. Types of transducers :
• Ultrasound transducers are basically divided into mechanical
transducers and electronic array transducers.

8. Mechanical transducer :
• A conventional single element transducers or a group of single
element transducers are mechanically moved to form images in real
time.

9. Electric array transducers :
• These consist of an array of small red angular transducers arranged
adjacent to one another. They do not move but are activated
electrically so that the ultrasound beam sweeps across the patient.

10. Electric array transducers :
• Linear array transducers : In these the rectangular transducer elements are
arranged in a line. These are 64-200 transducers forming an array 4-10cm long.
The transducer elements are pulsed in groups of 4 at slightly different times to
achieve a focused image. The scans from this transducer are rectangular in
format. These are primary used with high frequency (5-7 MHz) for evaluating soft
tissue-breast, thyroid etc.
• Phased or steered array transducers : This transducer contains 32 elements and
operates at a frequency of 2-3 MHz. In this all the elements are pulsed to form
each line of the image. The scans obtained are fan shaped or sector scans. This is
advantageous when scanning has to be done through a small acoustic window as
in upper abdomen and cardiologic examinations.
• Convex transducers : Convex transducers of 3.5 MHz and focus of 7-9 cm are best
for general purpose ultrasound examinations. In case of thin adults or children
5MHz transducer with a focus of 5-7cm is ideal.

11. ULTRASOUND DISPLAY

12. A mode (Amplitude mode) :
• In this mode the echoes returning from the interfaces are displayed
as spikes projecting from a baseline.

13. TM mode (Time – Motion mode) :
• In the TM mode the echoes are displayed as dots. It is used in the
imaging of moving structures like heart. The dots move back & forth
indicating the movements of tissue interface.

14. B mode (Brightness mode) :
• In the B mode echoes are displayed as dots. The B mode produces an
image of a slice of tissues.

15. Grey scale imaging :
• In the grey scale imaging the echoes are displayed as dots in various
shades of grey.

16. Knobs and controls in the ultrasound machine
:
• Power : A knob that varies the amount of energy that the transducer
transmits to the patient.
• Annotation keys : Allows labeling of image.
• Cineloop : The system memory stores the most recent sequence of
image in a cineloop before the freeze button is pressed.
• Dual image : The screen can be split in order to display two views of
an image or to compare the anatomy of the abnormal side with that
of normal side.

17. • Field of view : Gives 4 or 5 choices to the sonographer to make
maximal use of the screens resolution and yet display all the relevant
area.
• Freeze : All display data start and stop with this control.
• Zoom : The zoom button can be used to expand the image.
• Track ball : Controls the movements of the annotation markers, the
distance markers and cineloop.
• There are various controls in the ultrasound machine, which are used
for the optimal visualization of the image.

18. Time gain compensator (TGC) :
• The echoes from the deep tissues will be very small and attenuated.
TGC amplifies these signals and the echoes from different tissue
interfaces are made up into echoes with similar amplitude.
• Near and far gain controls – these alter the surface (near) echoes and
the deep (distant) echoes.

19. Recording the image : various methods are-
• Recording the image on a x-ray film. This requires an image
processing unit and a special camera.
• Self processing camera and film specifically designed to be attached
to the ultrasound.
• Imaging recording units that print the image on special paper. This
method is much less expensive than using film.

20. Controls present on a Doppler machine in
addition to the ultrasound controls are:
• Doppler gain : similar to the time gain control but increases the intensity of color signal from the
flowing blood.
• Angle control : manipulates angle of the ultrasound beam to obtain correct flow velocities.
• Color flow control : places the Doppler area on the ultrasound image to see for flow.
• Range gate cursor : cursor, which may be presented as a box or two parallel bars, shown in the
screen which indicates the depth and area from which the Doppler signal is obtained.
• Doppler cursor : places the cursor in the Doppler field, which can then be moved to the desired
position to record velocities.
• Power Doppler : switches to power Doppler mode.
• Sweep speed : this control helps to adjust the rate at which the spectral information is displayed.
Three speeds – slow, moderate and high can be selected.
• Wall filter : to filter out the noise and artifact caused by the patient respiration and vessel
motion. The higher the filter setting, lesser the information displayed in the Doppler signal.

21. ULTRASOUND IMAGING ARTIFACTS
• An artifact is an additional, missing or distorted image which does not
represent to the real image of part being examined.

22. ULTRASOUND IMAGING ARTIFACTS
• Reverberation artifact :
• If strong reflecting boundaries are present sound wave can reflect back and forth before
they eventually return to the transducer. This delay in registering the echo leads to reverberation.
• They give false impression of solid structure in areas when fluid is present.
• They can be prevented by changing the scanning angle.
• 2. Refraction :
• When the transducer is placed on the abdominal midline refraction occurs producing
double image of single object.
• Eg : Single pregnancy may appear as twins.
• 3. Side back artifacts :
• These occur when echoes are received that originate not only from the centre of the beam
but also from off centre.
• Eg : It can mimic sludge or layered material in the urinary bladder.
• 4.

23. • Mirror image :
• Mirror image artifacts presents structure that lie on one side of a strong reflector on the other side
of an image as well.
• This commonly occurs around the diaphragm and pleura because of total reflection from an air filled
lung.
• 5. Side lobes :
• The extra beams emitted from an array transducer side are weaker than primary beam and do not
normally produce echoes that are imaged. However if they encounter a strong reflector (bone or gas) their
echoes may be imaged particularly if they fall within an echoic region.
• 6. Acoustic shadowing :
• This appears as a zone of reduced echogenecity formed behind a strongly reflecting structure such
as bone. Thus visualization of soft tissue structures in the upper abdomen is obscured by overlying ribs.
• This effect is however useful is detecting calculi (stones).
• Similar shadowing can be caused by air in the lung or intestinal tract. Evaluating structures behind air
containing bowel loop is difficult.
•

24. • Artifacts can occur due to poor penetration, poor scanning angle and
poor resolution. If gain is too low, a solid mass can appear cystic. If
gain is too high fluid filled structure may resemble solid mass.
Therefore near and far gain should be adjusted carefully.
•

25. SUMMARY :
• The basic components of the ultrasound machine are transmitter
/pulser, transducer, receiver and processor , display and recording of
ultrasound image.
• Transducer is the most important component of ultrasound
machine and is made of piezo eclectic crystals. Transducer converts
electric energy to ultrasound energy and the reflected ultrasound
waves back to electrical energy.
• Radiographer / Radiologist uses different knobs and controls
in the ultrasound machine to get the optimal image.