• Attenuation can happen via 

  • Energy transfer into heat.

  • Scattering of wave.

• Piezoelectric material (PM) at front of transducer is responsible for creating US waves, receiving returning echoes, and converting that energy into an electrical signal that can be converted into a digital image on US machine.

  • Made up of PZT crystal that has unique characteristics. PZT crystal is sandwiched between 2 electrodes which can compress the PC and induce a current.

  • Region of compression comes towards PZT material and compresses it to induce a current within the material. An electronic signal will head back up to the wiring towards the US machine. This compression changes the orientation/shape of the PZT material and causes an electronic signal to be induced.

  • Compression of PZT material will induce a current around the material which will induce movement.

Sound waves comes towards PZT material -> current will be induced around material -> induce movement in material -> electronic signal will head back into US machine.

We can either run an alternating current and cause the shape to change continuously and propagate the wave at a set f through tissue or we can apply a large electric current over a short period of time over this PZT material which will then resonate at a set frequency. This resonance will cause the soundwave to propagate throughout the tissues.

Think about PZT crystals in 2 ways:

• One PZT crystal unit is a drum set. When u hit it that symbol will resonate at a set frequency. The wider / bigger the radius of the symbol, the lower the frequency of that sound when we hit it. The smaller the symbol gets, the higher the pitch, the higher the frequency will be when we hit that symbol. 

• Second way is to think of PZT material as a guitar string that is held b/n 2 fixed points. When it is strummed a certain note is played. No matter how hard you strum the guitar string, the same note will be played. The same frequency will come from strumming that guitar string. 

• Frequency of transducer is determined by 2 things

  • Speed of sound through PM

  • Thickness of PM

    • Thinner material -> higher the f -> higher attenuation

    • Thicker material -> lower the f  -> lower attenuation

  • f = c / wavelength

  • f = c / 2 x PZT

Matching layer 

• Located at front end of transducer

• Has acoustic impedance that lies b/n that of PZT crystals and ST in order to provide a smooth transition so fewer US waves will be directly reflected back and more will make it into our patient's tissue.

• The ideal thickness of matching layer can be calculated by using this formula: 1/4 x wavelength = ML

• Dampening block

  • Function 1: Shortens the period of time the PZT crystal is resonating, thus shortening the SPL

    • Improved axial resolution

    • Increased receiver time allowing us to image deeper tissue.

  • Function 2: Prevents US waves from coming into transducer and helps waves be transmitted forward into patient's tissue.

  • Think of a wet rag on a symbol. The symbol will not resonate for a long time and the note would not be very pure -> a set frequency will get a wider range.

  • No dampening block would result in a long SPL that is relatively pure and has the same f with very little variation in that frequency. 

  • High quality waves: light dampening with pure f (narrow bandwidth)

    • As bandwidth increases, QF decreases.

    • Good for pulse echo but bad for doppler.

• Quality factor = f0 / f (range)

  • Purity of f's that are coming out of transducer; does not determine quality of image. 

  • Determines how narrow our bandwidth

  • Increased dampening = increase in bandwidth (range of f) 

  • Less dampening -> lose axial resolution

• PZT crystals can be individually fired or fired as groups depending on how we are trying to image the structures.

• Green and purple lines represent the timging of firing of crystals. 

  • If fired at different times, waves introduced will interfere with one another and cause focusing of beam onto a set point. Focus distance can be determined by the firing. 

  • If fired in sequential manner, waves they will interfere with one another and steer the beam into a different direction. 

• US MODES

  • A mode (Amplitude): frequency 

    • Single line in field is used; 1 transducer element or group firing and receiving 

    • Attenuation is lost via heat loss or scatter

    • A mode helps us determine where the tissue boundaries are; only looks at amplitude of returning echoes. If no difference in acoustic impedance, no reflection. 

    • Used in ophthalmology and if midline shift in neonate.

    • Gray scale values correspond to amplitude. Higher the amplitude the lighter the gray scale value.

  • B mode (brightness)

  • M mode (motion)

Transducer types

• How we go about creating and propagating the wave.

• Single element and array transducers.

  • Single element = 1 PZT crystal; old news

  • Array = multiple PZT crystals in a row

    • Linear array = sequentially fires either individual or small groups of transducer crystals at a time and stitches together the A mode data to make B more gray scale. Then it shifts down to the next crystals.

    • Phased array = Firing entire array of elements. the order / sequence that current is applied will determine how beam is steared within patient. If current is applied to all elements at the same time then it will be more uniform and straight. 

• 4 mechanisms for changing the focal depth (from L to R)

  • Acoustic lens: material goes on front of transducer that focuses US beam to set focal depth. Like a magnify glass. 

  • Curved transducer array: shape will focus US beam to a set focal depth.

  • Manipulating diameter of transducer elements: Firing yellow + red elements are wider than just firing red elements resulting in different focal zones. These can be superimposed upon each other to form the image. 

  • Electronic focusing: delaying central transducer element firing, and firing the peripheral transducer elements first. US wave is first created on the periphery. Delaying the central firing will increase angle of US focusing and bring focal depth closer to transducer. 

AXIAL RESOLUTION

• Axial resolution: ability to differentiate 2 objects by depth; determined by SPL

  • Transmit time is when US pulses are generated

  • Pulse encounters tissues will reflect back to US machine. These echoes provide the data. 

  • SPL is distance of single pulse. Time that takes is pulse duration. 

    • SPL = # cycles x wavelength

      • Increasing dampening (quality factor) -> reduce the # of cycles released in pulse -> improve AR

      • Thinner PZT material -> shorter wavelength -> higher frequencies (worse depth)

      • Thicker PZT material -> longer wavelength -> lower frequency wave

  • Limit of axial resolution is 1/2 of SPL.

    • If any 2 objects are closer than 1/2 SPL within the axial plane, objects will not be able to be differentiated.

  • Axial resolution remains the same throughout the depth.

LATERAL RESOLUTION

• Lateral resolution (LR)

  • Ability to differentiate 2 discrete objects that are at same depth but on different lateral planes.

  • Relies heavily on beam focusing 

  • Width of the focal zone is roughly half the width of the diameter of the elements creating the US beam.

  • LR = beam width

    • 2 objects need to be further apart than beam width to be registered as separate. 

    • Beam width does change with depth unlike SPL.

    • Phases arrays use timing differential in firing b/n lateral and central elements. 

      • If large delay, closer the focal zone

      • If small delay, farther the focal zone

      • These can be superimposed upon each other to create a greater field of depth where the LR is improved. This comes at the cost of the temporal resolution. 

      • Want to reduce side lobes to improve LR

        • Increasing dampening improves AR and LR

        • Make transducer elements thinner. <1/2 wavelength of wave, get reduction in sidelobe production

ELEVATIONAL RESOLUTION

• Elevational resolution

  • Worst plane for resolution, same depth in different height plane.

  • Elevational height narrows down at the focal zone depending on the height of transducer elements

  • Can place an acoustic lens to focus beam height to a certain depth. 

  • If we add more transducer elements in elevational plane, we get 1.5D transducer array. Fewer rows than columns. 5 to 7 rows in a 1 D.  

  • 2D if  same # of rows as columns, 

  • Needles are placed in elevational plane.

• Temporal resolution

  • Ability to detect and display real-time movement

  • Synonymous with frame rate

    • Higher frame rate allows for better detection of fast motion

    • Rate of 24 frames per sec is required for smooth motion.

  • Frame rate = # frames / sec

  • Frame rate = 1 / T (frame)

  •