RESOLUTION AND DIFFERENT TRANSDUCERS

Dr Raghu Kishore Galla

RESOLUTION• Image resolution is the detail an image holds.

• Imaging resolution has three aspects: 1. Detail 2. Contrast 3. Temporal

Contrast and temporal resolutions relate more directly to instruments. Detail resolution relates more directly to transducers.

• If two reflectors are not separated sufficiently, they produce overlapping (not distinct) echoes that are not separated on the instrument display. Rather, the echoes merge together and appear as one.

• Thus the echoes are not resolved. If distinct (separated by a gap) echoes are not generated initially in the anatomy, the reflectors will not be separated on the display.

• In ultrasound imaging, the two aspects to detail resolution are axial and lateral, which depend on the different characteristics of ultrasound pulses as they travel through tissues.

Axial Resolution• Also known as linear, range, longitudinal, or depth resolution.• It is the minimum reflector separation required along the

direction of sound travel (along the scan line) to produce separate echoes.

• The important factor in determining axial resolution is spatial pulse length.

• Axial resolution is equal to one half the spatial pulse length. Axial resolution = ½ SPL• Axial resolution is the minimum reflector separation necessary

to resolve reflectors along scan lines. • Axial resolution (millimeters) equals spatial pulse length

(millimeters) divided by 2.

The SPL is the number of cycles emitted per pulse by the transducer multiplied by the wavelength.

Shorter pulses, producing better axial resolution, can be achieved with greater damping of the transducer element (to reduce the pulse duration and number of cycles) or with higher frequency (to reduce wavelength).

Objects spaced closer than ½ SPL will not be resolved

• For imaging applications, the ultrasound pulse typically consists of three cycles. At 5 MHz (wavelength of 0.31 mm), the SPL is about 3 x

0.31 0.93 mm, which provides an axial resolution of /2(0.93 mm) = 0.47 mm.

• At a given frequency, shorter pulse lengths require heavy damping and low Q,broad-band width operation. For a constant damping factor, higher frequencies (shorter

wavelengths) give better axial resolution, but the imaging depth is reduced.

Axial resolution remains constant with depth.

Lateral Resolution

• Also known as Lateral, Angular, Transverse, Azimuthal resolution.

Lateral resolution is the minimum reflector separation in the direction perpendicular to the beam direction (that is, across scan lines) that can produce two separate echoes when the beam is scanned across the reflectors

• Lateral resolution is equal to the beam width in the scan plane. Lateral Resolution = Beam Width (mm)

For both single element transducers and multi element array transducers, the beam diameter

determines the lateral resolution

• Since the beam diameter varies with the distance from the transducer in the near and far field, the lateral resolution is depth dependent.

• The best lateral resolution occurs at the near field—far field face.

• Lateral resolution is improved by reducing the beam diameter, that is, by focusing.

• The best resolution is obtained at the focus.

• Diagnostic ultrasound transducers often have better axial resolution than lateral resolution, although the two may be comparable in the focal region of strongly focused.

• At this depth, the effective beam diameter is approximately equal to half the transducer diameter.

• In the far field, the beam diverges and substantially reduces the lateral resolution.

• The typical lateral resolution for an unfocused transducer is approximately 2 to 5 mm.

• A focused transducer uses an acoustic lens (a curved acoustic material analogous to an optical lens) to decrease the beam diameter at a specified distance from the transducer.

Section thickness: Elevational resolution

• The Elevational or slice-thickness dimension of the ultrasound beam is perpendicular to the image plane.

• Can be considered a third aspect of detail resolution

and therefore is sometimes called Elevational resolution.

• Elevational resolution contributes to section thickness artifact, also called partial-volume artifact.

• This artifact is a filling in of what should be anechoic structures such as cysts.

• This filling in occurs when the section thickness is larger than the size of the structure. Thus echoes from outside the structure are included in the image, and the structure appears to be echoic.

• The thinner the section thickness, the less is its negative impact on sonographic images.

• Focusing in the section-thickness plane reduces section thickness artifacts.

• Slice thickness is typically the worst measure of resolution for array transducers.

• Use of a fixed focaI length lens across the entire surface of the array provides improved elevational resolution at the focal distance.

Elevational resolution is

dependent on the transducer

element height in much the same

way that the lateral resolution is dependent on the transducer element width.

Multiple linear array transducers with five to seven rows, known as 1.5-dimensional (1.5-D) transducer arrays, have the ability to steer and focus the beam in the elevational dimension.

The axial, lateral, and

elevational (slice thickness)

dimensions determine the

minimal volume element.

Contrast resolution• It is the ability of a gray-scale display to distinguish between

echoes of slightly different intensities. • Contrast resolution depends on the number of bits per pixel in

the image memory.• Increasing the number of bits per pixel (more gray shades)

improves contrast resolution. • ULS pulses DO NOT have a constant amplitude ,Overlapping

occurs. Better Contrast Resolution = Better Detail Resolution• An image with many shades of gray has better contrast

resolution.• An image with less shades of gray degrades contrast resolution.• It is your final production,what you are looking at the display

screen.

Temporal resolution• The ability of a display to distinguish closely spaced

events in time.

• Improves with increasing frame rate.

• Frame is made of scan lines/individual

• When a frame is frozen, keeps the temporal resolution of the frame when generated.

PRF = # focuses x lines per frame x frame rate

• # of focuses increase - PRF increase

• Lines/frame increase - PRF increase

• Frame rate increase - PRF increase

• Penetration increase - PRF decrease

• Frame rate decrease- displayed depth increase

Spatial resolution

• More lines per frame, therefore decrease spatial resolution and great detail Ability to image fine detail and distinguish two closely spaced structures.

• Overall detail of image.

• Line density determines spatial resolution

• Low line density: lines spaced far apart meaning less lines per frame, therefore increase spatial resolution and poor detail.

• High line density: lines closely packed meaning.

TRANSDUCERS

• TRANSDUCERS - have the ability to convert one form of energy to another.

• Ultrasound transducers perform two functions 1.During transmission, electrical energy from the system is

converted into sound. 2.During reception, the reflected sound pulse is converted

into electricity.

• Ultrasound is produced and detected with a transducer, composed of one or more ceramic elements with electromechanical (piezoelectric) properties.

Piezoelectricity • This effect describes the property of certain materials to

create a voltage (evp) when they are mechanically deformed by a pressure.

• These materials change shape when voltage is applied.

Piezoelectric Effect • Ability of a certain material to convert electrical energy to

mechanical energy and vice versa. • Describes property of certain materials to create a voltage

when they are mechanically deformed or when pressure is applied.

• When a pressure is applied the material expands and contracts.

Piezoelectric Material (PZT) 1. Natural material Quartz Tourmaline 2. Synthesized Material PZT Lead Zirconate Titanate Forms of ceramics

• TRANSDUCER - is also known as ceramic, active element cristal, Element, Disc.

• PROBE - is the assembly that holds all the components in. Often referred to as the Transducer.

• Over the past several decades, the transducer assembly has evolved considerably in design, function, and capability, from a single-element resonance crystal to a broadband transducer array of hundreds of individual elements.

• A simple single-element, plane-piston source transducer has major components including the

• Piezoelectric material• Matching layer • Backing block • Acoustic absorber • Insulating cover• Sensor electrodes and • Transducer housing.

• An electrical dipole is a molecular entity containing positive and negative electric charges that has no net charge.

• When mechanically compressed by an externally applied

pressure, the alignment of the dipoles is disturbed from the equilibrium position to cause an imbalance of the charge distribution.

• A potential difference (voltage) is created across the element with one surface maintaining a net positive charge and one surface a net negative charge.

• Surface electrodes measure the voltage, which is proportional to the incident mechanical pressure amplitude.

• Conversely, application of an external voltage through conductors attached to the surface electrodes induces the mechanical expansion and contraction of the transducer element.

• Ultrasound transducers for medical imaging applications employ a synthetic piezoelectric ceramic, most often lead-zirconate-titanate (PZT).

• The piezoelectric attributes are attained after a process of

• Molecular synthesis • Heating• Orientation of internal dipole structures with an

applied external voltage, • Cooling to permanently maintain the dipole orientation

and • Cutting into a specific shape.

• For PZT in its natural state, no piezoelectric properties are exhibited; however, heating the material past its “Curie temperature” (i.e., 3280 C to 3650 C) and applying an external voltage causes the dipoles to align in the ceramic.

• The external voltage is maintained until the material has cooled to below its Curie temperature.

• Once the material has cooled, the dipoles retain their alignment.

• At equilibrium, there is no net charge on ceramic surfaces.

• When compressed, an imbalance of charge produces a voltage between the surfaces.

• Similarly, when a voltage is applied between electrodes attached to both surfaces, mechanical deformation occurs.

• The piezoelectric element is composed of aligned molecular dipoles.

• Under the influence of mechanical pressure from an adjacent medium (e.g., an ultrasound echo), the element thickness

Contracts (at the peak pressure amplitude), Achieves equilibrium (with no pressure) or Expands (at the peak rarefactional pressure),

This causes realignment of the electrical dipoles to produce positive and negative surface charge.

• Surface electrodes measure the voltage as a function of time.

• An external voltage source applied to the element surfaces causes compression or expansion from equilibrium by realignment of the dipoles in response to the electrical attraction or repulsion force.

Resonance Transducers• Resonance transducers for pulse echo ultrasound imaging are

manufactured to operate in a “resonance” mode, whereby a voItage (commonly 150 V) of very short duration (a voltage spike of 1 msec) is applied, causing the piezoelectric material to initially contract, and subsequently vibrate at a natural resonance frequency.

• This frequency is selected by the “thickness cut,” due to the preferential emission of ultrasound waves whose wavelength is twice the thickness of the piezoelectric material.

• The operating frequency is determined from the speed of sound in, and the thickness of, the piezoelectric material.

For example, a 5-MHz transducer will have a wavelength in PZT (speed of sound in PZT is 4,000 m/sec) of

mmmetersm

fc

80.0108sec/105sec/4000 4

6

A short duration voltage spike causes

the resonance piezoelectric

element to vibrate at its natural

frequency, fo, which is determined by the

thickness of the transducer equal to

1/A.

• To achieve the 5-MHz resonance frequency, a transducer element thickness of ½ X 0.8 mm = 0.4 mm is required.

• Higher frequencies are achieved with thinner elements, and lower frequencies with thicker elements.

• Resonance transducers transmit and receive preferentially at a single “center frequency.”

Damping Block

• The damping block, layered on the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing.

• This component also dampens the transducer vibration to create an ultrasound pulse width and short spatial pulse length, which is necessary to preserve detail along he beam axis (axial resolution).

• The “Q factor” describes the bandwidth of the sound emanating from a transducer as

where fo is the center frequency and the bandwidth is the width of the frequency distribution.

Bandwidthf

Q o

• A “high Q” transducer has a narrow bandwidth (i.e., very little damping) and a corresponding long spatial pulse length.

• A “low Q” transducer has a wide bandwidth and short spatial pulse length.

• Imaging applications require a broad bandwidth transducer in order to achieve high spatial resolution along the direction of beam travel.

• Imaging applications require a broad bandwidth transducer in order to achieve high spatial resolution along the direction of beam travel.

• Blood velocity measurements by Doppler instrumentation require a relatively narrow-band transducer response in order to preserve velocity information encoded by changes in the echo frequency relative to the incident frequency.

• Continuous-wave ultrasound transducers have a very high Q characteristic.

• While the Q factor is derived from the term quality factor, a transducer with a low Q does not imply poor quality in the signal.

Matching Layer• The matching layer provides the interface between the

transducer element and the tissue and minimizes the acoustic impedance differences between the transducer and the patient.

• It consists of layers of materials with acoustic impedances that are intermediate to those of soft tissue and the transducer material.

• The thickness of each layer is equal to one-fourth the wavelength, determined from the center operating frequency of the transducer and speed of sound in the matching layer.

• For example, the wavelength of sound in a matching layer with a speed of sound of 2,000 m/sec for a 5-MHz ultrasound beam is 0.4 mm.

The optimal matching layer thickness is equal to ¼ = ¼ x 0.4 mm = 0. 1 mm.

• In addition to the matching layers, acoustic coupling gel (with acoustic impedance similar to soft tissue) is used between the transducer and the skin of the patient to eliminate air pockets that could attenuate and reflect the ultrasound beam.

Nonresonance (Broad-Bandwidth) “Multifrequency” Transducers

• Modern transducer design coupled with digital signal processing enables “multifrequency or “multihertz” transducer operation, where by the center frequency can be adjusted in he transmit mode.

• Unlike the resonance transducer design, the piezoelectric element is intricately machined into a large number of small “rods,” and then filled with an epoxy resin to create a smooth surface.

• The acoustic properties are closer to tissue than a pure PZT material, and thus provide a greater transmission efficiency of the ultrasound beam without resorting to multiple matching layers.

• Multifrequency transducers have bandwidths that exceed 80% of the center frequency.

• Excitation of the multifrequency transducer is accomplished with a short square wave burst of 150 V with one to three cycles, unlike the voltage spike used for resonance transducers.

• This allows the center frequency to be selected within the limits of the transducer bandwidth. Likewise, the broad bandwidth response permits the reception of echoes within a wide range of frequencies.

• For instance, ultrasound pulses can be produced at a low frequency, and the echoes received at higher frequency.

“Harmonic imaging” is a recently introduced technique that uses this ability

• lower frequency ultrasound is transmitted into the patient, and the higher frequency harmonics (e.g., two times the transmitted center frequency) created from the interaction with contrast agents and tissues, are received as echoes.

• Native tissue harmonic imaging has certain advantages including greater depth of penetration, noise and clutter removal, and improved lateral spatial resolution.

Transducer Arrays• The majority of ultrasound systems employ

transducers with many individual rectangular piezoelectric elements arranged in linear or curvilinear arrays.

• Typically, 128 to 512 individual rectangular elements compose the transducer assembly.

• Each element has a width typically less than half the wavelength and a length of several millimeters.

Two modes of activation are used to

produce a beam. These are the “linear”

(sequential) and “phased”

activation/receive modes.

Linear Arrays

• Linear array transducers typically contain 256 to 512 elements; physically these are the largest transducer assemblies.

• In operation, the simultaneous firing of’ a small group of 20 adjacent elements produces the ultrasound beam.

• The simultaneous activation produces a synthetic aperture (effetive transducer width) defined by the number of active elements.

• Echoes are detected in the receive mode by acquiring signals from most of the transducer elements.

• Subsequent “A-line” acquisition occurs by firing another group of transducer elements displaced by one or two elements.

• A rectangular field of view is produced with this transducer arrangement.

• For a curvilinear array, a trapezoidal field of view is produced.

Phased Arrays

• A phased-array transducer is usually composed of 64 to 128 individual elements in a smaller package than a linear array transducer.

• All transducer elements are activated nearly (but not exactly) simultaneously to produce a single ultrasound beam.

• By using time delays in the electrical activation of the discrete elements across the face of the transducer, the ultrasound beam can be steered and focused electronically without moving the transducer.

• During ultrasound signal reception, all of the transducer elements detect the returning echoes from the beam path, and sophisticated algorithms synthesize the image from the detected data.

Beam properties

• The ultrasound beam propagates as a longitudinal wave from the transducer surface into the propagation medium, and exhibits two distinct beam patterns

- a slightly converging beam out to a distance specified by the geometry and frequency of the transducer (the near field), and

- a diverging beam beyond that point (the far field).

For an unfocused,

single-element transducer, the

length of the near field is

determined by the transducer diameter and

the frequency of the transmitted

sound.

• For multiple transducer element arrays, an “effective” transducer diameter is determined by the excitation of a group of transducer elements.

• Because of the interactions of each of the individual beams and the ability to focus and steer the overall beam, the formulas for a single-element, unfocused transducer are not directly applicable.

The Near Field• The near field, also known as the Fresnel zone, is adjacent to

the transducer face and has a converging beam profile.

• Beam convergence in the near field occurs because of multiple constructive and destructive interference patterns of the ultrasound waves from the transducer surface.

• Huygen’s principle describes a large transducer surface as an infinite number of point sources of sound energy where each point is characterized as a radial emitter.

By analogy, a pebble dropped in a quiet pond creates a radial wave pattern.

As individual wave patterns interact, the

peaks and troughs from adjacent sources

constructively and destructively interfere,

causing the beam profile to be tightly

collimated in the near field.

• The ultrasound beam path is thus largely confined to the dimensions of the active portion of the transducer surface, with the beam diameter converging to approximately half the transducer diameter at the end of the near field.

• The near field length is dependent on the transducer frequency and diameter:

where d is the transducer diameter, r is the transducer radius, and is the wavelength of ultrasound in the propagation medium.

22

4rd

lengthfieldNear

• In soft tissue, = 1.54mm/f(MHz), and the near field length can be expressed as a function of frequency:

• For a 10-mm-diameter transducer, the near field extends 5.7 cm at 3.5 MHz and 16.2 cm at 10 MHz in soft tissue.

• For a 15-mm-diameter transducer, the corresponding near field lengths are 12.8 and 36.4 cm, respectively.

mmMHzmmd

lengthfieldNear22

54.14

A higher transducer frequency (shorter

wavelength) will result in a longer

near field, as will a larger diameter

element.

• Pressure amplitude characteristics in the near field are very complex, caused by the constructive and destructive interference wave patterns of the ultrasound beam.

• Peak ultrasound pressure occurs at the end of the near field, corresponding to the minimum beam diameter for a single-element transducer.

• Pressures vary rapidly from peak compression to peak rarefaction several times during transit through the near field.

• Only when the far field is reached do the ultrasound pressure variations decrease continuously.

The far field• The far field is also known as the Fraunhofer zone, and is where

the beam diverges.

• For a large-area single-element transducer, the angle of ultrasound beam divergence, 0, for the far field is given by

where d is the effective diameter of the transducer and is the wavelength; both must have the same units of distance.

d

22.1sin

• Less beam divergence occurs with high-frequency, large-diameter transducers.

• Unlike the near field, where beam intensity varies from maximum to minimum to maximum in a converging beam, ultrasound intensity in the far field decreases monotonically with distance.

Transducer Array Beam Formation and Focusing

• In a transducer array, the narrow piezoelectric element width (typically less than one wavelength) produces a diverging beam at a distance very close to the transducer face.

• Formation and convergence of the ultrasound beam occurs with the operation of several or all of the transducer elements at the same time.

• Transducer elements in a linear array that are fired simultaneously produce an effective transducer width equal to the sum of the widths of the individual elements.

• Individual beams interact via constructive and destructive interference to produce a collimated beam that has properties similar to the properties of a single transducer of the same size.

• With a phased-array transducer, the beam is formed by interaction of the individual wave fronts from each transducer, each with a slight difference in excitation time.

• Minor phase differences of adjacent beams form constructive and destructive wave summations that steer or focus the beam profile.

Common Transducers used in clinical setting

ELECTRICAL AND MECHANICAL TRANSDUCERS

Real – time Transducers

• Scanning of element happens within the assembly • Performed electronically• Gives rapid, dynamic imaging • Imaging with a rapid frame sequence display• Performed with arrays• Dominate transducer today

MECHANICAL TRANSDUCERS (Fixed focus, Conventional)

• Single element Disc shaped Fan or Sector image Fixed focal depth

• Two ways to focus: 1.External focusing – mirror or acoustic lens 2.Internal focusing – curving the crystal

• Rotating mirror, ducks and wheel

• Defective crystal destroy entire image

LINEAR SEQENTIAL ARRAY

• Multiple elements arranged in a line• Approximately 1 wavelength long • Display=rectangular image• Beams go out parallel to each other• Generally directed straight out from element• Large footprint• Big aperture

• Elements are stimulated individually in rapid succession (basic sequential)

- Data is collected = one line of sight (scan line) - 120-150 elements - Automatic - no moving parts - Focused - electronically

• Disadvantage – small crystal size = short narrow near field with rapidly diverging far field

• Defective crystal – drop out of line, top to bottom

Clinical applications

• The straight linear array probe is designed for superficial imaging.

• The crystals are aligned in a linear fashion within a flat head and produce sound waves in a straight line.

• The image produced is rectangular in shape.

•This probe has higher frequencies (5–13 MHz), which provides better resolution and less penetration.

•Therefore, this probe is ideal for imaging superficial structures and in ultrasound-guided procedures.

• Vascular access

• Evaluate for deep venous thrombosis

• Skin and soft tissue for abscess, foreign body

• Musculoskeletal—tendons, bones, muscles

LINEAR SEGMENTAL ARRAY

• Elements arranged in a line(linear)• Group or segment of crystals are stimulated instead

one at the time• Voltage pulses are applied to groups in succession 1-

4, 2-5, 3-6• Display =Rectangular Image• Scan lines are parallel• Beams sent straight out from element• Beams are sent out parallel to each other• Large footprint

• Large aperture of assembly• Only scans across face of probe - Focusing - No Steering• Great penetration• Real time results if repeated rapidly enough• Results in a deeper near field – les divergent Far Field• This type of array is no longer in use

LINEAR PHASED ARRAY• Elements arranged in a line (linear)• Array=more than one element• 100-300 elements side by side• ¼-1/2 wavelength• Display=sector image• Beams sent in different directions=steering • Display lines are not parallel• Angle large as 90 or as small as 30 degrees results in higher

frame rate and increased in line density.• Small footprint• Compact - Small footprint+ Sector Image(steering)=good for

tight spaces• Automatic –no moving parts

• Stimulates all elements at once• All elements work as one channel• Electronic steering• Stimulates each crystal with small time delay• Variations in pattern steer the beam in various directions• Time difference is very small <1 microsecond• Electronic Shaping(focusing)• Can modify depth of focus• Allow multi-focusing - Multiple beams down same line of sight• Sweeping is required• Defective element – erratic steering and focus

ANNULAR PHASED ARRAY • Elements arranged in Concentric Rings• Common center• Display - sector image• Steers mechanically – reflecting the beam witch

moving mirror, mechanically rotating transducer. • Cannot steer without mirror!• Large footprint• Heavy snow cone• Due to large aperture must apply great pressure to

get image

• Electronic Shaping(focusing)• Ring shape allows multiple transmit focal zones• Smaller diameter elements=shallow focus• Larger diameter elements=deeper focus• Each ring gets deeper - Great lateral resolution - Reduction of section thickness artifacts - Defective crystal – horizontal dropout

CURVILINEAR ARRAY

• Convex, curved, radial• Send pulses out in different directions from different

points across the curved array surface. • Operates identical to the linear array, only has curved

construction• Large footprint• Lines of site perpendicular to the array surface• Increase in curved pattern• Greater time delays between elements• Moves focus closer to source• Decrease in curved delay pattern

• Shorter delays, less time between elements• Moves focus far from source• Consist of 128 – 256 crystals• Crystals arranged along in arc• Radius of curve varies between 25-200mm• Larger radius=larger aperture• No loss of focus on the edge pulses but limit to depth• Similar to sector scanning, line density is decreased

at depth with curvilinear – contributing to a loss of lateral resolution

Clinical Applications

• The image generated is sector shaped. These probes have frequencies ranging between 1 and 8 MHz, which allows for greater penetration, but less resolution. These probes are most often used in abdominal and pelvic applications.

• They are also useful in certain musculoskeletal evaluations or procedures when deeper anatomy needs to be imaged or in obese patients.

• Abdominal aorta

• Biliary/gallbladder/liver/pancreas • Abdominal portion of FAST exam

• Kidney and bladder evaluation

• Transabdominal pelvic evaluation

ENDOCAVITARY PROBE

• The endocavitary probe also has a curved face, but a much higher frequency (8–13 MHz) than the curvilinear probe.

• This probe’s elongated shape allows it to be inserted close to the anatomy being evaluated.

• The curved face creates a wide field of view of almost 180° and its high frequencies provide superior resolution .

• This probe is used most commonly for gynecological applications, but can also be used for intraoral evaluation of peritonsillar abscesses.

• Transvaginal ultrasound

• Intraoral

VECTOR ARRAY• Flat top with small footprint – ideal for intercostals scanning• Sends out pulses in different directions from different starting

points• Center is perpendicular• Steer – electronically• Focus - electronically• Display – trapezoidal• Phasing can be applied to each element group in a linear

sequenced array to : a) Steer pulses in various directions b) Initiate pulses at various starting points across the array• Smaller footprint than curved

IVUS PROBE

• IVUS is a miniature ultrasound probe positioned at the tip of a coronary catheter.

• The probe emits ultrasound frequencies, typically at 20-45 MHz, and the signal is reflected from surrounding tissue and reconstructed into a real-time tomographic gray-scale image.

Thank you