Lesson 07: Ultrasound Transducers This lesson contains 73 slides plus 16 multiple-choice questions. This lesson was derived from pages 33 through 42 in the textbook:
Ultrasound Transducers
Ultrasound Transducers Real-time B-mode systems may use transducers that produce images that are displayed in linear formats, which represent sections of the scanned structures as either rectangles (standard linear) or parallelograms (steered linear). The linear format is created by sequentially transmitting a series of acoustic lines, each in a direction that is parallel to the previous acoustic line. Each acoustic line is generated by a single sound pulse leaving the transducer.
Ultrasound Transducers Some transducer configurations produce B-scan images in a sector format. The sector format displays wedge-shaped sections, that are categorized as pieshaped, blunted-pie, or trapezoidal. The sector format is created by transmitting a series of acoustic lines, each at an angle that is different from that of the previous acoustic line.
Ultrasound Transducers field-of-view: the scanning plane visible from a specific transducer configuration Linear images have the same field-of-view in both the near field, the area in a patient that is near the face of the transducer, and in the far field, the area in a patient that is farthest from the face of the transducer.
Ultrasound Transducers Sector images have a limited field-of-view in the near field, but the field-of-view in the far field is usually greater than that of a linear image. Also, spaces between individual acoustic lines in the far field are greater than in the near field, but interpolation in the image is used to overcome this disadvantage.
Ultrasound Transducers A real-time B-scan ultrasound image is updated many times a second to produce the live display. Temporal resolution is a function of the real-time frame rate, which represents how often updating occurs. A low frame rate (lower temporal resolution) is acceptable for abdominal imaging since there is not much movement of the scanned structures. Temporal variations may be lessened by using frame averaging, which is the addition of consecutive frames.
Ultrasound Transducers THE NUMBER OF ACOUSTIC LINES IN A REAL TIME IMAGE: PULSE REPETITION FREQUENCY FRAME RATE For a flicker-free real-time image, the frame rate should be at least 10 to 15 Hz. Low frame rates allow an increase in the number of acoustic lines, the individual sound beams that are produced throughout the scanned crosssection. Image quality is a function of line density, which is proportional to the number of acoustic lines.
Ultrasound Transducers THE NUMBER OF ACOUSTIC LINES IN A REAL TIME IMAGE: PULSE REPETITION FREQUENCY FRAME RATE Higher frame rates (higher temporal resolution) must be used for echocardiography because of the rapid movements of structures in the heart, but the use of higher frame rates, however, may result in reduced image quality due to the reduction in the number of acoustic lines.
Ultrasound Transducers THE NUMBER OF ACOUSTIC LINES IN A REAL TIME IMAGE: PULSE REPETITION FREQUENCY FRAME RATE Some manufacturers have been able to use high frame rates and still achieve what appears to be increased line density. The high frame rates in these systems produce fewer acoustic lines but each line is sampled multiple times during the receive mode from different angles, effectively increasing the number of displayed lines.
Ultrasound Transducers THE NUMBER OF ACOUSTIC LINES IN A REAL TIME IMAGE: PULSE REPETITION FREQUENCY FRAME RATE Depending on the system, frame rates can be fixed or operator-selectable or can vary automatically. Frame rates that vary automatically often depend on the transducer frequency, the display depth, and focal-zone settings.
Ultrasound Transducers THE NUMBER OF ACOUSTIC LINES IN A REAL TIME IMAGE: PULSE REPETITION FREQUENCY FRAME RATE A decrease in the DISPLAY DEPTH setting increases the pulse repetition frequency (PRF) to produce an increase in the number of acoustic lines. High PRF values usually result in depth ambiguity, which decreases the maximum depth that can be accurately imaged. To prevent depth ambiguity, a high PRF is only possible when the display depth is decreased.
Ultrasound Transducers ACOUSTIC LINES = PRF FRAME RATE PRF FRAME RATE = ACOUSTIC LINES 1000 Hz 10 Hz 100 1000 Hz 20 Hz 50 1500 Hz 10 Hz 150 1500 Hz 20 Hz 75 2000 Hz 10 Hz 200 2000 Hz 20 Hz 100 Decreasing the angle of a sector display, while not actually changing the number of acoustic lines, increases the line density without affecting the temporal resolution. Line density affects spatial resolution, but does not affect lateral resolution.
Ultrasound Transducers ACOUSTIC LINES = PRF FRAME RATE DISPLAY DEPTH PRF CHANCE OF DEPTH AMBIGUITY FRAME RATE ACOUSTIC LINES Increase Decrease Decrease - - Decrease Decrease Increase Increase - Increase Increase Decrease Decrease Increase
Flat-linear array PHOTO OF LINEAR ARRAY CONVENTIONAL LINEAR ARRAY Linear scanning formats are produced by flat-linear array transducers. The flatlinear array (often identified as linear array without the flat classification) contains a large number of linearly arranged piezoelectric elements. The elements, which are pulsed sequentially produce parallel acoustic lines to form a rectangular or parallelogram shaped image.
Flat-linear array footprint: the footprint is the region on the face of an ultrasound transducer that is in contact with the patient PHOTO OF LINEAR ARRAY LINEAR ARRAY SLICE PATTERN The linear scanning format displays a large field-of-view of structures close to the transducer. However, the transducer s large surface area, or footprint, often makes it difficult to obtain images of structures that are located beneath obstructions, such as the heart, which is located beneath the ribs. Flat-linear array transducers can be used for abdominal, obstetrical, small parts, vascular, and musculoskeletal imaging.
Flat-linear array
Flat-linear array CONVENTIONAL LINEAR ARRAY FETAL IMAGE
Flat-linear array CONVENTIONAL LINEAR ARRAY BREAST IMAGE
Flat-linear array UMBILICAL CORD WITH COLOR DOPPLER
Flat-linear array
Flat-linear array
Flat-linear array
Flat-linear array TRANSMIT FOCAL ZONES THYROID AND LEFT CAROTID Flat linear arrays, like other arrays, have selectable transmit focal zones for lateral resolution improvement along the two-dimensional multiple-element plane.
Flat-linear array MULTI-DIMENSIONAL LINEAR ARRAY ELEMENT CONFIGURATION A linear transducer may be configured as a multi-dimensional array (also called 1.5 dimensional array), with a matrix of elements along the width plane (often called elevation or z-axis ) to improve elevational resolution by reducing the slice thickness, often called section thickness. A reduced slice thickness decrease chances of a tissue averaging artifact.
Flat-linear array
Flat-linear array (Steered) STEERED LINEAR ARRAY VASCULAR IMAGE
Flat-linear array PROSTATE
Curved-linear array PHOTO OF CURVED-LINEAR ARRAY A sequentially pulsed transducer that produces a real-time sector image is the curved-linear array, which is often called curvilinear array or convex array.
Curved-linear array CURVED-LINEAR ARRAY ELEMENT CONFIGURATION Similar to the flat-linear array, a curved-linear array also contains a large number of linearly arranged piezoelectric elements. Since the surface of the curved-linear array is not flat, each acoustic line is transmitted at an angle, which is different from that of the previous acoustic line.
Curved-linear array CURVED-LINEAR ARRAY SLICE PATTERN Depending on the transducer s footprint, the field-of-view in the near field could be much greater than that of many other sector transducer configurations. Curved-linear array transducers can be used for abdominal, obstetrical, gynecological, and small parts imaging.
Curved-linear array
Curved-linear array LIVER AND RIGHT KIDNEY
Curved-linear array ABDOMEN WITH COLOR DOPPLER
Curved-linear array
Curved-linear array
Curved-linear array FIBROID UTERUS
Curved-linear array LIVER HEPATIC VESSELS
Curved-linear array ASCITES
Curved-linear array ENDOVAGINAL PROBE
Sagittal scan planes TRANSABDOMINAL PROBE ENDOVAGINAL PROBE
Curved-linear array ENDOVAGINAL UTERUS
Curved-linear array PROSTATE IMAGE
Curved-linear array NEONATAL BRAIN IMAGE
Curved-linear array HANDHELD ULTRASOUND SCANNER
Phased array PHOTO OF PHASED ARRAY Sector real-time imaging is also possible with a phased array transducer, which contains a large number of linearly arranged piezoelectric elements along a small scanning surface.
Phased array The sweeping, or steering, of each acoustic line is performed electronically by pulsing all of the elements in the array as one group, but with small time or phase differences between them.
Phased array PHASED ARRAY SLICE PATTERN The footprint of a phased array transducer is small compared to the curvedlinear array, and the pie-shaped image that is produced has a more limited field-of-view in the near field.
Phased array
Phased array ABDOMEN
Phased array FETAL HEAD
Phased array
Phased array ENDOVAGINAL UTERUS
Phased array
Phased array
Phased array CARDIAC IMAGE
Phased array TRANSESOPHAGEAL ECHOCARDIOGRAPHY
Phased array PROSTATE IMAGE
Vector array PHOTO OF VECTOR ARRAY Some manufacturers of ultrasound equipment provide compound array transducers, which combine flat sequenced linear array and phased array techniques to provide a trapezoidal imaging format.
Vector array VECTOR ARRAY SLICE PATTERN The footprint of a compound array (often termed trapezoidal array or vector array) transducer is slightly larger than that of a normal phased array transducer, and the sector image that is produced has a wider field-of-view in the near field. Phased arrays and compound arrays are routinely used for echocardiography, abdominal, pelvic, vascular, transcranial, and neonatal brain imaging.
Vector array ABDOMEN
Vector array HEART, 4-CHAMBER VIEW
Vector array CAROTID ARTERY
Transducer arrays Flat-linear array, curved-linear array, phased array, and trapezoidal array transducers have no moving parts. They can be rapidly switched back and forth between different modes of operation to provide simultaneous display modes including real-time only, real-time with M-mode or real-time with Doppler. Additionally, transducers containing arrays have electronic transmit, receive, and dynamic beam forming capabilities, which aid in the improvement of resolution. Most arrays also use acoustic lenses to further improve elevational resolution.
Mechanically steered Mechanical transducers, described as a transducer filled with a fluid and containing a motor, may also create real-time images.
Mechanical wobbler The motor rotates or sweeps a piezoelectric element through an arc to transmit acoustic lines that travel at different angles to produce a live sector image. Only a few ultrasound systems use this transducer configuration, which is relatively low in cost.
Mechanical wobbler Unlike transducer configurations that do not contain moving components, mechanical transducers cannot provide simultaneity of live displays (live 2-D with live M-mode or live 2-D with live spectral Doppler).
Mechanical wobbler annular array: a configuration consisting of concentric, ring-shaped piezoelectric elements that are used in some mechanically steered sector transducers SINGLE-ELEMENT ANNULAR ARRAY Some mechanically steered transducers contain annular arrays, which, like linear and phased arrays, can be electronically focused. This aids in the improvement of resolution by providing a greater depth-of-focus with slice thickness equal to beam width.
Mechanically steered HEPATIC VEINS
Mechanically steered
Mechanically steered EYE, DETACHED RETINA
USB powered CURVED LINEAR ARRAY TRANSDUCER AND USB ADAPTER USB POWERED MECHANICALLY STEERED PROBE A recently introduced scanner configuration defined as USB powered uses self-contained probes that can be plugged into the USB port of a PC (e.g., laptop, notebook, tablet, PDA) either directly or through an adapter. The probe or adapter contains all the electronics needed to drive the transducer and process the returning echoes.
Fluid-delay TRANSDUCER WITH A BUILT-IN FLUID DELAY A fluid-delay or stand-off may be used with any transducer configuration to permit better visualization of superficial structures.
Fluid-delay FLAT-LINEAR ARRAY TRANSDUCER WITH A REUSABLE EXTERNAL STAND-OFF PAD A transducer may have a built-in fluid delay, or a reusable or disposable standoff pad may be used.
Answers to the following SIXTEEN practice questions were derived from material in the textbook:
Question 1 For a real time image to be flicker-free, the minimum image frame rate should be 15 Hz 1000 Hz 1 Hz 100 Hz 1 MHz Page 34
Question 2 If the real time frame rate is 20 Hz, the pulse repetition period is 1/20 second image is updated every 1/1000 second number of acoustic lines is 1000 number of acoustic lines is 20 image is updated every 1/20 second Page 34
Question 3 If the real time frame rate is increased but the lines per frame are unchanged, what else must happen? speed of sound increases imaging depth increases transducer frequency increases the pulse repetition frequency decreases imaging depth decreases Page 34
Question 4 If the lines per frame changed but the imaging depth remained the same, what else must have changed? frame rate pulse repetition period pulse repetition frequency duty factor resolution Page 34
Question 5 If the imaging depth is increased and the sector angle and line density remain the same, what must have taken place? PRF increases transducer frequency increases frame rate decreases PRP decreases frame rate increases Page 34
Question 6 A real time transducer with a frame rate of 10 Hz produces 100 acoustic lines. If the PRF is NOT changed, a higher frame rate will increase the line density a frame rate of 20 Hz will produce 200 acoustic lines a lower frame rate results in more updated images per second the pulse repetition period is 100 milliseconds a frame rate of 20 Hz will produce 50 acoustic lines Page 34
Question 7 Which transducer configuration produced the image? flat linear array curved linear array convex array phased array vector array Pages 35 and 36
Question 8 Which transducer configuration produced the image? flat linear array curved linear array convex array phased array vector array Pages 39 and 40
Question 9 Which transducer configuration produced the image? flat linear array curved linear array non-curved linear array phased array vector array Pages 37 and 38
Question 10 Which transducer configuration produced the image? flat linear array curved linear array non-curved linear array phased array vector array Pages 39 and 40
Question 11 Dynamic focusing is possible only with transducers with frequencies above 5 MHz when the dynamic range is maximum only with transducer arrays with two-element CW Doppler probes with single piezoelectric elements Page 41
Question 12 With phased array transducers, the transmitted sound beam is swept by mechanically sweeping the piezoelectric elements mechanically rotating the piezoelectric elements varying the timing of pulses to the individual piezoelectric elements varying the voltage of pulses to the individual piezoelectric elements varying the frequency of pulses to the individual piezoelectric elements Page 39
Question 13 Which pulsing pattern provides an on-axis beam with focusing? Page 39
Question 14 The use of a water path permits the use of increased output power from the transducer higher frequencies to be used higher duty factors higher pulse repetition frequencies to be used better visualization of superficial structures Page 42
Question 15 Which element arrangement would only be used in mechanically steered transducers? phased linear array convex array non-curved sequenced array sequenced array annular array Page 41
Question 16 The advantage of an annular array over a single element transducer is reduced output power from the transducer higher frequencies are possible improved axial resolution greater depth of focus lower cost Page 41
END OF LESSON 07