Lesson 06: Pulse-echo Imaging and Display Modes. These lessons contain 26 slides plus 15 multiple-choice questions.

Lesson 06: Pulse-echo Imaging and Display Modes These lessons contain 26 slides plus 15 multiple-choice questions. These lesson were derived from pages 26 through 32 in the textbook:

ULTRASOUND IMAGING AND INSTRUMENTATION

Pulse-echo Imaging Voltage Sound Rectification Amplification Compensation Demodulation Compression Rejection Scan conversion Preprocessing Postprocessing Magnification All pulse-echo ultrasound systems contain the same basic components. The major components and their functions are best understood by referring to a block diagram.

TRANSDUCER EXCITATION AND OUTPUT POWER TRANSMITTER TRANSMIT POWER OUTPUT ACOUSTIC POWER ENERGY OUTPUT coded excitation: a method used to energize transducers by transmitting a long broadband pulse containing coded waveforms in order to increase the signal-to-noise ratio without loss of resolution The BEAMFORMER section of a pulseecho system provides the excitation to the TRANSDUCER. The excitation voltage from the beam former can be varied in some ultrasound systems. Varying the transducer's excitation voltage affects the amount of acoustic energy leaving the transducer. Various excitation methods are in use today including coded excitation, which improves signal-to-noise ratio. The frequency of the sound is not affected.

TIMING PRF >1000 Hz The rate of recurrence of the beamformer s excitation to the transducer is the pulse repetition frequency (PRF), which is determined by the TIMING section. The timing section also provides synchronization to the rest of the system so that the returning echoes will be processed and displayed according to their proper axial positions (along the path of propagation).

RECEIVER TGC GAIN MASTER GAIN digital: a signal occurring in discrete steps over time and in sequence; signals converted into multiple discrete numerical values OVERALL GAIN The RECEIVER is used to provide the initial processing of the radio frequency echo information. After rectification, which eliminates either the positive or negative half of the received analog signal, some ultrasound systems immediately convert the information into digital signals prior to further receiver processing. The receiver s sensitivity is a measure of the weakest echoes that it can detect.

TIME GAIN COMPENSATION amplification: the process of increasing smaller voltages to larger ones Time Gain Compensation (TGC), often called Depth Gain Compensation is a receiver function that is used to equalize differences in received echo amplitudes due to reflector depth. TGC provides gradually increasing amplification with depth. The TGC control is just one of the receiverassociated controls that affect the amplification of echoes. The Overall Gain control affects all echoes regardless of depth. The actual names of the various controls will vary with each manufacturer.

TIME GAIN COMPENSATION Many manufacturers incorporate a group of sliding potentiometers to control the amplification of received echoes. Each TGC potentiometer is programmed to affect echoes returning from a specific depth. The TGC Curve is a graphic display of the settings of the receiver controls. More prominent TGC slopes are required when using higher frequency transducers.

GAIN vs. OUTPUT Nearly identical images may be obtained by using a low transducer outputpower setting along with high receiver-gain settings or by using a high transducer output-power setting along with low receiver-gain settings. From a safety standpoint, it is better to use higher gain settings and a lower outputpower setting.

TGC INCORRECT SETTINGS

DYNAMIC RANGE DYNAMIC RANGE COMPRESSION LOG COMPRESSION COMPRESS Another RECEIVER function is Dynamic Range, which is the ability to display both strong and weak echoes. It is the spectrum of values between minimum and maximum echo signal amplitudes.

DYNAMIC RANGE Increased dynamic range; decreased compression; wider range of displayed gray levels Decreased dynamic range; increased compression; smaller range of displayed gray levels contrast resolution: the ability to distinguish between shades of gray 50 db 30 db A wider dynamic range, which is often expressed in db, increases the contrast resolution by ensuring a wider range of displayed gray levels. Compression is a related function that decreases the difference between small and large amplitude signals. Compression effectively reduces the dynamic range of the receiver and the contrast resolution of the image.

NOISE REDUCTION REJECT low-level echoes (noise) anechoic noise: signals conveying unwanted echoes anechoic: the property of being echo-free or without echoes BEFORE REJECT AFTER REJECT In some receivers, a REJECT control is used to establish a threshold to suppress noise or background information. These unwanted echoes, occasionally appearing in an area that is normally anechoic, are often caused by electrical interference.

NOISE REDUCTION Other methods commonly used to suppress noise and improve signal-to-noise ratio include: Frame averaging (persistence), which reduces image noise by averaging and overlapping sequential real-time frames to provide spatial smoothing of the image. Frequency compounding, which is a method of transmitting a single broadband pulse and then using different receive frequency sub-bands. It reduces speckle and electronic noise to improve axial and contrast resolutions. Spatial compounding, which is the process of steering ultrasound beams offaxis to provide multiple transmit angles, or lines of sight while combining them in real-time during a single cross-sectional scan. Tissue interfaces are encountered from numerous directions rather than from a single direction. This helps eliminate certain artifact patterns to provide a more realistic anatomic representation. Reducing the acoustic shadows enables the scanner to essentially see around obstructions.

HARMONICS harmonic: a wave whose frequency is a wholenumber multiple of that of another Harmonic echoes are non-linear, high frequency signals created when a contrast agent or tissue interacts with ultrasound energy during pulse-echo and Doppler studies. Some harmonics are native to specific types and characteristics of tissue and are often produced without the use of a contrast agent. These tissue harmonics are decreased when lower transmit power is used.

HARMONIC IMAGING frame rate: the number of complete real-time images per second Harmonic imaging is a procedure in which the receiver detects only echoes at the second harmonic, which is twice the fundamental (transmitted) frequency. Harmonic imaging, to be effective, requires the use of broadband transducers. Harmonic imaging, by reducing unwanted artifacts caused by interaction with the fundamental frequency sound waves, provides improved contrast resolution, and reduced visible noise. By reducing side lobes and slice thickness, it improves lateral resolution. However, the lower fundamental frequency produces a longer spatial pulse length resulting in somewhat degraded axial resolution. Pulse inversion harmonic imaging is a non-linear imaging method specifically made for enhanced detection of microbubble ultrasound contrast media. Pulse inversion harmonic imaging has half the frame rate as conventional imaging. Axial resolution is somewhat improved compared to fundamental harmonic imaging.

DISPLAY MODES In an ultrasound imaging system, there are two basic modes that are used for the display of echoes that return to the transducer. The A-mode, which was routinely used in the early days of ultrasound, provides an amplitude-modulated display. The escalation of the displayed spikes is a relative indication of the intensity, or strength, of returning echoes. Although some ultrasound systems have A-mode display capabilities, its current use is limited. A-mode, when used for ophthalmic ultrasound applications, is called A-scan.

DISPLAY MODES B-mode provides a brightness-modulated display in which there is a change in spot brightness for each echo that is received by the transducer. In a B-mode ultrasound imaging system, the returning echoes are eventually displayed on a television monitor as shades of gray, which are discrete brightness levels. Typically, the brighter gray shades represent echoes with greater intensity levels.

DISPLAY MODES M-mode (motion-mode), which is often called TM-mode for time motion, produces a graphic B-mode pattern that represents the motion of structures along a single dimension time display, penetrated by a single ultrasound beam.

M-mode B-SCAN (2-D) M-MODE To assure accuracy of transducer positioning, a single-line cursor representing the M-mode line-of-sight is positioned on a 2D image to guide the M-mode ultrasound beam. M-mode, which is commonly used during echocardiographic studies, is often provided as an option on general-purpose ultrasound systems.

M-mode Although cardiac structures in an M-mode display are less identifiable than in a two-dimensional image, the resolution of the M-mode display is far superior. This is due to the typically narrow dimensions of the single M-mode ultrasound beam. M-mode is often preferable for evaluating subtle changes or rapid movements of the heart, which are too fast for the eye to see during twodimensional echocardiography. The use of M-mode is also dictated when precise timing of cardiac events is required.

B-scan B-mode is most commonly used to produce B-scans, or B-mode slices, which are two-dimensional (2-D) cross-sectional displays of objects through scanning planes. Real-time B-scanners produce live cross-sectional images.

B-scan Real-time B-scanners produce live cross-sectional images.

PATIENT-ORIENTED B-SCAN PLANES

ORGAN-ORIENTED B-SCAN PLANES

B-SCAN WITH A-SCAN OPHTHALMIC IMAGE

OBSTETRICAL IMAGES 2D 3D Scanners with 3-D capability collect volumetric data by manually (freehand) or automatically storing a series of 2-D slices in real-time and then performing the processing necessary to produce static (frozen) 3-D images. Live 3-D images are called 4-D.

Answers to the following FIFTEEN practice questions were derived from material in the textbook:

Question 1 Which of the following controls is part of the receiver in a pulse-echo ultrasound system? BRIGHTNESS XMTR PWR RES TGC POST PROCESSING Page 28

Question 2 The sensitivity of an ultrasound system may be determined by measuring the duty factor strongest echoes that are received bandwidth amplitude range of the received echoes weakest echoes that are received Page 27

Question 3 If the OVERALL GAIN is decreased, only the brightness of the near echoes will decrease the energy to the patient is decreased the brightness of all echoes will decrease equally the frequency will decrease only the brightness of the far echoes will decrease Page 28

Question 4 If the ultrasound system displays only the echoes from strong reflectors and nothing else, the sonographer should increase the lateral resolution decrease the output power increase the overall gain decrease the slope of the TGC adjust the far gain Page 28

Question 5 The control that is used to suppress unwanted, low level echoes or background information is TGC POST PROCESSING DYNAMIC RANGE REJECT WRITE MAGNIFICATION Page 29

Question 6 Electrical interference could appear in an image as shadowing behind a poorly attenuating structure a decrease in far field penetration enhancement behind a highly reflective structure low-level echoes within a cyst a loss of axial resolution Page 29

Question 7 The receiver dynamic range that provides the best opportunity for the display of a wide range of gray shades is 3 db 6 db 9 db 60 db 0 db Page 29

Question 8 Compression is used in the receiver of an ultrasound system to reduce the duty factor increase the difference between small and large amplitude signals reduce the range of signal amplitudes provide post processing reduce the sound energy entering the patient Page 29

Question 9 The OUTPUT control on a pulse-echo ultrasound system controls the amount of amplification in the receiver varies the dynamic range in the receiver is used to equalize differences in received echo amplitudes due to differences in the depths of the reflectors varies the voltage that the beamformer supplies to the transducer does not have any affect on the amount of sound energy that enters a patient Page 26

Question 10 The OUTPUT control in a pulse-echo system does NOT affect the excitation voltage that is applied to the transducer frequency of the sound that leaves the transducer energy that enters the patient amount of energy leaving a transducer overall gain that may be required Page 26

Question 11 From a safety standpoint, which one of the following methods is BEST? Low transmitter output and high receiver gain High near gain and low far gain High reject and high transmitter output High transmitter output and low receiver gain Low near gain and high far gain Page 28

Question 12 The dynamic range of the receiver of an ultrasound system refers to the ability of the receiver to track a rapidly moving structure range of echo signal frequencies that can be processed without distortion speed with which the receiver recovers following the excitation pulse to the transducer depth range in tissue over which moving echoes can be received range of echo signal amplitudes that can be processed without distortion Page 29

Question 13 Which of the following results in an increased acoustic exposure to the patient? application of reject increase in the swept gain slope rate increase in the television monitor brightness increase in the beamformer voltage to the transducer increase in the overall gain Page 26

Question 14 Which of the following is NOT a brightness-modulated display, based on the amplitude of received echoes? B mode B scan A mode Real time gray scale M mode Page 31

Question 15 Increasing the gain of a pulse-echo system results in higher A-mode echoes. This is due to increased amount of sound emitted by the transducer increased amount of sound reflected increased efficiency of transducer conversion of sound into electricity increased amplification in the receiver decreased amplification in the receiver Pages 28 and 31

END OF LESSON 06