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PHYSICS I LECTURE 5
Static-Imaging Principles and Instrumentation
| Questionl | Answer |
|---|---|
| Static Imaging Principles and Instrumentation Modes are | .A-mode .M-mode .B-Mode .C-Mode |
| A- mode scanning (amplitude mode) | can be described as a depiction of the amplitude or strength of the returning echoes as a function of time along a single scan line or line of sight. |
| A- mode scanning (amplitude mode) | display shows the amplitude of the returning echo as a spike in the vertical dimension versus depth or time of the signal in the horizontal dimension (spatial information) |
| A-mode part 1 edleman | As the pulse is emitted by the transducer a dot moves at a constant speed across the systems display. |
| A-mode part 2 edleman | When a reflection returns to the transducer it is processed and the moving dot is deflected upward on the screen. |
| A-mode part 3 | Depth and time are interchangeable because they are directly proportional. Only the reflectors that lie along the path of the beam are imaged. |
| A -mode PART 4 | The sampling of this beam path is called the line of sight or scan line. Multiple interfaces along the scan line are detected by a series of echoes. |
| A-mode part 5 | .The height of the upward deflection is proportional to the amplitude of the returning echo. Strong echoes create tall spikes, whereas weak reflections create short spikes |
| A-mode part 6 | X- axis ( horizontal - depth of reflector (derived from time of flight.) .Y-axis (vertical) amplitude of the echo. |
| A-modes are accurate in determining depth reflectors | Or transducer to reflector distance measurements. This is used in ophthalmology where accurate measurements of depth are critical. The A-mode can also display information about the overall echogenicity of a structure (whether it is cystic or solid). |
| M-mode | .M-mode is a presentation of changing reflector position (motion) versus time. .This is used in echocardiography .This is also called TM-mode (time-motion mode) and PM mode (position-motion mode |
| M-Mode M-modes appear as a group of horizontal wavy lines. M-mode displays are created as follows: | .As the pulse is emitted by the transducer photo sensitive paper is moved at a constant speed across a B-mode display .Various squiggly lines represent depth of the reflecting surfaces and are produced on paper. |
| M-Mode | .A line that moves up and down on the display indicates that a reflector is moving closer to and further away from the transducer, whereas a straight horizontal line indicates a stationary reflector, hence the name motion mode. |
| M-Mode | .The X-axis (horizontal) of an M-mode display represents time .The y-axis (vertical) represents reflector depth which is derived from the time of flight information of the sound pulse. .A series of dots 0.5 seconds apart are used for calibration |
| B-Mode the first form of grayscale imaging. | .manual scanners first used .a single transducer was attached to a mechanical arm .to produce an image ,you would sweep the beam by moving the transducer across the skin of the patient |
| conversion of A-mode to B-mode | .required a new technique. .A-mode the amplitude was presented simply as the height of the waveform on a graph. |
| (B-MODE) AKA. brightness mode | each amplitude is mapped to a greyscale level or brightness. For conventional imaging maps a high amplitude signal is mapped to a bright white level |
| (B-MODE) AKA. brightness mode | .Weaker signals are mapped to a gray .very weak signals are mapped to dark grays .the absence of signal is mapped to almost black. |
| Static B-Mode instruments: | .In B-mode the amplitude of the returning echo is represented by the brightness of a dot. (brightness mode) .The position of the dot represents the depth (time) of the interface from the transducer. |
| Static B-Mode instruments: | Weaker reflections appear as darker gray dots whereas stronger reflections appear as brighter white dots. B-mode has a large field of view |
| PROBLEMS associated with B mode scanning (you do not have with A- mode). | It took several seconds to produce each image; These devices became known as “static” imagers because they worked best on stationary structures. The scan arm was cumbersome and limited the available anatomical views. |
| problems associated with B-mode | .There was a higher dependence of image quality on the skill of the .sonographer than with modern scanners. B-Mode has poor temporal resolution |
| Static B- Mode Scanning | When performing a static scan the transducer must be moved in order to acquire the many scan lines necessary to build up the image |
| Static B- Mode Scanning | This is not the case with real time transducers you can hold the transducer still and an image will be acquired because the transducer elements are moving. |
| Static B- Mode Scanning | .If you held the transducer still you would only get one line of sight. Because the scanning arm must adjust to many orientations it must be flexible but it must be very stable to accurately correlate the collected information |
| Static B- Mode Scanning | The interface location(dot) on the display is moved to the appropriate location by biasing the X and Y deflection plates with voltages from the position generator |
| Static B-Mode | The registration arm is responsible for ensuring that the echo information from a particular interface is displayed at the same location regardless of transducer orientation |
| Static B-Mode | That is the interface should be in exactly the same position on the display when the subject is viewed from many different directions |
| Registration | .Registration accuracy is the ability of the system to place reflections in proper positions while imaging from different orientations |
| Registration | For accurate localization of an interface the horizontal and vertical position of the transducer and also its angulation must be known |
| Registration | In static scan instruments the transducer is located on a special registration arm that can determine the precise location of the transducer so the depth measurements are accurately displayed. |
| Registration | The accuracy of reflector depth positioning is called depth calibration |
| One major problem with B-mode scanners | .the registration arm could easily become out of alignment and the displayed images were not accurate .Because the image in a static scan must be built up (several scan lines acquired) the individual signals must be able to be stored |
| One major problem with B-mode scanners | If you weren't able to store the scan lines one would be wiped out when the next was acquired. |
| Temporal Resolution | Temporal resolution- this is the ability to distinguish dynamics or changes over time. The major limit of temporal resolution is the frame rate. Every factor that lowers frame rate lowers the temporal resolution |
| Temporal Resolution examples | Examples are: imaging depth, the imaging frame or sector size, how many foci are being used, the line density and the number of acoustic lines per display line (Packet size in color) |
| major problem with B-mode is poor temporal resolution. | Moving interfaces create blurred images. The inability of static B-mode scanning to rapidly update the displayed image with new scan data has caused the almost total replacement of this modality by real time imaging. |
| temporal resolution trade offs | we must decide what is most important excellent image quality (A good Photo) or excellent temporal resolution ( a Good movie), some systems produce a excellent photograph and are inconsistent with producing an excellent movie! |
| temporal resolution To obtain a higher frame rate we must: | .Have s shallow image .Single focus .Narrow sector .Low line density .Lower frame rates result from: .Deeper imaging .Multiple focal points .Wide sector .High line density |
| More on Resolution | Detail resolution is a combination axial and lateral resolution together Detail resolution is primarily dependent on transducer characteristics; however system electronics and the display itself can have an effect. |
| Resolution | Contrast resolution- is the ability to distinguish structures based on variations of brightness, the restriction for contrast resolution is the number of bits per pixel in the display (bit depth) |
| Cathode Ray Tubes | .The display is the essential link between the US system and the medical staff. What we see determines what we conclude. DUH? .A television tube which is a special form of cathode ray tube (CRT) is a glass vacuum tube shaped like a funnel |
| CRT | Located in the narrow end is a gun that shoots out a stream of charged particles called electrons, which contain the video information |
| CRT | The interior surface of the screen is coated with phosphors which flow when struck by electrons. The problem with CRT’s operate only in the off or on mode that doesn’t allow an image to be displayed in anything other than black or white(no gray scale) |
| CRT | These lost a great deal of information about the internal detail of organs that come from the scattering of the sound and give organs their texture To better the diagnostic information the analog converter system was developed |
| The Bi-Stable Image | .composed of only 2 shades black and white Grayscale displays present multiple levels of brightness(white, light gray, medium gray). |
| Bi-Stable Image | The numerous levels allow the system to assign different gray shades to different echo amplitudes and to differentiate biologic tissue of different reflectivity. |
| Analog Scan Converter | Most US scanners write image data to a memory device called a “Scan Converter”. |
| Analog Scan Converter | The scan converter plays 2 important roles during imaging: It stores images during scan build-up for viewing and recording It performs scan conversion enabling image data to be viewed on video monitors |
| Analog Scan Converter | The analog scan converter is a funnel shaped vacuum tube with an electron gun located within its smaller end. A stream of charged particles called electrons which contain the image information is shot out of the electron gun. |
| Analog Scan Converter | The larger end of the tube contains a silicon wafer or dielectric matrix, the electrons strike the matrix and are stored |
| Analog Scan Converter | The scan converter accepts echo signal data from the scanner stores this information in an internal memory and reads out the data to a TV monitor |
| Analog Scan Converter | Temporary image storage is an obvious role of the scan converter When the scan converter is “unfrozen” it operates in a continuously refreshed state that is echo information is updated in real time as it is being displayed |
| Analog Scan Converter | In the “freeze” mode scanning operations are suspended and only the readout function is active. Scan conversion is necessary because the display and image acquisition occur in different formats |
| Analog Scan Converter | Ultrasound images displayed on TV monitors require image data to be ordered in a raster scanned format. |
| Analog Scan Converter | The echo image signals produced by the scanner arrive at the memory in a completely different order and at different data rates than those required by the monitor. |
| Analog Scan Converter | .The scan converter acts as a buffer between the two very different data formats during the writing and reading operations |
| Analog Scan Converter | The scan converter accepts echo information in the format and at the data speeds presented by the scanner it writes these data into its memory as an image |
| Analog Scan Converter | It must be able to do this for any transducer type, linear, curved and phased arrays and mechanical sectors supported by the scanner |
| RASTER SCANNING | .Raster scanning- Readout of the image matrix follows row-by-row sequence .The electron beam in the cathode ray tube is scanned across the screen in a row-by-roe sequence |
| Analog Scan Converter | .Analog storage tubes were subject to instability and drift. Images frequently became fuzzy and defocused sometimes even after only a few hors of scanning. |
| Analog Scan Converter | Gray levels drifted so much that image recording devices needed frequent re-adjusting to compensate |
| Analog Scan Converter | .The dielectric matrix may be thought of as a picture divided into millions of tiny dots each containing an electrical storage element. |
| Analog Scan Converter | The spatial resolution or image detail of an analog scan converter is excellent because it is determined by the number of TV scan lines per frame. More lines better spatial resolution |
| Analog Scan Converter | Basically the scan converter translates the information from the spoke format into the video format. |
| Spatial Resolution | We already know that a pulse of US emitted by a transducer travels through the medium in a well directed beam. To produce the image the US beam is swept like a searchlight |
| Spatial Resolution | We already know that a pulse of US emitted by a transducer travels through the medium in a well directed beam. To produce the image the US beam is swept like a searchlight |
| Spatial Resolution | Spatial resolution refers to how closely positioned to reflectors or scattering regions can be to one another and still be identified as separate reflectors on the display. It is really broken into 2 types, axial and lateral |
| Spatial Resolution | Axial is determined by pulse duration Lateral is determined by beam width |
| Spatial Resolution | .Spatial resolution is closely related to the ultrasound frequency, it improves as the frequency is increased .So as Spatial resolution increases so does frequency .As frequency increases so does spatial resolution |
| Analog Scan Converter | Limitations of analog: Image fade- stored charges on the silicon wafer dissipate over time Image flicker- switching between read and write modes Instability- too many factors depend on image quality Deterioration-image degrades as device ages |
| Digital Scan Converter | Current ultrasound instruments use digital technology for storage and manipulation of data |
| Digital Scan Converter | Digital devices are extremely stable, also when images are stored in digital format they can be easily manipulated and processed using different mathematical functions |
| Digital Scan Converter | The digital scan converters are essentially solid state computer memories that have proved to be inexpensive reliable and versatile (A digital watch vs a watch with a dial is an example of the difference |
| Digital Scan Converter | Digital scan converters use computer technology to convert images into numbers a process called digitizing |
| Digital Scan Converter | The image is stored in computer memory as a series of 0’s and 1’s. The numbers are processed and then re-translated into an image prior to display on the monitor |
| Digital Scan Converter Advantages of digital scan converters: | Uniformity- consistent gray scale throughout image Stability- does not fade or drift Durability- not affected by age or heavy use Speed- nearly instant processing Accuracy- error free Important elements of digital: Pixel Bit |
| Gated C Mode Scanning | C-Mode (Constant depth mode) the US system electronics employs the distance equation to turn the receiver electronics on and off (gating) as to listen to echoes returning only from a specific depth |
| Gated C Mode Scanning | This technique can be used to acquire an image at a specific slice at a particular depth by moving the transducer back and forth over the patient and developing the image over time (This technique is rarely used) PW is the most closely related to C-Mode |
| Gated C Mode Scanning | The gating electronics in c-mode scanning rejects all returning echoes except those received at a specific time interval So only the echoes returning from a specific depth are used for the image |
| Gated C Mode Scanning | The other echoes returning before the specific time or after the specific time are not amplified or processed and displayed This gives information only form the area of interest the gate can be adjusted for both location and slice thickness |
| Electrocardiograph Gating | .E-gated scanning is designed to reduce motion artifacts from static .b- mode scanning of the heart .The machine only uses scan data from a specific depth and discards all other incoming echoes |
| Electrocardiograph Gating | This prevents blurring of the heart walls due to motion Neither c-mode or ECG mode scanning are used much anymore |
| Transmission Mode Scanning | Transmission mode scanning is the only ultrasound method that detects the transmitted beam through the patient |
| Transmission Mode Scanning | Due to the ultrasound beam being almost totally attenuated as it passes through the body this method has not shown much promise except for breast and scrotal scanning |
| Transmission Mode Scanning | The transmitting and receiving transducers are separated by an angle of 180 degrees and move in concert This is comparable to CT scanning |
| Reflux Transmission Scanning | Reflux transmission imaging was developed for use in conjunction with ultrasound lithotripsy systems for the identification of stones before during and after lithotripsy |
| Reflux Transmission Scanning | Lithotripsy: like a shock wave it breaks up kidney stones |
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