Ultrasound Physics Ch. 14 & 15

Interlaced Display

525 horizontal closely spaced lines. The odd lines are written by the electron beam, then the remaining even lines are written

Transducer Output

AKA: power, output gain, acoustic power, pulser power, energy output, transmitter output -Determined by the excitation voltage from the pulser Piezoelectric crystal vibrates with a magnitude related to pulser voltage Adjusted by sonographer? Yes! When transducer output changes, the strength of every pulse transmitted to the body changes, as a result all reflections from anatomic structures also change. The brightness of the entire image changes ** Too much voltage can effect patient, refer to bioeffect

Output Power Vs. Receiver Gain

Adjustments to either output power or receiver gain can change the brightness of the entire image. Output Power: Affects brightness by adjusting the strength of the sound pulse sent to the body from the transducer When the pulse is more powerful, all of the returning echoes from the body are stronger, and the image is brighter. When the image is too bright due to high output power, the lateral and longitudional resolution degrade Receiver Gain: (SAFER) Affects the brightness by changing the amplification of the electronic signals after returning to the receiver. When amplification is increased, the electronic signals in the receiver are boosted and the image will be brighter Which one to choose? Look at its description. When the term suggests outgoing, the function is probably output power. When the word indicated reception or incoming, its likely receiver gain. ** when an entire image is too bright or too dark, first option should always be ALARA As Low As Reasonably Achievable

Analog- Digital Conversion

Analog-to-Digital Conversion: Electrical signal from the transducer is analog. But a digital scan converter can only process computer information. So, the analog signal must be converted into digital form for input into the scan converter. This is done by A to D Converter (to store). Digital-to-Analog Conversion: Information in the scan converter is digital, but the televisions are analog, so the signal must be reconverted to analog to form prior to display on the TV. Accomplished by the D to A Converter (to display)

Storage

Archives the ultrasound studies. Typical storage devices (or media) include computer hard drives, CD, DVD, videotape, magneto-optical discs, paper printouts, photographs, and USB drives

Bistable vs. Gray Scale Displays

Bistable: black and white (high contrast) Gray-scale: multiple levels of brightness. Different shades of gray are assigned to different echo amplitudes to facilitate differentiation the various biological tissue. Display controls: Contrast: Determines the range of brilliancies within the displayed image. Brightness: Determines the brilliance of the displayed image. Contrast Resolution: controlled by compression. The ability of the machine to recognize different shades of gray. Determined by number of bits in each pixel. More bits →more shades of gray →better contrast resolution. Less bits →less shades of gray →bad contrast resolution.

Bit

Bit—binary digit, the smallest amount of digital storage. Bistable, has a value of either zero or one. Binary number A group of "bit" is assigned to each pixel to store the gray-scale color assigned to that pixel. The more bits per pixel, the more shades of gray, and the better is the contrast resolution. The ability of the machine to recognize different shades of gray. More bits →more shades of gray →better contrast resolution. Less bits →less shades of gray →bad contrast resolution. How to calculate the number of gray shades Find how many bits are assigned to each pixel. Multiply the number 2 by the power of how many bits. Example: 3 bits/pixel 23 = 2 x 2 x 2 = 8 *Hint; use your fingers!

CRT (Cathode-Ray tube)

CRT (Cathode-Ray Tube): an electron beam emitted from the gun flies through a time varying magnetic field. The electrons are swept across the face of the screen and strike the phosphor coating on the screen. The phosphor glows and turns electrons into light

Spatial Compounding

Combining frames from different angle to form an image. Is a method of using sonographic information from several different imaging angles to produce a single image. The more frames in the compound acquisition sequence, the better the compound image quality, it reduces speckle and minimized shadowing artifacts. Limitations of the technique; reduce frame rates and reduce temporal resolution.

Digital Scan Converter

Computer world: A variable attains only discrete values Uses computer technology to convert images into numbers, a process called digitalizing. The image is stored in computer memory as a series of zeros and ones. The numbers can be processed and then retranslated for display as an image on a monitor. Advantages: Uniformity--consistent gray-scale quality throughout the image. Stability—does not fade or drift. Durability—not affected by age or heavy use. Speed—nearly-instant processing. Accuracy—error-free

DICOM

Digital Imaging and Computers in Medicine A set of rules of protocols that allows imaging systems to share information on a network

Frequency Compounding

Divides the reflection into sub-bands of limited frequency, and an image is created from each of these sub-bands Then, the images are averages, improving signal-to-noise ratio - speckle and clutter artifacts are reduced - spatial resolution (detail) is improved

Transducer

During transmission, the transducer transforms electrical energy into acoustic energy. During reception, it converts the returning acoustic energy into electrical energy. Channel—single PZT element in the transducer, the electronics in the beam former/pulser, and the wire that connects them. The number of elements in an array transducer that can be excited simultaneously is determined by the number of channels in the ultrasound system. Most systems have between 32 - 256 channels.

Speckle

Grainy or granular appearances in tissues that are really homogenous. Speckle does not represent actual tissue texture Created by interference effects of scattered sound, both constructive and destructive, from the many time tissue reflectors * Speckle reduces image contrast and spatial resolution and may negatively impact the diagnostic accuracy of our images ** look for the words- grainy, granular, and interference effects

Edge Enhancement

Increases the contrast at a boundary to make an image appear sharper; most useful to emphasize different tissues. Ideally suited to distinguish interfaces between structures with different gray-scale characteristics. The top of the image is without edge enhancement; the bottom half has edge enhancement.

Fill- In Interpolation

It is preprocessing. Improves image detail (spatial resolution) by filling in the missing data, especially for deeper parts of the sector shaped image. Example: The edges of a circular structure will be better defined. Images with a low line density are most improved with fill-in interpolation. Images with a low line density generally have a high frame rate (good temporal resolution).

Frame Rate

It takes 1/60 of a second to create one field, thus on typical monitors, a compete image is created in 1/30th of a second and the frame rate is 30 Hz 30 frames to produce flicker free image

Master Synchronizer

Maintains and organizes the proper timing and interaction of the systems components.

Recording and Archiving Techniques

Many different technologies are used to record and store ultrasound images and reports. Each form of data storage has strengths and weaknesses Paper media: charts from pen writers - Advantages: portability, does not requires a device to read - Disadvantages: Bulky, hard to store, difficult to make copies, cannot display dynamic images Magnetic Media: Computer discs, computer memory, magentic tape, video tape - Advantages: able to store large amounts of info efficiently, can store and play dynamic images, can record color -Disadvantages: Can be erased by strong magnetic fields Chemically Mediated Photographs: Photographs, flat films, multiformat camera film -Advantages: High resolution, accepted in medical community , can produce color images - Disadvantages: Bulky, difficult to store and retrieve, requires chemical processing, artifacts can arise from dirt or chemical contmination Optical Media: Laser discs, compact discs -Advantages: store huge amounts of data, inexpensive, not erased by exposure to magnetic fields - Disadvantages: Requires a display system, no standardized format for image display and storage

Temporal Resolution

OR persistence →frame averaging/compounding Combining frames from different time to form an image. An image processing technique that continues to display information from older images. The displayed image contains "history" from earlier images. A smoother image will reduce noise, higher signal-to-noise ratio and improved image quality is produced. Reduces temporal resolution. It is most effective with slow moving structures.

PACS

Picture Archiving and Communications System describes the digital ultrasound laboratory in which images and additional medical info are digitized and stored on a larger computer network 3 major advantages 1. Virtually instant access to archived studies 2. No degradation of data 3. Store and Forward telemedicine, the ability to electronically transmit images and reports to remote sites

Display

Presents processed data. The display may be a flat screen monitor, a transparency, a spectral plot, or a variety of other forms Essential link between ultrasound system and medical staff.

Dynamic Frequency Tuning

Produces a wide bandwidth transmit and then selectively filters the receive frequency based on imaging depth

Analog Scan Converter

Real world: a variable attains a continuum of values Originally made gray-scale display possible. Divides picture into a 1000 x 1000 matrix with an electrical storage element. Electrons from the CRT gun strike these elements and the "charge" is stored. Stored changes later read to retrieve information. Advantage: excellent spatial resolution, matrix grid provides superb image detail. Disadvantages: 4 shortcomings made analog scan converter obsolete. Image fade—charges on silicon wafer dissipate. Image flicker—constant switching between read and write modes. Drift—inconsistent pictures from day to day. Deterioration—tube ages and images degrade

Signal to Noise Ratio

Signal- Meaningful portion of the data, signal is good. Noise- Inaccurate portion of the data which degrades the quality of our information, noise is bad. High signal to noise ratio: With a high S/N ratio, the meaningful part of our data is much stronger than the inaccurate portion. A high quality image is created Low signal to Noise ratio: With a low S/N ratio, the inaccurate part of our data is much stronger than the meaningful part. The noise contaminates the good data ans degrades the image ** Increasing transducer output improves signal to noise ratio.

Preprocessing

The image can be manipulated in between the "storing" and the "displaying" of the data. Once preprocessing occurs, the data is alerted forever Preprocessing cannot be reversed or "undone." TGC Log compression Write magnification: Improves anatomic detail by enlarging a chosen portion of an image (region of interest) to fill the entire screen. Acquires new data Identical pixel size More pixels than the original ROI Improved spatial resolution Possible improved temporal resolution

Elastography

The theory that tissues will deform differently following the application of force. Stiffness data and ultrasound reflections are combined into elastogram images. It can be combined with anatomical images and Doppler to add to the diagnostic value of the exam.

Pulser and Beam Former

This created and controls the electrical signals sent to the transducer that generate sound pulses. The pulser determines the amplitude, pulse repetition period, and pulse repetition frequency, while the beam former determines the firing delay patterns for phased array systems. -The beam former determines the firing delay patterns for phased array systems. Pulser Modes: Continuous Wave- constant electrical signal in the form of a sine wave, electrical frequency= sounds frequency Pulsed wave, single crystal- Short duration elecrical spike, one electrical spike per ultrasound pulse Pulsed wave, arrays- Many elements fired for each ultrasound pulse

Receiver

This transforms the electrical signals from the transducer (produced by the reflected sound) into a form suitable for display. There are 5 operations that must be performed in the appropriate order for the system to function properly. Amplification (1st function) : Also called the receiver gain. This increases the strength of all electrical signals in the receiver prior to further processing. The sonographer can adjust the amplifier. The units are dB, The ratio of the output electrical signal strength to the input electrical signal strength of the amplifier. The effect upon the image: Every signal is treated identically ("uniform amplification") Thus, amplification changes the brightness of the entire image. By itself, increasing receiver gain cannot create an image with uniform brightness. The signal to noise ratio is unchanged when amplification levels change because the signal and noise change equally. Each electrical signal returning from the transducer is made larger with an equal amount of amplification. * Preamplifier- Alters the signal before its amplified, often performed in the probe. Designed to prevent electrical noise from contaminating the tiny electric signals Compensation (2nd function): Also called time gain compensation (TGC), depth gain compensation (DGC's) and swept gain.This is used to create an image of uniform brightness from top to bottom. SInce deeper pulses undergo more attenuation, eschoes returning from greater depths have lower amplitudes than those returning from shallow depths. Compensation makes uniform brightness from top to bottom. This can be adjusted by the sonographer. Compensation treats echoes differently, depending on the depth at which they are coming from. Compression (3rd function): Also called log compression, or dynamic range. This reduces the total range of signals from the smallest to the largest. This changes the gray scale mapping, and allows us to see all gray shades. Yes, this can be changed by the sonographer! With compression, decibels add or subtract. Compression is performed twice; first compression keeps the electrical signals level within the accuracy range of the systems electronics. Second, compression keeps an images gray scale content within the range of detection by the human eye. Demodulation (4th function): This is a 2-part process that changes the electrical signals within the receiver into a form that is suitable for display on the monitor, ( conversion of echo voltages from radio frequency to video form). Cannot be changed by the sonographer. First step is Rectification; turning all of the negative voltages into positive ones. Corrects for or eliminates negative voltages. Second step: Smoothing (or enveloping), putting an envelope around the bumps to even them out Rejection (5th function): Also called suppression, threshold, filter, negative. This eliminates low-level noise images. Allows a sonographer to control whether the low-level data will appear on the display image. This affects all low level signals everywhere on the image, but does not affect bright echoes. Yes, the reject can be adjusted by the songrapher.

Coded Excitation

Traditional imaging uses a very short pulse to create images. Coded excitation is a sophisticated method of improving image quality. It was developed within the context of bioeffects. It creates very long sound pulses containing a wide range of frequencies. It keeps peak intensity below the FDA's limit. Takes place in the pulser for deeper penetration. Improves signal-to-noise ratio. Improves spatial and contrast resolution.

Magnification

With magnification, the sonographer can improve a visualization of anatomic detail by enlarging a portion of an image to fill the entire screen 2 different types of maginifcation: 1. Read Magnification:Improves anatomic detail by enlarging a chosen portion of an image (region of interest) to fill the entire screen. Uses old data Larger pixel size Same number of pixels as the original ROI Unchanged spatial resolution Unchanged temporal resolution 2. Write Magnification: Improves anatomic detail by enlarging a chosen portion of an image (region of interest) to fill the entire screen. Acquires new data Identical pixel size More pixels than the original ROI Improved spatial resolution Possible improved temporal resolution

Progressive Scan Display

an alternative format presents all the lines in sequence. Also called non-interlaced display

Postprocessing

manipulating the data after it has been stored in the scan converter memory but prior to display. Increases versatility of the display process. This can be undone. Done on a freeze frame. Black/white inversion 3-D rendering Contrast variation

Switch

protects the delicate receiver components from the powerful signals that are created for pulse transmission. Also directs the electrical signals from the transducer to the appropriate electronic and processing components within the ultrasound system.

Pixel

smallest element of a digital picture. If we divided a picture into a grid, each square is a pixel. Pixel density is the number of picture elements per square inch. The more pixels per square inch, the greater the details in the image. The detail is called, "spatial," or "details resolution." The best spatial resolution is obtained with more elements or pixels in the image matrix, regardless of whether the converter is digital or analog. Low Pixel Density: Few pixel/inch Larger pixels Less detailed image Lower spatial resolution High Pixel Density Many pixels/inch Smaller pixels More detailed image Higher spatial resolution

Ultrasound System

the entire device that produces sound beams, retrieves the echoes and produces visual images and audio signals. 2 major functions 1. The preparation and transmission of electrical signals to the transducer, which creates a sound beam 2. The reception of electrical signals from the transducer, with subsequent processing into clinically meaningful images and sounds.

Scan Converters

translates the information (image data) from the spoke format into the video format to store (write) or display (read).