Transcript No Slide Title

Saudi Board of Radiology: Physics Refresher Course
Ultrasound Instrumentation
Kostas Chantziantoniou, MSc2, DABR
Head, Imaging Physics Section
King Faisal Specialist Hospital & Research Centre
Biomedical Physics Department
Riyadh, Kingdom of Saudi Arabia
Echo Display Modes
A-Mode (Amplitude Mode)
• A-mode displays depth on the horizontal axis and echo intensity (pulse amplitude) on
the vertical axis
• display depth is computed from:
depth (m) =
speed of sound in tissue (m/s) • pulse round time (s)
2
• the reason that the factor 1/2 appears is that the round-trip time is the time for the
pulse to travel to reflector and back (true distance is only half that)
• the speed of sound in tissue is assumed to be 1,540 m/s
• A-mode systems have no memory, and a permanent record is obtained by
photographing the CRT monitor
• A-mode may be used in ophthalmology or when accurate distance measurements are
required
• because A-mode only provides information along the line of sight of a fixed
transducer little use is made of the spike amplitude and has been superseded by
M-mode and B-mode imaging
B-Mode (Brightness Mode)
• B-mode is the electronic conversion of the A-mode and A-line information into
brightness-modulated dots on the display screen
• the brightness of the dots is proportional to the echo signal amplitude
• the B-mode display is used for M-mode and 2D gray scale imaging
M-Mode or T-M Mode (Time-Motion Mode)
• M-mode or T-M mode displays time on the horizontal axis and depth on the vertical
axis
• the spikes of A-mode are converted into dots, and brightness replaces amplitude
• sequential ultrasound pulses are displayed adjacent to each other, allowing the
change in position of interfaces to be seen
• a camera records the entire (horizontal) sweep to produce an image
• M-mode thus displays time-dependent motion
• longer times may be recorded with strip chart recorders
• M-mode is valuable for studying rapid movement, such as cardiac valve motion
Scan Converter
• the function of the scan converter is to displays a static two-dimensional image of a
section of tissue
• the gray scale is used to display the intensity of an echo from a given region
• the transducer functions both as a transmitter (in pulse mode) and a receiver
• the scanning technique involved either leaving the transducer in one place or
physically moving it (compound motion)
• this compound motion is required because anatomic structures in the body presents
various angles from which the ultrasound waves are reflected
• a single image or scan (frame) is created by adding together individual scan lines
• each scan line represented a series of echoes returning from a pulse traveling
through the tissue (one scan line is acquired for each emitted ultrasound pulse)
• the line density is the number of scan lines per FOV (frame)
Single Scan Line
Single Frame (Scan)
• the span between emitted pulses allows for the returning echoes to be received and
provides information about the depth of an interface
• the strength of returning echoes provides information about differences between the
two tissues
• images are normally displayed on a television monitor and may be digitized and
stored in a computer
• during image acquisition the digital signals are inserted into the matrix at memory
addresses that correspond as close as possible to the relative reflector positions in the
body
• ultrasound images generally have a 512 matrix size, with each pixel being 8 bits
deep (1 Byte), allowing 256 (28) gray levels to be displayed
• each ultrasound frame therefore contains 0.25 MB of information
Real Time Imaging
In modern B-mode scanners, the image is automatically scanned in a succession of
frames rapidly enough to follow the motion of tissue. The frame rate (frames per
second) for real-time imaging is between 15 to 40 frames/s.
In the process of acquiring an image, ultrasound beams are emitted in brief pulses
with a duration of 1 m and are typically repeated every millisecond. Many transducer
designs have been used to sweep back and forth (or steer) the ultrasound pulses
(scan lines) during the acquisition of a frames.
There are two basic techniques for producing real-time ultrasonic images:
Mechanical Scanning Transducers
Mechanical scanning may be accomplished by oscillating a transducer in angle, by
rotating a transducer or a group of transducers, by oscillating a reflector, or by
linearly translating a transducer.
• in most mechanical transducers, the rotating or oscillating component is immersed in
an acoustic coupling liquid within the transducer assembly
• the sound beam is thus swept at a rapid rate without movement of the entire transducer
assembly
• the most common methods of mechanical scanning currently used are the oscillating
transducer and rotating group driven by a motor
NOTE
• a mechanical transducer are cheaper
• depending on their design a rotating transducer can generate a sector or a trapezoid
shaped image
• focusing is accomplished by the use of an acoustic lens
Oscillating Transducer
Rotating Transducer
Electronic Scanning Transducers
Electronic transducers use an array of small rectangular piezoelectric elements arranged
either in a straight line or an annular design that do not move and are under electronic
control.
Linear Sequence Array (Linear Array)
The linear array is operated by applying voltage pulses to groups of elements in
succession (from right to left), in this way each group acts like a larger transducer
element. Firing each element individually would cause excessive beam divergence
and poor resolution (small aperture - each element can be approximately 1 mm in
size).
• the aperture (diameter) is the
size of the group of elements
energized to produce one pulse
• the width of the image is equal
to the length of the array
• these transducers produce a
rectangular shaped image
In linear array systems focusing is done electronically in one direction and with the use
of an acoustic lens in the other dimension.
Electronic focusing
Annular Sequence Array (Annular Array)
The elements instead of being arranged linearly they can also be ring-shaped and
arranged concentrically (annular).
• instead of firing a group of elements
like in a linear array each element is
fired individually
• these transducers produce a sector
shaped image
Convex Sequence Array (Curvilinear Array)
A convex array is constructed as a curved line of elements rather than a straight line. Its
operation is identical to that of the linear array (sequencing groups of elements), but
because of the curved construction, the pulses travel out in different direction,
producing a sector type image.
Vector Array
Phasing can be applied to a linear array to steer pulses in various directions, this type
of linear array is called a vector array. Scan lines originate from different points across
the top of the display and travel out in different directions. The image format is similar
to that of the convex array except that the contact surface is smaller and the top of the
display is flat.
Linear Phased Array (Phased Array)
A linear phased array is operated by applying voltage pulses to all elements (not a small
group) in the assembly as a complete group, but with small time differences, so that the
resulting sound pulse may be sent out in a specific path direction. If the same time
difference were used each time the process was repeated, the same direction would
result repeatedly. However, the time differences (phasing) are changed with each
successive repetition so that the beam direction can continually change.
• the phased array is sometimes called the electronic sector transducer
• this type of transducer produces a sector type of image
In addition to steering the beam by phasing, the phased array can focus the beam by
phasing as well.
Phased Array (Multiple Focus Mode)
• in order to create a multiple focus beam, phased arrays use multiple pulses in a given
scan line, each pulse being focused at a different depth
• in multiple focus mode, echoes from the focal region of each pulse are imaged and the
rest are discarded
• using multiple pulses per scan line takes more time, and the frame rate is reduced
• temporal resolution (resolution as a function of time) is sacrificed for an improvement
in detail resolution
Phased Array (Dynamic Aperture)
In order to maintain a comparable focal beam width, more array elements are used
(increasing aperture) as the focus is moved deeper.
• not all elements of a phased array are used to generate all pulses, smaller groups
are used for short focal lengths,whereas incrementally larger groups are used for
focuses of increasing depth
Image Acquisition
Pulser : The pulser produces electric pulses that drives the transducer (T) through the
beam former. It also includes a clock that determines the pulse repetition
frequency (PRF) and synchronizes the various components of the instrument.
Beam former : The beam former performs all the tasks necessary for beam steering,
transmit focusing, dynamic aperture and any other additional timing
requirements for phase arrays.
Receiver : The returning echoes (voltages) go through the beam former to the receiver,
where they are processed to a form suitable for input into the memory.
Memory : Electric information from memory drives the display, which produces a visual
image of the cross-sectional anatomy interrogated by the system.
Receiver
The receiver performs the following functions:
(1) amplification
(2) compensation
(3) compression
(4) demodulation
(5) rejection
Amplification
Amplification is the conversion of the small voltages received from the transducer to
larger ones suitable for processing and storage.
Power ratio =
(Voltage Out)2
(Voltage In)2
Gain (dB) = 10 • log (power ratio)
• the gain control determines how much amplification is accomplished in the receiver
• too little gain, weak echoes are not imaged
• too much gain, saturation occurs (i.e.: most echoes appear bright and differences in
echoes strength (contrast resolution) is lost
Compensation (Gain Compensation or Depth Gain Compensation)
Compensation equalizes differences in received echo amplifications because of reflector
depth.
• because attenuation depends on depth, reflectors with equal reflection coefficients will
not result in equal amplitude echoes arriving at the transducer if their travel distance
are different
NOTE
Image depth is
determined by the
attenuation and
maximum depth gain
compensation
Compression
Compression is the process of decreasing the differences between the smallest and
largest amplitudes, which is accomplished by logarithmic amplifiers that amplify weak
inputs more than strong ones. In other words, compression lowers the systems’ dynamic
range.
ASIDE
A viewing display can only handle a power ratio in the order of 100 (20 dB), we get
signals that have power ratio’s over 1,000,000,000 (90 dB).
Demodulation
Demodulation is the process of converting the voltage delivered to the receiver from one
form (radio frequency, RF) to another (video). This is done by rectification and
smoothing.
Rectification
Smoothing
Rejection
Rejection (also called suppression or threshold) eliminates the smaller amplitude voltage
pulses produced by weaker echoes (multiple scattering from within tissue) or electronic
noise.
Overall Receiver Process
Digitization (Pre-processing)
To store echo information in digital memory the demodulated voltage amplitudes
representing echoes must pass through an analog-to-digital converter (ADC). Digital
pre-processing is performed to assign numbers to echo intensities.
Contrast Resolution
For linear pre-processing assignments, the echo dynamic range (40 dB below) is
equally divided throughout the gray levels of the system. The more gray levels
(bits/pixel) that are used, the better the contrast resolution between adjacent pixels.
Image Memory
Image memory used in ultrasound instrumentation are of the digital type. These
memories are some times called digital scan converters because they provide a means
for displaying, using a television scan format information acquired in a linear or sector
scan line format. The image plane is divided into a 512 x 512 pixel matrix, with each
pixel being 8 bit (256 gray values) deep.
Image Storage
• US images are typically composed of 640 x 480 or 512 x 512 pixel matrices
• each pixel typically has a depth of 8 bits (1 byte) of digital data, providing up to 256
levels of gray
• image storage (without compression) is typically 0.25 MB/image
• for real-time imaging (10 to 30 frames/s) this can amount to hundreds of megabytes
• color images used for Doppler studies increase the storage requirements further
because of larger numbers of bits needed for color resolution (full fidelity color
requires 24 bits/pixel, 1 byte each for the red, green and blue primary color)
Image Display (Post-processing)
Digital post-processing is performed to assign specific display brightness to numbers
derived from specific pixel locations in memory. Since monitors are analog output
devices a digital-to-analog (DAC) must be used to convert the digital pixel value to a
brightness value on the monitor. Look-up tables (LUT) can be used to alter the way
the image can look on the monitor.
Display and Temporal Resolution
Scan Line Density (lines/frame)
Since the patient’s tissue is in effect ‘sampled’ along a number of scan lines, each frame
must be made up of a sufficiently large number of scan lines in order to obtain good
lateral resolution.
Frame Rate (frames/second)
To follow moving tissues, a sufficiently large number of frames must be scanned each
second. The frame repetition frequency depends on the number of lines/frame, and is
increased by increasing the PRF (number of pulses sent out every second):
frame rate • lines/frame = PRF
Depth of View
To image structures at depth, each pulse must have time to make the return journey from
the deepest tissue before the next pulse is generated. The depth of view is increased by
reducing the PRF:
depth of view = 0.5 • sound velocity
PRF
It is therefore not possible to achieve both a high pulse frame rate (frame repetition
frequency) and a high scan line density and at the the same time produce an image
with a large depth of view. One or more aspects have to be compromised.
It can be shown that:
penetration (cm) • number of focuses • lines/frame • frame/s < 77,000 cm/s
where 77,000 is the one half of the average speed of sound in tissue (154,000 cm/s)