Complete ultrasound sonography

Sonography has also known as Ultrasonography, Medical Ultrasound, Diagnostic sonography.

Nowadays almost every person or their family person went through the sonography using an ultrasound machine.

Especially during pregnancy, multiple time sonography is done to monitor fetus growth.

Now sonography is one of the most important methods for diagnostic imaging.

Sonography machine is compact as compared to other diagnostic imaging devices like CT Scan and MRI.

Because there is not any harmful radiation used for ultrasound imaging sonography as compared to ionic radiation like X-Ray.

It is safe even for pregnant women and infants and never produce heat inside the tissues if used in the limits.

Here will try to explain everything about sonography concerning technical and diagnostic parts.

What is Ultrasound Machine based sonography

It can define as a machine, device, process, or Scanner which uses the properties of ultrasound waves and produce 2D, 3D, or 4D view/images of internal organs of the body.

Usage the same echolocation methods which are used by whale or bat naturally.

Wave frequency range above 20,000 Hz (20kHz) higher than audible sound waves.

For sonography medical ultrasound uses within the range between 2MHz to 18MHz.

The wave need medium to travel from one place to another.

Descending order of wave speed in the medium is Solid, Liquid and Gas.

The speed of ultrasound wave in body tissue is 1540 m/s.

History of Sonography and Ultrasound

The story of the device truly starts when the Curie brothers discovered the piezoelectric effect in the year 1880.

When the mechanical force is applied to some specific materials they produce electricity for applied mechanical force.

The effect may be reversed when providing electricity then it starts to vibrate.

Some common natural piezoelectric materials are crystals, some ceramics, bone, etc.

The first application occurs in world war I. When Paul Langevin develops a sonar system for submarines to measure the distance of objects present in the path.

In this device, he put quartz crystal along with the glue in between two steel plates. It works as a transducer.

While using the device applied high-frequency electricity on steel plates.

Thus quartz crystals generate sound waves that travel forward and bounce back from an object.

Using hydrophones hear returning sound waves and note the time interval.

The calculation of sound speed and time is taken for returning gives the measurement of the distance between the submarine and the object.

It was the earlier versions of SONAR systems still used in submarines and oceanography.

During this time Diode and Triode tubes were already invented and using these components the amplification of waves become possible.

During the world war, II lot’s research work has been done in every major country like Russia, the USA, Japan.

They develop synthetic materials like barium titanate and lead zirconate titanate.

Which has a piezoelectric effect many times higher than natural materials.

It gives the boost to develop ultrasound transducers based machines/devices and applications.

Because now the customized small size transducer fabrication was easy and their efficiency was many times higher.

Thus the performance of transducers reaches the level. It can be used as an imaging tool for organs inside the human body.

Beginning of ultrasound machine based sonography

From 1920 to 1940 started to experimenting and find out the application of ultrasound in the field of medicine.

But neurologist Karl Dussik credited in 1942 for using ultrasound machine perform sonography for medical diagnosis.

He use his machine to diagnose brain tumor.

Later in 1948 George D. Ludwig design and develop an A-mode ultrasound machine for gallstone sonography.

The research was kept confidential by the US naval department for a long time.

After that Douglas Howry and Joseph Holmes together make a B-mode ultrasound sonography machine in the year 1951. Which makes possible 2D sonography.

In the year 1986 the university of japan research and develop 3D technology. With this technology possible to visualize three-dimensional images of the fetus.

Later it becomes 4D includes a real-time 3D image in the year 1990. After that as soon as technology sonography machine and technology getting advance.

Ultrasound Machine for Sonography

The modern ultrasound machine is equipment for sonography which can configure to perform all kind of diagnosis related to various human body organs like obs/GYN, cardiac, etc.

Also able to produce 3D and 4D images of internal body organs and can find blood flow directions using the Doppler effect.

An ideal machine have following unit or parts present :

1. Probes or Ultrasound transducers.

2. Acquisition system to interface and process transducer TX / RX data.

3. Drivers, Operating system and Application software.

4. Interface and control devices like keyboard and trackball.

5. Scan converter to process and convert image data for output devices.

6. Display Monitor and printer.

Ideally, the machine has a probe ( ultrasonic transducer ) to put on the body part need to be scan.

secondly, one or more display units ( monitor ) to view output in real-time.

The third one is a thermal printer to print output images on thermal paper and the rest is keyboard/trackball including the processing unit.

What is Channels in Ultrasound Machine

Channels are the number of transducers data processing capacity in parallel or simultaneously.

More common examples are 8, 16,32,64 Channels, and more. An increase in channels increases the hardware size and cost.

Probe / Transducer

Sonography probes are closely arranged array of transducers.

Each transducer element connected individually through wires to provide high voltage ( +150V to -150V ) pulses to generate ultrasound waves.

Also when working as a receiver convert ultrasound waves reflected from deferent organs at different depths into electrical pulses.

Nowadays probe has an average of 128 transducers (piezoelectric ) elements and operates on 2 MHz to 20 MHz.

Two points need to consider firstly higher wave frequency penetrates less which means can show body organs less in depths.

Secondly, the wave frequency is inversely proportional to the probe resolution.

The selection of probes depends on what diagnostic purpose using for the depth and size required for diagnosis.

And using for any surgical procedure or diagnose cavity.

There are many types of sonography probes exist. According to their diagnostic technique or function and organ need to scan.

Mainly classified into following four category.

1 Phased Array or Sector Probe

2 Linear Array

3 Curved Linear Array

4 Endocavitary Ultrasound Probe

5 3D / 4D probe ( will discuss in imaging / mode section )

Sector ( phase array ) Probe

Mostly known as a cardiac probe. Its size and specification make it perfect for cardiac sonography diagnosis.

During the construction of the probe transducer elements are layered and pack into a small area.

Which makes it to put in small body areas like between ribs for cardiac investigation.

Probe frequency ranges between 1MHz to 5 MHz and works similarly to a curved linear probe but with less width ( lateral resolution).

Linear Array Probe

In all kinds of probes, linear probes have the best resolution because of using a high frequency between 5MHz to 15MHz.

Due to less penetration and high resolution, it is suitable to use below 8cm.

Curved Linear Array Probe

Mostly used for abdominal and pelvic sonography diagnosis due to the advantage of covering a wide area with good lateral resolution.

Works on 2MHz to 5MHz frequency provide high resolution and wide curved transducer array provide wide area coverage.

Endocavitary Ultrasound Probe

The common application of this probe is a peritonsillar abscess ( intraoral ), early pregnancy, ovarian torsion, cyst, fibroids, ectopic pregnancy like transvaginal diagnosis.

Probe array surface is similar to a curved linear probe but small in size and long extension to reach internal organ surface need to scan.

Unlike the curved linear probe operates on 8MHz to 13MHz which is a much higher frequency than the curved linear probe.

Transmitting Beam and Receiving Focusing

Here is important to keep the observable region in the clear visible image range and improvement in lateral resolution.

To achieve this goal beam focusing technique is used. The beam focusing is the default function of modern days ultrasound machines.

Transmitting Beam Focusing

For easy understanding can assume ultrasound beam focusing is similar to focusing sunlight on a point using a lens or mirror.

Beam managed to focus on a point achieved by different pulse delays apply to individual transducer elements.

All the pulse delay sequences are managed by electronic circuits and also able to make desired changes.

Dynamic Receiving Focusing

Once beam leaves transducer and strikes to organs/tissue. Some of the waves are transmitted through the organ.

Few waves scatter in the surrounding and some parts of the beam reflect towards probe transducers.

During dynamic focus receiving electronics create calculated delay for each channel of the transducer element.

So that focal point reshapes and focuses point moves away from probe array at the rate of half-speed of the ultrasound wave.

What are the Modes of Sonography

There are several methods for sonography of internal organs simply called modes.

Some modes are an essential part of sonography, some of them are rarely used and rests are not in use.

A Mode ( Amplitude Mode )

The simplest and oldest form of sonography in which the single transducer is used.

A single pulse generated travels through the medium (body ) and reflects towards the probe.

Received echo’s image plotted in the form of amplitude peak reflected from different tissues.

B Mode or 2D Mode

An array of A mode transducers are used to create a two-dimensional image.

Usually, the probe array consists of 100 to 300 transducers parallelly scan to form an image.

M Mode (Motion Mode)

Representation of motion versus time of the B-mode image as per the chosen line.

The movement is represented along Y-axis and time is represented by the X-axis.

The M mode is used for calculating fetal heart rate for obstetrics, evaluate for lung sliding, and rule out pneumothorax.

Doppler Modes Ultrasonography

The main purpose of using the Doppler effect is to measure blood flow direction.

If there is any motion between sound or light like wave source and observer.

Then the frequency of wave increase or decrease according to their respective motion direction.

The frequency of wave increase if the source or observer coming closer respectively.

In opposite the frequency of wave decreases. Change in the wave frequency is also called the Doppler shift.

According to methods used to apply doppler application classified into the following categories.

Pulse Wave Doppler ( PW )

A similar pulse sequence applied as used in 2D sonography. All the transducer elements work as transmitters and receivers.

It gives the view of velocity, direction, and quality of observable flow.

This method can sample at any depth of image along with a doppler line known as sample volume (Gate).

The Doppler signal mostly aliases at a certain level to probe frequency.

For a good doppler image, there should be no noise and a clean window. For such an image need to select parameters carefully.

AUX Continuous Wave Doppler (AUX CW)

In this method instead of a pulse wave continuous signal generated by one element and a reflected wave received from another element.

In such a way no 2D image display but can measure flow at any velocity without aliasing with probe frequency.

Samples are collected along the doppler line but not at any specific depth.

Steered CW Doppler (CW)

Utilize the phase array probe transducer elements to produce the CW doppler image.

Specifically useful in cardiac diagnosis.

Aliasing

When doppler shift frequency is more than probe frequency then higher amplitude peak of doppler frequency cut off.

It shows on another window below baseline.

Color Doppler Mode

Colors are used to show blood flow direction and speed. Also separate blood flow from surrounding anatomy.

Using the pulse-echo method 2D image also possible and color doppler data superimpose on 2D black and white image.

Unlike PW or CW the color doppler uses the Autocorrection method in witch estimate velocity by averaging the value.

Every reflected receiving pulse compare with the previous pulse to calculate the velocity of motion.

Power Doppler Mode

Also known as Color Power Angio (CPA) uses the doppler effect to show the flow region in color over a grayscale background.

Imaging Techniques

Although imaging techniques fall under the Sonography Modes but due to the advancements in those techniques here discussing separately.

3D Imaging Mode

Collection of 2D images from different angles and apply surface rendering on them produce a 3D image.

For collecting 2D images at different angles the transducer element array inside the probe move left to right and vice versa.

Real Time 3D or 4D imaging Mode

Sure they are 3D images but instead of still 3D images continuous update live which makes it possible to visualize the movement.

For 4D imaging 3D Matrix transducer probe is used alternatively 3D mechanical array probe also capable to perform 4D sonography.

SonoCT Imaging

Like tomography images acquired in one plane from different viewing angles then combined to form a single image.

To perform the above task uses transmit beam-steering technology. It is a unique approach to overcome artifacts in conventional methods.

Harmonic Imaging Mode

Commonly used with micro-bubble contrast media to improve detection of blood flow in small vessels.

By selectively enhancing the signal from blood flow and at the same time minimizing the echoes from surrounding organs.

Here the receiving central frequency of the probe is twice that transmitting wave pulse frequency.

This means at the time of reception non-linear ( scattered ) higher harmonic received.

While the same method is used with 2D imaging to improve tissue image quality and resolution is called Tissue Harmonic Imaging (THI ).

Panoramic Imaging Mode or C-Mode (Cine mode)

Almost every ultrasound machine comes with this function. To get a panoramic image need to move the probe at a constant velocity.

Thus able to generate a single wide image of organs. After completing the display shows the complete sored image.

The advantage of the technique is to study complete abnormality with qualitative and quantitative information.

Image Quality and Resolution

For any ultrasound machine while performing sonography the following parameters define its image quality.

Uniform Contrast

Contrast level should be at same level in the entire image.

Frame Rate

The speed of electronics and transducers should be higher to acquire frames, processes, and displays on the monitor.

Bandwidth and Dynamic Range

Wide range with good sensitivity.

Spatial Discrimination

Must be capable to suppress reflected waves from other than imaging objects and cancel noise to prevent artifacts.

Resolution

In any sonography study, good resolution of the ultrasound machine included the following point to evaluate.

Contrast

The capability of the system to find out different tissue types clearly. The system should be identified tissues with a low level of less than 3dB.

Axial Resolution of the probe

The minimum separation between two anatomies the probe can distinguish parallel to the beam path.

Lateral Resolution of the probe

Least separation from other tissue the probe can distinguish in a plane perpendicular to the beam.

Transverse Resolution of the probe

Ability to differentiate anatomy side by side within the beam across the image plane.

Artifacts During Sonography

Before discuss artifacts need to understand few terms about ultrasound wave.

When it travels from one medium to another how it affects the acquired image.

Acoustic Impedance (Z): Resistance to the wave propagation when it travels inside the tissue.

Impedance = Density of matter sound wave passing through X Speed of sound wave

Reflection: When wave crosses through the two different medium or tissue types.

Then wave reflects in the proportion of the difference in impedance (Z) of both mediums.

Refraction: When the wave enters from one tissue to another with varying impedance (Z) then there is a change in the speed of the wave occur.

It can be cause of double-image artifacts.

Attenuation or Absorption: When tissue, organ, or body fluid absorb waves fully or partially and convert into heat and another form of energy.

Air and bone are the highest amounts of attenuation show through complete reflection and absorption.

Mirror Image Artifact

Highly reflective surface combine with air generate mirror artifact.

Clearly visible while performing cardiac ultrasound as the waves, interacts with the pleural-pericardium.

Acoustic Shadowing Artifact

Such organs or tissues with high attenuation coefficient are liable to generate this artifact.

Most common reasons are bones, ribs and gallstones.

Posterior Acoustic Enhancement Artifact

The exact opposite the acoustic shadowing. When waves travel inside significantly low attenuation such as blood or fluid-filled structures.

Edge Shadowing Artifact

Wave refracted while traveling one medium to another with variation in impedance.

Then wave deflected from the original path when cross curved smooth wall structure.

Mostly in the rounded interfaces between a fluid-filled circular structure surrounded by soft tissue.

Edge artifact ( black lines ) seen at the edge of fluid-filled structures for example the gallbladder, cyst, vessels, and bladder.

Reverberation Artifact

Beam start to travel from the probe then strike to highly reflective anatomy and again the cycle repeated.

This is the reason to generate multiple echos and clearly visible in the image as a highly reflective pleural line.

Comet Tail Artifact

The reason behind the artifact is similar to the Reverberation but it occurs when two reflective surfaces are close enough.

The difference between ring down artifact and comet-tail artifact is it dissipates with depth and has a triangular and tapered shape.

Can be seen when a metallic needle is inserted inside the diagnosis region.

Ring Down Artifact

While body fluid is trapped inside the tetrahedron of air bubbles which reflects the beam infinitely.

Due to this effect as result, an infinitely long vertical echogenic line displays an image.

Unlike the comet tail artifact line never dissipates with depth and remains constant till the image ends.

Side Lobe Artifact

when the beam of an off-axis side lobe strike with organ and returns like coming from the main beam.

This creates a duplicate structure on the screen but in a different area.

Other than Sonography Medical Applications

Ultrasound computer tomography (UCT ) is the latest application developing for breast cancer diagnosis which is another diagnostic application apart from the ultrasound machine.

Ultrasound attenuation by tissue produces heat energy and this property is used for some non-diagnostic treatments like surgery.

The beam needs to focus on the tissue witch produces heat and thus the tissue burns without any single cut.

For focusing and monitoring ultrasound beam CT Scan or MRI guided procedures are performed.