Ultrasound Scanning Techniques

Introduction

Imaging systems employ pulsed ultrasound or the pulsed Doppler mode. The instrumentation system consists of:

• An electrical signal source to vibrate the crystal
• Piezoelectric crystal (transmits ultrasound)
• A second crystal or the same crystal to receive echoes
• Amplifier
• Display device

The display can be in several different modes e.g.

• A scan
• B scan
• T-M scan
• Amplitude
• Brightness
• Time motion mode

Generation and Detection of Ultrasound

The physical mechanism normally used to generate and detect ultrasonic waves is the piezoelectric effect exhibited by certain crystalline materials which have the property to develop electric potentials on definite crystal surfaces when subjected to mechanical strain. The converse is also true, which means that mechanical displacement is produced when electrical charges are put on their surface. This effect is demonstrated by crystals of materials like Quartz, Tourmaline, and Rochelle salt.

It is difficult to establish the appropriate axis and cut the Quartz crystal to the required from, therefore, Quartz has generally been replaced by synthetic piezoelectric materials namely Barium nitrate and Lead zirconate titanate. They offer several advantages because they are far cheaper to produce and are much easier to construct transducers of complex shape and large areas. They can also be moulded to any shape to obtain a better focusing action for producing high intensity ultrasonic waves.

The choice of piezoelectric materials for a particular transducer depends upon its applications. Materials with high mechanical Q factor are suitable as transmitters whereas those with low mechanical Q and high sensitivity are preferred as receivers. Lead zirconate titanate (PZT) crystals are much better than Quartz crystals up to a frequency of about 15 MHz, because of its high electromechanical conversion efficiency and low intrinsic losses. PZT can operate at temperatures up to 100 °C or higher and it is stable over long periods of time. It is mechanically strong and can be machined to various shapes and sizes. At frequencies higher than this (15 MHz), Quartz is normally used because of its better mechanical properties. Polyvinylidene difluoride (PVDF) is another Ferro-electric polymer that can be used effectively in high frequency transducers. The surface of the synthetic crystals is normally silvered for making external electric connections.

There 3 parameters that are important in optimizing transducers for various types of applications, they are:

• Frequency
• Active element diameter
• Focusing

Frequency

With increase in frequency, the sound beam becomes more directional and the axial resolution improves, however due to attenuation of higher frequency sound waves in the tissues, the penetration decreases. For most abdominal ultrasound examinations, the frequencies used are in the range of 1.5 MHz, whereas the wavelength is the range of 1 mm. Higher frequencies (10 -15 MHz) are used for superficial organs, such as the eye, where deep penetration is not required and where the advantage may be taken of the 0.1 mm wavelength to improve resolution.

The following general rules apply to frequency:

• ↑Frequency ↑Axial Resolution ↑Lateral Resolution ↓Penetration
• ↓Frequency ↓Axial Resolution ↓Lateral Resolution ↑Penetration

Frequency also influences lateral resolution by affecting beam divergence. The following rule applies, assuming all other factors remain constant:

↑Frequency ↓Beam divergence ↑Lateral Resolution

Active Element Diameter (AED)

As the transducer face diameter increases, the beam width decreases and therefore the lateral resolution improves.

↑ A.E.D ↓ Rate of Divergence ↑ Lateral Resolution

The choice of which element size to use is generally based on two considerations:

• Where on the patient’s body the transducer is to be positioned.
• The depth in the body to the structures of interest.

When the structures of interest lie deep in the body, large diameters are advised, since the decreased rate of beam divergence becomes important at greater tissue depths.

Focusing

Focusing a transducer is a means of minimizing the beam width and adjusting the focal zone to give optimum results for a particular examination. Acoustic lenses can be used to shape the ultrasonic beam pattern. The width of the beam can be made narrow with the result that better lateral resolution can be obtained. The focal point can be selected at different depths from the face of the transducer. The ability to select different focal points allows for the optimization of transducer for a particular type of studies.

Basic Pulse-echo Apparatus

The basic layout of the apparatus based on this principle is shown in the figure below:

The transmitter generates a train of short duration pulses at a repetition frequency determined by the pulse repetition frequency (PRF) generator. These are converted into corresponding pulses of ultrasonic waves by a piezoelectric crystals acting as the transmitting transducer. The echoes from the target or discontinuity are picked up by the same transducer and amplified suitably for display on a Cathode Ray Tube (CRT). The X plates of the CRT are driven by the time base which starts at the instant when the transmitter radiates a pulse. In this way, the position of the echo along the trace is proportional to the time taken for a pulse to travel from the transmitter to the target and back again. Knowing the velocity of ultrasonic waves and the speed of the horizontal movement of trace on CRT, the distance of the target from the transmitting end can be estimated.

Probe

The transducer consists of a piezoelectric crystal which generates and detects ultrasonic pulses. The piezoelectric materials commonly used are Barium titanate and Lead zirconate titanate. The crystal is cut in such a way that it is mechanically resonant of an increased efficiency of conversion of electrical energy to acoustic energy. It is usually one half wavelength thick for the particular frequency used.

When the transducer is excited at its resonance frequency, it will continue to vibrate mechanically for some time after the electrical signal ceases. The effect is known as ‘’after ringing’’ and destroys the precision with which the emission or detection of a signal can be timed. To reduce it, the transducer must have a good transient response and consequently a low Q is desirable. To achieve this, the transducers are usually damped. This can be achieved by controlling the rear surface to have high impedance and high absorbing of ultrasonic waves. This is to ensure that the energy radiated into it does not return to the transducer to give rise to spurious echoes. Backing material, therefore, is an important consideration in transducer construction. It is generally an Epoxy resin loaded with a mixture of Tungsten and Aluminium powder. Backing material is made thick enough for complete absorption of the backward ultrasonic waves.

The probes are designed to achieve the highest sensitivity and penetration, optimum focal characteristics and the best possible resolution. This requires that the acoustic energy be transmitted efficiently into the patient. It is thus desirable to reduce the amount of reflected acoustic energy at the transducer body interface. The single quarter wavelength matching layer accomplishes this by interposing a carefully chosen layer of material between the transducer’s piezoelectric element and body tissue. A material with acoustic impedance between tissue and piezoelectric ceramic is selected to reduce the level of acoustic mismatch at the transducer body interface. A uniform thickness of one-quarter wavelength for a frequency at or near the transducer’s centre frequency results in higher acoustic transmission levels because of the favourable phase reversals within the layer.

The single quarter wavelength design however provides optimal transmission of ultrasonic energy at a particular wavelength only. This presents a problem for diagnostic pulse-echo ultrasound which is characterized by very short pulses containing a broad band of frequencies. In addition, the single quarter wavelength matching layer transducer has a face with a concave curvature. Occasionally this can lead to air bubble entrapment or patient contact problems. Multilayer matching technology overcomes these problems by interposing two layers between piezo-element and body. Two materials are chosen, with acoustic impedances between the values for ceramic and tissue. A stepwise transition of impedance from about 30 for ceramic to about 1.5 for tissue allows even further reduction of this acoustic impedance mismatch. The concavity can be filled with a material which is acoustically transparent as possible hence yielding a transducer with a hard flat face for good patient contact, while minimally affecting the ultrasound beam.

Pulse Repetition Frequency (PRF) Generation

This unit produces a train of pulses which controls the sequence of events in the rest of pulse-echo equipment. The PRF is usually kept between 500 Hz – 3 kHz.

Examples of circuits used to produce this type of waveforms are the: Blocking Oscillators and Astable multivibrator. The Astable multivibrator is preferred because the pulse duration can be more conveniently varied and the circuit does not require the use of a pulse transformer.

Transmitter

The transmitter crystal is driven by a pulse from the PRF generator and is made to trigger an SCR circuit which discharges a capacitor through the piezoelectric crystal in the probe to generate an ultrasonic signal. The typical circuit is shown below:

Receiver

The function of the receiver is to receiver the signal from the transducer and to extract from it the best possible representation of an echo pattern. To avoid significant worsening of the axial resolution, the receiver bandwidth is about twice the effective transducer width.

Swept Gain Control

Stronger echoes are received from the more proximal zones under examination than from the deep structures. The receiving amplifier can only accept a limited range of input signals without overloading and distortion. Abrupt changes in tissue properties that shift the acoustical impedance can cause the echo amplitudes to vary over a wide dynamic range like 40-60 dB, to avoid this; the amplifier gain is adjusted to compensate for these variations. This reduces the amplification for the first few centimetres of body tissue and progressively increases it to maximum for the weaker echoes from the distal zone.

Detector

After the logarithmic amplification, the echo signals are rectified in the detector circuit. The detector could be a conventional diode-capacitor type with inductive filter. In the rectification process, the negative half-cycles in the echo voltage waveforms are converted into positive half-cycles. This is followed by a demodulation circuit in which the fundamental frequency signal, upon which the echo amplitude information has been riding, is eliminated. The output of the demodulator circuit is in the form of an envelope of the echo signal. The output of the demodulator is the information that is desired, that is, the amplitude of the echo signal and its time delay from the transmission pulse.

Video Amplifier

The signal requires further amplification after its demodulation in the detector circuit before it can be given to Y-plates of the CRT. The output of the detector circuit is typically around 1V but for display on the CRT, the signal must be amplified to about 100 V to 150 V. In addition to this, the amplifier must have a good transient response with minimum possible overshoot. The commonly used video amplifier is the RC coupled type, having an inductance in series with the collector load. The collector helps in extending the high frequency response to the amplifier.

Time delay unit

The time delay unit is sometimes required for special applications. Normally, the time base will begin to move the spot across the CRT face at the same moment as the SCR is fired. If desired (in special cases), the start of the trace can be delayed by the time delay unit so that the trace can be expanded to obtain better display and examination of a distant echo.

Time base

The time base speed is adjusted so that the echoes from the mostly deep structures of interest will appear on the screen before the beam completely transverses it.

In many applications, distance markers appropriate to each time base setting appear directly on the screen, which greatly simplifies distance measurements.

Time marker

The timer marker produces pulses that are known time apart and therefore corresponds to a known distance a part in human tissues. These marker pulses are given to the video amplifier and then to the Y plats for display along with the echoes.

Display

After amplification in the video amplifier, the signal is given to the Y plates of the CRT. CRT is a fast acting device and gives a clear presentation of the received echo signals.

We have 2 key settings on pulse-echo instruments: the Reject and Damping controls. The reject setting controls the threshold above which the echo signal amplitude must rise to be visible on the A-scope and write on the B and M-mode displays. This control rejects/removes small amplitude inconsequential echoes that would otherwise produce noise signals in the display or recordings. The damping control adjusts either the amplitude of excitation to the transmitting transducer element or the electrical load on the transducer in order to reduce the acoustic output. The effect is desirable enough to improve the echo resolution for near field interfaces, because reducing the transmitter excitation shortens the effective pulse duration.

You can also read: Positron Emission Tomography (PET)

A-scan

This type of scan offers only one-dimensional information. The echo signals are applied to the Y-deflecting plates of the CRT so that they are displayed as vertical blips as the beam is swept across the CRT. The height of the vertical blip corresponds to the strength of the echo and its position from left to right across the CRT face corresponds to the depth of its point of origin from the transducer.

Key Considerations for A-scan

• The CRT is usually of the electrostatic deflection type.
• It is better to use CRT with post-deflection acceleration of the electron beam so that a very bright trace is obtained with lower deflecting voltages on the plates.
• The CRT should be a flat face type to eliminate screen curvature error.
• A variable persistence scope with storage facilities would be useful for prolonged viewing.

Medical Applications of A-scan

Echoencephalograph

• Echoencephalograph: This is an instrument used to diagnose and detect the mid-line of the brain. In the normal brain, the mid-line surfaces are parallel to the flat areas of the bone near the ear. When there is a head injury, the brain gets tilted to one side or the other due to bleeding, but it still retains its normal shape. In such cases, the echoes can easily obtained but they are placed at different distances from the probe i.e. the probe is placed first on one side and then the other side of the skull.

Even when a tumour grows in the brain, the anatomy of the brain is gently altered and there is usually considerable tilting and displacement of the brain ventricles. Ultrasonic mid-line echo, in such cases immediately establishes the abnormalities of the brain due to its shift to one side from the centre.

The Echoencephaloscope usually incorporates measuring range of 0 – 18 cm of tissue depth. The normally used frequency range is 1 – 3 MHz; the probe for 2 MHz with the diameter of 15 – 20 mm is the common and gives a good resolution. The probe for a 1 MHz allows for deep penetration and may preferably be employed for the elderly patients whose skulls are strongly calcified.

Echo-ophthalmoscope

• Echo-ophthalmoscope: A-scan ultrasonic technique is used in ophthalmology for the diagnosis of retinal detachments, intra-ocular tumours, vitreous opacities, orbital tumours, and lens dislocation. It helps in the measurement of axial length in patients with progressive myopia, localization of intra-ocular foreign bodies and extraction of non-magnetic foreign bodies. Echo-ophthalmoscopy employs a 7.5 – 15 MHz pencil type transducer. The transmitted pulse should be very small (nanoseconds) and range. In the normal eye echoes can be obtained from the following structures when performing an anterior-posterior examination along the optical axis:

• The surface of the lid (corresponding with the zero markers that appears constantly on the trace).
• The surface of the cornea.
• The anterior lens capsule.
• The posterior lens capsule.
• The posterior wall of the globe (usually appearing as a complex of echoes originating from the retina, choroid and sclera as a unit
• Retro-bulbar fat

An example of Echoes of a normal eye is shown below:

Echocardiograph (m-mode)

• Echocardiograph (m-mode): Echocardiograph is used to carry out cardiac examination and assessment of many congenital and acquired cardia diseases. By using this instrument it is possible to detect intra-cardia structures. The movement of these structures can also be recorded with better resolution than with the angiographic diagnostic technique. The instrument presents time-versus-motion information about heart structure on slow speeds normally used in electrocardiogram recordings. When an ECG trace is super-imposed on the ultrasonic display, the movement of structures detected ultrasonically can be conveniently correlated with known events in the cardia cycle. Phonocardiogram is also often recorded simultaneously.

The Echocardiogram is the best method for the diagnosis of mitral stenosis. Echocardiography is also used to study the aortic valve, tricuspid valve, and pulmonary valve. This instrument also finds importance use in the detection of pericardial effusion, which is the abnormal collection of fluid between the heart and the pericardial sac.

In A-mode display, ultrasonic echoes produce vertical displacements of a horizontal trace on a CRT. The amount of vertical displacement is proportional to the strength of the echo; and the distance along the horizontal trace represents the time of sound travel in human tissue hence permitting accurate measurements of tissue depth between any two echoes sources. By electronically rotating the A-mode echoes 90° towards the viewer, the echoes can be presented as bright dots of light along an imaginary horizontal base line as shown in the diagram below.

The distance between the dots again represents time or tissue depth and the intensity of the dots represents the strength of the echoes. If one of the echoes sources is a moving structure, then the dots of light from that structure will also move back and forth. If the dots are made to move with an electronic sweep, from bottom to the top of the screen at a pre-selected rate of speed, the moving dots will trace out the motion pattern of the moving structure. This display is known as the M-mode display.

If a photographic film is continuously exposed to one sweep cycle of this display, a composite picture will result, providing a waveform representation of the motion pattern of the moving structure. Alternately, thermal video printers are used for recording an echocardiograph. Currently, Digital echocardiography which involves recording, displaying and storing of an echocardiogram digitally has largely replaced the video tape recording which have limitations like the inability to quickly review the Echocardiogram also digital techniques makes measurements and interpretation much easier than video tape technology.

To take measurements with Echocardiography, the transducer is placed between the third and fourth ribs on the outer chest wall where there is no lung between the skin and the heart. From this probe, a low intensity ultrasonic beam is directed towards the heart area and echo signals are obtained. The probe position is manipulated to obtain echoes from areas of interest in the heart.

B-scanner

A-scope can be difficult to interpret when many echoes are present simultaneously. A pictorial display can be conceived as a means of simultaneously presenting the echo information as well as information about the position of the probe and the direction of propagation of the sound. This is achieved in B-scan display which results from brightness modulation with amplitude of the echoes obtained for various probe positions and orientations to produce a cross-sectional image of the object integrated by a storage display from individual scans.

The figure above shows the difference between A-scan and B-scan.

In order to record cross-sectional pictures of internal structures, the ultrasonic probe is mounted on a mechanical scanner which allows movement in two directions and which links the direction and position of a B-scope time base on a CRT to those of the ultrasonic beam within the patient.

B-scan is based on the fundamental information provided by echoes in an A-scan mode which is used to modulate intensity of the CRT electron beam instead of deflecting it vertically. To obtain a 2 –dimensional cross-sectional image (B-scan), it is necessary to know the transducer position and its orientation. To achieve this, the transducer is attached to either an articulated arm that allows a scan over a sector or a rectilinear gantry that produces a linear scan. The transducer is moved manually for scanning the region of interest on the body. The transducer is coupled by shaft encoders and position sensors to the co-ordinate generator so as to correlate the origin of the echoes from various structures of the body on the CRT scan.

B-scanning of static objects gives two-dimensional images which allow assessment of size, shape and position of the examined structures. For moving objects, the quality of the pictures may be degraded at a degree that is proportional to the range and velocity of the movement. Currently, fast working real time scanners have been developed to study both static and dynamic structures in the human body.

Advantages of Ultrasound Scanning

• Gives a clear picture of soft tissues
• There are no known harmful effects
• The general clarity of the image is better than other techniques
• Real time imaging
• Compared to other imaging techniques, it is cheaper

Disadvantages of Ultrasound Scanning

• Head and skeletal system cannot be examined as ultrasound cannot penetrate the bones
• Almost 100 % reflective at air interfaces, hence a coupling medium is required. Images beyond air spaces or lungs are difficult to obtain
• Scanning takes a long time and requires a skilled person