Ultrasonography

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17 Ultrasonography

Ultrasonography is the formation of a visible image from the use of ultrasound. A controlled beam of sound is directed into the relevant part of the body and the reflected ultrasound is used to build up an electronic image of the various structures which can be viewed on a monitor

Equipment

Piezoelectric Crystal	• Quartz crystal • When electric current is applied it vibrates and emits sound waves • When a sound wave hits the crystal it emits an electric current
Transducer (Probe)	• Produces and sends sound waves • Receives reflected echoes from boundaries between tissues • Has a sound-absorbing backing to stop sound being reflected from the probe • An acoustic lens to focus the sound waves • Shape of the probe gives the field of view (footprint) • Frequency of the sound wave determines the depth the wave will travel and the image resolution • The higher the frequency the better the resolution (but more of the beam is absorbed by the tissues therefore the depth of penetration of the beam is reduced) • High frequencies (7.5 megahertz, MHz) are used for superficial organs, e.g. breast and thyroid • Lower frequencies (3.5 megahertz, MHz) for the abdomen • Can contain one or more than one crystal element • Each crystal has its own electrical circuit • In multiple element probes the individual crystals can receive signals at different times • Smaller probes can be used in the vagina, rectum, oesophagus, etc.
Transducer Pulse Controls	Allow the setting of: • The frequency of the pulses • The time of the pulses • The scan mode of the machine
Central Processing Unit	Contains • A computer – containing a microprocessor and memory • Amplifiers • Power supplies to the microprocessor and transducer
Central Processing Unit	Function • Sends electrical currents to the transducer • Receives electrical pulses from the transducer – created by the returning echoes • Processes the data • By using the speed of sound through tissue (1540 metres per second) and the time taken for the echo to return • Displays the image on the monitor • Stores the final image
Display	Can demonstrate • Shades of grey • Colour • Movement • Two dimensional images • Three dimensional images • Surface rendering • Transparency mode • Real time images

Terminology

Acoustic Impedance	• A value given to a substance • Is calculated by multiplying the density of the medium by the velocity of the ultrasound travelling through the medium • It is independent of frequency • When a sound wave hits a substance with a different acoustic impedance part of the wave is reflected back
Acoustic Window	• An area of the body used to allow imaging of underlying structures, e.g. the spaces between the ribs, the liver
Aliasing	• When high velocities in one direction appear as high velocities in the opposite direction • Occurs when an analogue signal is sampled at a frequency which is lower than half its maximum frequency • All the frequency above half of the sampling frequency is projected below the base line (backfolded) in the low frequency region causing artifacts on the image
Amplitude	• The maximum value of either positive or negative current or voltage that occurs on an alternating current waveform • The magnitude (height) of the ultrasound beam • The ultrasound pulse is very brief so the power values arranged over a period of time will be low compared to peak intensity
Coupling Gel	• A gel put on a patient’s skin to exclude any air between the transducer and the skin surface • Done to enable the transmission of ultrasound waves between the transducer and the patient
Doppler Effect	When imaging a moving object: • The frequency of the reflected beam is changed by the movement • If the object is moving towards the probe the frequency will be increased • If the object is moving away from the probe the frequency is decreased • The speed of change of the frequency indicates how fast the object is moving
Doppler Scanner	• Equipment used to monitor a moving substance • Measures the change in frequency of the reflected echoes to determine the speed of movement of the object, e.g. the fl ow of blood or the beating heart
Echo	• The refl ection of an ultrasound wave back to the transducer • Occurs when the beam hits a surface at right angles
Fourier Transform	• A method of mathematically changing data, e.g. changing spatial data to frequency data • Spatial data gives the position of the varying intensities (brightness) across an image • Frequency data is number and frequency of sine and cosine waves forming the image
Frequency	• The number of cycles of alternating current that occur in one second • Measured in Hertz (Hz) • Ultrasound is frequencies beyond 20 kilohertz (kHz)
Nyquist Theorem	• States that an analogue signal waveform may be reconstructed without error from a sample which is equal to, or greater than, twice the highest frequency in the analogue signal, e.g. to digitally convert a 2 MHz signal a sample must be taken at 4 MHz
Phased Array	• A sector field of view with multiple transducer elements • Formed in precise sequence and under electronic control • This gives a wide field of view using a small transducer • Used in, e.g. cardiac or paediatric head scans
Piezoelectric Effect	When an electric current is produced by certain materials when pressure is applied to their surface
Pixel	• Value stored in the memory of a digital scan converter • Dictates the shades of grey on the monitor • Picture cell • The dots which can be used by a character on a digital image display screen • The smaller the pixel the greater the image quality
Probe (Transducer)	• A hand-held instrument • Composed of multiple elements of piezoelectric material each with its own electrodes
Pulse Repetition Frequency	The number of pulses occurring in one second expressed in kilohertz (kHz).
Rarefaction	The opposite of compression
Real Time Scan	• The image is updated after each ultrasound sweep • Therefore movement can be seen on the image
Scattering	• If the ultrasound encounters air in the bowel the beam will be scattered in all directions causing artefacts on the image which results in a non-diagnostic scan • If ultrasound encounters rough surfaces or small objects it will also be scattered
Sound Wave	• The result of mechanical energy travelling through matter as a wave • Producing alternating compression and rarefaction resulting in vibration • The rate of vibration is measured in hertz (Hz)
Three Dimensional Ultrasound	• The creation of a computerised, reconstructed image which represents the anatomical structure being investigated, e.g. used to visualise the fetal face and the adult heart valves
Ultrasound	• Sound waves with a frequency of over 20 000 Hz • Not audible to the human ear • Diagnostic ultrasound uses 1–10 MHz frequencies
Voxel	A three dimensional pixel

Development

‘A’ Scan	• A one dimensional display • Shown as a graph • The amplitude of the returning echo on the vertical axis • The time of the returning echo on the horizontal axis
‘B’ Scan	• Good to demonstrate anatomy • Poor for vessels Partial volume averaging due to slice thickness Thermal noise Grating lobes Reverberations
Two Dimensional B Scans	• Obtain a rapid series of slices through the body • Slices are displayed in sequence • Gives a real time image composed of pixels • When a new image is stored the value of the pixel in the memory is updated and the image changes (real time scanning) therefore one frame equals one image • In freehand systems the spacing of the planes depends on the speed the probe is moved • If too fast there will be insufficient slices to reconstruct the image • Difficult to assess volume • Assumes objects have regular shapes and the axes are measured • The volume is then calculated mathematically • Two dimensional images can be difficult to interpret
Three Dimensional Scanning	• Aim is to acquire a series of two dimensional slices through a volume of tissue • Each slice is adjacent to the previous slice • The images are stored in the memory as voxels • Each voxel represents the signal from a specific volume of tissue • Therefore a slice can be taken through the volume in any direction • The exact relationship of the two dimensional slices must be maintained to give an accurate three dimensional image Fig. 17.1 Diagrammatic representation of a three dimensional volume of tissue.
Freehand Three Dimensional Scanning	• The probe with the position-sensing device attached is operated manually • Therefore the transducer must be moved in a smooth and regular manner • Information on space location and orientation is sent to the position sensor
	Advantages • The equipment can be added to an existing system • A normal probe is used • It is possible to scan through a small acoustic window • Information can be obtained by rotating an inserted probe
	Disadvantages • The image planes are not in a constant relationship • Overlapping of the scanned planes may occur • Spacing of the planes depends on the speed the probe moves • If the probe is moved too quickly insufficient slices will be captured to ensure accurate image reconstruction
Mechanical Three Dimensional Scanning	• A specialist probe is used to give precise movement • Two dimensional images are acquired at predetermined intervals • During the acquisition process the probe is manually held in a stationary position and moves in an arc • Or the transducer can move in a linear direction when the scanner head is mechanically driven over the area of interest
	Advantages • No overlap of the scanned planes • The angular relationship between the planes is constant
	Disadvantages • The probe is very large • It has limited use if the acoustic window is small
Viewing the Three Dimensional Image	Two dimensional slices
	• Most systems give a two dimensional display • Different perspectives can be viewed horizontally or vertically to give a coronal, sagittal or axial view of the patient
	Surface rendering
	• Requires distinct boundaries to be recognised by the system, e.g. fl uid and tissue • The area being imaged must not move during the acquisition
	Transparency mode
	• Projections ‘inside’ structures, e.g. the fetal skeleton • Requires distinct acoustic properties between adjacent structures, e.g. bone and tissue
The Advantages of Three Dimensional Scanning	• Time saving – once stored the image can be manipulated • Different planes can be viewed • Volume analysis can be undertaken – important to show small changes • Teleradiology is possible • It is possible to see special relationships in three dimensions, e.g. in obstetrics showing the fetus to the parents
	Application
	• Echocardiography • Obstetrics • Urology • Vascular work
Doppler Ultrasound	• Demonstrates movement, e.g. blood fl ow in vessels • Using the Doppler effect
Continuous Wave Doppler	• Transducer has two crystals • One to send echoes and one to receive echoes • Audible sound • Either on an analogue recorder or • Spectral parts analyser – separates the signal into the various parts and assigns importance to each part • Analysis by fast Fourier transform
	Advantages
	• High sensitivity to low velocities • Detection of high velocities without aliasing
	Disadvantages
	• Cannot distinguish between sending and receiving signals and extraneous echoes • Does not produce a precise image
Pulsed Wave Doppler	• Uses a pulsed echo system • The transducer sends and receives the signal • Sends in short bursts • Receives when not sending • Return signal is ‘gated’ – therefore only information from a predetermined depth is transmitted • The choice of gate is determined by the Nyquist Theorem
	Advantages
	• Due to gating, unwanted structures are eliminated from the image
	Disadvantages
	• Cannot exceed one half pulse repetition frequency or aliasing will occur
Colour Flow Duplex	• Combines B scanning with Doppler and colour to demonstrate motion, direction and velocity • Superimposes a colour image of moving blood over the real time scan • Colour gives the direction of fl ow • Intensity gives velocity strength • Linear or phased array transducers • Signal analysed by: Amplitude Phase – describes the direction and presence of motion Frequency shift – velocity of motion shown with various colour intensities Non-moving objects – shown as grey, the shade depends on signal strength
Tissue Harmonic Imaging (THI)	• Sound wave becomes distorted as tissue expands and contracts • At certain energy levels additional frequencies are generated called harmonics • Harmonics can be two or three times the frequency of the original sound wave • Travel from the originating tissue to the transducer
	The harmonic signal
	• Is weaker than the original sound wave • Purer than the original as it has less noise because: It only travels from the tissue to the transducer Can be isolated by filtering out the original signal • Demonstrates enhanced contrast and grey tone differences
	Application
	• Scanning structures in the middle distance range
Micro Vascular Imaging (MVI) Software	• Pulse-inversion harmonic ultrasound • Images individual micro-bubbles • Captures and stores 10 images per second • Processing techniques remove noise and subtract background information • Used to identify angiogenesis (growth of a network of blood vessels that indicates a tumour)
SonoElastography Software	• Tumours and some infl ammatory conditions can lead to loss of tissue elasticity • This technique demonstrates changes in tissue elasticity • Relative elasticity is automatically calculated and a colour overlay is produced on a conventional B mode scan, e.g. stiffer tissue is blue, more elastic tissue is red or benign tumours green, malignant tumours are blue
Real-time Virtual Sonography (RVS) Software	• A magnetic positioning unit detects the position (spatial location and orientation) of the transducer • Calculates the multiplanar construction images from the stored data • At the beginning of scanning, position adjustment is done at the xiphoid process • Displays on one monitor: Real time ultrasound image Corresponding CT or MR image from a stored data set
Real-time Virtual Sonography (RVS) Software	Used for: • Direct comparison of lesions • Monitoring of intervention procedures

Sources

	Baines P 2000 3D ultrasound; how does it work and what is it used for? Synergy, March
	Hughes J 2001 3D ultrasound imaging; an expanding technique. Synergy, October
	Powers J 2003 Micro Vascular Imaging (MVI). Synergy, April