Bats make high-pitched chirps which are too high forhumans to hear. This is called ultrasoundLike normal sound, ultrasound echoes off objectsThe bat hears the echoes and works out what causedthem•Dolphins also navigate with ultrasound•Submarines use a similar method called sonar•We can also use ultrasound to look inside the body…
• Ultrasound – Cyclic sound pressure with a frequency greater than the upper limit of human hearing. • Human Ear Audible Range Frequency?
The human ear can onlyThe human ear canonly respond to theaudible frequency range~ 20Hz - 20kHz to theaudible frequency range ~20Hz - 20kHz
Medical sonography(ultrasonography) Ultrasound-based diagnostic imaging technique used to visualize muscles and internal organs, their size, structures and possible pathologies or lesions. APPLICATIONS? ADVANTAGES & DISADVANTAGES?
Diagnostic applications• Cardiology• Gynaecology & Obstetrics• Ophthalmology• Abdomen• Urology- to determine, for example, the amount of fluid retained in a patients bladder.• Musculoskeletal - tendons, muscles, and nerves• Vascular - arteries and veins• Interventional biopsy - emptying fluids, intrauterine transfusion
Therapeutic applications• Therapeutic applications use ultrasound to bring heat or agitation into the body.• Therefore much higher energies are used than in diagnostic ultrasound.
ULTRASOUND PHYSICSFormat What is sound/ultrasound? How is ultrasound produced Transducers - properties Effect of Frequency Image Formation Interaction of ultrasound with tissue Acoustic impedance Image appearance
Sound? Sound is a mechanical, longitudinal wave that travels in a straight line Sound requires a medium through which to travel
CATEGORIES OF SOUND Infrasound (subsonic) below 20Hz Audible sound 20-20,000Hz Ultrasound above 20,000Hz Nondiagnostic medical applications <1MHz Medical diagnostic ultrasound >1MHz
In 1826 DanielColladon, a Swissphysicist, and CharlesSturm, a Frenchmathematician,accurately measured itsspeed in water. Using along tube to listenunderwater (as Leonardoda Vinci suggested in1490), they recorded howfast the sound of asubmerged bell traveledacross Lake Geneva.Their result--1,435meters persecond in water of 1.8degrees Celsius (35degrees Fahrenheit)--wasonly 3 meters per secondoff from the speedaccepted today.
What is Ultrasound? Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz. Medical Ultrasound 2MHz to 16MHz
ULTRASOUND – How is it produced?Produced by passing an electrical current through a piezoelectrical (material that expands and contracts with current) crystal
Human Hair Single CrystalMicroscopic view of scanhead
In ultrasound, the following events happen:1. The ultrasound machine transmits high- frequency (1 to 12 megahertz) sound pulses into the body using a probe.2. The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).3. Some of the sound waves reflect back to the probe, while some travel on further until they reach another boundary and then reflect back to the probe .4. The reflected waves are detected by the probe and relayed to the machine.
1. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1540 m/s) and the time of the each echos return (usually on the order of millionths of a second).6. The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.
Piezoelectric material ACapplied to a piezoelectric crystal causes it to expand and contract – generating ultrasound, and vice versa Naturally occurring - quartz Synthetic - Lead zirconate titanate (PZT)
Ultrasound Production Transducer produces ultrasound pulses (transmit 1% of the time) These elements convert electrical energy into a mechanical ultrasound wave Reflectedechoes return to the scanhead which converts the ultrasound wave into an electrical signal
Piezoelectric Crystals Thethickness of the crystal determines the frequency of the scanhead Low Frequency High Frequency 3 MHz 10 MHz
Frequency also affects the QUALITY of the The frequency vs. Resolution ultrasound image The HIGHER the frequency, the BETTER the resolution The LOWER the frequency, the LESS the resolution A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz Low Frequency High Frequency 3 MHz 12 MHz
Broadband vs. NarrowbandNerve Visualisation: 5-10 MHz 6-13 MHz By altering the transmit frequencies one transducer replaces several transducers View a range of superficial to deep structures without changing transducers
Transducer DesignSize, design andfrequencydepend upon theexamination
Image FormationElectrical signal produces ‘dots’ on the screen Brightness of the dots is proportional to the strength of the returning echoes Location of the dots is determined by travel time. The velocity in tissue is assumed constant at 1540m/sec Distance = Velocity Time
Interactions of Ultrasound with Tissue Reflection Refraction Transmission Attenuation
Interactions of Ultrasound with Tissue Reflection The ultrasound reflects off tissue and returns to the transducer, the amount of reflection depends on differences in acoustic impedance The ultrasound image is formed from reflected echoes transducer
Refraction reflective refractionScatteredechoes Incident Angle of incidence = angle of reflection
Interactions of Ultrasound with Tissue Transmission Some of the ultrasound waves continue deeper into the body These waves will reflect from deeper tissue structures transducer
Interactions of Ultrasound with Tissue Attenuation Defined - the deeper the wave travels in the body, the weaker it becomes -3 processes: reflection, absorption, refraction Air (lung)> bone > muscle > soft tissue >blood > water
Interactions of Ultrasound with Tissue• Acoustic impedance (AI) is dependent on the density of the material in which sound is propagated - the greater the impedance the denser the material.• Reflections comes from the interface of different AI’s • greater ∆ of the AI = more signal reflected • works both ways (send and receive directions) Transducer Medium 1 Medium 2 Medium 3
Interaction of Ultrasound with Tissue• Greater the AI, greater the returned signal • largest difference is solid-gas interface • we don’t like gas or air • we don’t like bone for the same reason GEL!!• Sound is attenuated as it goes deeper into the body
Reflected Echo’s No Reflections = Black dots Fluid within a cyst, urine, blood ‘Hypoechoic’ or echofree
What determines how far ultrasound waves can travel? The FREQUENCY of the transducer The HIGHER the frequency, the LESS it can penetrate The LOWER the frequency, the DEEPER it can penetrate Attenuation is directly related to frequency
Ultrasound Beam Depth• Need to image at proper depth• Can’t control depth of beam • keeps going until attenuated• You can control the depth of displayed data
Ultrasound Beam Profile Beam comes out as a slice Beam Profile Approx. 1 mm thick Depth displayed – user controlled Image produced is “2D” tomographic slice assumes no thickness You control the aim 1mm
Goal of an Ultrasound System The ultimate goal of any ultrasound system is to make like tissues look the same and unlike tissues look different
Accomplishing this goal dependsupon... Resolving capability of the system axial/lateral resolution spatial resolution contrast resolution temporal resolution Processing Power ability to capture, preserve and display the information
Types of Resolution Axial Resolution specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects frequency (wavelength) affects axial resolution – frequency resolution
Types of Resolution Lateral Resolution the ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects beamwidth affects lateral resolution
Types of Resolution Spatial Resolution also called Detail Resolution the combination of AXIAL and LATERAL resolution - how closely two reflectors can be to one another while they can be identified as different reflectors
Types of Resolution Temporal Resolution the ability to accurately locate the position of moving structures at particular instants in time also known as frame rate
Types of Resolution Contrast Resolution the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects - dependant on the dynamic range