Ultrasound Etc Flashcards
Unit of acoustic impedance
Rayl (Ry)
Acoustic impedance (Z) equation
Acoustic impedance (Z) = Medium density (p) x Ultrasound velocity in medium (v)
Wave equation
Velocity (v) {m/s-1}= Frequency (f) {Hz} x Wavelength (lamda) {metres}
High frequency waves
-Shorter wavelengths
-Higher interaction with materials and so it loses energy (attenuation)
What are higher frequency waves used for
To detect smaller features inside the body
Attenuation of ultrasound
I=I0e^-mewx
-I=Intensity (Watts, Joules)
-I0=original intensity
-mew=attenuation coefficient
-x=distance travelled
B scans
-Brightness scan
-Gives 2D image based on A scan information
-Built up from many A scans
A scan
-Amplitude scan
-Gives numeric information
Doppler effect
The artificial modification in apparent wavelength due to relative motion between source and observer
Key bits in discharging tube
-Cathode
-Evacuated chamber
-Vaccum
-Bevel anode with tungsten target
-Voltage
-Electron beam
-Protective lead shield
Cathode in discharging tube
Gives out electrons which are attracted to anode
Evacuated chamber
The air is removed to prevent collisions (energy loss) between electrons and air molecules
High voltage
To give electrons enough energy to leave the cathode
Bevel anode
Forces x-rays to exit the evacuated envelope
Electron beam
High energy electrons, energised off the cathode, accelerated across potential difference gap & strike tungsten target with high kinetic energy
What is the potential difference between electrodes
Approximately 150kV
What happens in x-ray (discharge tube)
Electrons are accelerated via high voltage from the cathode and pass through the potential difference gap, strike the bevel anode and slow down. The energy loss is converted to x-rays
What else increases as kinetic energy does?
Speed squared.
^KE=^ (speed)^2
What are continuous x-rays also called?
Bremsstrahlung x-ray photons/Braking x-rays
Ultrasound frequency range
Medical 2-18 MHz
US is anything above 20kHz
Transducer
A device which converts one form of energy into another
How piezoelectric crystals act as transducers
Convert pressure to energy, due to net zero charge the crystals are not pushed. This is due to single + charge of silicon balancing two - charge in oxygen at the back. When crystal pushed, the force pushes the ions together, causing a difference in charge, which generates an electric charge.
Mineral with piezoelectric properties?
Quartz crustal
Acoustic matching
Increases the transmission of ultrasound waves via a coupling agent (gel) due to air
Acoustic impedance of air
4.29x10^2
Acoustic insulator
Prevents interference from external sources (e.g. Someone speaking)
Plastic nose
Barrier between patient and instrument
Backing block/damper
Prevents returning signal resonating in the probe as it blurs image
Power cable
Alternate current with high frequency at 1-20MHz
Electrodes
Take Alt current and transit it to the crystal and also take the signal from the reflected pulse and transit to the cable
Acoustic impedance
How much the US wave is slowed down in a medium
Characteristic radiation
Bremstrahlung. Caused by electrons being ejected and another dropping down energy level. Only few discrete transitions thag can result in a characteristic peak
Why the reflection coefficient (alpha) is unitless?
The units cancel each other out?
Continuous/Bremsstrahlung
Energised electrons hit atoms in tungsten anode and are slowed down. Each electron interacts different with target nuclei so attenuation varies in continuum
X-ray beam
Intensity
Controlled by the electron beam current given in the machine. More current means more electrons per second so more x-ray photons. A higher current results in higher beam intensity, decreasing image production time
Alpha and Kbeta
Produced by electrons transitioning from one energy level to another
X-ray beam photon
Controlled by voltage. High voltage produces high energy photons with shorter wavelengths and high frequency. Lower voltage produces low energy photons with longer wavelengths and lower frequency.
How frequency affects ultrasound
Low frequency gives clearer image but low level of penetration
High frequency ultrasound are not as penetrating as they get attenuated more
Characteristic spectra
Peaks appear at high voltage when accelerated electrons collide with tungsten target atom’s electrons in orbitals and knows them out of their energy levels. As they move down energy levels, characteristic spectrum is emitted
Kalpha
Quantum 2 to 1
Kbeta
Quantum 3 to 1
Tungsten target
High melt point and high atomic number to increase positive charge, collisions and x-rays
Why acoustic matching?
Greater difference in acoustic impedance of tissues causes more reflection
